FINAL DEGREE PROJECT
Chemical Engineering TREATMENT OF CONTAMINANTS OF EMERGING CONCERN WITH ADVANCED OXIDATION PROCESSES. Study on the state of art of dosage and modeling in the photo-Fenton process.
Report and Annexes
Autor: Nora Pujol Vidal
Director: Montserrat Pérez-Moya
Call: June 2020
Escola d’Enginyeria Barcelona Est (EEBE)
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Study on the state of art of dosage and modeling in the photo-Fenton process.
RESUM
Una de les preocupacions més generalitzades d’avui en dia recau sobre la situació ambiental del planeta, i concretament sobre la contaminació de l’aigua. La industrialització ha generat un augment en la quantitat de components residuals que son difícils d’eliminar per mitjà de mètodes convencionals en plantes de tractament d’aigua.
El present treball es centra en els processos d’oxidació avançada, concretament en els processos Fenton i foto-Fenton. Estudia la seva posició actual en el sector de la recerca així com les temàtiques en els que manca recerca o no hi ha consens. Presenten punts pendents de resoldre.
Un primer estudi bibliomètric s’ha dut a terme per tal de detectar, i posteriorment analitzar, aquests temes sense estudis concloents. Un cop decidit que el dosatge i el modelatge són temes sense conclusions fermes, s’ha realitzat una comparació bibliogràfica basada en aquests dos aspectes.
El projecte es centra primerament en fer un estudi generalitzat del procés. Seguidament en una posada en marxa de la realització experimental i, finalment en un anàlisi més detallat dels temes escollits per aprofundir. Pel que fa el dosatge, gran part de les publicacions es centren en la proposició de diverses tècniques d’addició, tant de reactius com de catalitzadors, que poden optimitzar-ne el consum i, en ocasions, millorar l’efectivitat de les reaccions per a contaminants concrets. No obstant, en cap cas s’ha trobat una proposta general i extrapolable a diferents equips, contaminants, reactius, etc.
D’altre banda, en un estudi fet referent a la modelització es pot observar que és possible fer una classificació dels models segons principis bàsics, models empírics i models semi-empírics. A més a més, s’hi menciona la inclusió del modelatge per l’automatització i control dels processos, un camp en el que clarament cal seguir fent recerca.
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RESUMEN
Una de las preocupaciones más generalizadas de hoy en día recae sobre la situación ambiental del planeta, y concretamente sobre la contaminación del agua. La industrialización ha generado un aumento en la cantidad de componentes residuales que son difíciles de eliminar por los métodos convencionales en plantas de tratamiento de aguas.
El presente trabajo se centra en los procesos de oxidación avanzada, concretamente en los procesos Fenton y foto-Fenton. Estudia su posición actual en el sector de la investigación, así como las temáticas en las que manca investigación o no existe un consenso, aquellas que presentan puntos pendientes de resolver.
Un primer estudio bibliométrico se ha realizado por tal de detectar, y posteriormente analizar, dichos temas sin estudios concluyentes. Una vez decidido que el dosaje y el modelaje son términos sin conclusiones firmes, se ha llevado a cabo una comparación bibliográfica centrada en estos dos aspectos.
El proyecto se centra primeramente en hacer un estudio generalizado del proceso. Seguidamente en una puesta en marcha de la realización experimental y, finalmente en un análisis más detallado de los temas elegidos para profundizar. Por lo que el dosaje respecta, una gran parte de las publicaciones se centran en la proposición de diversas técnicas de adición, tanto de reactivos como de catalizadores, que pueden optimizar su consumo y, en ocasiones, mejorar la efectividad de las reacciones para contaminantes concretos. No obstante, en ningún caso se ha encontrado una propuesta general y extrapolable a diferentes equipos, contaminantes, reactivos, etc.
Por otro lado, en el estudio referente a la modelización se puede observar que es posible hacer una clasificación de los modelos según principios básicos, modelos empíricos y modelos semi empíricos. Además, en el estudio se menciona la inclusión del modelaje para la automatización y control de los procesos, campo en el que claramente es necesario seguir investigando.
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ABSTRACT
The actual framework of water contamination in our planet is one of the major concerns nowadays. Industrialization has generated an increase in the amount of residual components which are difficult to remove with the methods applied in the conventional water treatment plants.
The present work is focused on the advanced oxidation processes, concretely on the Fenton and photo-Fenton processes. It studies their actual situation in the research sector as well as the subjects lacking investigation or having no consensus, those presenting open issues.
A bibliometric study has been done in order to detect, and furthermore analyze, the mentioned themes without conclusive studies. Once decided that dosage and modeling are terms with no solid conclusions, a bibliographic comparison centered on the above-mentioned aspects has been carried out.
The project aims, first, at making a general study of the process. Successively, at developing the experimental set-up and, lastly at a detailed analysis on the subjects chosen to go in depth with. Regarding dosage, the majority of the publications found in literature are focused on the proposal of diverse adding techniques applied for both reactants and catalysts, which are able to optimize the consumption and, occasionally, improve the effectiveness of the reactions. Nonetheless, in no case a general and extensible suggestion has been offered for different equipment, contaminants, reactants, etc.
On the other hand, in the study regarding modeling it can be observed that it is possible to classify the models in first principles, empirical, and semi-empirical models. In addition, the involvement of modeling into automatization and control of the processes is also reported, which is a field that clearly needs investigation in a greater extent.
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AKNOWLEDGEMENTS
In the first place, I would like to thank my parents and my brothers for their constant support, help and patience during this period of my life. For making me feel confident in my abilities not only during this project, but for the entire degree studies. It surely would not have been possible without them.
Above all, special thanks to my friends for their persistent encouragement and faith in me.
Thanks to my project tutor, Montserrat Pérez-Moya, for her guidance and dedication. I would like to express my gratitude to her for giving me the opportunity to be a part of this work and for her help and understanding given the exceptional circumstances. I must also acknowledge my project to Saidy Cristina Ayala, for teaching me with patience everything the experimental part of the project required.
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TABLE OF CONTENTS
RESUM…………...... i
RESUMEN……...... ii
ABSTRACT……...... iii
AKNOWLEDGEMENTS ...... iv
CHAPTER 1 INTRODUCTION ...... 1
1.1 Motivation ...... 2
1.2 Objective ...... 4
1.3 Scope of the project ...... 4
1.4 Sources of water pollution ...... 6 1.4.1 Contaminants of emerging concern (CEC’s) ...... 7 1.4.2 Pharmaceutical wastewater ...... 8 1.4.3 Agricultural wastewater ...... 8 1.4.4 Textile industries wastewater ...... 9 1.4.5 Paper mill industry ...... 10
1.5 Water-quality indicators ...... 10 1.5.1 Chemical Oxygen Demand (COD) ...... 11 1.5.2 Biological Oxygen Demand (BOD) ...... 12 1.5.3 Biodegradability ...... 12 1.5.4 Toxicity ...... 13
CHAPTER 2 TREATMENTS ...... 15
2.1 Conventional treatments ...... 16 2.1.1 Conventional wastewater treatment plants ...... 16
2.2 Advanced Oxidation Processes (AOPs) ...... 18 2.2.1 Introduction ...... 18
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2.2.2 Fenton and photo-Fenton reaction mechanisms ...... 20 2.2.3 Effects of main operational conditions ...... 23 2.2.4 Analytical methods for photo-Fenton treatments...... 29
2.3 Combination of water treatments ...... 36
CHAPTER 3 EXPERIMENTAL METHOD AND RESULTS ...... 41
3.1 Experimental Assembly ...... 42 3.1.1 Reactor ...... 42 3.1.2 Materials and reactants ...... 42
3.2 Experimental method ...... 42
3.3 Analytical calibration ...... 44 3.3.1 TOC measurement ...... 44 3.3.2 Contaminant determination ...... 45
3.3.3 Calibration for the H2O2 determination ...... 47
3.4 Design of experiments ...... 50
3.5 Project reorientation ...... 51
CHAPTER 4 BIBLIOGRAPHIC COMPARISON ...... 52
4.1 Introduction ...... 53
4.2 Dosage ...... 58 4.2.1 General bibliographic analysis of operational conditions...... 59
4.2.2 Analysis of hydrogen peroxide dosage ...... 81 4.2.3 Analysis of catalyst dosage ...... 87
4.3 Modelling ...... 94 4.3.1 First-principles models ...... 97 4.3.2 Empirical models ...... 99 4.3.3 Semi empirical models ...... 107
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CHAPTER 5 ENVIRONMENTAL IMPACT ...... 112
5.1 Experimental impact ...... 113
5.2 Computational impact ...... 114
CHAPTER 6 CONCLUSIONS ...... 115
6.1 Project conclusions ...... 116
6.2 Future works ...... 118
CHAPTER 7 ECONOMIC EVALUATION ...... 119
7.1 Introduction ...... 120
7.2 Experimental budget estimation ...... 120 7.2.1 Materials and reactants costs...... 120 7.2.2 Services costs ...... 122 7.2.3 Personnel costs ...... 123 7.2.4 Amortizations ...... 124 7.2.5 Final cost ...... 125
7.3 Research cost estimation ...... 125
CHAPTER 8 REFERENCES ...... 127
CHAPTER 9 ANNEXES ...... 140
9.1 Safety data sheets ...... 141
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CHAPTER 1 INTRODUCTION
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1.1 Motivation
Water is the most essential element for all the living beings. The rapid global economic growth and the enlargement of industrialization in the actual society has generated an overexploitation of freshwater resources, a clear water crisis. (Ortega-Gómez, Fernández-Ibáñez, et al. 2012; Rahim, Abdul, and Wan Daud 2015). This is leading to one of the most serious problems facing the world: water scarcity and the lack of access to safe drinking water.
Accordingly, one of the most important issues the society is facing nowadays is the continuous accumulation of pollutants poured in water effluents which, consequently, turns it into contaminated water. Hence, the main concern for water resources is the disinfection of drinking water as well as wastewater, used in multiple sectors such as agriculture and aquifer recharging.
Due to the constant growth of the population demand on decontaminating waters, the important accomplishment of new regulations and the fact that water is a limited resource, it has become necessary to develop new purification technologies. These have the aim to ensure water quality requirements as set out by water reuse legislation.
Apart from domestic water use, petroleum, textile, agriculture, or pharmacy are just a few out of the several industrial sectors responsible of the water consumption and pollution. Many released waters -mostly from industrial and agriculture- contain substances which are not easily removed, most of them are recalcitrant compounds; chemicals that are to various degrees resistant to microbiological degradation in soil and water. Domestic-use water will not be discussed in the course of this work.
Agricultural water represents the more than two thirds of the global use of water (Figure 1.1), and can represent more than 90% of the consumed water in developed countries. Nevertheless, agricultural and industrial wastewater contain the most concerning contaminants for the environment.H2O2
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Figure 1.1: Trends in the global water use by sector. Source:(Diop et al. 2002)
Considering the expected population growth, water demand will keep increasing over the years. Moreover, conventional water treatments (i.e. biological treatments) are not suitable for this kind of contaminants and European environmental legislation (Kallis and Butler 2001) set specific boundaries in this area that cannot be overpassed. Based on these facts, the research of alternative methods and technology improvements has intensified in other to obtain re-useful water and amend water treatments. (Ballés i Canals 2018)
Advanced Oxidation Processes (AOPs) are considered as powerful methods for degradation of these pollutants due to their ability for removing almost every organic pollutant.
No evidence has been reported for the complete effectiveness of these processes and new features are still under development. (Cabrera Reina et al. 2012; Navalon, Alvaro, and Garcia 2010).
Extensive research has been done but further studies are required due to the constant discovery of new harmful pollutants. (Miralles-Cuevas et al. 2013)
Among the Advanced oxidation processes, Fenton and photo-Fenton account for many of the reported works in this field. Advances in the efficient, economical and sustainable fields of these oxidations are being reviewed. Hence, in this study the focus is set on the analysis of different operational conditions that manage to obtain finest results.
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1.2 Objective
The aim of this work was to broaden the state of art of the Advanced Oxidation Processes (AOPs), emphasizing on the Fenton and photo-Fenton processes. For that, an accurate research has been carried out among the different studies realized until now.
The aims of this project are:
• Evaluate the actual framework of water treatment and photo-Fenton treatment. • Study and analyze the photo-Fenton processes and emphasizing the parameters affecting the system. • Evaluate the viability of treating Venlafaxine with photo-Fenton process. • Generate a design of experiments (DOE) to find out the best operational conditions. o The exceptional working conditions stablished on behalf of COVID-19 hindered the laboratory activities and thus, the development of experiments. Supplementary objectives were formulated. • Make a preliminary research of the photo-Fenton system and determine the main research gaps. • Study the system variables influencing the final effectiveness of the process, calling attention on the open issues. • Make a brief review on these knowledge niches. Study the quantity of researches done in these subjects.
1.3 Scope of the project
The below-displayed flow chart diagram is a schematic representation of the project scope. It is efforted by the author, trying to make more visual the reorientation of the work and the principal points to which the project is focused on. The initial project approach is colored in grey, and the subsequent plan characteristics are further developed.
The scope of this reoriented project is to analyze the current situation of water. Focusing on the principal pollutants, contamination indicators and removal treatments applied. The below- displayed flow chart is representative of the area embraced in the project.
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Study on the state of art of dosage and modeling in the photo-Fenton process. Current framework: water
Pollutants Indicators Treatments
• CEC’s • COD Conventional AOPs Others • Pharmaceutical wastewaters • BOD Combination with • Textile wastewaters • Biodegradability • Agriculture wastewaters • Toxicity Fenton and photo- • Paper mill industry Fenton
Operational conditions Experimental methodology Kinetics
1. [Catalyst] • pH • Preliminary experiments • Determination of study Modelling 2. [H2O2] Dosage • Reagents variables 3. Ratios • Homogeneous vs Heterogeneous • Design of experiments Simulation • Temperature • Analysis of the experimental results Validation • Inorganic ions Optimization • Chelating agents
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1.4 Sources of water pollution
Due to the extent industrialization held in our planet, many concerning contaminants emerge from industries wastewaters. This work will be focused on the study of four main wastewater sources: pharmaceutical, agrochemical, textile and paper mill industries. A brief resume of the general contaminants of emerging concern (CECs) and the mentioned sectors of industry is explained consecutively.
The awareness of these effluents has risen popularity among literature in the past years. The below- showed pie chart, represents the percentage of research articles, reviews and conference abstracts mentioning the indicated sources of contaminants.
CECs pharmaceutical agricultural textile papermill
5%
25%
46%
12%
12%
Figure 1.2: Pie Chart representing the percentages of papers published on each sector since 2010. Data obtained from (ScienceDirect 2020). Data from 2020 is only considered until June.
As it can be observed CECs is the most researched topic followed by textile wastewater. Papermill wastewater has been the less investigated in the last decade.
Throughout the years investigations about emerging contaminants have been rising. Nevertheless, within a broader context, one could set the focus on emerging contaminants (ECs) or contaminants of emerging concern (CECs). (Sauvé and Desrosiers 2014). The first could be understood as the newly appeared pollutants, while the second concept applies to contaminants for which concern have emerged recently, but they have in fact been in the environment singe long time ago. This argue could be blurring as some authors talk about ECs when they actually referring to CECs.
In Figure 1.2 the CECs group is actually containing the research done by ECs and CECs words separately. It has been considered all the same. Further specification on this contaminants is subsequently added.
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1.4.1 Contaminants of emerging concern (CEC’s)
The correct definition of CECs refers to all the chemicals substances that are continuously discharged in water, soil or air that had not been previously detected or were present in insignificant amounts. (US EPA 2019). These environmental pollutants come from domestic and industrial sewage systems, and have become a major concern for its harmful ecological and human effects (Herrero et al. 2012). However, despite the research efforts which arose with the worry for the scientific community, limited information is available about the consequences of the emerging contaminants.
CEC’s are a group of chemicals including pharmaceuticals, personal care products (PPCPs), hormone disrupting substances, brominated flame retardants, toxic elemental species, etc. (Vandermeersch et al. 2015; Vidal-Dorsch et al. 2012; Audino et al. 2019) , which are more and more being detected at low concentrations in waters. Relevant examples of such emerging compounds are those which can cause a determinant effect on the environment without persisting in it, their continuous introduction in the environment compensates the high removal rate (Oller, Malato, and Sánchez- Pérez 2011). Conventional sewage plants are not able to remove this kind of contaminants (US EPA 2019).
According to C.G. Sousa et al (Sousa et al. 2018), surface water should be protected through risk assessment, alleviation measures for the release of contaminants, monitoring programs, etc. For example, control the micropollutant releases in the sources of their production, implementation and discharge as done by Kolpin et al. (Kolpin et al. 2002). In this regard, in 2000 a structure for an EU action in the water police was stablished by Directive 2000/60/EC (European Parliament and Council of the EU 2000). Subsequently, a list of 33 priority substances considered to be monitored was published in the 2008/105/EC directive (European Parliament and Council of the EU 2008), altered from the previous one. Additional directives were settled over the following years until 2013, when a first Watch list was proposed (Directive 39/2013/EU, (European Parliament and Council of the EU 2013)). The Watch List is a guideline of substances for which Union-wide monitoring data need to be gathered for the purpose of supporting future prioritization exercises in the EU, the complete list being published in the Decision 2015/495/EU (2015) (EU Comission 2015).
The report proposes the following seven substances/groups of substances to complete the first Watch List: ▪ Oxadiazon ▪ Methiocarb ▪ 2,6-ditert-butyl-4-methylphenol ▪ Tri-allate
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▪ Imidacloprid, Thiacloprid, Thiamethoxam, Clothianidin, Acetamiprid ▪ Erythromycin Clarithromycin, Azithromycin ▪ 2-Ethylhexyl 4-methoxycinnamate
1.4.2 Pharmaceutical wastewater
In the last two decades, many antinflammatory, analgesics, betablockers, lipid regulators, antibiotics, anti-epileptics and estrogens have been detected in soils, surface waters, ground waters and drinking waters. Parent compounds can reach the environment not only via pharmaceutical industries or domestic sewage treatment plant effluents, but also from health center waste waters or landfill leachates.
It is estimated that the global consume of these products is 10.000 tones/year (Joss et al. 2006). The main consequences are the massive usage in industries as well as in daily life.
This industrial sector makes use of toxic solvents and intermediates which are discharged to wastewaters after their usage in the processes. (Gustavo TrovóTrov, Alessandra Santos Melo, and Fernandes Pupo Nogueira 2008). These components are normally lipophilic1 and non- biodegradable regarding the accepting media. Therefore, it is one of the major concerns in relation with water pollution nowadays.
Even though they appear in aquatic environments with a concentration in range of micro to nano quantities, they are toxic in nature and can exert severe results due to its long period of presence in the environment.
Pharmaceutical substances and care products are spreading increasingly in the environment and yet are difficult to treat using conventional technologies. Antibiotic wastewaters, for instance, has a high chemical oxygen demand (COD) and a low biological oxygen demand (BOD)- concepts that will be further explained. Consequently, biological processes are unsuitable for wastewater treatment. (Elmolla and Chaudhuri 2009a)
1.4.3 Agricultural wastewater
Pesticides including herbicides, insecticides and fungicides are the dominant species contaminating agronomical waters that are further joined with ground waters or surface waters.
1 Lipophilic: with the ability to dissolve in fats, oils, lipids and non-polar solvents.
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These products are principally used for agricultural and health proposal, and a large quantity are currently used throughout the world. Most of them are considered a priority substance in the European Community Directives because of their hazardousness and toxicity towards the environment (El Bakouri et al. 2009; EU 2001).
(EU 2001)Not only the cultivated areas are the sources of this type of water pollutants but also the discharges from pesticide production plants can have direct effect on the health of the living organisms even at micro-concentrations (Rahim, Abdul, and Wan Daud 2015).
To the same extent with the other mentioned pollutants, the removal of these contaminants from water involves the implementation of more complex treatments than the conventional ones.
1.4.4 Textile industries wastewater
The textile industry is one of the industrial sectors of major complexity since it is a fragmented heterogeneous sector, based mostly on little or medium companies.
The categorization of wastewaters proceeds through a consideration of the nature of all the processes employed and the chemicals associated. This industry covers an extent industrial chain which arises chemical pollutants in a large range of processes that include: sizing and desizing, weaving, scouring, bleaching, mercerizing, carbonizing, fulling, dyeing and finishing (Correia, Stephenson, and Judd 1994).
The effluents released from the textile industries contain chemicals such as dyes, dispersants or leveling agents which can be biodegradable and not biodegradable. The composition of the wastewater produced is subject to the possible change coming from the diversity of the textiles employed and the range of chemicals within each industrial category (Singh et al. 2019).
Bleaching and dyeing are the processes containing more stages and therefore have the possibility to be more involved in the pollution issue. Evidently, there is a difference on the treatment applied to each stage. These two specific processes contain appreciable amounts of organic compounds. Biological or conventional treatments are not amenable for degrading the pollutants. Therefore, advanced treatments such as photo-Fenton processes are used to treat these wastewaters.
The main environmental impacts are generated from water and energy consumption, suspended solids and atmospheric emissions. Among these, the strongest impact is related to water consumption (80-100 m3/ton of finished textile) , wastewater discharge characterized by low biodegradability, which is composed by 115-175 kg of COD/ton-of-finished-textile and organic chemicals (Oller, Malato, and Sánchez-Pérez 2011; Savin 2008). Given this circumstance, the reuse of effluents is an economic and ecological challenge for the textile sector.
The release of these effluents is very problematic for the aquatic life and the natural environment as well as mutagenic to human beings. The conventional treatments applied such as
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coagulation/flocculation, membrane separation or elimination by activated carbon adsorption are only useful for a phase transfer of the contaminant. Moreover, as it happens with the pharmaceutical contaminants, the biological treatments are not enough to fully degrade the pollutants.
1.4.5 Paper mill industry
Various stages of pulping and paper making activities are involved in the production of paper. Within these, bleaching process is one of the most contaminants stages. Accordingly, large amount of fresh water is consumed and wastewater released, creating an impact on the environment due to the chemicals and substances present during the production activities. Moreover, these substances are a threat for the well-being of the wild and human life (Ashrafi, Yerushalmi, and Haghighat 2015).
Paper mill effluent is a combination of organic (acids, phenolic compounds, sugars) and inorganic contaminants raised from tannins, lignin, resins, etc., derived from the wood pulp and the chemical additives (Ashrafi, Yerushalmi, and Haghighat 2015; Buzzini and Pires 2007).
Today, most paper mills use aerobic and anaerobic conventional processes to deal with the pollutants. These involve physical and biological techniques that do not lead to the complete degradation of recalcitrant organic matter (Lucas et al. 2012a).
Suitable internal management and external recycling and reusing is a highlighted necessity nowadays as the production and the demand of water supplies keeps increasing. This industry faces a shortage of available water supply. Thus, freshwater consumption must be reduced. Hence, improving wastewater discharge quality and reusing wastewater as a process water requires the implementation of advanced treatments such as AOPs.
1.5 Water-quality indicators
There is a difference between quality indicators stablished by the pouring regulations and the concrete contaminant or substance indicators.
The former is normally referred to percentages or ranges and centered on measuring the contaminant as a lumped parameter. That is to say, parameters such as toxicity, biodegradability,
CO2, BOD or COD are regulated by ordinances and are normally measured to make sure that the dealing water accomplishes these regarding directives.
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A brief explanation of these lumped parameters is reported in this section as they are the parameters normally cited and/or requested by the general normative.
On the other hand, specific regulations are stablished for contaminants itself. Particular water- quality indicators are determined for very toxic substances. Each treatment employed considers different quality indicators depending on the reagents and catalysts used. Regarding this work, indicators will be described for the specific photo-Fenton substances (section 2.2.4).
1.5.1 Chemical Oxygen Demand (COD)
The organic matter in water is consumed by the bacteria and algae normally present in healthy water sources and it leads to an increased concentration of these microorganisms. Moreover, wastewater also contains oxidizable organic and inorganic compounds which consume the oxygen available in the ecosystem.
COD indicator can be described as the amount of oxidant that reacts with the sample under controlled conditions (Jenkins 1982). The quantity of oxidant consumed is expressed in terms of its oxygen equivalence represented in mg of O2 per liter.
Although it is not a specific compound, it has been used worldwide to gauge overall treatment plant efficiencies (Moreno-Casillas et al. 2007). It can also be used as a lumped indicator for the degree of pollution in the effluent and to evaluate the environmental impact of the bodies of the wastewater.
COD can be theoretically calculated in order to compare it with the sequential experimental results (Eq. 1.1):
푚푔 푠푢푏푠푡푎푛푐푒 ( ) · 32 · [푁표. 퐶] Eq. 1.1 [ 퐿 ] 퐶푂퐷푡 = 푀푊푠푢푏푠푡푎푛푐푒
Where 퐶푂퐷푡 corresponds to the theoretical COD calculated and 32 corresponds to the molecular weight of Oxygen.
In order to have an idea of the amount of organic and inorganic compounds present in water that could be liable to be oxidized, an oxidation test is done using potassium dichromate (COD-Cr) and potassium permanganate (COD-Mn). These are agents with a strong oxidant power in acidic environment. The first is often used for residual waters whereas COD-Mn for the analysis of natural and mineral waters.
The extent of sample oxidation test can be affected by some factors such as reagent strength, specific COD of the sample or digestion time.
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1.5.2 Biological Oxygen Demand (BOD)
In order to be able to evaluate the type of treatment that must be applied, the residual charge in water is measured. It corresponds to the Biochemical Oxygen Demand (BOD), an empiric determination in order to measure the oxygen requirements in wastewaters, effluents and contaminated waters.
It consists on the quantification of the molecular oxygen used during a specific incubation period for the biological degradation on the organic matter and the oxygen needed for the oxidation of inorganic matter. It is measured in units of mg of O2/L (ppm of O2). This oxygen demand can also be estimated for the oxidation of reduced nitrogen forms (Srandard Methods 2020).
Three different measurement methods can be considered depending on the test period. Oxygen consumed in a period of 5 days (BOD5), oxygen consumed after 60 to 90 days of incubation (ultimate BOD), and continuous oxygen uptake (respirometric method). Nevertheless, many other variations of oxygen measurement exist including shorter and longer incubation times.
1.5.3 Biodegradability
As mentioned above, organic matter content in wastewaters is measured with the BOD and the COD indicators. The ratio between these two parameters is an indicator of the degree of degradation of the wastewater. It is also called biodegradability ratio and it evaluates the scope of degradation by microbes.
Numerous reviewers have studied the optimum value or range of values that would correspond to an adequate biodegradation. Accordingly, the lower the BOD/COD ratio, the less biodegradable is the substance. Biodegradation tests should be conducted to ensure the effectiveness when applying biological treatments.
The previously mentioned BOD5, which is a percentage of the COD (Eq. 1.2), is considered proper indicator for the substance biodegradability.
퐵푂퐷5 = (%) · 퐶푂퐷 Eq. 1.2
Often, the BOD/COD ratio is measured regarding the BOD5/COD. Nevertheless, the same behavior is followed. The closer the result is to the unit, the more biodegradable is the substance since the major part of organic matter to be oxidized is organic, thence it can be decomposed by microorganisms.
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The procedure to obtaining a measure of biodegradability implies the quantification of the involved parameters. Regularly, the values are expressed as ranges. An example of pharmaceutical wastewater biodegradability stages is presented in the following Table 1.1:
Table 1.1: Pharmaceutical wastewater BOD5/COD ratios. Source: (Nelly et al. 2012)
BOD5/COD Characterization >0.8 Easily biodegradable 0.7 – 0.8 Average biodegradable 0.3 – 0.7 Slowly biodegradable <0.3 Not biodegradable
Even though biodegradability is not directly connected to toxicity, it could be deduced that, in the case of an advanced pretreatment followed by a biological process, if the toxicity is enhanced during the pretreatment then biodegradability of the solution should decrease. (Mirzaei et al. 2017)
1.5.4 Toxicity
The current guidelines and directives have been based on the detection of specific pollutants in water, included in a list of priority concern. The presence of these contaminants in water is what is described as toxicity (Hernando et al. 2005).
Certain analytical methods such as gas or liquid chromatography coupled with spectrometry have been implemented for the monitoring of these priority contaminants. In addition to emerging pollutants, in the last few years, several articles have reported the presence of substances coming from pharmaceutical and agriculture residues, from textile or paper industries and even from domestic residual waters which are considered of major concern.
The quality control can be based on many chemical measures such as the presence of Total Organic Carbon (TOC). Nevertheless, some authors point out that the detection of specific pollutants is not enough to determinate the risks.
Specifying the grade of toxicity in a residual water is currently an open issue due to the difficulty of determining which are the parameters that must be considered to measure the toxicity effects on the ecosystem.
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Some of the above-mentioned pollutants have been described as dangerous for the human and animal health (i.e., carcinogenic) and bio-cumulable, meaning exceedingly difficult to degrade (i.e., DDT2).
In addition, once in the environment, substances undergo biotransformation, a process which releases degradation products and metabolites of these emerging compounds. The produced by- products during the degradation can be more toxic than the parent compound (Oller, Malato, and Sánchez-Pérez 2011; Prieto-Rodríguez et al. 2013; Mirzaei et al. 2017).
Toxicity assessment is crucial in pretreatment processes. Assays may be applied as a criterion to select the type of pretreatment process of biotreatment operations.
Various tests such as D. magna, Selenastrum capricornutum, V. fischeri, Pseudomonas, Staphylococcus aureus, Escherichia coli, Phaeodactylum tricornutum, Pseudokirchneriella subcapitata and Lepidium sativum have been used for toxicity assessment (Mirzaei et al. 2017; Santos-Juanes et al. 2011).
It must be mentioned that the generated intermediates can be substantially altered by the water matrix and hence the toxicity may also present variations.
2 DDT: Dichlorodiphenyltrichloroethane, commonly known as DDT, is a colorless, tasteless, and almost odorless crystalline chemical compound. Originally developed as an insecticide.
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CHAPTER 2 TREATMENTS
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2.1 Conventional treatments
Conventional wastewater treatments are a combination of physical, chemical and biological processes to remove organic matter or solids from wastewaters. A rapid and little detailed explanation is done on these treatments in order to contextualize because they are not considered within the key sections of the project scope.
Different degrees of treatment can be described embracing preliminary, primary, secondary and tertiary or advanced wastewater treatments.
2.1.1 Conventional wastewater treatment plants
• Preliminary processes
The aim of the preliminary treatment is to remove large and rough materials found in the water that will be treated. The removal of these materials is necessary to enhance the operation and maintenance of the subsequent treatment units.
• Physical or primary processes
Based on unit operations of separation, the physical processes do not imply any alteration on the chemical structure of the contaminant. The main targets are to split apart the particles of suspended solids present in the treated water and the removal of materials floating (scum).
During this treatment approximately 25 to 50% of the incoming BOD5, 50 – 70 % of the suspended soils and almost all grease and fat are eliminated (Terán 2016; FAO 2020), while colloidal and dissolved constituents are not affected.
Some examples are decantation, homogenization, filtration, coagulation, flocculation, precipitation or neutralization.
• Biological or secondary processes
Processes in charge of removing the biodegradable organic matter using aerobic or anaerobic biological treatment processes.
By spreading the water throughout a film of microorganisms the organic matter is degraded. Microorganisms use the organic matter coming from the effluent as nutrients and generate sub- effluents which are posteriorly treated or eliminated by sedimentation. The biological solids removed during this treatment are called biological sludge and can be processed together with the residues coming from the primary process.
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Nonetheless, not all the organic matter present in the effluent is biodegradable, thus a part of it remains without being oxidized. Therefore, a tertiary treatment is applied.
• Chemical or tertiary processes
When specific wastewater constituents cannot be removed by the secondary treatment, tertiary treatment is employed. There is a large number of encountered microorganisms and other contaminants which have a low degradation rate in water plant treatments. Some of these difficult- to-remove substances are (FAO 2020):
- Nitrogen - Phosphorus - Refractory organics - Heavy metals - Dissolved solids
Removal through membranes or using oxidant agents such as chlorine or ozone has been employed recently, Jiang and Lloyd (Jiang and Lloyd 2002) reported some of the oxidizers and their redox potential (Table 2.1).
Table 2.1: Redox potential for oxidants used in water and wastewater treatments. Source: (Jiang and Lloyd 2002) Oxidant reagent Eº (V) Chlorine 1.358 / 0.841 (depending on the reaction) Hypochlorite 1.482 Chlorine dioxide 0.954 Perchlorate 1.389 Ozone 2.076 Hydrogen peroxide 1.776 Dissolved oxygen 1.229 Permanganate 1.679 / 1.507 (depending on the reaction) Ferrate (VI) 2.20
Because of its advanced treatment it is sometimes used directly in place of secondary processes.
In the present day three subgroups of chemical treatment exist (Terán 2016):
- Classical oxidation processes - Electrochemical processes - Advanced oxidation processes (AOPs), which will be extensively explained in this work.
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2.2 Advanced Oxidation Processes (AOPs)
2.2.1 Introduction
The application of water treatment methods sets its focus on the nature and physicochemical properties of the effluents.
Polluted water, usually a result of human activity, can be efficiently processed by biological treatments, active-carbon adsorption or treated with conventional chemical treatments (i.e., biological treatments). Nevertheless, these treatments are not efficient enough to reach the levels of purity that the legislations or the ulterior usages require. Therefore, in most industrialized countries the application of AOPs is becoming increasingly popular.
Advanced oxidation processes were firstly concocted by Glaze et al. in 1987 (Glaze, Kang, and Chapin 1987) and include a group of techniques capable of generating highly oxidant species in situ. The oxidants consist mainly in ·OH radicals, but lately, the AOP concept has been extended to - oxidative processes with sulfate radicals (SO4 ). AOPs oxidants are mainly applied for the elimination of organic and inorganic pollutants while other common oxidants as Chlorine or Ozone can be applied in a dual role; decontamination and disinfection. AOPs radicals are rarely used for disinfection as they have too short life for the required detention time. Thus, when these oxidants are applied to wastewater treatments, they are expected to destruct the contaminants and transform them to less-toxic products.
Hydroxyl radicals are the most reactive oxidizing agents in water treatments, which have an elevated oxidative potential (Eº = 2.8 V vs Normal Hydrogen Electrode (NHE)) and a great capacity to oxidize organic compounds in a non-selective way and to keep the velocity of reaction fast. The hydroxyl radical can be created through photochemical means or by other forms of energy.
The properties are advantageous to complete the mineralization of the contaminant or to degrade it into substances that are easily biodegradable. Generally, mineralization end products are CO2, water and inorganic ions.
AOPs can be categorized in photochemical and non-photochemical methods. The most used in the recent years appear in Table 2.2.
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Table 2.2: Classification of photochemical and non-photochemical AOPs. Source: (José Blanco Jurado 2009)
Photochemical processes Non-photochemical processes - Photolysis in vacuum UV (UVV) Alkaline ozonation, pH>8.5 (O3/OH )
UV/ H2O2 O3 / H2O2 2+ UV/O3 Fenton system (Fe /H2O2)
UV/ O3 / H2O2 Electrochemical oxidation 2+ Photo-Fenton system (Fe /H2O2/UV)
Photocatalytic oxidation (UV/TiO3)
They can also be classified depending on the method used to generate the radical species. Among all of the systems, the following account for almost 75% of the reported works in this area (Cabrera Reina et al. 2012):
- Heterogeneous photocatalysis with TiO2. - Ozonation.
- H2O2 systems. - Fenton type reactions (Fenton and photo-Fenton processes).
The advantages of this type of treatment with respect to other methods are (Montano García 2007; José Blanco Jurado 2009; Castaño Cid 2014):
- Not only do the pollutants change their phase but they are transformed chemically. In most cases the complete mineralization is accomplished. - These processes do not generate large amounts of sludge, which would require a posterior treatment. This fact saves time and money. - It is useful when having low concentration of the pollutant. - Consumes less energy than other methods such as the incineration. - As it is a non-selective process, the sub-products created are also removed by the same process. - Transforms the refractory compounds into others that can be treated with easier and cheaper processes. - Appropriate to decrease the concentration of compounds created in previous treatments. - They improve the organoleptic properties of treated water.
Nevertheless, using AOPs includes some drawbacks. The main disadvantage is the operational cost. A process of this type requires high electrical energy input (particularly UV radiation generation) 2+ and expensive chemicals demand (H2O2, Fe , …) which is worth an elevated amount of money. In fact, only wastewaters with relatively small concentrations (COD ≤ 5 g/L) can be economically treated with these technologies (Montano García 2007).
This work will set its focus on the Fenton and Photo-Fenton processes.
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2.2.2 Fenton and photo-Fenton reaction mechanisms
• Fenton process
Fundamental chemistry
It can be described with the “classical mechanism” proposed by Haber and Weiss in 1934 (Montano García 2007), which consists in the direct addition of hydrogen peroxide and iron salts into the water, working in acidic conditions.
In Fenton reaction (Eq. 2.1) firstly reported by H.J.H Fenton in 1894, the ferrous iron initiates and catalyzes the decomposition of H2O2, resulting in the generation of hydroxyl radicals which will subsequently attack the organic compounds in the solution (Castaño Cid 2014; Pera-Titus et al. 2004; Herney-Ramirez, Vicente, and Madeira 2010). Nowadays it is a widely used and studied catalytic process based on the electron transfer between the reagent and a metal acting as a homogeneous catalyst.
2+ 3+ − 퐹푒 + 퐻2푂2 → 퐹푒 + 퐻푂 · +퐻푂 Eq. 2.1
The radicals oxidize the organic matter as showed in Eq. 2.2. They also react with ferric ions, hydrogen peroxide and even with other radicals. Showed below in Eq. 2.3, Eq. 2.4 and Eq. 2.5 respectively.
푅퐻 + 퐻푂 · → 퐹푒3+ + 퐻푂− → 표푥𝑖푑𝑖푠푒푑 푝푟표푑푢푐푡푠 Eq. 2.2
퐹푒2+ + 퐻푂 · → 퐹푒3+ + 퐻푂− Eq. 2.3
퐻2푂2 + 퐻푂 · → 퐻2푂 + 퐻푂2 · Eq. 2.4
퐻푂2 + 퐻푂 · → 푂2 + 퐻2푂2 Eq. 2.5
Hydroxyl radicals permit the oxidation of many classes of organic compounds to CO2, H2O and inorganic ions from heteroatoms present, or at least the oxidation of contaminants to less harmful and biodegradable compounds (Nogueira, Silva, and Trovó 2005).
The radical HO2 · has not as much oxidation power as the hydroxyl power. It barely contributes in the degradation of organic matter.
As mentioned before, Fe2+ Is the used catalyst for the Fenton process. There are existing ways to obtain the same target as Fenton process but using also other metal ions as catalysts under the
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name of Fenton-like reactions. Cu2+ is an example of catalyst that could be implemented for the decomposition of H2O2. The main drawback of using these other metals such as copper as catalyst is related to the generation and necessary management of the hazardous sludge formed after the neutralization step (Primo, Rivero, and Ortiz 2008).
• Photo-Fenton process
Fundamental chemistry
In dark Fenton processes, Fe3+ ions are accumulated in the system and thus, once Fe2+ is consumed, the reaction cannot proceed.
The degradation rate of organic matter in the Fenton process can be improved by adding an irradiation source. This positive effect is due to the photolysis of ferric ion complexes. Fe3+ is reduced to Fe2+ producing new hydroxyl radicals and regenerating Fe2+ ions that can promote the reaction with more H2O2 molecules (M. Pérez-Moya et al. 2008). This mechanism is showed in the figure below:
Figure 2.1: Reaction cycle for the photo-Fenton process. Source: (Hartmann, Kullmann, and Keller 2010)
The hydroxyl radicals have a non-selective action over the organic compounds. They can “attack” in three different ways:
- The radical takes out a hydrogen atom formatting water. - Electrophilic addition of the HO· to a double bond. - Electrophilic transference.
Accordingly, the photo-Fenton process is the combination of Fenton reagents and UV-vis radiation that gives rise to extra HO· radicals by two additional reactions: Eq. 2.6 presents the photoreduction of Fe3+ to Fe2+ ions and Eq. 2.7 which shows the peroxide photolysis (Rahim, Abdul, and Wan Daud 2015).
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[Fe(OH)]2+ + hv → Fe2+ + HO · Eq. 2.6
H2O2 + hv → 2 HO · Eq. 2.7
The use of light involves the activity of iron complexes. Photoreduction of dissolved ferric iron entails a ligand-to-metal charge-transfer (LMCT). The ligands can be any Lewis bases able to form a - 2- 3- 3- complex with ferric iron such as: OH , H2O, HO , HSO /SO2 , Cl, carboxylates, etc. (Pignatello, Oliveros, and Mackay 2006). Therefore, intermediate complexes dissociate as shown in Eq. 2.8:
[퐹푒3+ + 퐿] + ℎ푣 → 퐹푒2+ + 퐿′ Eq. 2.8
Depending on the ligand, the product may be an hydroxyl radical (Eq. 2.6 and Eq. 2.9) or another derived from the ligand. Eq. 2.10 corresponds to a direct oxidation of the organic ligand (Santos- Juanes et al. 2011).
3+ 2+ + [Fe(H2O)] + hv → Fe + HO · + H Eq. 2.9
2+ 2+ [퐹푒(푂푂퐶 − 푅)] + ℎ푣 → 퐹푒 + 퐶푂2 · + 푅 · Eq. 2.10
Photolysis reactions of these complexes yields to HO· and regeneration of Fe2+ ions which further participate in the principal reaction giving more HO· radicals and thus, increasing the oxidation rate. The most photoactive species is [Fe(OH)]2+ due to a combination of its relatively high absorption coefficient and concentration relative to other Fe3+ species (Pignatello, Oliveros, and Mackay 2006).
The fate of the carbon-centered free radicals (R·) is to react prolonging the complex chain mechanism.
푅 · +푂2 → 푅푂2 · Eq. 2.11
푅푂2 · + 퐻2푂 → 푅푂퐻 + 퐻02 · Eq. 2.12
3+ 2+ + 퐹푒 + 퐻2푂 → 퐹푒 + 퐻푂2 · + 퐻 Eq. 2.13
퐻푂 · + 퐻2푂2 → 퐻2푂 + 퐻푂2 · Eq. 2.14
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Study on the state of art of dosage and modeling in the photo-Fenton process.
They react − + Eq. 2.15 퐻푂2 · ↔ 푂2 · + 퐻 with dissolved − 3+ 2+ Eq. 2.16 oxygen 푂2 · + 퐹푒 ↔ 푂2 + 퐹푒 following the − − Eq. 2.17 푂2 · + 퐻2푂2 → 푂2 + 퐻푂 · +퐻푂 Dorfman
퐻푂2 · ↔ 퐻2푂2 + 푂2 Eq. 2.18 mechanism (Eq. 2.11 and Eq. 2.12). (Dorfman, Taub, and Bühler 1962). This generates HO2, peroxyl radicals (R-O2) or oxyl radicals (R-O·). The hydroxy peroxide radical (HO2·) is given by Eq. 2.13 and
Eq. 2.14. These can regenerate H2O2 (Eq. 2.18). (Santos-Juanes et al. 2011)
Furthermore, reaction Eq. 2.15 shows another pathway for the reduction of Fe3+ to Fe2+, happening thanks to superoxide -the conjugate base of HO2- while Eq. 2.16 and Eq. 2.17 determinate the generation of hydroxyl radicals.
When the radicals react with organic matter, with H2O2 or other radicals, the radical consumption is non-efficient. Normally these reactions produce oxygen, whereas oxidation steps generally consume oxygen. Hence, the evolution of oxygen concentration is an important parameter to be aware of. An increase of O2 concentration during the process means hydrogen peroxide is decomposing without reacting with organic matter, which means the reactant is being inefficient.
On the other hand, a decrease on the oxygen concentration indicates a lack of H2O2 (Ortega-Gómez, Fernández-Ibáñez, et al. 2012; Santos-Juanes et al. 2011; Castaño Cid 2014).
2.2.3 Effects of main operational conditions
The key parameters affecting the Fenton and photo-Fenton processes are believed to be the reaction conditions (pH, temperature, irradiance or quantity of organic or inorganic constituents) and the characteristics of the reagents such as the amount of dissolved iron, the H2O2 concentration and their stoichiometric relation as well as the presence of inorganic ions. (Ortega-Gómez, Fernández-Ibáñez, et al. 2012; Neyens and Baeyens 2003).
• pH
The control of pH is one of the major issues of AOPs.
The optimum pH for photo-Fenton processes ranges from 2 to 4. Some studies state that the highest velocity of reaction happens when the pH =2,8 (Pignatello, Oliveros, and Mackay 2006; Neyens and Baeyens 2003). It is in these conditions where the hydroxy complex Fe(OH)2+ is more
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2+ present, which has benefits on the process as it has been mentioned. In fact, [Fe(OH)(H2O)5] is the most common ferric species present in the solution at pH 2-3 and it has the largest light absorption coefficient and quantum yield for radical production (Benkelberg and Warneck 1995).
If the pH is higher (pH≈4) the Fe2+ are easily transformed into Fe3+, generating hydroxyl complexes and causing instability in the solution due to iron precipitation in the shape of Fe(OH)3. (Elmolla and Chaudhuri 2009a)
3+ Moreover, in a very basic pH, the predominant substance is [Fe(H2O)6] (Elmolla and Chaudhuri
2009a). Hydrogen peroxide loses its oxidative properties because it is dissociated into O2 and water + + (Eq. 2.19). Moreover, H2O2 together with high concentration of H generates oxonium ions (H3O2 ). Accordingly, the reactivity between reagent and catalyst is reduced. (Fenton et al. 1998; Elmolla and Chaudhuri 2009a). Thus, the pH during the photo-Fenton processes needs to be adjusted low to maintain the efficiency.
2H2O2 → 2H2O + O2 Eq. 2.19
The conditions of greatest process efficiency include working in a neutral or near-neutral pH. Natural organic matter (NOM) present in the treated water, improves the neutrality of pH. This environment would have a positive effect on the generation of Fe3+- organo complexes, which absorb light spectrum and are stable at a neutral pH. Carboxylate and polycarboxylate are the most common NOM functional groups present in natural waters, and they create strong Fe3+ complexes that permit operating at neutral conditions.
A research carried out by Alfano et al. (Conte et al. 2014) studies the homogeneous Fenton and photo-Fenton degradation of an herbicide employing different sources of iron in water solution. It investigates the sulphate, oxalate and citrate complexes under diverse operational conditions of pH, employing experiments at values of pH 3 and 5, and reaction Temperature, either 25ºC or 35ºC.
The behavior of the complexes was studied with two parameters: the photon and the quantum efficiency in degradation and mineralization of the contaminant. The degradation efficiency is centered on the conversion of the pollutant concentration while mineralization efficiency is calculated with the degradation of TOC with time.
In the case of pollutant conversion, best results are obtained using pH 3 (considering the three complexes and T=35ºC). The same happens with TOC degradation: ferric oxalate and citrate show the best results, obtaining almost 100% of TOC degradation with only 45 min.
It is also important to evaluate the consumption of hydrogen peroxide during the treatment. In the study mentioned previously, the parameters used for this testing are the “initial specific consumption of oxidizing agent” and the “minimum consumption of H2O2 needed to achieve mineralization”. The first one represents the relation between the consumption rate of H2O2 and the one from TOC. The higher this value gets, the faster is the reagent consuming compared to the pollutant, which is not convenient for the process.
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The results obtained showed that experiments carried out at lower pH have a lower consumption of oxidizing agent.
Note that, as mentioned above, when working at pH 5, H2O2 can be additionally decomposed by iron oxides generated during the iron compounds precipitation at pH higher than 3.
• Reagent and catalytic concentration
The dose of H2O2 has a fundamental effect on the reaction efficacy while the concentration of ferric ions has an impact on the kinetics of the reaction (Pignatello, Oliveros, and Mackay 2006). Because of their importance in the determination of the overall efficiency of the experiment, the relation between this parameters must be strongly considered in terms of hydroxyl radical production and consumption (Neyens and Baeyens 2003) . The optimum treatment conditions will depend on the degree of degradation that is to be achieved as well as the concentration of pollutants in the specific treated water.
On the one hand, an increase of ferric or ferrous iron concentration, increases the reaction velocity. However, there is a point where a large dose of iron could reduce the efficacy of the reaction due to the rising of solution turbidity, which is a hinder to the absorption of light in the case of photo- Fenton systems. (Ebrahiem, Al-Maghrabi, and Mobarki 2017) Fe2+ is reacting with OH· radicals as a scavenger.
There is a value for the applied radiation corresponding to the activation of all the iron. More photon flux would only achieve results related to the possible direct photolysis of the by-products formed.
A study carried out by Santos-Juanes et al. (Santos-Juanes et al. 2011) on a photo-Fenton experiment with paracetamol and performed under solar irradiation, proposed a concentration of 0.36 mM of Fe as the suitable concentration under direct solar light. More than 60 W/m2 is never attained on Mediterranean area.
Iron salts used can be both Fe2+ and Fe3* but the first one is the commonly used because it is less corrosive, more economic and more soluble (Nogueira, Silva, and Trovó 2005; Paula, Batista, and Nogueira 2012).
As mentioned previously, iron ions coming from iron salts are not the only catalysts applied in AOPs. Many researchers focus their work on the application of iron complexes as catalysts. The advantages and consequences of this variations will be further studied in this work.
On the other hand, if the focus is set on raising degradation, an increase of H2O2 concentration at the initial phase is favorable (Elmolla and Chaudhuri 2009b; Oscar González, Sans, and Esplugas 2007). Nevertheless, this happens only up to a point where the increasing of concentration is no
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longer appreciable or even disadvantageous for the degradation. The advantageous tendency stops when low molecular weight acids are formed, which happens commonly at the end of the reaction system.
Higher oxidant concentration has the same positive effect on the degradation velocity as when increasing the pH because it enhances the production of more HO·. Again, the dose increment has beneficial limit, where the degradation rate starts decreasing due to the scavenging of OH· as it is expressed in Eq. 2.20 and Eq. 2.21 (Herney-Ramirez, Vicente, and Madeira 2010).
H2O2 + HO · → HO2 · + H2O Eq. 2.20
HO2 · + HO · → H2O + O2 Eq. 2.21
The dosage of peroxide is an important factor to take into consideration when putting the process into effect because it is one factor directly affecting the economic impact as it is an expensive reactive. High concentrations of H2O2 increment the possibility of reaction between radicals and hydrogen peroxide (Eq. 2.14 and Eq. 2.17 ), and consequently, the reagent is less available for the pollutant oxidation. This effect can be noticed checking the kinetic constant for the pollutant degradation using different H2O2 concentrations.
Two statements must be taken into consideration under any circumstances. The first is that large doses of both iron and hydrogen peroxide, could result on these species acting as radical scavengers, while the second regards the quantity of added iron accepted by legislation. The maximum legal value in effluents in Spanish legislation is 10 mg/L (Generalitat de Catalunya 2003). If the final solution contains more than the stated value of iron, posterior treatments are mandatory in order to reduce the concentration which would, consequently, increase the final project budget.
• Homogeneous vs heterogeneous systems
Iron species used for both Fenton and photo-Fenton processes are dissolved in aqueous solutions, hence, in the same phase with reactants. Therefore, there is no mass transfer limitation (Rahim, Abdul, and Wan Daud 2015).
The large amount of studies realized on AOPs for wastewater treatments clearly prove that homogeneous processes such as Fenton and photo-Fenton are effective in degradation and mineralization as well as in improving the biodegradability. Further investigations are done on the comparison between this method and the heterogeneous processes (i.e. TiO2 photocatalysis and UV/ZnO) (Elmolla and Chaudhuri 2010).
However, during the homogeneous treatment process iron sludge is formed, and there could be serious drawbacks concerning the removal (Kavitha and Palanivelu 2016). It consists mainly on
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Study on the state of art of dosage and modeling in the photo-Fenton process.
ferric-hydroxide sludge operating at pH values higher than 4 (Hermosilla, Cortijo, and Huang 2009) which remain in water posing adverse effects on the environment and waste disposal issues. Moreover, a large amount of catalytic metal is misplaced in the sludge and regeneration of catalyst becomes almost impracticable.
Nowadays, extent literature is dedicated to heterogeneous systems because, in the case of needing more iron concentration than the accepted by legislation, this alternative prevents the formation of the previously-mentioned iron sludge.
Heterogeneous catalysts are designed to overcome the constrains. These are generally solid and have granulated forms (Kavitha and Palanivelu 2016).
The immobilization of transition metals (especially iron species) are the focus of many research groups in order remove iron from water after the process, fulfilling with the legislations. In addition, to avoid the loss of catalyst (Melero et al. 2009). In this case, iron can effectively participate in the degradation of recalcitrant compounds without the generation of ferric hydroxide precipitation.
Notwithstanding, heterogeneous catalysis is of slower oxidation rate than homogeneous reaction (Punzi, Mattiasson, and Jonstrup 2012). This is on account of the small fractions of iron present in the catalytic surface. On this basis, a smart advance would be the development of hetero-catalysts with a larger surface area and further action on the degradation of the pollutant. The latest Fenton and photo-Fenton investigations have set its focus on the development of the above-mentioned catalysts.
Amongst the large studies realized about this topic, Rahim et al. (Rahim, Abdul, and Wan Daud 2015), point out three mechanisms that have been proposed for the heterogeneous catalysis of Fenton processes:
- Leaching of iron to the reaction solution and activating H2O2 with a homogeneous pathway - Merging iron with hydrogen peroxide in the surface of the catalysts to accomplish its decomposition to HO· radicals. - Chemisorption of probe molecule in the catalyst surface.
These techniques can be combined in one unique experimental process.
Some heterogeneous catalysts used for the Fenton and photo-Fenton processes include materials such as Hematite, Goethite, Magnetite, Fe silicate or Fe2O3/Al2O3/silica which have specific active sites or components taking part in the reactions. Vorontsov (Vorontsov 2018) cities some of the most used materials in his article, relating each of them to their active components.
Lastly, it should be noticed that heterogeneous treatments are not practically efficient when the treated water is highly polluted. The reason of inefficiency is the inner filtration effects related to the large absorbing molecules and the inhibition of photons absorption by Fe ions.
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• Temperature
Most of the photo-Fenton processes carried out in the experiments take little consideration on the Temperature. Nevertheless, in Fenton systems carried out under the same irradiation conditions, temperature markedly influences the degree of TOC removal (Pérez et al. 2002).
The experiments are carried out at atmospheric pressure. Fenton reaction is endothermic, the reaction velocity and mineralization rate increases with the temperature (Zhang, Choi, and Huang 2005) which makes the treatment highly dependent on the temperature.
The higher the temperature, the better results will be obtained. Nevertheless, the heating requires large energy application, which implies higher cost on the project. Having knowledge on the temperature effects is advantageous when working with effluents coming from industrial processes carried out at high temperatures as no heating needs to be applied. An example of incoming hot effluents are the textile wastewaters.
The efficiency of the process is only affected if the temperature overcomes 40ºC (Litter and Slodowicz 2017). Too high values may decompose the hydrogen peroxide and water.
• Inorganic ions concentration
Fenton and photo-Fenton oxidations of organic compounds are inhibited by inorganic ions such as carbonates, bicarbonates, chlorides, fluoride, bromide, phosphate and sulfate, which may be present in the treating water or generated via degradation process (Pignatello, Oliveros, and Mackay 2006; Rahim, Abdul, and Wan Daud 2015).
The changes that this factor origins in the kinetics of the reaction depends on the iron type, and concentration. It has effects such as: HO·-radical scavenging and generation of less reactive radicals, coordination of Fe3+ to form less reactive complexes and lessening the activity of iron species, generation of by-products containing these ions that can be more toxic in some cases, competitions with organic compounds for active sites or effects on ferrous ion recovery.
• Light intensity
Photo-Fenton reactions can take place in a wavelength range from 300nm to visible. The irradiance has the role of activating the catalyst. Inside the accepted radiation ranges, the greater light intensity, the higher the degradation rate. Nonetheless, once all the iron is activated, more photon
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Study on the state of art of dosage and modeling in the photo-Fenton process.
flux does not achieve better results apart from those related to the possible direct photolysis of the formed by-products (Santos-Juanes et al. 2011).
When ferrous complexes appear, different light absorption properties appear for each ligand. These Fe3+ chelates make more efficient the use of photons up to the visible, meaning that a considerable part of the solar spectrum can be used (Litter and Slodowicz 2017).
However, the presence of iron complexes in the media can promote a lesser photodegradation of organic contaminants due to their stronger capacity to absorb radiation (Schwingel De Oliveira et al. 2007).
Generally, the range of absorption used in photo-Fenton contains the wavelength corresponding to solar light. This is a big advantage for this kind of treatment as radiation sources have a major weight on the final budget. The trouble could be found in the design of the reactor. It must be a reactor capable of receiving the maximum radiation from solar light and, if possible, to diverse sectors of the solution inside the recipient. Overcoming this little drawback makes the process highly sustainable.
• Chelating agents
Even though photo-Fenton processes have demonstrated acceptable scores for the contaminant degradation in acidic solutions, many investigations have been lately focused on employing organic and inorganic ligands to improve the efficiency and to increase the oxidation rate. (Rahim, Abdul, and Wan Daud 2015). The positive effects can be attributed to:
- Higher quantum yield of HO· radicals. - Increasing the reduction of ferric ions and consequently, generating higher amounts of hydroxyl radicals.
2.2.4 Analytical methods for photo-Fenton treatments.
The process performance is evaluated in every experiment by different ways. Generally, by withdrawing aliquots and measuring the necessary variables at regular interval times.
Measurements of contaminant concentration, total organic carbon (TOC), H2O2, and iron species concentrations are the most performed in researches.
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• Total Organic Carbon (TOC)
The measurement and determination of the Total Organic Carbon (TOC) permits the assessment of the mineralization of the organic matter present in the studied contaminant.
It is normally used as an indicator for the quality of water and it is measured as the amount of carbon dioxide (CO2) generated during the oxidation of organic matter in specific conditions.
It can be considered as the alternative to evaluate the organic charge instead of COD determination. In the case of photo-Fenton, TOC is used because COD methodology involves redox reactions, which could have interference in the peroxide and iron value determinations.
It consists of a fast and accurate measure of the organic content as it does not depend on the initial oxidation state of the matter nor on the determination of other organic bonds (i.e. nitrogen bonds) or inorganic.
The tracking of this parameter is a good indication of the mineralization rate acquired, i.e. in wastewater treatment experiments, the degradation of TOC is a commonly studied parameter to find out if the mineralization of water is fully completed.
Both BOD and COD can be applied as techniques to define which TOC fraction could be biologically or chemically oxidized, respectively.
TOC device
The mineralization level of the studied components it is monitored with a Shimadzu TOC-Vcsn. For the calibration standards it is used a known potassic ftalate solution, a known solution of sodium carbonate and a sodium nitrate solution.
For the carbon analysis, all the existing carbon in the component is submitted to a catalytic oxidation at 680ºC. This combustion generates both vaporized water, which it is later condensed, and carbon, which is all oxidized to CO2. Afterwards, the CO2 is dragged into a non-dispersive infrared detector (NDIR).
The TOC analyzer does not differentiate the carbon present in the different chemical substances, but it does discern whether it is organic or inorganic. Thus, once the total carbon is obtained, one more sample is injected in order to calculate the inorganic carbon which basically considers de dissolved carbon dioxide, carbonates and bicarbonates. So, while the total combustion in the catalytic oven measures the totality of the organic carbon, the inorganic carbon is volatilized through acidification of the solution.
Finally, the TOC is obtained by the difference between the measurements of TC and IC (Eq. 2.22) and expressed as [mg/L] or [ppm] of carbon.
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TOC = TC − IC Eq. 2.22
Figure 2.2 is an image of the specific instrument employed for this determination.
Figure 2.2: Shimadzu TOC-Vcsn in EEBE laboratory. Source:(Ballés and Canals 2018)
Analytical technique for TOC evolution is detailed in CHAPTER 3.
• Contaminant concentration (by HPLC-UV)
The removal of contaminants from water is the principal target of water treatments. Therefore, the evolution is a key parameter to be observed.
High-Performance Liquid Chromatography (HPLC) is an analytical method used to follow the degradation of the studied model pollutants. It uses a technique where the components of the sample are divided in two phases: the stationary phase and the mobile phase.
The chromatographic system employed consists of a column containing the stationary phase (gel) through which the sample passes together with the mobile phase. The diverse components interact dissimilar with the phases and in consequence they go through the static phase with different velocities. As a result, these are separated depending on the retention rate of the sample with both phases.
The instrument used for determination is composed by the quaternary system described below:
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Figure 2.3:External structure and parts of the HPLC in EEBE laboratory
It offers (Agilent Technologies 2003):
The specific characteristics of the instrument regarding its manual are:
- Gradients of up to 4 different solvents. - Pressure range up to 600 bar. - Sophisticated pump control to deliver very low chromatographic noise and very low acoustic noise for better results and better working environment. - Degasser and automatic purge valve integrated into pump module. - Variable volume autosampler with reduced delay volume, reduced carryover and the option to operate as a fixed loop autosampler. - Thermostatic column compartment with a pressure range up to 600 bar - Choice of detectors (a set of different flow cells is available for different detectors to fit application needs regarding flow ranges (nano scale, micro scale, standard and preparative applications) and pressures): ▪ Diode-array detector with greatly enhanced sensitivity and baseline stability using cartridge cell system with optofluidic waveguides (data collection rate up to 80 Hz with full spectral information) or ▪ Variable wavelength detector.
The measured parameters in this technique are firstly, the retention time, corresponding to how long it takes for a component to exit the column. A final detector located right after the column,
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oversees giving an electrical signal which is digitalized and recorded against time by the software generating peaks. This depends on the concentration and the type of chemical.
For this signal quantification there exists a linear relationship between the peak area and the contaminant concentration in the sample, which is obtained by calibration with standard solutions of the analyte.
The detector implemented in this practice is a diode array consisting on 1024 diodes and a slit of 1nm, with the possibility to be programed from 1 to 16 nm. Deuterium lights are used for UV (190 to 350 nm) while tungsten lights for the visible light range (350 to 950).
The device is equipped with a quaternary bomb, a manual injector, a thermostat for the column, a degasser for the mobile phase and a UV diode array detector.
Figure 2.4: HPLC Machine in EEBE laboratory. Source: (Jaén 2011)
• H2O2 determination
Being aware of the amount of hydrogen peroxide left in every moment and its consumption is an indicator to evaluate the efficiency of the chemical oxidation. When the H2O2 is used up, no more radicals are produced, and it comes to a standstill.
The residual concentration is measured at regular time intervals to be able to identify its behavior and influence on the contaminant degradation and TOC mineralization.
Diverse methods have been used to determinate the peroxide during the degradation of organic compounds in AOPs systems. The most commonly used in photo-Fenton processes are the iodometric or permanganate titration but spectrophotometric methods using titanium sulfate or oxalate and N, N diethyl-p-phenylenediamine (DPD) have been reported recently.
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However, they present several shortcomings. Iodometric titration is subjected to possible errors due to volatilization and hydrolysis of I2 and air oxidation of the iodine anion. In addition, it is more time-demanding than spectrophotometric determination. DPD use high cost reagents.
Permanganate titration is a good method to follow the H2O2 evolution, but it is not applicable to photo-Fenton processes because, in the reactions, Fe2+ reacts with permanganate interfering in the peroxide determination.
In short, the chosen analytical method for the hydrogen peroxide in this case is the spectrophotometric determination. Details on this method used are defined later in this work (CHAPTER 3).
The non-reacted hydrogen peroxide concentration is monitored with a device called Hitachi U-2001 UV-VIS spectrophotometer.
UV-VIS spectrophotometer
The spectrophotometer is an optical instrument used to measure the intensity of light relative to wavelength. It is useful to make quantitative and qualitative analysis in the UV-Vis range.
The spectrophotometry is based on the capacity of the molecules to absorb radiation. The wavelength and the efficiency with which the molecules absorb the radiation depend on the atomic structure and the environmental conditions.
Molecules can absorb and store light energy. When this happens, a jump is done from the fundamental energetic stage to an excited stage. Each molecule has string of energetic bands distinguished from the others that makes it a sign of identity.
This analysis allows the comparison of the absorbed radiation between two samples: one with an unknown quantity of solute and the other containing a specific known quantity of the same substance. The absorption of the radiation deals with visible light, ultraviolet, and infrared. This study uses UV radiations from 80 to 400 nm and visible light from 400 to 800 nm.
The functioning is based on the Bourguer, Lambert and Beer laws which stablish the relation between the intensity of the transmitted light for a specific sample and the thickness or concentration of the same.
The elements that compose the apparatus are shown in figure XX. It consists in a radiation source, a monochromator, two compartments to place the cuvettes and a detector. Moreover, the implemented spectrophotometer uses a double beam of light where the ray is divided in two using a chopper. One passes through the reference and the other one through the sample. Moreover, it is also important to consider the material of the cuvette. This must be the proper one to avoid interferences with the working radiation.
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Figure 2.5: Diagram of the Hitachi U-2001 UV-VIS spectrophotometer. Source: (Orozco 2014)
In the following image (Figure 2.6) the instruments employed for the determination of hydrogen peroxide are shown:
Figure 2.6: Spectrophotometer used in EEBE laboratory. Source: (Jaén 2011)
• Iron measurement
Iron plays the catalyst role in most of the Fenton and photo-Fenton processes. The tracking procedure is distinct for every type of catalyst used. Iron can be dissolved in the solution or integrated in particles which interact in the reaction and are further physically separated (Klamerth 2011).
One of the methods to determinate iron is the spectrophotometric method IS0 6332 (ISO 1998) using 1,10-phenanthroline. The dissolved ferrous iron forms a colored chelate complex with 1,10-
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phenanthroline which does not change color between pH=3 and pH =9. H2O2 and other chemicals interfere with the test, as they oxidize Fe2+ to Fe3+, which does not complex with the 1,10- phenanthroline.
Interferences in the sample can happen because of the heavy metals that can form complexes with 1,10-phenanthroline (Co2+ Cr3+, Cu2+, Ni2+, Zn2+) and some others that might cause its precipitation 2+ 3+ 2+ 2+ 2- (Ag , Bi , Cd , Hg , MoO4 ). Nevertheless, in urban waste waters these interferences can be excluded as the mentioned metals do not and should not be present significantly.
• Toxicity of biodegradability detection (H2O2 depleted by catalase)
H2O2 depletion must only be considered if the biodegradability is willed to be measured. It is a type of toxicity test that could be implemented. However, it is only mentioned in this section as no application will be employed after the process.
The remaining hydrogen peroxide in the samples must be removed because it can have an influence on the BOD measurements. Bovine liver catalase is used to remove the H2O2 present. These enzymes are created in aqueous phases containing dissolved oxygen and act as catalysts for the degradation of H2O2 to water and oxygen.
The catalase solution is not stable for more than 24 hours. The solution is prepared with 100mg/L of catalase and ultra-pure water. The sample must be neutralized (pH = 6-7) before the addition of catalase as the enzyme is deactivated at pH lower than 5 and higher than 8.
2.3 Combination of water treatments
Advanced oxidation processes are considered a highly competitive water treatment Technology for the removal of organic contaminants with high chemical stability or/and low biodegradability which are not treatable by conventional or Standard methods (Oller, Malato, and Sánchez-Pérez 2011).
The combination of AOPs with conventional treatments can be economically favorable. AOPs are expensive treatments but conventional contaminant removal implies less costs. When the concentrations are large, conventional treatments do not fulfill the degradation and, as a consequence, AOPs can be employed. Nevertheless, instead of obtaining a full degradation with AOPs, these could be used to degrade the pollutants up to the point where conventional treatments can be involved. In this way costs would be reduced.
Biodegradation and photodegradation are the main paths implemented for the elimination of toxic compounds. On the one hand, photodegradation implies the direct or indirect photolysis. A process where a photosensitizer absorbs the energy from light and transfers it to the pollutant, which
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otherwise would not react photochemically because their absorption capacity does not match the wavelength interval of sun light.
On the other hand, biodegradation refers to the elimination of the pollutant by the metabolic activity of living organisms that live in natural water and soil. (European Parliament and EU Comission 2003). As it has been mentioned previously in this work, satisfactory results are not always obtained if only implementing conventional biological processes, especially for industrial wastewater treatments, since many of the substances released by the chemical industry are resistant to biological treatment.
Chemical oxidation for complete mineralization is generally expensive. One attractive alternative is to apply the chemical oxidation as a pre-treatment just to convert the initially organic compounds to more biodegradable compounds which would be easily treated in a further biological oxidation. During the pre-treatment stage, the work should be focused only on the maximum removal of the contaminant. Mineralization should be minimal in order to avoid wasting unnecessary chemicals and energy which would have a negative effect over the operation costs. (Oller, Malato, and Sánchez-Pérez 2011)
There are real cases in which de combination strategy is contrary. First it takes place the elimination of the biodegradable part and afterwards the recalcitrant compounds are removed by a post- treatment (AOP).
The combination of bioremediation technologies and AOPs, as a pre or post treatment, has been widely reported with the aim of reducing the operating costs that a chemical oxidation for complete mineralization implies. Figure 2.7 represents which are the possible combinations as a pre or post treatment. The expensiveness comes from the resistance of some oxidation intermediates degradation, which not only do they consume energy but also chemical reagents and catalysts (Muñoz et al. 2005).
Figure 2.7: AOP as pretreatment (a.) and post-treatment (b.) of biological system. Source: (Naddeo, Cessaro, and Belgiorno 2013)
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The combination of appropriate techniques is an important issue as the goal is to provide economically and technically feasible options.
The wide difference in the cost of the processes states that even though the system combination is centered on the order of the treatments, the overall purpose will be minimizing the AOPs and maximizing the biological state. The key issue is to design a combined process whose performance diminishes the ecological and economic impacts.
Oller et al., (Oller, Malato, and Sánchez-Pérez 2011) published a detailed review based on the research combining AOPs and biological technologies. Figure 2.8 recapitulates and schematizes the necessary steps to evaluate the AOP/biological treatment strategy for toxic and/or non- biodegradable industrial wastewater.
The diagram depicts:
- The conceivable situations and associations that could happen depending on the characteristics of the treated wastewater. - The necessary analysis (both chemical and biological) that must be performed in a industrial wastewater treatment line.
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Figure 2.8: Strategy for the selection of the best treatment option for a specific toxic and/or non-biodegradable industrial wastewater. Source: (Oller, Malato, and Sánchez-Pérez 2011)
Regarding the diagram features, organic carbon concentration (TOC) and chemical oxygen demand (COD) are the head variables used to describe chemical oxidation, but another series of parameters related to the operational process conditions should be monitored still. These include some of the previously mentioned such as catalyst and/or hydrogen peroxide concentration, radiation intensity or pH.
When combining AOPs with biological treatments it is worthwhile finding the point during the photo-Fenton process at which the wastewater starts being biodegradable and thus permits an optimal combination with a biological treatment. One possible way to study this point is o perform a respirometric test at different stages of the process.
As an example, Lucas et al., (Lucas et al. 2012a) included in their experiment the study of the biodegradability of the activated sludge following the previously explained methods. Figure 2.9 shows the biodegradability evolution during a photo-Fenton process and the different possible ranges from non-biodegradable to high biodegradability. The results were obtained by the respirometric test using rbCOD/COD and the consumption of H2O2 (taking the concentration
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needed for a 90% mineralization). Observing the evolution based on the H2O2 consumption it is also a way to check its efficiency.
Figure 2.9: Biodegradability evolution along a solar photo-Fenton experiment. Source: (Lucas et al. 2012b)
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Study on the state of art of dosage and modeling in the photo-Fenton process.
CHAPTER 3 EXPERIMENTAL METHOD AND RESULTS
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3.1 Experimental Assembly
3.1.1 Reactor
The experimental set up of the photo-Fenton tests will be carried out in a 0.5L lab-scale reactor.
It consists on a jacketed reactor in a batch mode, collocated on top of a magnetic stirrer that permits its constant mixing. Thanks to the jacketing, the temperature is constant all the time. In addition, two sensors are collocated inside; a Ph-meter, and a thermometer, which take on-line measures.
3.1.2 Materials and reactants
In addition to what has been mentioned above, the material necessary to carry out the experiments is showed in the following tables:
Table 3.1: Laboratory devices employed.
Devices Model TOC analyzer Shimadzu TOC-V CSH/CSN Spectrophotometer Hitachi, U-2001 HPLC Chromatograph Agilent Technologies, Series 1200 pH-meter Crison, GLP22 Magneto- thermic agitator Balance
The laboratory material and the reactants used for the experiments are reported later in this work (CHAPTER 7) in a complete table together with its units and budget value.
3.2 Experimental method
The development of the experimental part was conceived as it is shown in Figure 3.1:
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•Prepare the analytical equipment that will be used to obtain measurements Analytical in the following experiments. calibration
•Verify the application of photo-Fenton treatment to the specific polluted solution. Preliminary •All the reactants present but in different concentrations = verification of Experiments their influence in the degradation.
•Aimed at proving the influence of each reactant. A test is done for every 2+ 2+ 2+ Blank considered reactant: H2O2, Fe , UV, H2O2/ Fe , H2O2/ UV, H2O2/ Fe /UV. Experiments
•With the previous results, choose the adequate variables to operate with Operational and the conditions that must be applyed. conditions
•Design a set of experiments for the studied variables in the process. •Central composition design could be applied in order to study the best Design of compositions and loads of reactants. Experiments
•Obtain results, study the behavior of the system, draw some conclusions and observe future alternatives or extensions. Results
Figure 3.1: General flowchart of the experimental methodology.
The lay out of the experiments was initially set as divided in the above-represented stages.
In March 2020 the state of alarm was declared in Spain due to COVID-19. A nationwide lockdown was officially called, interdicting the possibility of working in the laboratory facilities. In consequence, no experimental work could be done apart from the analytical calibration.
Analytical methodology consisted in calibrating the equipment regarding the studied pollutant and the reactants used. For the reaction monitoring, the chosen analytical methods are TOC
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determination, UV-Visible Spectrophotometer for the H2O2 concentration measurements, temperature, pH measurement, HPLC chromatographer to define the contaminant concentration.
Only methods requiring specific calibration will be more extensively explained.
3.3 Analytical calibration
3.3.1 TOC measurement
The samples used for the TOC calibration are the ones previously prepared for the degradation measurement of the corrupting element.
In order to determine the TOC in a sample it is highly recommended to make a previous calculation on the corresponding carbon present in that sample (Eq. 3.1). It must be taken into consideration if the dealt carbon is coming from a single contaminant or from a mix of pollutants, because the molecular masses and the number of Carbons are essential for the measure.
The theoretical calculation of TOC follows the equation below: 푚푔 [푠푢푏푠푡푎푛푐푒 ( )] · 12 · [푁표. 퐶] 퐿 Eq. 3.1 푇푂퐶푡 = 푀푊푠푢푏푠푡푎푛푐푒
Where 푇푂퐶푡 corresponds to the theoretical TOC calculated and 12 corresponds to the molecular weight of Carbon.
In the specific case of Venlafaxine (C17H27NO2) the amount of carbon is evaluated considering the concentration in the sample, the molecular mass of VEN (277,4 g/mol), and the molecular mass of Carbon (12 g/mol).
An example of this measure is shown in the below expression with a sample of 20 mg/L of VEN:
1 푚푚표푙 푉퐸푁 20 푚푔 푉퐸푁 17 푚표푙퐶 12 푚푔 퐶 푚푔 퐶 푇푂퐶 = · · · = 14,71 푡 277,4 푚푔 푚푚표푙−1 1퐿 1 푚푚표푙푉 퐸푁 1 푚푚표푙 퐶 퐿
The steps to be followed for the use of TOC instrument must be the ones specified in the corresponding PNT paper (CEPIMA 2012b).
In this way, an orientation number will be obtained for the further work with the instrument and therefore, time and reactants could be saved.
The calibration curve is prepared with the concentrations in Table 3.2, which correspond to a theoretical value of TOC, and with the obtained values using the TOC device.
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Table 3.2: Theoretical and experimental TOC data used for the calibration.
VEN [mg/L] TOCt TCexp ICexp TOCexp 30 22,06 20,60 -0,076 20,68 20 17,71 14,93 -0,044 14,97 10 7,35 7,78 0,009 7,77 4 2,94 3,45 -0,069 3,52
y = 0,8514x + 1,08 R² = 0,987 25
20
15
10
Experimental TOC [mg/L] 5
0 0 5 10 15 20 25 Theoretical TOC [mg/L]
Figure 3.2: Calibration curve for the TOC determination.
The TOC device has already the calibration curves inserted. These could not be represented because the experimental part was stopped. Nevertheless, the relationship between the theoretical and the experimental values of TOC gives an estimation of how accurate are the measures taken with this instrument.
3.3.2 Contaminant determination
For the specific case of Venlafaxine, a chromatographic HPLC is used to measure the concentration.
The chromatographic system employed is reported in section 2.2.4. The results obtained for the laboratory measurements are subsequently shown.
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Table 3.3: Results obtained from de HPLC on the VEN concentration determination.
VEN [mg/L] Amed [mAu] Deviation 30 1050,8 26,3 20 729,6 8,1 15 523,0 10,4 10 343,8 8,9 8 266,2 7,9 6 198,8 2,6 4 134,0 4,0 2 68,1 2,3 1 30,4 1,6 0,5 14,5 1,0
Assuming the data obtained (Table 3.3), a calibration curve can be depicted as well as its corresponding equation.
y = 35,6x - 7,6113 R² = 0,9991 1200
1000
800
600
Area Area [mAu] 400
200
0 0 5 10 15 20 25 30 35 Concentration VEN [mg/L]
Figure 3.3: Venlafaxine calibration curve.
Table 3.4: VEN calibration curve characteristics.
Standard LD LQ Lineal range Calibration curve R² Deviation (mmol/L) (mmol/L) (mmol/L)
y = 35,6x – 7,6113 0,9991 3,54 0,5 1 0,5 – 30
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The linear correlation obtained is considered as adequate because the value is really close to the unit (R² =0.9991). Thusly, it allows the calculation of Venlafaxine concentration.
3.3.3 Calibration for the H2O2 determination
The spectrophotometric determination mechanism is based on the following reaction:
− + 3+ − + 3+ Eq. 3.1 푉푂3 + 4퐻 + 퐻2푂2 → 푉푂2 + 3퐻2푂 푉푂3 + 4퐻 + 퐻2푂2 → 푉푂2 + 3퐻2푂
Hydrogen peroxide reacts with ammonium metavanadate in acid environment. The peroxide is − 3+ reduced to water while the vanadate (푉푂3 ) is oxidized to peroxovanadium cation (VO2 ). This last element presents its maximum absorbance at 450 nm, wavelength at which the process will take place.
An excess of ammonium metavanadate is added so that all the hydrogen peroxide reacts. As a result, the final concentration of peroxovanadium equals the initial concentration of H2O2.
The standardization of H2O2 is normally done with permanganate. Neverhteless, this method is not favorable for Fenton or photo-Fenton experiments because the iron can react with the permanganate as well, interfering in the determination of peroxide.
The employed equipment has been described in CHAPTER 2.
The development is recounted with detail in the corresponding PNT paper (CEPIMA 2012a). However, basic steps are rendered in four blocks:
1. Preparation of reagents
a) Solution of H2SO4 [9M]
b) Solution of H2SO4 [0,58M] and ammonium metavanadate [0,062M]. (1L)
2. Preparation of pattern dissolutions for the analytical curve.
Note: this part does not correspond with the instructions given in the PNT because metavanadate is very toxic and difficult to degrade. Thus, it is believed that the quantity used must be reduced.
The H2O2 used corresponds to a 33% w/v. Therefore, the concentration is:
3 푤 33푔 10 푚퐿 1 푚표푙 퐻2푂2 푚표푙 33% = · · = 9,706 Eq. 3.2 푣 100푚퐿 1퐿 34 푔 퐻2푂2 퐿
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The development of this second part has been divided into 2 different subsections (A, and B):
• A: Preparation of H2O2 solutions with different concentrations.
To get a dissolution with a concentration of 500 mmol/L:
퐶 · 푉 = 퐶 · 푉 1 1 2 2 Eq. 3.3
푚푚표푙 푚푚표푙 9706 · 푉 = 500 · 25푥10−3퐿 퐿 1 퐿 Eq. 3.4 푉1 = 1,3 푚퐿 (𝑖푛 푎 푓푙푎푠푘 표푓 25푚퐿)
V1 corresponds to the volume that we need of the principal dissolution to make a diluted one with a concentration of 500 mmol/L.
Afterwards, the pattern dilutions are prepared following Table 3.5:
Table 3.5: Volumes for the corresponding pattern solutions.
H2O2 [mmol/L] V [µL] 1 50 2 100 4 200 6 300 8 400 10 500 15 750 20 1000
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Figure 3.4: Pattern solutions for the UV-Vis calibration.
• B: Pattern dissolutions with 1mL of each of the previous solutions and 1ml of
metavanadate. Diluted in a 10ml volumetric flask. (1ml of metavanadate + 1ml of H2O2 + water to fill it up to 10 mL)
3. Read the absorbances at 450 nm.
Instructions given to use the spectrophotometer must be accurately followed.
Two cuvettes are used: one for the blank solution and the other one for the solution of the correspondent concentration.
The lecture is done three times to make it more reliable. The results are sown in Table 3.6 below:
Table 3.6: Results of the H2O2 spectrophotometry with the pattern solutions.
H2O2 [mmol/L] Absmed [u.A] Deviation 1 0,285 0,006 2 0,566 0,011 4 1,118 0,004 6 1,677 0,009 8 2,160 0,004 10 2,697 0,003 15 3,916 0,026 20 5,327 0,008
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4. Generate the calibration curve.
Further usage of the calibration is possible through the following equation and its representation:
y = 0,2623x + 0,0546 R² = 0,9995 6
5
4
3
2 Absorbance [u.A]
1
0 0 5 10 15 20 25
H2O2 concentration [mmol/L]
Figure 3.5: Hydrogen Peroxide calibration curve
Table 3.7: H2O2 calibration curve characteristics.
Standard LD LQ Lineal range Calibration curve R² Deviation (mmol/L) (mmol/L) (mmol/L)
y = 0,2623x + 0,0546 0,9995 0,023 0,3 1 1,0 – 20
3.4 Design of experiments
A design of the expected experiments should have been done in order to obtain the adequate diversity in the results.
Firstly, preliminary and blank experiments would have taken place, from which certainty on the Fenton and photo-Fenton process would have been proven.
Afterwards, two or three variables are chosen based on the results.
On the one hand, if two variables are chosen, it is needed a 22 design where two factors are studied at two levels. On the other hand, if three variables are studied a 23 design would be implemented,
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at two levels. Both the star and the spherical shaped experiments consider the replication of the central points to give statistical results.
Once the number of experiments has been decided, the same runs should be repeated three times in order to obtain more reliable results.
3.5 Project reorientation
At this point, the experiments could be carried out obtaining results. However, given the mentioned circumstances with regard to COVID-19, there was the need to redirect the project. The new objectives focus was reset into an analysis of the literature about Fenton and photo-Fenton treatments, emphasizing on the second one.
The subsequent chapter includes all the information regarding this literature comparison. It is based on the parameters which are generally thought to be more concerning nowadays: dosing and modeling.
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CHAPTER 4 BIBLIOGRAPHIC COMPARISON
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4.1 Introduction
An increasing trend in the concern of polluted waters with recalcitrant characteristics is showed in the lectures. The global water situation awakes concern over the scientific sector and thus, researchers are seeking for advances in water treatments systems.
This work is mainly centered on analyzing Fenton and Photo-Fenton papers in order to figure out which characteristics could contribute improvement of these systems.
A work carried out by Giménez (Giménez et al. 2015) claims that a strong interest on the application of AOPs has been shown in recent years. He reinforces his hypothesis by looking at the literature evolution.
Only in the period between 2005 and 2014, 56059 publications (according to SCOPUS data) have been done in the field of photocatalysis and 17206 (according to Web of Science data), which represents the efforts committed in the research. In Table 4.1 the search results of the different works cited in Scopus (SC) and Web of Science (WS) are shown.
Table 4.1: Photocatalysis Works Cited in Scopus and Web of Science. Document type: all. Source: (Giménez et al. 2015)
Photocatalysis and Life Cycle Photocatalysis and Photocatalysis and Photocatalysis Assessment cost estimation cost evaluation Year SC WS SC WS SC WS SC WS 2005 2254 752 5 1 8 - 30 1 2006 2538 829 5 2 4 - 40 - 2007 3131 1135 6 1 14 - 45 1 2008 3535 1263 15 - 12 - 54 2 2009 4611 1556 34 1 23 - 92 - 2010 4982 1508 29 2 18 - 96 4 2011 6740 1981 37 2 22 - 180 2 2012 8011 2420 41 0 33 - 173 4 2013 9485 2719 70 2 43 1 290 3 2014 10772 3043 73 1 52 - 313 3 TOTAL 56059 17206 315 12 229 1 1313 20
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The general search is the more numerous, as expected. The representation of each subject related to photolysis among the total is represented in Figure 4.1. The SC and the WS results are showed together for each part.
Life cycle assesment Cost estimation Cost evaluation
17%
12%
71%
Figure 4.1: Representation of the number of articles published on photolysis and life cycle assessment, photocatalysis and cost estimation and photocatalysis and cost evaluation. Data based on the results showed in Table 4.1.
Cost evaluation is, by far, the major concern among authors regarding photocatalysis.
Liu et al. (X. Liu et al. 2018) did a similar research also obtaining the data from Web of Science, updated in November 2017. Figure 4.2 presents the trend in the number of publications of antibiotics treatment with some Fenton and Fenton-like treatments.
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Figure 4.2: Trends in the number of publications on the topic of “Fenton and antibiotic”, “electro Fenton and antibiotic” and “photo Fenton and antibiotic” (data obtained from Web of Science on November 11, 2017). Source: (X. Liu et al. 2018)
In order to observe the interest on these processes and the relation to its parameters, a bibliometric study is proposed. It is based only on the quantity of studies done in the area.
The considered data is obtained from the ScienceDirect searching website: (ScienceDirect 2020), and it is based on a search by key words. Finding the appropriate key words for each case has been the main trouble during the study.
A first general search is done on the topic of water pollution, tertiary treatments, and advanced oxidation processes (Figure 4.3). From the whole literature, only research papers, reviews and conference abstracts have been selected.
It can be observed that Advanced oxidation processes play an important role inside the tertiary treatments.
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3500
3000
2500
2000 AOPs 1500 Tertiary treatment water pollution
1000 Numberpublications of
500
0 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Year
Figure 4.3: Trends in the number of publications in the last decade on the topic of water pollution, AOPs and Tertiary treatment. Data obtained from (ScienceDirect 2020). Data from 2020 is only considered until June.
As it can be observed, the three studied key words have the same evolution trend, increasing among the years.
Specific search was done on the photo-Fenton key word to provide information about this concrete Advanced Oxidation Process. Figure 4.4 represents the progression since 2010.
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2500
2000
1500
1000 Fenton and photo-Fenton AOPs Numberpublications of 500
0 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Years
Figure 4.4: Trends in the number of publications in the last decade on the topic photo-Fenton and AOPs. Data obtained from (ScienceDirect 2020). Data from 2020 is only considered until June.
As it is expected, similar general direction is shown by the Fenton and photo-Fenton, and de AOPs. This graphic represents the relevance of these processes. As it is expected, the number of AOPs publications are always higher than Fenton and photo-Fenton, but both items show a similar general direction. The average representation of Fenton and photo-Fenton publications correspond to a 62% of the whole AOPs articles, reviews and conferences. In the worst scenario, which would be 2019, the Fenton and photo-Fenton publications represent a 63% of the entire AOPs papers.
Given these generic representations which clearly show the actual dedication in the water treatment sector, emphasis will be settled upon research working with subtopics which are claimed to have important influence on the process and for which there are no clear-cut conclusions yet.
After analyzing the literature, it is clear that there are specific topics on which many investigation efforts are dedicated. All of them arrive at similar conclusions and present issues that lack studies or are not concluded. Therefore, it was decided to aim attention at two subtopics: the reactants dosage and the process modeling. Bibliometrics is employed again to study their significance and, afterwards, report a summary of what has been already addressed and the points that need farther work, the actual the influence and concern of the selected themes.
The search is done comparing each specific subject with photo-Fenton in general. Figure 4.5 make noticeable the continuous concern on this area, which is an indicator of the non-definite outcomes.
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The exact key words searched are3:
• Photo-Fenton: “photo-Fenton” • Dosage: "photo-Fenton" AND dosage • Process modeling: "photo-Fenton" AND modeling W/10 process.
700
600
500
400 Dosage 300 Modeling
200 photo-Fenton Numberpublications of
100
0 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 Year
Figure 4.5: Trends in the number of publications in the last decade on the topic dosage, modeling and photo-Fenton. Data obtained from (ScienceDirect 2020). Data from 2020 is only considered until June.
With regard to the graphic above, 37.5% of the research paper containing the word photo-Fenton, also discuss the dosage. Separately, 55.7% talk process modeling out. It is an indicator that both topics are relevant and awake concern over the research groups.
Detailed bibliographic study on these specific subjects is found in the ensuing sections. The discussion is based on corresponding articles and reviews.
4.2 Dosage
Dosage is acknowledged as an important issue in all the Fenton processes (Chu et al. 2007; Zazo et al. 2011; Yamal-Turbay et al. 2013). Fenton and photo-Fenton processes require specific catalyst
3 The searching is done with Boolean operators. To search for a specific phrase, enclose the terms in double quotes (" "). Use W/n to specify how far apart terms may appear in documents. W represents "within", and n represents the maximum number of words between the terms. (Van der Walt 2020)
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and oxidant concentrations to target the desired degradation of the contaminant and often a further mineralization.
As it has been mentioned in section 2.2.3, the addition of more or less catalyst and reagent has a predominant effect in the treatments. For instance, the amount of H2O2 used is a primordial parameter to take into consideration because the process comes to an end when all the H2O2 is consumed.
Ferrous salts reaction with hydrogen peroxide is what produces the HO· radicals. However, due to the extremely instability of the radicals and the fact that are non-selective, it is often scavenged by undesired secondary reactions. Hence, many researchers propose the continuous production of radicals in the reaction media by applying a continuous addition of H2O2 into the system for a specific period of time.(Zazo et al. 2011; Prato-Garcia and Buitrón 2012; Yamal-Turbay et al. 2013; Ortega-Gómez, Fernández-Ibáñez, et al. 2012; Chu et al. 2007; I Carra et al. 2012)
Literature presented studies which stated that treatment for real wastewaters by Fenton’s reagents requires much higher doses than wastewaters used in a simulation bearing the same contaminant content and achieving the same treatment efficiency. (Gulkaya, Surucu, and Dilek 2006)
Iron and peroxide concentrations are found expressed in different ways among the bibliography, just as the concentration or related to one another throughout a ratio, with carbon oxygen demand (COD) or with the pollutant concentration.
An extent bibliographic research has been done aiming to compare different studies carried out throughout the last decades, focusing on the hydrogen peroxide and the catalyst (iron ion or different) dosage analysis.
4.2.1 General bibliographic analysis of operational conditions.
The following table contains a brief resume of 26 research articles. An effort has been done to summarize all the information in a single table in order to make more visual the differences and coincidences among the studies.
The selection among all the read papers was characterized by the availability of the full articles, the understandability of the essay, the structure of information and the presentation of clear and conclusive statements.
The table is sectioned in columns containing the most relevant information of each article. This include the treated contaminant, set up and process conditions, dosage scheme, objective function and final comments and results.
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The decision of focusing on these sectors is taken by the author after an extent analysis of many articles. It is considered significant information as it gives a general view on which are the differences on the treated contaminant, the employed equipment or the operational conditions. These are issues that each research group treat differently depending on the given facilities. For instance, specifying the contaminant allows visualizing if all the authors study pharmaceutical waters, and indicating the type of water gives information on whether the treated wastewater is an effluent or a solution made up just for the experimentation.
The comparative study has been divided considering the diverse water sources: pharmaceutical, agrochemical, textile, and paper mill industries. It can be noted that for every class of water source there are actualized references available. This is an indicator of the constant interest that this subject arises. However, it can be noticed that pharmaceutical wastewater has been the most popularly studied, which may be due to the concern that CECs have arisen. Most of the CECs are pharmaceutical or take-care products.
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Table 4.2: Literature resume of pharmaceutical wastewaters.
Contaminant Setup & Objective Dosage scheme Results Reference (Pharma) Process conditions function Paracetamol Tank reactor: reservoir tank Homogeneous TOC removal Best results using irradiation. (Audino et al. (PCT) Volume: 9L PCT removal 2019) 75푚푖푛 Unique initial addition. [H2O2] track Highest level mineralization, 푋푇푂퐶 = 68.5% Well-stirred annual photoreactor [Iron] 2+. t equipped with an actinic lamp. → Varying initial H2O2 and Fe Same conditions but ↑ [C o ] = ↑ TOC conversion but ↓ H2O2 Batch mode with recirculation. 1. [PCT] = ct. efficiency on PCT removal. Volume: 6L 2. [Cto ] = half and twice the stoichiometric dose. H2O2 Solution volume: 1.5 L to 3. [CFe2+ ] = 5 to 10* mg /L t t 4. [C o ]/[C o ] = 10.5, 21, 42. UVA-UVB lamp (λ=300 and 420 nm) H2O2 cont 5. 6. W/ and w/out irradiation. Iron species: Iron salts, FeSO4·7H2O
Solution characteristics:
[PCT]o = 40 mg/L
PCT & Phenol Pyrex-glass cell wrapped in aluminum Heterogeneous Phenol removal Best working conditions: (Rad et al. foil as reactor. PCT removal • pH = 3.5 2015) Volume: 500mL Unique initial addition • [Cto ] = 50 mmol/L H2O2 to • [CFe2+ ] = 0.2 g/L 4 UV lamps (15 W, λ=365 nm) Box-Behnken design (BBD): five factors at three levels. Phenol degradation= 95% Iron species = Cobalt ferrite PCT degradation = 85% nanoparticles 8. [PCT] and [phenol]= 20 – 100 mg/L Values in agreement with estimated values by the model. 9. [Cto ] = 30 – 70 mmol/L H2O2 T: 45ºC to 10. [CFe2+ ] = 1.0 – 0.3 mg /L pH: 3-4 11. 7. Contact time = 1h = ct.
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Sulfonamides: Up-flow glass reactor. Homogeneous SDZ and STZ removal ↑ pH = ↓ degradation rate (Paula, Batista, Sulfadiazine Mineralization Degradation rate ↑ using Fe-Oxalate. and Nogueira 2012) (SDZ) 15 W blacklight lamp. (λmax= 365 nm) Unique initial addition Degradation time: Irradiated volume: • Fe-Oxalate < Fe (NO3)3 Sulfathiazole • SDZ = 500mL Varying pH and [H2O2] : • pH = 6 and FE-Oxalate < pH = 2-3 and Fe (NO3)3 (STZ) • STZ = 280 mL 12. [Cto ] = 2.5 – 10 mmol/L (Δ 2.5 Mm) (pH=6 → 70% deg. but insignificant H2O2 to mineralization) 13. [CFe2+ ] = ct = 0.2 mM
Iron species: Fe(NO3)3 and Fe-Oxalate.14. [Cto ]/[Cto ] = 25, 50, 75, 100. H2O2 cont Complete SDZ, STZ degradation: to to 15. [C ]/[C 2+ ] = 12.5, 25, 37.5, 50. pH: 2.5, 4, 5, 6. H2O2 Fe • 8 min
• pH=2,5
Effluent characteristics: • Fe-Oxalate • [Cto ]/[Cto ] = 50 ([Cto ] = 5 mM) [SDZ, STZ] = ct = 25.0 mg/L H2O2 cont H2O2 • SDZ (TOC = 12.0 mg/L) • SDZ= 92% and STZ = 90% mineralization. (42 • STZ (TOC = 10.6 mg/L) min)
Amoxicillin Batch experiment: Pyrex reactor. Homogeneous COD degradation The best results from every experiment were fulfilled in the (Elmolla and (AMX) Volume: 600 mL BOD5/COD next conditions: Chaudhuri 2009a) Unique initial addition TOC degradation. • [CH2O2 ]/ [COD] = 1.5 Ampicillin UV lamp: nominal power = 6W. [H2O2]/ [COD] • [CH O ]/[CFe2+ ] = 20 (AMP) 2 2 (emissions at wavelength = 365 nm) Varying [H2O2], maintaining [COD]: • pH=3 Cloxacillin • [COD] = ct = 520 mg/L = 16.25 mM (ct) • [COD] = 520 mg/L (CLX) Iron species: Iron salts, FeSO4·7H2O • [CH2O2 ] = 15 – 40 mM Degradation achieved in 2 min. • [CH2O2 ]/ [COD] = 1, 1.5, 2, 2.5
T: 22 ± 2ºC 2+ • [CH2O2 ]/[CFe ] = 50 pH: 3.5 to study the ratios effects and: • 50 min irradiation BOD5/COD = 0.4, considerate adequate for biological 2, 2.5, 3, 3.5 and 4 to evaluate the pH treatment. effect. ퟐ+ [퐇ퟐ퐎ퟐ]/[ 퐅퐞 ] COD degradation = 80.8 %
Varying [CFe2+ ], maintaining [COD]: TOC degradation = 58.4 % • [COD] = 520 mg/L = 16.25 mM (ct)
• [CFe2+ ] = 0.16 – 2.4 mg/L Mineralization of organic carbon and Nitrogen.
2+ • [CH2O2 ]/[CFe ] = 10, 20, 50, 100, 150.
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• [CH2O2 ] = ct = 24.3 mM.
• [CH2O2 ]/ [COD] = 1.5 (best result tested) • 50 min irradiation
퐭퐨 [퐂퐚퐧퐭퐢퐛퐢퐨퐭퐢퐜퐬 ]
• [CH2O2 ]/ [COD] = 1.5
2+ • [CH2O2 ]/[CFe ] = 20 (best result tested) • pH = 3 • [COD] = 520 (AMX), 1229 (AMP), 2440 (CLX) mg/L
Amoxicillin Up-flow photoreactor using a glass Homogeneous AMX, PCT & BZF Best results for BZF and PCT using FeOx: (Gustavo (AMX) cylinder. degradation • 98% degradation after 5 min. TrovóTrov, Volume: 800 mL Unique initial addition TOC removal Alessandra Bezafibrate Santos Melo, Kinetics study Using Fe(NO3)3: (BZF) and Fernandes For irradiated experiments: 15W Fixed concentration of iron (0.20 mM) and of H2O2 • 89% (BZF) and 53% (PCT) of degradation. Pupo Nogueira Paracetamol black-light fluorescent • AMX & PCT = blacklight irradiation → For AMX, no difference (using Fe(NO3)3 or FeOx) Total 2008) (PCT) • BZF = solar irradiation degradation after 0.5 min. Iron source: Fe(NO3)3·9H2O and • Adding FeOx potassium ferrioxalate • Adding Fe(NO3)3 ↑ TOC removal when using FeOx → high quantum yield of (K3Fe(C2O4)3·3H2O), ferroxilate (FeOx). Fe(II) generation + radicals created during oxidation = enhance oxidative process.
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to [CAMX ] = 0.1 M = 42 mg/L (19 mg/L of Varying H2O2 using FeOx (presented better result ↑ [CH2O2 ] (1 to 2 mM)= TOC) on the previous test) • ↑ AMX degradation constant (from 0.91 to 2 to • -1 [CPCT ] = 0.1M = 15 mg/L (10 mg/L of [AMX], [PCT] = 1 – 5 mM min ), but not possible to calculate when • TOC) [BZF] = 3 – 10 mM [CH2O2 ] = 5 mM bc AMX is lower than the to detection limit after 0.5 min. [CBZF] = 0.055 M = 20 mg/L (13 mg/L
TOC) • ↑ AMX mineralization (from 67 to 85%); [CH2O2 ] = 5mM, no improvement (87%)
↑ [CH2O2 ] (3-10 mM) = • ↑ BZF degradation constant (0.61-1.5 min-1) • TOC not influenced
↑ [CH2O2 ] = • PCT degradation kinetics independent from
[CH2O2 ] • TOC not influenced
[C ] AMX Compound parabolic collector reactor Homogeneous ↑ H2O2 = ↑ degradation efficiency (Alalm, Tawfik, (CPC) depending on the solar light. Removal of: Optimal [C ] = 1.5 g/L and Ookawara AMP H2O2 • 2015) Fed volume = 4L Unique initial addition AMX • [C ] < 1.5 g/L → H2O2 totally consumed PCT H2O2 Light source: solar light. AMP after 45 min = not enough for complete Diclofenac Illumination time (t30W) used to 1 experiment x each PhAC. PCT degradation. compare instead of exposition time. → • [CH2O2 ] = 0.5 – 2 g/L Diclofenac • [CH2O2 ] > 1.5 g/L (≈2 g/L) H2O2 remains • Illuminated volume = 2.3 L. • [CFe2+] = 0.1 – 0.75 g/L Optimal conditions:
Iron source: iron salts, FeSO4 · 7H2O Kinetic study 2+ [CFe ] = 0.5 g/L and [CH2O2 ] = 1.5 g/L. Removals: • AMX = 97% pH = 3 • AMP = 88% • PCT = 100% Effluent characteristics: • Diclofenac = 94% • [AMX]= 100 mg/L • [AMP]= 100 mg/L • [PCT]= 100 mg/L • [Diclofenac]= 100 mg/L
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Atenolol (ATL) Pyrex UV reactor PhACs removal 100% removal of PhACs after 1 min. (Veloutsou, Bizani, and Metoprolol Volume = 1L Homogenous ↑ [CFe] = ↑ degradation TOC degradation Fytianos 2014) Light source: High pressure Hg lamp, Unique initial addition For Atenolol:
125W and λmàx=290 nm. Kinetic study • [CFe] = 5 mg/L 푡표 • [C ] = 100 mg/L [CFe ] = 2.8 – 5 mg/L H2O2 Iron source = Iron salts. Fe(ClO4)3 to • COD complete removal ≈ 150 min [CH O ]= 95 – 100 mg/L 2 2
Reaction time = 180 min For Metoprolol:
• [CFe] = 2.8 mg/L T = 30 – 35 ºC • [CH2O2] = 95 mg/L pH ≈ 2.9 • COD complete removal ≈ 150 min
Effluent characteristics:
• [ATL]o = 20 mg/L • [Metoprolol]o =20 mg/L
BLB-R Homogeneous Sulfonamides removal Best results with [Cto ] = 400 mg/L (O González et Sulfonamides H2O2 al. 2009) Reactor vessel placed in a Unique initial addition TOC degradation Complete remove of antibiotics in 50 min. photochemical chamber: 56.4 % TOC removal after 94 min. continuously stirred 푡표 [CFe ] = ct = 10 mg/L Energy consumption 82.2 COD removal after 94 min. Volume = 2L t [C o ]= 50, 100, 200, 300 and 400 mg/L Lab scale H2O2 Energy consumption ≈ 2.5 kJ/L.
Light source: Low pressure UV lamp of Hg vapor (8 lamps x 8W each). λ = 350 – 400 nm
Iron source: iron salts, FeSO4 · 7H2O
T = 23 – 31 ºC (not controlled) Reaction time = 100 min
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Sulfonamides 4-CPC R Homogeneous SMX, TOC, COD Complete remove of SMX in 100 min. (O González et removal 87 % TOC removal after 94 min al. 2009) Four CPC units. Reactor: continuously Stepped additions Biodegradability 96% COD removal after 94 min. stirred tank with recirculation. Nitrogen removal Energy consumption ≈ 19 kJ/L. Volume = 82 L 푡표 [CFe ] = ct = 10 mg/L Illuminated volume = 44.6 L BLB-R vs. 4CPC-R (300 mg H2O2/L) [CH2O2 ] TOTAL = 550 mg/L Pilot pant scale • Best TOC removal: BLB-R to [C ] = 50 mg/L, when it is completely • Best COD removal: 4CPC-R Light source: solar irradiation ≈ 121.6 H2O2 • Best SMX removal: BLB-R consumed, another 50 mg/L are added until a total W/m2. • of 550 mg/L Higher biodegradability: 4CPC-R (300 < λ < 500 nm) (This experiment was to determinate the maximum oxidation degree that could be reached 4 2 Iron source: iron salts, FeSO · 7H O based on the H2O2 consumed)
T = 26ºC To compare wit BLB-R (Lab scale) pH = 2.8 to [CH O ] = 300 mg/L (all at once) Reaction time = 100 min 2 2 To obtain a direct comparison with the BLB-R experiment.
Cylindrical borosilicate vessel, batch Homogeneous Drug removal [Cto ] = 22.5 mM (Giri and Dipyrone H2O2 mode with continuous stirring. Mineralization to Golder 2014) [C 2+ ] = 2.25 mM Volume = 1 L Unique initial addition Biodegradability Fe Fenton Drug solution volume = 400 mL. • 94.1 % of drug removal in 45 min Variation of reagents and pH • Initial faster drug removal followed by virtually Light source: UV lamp of 9 W (362 constant rate: 73.5 % of drug removal in 2.5 min nm) 푡표 [CFe ] = 1 – 3 mM • 28.4% and 42.78% TOC removal in 2.5 and 5 2 12 W/m of intensity. • (1, 1.5, 2, 2.25, 3) Mm min.
• Biodegradability: BOD5/COD = 0.62 Iron source: iron salts, FeSO4 · 7H2O [CH O ] TOTAL = 5 – 25 mM Photo-Fenton 2 2 • (5, 10, 15, 17.5, 22.5, 25) mM • 96.4 % of drug removal in 45 min. T: room temperature (13 – 25 ºC) • Initial faster drug removal followed by virtually pH= 3.5 constant rate: 83.2 % of drug removal in 2.5 min Reaction after 2h of initial mixing. • 53.6% and 56% TOC removal in 2.5 and 5 min.
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Efficiency of drug degradation and • Better results of biodegradability: BOD5/COD = mineralization is reported with 1.51 respect of [4-MAA] In absence of Fe ions, the PhAC removal was 74,4% in 45 Solution characteristics: min. • [BOD5] = 3.75 mg/L • [COD] = 41.3 mg/L • [DIPY] = 50 mg/L
Metoprolol Photochemical reactor, Pyrex- Homogeneous MET removal Adding RES (ligand iron complex) to the solution to perform (Romero et al. (MET) Jacketed thermostatic vessel. RES removal the reaction in neutral pH. 2016) Volume: 2L Unique initial addition TOC removal Resorcinol Biodegradability Working at highest concentrations: 푡 (RES) Light source: 3 x BLB of 8W (365 nm) 푡표 • [C 표 ] [CFe ] = 1, 2.5, 5 and 10 mg/L Fe = 5 and 10 mg/L
• [CH2O2 ] TOTAL = 125 and 150 mg/L
Iron source: iron salts, FeSO4 · 7H2O Fenton (10 mgFe/L, 150 mgH2O2, 20 min) [CH2O2 ] TOTAL = 25, 50, 125, 150 mg/L • 100% MET removal T = 25ºC [RES] was tested: 12.5, 25 and 50 mg/L to evaluate • 92% RES removal pH = 6.2 (neutral) the best % of ligand chelation with the minimum • 17.2% TOC removal in 60 min. quantity of ligand. • 33% COD removal in 120 min. - 100% MET degradation with 50mg/L Photo-Fenton (10 mgFe/L, 150 mgH2O2, 20 min) • 100% MET removal • 94.4% RES removal • 76% TOC removal in 60 min. • 45% COD removal in 60 min.
Photo-Fenton process gets better results.
Metoprolol Photochemical reactor, Pyrex- Homogeneous MET removal Photolysis alone, H2O2 alone and photolysis with iron were (Romero et al. Jacketed thermostatic vessel. TOC removal not able to eliminate MET. 2016) Volume: 2L Unique initial addition BLB lamp: Light sources: 푡표 • 100% MET elimination after 7 min. [CFe ] = 2.5 and 10 mg/L
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• 3 x BLB of 8W (365 nm) • 81.2% TOC removal after 90 min. • Solar irradiation [CH2O2 ] = 25 and 150 mg/L Artificial solar • Artificial solar irradiation • 97.3% of MET elimination after 7 min. • UVC lamp • 78.8% of TOC removal after 120 min. UVC Iron source: iron salts, FeSO4 · 7H2O • 100% of MET elimination after 20 min. • 17.6% of TOC removal after 60 min. T = 25ºC
pH = 3 Fenton (10 mgFe/L, 150 mgH2O2, 20 min) • 67% MET removal Experiment carried out to compare it • 8% TOC removal in 60 min. with the addition of Resorcinol (RES)
Photo-Fenton (10 mgFe/L, 150 mgH2O2, 20 min) • 100% MET removal • 25% TOC removal in 60 min.
PCT Open Pyrex glass vessel Homogeneous PCT removal Blank experiments to assure that the result of ph-F was not (Trovó et al. Volume = 2L TOC removal due to hydrolysis and/or photolysis. 2012a) Unique initial addition Toxicity Negligible photolysis of PCT at natural pH (4.2) after 300 (Trovó et al. Irradiation source: Solar radiation min and no hydrolysis at pH 2.5 and 4.5 during 48h. 2012b)
simulator [H2O2] = 120 mg/L (1100 W xenon arc lamp) PCT degradation favored in presence of FeSO4. [CH2O2 ]= 0.05 mM (based on previous work) For 100% oxidation of PCT: Iron source: iron salts, FeSO4 · 7H2O • 120 min with FeSO4. (FeSO4) • 180 min with FeOx
or K3Fe(C2O4)3 · 3H2O (FeOx) TOC removal after 300 min irradiation:
Temperature ¼ 25 _C • 79% with FeSO4. pH = 2.8 • 58% with FeOx. (contribution of 3.6 mg/L from Reaction time ≈ 300 min oxalate = 11% of initial TOC)
Initial solution characteristics: • [PCT] = 15 mg/L • [TOC] = 9.6 mg C /L
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Photo-Fenton solution characteristics: • [PCT] = 50 mg/L • [TOC] = 31.8 mg C /L
Combination CPC photoreactor, semi-batch mode Homogeneous EC removal Total ECs removal were achieved in less than 90 min. (Quiñones et of 6 CECs with recirculation. TOC removal al. 2015) Volume: 7L Unique initial addition Toxicity At the optimum point, 2.5-fold increase of the BOD5/COD Biodegradability ratio was shown after treatment. 3+ Solar light source [H2O2]/[Fe ] = 6.1(mass ratio) Almost 35% TOC removal after 5 h. [Catalyst] = 2.8 mg/L pH = 3 Reaction time: 5 h
Effluent characteristics: • Secondary effluent from a MWTP. • [EC’s] = 0.2 mg/L each one • [TOC]o = 20 mg/L from the secondary effluent + 0.8 mg/L from the ECs. • Volume : 5L
Phenol Magnetically stirred cylindrical quartz Unique initial addition Fe-Laponite type Better performance under irradiation than in the dark. (Iurascu et al. cells. Phenol degradation. 2009) Volume = 10mL Heterogeneous: Kinetic study. 1 g/L of catalyst Fe-Lap gave the best performance. (using 푡표 UV-A 40 W and 0.1 mM of phenol was used to [C퐻2푂2] = 50 mM) Light source: low pressure Hg lamps: determine the best catalysts for the process. • 15 W UV-C (λmax =254 nm) Higher doses of catalyst = lower phenol conversion. 푡 • 40W UV-A (λmax =365 nm) 표 [CCatalyst] = 0.5 - 2 g/L (turbidity) 푡표 Phenol is converted under both irradiation wavelengths. [C퐻 푂 ] = 20, 50, 100 mM Iron source: 2 2 UV-A << UV-C.
Homogeneous:
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푡표 • Heterogeneous: various [CFe ] = 2mg/L dissolved. <<< amount of Fe content Fe-Laponite catalysts, in the catalysts. differently treated. Comparable with the leached Fe from Fe-Laponite • Homogeneous: iron salts, systems measured in the solution (< 1mg/L) Fe(NO3)3·9H2O Kinetic study: pH = 3 With the optimal results obtained previously, T = 30ºC phenol degradation routes can be summarized in General Lumped Kinetic model.
PCT Pyrex cylindrical reactor. Heterogeneous PCT degradation No detectable adsorption onto Goethite (Mameri et Solution volume: 50 mL surface was observed. al. 2016) Unique initial addition Kinetic study Light source: solar light at 365 nm (I = With the 1 mM sensibility detection, Fe2+ 1840 mW/cm2 ) To find out the optimum catalyst amount, leachate was not detected. different were tested under neutral pH and pH = 3 atmospheric temperature. No degradation was shown after 3 h of direct T = 20ºC photolysis. 푡표 Reaction time = 180 min. [CCatalyst] = 0.5 – 1.5 g/L At optimum conditions, the first order Solution characteristics: reaction rate was 2.33x10-1h • [PCT] = 0.1 Mm 푡표 • Optimum [CCatalyst] = 1 g/L • Optimum [C푡표 ] = 5 mM H2푂2
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Table 4.3: Literature resume of agriculture wastewaters.
Contaminant (agro- Setup & Objective Dosage scheme Results Reference industry) Process conditions function
5 pesticides Compound Parabolic Collector (CPC) → Batch mode Homogeneous Pesticides removal. Reference experiment: (I Carra et al. 2012) with recirculation. • Best results with light than in dark Stepped iron addition. TOC removal conditions. Total treated volume: 7L kinetics • Complete removal = 7.5 min and at Irradiated volume: 4.55 L Total [Fe] changes. At the beginning is kept at 20% mineralization. 20, but later is increased. DO evolution • Total mineralization = 52% Water coming from activated sludge (previously to treated by aerobic treatment) Reference experiment: [H2O2] consumption [CFe2+ ] = 5 mg/L for 4 additions (still 20 mg • pH= 2.8 Fe/L): Solar light source. Measure with the normalized • [Cto ] = 650 mg/L ≈ 20mM H2O2 illumination time (t30W). average UV radiation = 20W. to • Not complete elimination of the • [C 2+ ] =20mg/L all at once Fe pollutants after the 60 min.
Comparison of Dark and Irradiated experiments. • Total mineralization = 8% (due to Others: the low concentration of iron
• pH = 7 during the first 15 min) Iron species: Iron salts, FeSO4·7H2O to • [C 2+ ] =5, 10 and 20 mg/L (4 Fe additions) at times = 0, 5, 10, 15 T: room temperature Consumption rate of H2O2 in reference > (min). Total reaction time = 60 min 5mg/L for 4 additions.
Different sequences: varying iron Variative sequences: Effluent characteristics: concentration in the system per addition as • Better mineralization when ↑ Fe pH = 7 well as the time of addition. amount and ↓ addition time. [TOC]o = 50 mg/LDE mineralized water • pH = 7 • TOC removal with variations ≈ • to [CFe2+ ] = 20-20-10-10 mg/L (4 reference
additions) • ↑Max mineralization rate when ↑ to • [CFe2+ ] = 20-20-10-10-10 mg/L (5 dissolved [Fe]. → Saturation point additions) = [Fe]mean ≈ 20 mg/L
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• Sequence 20–20–10–10–10 mg Fe/L = highest mineralization rate with the least dissolved iron at initial neutral pH. ATZ Lab-scale photochemical reactor → batch mode Homogeneous ATZ removal Removal efficiency of ATZ by ESFP = 3–10% (Chu et al. 2007) (herbicide) completely mixed higher than OSFP. Pyrex-glass cell wrapped in aluminum foil. Stepped peroxide additions Kinetic study Best result = H1 and H2 of 0,6+0,4 respectively. No irradiation: Fenton Process (FP) [CFe2+ ] = 0.1 mM [CH2O2]total = 0.1 mM → 2 different dosages. Iron species: Iron salts, FeSO4·7H2O Even-dose stepwise FP (ESFP): T: room temperature pH: 3 [Cto ] = 0.5 mM dose at the beginning + 0,5 H2O2 at times: 1.5, 2.5, 4.0, 5.0, 7.5, 10.0, 12.5, and solution characteristics: 15.0 min. • [ATZ] = 0.01 mM Asymmetrical-dose stepwise FP (ASFP) Variable stepwise doses
2 consequent additions of H2O2 (H1 and H2): • H1 = 0.03, 0.04, 0.05, 0.06 and 0.07 mM at the beginning. • H2= (0.1 – H1). At dosing time T. This T is set to 5 min.
OSFP experiment: As a reference. • [Cto ] = 0.1 mM all at once at the H2O2 beginning. • [Cto ] = H1 alone at initial time. H2O2
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General agro- Lab-scale Pyrex photoreactor → batch mode Homogeneous Color (UV 400) ↑ [H2O2] up to 3 mg/L = enhance kinetics and (Ahmed et al. 2011) industry Volume: 2 L removal efficiency: • wastewater. Unique initial addition COD removal = 85% Low pressure mercury vapor lamp (40W, 254 nm) Aromaticity (UV • Aromaticity removal = 82%
Variation of [H2O2]: 280) removal • Color removal = no significant to influence when ↑ [H2O2]. Iron species: Iron salts, FeSO4·7H2O • [C ] = 0.5-1.5-2-3-5 mg/L H2O2 to COD removal [H2O2] > 3 mg/L = no significant changes. • [CFe2+ ] = ct = 30 mg/L T: 26ºC Low [H2O2] = remove color but no satisfactory remove of COD. pH = 3 2+ TOC removal Variation of [Fe ]: [Fe2+] =30 mg Fe/L → Best results of color and Time = 6h to • [CH O ] = ct = 3 mg/L aromaticity removal. 2 2 to 2+ • [C 2+ ] = 0 – 60 mg/L (0-10-15-30- [Fe ] > 30 mg/L = does not yield higher AIW effluent characteristics: (1L of AIW) Fe 60) removals. • [COD] = 2000-7000 mg O2/L 2+ Under [H2O2] = 3 mg/L and [Fe ] = 30 mg/L: • [TOC] = 180-300 mg C/L • 92% of [COD] removed • 83% of [TOC] removed
Table 4.4: Literature resume of textile wastewaters.
Contaminant Setup & Objective Dosage scheme Results Reference (textile) Process conditions function Indigo blue Compound Parabolic Collector (CPC2D) → Batch Heterogeneous Color removal Aqueous solution (indigo blue) (Tatiana Almazán- Textile mode with recirculation. Catalyst = 1.5 g Sánchez et al. 2017) Unique initial addition [H2O2] = 0.5 M wastewater Total treated volume: 1L • With Mt-Fe = 98% removal in 3h Aqueous solution (indigo blue) and • With AC-Cu = 99% removal in 1.5h Solar light source. to to Textile wastewater: • [CFe2+ ] > [C_Cu ] but the Cu has • [Catalysts] = 0.5, 1, 1.5 g of Mt-Fe or better catalytic activity. Comparison of Dark and Irradiated experiments. AC-Cu. Fenton process (no irradiation) = 4.98% dye to removal. (3h) • [CFe2+ ] = 28.83, 57.66, 86.49 mg/L
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to Catalysts: • [C_Cu ] = 11.48, 22.96, 34.44 mg/L Iron species: Iron modified clay (Mt-Fe) • [Cto ] = 0.1, 0.3, 0.5 M Textile wastewater H2O2 Copper species: Copper modified carbon (AC-Cu)16. Catalyst = 1.5 g [H2O2] = 0.5 M T: room temperature • With Mt-Fe = 79% removal in 4h • With AC-Cu = 93% removal in 4h Aqueous effluent characteristics: Time wastewater > time aqueous solution due • pH = 7 to the high concentration of organic and • treatment time = 3h inorganic matter → slowed the oxidation of the dye. Textile wastewater effluent characteristics: • pH= 6.8 • Treatment time = 4h • [TOC]= 417 mg/L
Photoreactor with recirculation. Homogeneous Dye removal Optimal [Cto ] = 6Mm (Tarkwa et al. 2019) Orange G dye H2O2 Volume: 1.3 L Unique single addition to Optimal [C 2+ ] = 0.4 Mm Mineralization Fe • Kinetic constant = 1.05 min-1 Light source: low pressure Hg lamp (λ = 253.7 nm) [Cto ] = 1, 2, 4, 6 and 8 mM H2O2 • 92.2% mineralization degree in 180 min. 3+ Iron species: ferric iron (Fe ) to [C 3+ ] =0.1, 0.2, 0.3, 0.4 and 0.6 mM Optimal obtained ratio= when contact point Fe between lines: pH = 3 to to • [C ] = 6Mm & [C 2+ ] = 0.435 To minimize the ·OH wasting reactions (with a H2O2 Fe high rate constants), the suitable Molar Ratio Mm → R= 13.8 Solution characteristics: 3+ [H2O2]/[Fe ] (R): • Complete removal of Orange G dye. • [Orange G] = 0.1 mM • Experiments with fixed [Fe3+] or • 93.41% TOC removal in 180 min. [H2O2] and varying the other = linear relations between k and R.
FBL Fenton: Homogeneous Dyes Color removal efficiencies were excellent for (R. Liu et al. 2007) FBB degradation the 3 dyes. Jar-test with stirring. Unique initial addition S-RL dyes Solution characteristichs: TOC removal TOC removal > 95 % in FBB and FBL cases when [Cto ] = 90 mg. • Volume: 300 mL Fixed Fe and variation of H2O2 dosage: H2O2
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to • [COD]O= 500mg/L • [C ] = 90, 240, 390, 540, 690 and H2O2 840 mg in the 300 mL solution. TOC removal increased 66% for S-RL by t to o Iron source: Iron salts, FeSO4·7H2O [C ] = 840 g → increased with the photo- • [CFe3+ ] = 55 mg in the 300 mL H2O2 solution. Fenton process. 2+ Photo-Fenton: • [H2O2]/[Fe ] = range from 1.63 to 15.25 (g/g) Photoreactor Solution characteristics: Fixed H2O2 and variation of Fe dosage: • Volume: 4 L • [Cto ] = 90, 240, 390, 540, 690 and H2O2 • [COD]O= 500mg/L 840 mg in the 300 mL solution. to • [CFe3+ ] = 90, 180, 270, 360, 450 and Iron source: Iron salts, FeSO4·7H2O 540 mg in the 300 mL solution.
Light source: 12 x medium pressure Hg lamps. 15W of power each lamp. (λ = 254 nm) UV irradiation time = 35min.
(only Fenton treatment) Homogeneous COD removal to (Gulkaya, Surucu, and General COD in ↑ [CFe2+ ] = ↑ [COD] removal dye waters. Batch reactor Dilek 2006) V of solution: 100 mL Unique initial addition TOC removal • [Cto ] Fe2+ from 1.1 to 5.5 g/L = 40 to 95% COD removal. Iron source: Iron salts, FeSO4·7H2O [Fe] determination: to • [CFe2+ ] > 5.5 g/L → dominance of • [Cto ] = ct. = 385 g/L H2O2 oxidation, negligible coagulation. Temperature: Studied range (25-70 ºC) to • [CFe2+ ] = between 1.1 – 10.9 g/L pH: studied range (2-5.5) to ↑ [C퐻 푂 ] = ↑ [COD] removal To study dosage: 2 2 [H2O2] determination: • T = ct = 50ºC to t • [C ] = between 19.3 – 577.5 g/L • [C o ] from 19.3 to 385 g/L = 67 H2O2 퐻2푂2 • pH = ct = 3 to • [CFe2+ ] = ct. = 5.5 g/L to 95% COD removal. to • [C 2+ ] > 385 g/L → ferrous conc Solution characteristics: (*) Ranges are selected from preliminary Fe experiments. became deficient for reacting with [COD]o = 2400 mg/L H2O2. [TOC]o = 2000 mg/L Further experiments carried out to decrease the ↑ [Cto ] = ↑ [TOC] removal (almost all 퐻2푂2 reagents load but maintain the ratio value. removed in oxidation stage)
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The study differentiates between two stages: to to oxidation and coagulation. ↑ [COD] removal = ↑ [C ] /[C 2+ ] mass 퐻2푂2 Fe ratio (until 95) to to [COD] = ct = ↑ [C ] /[C 2+ ] from 95 to 290 퐻2푂2 Fe ↓ [COD] = ratio > 290
to to ↑ [C ] /[C 2+ ] = 95 to 290 = best COD 퐻2푂2 Fe removal ( =153 – 470 as molar ratio)
Mixture of Photocatalytic reactor covered with aluminum foil. Homogeneous COD removal. For conditions of 6 g/L H2O2, 0.05 g/L Fe2+ and (Módenes et al. colorants and Batch system. pH=3: 2012) Volume=500mL • 88-98% COD and color removal dyes. Unique initial addition. Decolorization.
Iron source: Iron salts, FeSO4·7H2O Factorial design with central concentrations TOC removal Although the optimal based on preliminary experiments: concentration of 0.05 g L= best COD removal, a Solar light and artificial light: 3 x high pressure Hg • [Cto ] = 1 – 7 g/L Cost evaluation non-significant difference in the color removal H2O2 vapor lamps (250 W, λ=350 nm) to when the • [C 2+ ] = 0.01 – 0.09 g/L Fe concentration ranged from 0.01 to 0.1 g/L
T ≈ 35ºC pH= varied for study. Irradiation time increases from 120 to 240 min, the same COD Effluent characteristics: removal (94%) was achieved for all iron concentrations tested.
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Table 4.5: Literature resume of paper mill wastewaters.
Contaminant Setup & Objective Dosage scheme Results Reference (paper mill) Process conditions function Fenton Mix of pulp Compound Parabolic Colector (CPC) → Batch mode Homogeneous Pollutants removal. (Lucas et al. 2012a) with recirculation. [DOC] removal = 4% (1.3 mg/L), 18% (20 (Lucas et al. 2012b) and mill Unique initial addition TOC removal mg/L), 36% (50 mg/L). wastewater Total photoreactor volume: 40L kinetics
Irradiated volume: 22L [COD]o = 899 mg O2/L Minor improvement when ↑ iron (20 to 50) →
[TOC]O = 348 MG/l TP removal Fe complexation and reverse reaction. Water coming from activated sludge (previously [TP] = 218 mg gallic acid/L (Total Polyphenols) treated by aerobic treatment) [H2O2] consumption H2O2 consumption = 17Mm (20 mg Fe/L) and Fenton experiments (lab scale): 35 mM (50 mg Fe/L). Solar light source. Measure of the accumulated UV to • [C 2+ ] = 1.3, 20, 50 mg Fe/L Toxicity energy received on any surface in the same position Fe • [Cto ] = ct = around 200-300 regarding to the sun, per unit of volume in the reactor. H2O2 mg/L Biodegradability
Iron species: Iron salts, FeSO4·7H2O
T: room temperature 17. pH: 2.8 – 3 Photo-Fenton experiments (pilot plant scale): Photo-Fenton: Experiment (1) ↑ of iron dosage = ↑ DOC degradation to (1) • [CFe2+ ] = 1.3, 5, 10 mg /L • [Cto ] = ct = around 200-300 Mineralization = 55% H2O2 mg/L • 1.3 mg Fe/L = 59 kJ/L + 29 mM H2O2. Experiment (2) Mineralization > 90% to • 5 mg Fe/L = 31 kJ/L + 50 mM H2O2 • [CFe2+ ] = 1.3, 20, 50 mg /L • 10 mg Fe/L = 26 kJ/L + 59 mM H2O2 • [pollutant] = reduced 50% (2)
Mineralization > 90%
• 20 mg Fe/L = 17 kJ/L + 50 mM H2O2
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• 50 mg Fe/L = 15 kJ/L + 50 mM H2O2
Main drawback = generation of sludge
Mix of pulp Lab-scale photochemical reactor → batch mode Primary studies to select an optimal pH: Color, TOC & AOX Best working conditions: pH = 5 (Catalkaya and completely mixed • pH = 3 – 5. → optimal = 5. removal • Color removal = 88.1 % Kargi 2007) and mill t Pyrex-glass cell wrapped in aluminum foil. • [C o ] = 50 mmol/L • TOC removal = 84.6 % H2O2 wastewater Volume: 2.2 L to • [CFe2+ ] = 5 mmol/L Fenton: Mercury vapor lamp (16 W, 254 nm) to Fenton and ph-Fenton process: Best [CFe2+ ] = 2.5 mM • 83% color removal Iron species: Iron salts, FeSO4·7H2O Effect of iron concentration • 87% TOC removal to 89% TOC removal with 5 Mm Fe(II) but 73% • [CFe2+ ] = 1 to 10 mg/L T: room temperature t color removal → inhibition effect of iron. • [C o ] = ct = 50 mM pH: variable H2O2 Effect of H2O2 concentration: to 2+ Best [C ] = 50 mM → [H2O2]/[Fe ] molar to H2O2 • [C 2+ ] = 2.5 mg/L Effluent characteristics: Fe ratio = 20. • [Cto ] = 5 to 100 mM pH = 7 H2O2 • 84.7% color removal [COD]o = 400 mg/L • 87.5% TOC removal [BOD]o = 240 mg/L Photo-Fenton process: to shorten irradiation • 89% AOX removal time and enhance removals. [TOC]o = 110 mg/L Higher H2O2 conc and ratio = percent removals [AOX]o = 1.94 mg/L decreased
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Photo-Fenton: to Best [CFe2+ ] = 2.5 mM • 83% color removal • 85% TOC removal in 5 min. to 2+ Best [C ] = 50 mM → [H2O2]/[Fe ] molar H2O2 ratio = 20. • 82.3 % color removal • 85% TOC removal • 93.5% AOX removal. After 5 min irradiation.
Higher H2O2 conc and ratio = high rates of free radical production.
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A few observations can be remarked from the above table regarding the dosage strategy.
One clear similarity between all the proposed articles is the concern for hydrogen peroxide and catalytic additions or/and loads. Most of the proposed research articles discuss the effect of main operational conditions over the effectiveness of the process. These typically include the effect of the initial reagent concentrations, pH, temperature, wavelength and power of the radiation source, etc.
The majority of the authors choose an article profile based on a specific contaminant which consists in a first introduction followed by the experimental and analytical procedure and ending with the obtained results, comments, and conclusions. Moreover, blank assays are often performed as a way to check the effects of the different parameters alone or combined. Accordingly, Fenton process is also proposed in most of the experiments which allows comparison and taking conclusions about the effect of radiation. Authors choose ranges of concentration values based on other previous studies or by making suppositions with the stoichiometric balances.
Just a few of the contemplated articles consider other objectives such as biodegradability, nitrogen removal (O González et al. 2009), or toxicity (Trovó et al. 2012b). Articles including this information are not only aimed at studying the operation of the Fenton and photo-Fenton process but could also be studying either the disinfection of the water or the possibility of applying subsequent biological treatments. An example of considering post treatments is the article published by Giri and Golder (Giri and Golder 2014). Articles researching on the combination of treatments are not specifically treated in this work. Nevertheless, a search with ScienceDirect showed that more than 650 articles on this field have been published since 1997 and, approximately 500 of which in the last 10 years.
Furthermore, there are several articles which refer to the removal kinetics, the time it takes to degrade the contaminants or mineralize the water. This information is employed both in the discussion on the kinetic behavior and in the decision making on the loads or ratios employed (Mameri et al. 2016; I Carra et al. 2012; Lucas et al. 2012b).
Focusing on the experimental means, only a few experiments consider the optimization of reagents by studying the mass/molar ratio between H2O2 and the catalyst. Most of them only consider either the catalyst load keeping the hydrogen peroxide constant, or the other way around.
Certain researches consider the stepwise dosage of both H2O2 and catalyst, but as it can be seen, it is a great minority. Later in this work, this strategy will be deeply discussed.
Another comparison could be done on the type of catalysts. Both homogeneous and heterogeneous assays are presented. It has been observed that heterogeneous processes imply extent work. This stems from the sometimes-laborious preparation of the catalysts, as it requires the incorporation of iron into different supports (pillaring, impregnation, cationic exchange, etc.), as well as from the
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subsequent need to eliminate the supports from the sample/effluent by physical approaches such as filtration.
It is also noticeable the larger reaction time implemented in heterogeneous systems compared to homogeneous systems. It could be stated that by directly adding the iron salts dissolved into the solution, the reaction time can be much lower, which is an advantage when considering the accumulated energy needed for radiation during the process.
It has been generally observed that hydrogen peroxide and the catalytic dosage play an important paper on the operational conditions. A bibliometric study has been done on the number of articles published in these areas to check their relevance in the system. The priority of the search is based on the interest on the quantity and not on the quality of these articles, because to measure the quality, it would be necessary to really specific with the key words and specify, for example, in which quartile are the articles found. This approach level has not been scoped in the present work. Results are showed continuously:
- “photo-Fenton” AND dosage W/10 reagents ▪ 927 results since 1992. ▪ 791 results since 2010.
- “photo-Fenton” AND dosage W/10 catalyst ▪ 1011 results since 2003. ▪ 2010 results since 2010.
As it can be noted, the quantities obtained are noteworthy. In the subsequent sections, specific articles regarding these topics are analyzed.
4.2.2 Analysis of hydrogen peroxide dosage
In Fenton-based processes, the presence of H2O2 is essential as it is the main source of radicals during the treatment. The optimal amount of H2O2 should be determined because, as mentioned before in this work, a high hydrogen peroxide concentration may have a negative effect on both the contaminant degradation and the general mineralization of the effluent/solution. The H2O2 consumption it is also a critical issue due to its major contribution to the operating costs (Zazo et al. 2011; Mirzaei et al. 2017).
Among others, radical scavenging effect, auto-decomposition of H2O2 and effectiveness loss in COD removal (Elmolla and Chaudhuri 2009b) are the main adverse outcomes of the addition of too high
H2O2 concentrations. Moreover, exceeding the optimal value would end up in residual unconverted reagent which cannot be further recovered and must be removed before discharging the final effluent as it possesses potential toxicity effects (Bautista et al. 2010).
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Another relevant issue is the Dissolved Oxygen (DO) concentrations variations, which have been deeply studied during the photo-Fenton processes. Following DO concentration throughout the course of the reactions is an indicator of the H2O2 consumption. Over the last 10 years, around 260 publications have mentioned dissolved oxygen evolution (ScienceDirect 2020).
Going deeply in this subject, variations in the evolution of O2 depend mostly of the initial amounts of iron and peroxide added for the reactions. The behavior of O2 during the process was studied by many authors. Among them, Santos-Juanes et al. (Santos-Juanes et al. 2011) focused on the oxygen dependency of the irradiation and of the iron and reagent content, which is represented in Figure 4.6. In the meantime, Figure 4.7 represents the higher generation of molecular oxygen in concert with higher quantities of initial H2O2.
Figure 4.6: Evolution of dissolved oxygen concentration under different photo-Fenton operating conditions. Source:(Santos-Juanes et al. 2011)
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Figure 4.7: Dissolved oxygen evolution (A) and oxygen generation rates (B) as a function of hydrogen peroxide concentration. Source: (Santos-Juanes et al. 2011)
Conversely, in the case of the lowest hydroxide peroxide concentration, the dissolved oxygen curve dropped to zero as the oxygen needed for organic matter oxidation was higher than the oxygen generated by the reactions (Eq. 2.16 and Eq. 2.18) or the O2 transferred from the atmosphere. This is not recommendable given the participation of oxygen in some mineralization steps (Eq. 2.11 and Eq. 2.12)
Elevated DO is an indicator of inefficient consumption of the reagent. H2O2 is participating in secondary reactions which do not concern the oxidation of organic matter, it is decomposing without reacting. A first maximum should be obtained before all the oxygen generation reactions finish, and from that point onwards, a decrease should be observed as oxidation reactions are oxygen consuming.
It is thanks to the H2O2 tracking that the mineralization performance of the system can be evaluated. The mineralization performance of the system is measured with the specific consumption of the oxidizing agent evaluated as follows:
t0 tf Eq. 4.1 cH O − cH O γ = 2 2 2 2 H2O2/TOC t0 tf cTOC − cTOC
Where tf is the final reaction time or the moment at which the oxidizing agent consumption occurs.
The higher this value is, the less efficient consumption of H2O2 as the difference of consumption between one and the other is larger.
Further investigations on this subject are directed, firstly towards the stepwise dosage of peroxide, and secondly towards the automation of hydrogen peroxide dosage by developing a process
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controller that minimizes its consumption and maximizes the contaminant mineralization rate (Ortega-Gómez, Moreno Úbeda, et al. 2012; Prato-Garcia and Buitrón 2012). Further detailing on this subject is treated later in this work.
It is worth mentioning the research by Prato-García et al. (Prato-Garcia and Buitrón 2012). The study is centered on the relation between these two parameters and how peroxide dosage could be optimized. The author proposes five experiment types, including an automation of H2O2 dosage:
- (1) H2O2 was always maintained between a specific concentration range. More peroxide was added when necessary. - (2) Based on the first experiment results. The total amount needed for a 60% DOC elimination was loaded at the initial time, all at once.
- (3) After the initial dose, specific amount of H2O2 was loaded into the system every time hydrogen peroxide was consumed.
- (4) H2O2 dosing system was taken from experiment 1. DOC was added by steps to confirm
the DO-H2O2 relationship observed in the preceding experiments.
- (5) Once the H2O2-to-DO ratio had been clearly defined, experiments were performed with automatically controlled hydrogen peroxide addition according to the DO. Three pairs of DO set points were selected. DO was recorded and small initial doses of hydrogen peroxide were added manually until DO dropped below the low set point, and the dosing pump was turned on automatically by the software. From this moment on it continued to work automatically. When the high DO set point was passed the pump stopped.
Experiments 1 and 2 correspond to the normally used operating conditions in photo-Fenton processes. Intermediate situation would be a manual addition in steps, taking place in experiment 3.
First two experiments were similar on the consumption of hydrogen peroxide required for a 60% mineralization. However, in experiment 3, the load of H2O2 resulted to be much lower for the same mineralization target. Consequently, more energy and longer processing time was required. This is a demonstration of how H2O2 is used more efficiently when added in steps.
Regarding the last automatic experiment, the main advantage was a higher H2O2 accumulation at the beginning which enhanced a rapid mineralization afterwards and reduced the accumulation when reaction rate slowed down.
On the converse, the extremely low concentration of hydrogen peroxide when the mineralization rate was maximum was considered a remarkable drawback, as it provoked the reaction slow down and the necessity of higher accumulated energy for accomplishing the 60% mineralization (as in the other experiments).
An optimized automatic dosing would avoid this spiked drop in H2O2 concentration. Moreover, higher mineralization rates would be reached using less accumulated energy.
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Finally, other concerning matter would be direct injection of oxygen to the reactor in order to favorize the Dorfman-mechanism at the beginning of the process and, in this way, reduce the H2O2 consumption.
Stepwise dosage
It is worth mentioning that most of the proposed models until now are based on batch operations, where the reagents are completely dosed at the beginning of the process. Notwithstanding, is has been proved that these kinds of strategy may lead to radical scavenging or non-efficient oxidizing agent consumption.
Only a few of the most recent studies (Chu et al. 2007; Yamal-Turbay et al. 2013; Zazo et al. 2011; Prato-Garcia and Buitrón 2012) have intended to apply and model different reagent dosage schemes with satisfactory results.
Typically, the H2O2 amounts needed for the oxidation are added at the beginning, promoting high radical production in the initial phase. It has been observed that incrementing the peroxide load leads to a better oxidation. Hence, if the initial dose is low, peroxide will be rapidly consumed and consequently, the reaction system detained.
Chu et al. (Chu et al. 2007) proved the Fenton performance can be improved by splitting the H2O2 load into portions dosed along the process.
In this study, three series of experiments were carried out. The schematic explanation of each, can be found in Table 4.3.
(I) One-Step Fenton process (OSFP) (II) Even-dose stepwise Fenton process (ESFP) (III) Asymmetrical-dose stepwise Fenton process (ASFP) o T is set to 5 min based on: the optimal time at which previous experiment noticed no more improvement because of the peroxide removal, and the reactor design; the shorter the retention time, the lower the cost.
Results are shown in Table 4.6 and Table 4.7 below.
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Table 4.6: Comparison of remaining ATZ by ESFP and OSFP at a reaction time of 25 min. Source: (Chu et al. 2007)
Table 4.7: Comparison of remaining ATZ by OSFP and OSFP at a reaction time of 25 min. Source: (Chu et al. 2007)
Numbers clearly reveal that the performance of stepwise-FP was found to be better than the FP.
As summarized in Table 4.6, in different times, the removal efficiency of ATZ by ESFP is 3 to 10% higher than that of OSFP. The reason could be likely to the reduction of the peroxide concentration peak in the solution, which enhanced the diminution of H2O2 reacting with the radicals and, thereafter, the production of less efficient radicals such as ·HO2. (Eq. 2.4)
With regard to the last experiment (Table 4.7), the overall performance was found to be improved by 15-22% of contaminant removal with respect to the OSFP using only H1, while 5-11% compared to the OSFP (0,1 mM all at the beginning). Best ASFP realization was obtained by H1 and H2 of 0,6+0,4 mM respectively, which is the closest to ESFP experiment. This suggests that both the dosing time T and combination of H1 + H2 are the dominant factors in determining the performance of the treatment.
Summarizing, H2O2 has crucial effect on the photo-Fenton process considering costs and optimization. Studies on these fields are emerging. An example could be the cost analysis study
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carried out by Carra et al. (Irene Carra et al. 2013) on different H2O2 supply strategies in the solar photo-Fenton processes. Nevertheless, research is still scant.
4.2.3 Analysis of catalyst dosage
Conventional photo-Fenton processes using iron salts, have problems with the solubilization of iron in the solution if the pH is not acid. Ferrous hydroxide in water have a poor solubility, and its precipitation could inhibit the Fenton reaction and consume the ferrous ions. In addition, turbidity of the solution would increase, reducing the absorbance capacity of the solution. (Alalm, Tawfik, and Ookawara 2015; I Carra et al. 2012).
Iron solubility depends, to a great extent, on the pH of the solution and on the presence of complexing or reducing agents. Temperature and the Ionic Strength (IS) of the solution are also important dependencies. The presence of organic and inorganic ligands favorizes solubilization but competing mechanisms for sites in the iron can have an effect. The effect of IS is also significant because the solubility increases when the interionic forces of the solution are low.
It has been proved that the larger the amount of iron, the highest the removal rates of contaminants as well as mineralization. Nonetheless, this amount becomes not as effective in a saturation point, where further addition would produce a non-significant improvement on efficiency. In addition, these quantities may be above the EU directives and produce large amounts of sludge which have to be subsequently treated.
Consider the explanation of homogeneous and heterogeneous processes. In the first, iron is dissolved, generates sludge, and it works within a limited range pH.
In order to avoid the disadvantages and to synthesize reusable catalysts, several Fe-containing heterogeneous catalysts have been tested for the Fenton and photo-Fenton processes.
Heterogeneous processes have risen most investigators interest. Working conditions include neutral or near-neutral pH as suitable for the development of the process as it avoids adding acidic substances to get to the desired pH value. In this way, stable iron chelates have been deeply studied. However, the use of chelates is not often recommended for Fenton processes applied to tertiary treatments which discharge the treated water directly to the receiving environment.
Iurascu et al. (Iurascu et al. 2009) studied the photo-degradation of phenol with laponite-based materials (Table 4.2) changing the load of catalyst and finding an optimum dose. Increasing the amount of heterogeneous catalyst has de same effect on the reaction rates as the homogeneous processes. With a higher dose, the peroxide decomposition increases, thus the radical production does so.
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The oxidation of 0.1 mM of phenol was first studied. Catalyst dose was varied from 0.5 to 2g/L, obtaining the best performance with 1 g/L. Lower concentrations resulted in lower degradation rates, as expected. However, for concentrations higher than 1 g/L, phenol conversion presented the same behavior, which would not be an expected result in a homogeneous process.
The deleterious effect observed for high amount of catalyst could be possibly explained with the fact that the volume of the solution is the same in all cases while the amount of solid catalyst is increasing. Hence, the decrease in degradation rate can be attributed to the turbidity. Consequently, a relevant fraction of the incident radiation is to be lost and therefore not being absorbed. Many other authors have corroborated such explanation (Bobu et al. 2008; Chen et al. 2009; Mameri et al. 2016).
Nonetheless, using heterogeneous solid catalysts is not the only way to get to work in a neutral or almost neutral pH.
For instance, Romero et al. (Romero et al. 2016) performed a research based on the degradation of Metoprolol (MET) and Resorcinol (RES). The later one is used as a ligand iron complex, which allows working at pH= 6,2 as it can surround the iron and hinder precipitation reaction. In this case the amount of Resorcinol added was evaluated to achieve the highest percentage of ligand chelation using the minimum ligand.
Thereupon, Romero realized another experiment without the RES as a ligand, working only with MET as a contaminant and acidic conditions (pH =3). The goal was to compare the results. On the one hand, for the case of Fenton process it is clear the improving influence of RES, as a complete removal of MET is obtained within 20 min while only a 67% is removed without RES.
On the other hand, for the same irradiation and dosage conditions in a photo-Fenton process, complete MET removal was obtained in both cases. Higher MET removal rates were observed in the first case, with the addition of RES the degradation of MET was faster. In addition, regarding the mineralization, higher TOC removal was obtained using RES and neutral pH.
In the same way that H2O2 stepwise dosage can be beneficial for the process, iron stepped dosage has also been contemplated. (I Carra et al. 2012) performed a study dedicated to sequential iron dosage in photo-Fenton process for permitting operation setting pH at neutral values. Different iron dosage combinations and addition times were used to mineralize a matrix water containing five different pesticides commonly found in Mediterranean intensive agriculture.
Table 4.2 shows a resume of the main operational conditions applied as well as brief conclusions and results, while Table 4.8 contains basically the same information but directly extracted from the article:
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Table 4.8: Operating conditions and results obtained after 60 min for the solar photo-Fenton process applied to demineralized and real wastewater. Source: (I Carra et al. 2012)
Reference experiments were carried out adding the total amount of iron at the beginning and working under acidic conditions. The results were used for comparison with the following step wise dosage experiments.
Figure 4.8: Results obtained after 1 h of solar photo-Fenton and dark Fenton (inset) with 50 mg DOC/L and 650 mg H2O2/L at pH 2.8 ( DO, Iron, DOC, H2O2 and pesticide concentration); with one initial iron addition of 20 mg Fe/L as a reference experiment. Source: (I Carra et al. 2012)
Iron salts first react with water molecules in a hydrolysis reaction which ends up precipitating iron hydroxides. If adding the total load at the beginning, after the hydrolyzation, some iron ions are still available as catalysts for the photo-Fenton reaction. However, when the first additions are as low as 5 mg/L, no iron is left to catalyze the following reactions. Moreover, it can be observed that when the next dose (5 mg/L) is added, hydrolysis still happens for a short period of time, confirming that the addition amount is rather low.
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Figure 4.9: Results obtained after 1 h of solar photo-Fenton and dark Fenton (inset) with 50 mg DOC/L and 650 mg H2O2/L ( DO, Iron, DOC, H2O2 and pesticide concentration) at initial neutral pH with four additions of 5 mg Fe/L. The arrows point out the time of addition. Source: (I Carra et al. 2012)
This sequence of 4 constant additions is repeated for 5, 10 and 20 mg/L, obtaining with the last, the best performance. When first addition was 20 mg/L, although iron precipitated as well, there was more dissolved iron in the solution to start the photolytic reactions.
Nonetheless, 20x4=80 mg/L of iron added in total. This amount is extremely high. Accordingly, once known that an initial concentration of 20 mg/L is adequate, the following loads are reduced in quantity as showed in Table 4.8 . Pesticides removal, which was fast from the beginning, H2O2 consumption and DO response attest activation of the process from the first addition onwards. In this case, the first addition after 5 min avoided the process slowing down. The next loads were added every 10 min.
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Figure 4.10: Results obtained after 1 h of solar photo-Fenton and dark Fenton (inset) with 50 mg DOC/L and 650 mg H2O2/L ( DO, Iron, DOC, H2O2 and pesticide concentration) and five additions of 20–20–10–10–10 mg Fe/L. The arrows point out the time of addition. Source: (I Carra et al. 2012)
Regarding TOC (DOC in the figures), a bigger shoulder appeared at the beginning compared to the reference. However, final TOC degradation result applying stepwise dosage was almost the same as in the reference experiment.
The DO tracking, as mentioned before in this work, is a good reaction-effectiveness indicator. In the experiment of 20-20-10-10-10, a first minimum appears due to the Dorfman mechanism. Afterwards, a DO decrease is shown when the first dose is added. The iron added in the first loading mostly precipitated and therefore, the oxidation of the contaminant was not completed. A similar behavior was shown when adding iron for the second time, indicating that oxidation attacks took place again. From that point onwards, the DO concentration increased showing that the Dorfman mechanism was over and that the oxygen-generating reactions predominated.
Accordingly, the remaining iron additions (15, 25, 35 min) were characterized by a maximum in the 2+ DO line. This fact is explained by the rapid reaction between Fe and H2O2, swiftly generating hydroxyl radicals, which partially reacted with organic matter. Nevertheless, high concentrations of
DO spell out the inefficient reactions with themselves or with H2O2.
The mineralization level was up to the same standard with the different operational conditions. However, the results showed different mineralization level for complete pesticide removal because of the different additions of iron, which lead to different averages of iron concentration for each case.
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H2O2 consumption rates and mineralization were dissimilar in the linear zone. Considering a constant UV source available and an excess of reagent, the process rates depend rather on TOC or iron. On the one hand, if TOC is too low with respect to the dissolved iron, there is a limitation on the production of iron complexes with the organic matter. On the other hand, if the periods between additions are too prolonged, there would be a lack of catalysts for the photo-Fenton process.
The relation between TOC degradation rate and dissolved iron is studied for the purpose of evaluating the saturation limits of the quantity of iron added.
Figure 4.11: Variation in the maximum DOC removal rate, with the mean dissolved iron concentration in the period of maximum mineralization rate. Source: (I Carra et al. 2012)
It can be observed that a linear behavior is followed. The increase of iron amount is proportional to the maximum mineralization rate. Thus, the higher amount of iron available in the system, the more radicals are generated under the relative conditions. The saturation effect is detected for iron amounts higher than 20 mg/L, where no significant improvement can be detected.
Accordingly, uppermost mineralization results were obtained with the addition sequence 20–20– 10–10–10 mg Fe/L with the least dissolved iron.
Results on this study have shown that the initial iron concentration and the time of addition are essential parameters to consider for the running of the process, mainly due to iron hydrolysis, substances present in the water (carbonates, phosphates) and the complex chemistry of iron.
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• Ratios
The addition of the adequate amounts of reagents in the processes is important. Nevertheless, the
H2O2/Fe molar ratio is the key pathway to optimization of the process. Generally, this ratio depends on the water matrix that is being treated.(Gulkaya, Surucu, and Dilek 2006)
Keeping such variable within an optimal range allows maintaining a good COD removal efficiency while the Fenton’s reagents concentrations decrease. Despite the fact that many authors have been working with this variable as a reference, there is a wide difference of ranges among research groups. As mentioned before, the ratio needed is highly dependent on the organic matter load present in water. The following studies are an example of investigations carried out using the molar ratio: Batista et al. (Paula, Batista, and Nogueira 2012)used ratios varying from 12.5:1 to 50:1, so increasing up to 4 times the ratio in the same research, Elmolla et al.(Elmolla and Chaudhuri 2009a) presents values from 10:1 to 150:1, three times bigger than the previous experiment and incrementing the value of the ratio up to 15 times in the same investigation.
In the same line, Gulkaya et al. (Gulkaya, Surucu, and Dilek 2006) affirmed that many different systems were applied by different research groups which have reported various H2O2/Fe ratio for efficient dye degradation. When comparison was made between the results obtained in this study and the reported by literature, the author observed that the acquire ratio values were remarkably high. This fact would be attributed to the higher/different organic concentration involved in the experiment and to the fact that, unlike others, there was presence of auxiliary chemicals in the used wastewater.
With the objective of reducing the added loads of both H2O2 and Fe up to the condition that the ratio is kept constant, a series of experiments were conducted where three values of ratios were selected (from a predetermined ratio range evaluated previously in the investigation). For each value, different doses of H2O2 and Fe were selected yielding the same ratio. According to the results, COD removal varied between 90 and 95% for specific observed ranges of peroxide and iron concentrations. Beyond those values, the COD removal decreased although ratio was kept constant. This is a noteworthy observation as it allowed noticing that sufficient amount of reagents is needed to produce enough radicals for the Fenton reactions.
As a conclusion, the results demonstrate the possibility of reducing the reagent load up to a point while keeping the ratio constant and for nearly the same treatment efficiency.
Alternative ways of finding the optimum value for these ratios have been found in literature. A study performed by Tarkwa et al. (Tarkwa et al. 2019) reported that the HO· wasting reaction had elevated rate constant. Therefore, to reduce the occurrence of these reactions and avoid the radical scavenging, a suitable ratio R was stablished using the apparent rate constant (kapp).
To accomplish this target, the author found out that the relationship between kapp and R is a straight line as a function of H2O2 or Fe for a fixed Fe or H2O2, respectively. In Figure 4.12 the result obtained
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can be observed. The optimal R value correspond to the intercept of the two straight lines. it matches the point at which the scavenging rates of HO· by H2O2 or Fe overdosed are at the lower limit.
Figure 4.12: Optimization of Fenton’s reagent molar ratio (R = [H2O2]/[Fe3+]) for oxidation Orange G solution using apparent rate constants obtained constant Fe3+ (filled square) constant H2O2 (filled pointed up red triangle).. Source: (Tarkwa et al. 2019)
4.3 Modelling
The complex character of the Fenton processes implies that there is much work ahead to do in order to determine and identify pathways and mechanisms to understand and predict the treatments behavior. Many efforts have been devoted to understanding this behavior. It is an exceedingly difficult task to determinate the degradation mechanisms.
Lately, many researchers have undertaken different approaches to obtain a reliable model for the Fenton and Photo-Fenton processes. According to ScienceDirect, since 1997 more than 2800 articles including modelling the process have been published. Figure 4.13 shows how the investigation on this sector has increased regularly, finding in the last 10 years the majority of articles.
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400
350
300
250
200
150
Numberpubliations of 100
50
0
2003 2015 1997 1998 1999 2000 2001 2002 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2016 2017 2018 2019 2020 Years
Figure 4.13: Articles published on process modelling. Data obtained from (ScienceDirect 2020). Data from 2020 is only considered until June.
As it can be observed, modelling is often employed in papers. The main objective when modeling a process is the optimization of it. The number of articles considering modeling for optimization is showed next:
- “photo-Fenton” AND optimization ▪ 2369 results since 1996. ▪ 1981 results since 2010.
Around a 47% of the published articles regarding photo-Fenton, take into consideration the optimization. However, it is worth mentioning that optimization is a very general work. One of the major topics discussed in this project is the dosage of reagents, and one of the targets of this study is to find the correct amount, time and way to make it optimal. For this reason, a more specific search has been done on the number of articles considering reagent’s optimization.
- “photo-Fenton” AND optimization W/10 reagents ▪ 1207 results since 1996. ▪ 984 results since 2010.
As it can be observed, in the last decade, almost half of the papers published consider the study of reagent’s optimization. Additionally, other subjects are willing to be optimized in the photo-Fenton process. For instance, cost is one theme related to reagents, as reducing its amount it is also reducing the final price of the project. Nevertheless, the elevated costs come from diverse parameters of the system. The elevated results regarding the search on optimizing costs and environmental impact affirm the importance of these topics in the engineering area.
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- “photo-Fenton” AND optimization W/10 cost ▪ 1157 results since 1993. ▪ 997 results since 2010.
- “photo-Fenton” AND optimization W/10 “environmental impact” ▪ 178 results since 1999. ▪ 154 results since 2010.
Again 50% of the photo-Fenton optimization publications mention the cost optimization which is an expected result as cost is not only a concern but can also become a limitation. As mentioned before, reagents optimization is majorly related to costs. However, the last presented search has been done to be more concrete on the articles that specifically talk about costs optimization.
Conversely, environmental impact optimization is not as popular. Only an 8% of the articles mention it targeting its optimization while 15% talk about it in a general way (296 articles, reviews and conference abstracts)
After this brief mention about optimization, emphasis is set in modeling again.
MODEL BASED APPROACH
First-Principle Empirical Semi-empirical Conte et al., 2012 Pérez-Moya et al., 2008 Cabrera Reina et al., 2012
Data Based Control Process Models applications optimization Ortega-Gómez, et al., Shokry et al. 2015 Moreno-Benito et al. 2012 2013
Figure 4.14: Classification diagram of the model approaches. Source: Prepared by the author.
After revising the literature regarding photo-Fenton process modeling, a classification of the models could be done as first-principle models, empirical models and semi-empirical models. These three approaches have been compared in terms of articles and reviews published. Figure 4.14 stands out an example for each of the models with the citation of a specific paper.
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4.3.1 First-principles models
Phenomenological models are meticulous approaches developed mostly in the last decades and based on mass and energy balances. First-principles models rely on the description of all the elementary steps. It is based on the fundamental laws of nature.
A complete and exhaustive understanding of the system (mass balances, kinetics, instrument specific characteristics, etc.) at a molecular level, is essential for the prediction of a first-principles modeling. Despite being remarkably complex, their evolution and expansion for chemical process is of extreme importance from both fundamental and applied points of view. Intensive work is required when predicting this type of model as several measurements of the system variables and detailed data must be done as well as solvation of large and complex mathematical optimization problems for the model fitting and parameter estimation. Moreover, it becomes a tool for the optimization and the design of more efficient reactors and processes.
Research into first-principle models include intricate reaction mechanisms and design parameters (reactor and irradiation type, application scale, etc.). The pseudo-first kinetic model is the most generally used, especially for the degradation of TOC (Turbay 2013). The main drawback relays on the complexity to determinate irradiation. Accordingly, FPMs might provide accuracy and understanding, but its elevated costs make it an unaffordable model recreation.
Fenton and photo-Fenton processes require the application of tangled chemistry which is difficult to determinate. Details on these processes have been well understood and reported in many papers, conferences, and reviews. However, the costs of the modelling may not be affordable. Costs mainly derivate from the need of expensive analytical instruments and material which allow a continuous monitoring of the chemical species and conditions that must be tracked during the reactions.
An example of First-Principle model design is the paper published in 2012 by Alfano et al., (Conte et al. 2012). He studied the degradation of 2,4-D herbicide in aqueous solution using photo-Fenton process and a solar reactor.
First, the reaction sequence of the specific treatment is described and, on this base, a kinetic model is proposed to predict the concentration of the pollutant, the main intermediate component (DCP), and the H2O2. In this case, the reaction sequence is based in other authors work. The three main tracked parameters are derived from certain assumptions which consider the negligence of some reactions and applicable approximations. Such quantifications are measured from the experiment taking place in an isothermal, well stirred tank reactor irradiated from the bottom.
Once the main characteristics are defined, the kinetics parameters are estimated using the model results and the experimental data. The estimations include a range of working temperatures,
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different [H2O2]/[2,4-D] molar ratios and ferric iron concentrations. Eventually, the developed kinetic model was implemented to predict the concentrations of the reacting species during the photo-Fenton degradation under different experimental conditions.
The reactor model includes de definition of first-order, ordinary differential equations for the resoluteness of mass balance as well as the LVRPA4 (Local Volumetric Rate of Photon Absorption) at every point inside the reactor, necessary for the establishment of irradiation parameters of the equation.
Design methods are requisites for the experimentation. When analyzing a process, experiments are often used to evaluate which process inputs have a significant impact on the final output and the level of these inputs to achieve a desired result. Designing experiments involves a set of operating variables.
In the case of Alfano et al. (Conte et al. 2012), a D-optimal design method was adopted. Considered variables were stablished: ranges of temperature, iron salt concentrations, hydrogen peroxide to 2,4-D initial concentration ratios and irradiation levels.
Generally, in all types of models, for the Design of Experiment (DOE), the variables are coded depending on an implemented model. The former research adopts a quadratic model with interactions and a minimum number of experimental runs.
From the values of the obtained parameters, an optimization procedure is employed to provide those values that minimize the difference between the model predictions and the experimental data. To compare these values, theoretical results are obtained solving the system of ordinary differential equations previously proposed.
Finally, predicted and experimental concentrations, temperatures and irradiation fluxes results are compared.
First-principles models are not induced for a cheap and usual measurement of the lumped parameters. This kind of models are not considered satisfactory for the required observation on the global evolution of the system. Hence, it must be taken into consideration what is the model intended for when evaluating the limiting scope to which first-principles modeling is affordable - which is the main purpose- . (M. P. Pérez-Moya et al. 2011).
4 Parameter determining the photons received in each differential volume of the solution inside the reactor. Is gives an approach on the distribution (homogeneous or not) of the irradiation.
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• State of art
A search on the published research articles, reviews and conferences has been done in order to observe the evolution and the state of the investigation done in this area. Searched key words are modeling and first principles. It is hard to achieve reliable results because the search only relies on the written key words, which may not appear exactly in a paper dealing with the subject.
- photo-Fenton” AND modeling W/10 “first principles” ▪ 20 results since 2008
It is obvious that more than 20 papers have been published for the last decade. However, as mentioned before, the word first-principles may not appear identical in all of them. As a consequence, it can be stated that the dilemma here is deciding the appropriate key words. Therefore, alternative words related to the first-principle modeling could be proposed.
All of the articles using first-principles as the modeling approach have to, at least, deal with kinetics. For this reason, a search has been especially dedicated to it.
- “photo- Fenton” AND modeling W/10 kinetics ▪ 1831 results since 1996 ▪ 1477 results since 2010
As it can be noticed, several articles are published considering kinetics compared to first-principles. This fact corroborates the hazardousness of the bibliometric study.
The same has been proposed for other possible themes studied such as mass balances or LVRPA as they can be some of the essentials in a first-principles model approach. For the former, around 155 articles appear published for the last decade, while for the last only 18 articles have been published, which is a very low value. This little number of articles could be given due to the extended complexity of mathematical calculations related to the LVRPA.
4.3.2 Empirical models
On the reverse side, empirical methods are often employed. The resulting models completely rely on a complete experimental approach and do not derivate from physical equations, so an extent work on data treatment is essential. Empiric models are mostly applicable to single-variable processes whilst Fenton and photo-Fenton treatments involve multivariable complex processes. To overcome this fact, many authors such as (Yamal-Turbay et al. 2013) use an accurate DOE. Posterior optimization approaches are applied such as response surface methodology (RSM) (Al Deen et al. 2016) or artificial neural network (ANN) (Speck, Ramesh, and Thivaharan 2016) regression models.
Statistical approaches such as RSM can be employed to maximize the production of a substance by optimization of operational factors. It has been prosperous in determining response trends and
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possible varying interactions (Fernández et al. 2002). Moreover, it has been proposed for practical black-box modeling (i.e. ANN) (M. P. Pérez-Moya et al. 2011).
Nevertheless, these modeling approaches are adjusted mathematically to an equation or behavior which gives sense to the process, but which may not be adequate for extrapolation and process scale-up. Furthermore, they have been shown to produce misleading conclusions regarding optimization. (M. Pérez-Moya et al. 2008).
In the empirical approach the work done by Pérez-Moya et al. (M. Pérez-Moya et al. 2008) is a representative example of regression models coupled with experimental design of experiment techniques (Cabrera Reina et al. 2012).
The process performance was modeled as a function of the conditions applied to the reactions. It was later studied by fitting different models to the degradation data. Afterwards, comparison and discussion of the models is done according to the degree of correlation. Finally, the performance of the degradation procedure is represented.
A second-degree polynomial model is the most employed to obtain an optimal response. Even though is easy to estimate and apply, it is just an approximation. Alternative models are proposed such as potential model or Hoerl equation.
To optimize degradation parameters in a photo-reactor many studies have focused on univariate approaches, one variable is varied each time. However, photo-assisted Fenton degradation is a multivariable system and thus, the characterization of the system requires taking into consideration the cross-factor effects. This methodology has been previously used in the experimental design of Fenton processes (Oliveros et al. 1997; Herrera, Lopez, and Kiwi 2000). Accordingly, Pérez-Moya et al. (M. Pérez-Moya et al. 2008) states that the use of DOE is suitable to:
- Identify the parameters affecting the multivariate process. - Fit the empirical models to experimental data. - Develop empirical models statistically.
As mentioned before, DOE is used to plan the measurements that must be done to evaluate the factors affecting the process (reagents initial concentration, temperature, irradiation, etc.).
Executing the DOE leads to a set of data including the input and output values. Next, a generalized correlation between the input and output values is done to predict the response of the system.
The first model stablished by the authors on this work is a quadratic model (Eq. 4.2)
2 Eq. 4.2 f(x) = a + ∑ bixi + ∑ cixi + ∑ ∑ dijxixj i i i j>i
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Second polynomial model is assumed as it results helpful in multivariate analysis. The different relation between the factors and their influence is captured in the linear, bilinear, and quadratic terms of the equation.
To build a model some steps must be followed:
- Model selection. - Parameter estimation: fitting the model to experimental data by searching the model parameters that best describe the data. Fitting the model means minimizing the error between experimental and simulated results. - Rate the fitting degree of the model to the data using correlation indexes (i.e. R2).
Nonetheless, to build a model further information and reasoning is required.
The exposed work proposes different models that could be suitable from the structural and implicit features and their consistency with the nature of the process. The author claims that the simple one-dimension case (the most used) presents weaknesses and failures that can be superseded by alternative models.
The shortcomings detected include the incapability of the model to suit the behavior of the system when the reaction extents, which may lead to the estimation of negative concentrations and inexistent minimum/maximum values. Therefore, the quadratic model may be a steady approximation for slightly non-linear intervals while deceiving for high non-linear intervals.
For multidimensional systems, same concepts apply. Extreme points are determined by solving the Hessian matrix5 of the model.
An example is explained to illustrate the analysis previously introduced. Consider:
- Perfect experimental data. - Kinetic rate affected by some extra conditions (other chemicals, temperature, etc.) that may be the object under investigation. - The real system simulation as shown in Figure 4.15 (a). Three series of experiments at different reaction times measuring the system response for different kinetic constants (k’s): 0.25, 0.5, 1, 2.
5 The "Hessian matrix" of a multivariable function f(x, y, z, …), organizes all second partial derivatives into a matrix. it is a matrix with functions as entries. In other words, it is meant to be evaluated at some point (x0, x1, x2, etc.). (Khan Academy 2020)
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Figure 4.15: Fitting of a quadratic equation to the data from different conditions. Source: (M.
If an investigator had no knowledge of these considerations (Figure 4.15Figure 4.7 (a)) , he would have gone to the lab first, and find data. Afterwards, that data would be used to represent Figure 4.15 (b) and try to adjust a model. The experimental data obtained is represented in dots and the approximation in lines.
In Figure 4.15 (b), the quadratic model could be considered as good estimation (based on the correlation) for the measurements obtained at time 1 (square dots). However, when the quadratic model is applied for higher times the predictions are inconsistent. For instance, for time 3 (rhombic dots) the correlation value obtained (R2=0.98) displays a parameter fitting that may seem reliable. Yet, Figure 4.15 (b) shows an absolute inconsistency: inexistent optimal condition is estimated for k=1.5 where a concentration value below zero is obtained.
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The previous example is used to illustrate how, even after the optimization of the parameter values and despite the adequate value of the correlation, the selection of the model may lead to an untrustworthy prediction capacity. Using this approximation, it is totally not possible to optimize the process and find the best operating conditions.
For instance, if maximum or minimum degradation values are the target, no use can be done of the graphic and neither of the nulling derivation process. That would present a max/min value for the model, but is not an optimal value form the system.
A comparative study is presented to analyze the estimations given by three different models: a quadratic model, a potential model and a model based on the Hoerl equation. The study is specifically addressed to the determination of extreme points as an evaluation of the capacity of the models to characterize the process.
Extreme points can be obtained both graphically or by nulling the corresponding derivate equations (using the Hessian matrix).
Fitting the parameters for each model to the degradation data for different reaction times allows obtaining the needed coefficients for each case.
The additional models proposed are defined by equations to the same extent as quadratic models. For the potential model (Eq. 4.3) while the last model is based on the classical Hoerl equation (Eq. 4.4).
Eq. 4.3 di f(x) = a + b ∏(xi + ci) i
Eq. 4.4 bi cixi bi cixi f(x) = ∏ (aixi e ) = A ∏ (xi e ) i i
These models are intended for a greater fitting flexibility. By adjusting their parameter values both models may be fitted to vastly different data and, while remaining non-linear and depending on their ratio, they will not be forced to present maximums and minimums.
The following indexes are used for the quantitative discussion between models.
• Residual values: it corresponds to the difference between an observed value and the corresponding fitted value. To obtain the optimal models’ parameters, the least squares criterion aims at minimizing these residual values.
• Coefficient of multiple determination (R2): it is necessary to establish the total residual sum of squares, corresponding to the regression sum of squares, SSR (the squared difference
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between the fitted value and the mean value) plus the residual sum of squares, SSE (described above). The closest to 1.0, the better the model fits the data.
• Adjusted R2: is a useful index to compare the quality of the different proposed models and balance their different degrees of freedom. The coefficient of multiple determination will increase as the number of parameters to be adjusted grows. In this way, a model with more terms (less degrees of freedom) appears to have a better fit. The Adjusted R2 comprises the number of experiments done and the fit coefficient number. Thus, Adjusted R2 will only increase its value id the addition of new parameter improves the model more than it would be expected at random.
Analyzing these indexes results in observations which demonstrate a better fitting using Models 2 and/or 3 (potential and Hoerl equation) than employing quadratic model. Additionally, Models 2 and/or 3 require less parameters and thereby may be used with less experimental data. These favoring considerations are merely statistical, further contemplations can only be done regarding a first-model principle.
Once the fitting and parameter estimation is complete, the models are studied regarding the estimations produced. The extreme points are estimated by the derivative test. For the potential and Hoerl equation models, these points are not physically explainable and, moreover, are completely out of the interpolation domain range. According to these models, extreme points are inexistent. Meanwhile, for the quadratic model four candidate extreme points arise: two beyond the boundaries of experimental interpolation and two inside the domain.
Two and three-dimensional response surface are represented for each model obtaining a graphical view on the process variables.
As a conclusion, the comparison of the models has revealed that the quadratic model may produce low residual values but when studying the extreme points, inconsistencies are revealed. Conversely, the Hoerl equation-based model achieves the requirement of less parameters while still presenting low residual values. The graphical analysis reveals the disparate estimations given by each model.
Empirical models are fast and give an understandable prediction. However, it is worth mentioning that a full comprehension of the validation range is strongly required, as well as the understanding of the mathematics applied behind the model.
Hence, further research is required in model selection to acquire trustworthy attributions of Fenton, Fenton-like and photo-Fenton treatment performance.
Data based models (DBM)
Generally, EM lack a clear and precise representation of the process behavior during the transformation of inputs to outputs. Many have proven successful in representing the complexity
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of the system within specific ranges of data used to calibrate them. Outside the range extrapolations are not accurate.
Most optimization and simulation models, in mathematical terms, try to find the interactions and processes that take place among the diverse components in the system.
Data comes from sets containing really extent information regarding the variables and parameters whose values are determined during calibration. Different from empirical models, the amount of data used in this case is remarkably numerous. It is employed with information of the system taken with extremely short periods of time. The more data available, the highest the options to reach a better model fit.
However, the gathering of these amount of data require rather advanced technologies such as quick sensors able to determine parameters during the process, or elevated human costs suggesting more people or more working hours. In addition, lots of data implies a major computational potential which could be challenging and greatly laborious.
These types of models are contrasted to what are typically called “black-box” models or statistical models which, as the EM, they attempt to convert inputs to outputs using only mathematical functions or equations that do the job.
Data modeling techniques provide efficient alternatives to construct simple and accurate data- based models (metamodels), saving time and cost of experimental work and reducing the complexity of fitting and using FPMs simulation. Artificial Neural Networks (ANN), Support Vector Regression (SVR), or Ordinary Kriging (OK) have been widely used in many areas. (Shokry et al. 2015)
Artificial neural network (ANN) is a well-known efficient method, used for nonlinear system modeling. The development and application of these “black-box” statistical models plagiarize larger, delineated, process-oriented models. In comparison with the other two mentioned methods, ANN is the most employed technique.
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Figure 4.16: A typical multi-layer ANN. Source: (UNESCO 2005)
The basic structure of ANN is represented in Figure 4.16. It is composed by numerous input layers nodes and less output layers nodes. The middle columns of nodes are called hidden layers, which together with the number of nodes in each layer are two of the design parameters.
• State of art
The same research carried out with the first principles models, has been done for the empirical approach. Again, modeling and empirical are the search key words.
- “photo-Fenton” AND modeling W/10 empirical ▪ 298 results since 1995 ▪ 227 results since 2010
Most of the research done in the area has been published in the last decade. This is an indicator of the increasing and continuous interest in the area. In the same way as in the first-principal investigation, there are articles which certainly write the specific word identically and there are others where the key word is not explicitly written.
Inside this area, the focus has been set on the response surface. Eventhough it is a subject that has been supported and criticized by many articles, it is a widely employed methodology. More specific search has set the focus on it, obtaining the following results:
- “photo-Fenton” AND modeling W/10 “response surface” ▪ 285 results since 1997 ▪ 243 results since 2010
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In addition, another methodology that has been somehow trendy is the Artificial Neuronal Network. Furthermore, the research group in the UPC has also worked with Support Vector Regression and Ordinary Kriging, but these have not been extensively found among literature.
4.3.3 Semi empirical models
In the case of a semi empiric model, the model approach relies on a limited number of physical equations which decompose the expansion of the process into consecutive steps. It is a balance between the empirical and the first-principles models. The last takes representation in the definition of the thermodynamic, kinetic and mass balance equations while the former when the parameters to these equations are estimated via the experiments.
Besides the variations that the model could have, there are parameters affecting the course of the process. For instance, the quantity of reagents used, the irradiation applied or other working conditions such as pH or temperature. A semi empirical model relies on the tuning of these parameters regarding experimental data.
The capability of the model to behave adequately in extrapolated conditions must be assessed as, in practice, the identification data base used for the calibration does not necessarily include the entire range of conditions that will be evaluated by the model.
Cabrera Reina et al. (Cabrera Reina et al. 2012) proposed a kinetics model with lumped components and TOC fractionation to track paracetamol degradation using phot-Fenton process. The author claims that kinetic aspects are not fully understood because of the great interrelation and coupling phenomena among the organic components used for the process.
Many models are being employed from key compounds or simple mixtures which give a great response on the prediction of contamination but with either unsuitable or no information concerning the oxygen evolution. In Cabrera Reina et al. (Cabrera Reina et al. 2012) study, simplified photo-Fenton reaction set is used to create a model to simulate TOC, H2O2 and O2 evolution. As mentioned earlier in this work, tracking the oxygen profile is a trusted indicator for the efficient use of intermediate radicals formed during the treatment.
The distinguish feature is that it grouped all the intermediate radical species in one variable. The process is defined by four large groups of reactions:
- The photo-Fenton cycle itself - Oxygen generation/consumption. - Organic matter oxidation (contaminant) as well as intermediate formation. - Final mineralization.
The model assumes nine processes and eight states:
- The two ferric species (Fe+2 and Fe+3)
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- Hydrogen peroxide - The radicals formed from peroxide (R) - Dissolve oxygen - Three states accounting for the organic matter, MX1, MX2 (partially oxidized) and M (organic matter not still degraded).
The dynamic model is based on mass balances for batch operation mode of the components in the model. Being the process carried out in a batch reactor, matter inside the reactor is constant. No matter enters or exits during the treatment.
As the author considers the O2 evolution during the process, a mass balance must be also defined for this component. Unlike the other components balances, the oxygen mass balance has to consider the oxygens transferred to air when its concentration is higher than de maximum DO concentration. The overall gas-liquid mass transfer coefficient (KLa) for O2 is obtained experimentally using the dynamic method. The resulted value for KLa is further used in all simulations.
In resume, the kinetics expressions for each reaction are described as well as the mass balances necessary to employ the model. Again, experiments are designed in order to obtain results and compare them with the ones attained using the simulation of the model.
Afterwards, optimization processes are attempted to obtain parameters. In this specific case, three different processes are proposed: sequential scanning with a quadratic objective function, and preliminary bounds on the limitations conducted in two steps; a Monte-Carlo direct search with percentage error as the objective function; and a final optimization using the Matlab toolbox with a weighted quadratic objective function.
Non-linear regression methods such as least squared regression methods are employed with the aim of obtaining a relationship between the dependent and independent variables. Plotting the experimental results together with the profiles obtained in the model simulated with the estimated parameters, provides great qualitative analysis of the procedure.
Control and optimization
Modeling the system allows optimizing and controlling the process. The design of the chemical process must take technological integration into account to ensure adequate operation in an industrial application. Therefore, further application of semi-empirical models involves automating the control system and optimizing the process. In the figure below, some articles/review references regarding these applications are shown.
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Control Process applications optimization
Ortega-Gómez, Moreno Úbeda, et al., 2012 Giannakis et al., 2017
Carra et al., 2013 Alalm, Tawfik, and Ookawara 2015
Irene Carra et al. 2013 Prieto-Rodríguez et al., 2011
Figure 4.17: Examples of articles published about Control Applications and process optimization
As it has been mentioned, hydrogen peroxide dosage method is one of the key factors affecting both effectiveness and costs of the treatment. Studies done in this sector, aim at automating the photo-Fenton process and, in this way, improve the final performance by reducing the reagent consumption compared to manual operation. Nevertheless, the dynamics of the Fenton processes strongly rely on the nature of the treated water. Thus, papers focused on this subject are limited to particular conditions.
One adequate indicator for the consumption of H2O2 is the dissolved oxygen concentration evolution, which reflects the variations in photo-Fenton process. Ortega-Gómet et al. (Ortega- Gómez, Moreno Úbeda, et al. 2012) carried out a research to develop a process controller which ensures the correct and automatic dosage of hydrogen peroxide based on the DO evolution.
Automatization involves the development and understanding of new technologies that could improve the course of the experiments. Among all of the technologies, some examples could be:
- Sensors to improve parameters detections and on-line measurements. - Sequential bombs programed to add specific loads at specific times. - Catalysts which avoid sludge and acidic conditions, and still provide fast degradation. - Photoreactors designed with lamps moving in order to irradiate the whole solution.
Nevertheless, the cost of their application is extremely high in relation to its proven trustworthiness.
The need of an automatization is chained to the process optimization. Nowadays, reducing costs, energy, or minimizing mistakes are some of the main concerns.
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• State of art
Study on the number of works published about semi empirical photo-Fenton modeling reveals the following results:
- “photo-Fenton” AND modeling W/10 “semi-empirical” ▪ 30 results since 2002
Regarding the control applications that are considered within this kind of model, the search results obtained are:
- “photo-Fenton” AND control W/10 dosage ▪ 960 results since 1992. ▪ 838 results since 2010.
- “photo-Fenton” AND automatic W/10 dosage ▪ 92 results since 2003. ▪ 80 results since 2010.
Comparing the results obtained in section 4.2.1 regarding only the dosage it could be claimed that the control of dosage it is being often applied because the number of articles is very similar. On the other hand, automatization is an application still under development.
It could be concluded that semi-empirical may not be a commonly used word among the studies as the number of articles with that specific name is far lower than the value captured for the other related key words.
As a conclusion, it could be stated that the implementation of an empirical model is, and has been, the most employed way to reach a simulation of the photo-Fenton system. Figure 4.18 represents the previously mentioned searches on the first principles, empirical and semiempirical modeling.
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Empirical First-Principles Semi-empirical
9% 7%
84%
Figure 4.18: Comparison between de different model approaches through the papers published in the last decade. Data obtained from (ScienceDirect 2020). Data from 2020 is only considered until June.
It is important to remark what has been reported before. The search done is highly dependent on the key words, and seeing this graphic it should be stated that maybe, the words chosen are not the most adequate. The selection could change or be more extent in order to obtain more reliable results. However, Figure 4.18 can be considered as a trend indicator.
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CHAPTER 5 ENVIRONMENTAL IMPACT
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5.1 Experimental impact
Security elements, general laboratory rules and proper handle of the reactants and materials must be considered when developing the experiments.
The safety data sheets of the employed components have also been considered. However, the only seriously dangerous chemical used is ammonium metavanadate.
The adequate deposition of materials is the factor affecting the most in the environment. Each generated solution must be correctly deposited in the corresponding container depending on the residual amount of reagents left when finishing the experiments.
Safety data sheets have been also considered (see CHAPTER 9). Nevertheless, the only dangerous chemical used is ammonium metavanadate.
The treated contaminant, Venlafaxine (VEN), is a type of antidepressant. A drug used to overcome depression, anxiety and panic attacks. The IUPAC name for Venlafaxine is (RS)-1-[2-dimethylamino-
1-(4-methoxyphenyl)-ethyl] cyclohexanol and its molecular formula is C17H27NO2. The corresponding structure is showed below:
Figure 5.1: molecular shape of Venlafaxine. Source: (Pubchem 2020)
Environmentally, its occurrence has been reported in surface waters (28 ng/L), in residual wastewaters (213.2 ng/L), as well as in treatment stations effluents (174 ng/L) (Aymerich et al. 2016). Another study in which ninety European WWTPs were monitored, 156 polar organic contaminants were determined and VEN was detected in more than 99% of samples from urban effluents. (Ayala Durán 2017). Many studies related to VEN toxicity state that it is harmful for many living beings and organisms in the ecosystem.
In this project, the idea was to treat water polluted with VEN with photo-Fenton process involving the necessary reactants.
Homogeneous processes imply a pH 3 in the working conditions. In order to get to this acid environment, acid is added. Therefore, the acid residual solution must be correctly thrown to the specific container.
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The quantities of iron applied do not overpass the limit marked by the Spanish legislation, which corresponds to 10 mg/L. If the solution has less than this value after the experimentation, there is no residual treatment problem. Otherwise, special attention must be put on the deposition.
With regard to H2O2, it is not a dealing issue as it is consumed during the reaction.
In conclusion, what can create most environmental concern is the residual acid waters or the possible intermediates created during the process, which in exceptional cases could be more toxic than the proper pollutant studied. In order to find out whether if these intermediates are dangerous or not, toxicity and biodegradability tests must be run.
On the other hand, for the TOC characterization no residues are generated. However, for the tracking of hydrogen peroxide, ammonium metavanadate is employed. This results in a residue that must be processed separately.
5.2 Computational impact
For the present project, little experimentation has been carried out and, instead, an extent work has been done using the computer. This is the reason why the environmental impact generated by the use of computer is considered.
It can be a controversy the decision of including or not this section. However, doing an estimation allows its consideration.
A laptop that is on for eight hours a day uses between 150 and 300 kWh and emits between 44 and 88 kg of CO2 per year. Table 5.1 considers the amount of energy that could cause environmental issues.
Table 5.1: Energy consumed during the project by the computer device. Average power (kW/h) Time (h) TOTAL ENERGY (kW) 250 900 225000
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CHAPTER 6 CONCLUSIONS
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6.1 Project conclusions
The project itself was initiated setting up objectives that have been altered throughout the course of the project because of the exceptional circumstances given by COVID-19.
Research done in the area of water pollution is clearly active. Over the last ten years investigation has been increasing continuously, proving that no ending point has been achieved or, at least not suitable enough conclusions have been reached.
Further conclusions have been drawn on this work. The principal and most forceful conclusion stated in this project corresponds to the affirmation that water contamination is a not-yet-settled issue.
Supplemental proposed objectives on this project have been achieved. Moreover, other collateral conclusions are mentioned:
• A study on the actual water treatment situation has been done, setting the focus on the tertiary treatments, concretely on photo-Fenton processes and parameters affecting the treatment effectiveness have been explained.
• Regarding catalysts, the implementation of heterogeneous components is advantageous in the sludge reduction and it permits more comfortable operational conditions such as working at neutral pH. Nevertheless, it has not been shown as contributing a significant increase in the final removing effect and, moreover, it requires previous catalyst preparation and posterior filtration. Moreover, the degradation time is longer.
• There are some issues willing to be solved. Some of the found among literature are: the addition of reagents and catalysts, the incorporation of solar light or the variation of irradiation, shorting the degradation and mineralization time, or diminishing the final sludge.
• Pending research issues have been determined thanks to a bibliometric study on the photo- Fenton process parameters. Dosage and modeling have been chosen to study in depth. The lack of conclusion statements related to those issues is noticeable as the interest is continuously increasing.
• A bibliographic study has allowed the analysis the number of investigation articles and reviews proposed on the open issues: dosage and modeling. This study has presented difficulties on defining the appropriate key words for each search. It also must be said that it is only focused on the quantity of articles.
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It is worth mentioning that comparing research articles and arriving to specific conclusions is a hassle because:
- Generally, there is no consensus on the specific objective function, and when there is, the experimental conditions are very dissimilar. - The employed equipment is different in every case, and this provides variation in the results. - Collocation of material in the studied field, provides great validation for a single experiment but most of the time less reliability on the results as these are “less general” and more adapted to what it is wanted to be obtained.
Apart from the conclusions regarding the general principal targets, specific conclusions have been done regarding dosage and modeling as they are the open issues that have been chosen to study.
Relative to the dosage method:
- It could be stated that the photo-Fenton process is proven to be more efficient when dosing the hydrogen peroxide by steps instead of adding its totality at the beginning of the reaction.
- After revising the dosing methods found in literature, it is assertive that stepwise dosage has positive effectiveness on degradation and, for this reason, it is being applied in many researches recently. ▪ Both time and percentage of the added load are interactive variables to take into consideration when planning the stepped dosage. ▪ It has been demonstrated that stepped dosage is more effective for the photo- Fenton treatments because less reactants are consumed while the same or more effectiveness is assumed.
- Among the studied literature, it has been found that some authors propose methods to
track the consumption of H2O2 along the reaction. Knowing about the hydrogen peroxide evolution is resulted to be very advantageous because the mineralization performance can be evaluated and, moreover, it helps designing the dosage method in order to use as less reagent as possible and, in this way, reduce the costs. ▪ The online parameter that has been found more implemented in the references studied, is the observation of DO evolution. A high concentration of dissolved oxygen is an indicator of secondary reactions taking place, which result in an
inefficient consumption of H2O2. On the other hand, very low concentrations of DO indicate lack of oxygen needed for the main oxidation steps.
- Other authors propose methods such as finding the optimal ratio of catalyst and reagent or focusing on improving the efficiency by adding the iron scaled and in different reaction times.
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Relative to modeling the process:
- Based on the bibliometric study done in this area, the most employed approach is the empirical. However, the search is only an approximation of reality.
- Many authors apply an empirical estimation and confirm an adequate model based on the correlation indexes which may represent a correct visual approximation but which are not suitable when defining the extreme points. It exists the possibility of finding valid models only based on the correlation. Nevertheless, further analysis on other parameters such as residual values are essential to ascertain if the found optimum values correspond with the hole system or only with the mathematical model.
- Bibliometric studies demonstrate that the modeling and control of the processes has been of major concern lately. It is believed among the authors that the application of control could reduce the human error and the amount of reactant loaded while achieving the same degradation. Even though automatization and control are demanded by many companies to apply these treatments to their wastewaters, the scale-up is very expensive.
6.2 Future works
On the basis of the previously reported conclusions, some future works are proposed in order to be more specific on the boundaries of the bibliometric study:
• Obtain a deepest knowledge on the way of searching, the key words implemented and their combinations.
• Go further in the search and make a more systematic and accurate exam of the publications. Focus not only on the interest of the scientific community but, once observing the quantity of studies performed, notice the quality of the selected articles. A way of doing this would be specifying the quartile to which they belong regarding every area.
Moreover, after the deep work done on these topics, the focus of the experimental research could be extended to:
• Consider the step-wise dosage for hydrogen peroxide and for iron.
• Approach a model of the experiments using mathematical Software tools to estimate the best parameters and make a statistical analysis to observe how the fitting process behaves considering different measures.
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CHAPTER 7 ECONOMIC EVALUATION
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7.1 Introduction
The project budget has been done for the initially formulated experimental project as well as for the proposed reoriented work. For the former, an estimation of the experimental cost has been done, while an evaluation of the research costs was considered for the finally developed one.
For the experimental procedure, it is considered that the University facilitate the laboratory installations. Thus, the price of the fixed energy values, all the cleaning services and tap water current will be not considered in the following evaluation.
No consideration of the previous factors would imply the additional related costs.
In the first case, spending is divided in:
- Materials and reactants costs - Services costs - Personnel costs - Amortization
While in the reoriented bibliographic work, only the working hours have been considered.
7.2 Experimental budget estimation 7.2.1 Materials and reactants costs
This section includes the tables containing the needed units and quantities of material and reactants respectively, related to their unit price as well as the total cost.
The reported information approximates what it is thought to be used for the experiments. As it has been mentioned before, the experimentation part was eliminated of the project. Thus, the materials and reactants, and their respective quantities have been taken from diverse previous similar works, projects with a similar magnitude order by means of the number of experiments run (between 20 and 100)
For the indicated number of tests, the following units and price have been estimated.
Table 7.1: Laboratory material unitary cost and total cost.
Total cost Laboratory material Units €/unit (€) Beaker (100 mL) 1 1.75 1.75 Beaker (250 mL) 1 2.10 2.10 Calibrated pipette (5 mL) 1 2.21 2.21
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Calibrated pipette (10 mL) 1 2.53 2.53 Calibrated pipette (20 mL) 1 3.12 3.12 Volumetric flask (10 mL) 10 1.91 19.1 Volumetric flask (100 mL) 5 2.46 12.30 Volumetric flask (1 L) 1 5.75 5.75 Volumetric flask in amber-colored glass (1 L) 1 32.01 32.01 Volumetric flask (2 L) 1 9.08 9.08 Volumetric flask (5 L) 1 12.35 12.35 Spectrophotometric quartz cuvette 2 69.50 69.50 Automatic pipette 1 105.00 105.00 Micro pipette tips 1 box 4.30 4.30 Latex gloves 1 box 6.70 6.70 Eppendorf 100 8.00 8.00 Centrifugate tubes 100 16.30 16.30 Syringe (10 mL) 1 1.20 1.20 Clamp holder 4 11.00 44.00 Clamps 4 6.60 26.40 Iron ring 2 20.00 40.00 Plastic wash bottle 1 2.80 2.80 Support 1 14.10 14.10 Chronometer 1 10.00 10.00 Micro-spatula 1 0.95 0.95 Stirring bar 1 1.20 1.20 TOTAL COST 452.75 €
Table 7.2: Cost of employed reactants.
Reactant Quantity Total cost (€) Venlafaxine (*) 25 g - Hydrogen peroxide (30% w/v) 1 L 26 Iron Sulphate (II) 50 g 14 Ammonium Metavanadate 50 g 45 Hydrochloric acid 1 L 20 Acetonitrile (HPLC) 5 L 158 Acetic Acid (HPLC) 1 L 59 TOTAL COST 322
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(*) The experiment can be carried out employing Venlafaxine powder for HPLC, which has a cost around 158 €/ 10 mg (Sigma-Aldrich 2020) or the active component of Venlafaxine, which is also useful and the cost corresponds to 1€/g. The drawback of the activated compound is that I can only be found in prescription pharmacies. However, no contaminant has been actually employed. For this reason, no specific value has been determined in the table. Moreover, generally the contaminant comes within the wastewater that needs to be cleared. Hence, it should have no cost attached.To summarize the total tangible experimental cost, a resume is shown in TABLE XXXX:
Table 7.3: Total cost of laboratory material and employed reactants.
Costs Price (€) Material 452.75 Reactants 322 TOTAL COST 747.75 7.2.2 Services costs
• Energy purchase
The energetic cost considers the fixed value of energy cost and the variable costs which correspond to the energy used by the devices.
In Table 7.4, the energy consumed by the laboratory devices and its corresponding costs are listed. As far as the time concerns, values found in Table 7.4 are an approximation because the real times are impossible to know as the experimental project has not been performed entirely. In fact, all the values proposed are taken from various thesis performed in the same university laboratory.
According to Endesa (Endesa 2020), the price for the kW/h in terms of power equates to 0.1143 €/kW·day.
Table 7.4: Variable Cost of the energy consumed by the devices.
Devices Power (kW) Time (h) Cost (€/Kw·h) Final cost (€) TOC analyzer 1.20 90 0.1143 12.344 Spectrophotometer 0.25 12 0.1143 0.3429 HPLC Chromatograph 0.20 50 0.1143 1.1430 pH-meter 0.0066 135 0.1143 0.1018 Magneto- thermic agitator 0.0006 8 0.1143 0.0005 Balance 0.01 10 0.1143 0.0114 UV lamp (*) - - - - TOTAL COST (€) 13.944
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(*) The initial idea was to carry out the experiments with solar light. Nonetheless, synthetic irradiation was estimated to also be applied in some experiments in order to compare the reaction behavior.
The energy use cost is estimated considering both variable and fixed costs. However, the fixed costs will be on account of the university spending as the project is considered within the educational framework of the center.
Therefore, the only energetic cost contemplated correspond to the variable costs presented in the table above. • Water purchase
The development of the experiments involves the use of three different waters whose price and used volume are presented in Table 7.5.
Table 7.5: Water consumption costs.
Water Volume (L) Cost (€/L) Final cost (€) Milli-Q 10 0.50 5.00 Deionized 800 0.15 120 TOTAL COST (€) 125
In summary, the overall services cost is constituted by the sum of energy and water consumption:
TOTAL SERVICES COST = 13.94 + 125 = 138.94 €
7.2.3 Personnel costs
The calculation of the personnel costs is considered as if the work was ruled by a company compromised for two employees, one laboratory technician and one engineer.
Corresponding salary can be estimated as expressed in Eq. 7.1.
푆푎푙푎푟푦 = 퐺푆푌 + 푆푆 Eq. 7.1
Where GSY stands for Gross Salary per Year, and SS is the Social Security fee. Both terms are associated to each employee.
The corresponding salary for the scientific professional and the technician are based on what the Catalan Institute of Statistics (Idescat 2020) states, which is showed in Table 7.6.
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For a company in Spain, the SS corresponds to a 32% of the GSY.
A full-time job will be considered, corresponding to 8 h/day for 5 days a week, for 4 weeks per month and during 5 months (Feb – June).
Table 7.6: Annual salary for the project employees.
Personnel €/h GSY SS Salary (€/year) Chemical Engineer 22.44 17952 5744.64 23696.64 Laboratory technician 18.63 14904 4769.28 19673.28 TOTAL COST 43369.92
7.2.4 Amortizations
For the purchased devices used to develop the project the amortization costs are considered. The equation employed for its calculation is represented in eq XXXX
퐴푞푢𝑖푠𝑖푡𝑖표푛 푐표푠푡 (€) − 푅푒푠𝑖푑푢푎푙 푉푎푙푢푒 (€) Eq. 7.2 퐴푚표푟푡𝑖푧푎푡𝑖표푛 = 퐿𝑖푓푒 푠푝푎푛 (푦푒푎푟푠)
No residual value is considered in this case because the laboratory devices are used until break down. The life span is considered as an average of 15 years for all the devices.
In Table 7.7, a summary of all the amortization costs is represented.
Table 7.7: Amortization cost of the employed devices.
Devices Acquisition cost (€) Amortization cost (€) TOC analyzer 23400 1560.00 Spectrophotometer 9850 656.67 HPLC Chromatograph 30000 2000.00 pH-meter 1200 80.00 Magneto- thermic agitator 285 19.00 Balance 1000 66.67 TOTAL COST 65735 4382.34
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7.2.5 Final cost
Table 7.8 represents the total estimated cost for a project of this kind. It is worth mentioning that additional costs would be summed to this final amount if the job was done for a company and the residual solutions had to be processed in posterior treatments.
Table 7.8: Total estimation of the costs.
Section Cost (€) Materials and reactants 747.75 Services 138.94 Personnel 43369.92 Amortizations 4382.34 TOTAL (€) 48638.95
7.3 Research cost estimation
The development of this project is based on an extent literature research about the parameters affecting mainly Fenton and photo-Fenton process effectiveness. The cost of the general project relies on the hours dedicated to this research and to posterior data treatment.
For the present work the following literature has been analyzed:
- 110 articles and reviews. - 4 theses. - 5 webpages.
In order to evaluate the working time, it will be considered that 3 hours are required to select each piece of information for the analytical part of the project (most of the articles and reviews), 3 more hours are taken from the understanding, analysis and extraction of the information and approximately 250 hours are dedicated to elaborate the aggregate report. The time spent reading and gathering information from the other pieces of literature will be considered within these 250 hours.
The minimum salary set for a student in UPC is 8 €/h. Thus, the total cost will be considered regarding this price. Table XXXX corresponds to the calculations done.
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Table 7.9: Total hours and cost of the literature search and analysis.
Activity Hours Bibliography selection 330 Bibliography analysis 330 Report development 250 Total hours 910 TOTAL COST (€) 7280
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CHAPTER 8 REFERENCES
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Agilent Technologies. 2003. “Agilent Technologies Agilent 1260 Infinity Quaternary LC System Manual and Quick Reference Agilent 1260 Infinity Quaternary LC Manual and Quick Reference Notices.”
Ahmed, Bedoui, Elalaoui Limem, Ahmed Abdel-Wahab, and Bensalah Nasr. 2011. “Photo-Fenton Treatment of Actual Agro-Industrial Wastewaters.” Ind. Eng. Chem. Res 50: 6673–80. https://doi.org/10.1021/ie200266d.
Alalm, Mohamed Gar, Ahmed Tawfik, and Shinichi Ookawara. 2015. “Degradation of Four Pharmaceuticals by Solar Photo-Fenton Process: Kinetics and Costs Estimation.” https://doi.org/10.1016/j.jece.2014.12.009.
Ashrafi, Omid, Laleh Yerushalmi, and Fariborz Haghighat. 2015. “Wastewater Treatment in the Pulp- and-Paper Industry: A Review of Treatment Processes and the Associated Greenhouse Gas Emission.” https://doi.org/10.1016/j.jenvman.2015.05.010.
Audino, Francesca, Leandro Oscar Conte, Agustina Violeta Schenone, Montserrat Pérez-Moya, Moisès Graells, and Orlando Mario Alfano. 2019. “A Kinetic Study for the Fenton and Photo- Fenton Paracetamol Degradation in an Annular Photoreactor.” Environmental Science and Pollution Research 26 (5): 4312–23. https://doi.org/10.1007/s11356-018-3098-4.
Ayala Durán, Saidy Cristina. 2017. “Síntese e Avaliação de Heteroestruturas de Fe2O3/TIO2, Fe2O3/CeO2 E Fe2O3/V2O5 Como Catalisadores No Processo Foto-Fenton Heterogêneo Para a Degradação de Venlafaxina.” Journal of Chemical Information and Modeling. UNESP. https://doi.org/10.1017/CBO9781107415324.004.
Aymerich, I, V Acu ~ Na, D Barcel, M J García, M Petrovic, M Poch, S Rodriguez-Mozaz, et al. 2016. “Attenuation of Pharmaceuticals and Their Transformation Products in a Wastewater Treatment Plant and Its Receiving River Ecosystem” 100: 126–36. https://doi.org/10.1016/j.watres.2016.04.022.
Bakouri, Hicham El, José Morillo, José Usero, and Abdelhamid Ouassini. 2009. “Natural Attenuation of Pesticide Water Contamination by Using Ecological Adsorbents: Application for Chlorinated Pesticides Included in European Water Framework Directive.” Journal of Hydrology 364: 175– 81. https://doi.org/10.1016/j.jhydrol.2008.10.012.
Ballés i Canals, Ricard. 2018. “TFG- ESTUDIO DEL PROCESO FOTO-FENTON CON DOSIFICACIÓN DE H2O2 PARA EL TRATAMIENTO DE AGUAS RESIDUALES.” UPC.
Bautista, P, A F Mohedano, N Mené Ndez, J A Casas, and J J Rodriguez. 2010. “Catalytic Wet Peroxide Oxidation of Cosmetic Wastewaters with Fe-Bearing Catalysts.” Catalysis Today 151: 119–23. https://doi.org/10.1016/j.cattod.2010.01.023.
Benkelberg, Heinz-Jiirgen, and Peter Warneck. 1995. “Photodecomposition of Iron(III) Hydroxo and Sulfato Complexes in Aqueous Solution: Wavelength Dependence of OH and SO4-Quantum Yields.” J. Phys. Chem. Vol. 99. https://pubs.acs.org/sharingguidelines.
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CHAPTER 9 ANNEXES
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9.1 Safety data sheets • Hydrogen peroxide
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• Iron Sulphate
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• Hydrochloric acid
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• Ammonium Metavanadate
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• Acetonitrile
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• Acetic Acid
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