Word Ozein Meaning to Smell) by Schönbein, in 1840, Who Recognised It As a New Compound (Greenwood and Earnshaw, 1984B; Rocke, 1994)
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AN INVESTIGATION OF ADVANCED OXIDATION PROCESSES IN WATER TREATMENT GAVIN WYATT SCHWIKKARD MScEng (Natal); BScEng (Natal) Submitted in fulfilment of the academic requirements for the degree of Doctor of Philosophy in the School of Chemical Engineering, University of Natal, Durban. February 2001 ii ABSTRACT The deteriorating water quality in South Africa and changing legislation requiring the industrial implementation of waste minimisation and pollution prevention technologies has highlighted the need for the investigation of new effluent treatment technologies such as advanced oxidation processes. This investigation details the evaluation of ultrasound, an emerging advanced oxidation process, to degrade organic compounds during water treatment. The objectives of the investigation included the design of a suitable ultrasonic laboratory reactor to investigate ultrasound chemistry and the sub-processes occurring during sonication. Atrazine was used as a model compound to compare the performance of ultrasound with that of ozone and hydrogen peroxide, already established advanced oxidation processes. Recommendations have also been made for the scale-up of ultrasonic processes. A 500 mL ultrasonic cell containing an ultrasonic horn as an energy source was designed and constructed. The measurement of hydrogen peroxide concentration was used as a tool to indicate the process conditions under which the formation of free radical reactions during sonication are enhanced. These include the application of oxygen and air sparging or the addition of a commercial source of hydrogen peroxide. It was found that oxygen sparging and a high acoustic power input should be used in ultrasonic processes with a short retention time, and conversely, that air sparging and a lower acoustic energy source should be used in processes with a long retention time. A flow loop system should be considered to maximise oxidation both within and beyond the sonicated zone, gas sparging should only occur within the sonication zone else the degradation of hydrogen peroxide is encouraged. Ultrasound is most effectively applied in water treatment as a pretreatment stage in combination with other technologies and not as a stand-alone process. Atrazine was used as a model compound to compare the performance of ultrasound with ozone because of its persistence in the environment and resistance to degradation. Atrazine was degraded during sonication and ozonation, degradation increased with the addition of hydrogen peroxide. Ozone decomposition (and hence free radical reactions) was enhanced when ozone was combined with ultrasound or hydrogen peroxide. Enhanced ozone decomposition during ozonation combined with sonication is due to the conditions (high temperatures and pressures) as well as the free radical reactions occurring within the collapsing cavitation bubbles and at the gas-liquid interface. The enhancing effect of combining ultrasound with ozone was greatest at the low ozone concentrations typically applied during water treatment. Atrazine degradation during sonication and ozonation is predominantly due to the reaction with hydroxyl radicals. Atrazine degradation products identified using gas chromatography and mass spectrometry were deethylatrazine, hydroxyatrazine and deethyldeisopropylatrazine (tentatively identified). iii PREFACE I, Gavin Schwikkard, declare that unless indicated, this thesis is my own work and that it has not been submitted, in whole or in part, for a degree at another University or Institution. GW SCHWIKKARD February 2001 iv ACKNOWLEDGMENTS I would like to express my thanks and appreciation to the following people and organisations who have contributed to the investigation: The Water Research Commission and Pollution Research Group for funding the investigation. Diarec Diamond Sales cc for assistance in acquiring the ultrasonic horn. Umgeni Water for the loan of an ozone generator. The Foundation for Research Development for personal funding during the investigation. Sasol for the postponement of my bursary obligation. My supervisor, Prof. C Buckley, for his dedication, advice and commitment to excellence. Mr C Brouckaert for advice on process modelling. Prof. D Mulholland of the School of Chemistry and Applied Chemistry for the use of HPLC equipment. Mr K Robertson and Mr L Henwood from the workshop of the School of Chemical Engineering for the construction of the ultrasonic cell and for assistance with equipment maintenance. The laboratory technicians, Ms D Harrison, Ms S Wadley and Mr B Parel, for their technical support. Ms S Freese from Umgeni Water for advice on setting up the ozone system. Ms A Hansa from the ML Sultan Technikon for assistance with the identification of degradation products. Mr R Coetzer from Sasol Technology for assistance and advice with statistical modelling. Mr I Vorster from the Rand Afrikaans University for mass spectroscopy analysis. My fellow graduate students for their friendship and experiences we shared learning from one another. My family, David, Dorothy, Mandy and Joanne, for their continual encouragement and support throughout the investigation. My wife, Sianne, for performing the HPLC atrazine analysis, assisting with the interpretation of mass spectra and for the sacrifices she made during the writing of this thesis. And to God, the Father, Son and Holy Spirit, who add meaning to life. v CONTENTS Figures xiii Tables xxi Nomenclature xxxiii Abbreviations xxxvi Glossary xxxvii 1. INTRODUCTION 1.1 Water resources in South Africa 1-1 1.2 Water legislation in South Africa 1-3 1.3 Advanced oxidation technologies 1-6 1.4 Project background 1-8 1.5 Project objectives 1-9 1.6 Thesis outline 1-11 2. ULTRASOUND 2.1 History of ultrasound 2-1 2.2 Cavitation 2-2 2.2.1 Formation of ultrasonic cavitation 2-2 2.2.2 Classification of cavitation 2-3 2.2.3 Equations of cavitation bubble dynamics 2-5 2.2.4 Cavitation phenomena 2-11 2.2.4.1 Sonoluminescence 2-11 2.2.4.2 Cavitation noise 2-16 2.3 Sonochemistry 2-17 2.3.1 Chemical effects 2-17 2.3.1.1 Site of sonochemical reactions 2-18 2.3.1.2 Effects of dissolved gas 2-24 2.3.2 Physical effects 2-26 vi Contents cont. 2.4 Applications of ultrasound 2-28 2.4.1 Physical applications 2-28 2.4.2 Chemical applications 2-29 2.4.2.1 Organic synthesis 2-31 2.4.2.2 Sonocatalysis 2-34 2.4.2.3 Polymer chemistry 2-36 2.4.2.4 Sonoelectrochemistry 2-38 2.4.2.5 Sonocrystallization 2-39 2.4.3 Medical applications 2-40 2.4.4 Industrial applications 2-41 2.4.4.1 Water and effluent treatment 2-42 2.4.4.2 Textile industry 2-48 2.4.4.3 Food industry 2-49 2.4.4.4 Petroleum industry 2-50 2.4.4.5 Membrane processes 2-51 2.5 Sonochemical equipment 2-51 2.5.1 Transducers 2-52 2.5.1.1 Piezoelectric transducers 2-52 2.5.1.2 Magnetostrictive transducers 2-53 2.5.2 Ultrasonic baths 2-53 2.5.3 Ultrasonic horns 2-56 2.5.4 Ultrasonic reactors 2-59 2.5.5 Equipment design and reactor modelling 2-66 2.6 Concluding remarks 2-70 3. OZONE AND HYDROGEN PEROXIDE CHEMISTRY 3.1 Ozone fundamentals 3-1 3.1.1 Chemical and physical properties 3-2 3.1.2 Classification of ozone reactions 3-3 3.1.3 Kinetics of ozone decomposition 3-6 3.2 Ozone in water treatment 3-13 3.2.1 History of ozone use 3-14 3.2.2 Ozone generation and transfer 3-15 vii Contents cont. 3.2.3 Application 3-19 3.2.3.1 Potable water treatment 3-19 3.2.3.2 Industrial effluent treatment 3-23 3.3 Hydrogen peroxide 3-24 3.3.1 Chemical and physical properties 3-26 3.3.2 Application in water treatment 3-27 3.4 Concluding remarks 3-29 4. ATRAZINE CHEMISTRY 4.1 Characteristics of atrazine 4-1 4.1.1 Discovery 4-2 4.1.2 Chemical and physical properties 4-2 4.1.3 Toxicity 4-3 4.2 Atrazine in the environment 4-4 4.2.1 Application and usage 4-5 4.2.2 Atrazine in soil 4-6 4.2.2.1 Transformation 4-6 4.2.2.2 Retention 4-11 4.2.3 Atrazine in aquatic systems 4-13 4.2.3.1 Groundwater 4-13 4.2.3.2 Surface water 4-14 4.2.4 Mode of action in plants 4-16 4.3 Atrazine in water treatment 4-19 4.3.1 Ozonation 4-23 4.3.2 Ultraviolet radiation 4-33 4.3.3 Ultrasonic degradation 4-39 4.3.4 Biological treatment 4-41 4.4 Concluding remarks 4-45 5. EXPERIMENTAL DESIGN 5.1 Ultrasonic equipment 5-1 5.1.1 Ultrasonic horn 5-1 viii Contents cont. 5.1.2 Ultrasonic cell 5-4 5.1.2.1 Frequency 5-6 5.1.2.2 Acoustic power 5-6 5.1.2.3 Horn shape 5-7 5.1.2.4 Volume 5-8 5.1.2.5 Vessel type 5-9 5.1.2.6 Pressure 5-11 5.1.2.7 Mixing 5-11 5.1.2.8 Temperature 5-12 5.1.2.9 Gas saturation 5-13 5.1.2.10 Sampling 5-14 5.2 Ozone equipment 5-14 5.3 Experimental procedures 5-17 5.4 Analytical procedures 5-18 5.4.1 Hydrogen peroxide measurement 5-18 5.4.2 Ozone measurement 5-19 5.4.1.1 Iodometric method 5-19 5.4.1.2 Indigo colorimetric method 5-20 5.4.3 Dissolved oxygen measurement 5-20 5.4.4 Atrazine measurement 5-21 5.5 Statistical modelling 5-21 6. ULTRASOUND PROCESS INVESTIGATION 6.1 Dissolved oxygen concentration 6-2 6.2 Hydrogen peroxide formation 6-5 6.2.1 Effect of dissolved gas 6-5 6.2.2 Acoustic power 6-8 6.2.3 Regression analysis 6-12 6.3 Hydrogen peroxide degradation 6-15 6.4 Interval experiments 6-17 6.4.1 Sonication period intervals 6-18 6.4.2 Gas intervals 6-20 ix Contents cont.