Chlorine Cycling in Electrochemical Water and Wastewater
Treatment Systems
by
Linxi Chen
April 3, 2014
B.S., Environmental Engineering, Wuhan University of Technology (2007)
A dissertation submitted to the
Graduate School of the
University of Cincinnati
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
Environmental Engineering Program
College of Engineering and Applied Science
Committee Chair: Dr. Margaret Kupferle, Ph.D., P.E.
Abstract
In this study, phenol was used in a sodium chloride or sulfate matrix as a representative pollutant to systematically study which operating conditions have the largest impact on chlorine forms in an electrochemical treatment system. Initially, an HPLC method was developed and validated to simultaneously determine phenol and potential intermediates from hydroxylation and hypochlorination pathways during electrooxidation in the presence of chloride.
In a combined-reactor configured with a boron-doped diamond (BDD) anode, samples were analyzed to identify and quantify organic intermediates and inorganic chlorine species generated during the electrooxidation of phenol. Ionic strength was kept constant at 50 mM and the applied current density was 12 mA/cm2. The effects of chloride-to-phenol ratio on contaminant removal efficiency and byproduct formation were studied. Experimental results showed that phenol was removed faster at higher chloride-to-phenol ratios but more chlorinated intermediates and chlorate were produced.
The impact of initial chloride concentration on the chlorate formation rate was stronger than its impact on phenol removal rate.
Analysis of variance (ANOVA) was used to evaluate the statistical significance of operational factors in a full 24 factorial design. Factors studied were anode type (BDD vs. graphite), initial phenol concentration (0.25 – 0.5 mM), initial chloride concentration
(5 – 50 mM) and applied current density (12 – 25 mA/cm2), on responses such as phenol
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removal rate and chlorate production rate. Results showed that anode type and chloride concentration had the most significant effects either individually or interactively on the phenol removal rate, and that chloride concentration had a considerable effect on the chlorate production rate. Additionally, applied current density had a significant effect on the free chlorine production rate after breakthrough if and when it occurred with BDD in the presence of excess chloride. A 23 factorial design with a given reactor configuration with either BDD or graphite anode was optimized using response surface methodology
(RSM) with respect to phenol removal and control of chlorate production. Linear regression results showed that the phenol removal rate was highest at low phenol concentration and high chloride concentration, whereas low chloride concentration minimized chlorate production.
In addition to the ANOVA analyses, kinetics of the electrochemical oxidation of phenol and intermediates formed in the presence of chloride were explored for the different anodes at various chloride-to-phenol ratios. Comparison of rate constant k values of the first-order reactions showed that hypochlorination and hydroxylation pathways were in competition and hypochlorination pathway was more favored and 2- chlorophenol was the most dominant species in most cases.
Mass balances around carbon and chlorine for measured species were also considered. Lack of closure for both indicated possible formation of other by-products that were not identified by HPLC. LC-QTOF-MS was used to qualitatively investigate the unknown by-products formed during phenol electrooxidation in the presence of chloride at two levels (5 mM and 50 mM) using the BDD anode. Results showed
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formation of chlorinated dimers and trimers of phenol, including potential formation of polychlorinated dibenzo-p-dioxins (PCDDs).
Keywords: electrooxidation; phenol; chloride; HPLC; BDD; ANOVA; factorial design; optimization; kinetic modeling; LC-QTOF-MS.
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Acknowledgements
I would like to express my gratitude to:
• Dr. Margaret Kupferle, my advisor, for her generous support and endless patience. Without her devotion to me and my work, none of this would have been possible. She was always there to encourage me, support me, and keep me on the right track. In the
future I will strive to be as good a leader and mentor as she was to me.
• The National Science Foundation, for their generous funding and support of this research. They saw the potential in this project and provided the means necessary to make my dream come true.
• Dr. Am Jang, for his support and mentoring during my early stages as a graduate student researcher.
• Dr. Woohyoung Lee, Dr. Dionysious Dionysiou and Dr. George Sorial for their willingness to serve on my committee, their invaluable comments and guidance
through my work.
• Dr. Pablo Campo, for his willingness and patience to assist me in instruments and
time commitment on reviewing my manuscript.
• Jiefei Yu, Xuexiang He, Mugdha Mathure, Liang Yan, Geshan Zhang, Lijuan Sang, Xiaodi Duan, Qingshi Tu and all of my colleagues and friends for their
encouragement and motivation both in and out of the lab.
• Greg, my colleague, my best friend, and my life partner for his tireless devotion to keeping me motivated, and his willingness to share with me the best times and help me through the worst times.
• At last, my family, for their unconditional love and infinite support.
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Table of Contents
Chapter 1 Introduction ...... 1 1.1 Background Introduction ...... 1 1.1.1 Electrochemical Treatment Technology ...... 1 1.1.2 Phenol as a Representative Organic Waste ...... 2 1.1.3 Anode Materials ...... 2 1.1.4 Chlorine Chemistry ...... 4 1.1.5 Operational Conditions Parameter Study ...... 7 1.1.6 Reaction Kinetics and Mechanism of Electrochemical Oxidation of Phenol . 8 1.2 Research Objectives ...... 12 1.3 Approach and Relevance ...... 13 1.4 References ...... 15
Chapter 2 Development, Validation and Application of an HPLC Method for Phenol Electrooxidation Products in the Presence of Chloride ...... 19 2.1 Introduction ...... 19 2.2 Method Development...... 22 2.2.1 Materials ...... 22 2.2.2 Development Details ...... 22 2.3 Method Validation ...... 26 2.3.1 Calibration Curves ...... 26 2.3.2 Precision (Repeatability and Reproducibility) ...... 28 2.3.3 Accuracy ...... 31 2.3.4 Method Detection Limit ...... 31 2.3.5 Column to Column Comparisons ...... 32 2.4 Method Application to Electrooxidation of Phenol ...... 33 2.4.1 Experimental Method ...... 33 2.4.2 Results and Discussion ...... 34 2.5 Conclusion...... 37 2.6 References ...... 38
Chapter 3 Effect of Chloride-to-Phenol Ratio on Phenol and Intermediates Conversion during Anodic Oxidation ...... 40 3.1 Introduction ...... 40 3.2 Experimental ...... 42 3.2.1 Chemicals and Reagents ...... 42 3.2.2 Experimental Setup ...... 43
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3.2.3 Analytical Methods ...... 44 3.2.4 Operating Conditions ...... 45 3.3 Results and Discussion...... 45 3.3.1 Effect on Phenol Removal and Intermediates Conversion ...... 45 3.3.2 Effect on Undesirable Byproduct ...... 50 3.3.3 Effect on Removal of TOC and COD ...... 53 3.3.4 Effect on Current Efficiency ...... 56 3.4 Conclusion...... 57 3.5 References ...... 58
Chapter 4 Electrochemical Oxidation of Organic Waste: Statistical Studies and System Optimization ...... 61 4.1 Introduction ...... 61 4.1.1 Identifying Operational Parameters ...... 62 4.1.2 Analysis of Variance (ANOVA) ...... 63 4.1.3 Response Surface Methodology (Optimization) ...... 66 4.2 Materials and Methods ...... 67 4.2.1 Experimental Setup ...... 67 4.2.2 Analytical Measurements ...... 69 4.3 Results and Discussion...... 70 4.3.1 Analysis of Variance ...... 70 4.3.2 Surface Response Methodology ...... 74 4.4 Conclusion...... 82 4.5 References ...... 83
Chapter 5 Kinetics of the Electrochemical Oxidation of Phenol and Intermediates in the Presence of Chloride under Different Reaction Conditions ...... 86 5.1 Introduction ...... 86 5.2 Experimental Section...... 87 5.2.1 Chemicals and Materials ...... 87 5.2.2 Experimental Setup ...... 87 5.2.3 Analytical Methods ...... 88 5.2.4 Experimental Conditions ...... 89 5.3 Kinetic Modeling on Intermediate Conversion ...... 90 5.3.1 Modeling Basics ...... 90 5.3.2 First-Order Reaction Differential Equations ...... 91 5.4 Results and Discussion...... 92 5.4.1 Experimental Results...... 92
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5.4.2 Calculatation of Reaction Rate Constants ...... 101 5.4.3 Evaluation of the Model Fit ...... 108 5.5 Conclusion...... 110 5.6 References ...... 112
Chapter 6 Identification of Intermediate Products during Anodic Oxidation of Phenol using Liquid Chromatography Quadruple Time-of-Flight Mass Spectrometry ...... 113 6.1 Introduction ...... 113 6.2 Experimental Section...... 115 6.2.1 Chemicals and Reagents ...... 115 6.2.2 Experimental Setup ...... 115 6.2.3 Analytical Methods ...... 116 6.2.4 LC-QTOF-MS Analysis ...... 117 6.3 Results and Discussion...... 118 6.3.1 Intermediates Identified by HPLC ...... 118 6.3.2 Identification of Unknown By-products...... 122 6.3.3 Interpretation of Triclosan Standard MS/MS Spectrum ...... 123 6.3.4 Structural Confirmation of By-products...... 124 6.3.5 Proposed Reaction Pathways for Electrooxidation of Phenol in the Presence of Chloride ...... 129 6.3.6 Influence of Chloride Concentration on By-product Formation ...... 130 6.4 Conclusion...... 131 6.5 References ...... 132
Chapter 7 General Conclusions ...... 138 7.1 Summary of Conclusions ...... 138 7.2 Future Study ...... 140
Appendices
A Limiting Current Measurement: Determine the Mass Transfer Coefficient km... ….A-1 B Cyclic Voltammetry Studies………………………………………………………...B-1 C Supporting Information for Chapter 2………………………………………………C-1 D Preliminary Data (Chapter 3)……………………………………………………… D-1 E R Script for RSM (Chapter 4)……………………………………………………….E-1 F Preliminary Data (Constant Voltage Experiments)………………………………….F-1 G Supporting Information for Chapter 5………………………………………………G-1 viii
H Supporting Information (Chapter 6)………………………………………………...H-1 I Impact of Different Electrode Materials on Intermediates Formed during Electrochemical Oxidation of Phenol in the Presence of Chloride Ions…………………………………...... I-1 J Kinetics of the Electrochemical Oxidation of Phenol and Intermediates in the Presence of Chloride under Different Reaction Conditions………………………………………...... J-1
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List of Tables
Table 1-1. Operational parameters based on full factorial design ------8
Table 2-1. Mobile solvent gradient------23
Table 2-2. Retention times and UV detector wavelengths ------25
Table 2-3. Results of calibration curves for intra-day (n = 3) and inter-day ------27
Table 2-4. Accuracy and intra- day (n = 5) and inter-day (n = 3) precisions for standards
------28
Table 2-5. Method detection limits for standards------32
Table 3-1. First-order rate constants for phenol removal, chlorate production, and TOC and COD removal ------46
Table 4-1. Factors and dependent variables studied in ANOVA ------64
Table 4-2. Independent Variables of the 24 Factorial Design of Experiments ------64
Table 4-3. Experimental Conditions for the 24 Factorial Design ------65
Table 4-4. One-way ANOVA Results ------71
Table 4-5. Two-way ANOVA Results ------73
3 Table 4-6. Design matrix of the 2 factorial design (BDD runs) and response factors Y1,
Y2 ------75
Table 4-7. ANOVA for the two response factors Y1 and Y2 (BDD runs)------80
Table 4-8. ANOVA for response Y2 (Graphite runs) ------81
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Table 5-1. Table of Run Conditions ------89
Table 5-2. Reaction rate constant k values when using BDD anode ------101
Table 5-3. Reaction rate constant k values when using graphite anode ------102
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List of Figures
Figure 1-1. (Adapted from Polcaro, A. 2009) Scheme of the anodic reactions at BDD anodes: (a) electrochemical chloride oxidation, (b) formation of hydroxyl radicals by water discharge, (c) formation of chlorate ions (reaction 11), (d) formation of chlorine dioxide (reaction 10), (e) oxygen evolution and formation of ROS...... 7
Figure 1-2. Flowchart of chapter outline and relevance...... 13
Figure 2-1. Two proposed reaction pathways for electrooxidation of phenol. Compounds shown here are standards used in HPLC method development...... 21
Figure 2-2. Example HPLC chromatogram for standards with corresponding peak numbers as shown in Table 2-2...... 26
Figure 2-3. Schematic graph of the electrooxidation system: 1. potentiostat; 2. reactor; 3.
BDD anode; 4. stainless steel cathode; 5. Ag/AgCl reference electrode; 6. pump; 7. recirculation tubing...... 34
Figure 2-4. Phenol degradation and organic intermediates conversion profiles during two electrochemical runs, using Na2SO4 17 mM (a); NaCl 5 mM in combination with Na2SO4
15 mM (b), as electrolytes. The other experimental conditions: Initial phenol concentration = 0.5 mM; constant voltage = 6 V; BDD anode, stainless steel cathode. .. 36
Figure 3-1. Experimental set-up showing BDD configuration...... 44
Figure 3-2. First-order phenol removal rates under different treatment conditions...... 47
Figure 3-3. First-order phenol removal rates are proportional to chloride-to-phenol ratio.
...... 47
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Figure 3-4. Phenol and intermediates measured (concentrations normalized with respect to the initial concentration of phenol) when phenol is treated at 12 mA/cm2 constant current density with BDD anode (a) 0.5 mM phenol in 50 mM sodium chloride, (b) 0.25 mM phenol in 50 mM sodium chloride, (c) 0.5 mM phenol in 5 mM sodium chloride and
15 mM sodium sulfate, and (d) 0.25 mM phenol in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths)...... 49
Figure 3-5. Chlorine-containing species measured when phenol concentration is treated when phenol is treated at 12 mA/cm2 constant current with BDD anode (a) 0.5 mM phenol in 50 mM sodium chloride, (b) 0.25 mM phenol in 50 mM sodium chloride,
(c) 0.5 mM phenol in 5 mM sodium chloride and 15 mM sodium sulfate, and (d) 0.25 mM phenol in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths)...... 50
Figure 3-6. Comparison of TOC removal...... 53
Figure 3-7. Actual TOC (solid symbol) vs. Calculated TOC (open symbol) degradation.
...... 54
Figure 3-8. Comparison of COD removal...... 55
Figure 3-9. Actual COD (solid symbols) vs. Theoretical COD (open symbols)...... 56
Figure 3-10. Impact of chloride-to-phenol ratio on ICE...... 57
Figure 4-1. One-way ANOVA plots with significant effects...... 72
Figure 4-2. Half-normal probability plot of effects on response Y1 (phenol removal rate) with BDD runs...... 77
Figure 4-3. Chloride - phenol concentration (AB) interaction graph for BDD runs...... 78
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Figure 4-4. (a) Response surface plot of phenol removal rate; (b) The contour plot. x1 is the coded variable for phenol concentration, x2 is the coded variable for chloride concentration...... 79
3 Figure 4-5. Half-normal plot of the factor effects from the 2 factorial for response Y2
(chlorate production rate) with BDD runs...... 81
Figure 5-1. Two proposed pathways: hydroxylation and hypochlorination...... 91
Figure 5-2. Phenol and intermediates measured when phenol is treated at 12 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 93
Figure 5-3. Phenol and intermediates measured when phenol is treated at 25 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 94
Figure 5-4. Phenol and intermediates measured when phenol is treated at 12 mA/cm2 constant current density in in 5 mM sodium chloride and 15 mM sodium sulfate
(equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 95
Figure 5-5. Phenol and intermediates measured when phenol is treated at 25 mA/cm2 constant current density in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 96
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Figure 5-6. Chlorine-containing species measured when phenol is treated at 12 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 97
Figure 5-7. Chlorine-containing species measured when phenol is treated at 25 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 98
Figure 5-8. Chlorine-containing species measured when phenol concentration is treated when phenol is treated at 12 mA/cm2 constant current density in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 99
Figure 5-9. Chlorine-containing species measured when phenol concentration is treated when phenol is treated at 25 mA/cm2 constant current density in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol...... 100
Figure 5-10. Comparison of phenol conversion rates for formation of initial intermediates. k1 and k2 follow hypochlorination and k21 and k22 follow hydroxylation. 104
Figure 5-11. Comparison of phenol secondary conversion rates in hypochlorination pathway: mono- to dichlorophenols (k3, k5 and k6), di- to trichlorophenol (k7, k8) and trichlorophenol to other products (k14)...... 105
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Figure 5-12. Comparison of predicted reaction rates for all pathways to “other products”: phenol (k9), monochlorophenols (k10, k11), dichlorophenols (k12 and k13), trichlorophenol
(k14), and hydroquinone, benzoquinone (k25 and k26)...... 106
Figure 5-13. Comparison between 2,4-dichlorophenol conversion rates (k3, k6, k7 and k12).
...... 107
Figure 5-14. Comparison between 2,4,6-trichlorophenol conversion rates (k7, k8 and k14) and mineralization rate (k20)...... 108
Figure 5-15. Experimental and model concentration profiles for phenol and intermediates.
Experimental results from G8: 0.5 mM phenol in 50 mM NaCl with graphite anode using
25 mA/cm2 current density...... 110
Figure 6-1. Normalized phenol and intermediates concentration progression with time
(a&b), TOC and COD removal (c&d), and the comparison of actual and calculated TOC
(e&f) in the presence of high chloride and low chloride concentration...... 121
Figure 6-2. Chlorine mass balance with high chloride (a) and low chloride concentration
(b)...... 122
Figure 6-3. Proposed pathway from measured by-products...... 130
Figure 6-4. Time-course progression of by-products formed in high-chloride (a) and low- chloride experiments (b)...... 131
Figure 7-1. Phenol and intermediates conversion with time when 0.5 mM phenol was treated in 25 mM/25 mM NaCl/NaBr using BDD anode at 12 mA/cm2 using the same reactor configuration as in the previous studies...... 142
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List of Symbols and Abbreviations
The following symbols and abbreviations are used throughout this dissertation:
® - registered trademark
ANOVA – analysis of variance
BBD – Box-Behnken design
BDD - boron-doped diamond
℃ - degree(s) Celsius
C, C-1 – coulombs, per coulomb
CCD – central composite design
CE – collision energy
COD – chemical oxygen demand
CV – coefficient of variation
CVD – chemical vapor deposition
DAD – diode-array detector
DBPs – disinfection byproducts
DCPs - dichlorophenols
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EDCs – endocrine-disrupting chemicals
ESI – electrospray-ionization
FAC – free available chlorine
FMF – find by molecular feature
GUI – graphical user interface
HPLC – high-performance liquid chromatography
IC – ion chromatograph
ICE – instantaneous current efficiency
k – reaction rate constant
LC-QTOF-MS – liquid chromatography quadruple time-of-flight mass spectrometry
LOQ – limit of quantification
LSE – least-squares error
M – mole or molar
MCLs – maximum contamination levels
MCPs - monochlorophenols
MDL – method detection limit
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mA - milliamps mg – milligram(s) min – minute(s) mL – milliliter(s) mM – millimolar mm, mm2, mm3 – millimeter(s), square millimeter(s), cubic millimeter(s) m/z – mass-to-charge ratio
µL – microliter(s)
NSF – National Science Foundation
ODS - octadecylsilane
PAHs - polycyclic aromatic hydrocarbons
PCBs – polychlorinated biphenyls
PCDDs – polychlorinated dibenzo-p-dioxins ppm – parts per million, mg/L
PVC – polyvinyl chloride
ROS – reactive oxidizing species
RSM – response surface methodology
TCP – trichlorophenol
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TIC – total ionic chromatogram
TOC – total organic carbon
TOX – total organic halogen
UV - ultraviolet
USEPA – United States Environmental Protection Agency
V – volt(s)
VOCs – volatile organic compounds
WHO – World Health Organization x, X – independent variables y, Y – response variables
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Chapter 1 Introduction
1.1 Background Introduction
1.1.1 Electrochemical Treatment Technology
The electrochemical treatment method has been receiving increased attention in the water and wastewater treatment field recently. It is useful in treating wastes that are not biodegradable since it is not biologically mediated. It is a clean technology in that it does not require addition of chemicals and does not produce sludge as a byproduct. If coupled with solar energy sources, electrochemical treatment may be a relatively inexpensive, efficient and environmentally friendly way to treat waste that is not biologically degradable.
The electrochemical technique has been applied in the past to the treatment of industrial effluents. For organic pollutants such as phenol, polycyclic aromatic hydrocarbons (PAHs), and endocrine disrupting chemicals (EDCs), which have been reported to have harmful impacts on human health, the treatment strategy of electrochemical oxidation has the advantages of being easy to control, low cost and high efficiency, outweighing other existing treatment methods (1). Alfaro et al. (2) reported that various studies in the past have been done with various anode materials, such as Pt,
Ti/IrO2, Ti/PbO2 and boron-doped diamond (BDD).
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1.1.2 Phenol as a Representative Organic Waste
Phenol was chosen as a model compound for this project because it is often found in a variety of organic wastes, such as dye industry, paper processing, and humic materials,
(1, 3-7). Phenol and its intermediates produced during oxidation are well-studied, and therefore well-developed analytical methods are known. The dominant phenol degradation pathway varies with electrode material (1), and other operational parameters.
In this dissertation, BDD and graphite were studied as anode materials.
1.1.3 Anode Materials
Graphite has long been an interest of electrochemists (8-11, 11-14). Its good electrical conductivity and semi-metal property make it a commonly used electrode material (12,
14). The low cost and wide availability of graphite has led to its use in many different applications, for example, anodic oxidation in wastewater treatment. Because graphite has low oxygen evolution overpotential, anodic oxidation can take place at a low current density or high concentration of chlorides (11). Due to its unstable surface, however, graphite can slowly degrade during electrolysis (13). Recently, graphite has been extensively studied in carbon nanotubes research (14).
Synthetic boron-doped diamond (BDD) thin film coating on a p-Silicon substrate is a new electrode material that has become one of the most popular electrode materials studied in recent years (15). Synthetic diamond thin films are usually grown on substrates by using hot-filament or microwave-assisted chemical vapor deposition (CVD) techniques (16). When diamond is doped with boron, an element with semi-metal properties, the conductivity of the material can be improved significantly.
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It has been proven that BDD exhibits inert and weak adsorption (2, 15). The hydroxyl radicals produced from water splitting during electrolysis in the vicinity of the
BDD anode may transfer to the bulk solution, therefore, contaminants can be degraded by direct electrochemical oxidation. The results show that the degree of deactivation of BDD electrodes is less than platinum and other more traditional electrodes, such as graphite,
IrO2, SnO2, and PbO2 (2).
Prior results from the literature suggest that BDD exhibits very high overpotentials and high anodic stability because of the inertness of the surface towards adsorption (2,
15). In addition, hydroxyl radicals produced by BDD have already proven to contribute to the complete oxidation of phenol to carbon dioxide (4). Compared with BDD, graphite is less expensive and more easily accessible. Graphite has a less stable surface than BDD, therefore anode degradation may take place during electrolysis (13). Graphite also has lower oxygen evolution overpotential indicating it is less efficient during anodic oxidation due to more useless oxygen production from side-reactions(10).
Research has found that BDD exhibits higher oxygen evolution overpotential than graphite, possibly because of its inert surface for impurity adsorption (17). BDD, therefore, shows higher efficiency than graphite pertaining to anodic oxidation at high current densities due to fewer oxygen evolution side-reactions (10). It can be concluded from the literature that compared to graphite, BDD has: (1) higher price; (2) higher physical stability, chemical stability, and electrical conductivity; (3) larger working potential window in conventional aqueous electrolyte solutions.
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1.1.4 Chlorine Chemistry
Chlorine chemistry is another important aspect of this project, since chlorides are ubiquitous in naturally occurring waters and can be expected to occur in the majority of wastewaters. Therefore, the element chlorine should not be overlooked in waste treatment design. Many recalcitrant and toxic organic compounds contain chlorine. For example, polychlorinated biphenyls (PCBs) have strong persistence in the environment due to the low biodegradability, and can cause many severe health effects. Disinfection byproducts (DBPs) can also be formed when disinfectants such as chlorine react with naturally-occurring organic matter in water. U.S. EPA has regulated a number of chlorine related contaminants by assigning maximum contaminant levels (MCLs) (18). Chloride may be oxidized to chlorine species during advanced oxidative treatment, including electrochemical treatment systems. In the electrochemical oxidation of phenol with the presence of chlorine, it is important to know if any undesirable chlorinated byproducts are produced in the system and how the chlorinated species and their concentrations are influenced by varying parameters, such as reactor configuration, anode material, applied current density and chlorine-to-contaminant ratio in the waste (19).
Chloride-containing electrolyte in waste treatment methods has been extensively applied in past studies (8, 20-24). The most important benefit to using chloride brines is that the generation of free available chlorine (FAC) adds additional oxidizing power, which has been widely applied as a disinfectant in drinking water treatment as well. (25-
29) When it comes to electrochemical waste treatment, one can expect a higher removal rate of contaminant when using a higher concentration of chloride brine. On the other hand, it is possible that higher chloride concentration will lead to more side reactions and
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higher production of chlorinated intermediates that have the potential to be toxic. It is important, therefore, to balance these factors in order to seek an optimal design that maximizes contaminant removal efficiency and minimizes the effluent toxicity.
Researchers have been studying the chlorine evolution mechanisms during chloride solution electrolysis since the 1970s (8). Hyunwoong et al. (20)studied rate constants of
· ·- - active chlorine species (Cl , Cl2 and HOCl/OCl ) generated as indirect oxidants in the presence of NaCl electrolyte. The behavior of chlorine species, including chlorate by- products, has been investigated in dilute chloride solutions (21). Polcaro et al. (22) later discussed Cl- , HClO concentrations as function with faradic yield (ε) by comparing experimental data and they developed a model. They observed that concentrations of
- - - electrogenerated oxidants (ClO /HClO) and chlorates (ClO3 , ClO4 ) increase with current density and chloride concentration. It was concluded that (1) high free chlorine generation leads to high removal rate and generation of undesired byproducts; (2) low current density, high mass transfer rate and low residence time contribute to maximizing oxidant concentrations and minimizing chlorates concentrations (22). However, the amount of chlorates produced under various conditions and their rate constants with respect to variables (e.g. time, current density, chloride concentration, etc.) have not been systematically studied so far. This information is critical to understanding the whole chlorine-cycling system.
The main reactions and side reactions are listed below:
Anode:
+ - Phenol + H2O → CO2 + Oxidized species + H + e (1-1)
- - 4OH → 2H2O + O2 + 4e (1-2)
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Cathode (reduction):
- - 2H2O + 2e → H2 + 2OH (1-3)
Bulk solution and/or near the anode surface (indirect oxidation)
Hydroxyl radical formation:
· + - H2O→OH + H + e (1-4)
Free chlorine:
- - 2Cl → Cl2 + 2e (1-5)
+ - Cl2 + H2O → HClO + H + Cl (1-6)
HClO ↔ ClO- + H+ (1-7)
Byproducts formation:
- - - + - 12ClO + 6H2O → 4ClO3 + 8Cl + 12H + 3O2 + 12e (1-8)
- - + - ClO3 + H2O → ClO4 + 2H + 2e (1-9)
· 3OH + HClO → ClO2 + 2H2O (1-10)
· - + 4OH + HClO → ClO3 + H + 2H2O (1-11)
- - + 2ClO2 + H2O → ClO3 + ClO2 + 2H (fast) (1-12)
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Figure 1-1. (Adapted from Polcaro, A. 2009 (22)) Scheme of the anodic reactions at BDD
anodes: (a) electrochemical chloride oxidation, (b) formation of hydroxyl radicals by water
discharge, (c) formation of chlorate ions (reaction 11), (d) formation of chlorine dioxide
(reaction 10), (e) oxygen evolution and formation of ROS.
1.1.5 Operational Conditions Parameter Study
The parameters shown in Table 1 have been considered in the literature and in previous
studies in the author’s research group (1, 11, 15, 30-35). Therefore, the parameter study
makes up research objectives 2 and 3. The parameter values chosen are based on the
proposal for this research by Kupferle (19) and preliminary experiments.
Electrolyte composition is an important factor because it provides conductivity for
electrolysis to take place. Reactive electrolytes, for example those containing Cl-, contribute to the oxidation, leading to a varying reaction pathway and production of undesirable byproducts. NaCl was chosen as the reactive electrolyte in this study. Non- reactive electrolytes (without a reactive ion), do not contribute to the oxidation. In this case, Na2SO4 was chosen because it is commonly used in the literature.
Anode material has a significant impact on the products because of the different surface structures of anodes used (graphite vs. BDD). The difference of oxygen evolution
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overpotentials between two materials results in different chemistry on the anode surface and side-reactions. This difference will affect the reaction pathway followed during electrolysis.
Chloride concentration and initial phenol concentration are considered because they represent the concept of chloride-to-waste ratio, which is an important factor in industrial waste stream treatment. These two parameters will be studied together in addition to control cases where one is present when the other is not.
Applied current density is considered as an operational measure since the reactor is given a constant current supply instead of voltage. The chemical reaction kinetics and mass transfer depend on the control of current density, which is discussed in Appendix A.
Table 1-1. Operational parameters based on full factorial design
Factors Variable 1 Variable 2 Variable 3 Electrolyte NaCl *Na2SO4 - w/ or w/o Cl- Anode material BDD Graphite - Chloride 50 25 0 Concentration (mM) Initial phenol 0.5 0.25 0 concentration (mM)
*Na2SO4 solution concentrations are 17 mM and 8 mM, which should keep the same ionic strength as NaCl solution at 50 mM and 25 mM, respectively.
1.1.6 Reaction Kinetics and Mechanism of Electrochemical Oxidation of Phenol
The pathways of phenol electrooxidation are associated with electrode materials and electrolyte composition (reactive or non-reactive electrolyte). However, very little
8
research has examined the comparison and competition between different pathways with respect to various operational conditions. Anode material, for example, plays a significant role in the specific interaction between the anode surface and the compounds.
Iniesta (15) studied complete and partial oxidation of phenol by investigating the total organochlorinated compounds (TOX) and gas evolution. Their results indicated that phenol was first oxidized to non-volatile chlorinated aliphatic acids and then further oxidized to a volatile chlorinated compound, chloroform (CHCl3). They concluded that chlorine presence contributed to phenol oxidation due to electrogenerated hypochlorite
(ClO-). Another study by Canizares et al. (3) found that, during the electrochemical oxidation of phenol using a BDD anode, quinonic intermediates (hydroquinone and benzoquinone) and carboxylic acids (maleic/fumaric acid and oxalic acid) may be produced. Other polyhydroxybenzenes and quinonic compounds were also detected.
Moreover, some volatile organochlorinated compounds (VOCs) formed according to
Canizares et al. (3); polymers have been found as well (36).
There are two main approaches in the literature for developing mathematical modeling during electrochemical oxidation. One is single-variable model based on chemical oxygen demand (COD) decrease with time during the oxidation process (3, 7,
15, 32, 33, 37). The other is multi-variable model to quantify the species including organic waste, intermediates, oxidizing species involved and final product during the oxidation process (38, 39). The first approach focuses on the macroscopic scale of the whole system and energy efficiency of the process, discussions are divided into two scenarios based on current /mass transfer controlled. The second approach focuses on detailed concentration profiles of the components with respect to time. The advantage of
9
the single-variable model is that it has only one variable, i.e. COD, making it very intuitive for model computation. And it widely applicable to many different reactor systems (the variable COD does not depend on reactor design). The disadvantage is that it does not provide the detailed progression of species involved.
The advantage of the second approach is that it provides an accurate and detailed picture of the complex electrochemical oxidation system. The comprehensive aspects on the oxidation process can be evaluated by the model, for example, mass transport, competitions between side-reactions, and so on. The disadvantage of the second approach is that many parameters have to be adjusted and assumptions have to be made according to different reactor conditions, thus making the model itself very complex and condition-specific.
Generally, kinetic mechanisms of an electrochemical oxidation system consist of three processes: (1) direct oxidation at the electrode surface, i.e., electron transfer of organic substances; (2) indirect oxidation mediated by electrochemically formed oxidizing species, such as OH radicals, and/or (3) free available chlorine species in the presence of chloride. The kinetic mechanism of electrochemical oxidation of phenol in the presence of chloride, being the most complex case, contains all the three types of processes.
1.1.6.1 Direct oxidation
Zhi and Wang (40) have investigated and quantified the pathway of direct electron transfer of phenol at the surface of BDD anode. They concluded that the direct electron transfer pathway was in competition with OH radical oxidation pathway and the extent of phenol degradation due to direct electron transfer decreased as applied potential
10
increased. At higher applied voltage, the reaction rates increase and once the adsorption site on BDD surface has been saturated (BDD is proven to be inert for adsorption), the
OH radical-mediated pathway becomes dominant, which is likely to apply to the work reported in this dissertation since applied voltage is high. When using non-reactive electrolyte (e.g. sodium sulfate) and inert anodes (e.g. BDD), OH-mediated oxidation is the primary mechanism, during which process, OH radicals are attached to the surface at anode to give oxygen (side reaction) or in the vicinity of electrode to oxidize organic and the two reactions are in competition (34). Highly active and short-lived OH radicals react with organics at high reaction rates. BDD is characterized to have weak adsorption to
OH radicals, resulting in a high reactivity of OH radicals towards organic and a low oxygen evolution (high oxygen evolution overpotential) (32).
1.1.6.2 Indirect oxidation mediated by OH radicals
Mascia et al. have previously studied kinetic modeling of electrooxidation of organic compounds (phenol, cyanuric acid and atrazine) at BDD anode surface with OH radicals involved (41). They employed a lumped reaction rate constant (kOH) in the reaction of water with OH radical (first-order reaction), assuming that OH radicals generated at the surface are present in excess and contribute to complete mineralization of pollutant
(where CO2 is the final product). Then the overall reaction is second-order with respect to organic pollutant and OH radical concentrations.
1.1.6.3 Indirect oxidation mediated by free chlorine when chloride is present
When the oxidation occurs in the presence of chloride, the electrochemically-generated free chlorine species play a mediating role during indirect electrolysis, as shown in Figure
1-1. To elucidate the oxidation reaction mechanism, the most fundamental step is to look
11
into free chlorine evolution with respect to anode material characteristics. Murata et al.
(23) used a BDD anode to quantitatively determine free chlorine generation during the process of anodic oxidation in the presence/absence of chloride ion. They reported a linear calibration of free chlorine in the range of 20-100 mg Cl L-1. They also found that an excess amount of Cl- enhanced the generation rate of free chlorine. Another aspect that is often overlooked is the formation of undesirable by-products, such as chlorate and perchlorate. Jung et al. (42) developed a kinetic model for chlorates formation during the chlorine cycling pathway which is promising for the purpose of controlling by-products, however, the computed parameters are very specific to their reaction conditions.
1.2 Research Objectives
Research Objective 1: To develop and validate an HPLC method that can simultaneously measure phenol and reaction intermediates from hypochlorination and hydroxylation pathways.
Research Objective 2: To investigate the amount and species of undesirable byproducts, such as chlorate, perchlorate, chlorinated organics, or other species that are produced in chlorine-cycling systems depending on various parameters, such as reactor configuration, electrolyte composition, anode selection, the initial chlorine-to-waste concentration ratio, applied current density and surface-to-volume ratio.
Research Objective 3: To apply a mathematical model for data analysis, by varying the above-mentioned parameters, and to propose pathways for phenol and its intermediate products, to predict mechanisms for formation of different intermediates and products in chlorine-cycling systems.
12
Research Objective 4: To suggest a treatment strategy that optimizes these operating
conditions to minimize the productions of undesirable byproducts and maximize
contaminant oxidation to carbon dioxide and other non-toxic or less toxic end products.
1.3 Approach and Relevance
Figure 1-2. Flowchart of chapter outline and relevance.
This dissertation is organized as according to the flowchart shown in Figure 1-2.
Chapter 2 reports the development and validation of an HPLC method that can
simultaneously measure intermediates during the electrochemical oxidation of phenol
13
from both pathways. This chapter is in line with Research Objective 1, which lays the foundation of the analytical work throughout the whole research.
Chapter 3 investigates effects of operational conditions during the treatment process under constant current with BDD was used as the anode. The formation of chlorate was examined under conditions of varying initial phenol concentration and chloride concentration. This chapter relates to the Research Objective 2.
Chapter 4 discusses the statistical significance of operational factors based on a full 24 factorial design. Analysis of Variance (ANOVA) was used. Response surface methodology (RSM) was also employed to further investigate the optimal operational conditions for the maximum treatment efficiency and the lowest byproduct formation.
Chapter 4 maps to the first part of Research Objective 3.
Chapter 5 proposes kinetic modeling of the intermediates conversions during electrochemical oxidation of phenol. Mathematica was used to calculate the first-order reaction rate constants. The model demonstrated favored pathway and dominant species when both pathways were in competition. Chapter 5 maps to the second part of Research
Objective 3.
Chapter 6 explores formation of previously unreported by-products using the state-of- art instrument LC-QTOF-MS and confirms speculative reports of polymer formation at the electrode surface.
Appendices A - H include preliminary data and other related results.
Appendices I and J are composed of two conference presentations.
14
1.4 References
1. Comninellis, C. and Nerini, A. Anodic oxidation of phenol in the presence of NaCl for wastewater treatment. J. Appl. Electrochem. 1995, 25 (1), 23-28.
2. Alfaro, M.A.Q.; Ferro, S.; Martínez-Huitle, C.A.; Vong, Y.M. Boron doped diamond electrode for the wastewater treatment. Journal of the Brazilian Chemical Society 2006, 17 (2), 227-236.
3. Cañizares, P.; Lobato, J.; Paz, R.; Rodrigo, M.A.; Sáez, C. Electrochemical oxidation of phenolic wastes with boron-doped diamond anodes. Water Research 2005, 39 (12), 2687-2703.
4. Andrade, L.S.; Laurindo, E.A.; De Oliveira, R.V.; Rocha-Filho, R.C.; Cass, Q.B. Development of a HPLC method to follow the degradation of phenol by electrochemical or photoelectrochemical treatment. J. Braz. Chem. Soc. 2006, 17 (2), 369-373.
5. Rajkumar, D.; Kim, J.G.; Palanivelu, K. Indirect electrochemical oxidation of phenol in the presence of chloride for wastewater treatment. Chemical Engineering and Technology 2005, 28 (1), 98-105.
6. Malpass, G. Decolourisation of real textile waste using electrochemical techniques: Effect of electrode composition. J. Hazard. Mater. 2008, 156, 170-177.
7. Chatzisymeon, E.; Xekoukoulotakis, N.P.; Diamadopoulos, E.; Katsaounis, A.; Mantzavinos, D. Boron-doped diamond anodic treatment of olive mill wastewaters: Statistical analysis, kinetic modeling and biodegradability. Water Res. 2009, 43 (16), 3999-4009.
8. Janssen, L.J.J. and Hoogland, J.G. The electrolysis of an acidic NaCl solution with a graphite anode-III. Mechanism of chlorine evolution. Electrochim. Acta 1970, 15 (6), 941-951.
9. Burns, R.M. and Hulett, G.A. SOME PROPERTIES OF GRAPHITE. J. Am. Chem. Soc. 1923, 45 (3), 572-578; 10.1021/ja01656a002.
10. Chen, G. Electrochemical technologies in wastewater treatment. Separation and purification technology 2004, 38 (1), 11.
11. Chen, G.G. Electrochemical Wastewater Treatment Processes, In Advanced physicochemical treatment technologies, Anonymous ; Humana Press: Totowa, NJ, 2007; Vol.5 pp. 57-106.
12. Chung, D.D.L. Review Graphite. J. Mater. Sci. 2002, 37 (8), 1475-1489.
15
13. Fugivara, C.C.S. Electrochemical decomposition of cyanides on tin dioxide electrodes in alkaline media. Analyst (London) 1996, 121 (4), 541.
14. Thostenson, E.T.; Ren, Z.; Chou, T. Advances in the science and technology of carbon nanotubes and their composites: a review. Composites Sci. Technol. 2001, 61 (13), 1899-1912.
15. Iniesta, J.; Michaud, P.A.; Panizza, M.; Cerisola, G.; Aldaz, A.; Comninellis, C. Electrochemical oxidation of phenol at boron-doped diamond electrode. Electrochimica Acta 2001, 46 (23), 3573-3578.
16. Xu, J., Granger, M. C., Chen, Q., Strojek, J. W., Lister, T. E., & Swain, G. M. Boron- doped diamond thin-film electrodes. Analytical Chemistry 1997, 69 (19), 591A-597A.
17. Martin, H.B.H.B. Hydrogen and oxygen evolution on boron-doped diamond electrodes. J. Electrochem. Soc. 1996, 143 (6), L133-L136.
18. U.S.EPA Drinking Water Contaminants.
19. Kupferle, M.J. Chlorine Cycling in Electrochemical Treatment Systems. 2008, NSF CAREER Proposal Award No. 0747602.
20. Park, H.; Vecitis, C.D.; Hoffmann, M.R. Electrochemical Water Splitting Coupled with Organic Compound Oxidation: The Role of Active Chlorine Species. J. Phys. Chem. C 2009, 113 (18), 7935-7945.
21. Polcaro, A.; Vacca, A.; Mascia, M.; Ferrara, F. Product and by-product formation in electrolysis of dilute chloride solutions. J. Appl. Electrochem. 2008, 38 (7), 979-984.
22. Polcaro, A.; Vacca, A.; Mascia, M.; Palmas, S.; Rodiguez Ruiz, J. Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides. J. Appl. Electrochem. 2009.
23. Murata, M.; Ivandini, T.A.; Shibata, M.; Nomura, S.; Fujishima, A.; Einaga, Y. Electrochemical detection of free chlorine at highly boron-doped diamond electrodes. J Electroanal Chem 2008, 612 (1), 29-36.
24. Ferro, S.S. Chlorine evolution at highly boron-doped diamond electrodes. J. Electrochem. Soc. 2000, 147 (7), 2614-2619.
25. Venczel, L.V. Inactivation of Cryptosporidium parvum oocysts and Clostridium perfringens spores by a mixed-oxidant disinfectant and by free chlorine. Appl. Environ. Microbiol. 1997, 63 (4), 1598.
26. Simpson, K.L. Drinking water disinfection by-products: an Australian perspective. Water research (Oxford) 1998, 32 (5), 1522.
16
27. Reller, M.E. A randomized controlled trial of household-based flocculant-disinfectant drinking water treatment for diarrhea prevention in rural Guatemala. Am. J. Trop. Med. Hyg. 2003, 69 (4), 411.
28. Richardson, S.D. Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection by-products in drinking water: a review and roadmap for research. Mutation research.Reviews in mutation research 2007, 636 (1-3), 178.
29. Arnold, B.F. Treating water with chlorine at point-of-use to improve water quality and reduce child diarrhea in developing countries: a systematic review and meta-analysis. Am. J. Trop. Med. Hyg. 2007, 76 (2), 354.
30. Pera-Titus, M.; Garcıá -Molina, V.; Baños, M.A.; Giménez, J.; Esplugas, S. Degradation of chlorophenols by means of advanced oxidation processes: a general review. Applied Catalysis B, Environmental 2004, 47 (4), 219-256.
31. Kapałka, A.; Fóti, G.; Comninellis, C. The importance of electrode material in environmental electrochemistry. Electrochim. Acta 2007, 54 (7), 2018-2023.
32. Kapałka, A.; Fóti, G.; Comninellis, C. Kinetic modelling of the electrochemical mineralization of organic pollutants for wastewater treatment. J. Appl. Electrochem. 2007, 38 (1), 7-16.
33. Panizza, M.; Michaud, P.A.; Cerisola, G.; Comninellis, C. Anodic oxidation of 2- naphthol at boron-doped diamond electrodes. J Electroanal Chem 2001, 507 (1-2), 206- 214.
34. Comninellis, C. and Chen, G. Ebook: Electrochemistry for the environment. 2010.
35. Sundaram, V. Operation and design impacts on efficiency and toxicity during electrochemical treatment of azo dye-containing wastewater. Master of Science Thesis. University of Cincinnati, 2005, https://etd.ohiolink.edu/
36. King, R.A.; May, B.L.; Davies, D.A.; Bird, A.R. Measurement of phenol and p-cresol in urine and feces using vacuum microdistillation and high-performance liquid chromatography. Analytical Biochemistry 2009, 384 (1), 27-33.
37. Wang, L.; Fu, J.; Qiao, Q.; Zhao, Y. Kinetic modeling of electrochemical degradation of phenol in a three-dimension electrode process. J. Hazard. Mater. 2007, 144 (1-2), 118- 125.
38. Mascia, M.; Vacca, A.; Polcaro, A.M.; Palmas, S.; Ruiz, J.R.; Da Pozzo, A. Electrochemical treatment of phenolic waters in presence of chloride with boron-doped diamond (BDD) anodes: Experimental study and mathematical model. J. Hazard. Mater. 2010, 174 (1–3), 314-322.
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39. Boudreau, J.; Bejan, D.; Bunce, N.J. Competition between electrochemical advanced oxidation and electrochemical hypochlorination of acetaminophen at boron-doped diamond and ruthenium dioxide based anodes. Can. J. Chem 2010, 88 (5), 418-425.
40. Zhi, J.; Wang, H.; Nakashima, T.; Rao, T.N.; Fujishima, A. Electrochemical incineration of organic pollutants on boron-doped diamond electrode. Evidence for direct electrochemical oxidation pathway. Journal of Physical Chemistry B 2003, 107 (48), 13389-13395.
41. Mascia, M.; Vacca, A.; Palmas, S.; Polcaro, A.M. Kinetics of the electrochemical oxidation of organic compounds at BDD anodes: Modelling of surface reactions. J. Appl. Electrochem. 2007, 37 (1), 71-76.
42. Jung, Y.J.; Baek, K.W.; Oh, B.S.; Kang, J. An investigation of the formation of chlorate and perchlorate during electrolysis using Pt/Ti electrodes: The effects of pH and reactive oxygen species and the results of kinetic studies. Water Res. 2010, 44 (18), 5345- 5355.
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Chapter 2 Development, Validation and Application of an
HPLC Method for Phenol Electrooxidation Products in the
Presence of Chloride1
2.1 Introduction
Many industrial waste effluents contain organic pollutants that are hard to degrade using traditional biological treatment methods, and electrochemical processes have received increasing attention as means for addressing this problem. Phenol was chosen as a target compound in this study since it represents a large category of organic pollutants from industrial effluents. The dominant products of phenol electrooxidation are impacted by electrode materials and electrolyte composition (reactive or non-reactive electrolyte). It has been confirmed that during electrooxidation of phenol, reaction intermediate pathways fall into two categories: hydroxylation and hypochlorination pathways (1, 2).
While each pathway has been studied independently, very little research has examined the comparison/competition between these pathways with respect to electrolyte composition in the presence (or absence) of chloride.
1 This chapter is an original article published in the International Journal of Environmental Analytical
Chemistry, 30 Jan 2014, copyright Taylor & Francis, available online at: http://www.tandfonline.com/doi/full/10.1080/03067319.2013.871715#.UzHYdK1dVIw
19
Analytical methods for quantification of phenol and its derivatives have been developed in different matrices using HPLC techniques. Most of these methods (3-12) utilize a C18 reversed phase column to provide a non-polar stationary phase paired with water/methanol or water/acetonitrile mixtures as the polar mobile phase. While other authors have developed HPLC methods to apply to phenol electrooxidation products, they have focused either on hydroxylation products (3) or hypochlorination products (13).
An HPLC method that can measure products of both potentially competing pathways in the same sample has not been developed to date to the authors’ knowledge. The possible compounds that may be found in the two reaction pathways are shown in Figure 2-1.
MarvinSketch version 6.0.2 (ChemAxon) was used for drawing all the chemical structures in the reaction pathways. The objective of this work was to build on previous methods to develop a method that can detect all of the compounds shown in Figure 2-1 and then demonstrate the application of the developed method to phenol electrooxidation in the presence and absence of chloride.
20
Figure 2-1. Two proposed reaction pathways for electrooxidation of phenol. Compounds shown here are standards used in HPLC method development.
21
2.2 Method Development
2.2.1 Materials
2.2.1.1 Chemicals and reagents
Standard compounds (benzoquinone, hydroquinone, catechol, 2-chlorophenol, 3- chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 2,3,6- trichlorophenol, 2,4,5-trichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol) were ordered from Sigma-Aldrich® except phenol, which was obtained from Fisher
Scientific. Standards were prepared as a 2 mM stock solution in a 50 mM sodium chloride (Fisher Scientific) matrix. Water purified by a Super-Q system (Millipore) was used for making all the dilutions and the HPLC mobile phase. Other compounds used in preparation of the mobile phase (sulfuric acid, formic acid, HPLC-grade acetonitrile,
HPLC-grade methanol and acetic acid) were purchased from Fisher Scientific. All chemicals were analytical grade unless otherwise specified.
2.2.1.2 Instrumentation
An Agilent 1100 HPLC system, containing a diode-array detector (DAD) and quadric pump with autosampler was operated at a column temperature of 30 ℃ with a mobile phase constant flow rate of 0.8 mL min-1 and sample injection volume of 10 µL. Data acquisition was carried out by HP Chemstation 1.0 software.
2.2.2 Development Details
Originally, when a reverse-phase C18 column (SUPELCOSILTM LC-18-DB from
SUPELCO, 100 mm × 3.0 mm) was used, it was observed that the resolutions of the peaks were unsatisfactory using water-acetonitrile as the mobile phase. Then 0.5 mM
22
sulfuric acid was added to the aqueous phase to adjust the pH to 3, and methanol was added to the organic phase (acetonitrile:methanol = 98:2, v/v). The chromatograms showed sharper peak shapes. However, peak separation was still poor and many of the chlorophenol isomers co-eluted.
A Hypersil ODS column (Thermo Scientific, 150 mm × 4.6 mm) with a gradient was tried next based on work published by Ross et al. who achieved clean separation of chlorophenol isomers in the method they developed using this column to study pentachlorophenol reduction (11). Acetic acid was added to the acetonitrile:methanol mixture to improve peak sharpness (90:5:5 acetonitrile:methanol:acetic acid by volume) while formic acid (0.1 %) was added to the pH-adjusted water to prevent algae growth.
For the gradient, the best peak separation was achieved with an initial percentage of aqueous phase at 90%, and a final percentage of aqueous phase at 10% in a run time of
15 min (Table 2-1).
Table 2-1. Mobile solvent gradient
Time (min) Aqueous phase (%) Organic phase (%)
0.00 90 10
1.00 85 15
3.00 40 60
6.00 30 70
9.00 20 80
12.00 10 90
Individual standards were injected first in order to determine the retention times, then a mixture of standards was injected to compare and confirm the elution order. The
23
retention times and chromatograms at four absorbance wavelengths for a mixture of standards using this gradient are shown in Table 2-2 and Figure 2-2, respectively. The elution order for the compounds is: hydroquinone < benzoquinone < catechol < phenol <
4-chlorocatechol < 2-chlorophenol < 4-chlorophenol < 3-chlorophenol < 2,6- dichlorophenol < 2,4-dichlorophenol < 2,3,6-trichlorophenol < 2,4,5-trichlorophenol <
2,4,6-trichlorophenol < pentachlorophenol. Note that compounds with higher hypochlorination levels elute later. DAD wavelengths at 270 nm (for bq and phenol),
280 nm (for cat, mcps, 2,6-dcp and 2,3,6-tcp), 290 nm (for 2,4-dcp, 2,4,5-tcp and 2,4,6- tcp) and 300 nm (for pcp) were chosen for quantitation based on maximum absorbance.
Standards prepared in Super-Q water were compared to standards in the saline matrix, and results showed that the presence of salt (up to 50 mM) does not affect the peak area or retention time. During the course of method development, three different Hypersil
ODS columns with the same order number and packing material but different lot numbers were used. While the first column was being tested, both hydroquinone and pentachlorophenol preparation were problematic and the repeatability and reproducibility studies helped identify these issues. When the second column study was performed, issues with the pentachlorophenol standard had been resolved, but hydroquinone continued to be problematic. A third column was in use after both standard preparation issues had been resolved. In the case of pentachlorophenol, solubility was improved by making the stock solution in methanol and using that solution to make the mixed standard in salt matrix. In the case of hydroquinone, the stock solutions were prepared with nitrogen-purged water in an anaerobic chamber and stored in sealed vials in the dark to prevent oxidation. The calibration of hydroquinone was done separately from the
24
mixture of other standards, and the same stock was used for the three-day reproducibility study. There were still some issues with possible oxidation as will be seen in the data presented for the third column in the following sections.
Table 2-2. Retention times and UV detector wavelengths
Peak No. Compound Abbreviation Retention Time Wavelength
1 hydroquinone hq (min)3.7 (nm)290 2 benzoquinone bq 5.3 270 3 catechol cat 5.5 280 4 phenol phenol 6.0 270 5 2-chlorophenol 2-cp 6.7 280 6 4-chlorophenol 4-cp 6.8 280 7 3-chlorophenol 3-cp 6.9 280 8 2,6-dichlorophenol 2,6-dcp 7.4 280 9 2,4-dichlorophenol 2,4-dcp 7.7 290 10 2,3,6-trichlorophenol 2,3,6-tcp 8.4 280 11 2,4,5-trichlorophenol 2,4,5-tcp 8.7 290 12 2,4,6-trichlorophenol 2,4,6-tcp 8.8 290 13 pentachlorophenol pcp 11.0 300
25
Figure 2-2. Example HPLC chromatogram for standards with corresponding peak numbers as shown in Table 2-2.
2.3 Method Validation
2.3.1 Calibration Curves
Standards containing mixtures of phenol and chlorophenol isomers were prepared at the concentrations of 0.005 mM, 0.01 mM, 0.025 mM, 0.05 mM and 0.2 mM in 50 mM NaCl matrix and injected into the HPLC. Another standard containing only phenol of 0.5 mM was made separately because 0.5 mM was going to be the initial concentration of phenol in later electrochemistry experiments. Response was measured as peak area after manual
26
integration using Chemstation 1.0 software. Peak area was plotted versus compound concentration and straight lines were observed for all compounds in the range tested.
Linear regression was used to fit the data to the formula,"#$% $'#$ = )*+"# ×
-+.-#./'$/0+. + 0./#'-#"/ and an 23 (regression coefficient) was calculated using
OriginLab software. The results for each compound were plotted and intra-day calibrations were done for three consecutive days, each based on mean peak areas of five injections. Intra-day (Day 1, 2 and 3) and inter-day (average of all three days) calibration curves with error bars for all thirteen compounds were generated, and these plots are given in supporting documentation. The intra-day and inter-day slope value, intercept and 23 for each compound are summarized in Table 3. Inter-day 23 values were from
0.9796 to 1.000. The slope values and intercepts do not reveal significant differences between days, except for the hydroxylation products (hq, bq and cat). The measured areas for hydroxylation products showed decrease over time especially at lower concentrations causing more inconsistency in slopes between days.
Table 2-3. Results of calibration curves for intra-day (n = 3) and inter-day
Day 1 Day 2 Day 3 Interday Compound Intercept Slope R2 Intercept Slope R2 Intercept Slope R2 Intercept Slope R2 hq 1.467 599.716 0.9999 0.322 593.119 1.0000 -0.576 793.832 1.0000 0.405 662.222 1.0000 bq -2.185 519.269 0.9957 -0.821 556.334 0.9994 2.653 358.8800.9943-0.118478.1610.9988 cat -6.281 1374.209 0.9981 -4.369 1353.673 0.9992 -1.616 1125.277 1.0000 -4.089 1284.387 0.9993 phenol 0.159 1006.347 1.0000 -0.237 1017.018 1.0000 -0.657 1010.342 1.0000 -0.245 1011.236 1.0000 2-cp -0.041 1379.482 1.0000 -0.296 1380.863 1.0000 -0.565 1346.057 1.0000 -0.301 1368.801 1.0000 4-cp -0.266 1071.780 1.0000 -0.492 1076.772 1.0000 -0.812 1053.750 1.0000 -0.524 1067.434 1.0000 3-cp -0.660 1176.676 1.0000 -1.024 1181.609 0.9999 -1.075 1143.509 1.0000 -0.920 1167.264 1.0000 2,6-dcp -0.948 1512.457 1.0000 -1.326 1522.612 0.9999 -2.438 1478.192 0.9999 -1.571 1504.421 0.9999 2,4-dcp -1.305 1489.331 1.0000 -1.776 1495.837 0.9999 -3.280 1455.898 0.9998 -2.120 1480.355 0.9999 2,3,6-tcp -1.457 1259.916 0.9999 -2.660 1275.285 0.9995 -5.384 1234.529 0.9989 -3.167 1256.577 0.9995 2,4,5-tcp -2.534 1817.611 0.9999 -4.906 1864.152 0.9994 -13.750 1813.483 0.9969 -7.063 1831.748 0.9991 2,4,6-tcp -1.488 1462.516 0.9999 -2.759 1486.411 0.9994 -4.304 1426.745 0.9994 -2.851 1458.557 0.9996 pcp -4.185 1385.962 0.9981 -6.430 1370.095 0.9963 -16.732 1263.352 0.9796 -9.115 1339.803 0.9934
27
2.3.2 Precision (Repeatability and Reproducibility)
Repeatability (within-run precision) was examined for standards by analyzing five times for each concentration within a day (n = 5). Reproducibility (between run precision) was evaluated for three consecutive days (n = 15). The intra-day coefficients of variation
) (CV) value was calculated by using formula 45 = 100 × :8̅ , where ) is the standard deviation of five replicates, 8̅ is the mean of five replicates at each concentration level of each standard at each day (14). A pooled CV was expressed by the average of CVs from three days as the inter-day precision. The results of the precision analysis are shown in
Table 2-4. Overall, the method was repeatable and reproducible with CV values below
5%, except for variation in the values noted for hydroquinone with higher CV values on
Day 1. Researchers applying this method should be aware of the sensitivity of these standards to oxygen.
Table 2-4. Accuracy and intra- day (n = 5) and inter-day (n = 3) precisions for standards
Day 1 Day 2 Day 3 Pooled Concentration (mM) CV (%) Accuracy CV (%) Accuracy CV (%) Accuracy CV (%)
0.005 98.443 83.710 14.685 96.017 1.741 107.434 38.290
0.01 73.872 95.563 11.196 99.139 3.510 103.199 29.526
0.025 hydroquinone 67.835 102.579 10.869 100.670 2.566 99.015 27.090
0.05 66.769 101.737 10.821 100.345 2.679 98.798 26.756
0.2 66.418 99.872 9.622 99.973 3.868 100.078 26.636
0.005 0.009 173.301 4.010 122.849 n.a. n.a. 2.010
0.01 benzoquinone 0.013 130.782 9.488 102.720 41.517 59.033 17.006
0.025 0.016 62.902 14.376 86.631 13.923 109.659 9.438
28
0.05 0.052 104.313 1.765 104.139 9.769 119.176 3.862
0.2 0.200 100.187 1.644 99.929 10.533 98.822 4.126
0.005 0.008 161.912 0.829 151.795 6.642 112.698 2.493
0.01 0.012 122.711 5.985 107.284 8.308 98.546 4.768
0.025 catechol 0.019 76.116 9.127 84.825 5.218 98.179 4.788
0.05 0.050 100.130 0.600 100.273 4.284 99.780 1.645
0.2 0.201 100.270 1.273 100.169 2.378 100.038 1.284
0.005 1.939 91.852 3.242 102.386 1.827 107.544 2.336
0.01 1.302 92.885 1.230 96.458 0.731 98.858 1.087
0.025 0.489 100.071 0.839 100.063 1.042 100.470 0.790 phenol 0.05 0.574 98.957 0.513 98.853 0.981 100.601 0.690
0.2 0.322 101.294 0.406 100.608 0.780 99.464 0.503
0.5 0.334 99.807 0.398 99.915 0.457 100.078 0.396
0.005 2.502 105.816 4.424 111.525 3.495 107.422 3.474
0.01 1.661 98.892 2.629 101.025 1.630 97.842 1.973
0.025 2-cp 0.680 100.701 0.757 99.825 0.560 98.702 0.666
0.05 1.521 99.116 0.522 98.344 0.682 100.339 0.908
0.2 0.596 100.043 0.540 100.096 0.816 100.000 0.651
0.005 2.254 107.000 2.459 108.656 1.604 110.760 2.106
0.01 1.318 98.706 1.749 100.144 0.744 99.336 1.270
0.025 4-cp 1.052 101.840 1.377 100.687 0.564 99.182 0.998
0.05 1.555 98.344 0.243 98.438 1.153 99.247 0.983
0.2 0.539 100.074 1.003 100.081 1.106 100.055 0.883
0.005 3-cp 1.313 106.685 0.684 110.272 2.539 109.405 1.512
29
0.01 0.549 96.693 0.712 100.636 0.868 98.195 0.710
0.025 1.048 101.399 0.976 100.834 0.731 99.293 0.918
0.05 1.769 99.153 1.580 98.017 1.071 99.647 1.473
0.2 0.670 100.035 0.928 100.103 1.163 100.032 0.920
0.005 1.451 110.841 0.903 116.637 2.531 122.761 1.628
0.01 0.973 99.873 0.406 103.411 1.052 102.896 0.810
0.025 2,6-dcp 0.794 100.028 1.059 99.614 1.227 98.030 1.026
0.05 0.848 98.607 0.983 97.198 0.884 97.457 0.905
0.2 0.395 100.080 0.475 100.162 0.151 100.168 0.340
0.005 0.616 111.944 1.147 118.075 1.328 129.561 1.030
0.01 0.926 101.159 1.546 103.709 0.811 104.465 1.094
0.025 2,4-dcp 0.439 100.081 0.889 98.963 0.558 96.244 0.629
0.05 0.645 98.106 0.540 97.316 0.621 97.217 0.602
0.2 0.395 100.107 0.681 100.163 0.774 100.203 0.617
0.005 1.541 118.970 1.671 136.837 0.867 160.798 1.360
0.01 0.987 102.445 1.429 110.180 1.217 114.326 1.211
0.025 2,3,6-tcp 0.481 98.961 1.275 96.652 0.584 92.627 0.780
0.05 0.748 97.521 0.542 94.585 0.947 92.768 0.746
0.2 0.209 100.153 0.828 100.342 0.728 100.493 0.588
0.005 2.032 119.308 1.058 140.418 1.446 201.902 1.512
0.01 1.218 104.090 1.660 111.189 1.586 124.799 1.488
0.025 2,4,5-tcp 2.005 98.809 0.844 96.372 1.409 87.746 1.419
0.05 1.177 97.148 1.771 94.028 0.666 87.618 1.205
0.2 1.765 100.175 2.703 100.377 1.470 100.840 1.979
30
0.005 1.997 122.818 0.505 137.398 2.997 143.558 1.833
0.01 1.988 103.872 1.546 111.140 1.536 108.196 1.690
0.025 2,4,6-tcp 2.206 97.747 0.504 97.609 1.794 96.389 1.501
0.05 0.917 97.367 2.099 93.711 0.992 94.367 1.336
0.2 2.455 100.176 2.937 100.379 3.142 100.361 2.845
0.0025 1.145 168.549 1.281 197.520 0.625 320.261 1.017
0.005 1.300 121.415 1.042 127.604 0.398 172.483 0.913
0.0125 pcp 0.827 94.674 1.024 92.991 0.762 82.435 0.871
0.025 0.795 88.771 0.579 84.418 1.376 63.250 0.917
0.1 0.338 100.689 0.717 100.953 0.795 102.252 0.617
2.3.3 Accuracy
Accuracy was assessed at five different concentration levels of standards by replicate measurements (n = 5) for three consecutive days. Percent recovery (%2) was calculated by dividing the mean measured concentration to the theoretical concentration (Table 2-4).
2.3.4 Method Detection Limit
Six spike concentration levels of mixture standards (0.001, 0.002, 0.0025, 0.003, 0.004,
0.005 mM) were attempted and experiment was repeated for seven times (twice for 0.002 mM spike level). Each standard was injected 10 times. The test procedure followed the
U.S. EPA method for Method Detection Limit (MDL) (40 CFR 136, Appendix B, revision 1.11). Calculation results were obtained by multiplying the standard deviation and the corresponding Student’s t-value (t = 2.821 was used for n = 10, at 99%
31
confidence level with 9 degrees of freedom). Limit of quantification (LOQ) was determined mathematically as equal to 10 times the MDL. The results of MDL and LOQ are summarized (shown in mM and mg L-1) in Table 2-5.
Table 2-5. Method detection limits for standards
Mean MDL Mean LOQ Mean MDL Mean LOQ Compound (mM × 104) (mM × 103) (mg L-1) (mg L-1) hq 5.100 5.100 0.056 0.562 bq 9.468 9.468 0.102 1.024 cat 2.961 2.961 0.033 0.326 phenol 4.497 4.497 0.042 0.423 2-cp 6.078 6.078 0.078 0.781 4-cp 4.314 4.314 0.055 0.555 3-cp 4.179 4.179 0.054 0.537 2,6-dcp 1.872 1.872 0.031 0.305 2,4-dcp 2.125 2.125 0.035 0.346 2,3,6-tcp 3.825 3.825 0.076 0.755 2,4,5-tcp 1.903 1.903 0.038 0.376 2,4,6-tcp 3.147 3.147 0.062 0.621 pcp 2.975 2.975 0.079 0.792
2.3.5 Column to Column Comparisons
Data for phenol and the mono-, di- and trichlorophenols are available for three columns with different lot numbers as discussed in Section 2 Method Development. The same three-day analysis protocol of standards measurement was performed for these compounds using the three columns. Calibration curves were obtained for each column at each day and data are provided in Supplemental Material. Pentachlorophenol data are
32
available for the second and the third Hypersil ODS columns as well. In general, the three different columns performed comparably except for some issues noted on the second column for separation of 2,4,5-trichlorophenol and 2,4,6-trichlorophenol peaks.
This issue was not a problem on the first and third columns and in subsequent application of the method, so there may have been a quality control issue with the second column.
2.4 Method Application to Electrooxidation of Phenol
2.4.1 Experimental Method
The electrooxidation of phenol was carried out in an undivided electrochemical reactor
(Figure 2-3). A potentiostat CHI 1100b (CH Instruments) was used as an external power supply providing constant voltage at 6 V throughout all the experiments. The reactor was made of Pyrex® glass having a working volume of 1L and an overall capacity of 1250 cm3. A Teflon-lined screw top plastic lid with openings to accommodate electrodes was used to cover the reactor. PVC holders were used to maintain the two electrodes at the required height. The anode used was boron-doped diamond (Adamant Technologies) with dimensions 5.0 × 2.5 × 0.2 cm. Stainless steel foil (Alfa Aesar®) with dimensions
5.0 × 2.5 × 0.05 cm was used as the cathode. The reference electrode used was a silver/silver chloride (Ag/AgCl) electrode purchased from BASi. The electrodes were immersed into the electrolyte solution to a depth of 12.2 cm. The distance between anode and cathode was 3.4 cm. A peristaltic pump (Cole-Parmer®) with Masterflex® tubing
(Cole-Palmer®) was used for mixing via recirculation of the contents of the reactor at
1590 mL min-1. The undivided electrochemical reactor was operated at room temperature, 22 ± 0.5 ℃, for all experiments. The pH was not buffered but was
33
monitored throughout the experiments, and it ranged between 4.7 to 7.1. Samples were taken before the current was turned on and then at every 18 min (or multiples of 18) to get immediate measurements for phenol and its intermediates by using HPLC with the method described above.
Figure 2-3. Schematic graph of the electrooxidation system: 1. potentiostat; 2. reactor; 3.
BDD anode; 4. stainless steel cathode; 5. Ag/AgCl reference electrode; 6. pump; 7. recirculation tubing.
2.4.2 Results and Discussion
Figure 2-4 shows the comparison of phenol removal rates and variation of intermediates when using Na2SO4 only versus NaCl in Na2SO4. It can be seen that when chloride is present, phenol loss occurred much faster than in the absence of chloride. This finding is in line with previously reported findings that the degradation rates of organic pollutants may be greatly increased in the presence of chloride ions (6, 13, 15), with increasing
34
rates correlating to increasing chloride concentration (15). This is because the generation of free chlorine contributes to oxidation in the bulk phase in addition to the direct electrooxidation of phenol on the anode.
Benzoquinone was the only identified intermediate when using Na2SO4.
Benzoquinone, catechol (hydroxylation products) and chlorophenol (hypochlorination products) intermediates were identified when there was chloride present in the matrix.
Due to contribution of free chlorine, phenol was converted to intermediates faster when chloride was present. More types of intermediates such as chlorophenols may be formed in the presence of chloride, which may increase the potential for persistent toxicity of the effluent. Since chloride is ubiquitous in natural waters and industrial effluents, process design must target control of unwanted chlorine-related byproducts. The advantage of using the established HPLC method is that trade-offs between both competing pathways can be examined in the same sample, which eliminates confounding effects of comparing different samples and methods and helps to select the optimal treatment conditions that balance removal efficiency against production of unwanted side reactions and endproducts.
35
0.5
0.4
0.3
phenol bq 0.2 Concentration (mM) Concentration 0.1
a 0.0 020040060080010001200 Time (min)
0.5
0.08
0.06
0.4 0.04 0.02 Concentration (mM) Concentration 0.00 0200400600800 Time (min) 0.3 phenol bq
cat 0.2 2-cp 4-cp 2,6-dcp Concentration (mM) 2,4-dcp 0.1 Free Chlorine
b 0.0 020040060080010001200 Time (min)
Figure 2-4. Phenol degradation and organic intermediates conversion profiles during two electrochemical runs, using Na2SO4 17 mM (a); NaCl 5 mM in combination with Na2SO4 15 mM (b), as electrolytes. The other experimental conditions: Initial phenol concentration = 0.5 mM; constant voltage = 6 V; BDD anode, stainless steel cathode.
36
2.5 Conclusion
The developed HPLC method can analyze products from both hydroxylation and hypochlorination pathways of phenol electrooxidation simultaneously, and this is useful in more completely characterizing reactor performance in the presence of chloride. Good linearity, intra and inter-day precision and accuracy were achieved with the new method although sensitivity of hydroquinone stock solutions to oxidation increased variability for this compound. Method detection limits were obtained in the 0.03-0.1 mg L-1 range.
Supplemental material
Supplemental material is available in Appendix C, as indicated in the text, for raw data of all three columns. Figures C1-3 and Tables C1-4 are calibration curves and results for all three columns (intra- and inter-day). Table C-5 is MDL data for all three columns.
Tables C6-9 are results of accuracy and precisions for all three columns. Figure C-4 and
Table C-10 are example chromatograms and method information acquired from the other column (LC-18-DB).
37
2.6 References
1. Boudreau, J.; Bejan, D.; Bunce, N.J. Competition between electrochemical advanced oxidation and electrochemical hypochlorination of acetaminophen at boron-doped diamond and ruthenium dioxide based anodes. Can. J. Chem 2010, 88 (5), 418-425.
2. Körbahti, B.K. and Tanyolaç, A. Kinetic modeling of conversion products in the electrochemical treatment of phenolic wastewater with a NaCl electrolyte. Ind. Eng. Chem. Res 2003, 42 (21), 5060-5065.
3. Andrade, L.S.; Laurindo, E.A.; De Oliveira, R.V.; Rocha-Filho, R.C.; Cass, Q.B. Development of a HPLC method to follow the degradation of phenol by electrochemical or photoelectrochemical treatment. J. Braz. Chem. Soc. 2006, 17 (2), 369-373.
4. Ruana, J.; Urbe, I.; Borrull, F. Determination of phenols at the ng/l level in drinking and river waters by liquid chromatography with UV and electrochemical detection. J. Chromatogr. A 1993, 655 (2), 217-226.
5. King, R.A.; May, B.L.; Davies, D.A.; Bird, A.R. Measurement of phenol and p-cresol in urine and feces using vacuum microdistillation and high-performance liquid chromatography. Analytical Biochemistry 2009, 384 (1), 27-33.
6. Comninellis, C. and Nerini, A. Anodic oxidation of phenol in the presence of NaCl for wastewater treatment. J. Appl. Electrochem. 1995, 25 (1), 23-28.
7. Zhu, X.; Shi, S.; Wei, J.; Lv, F.; Zhao, H.; Kong, J.; He, Q.; Ni, J. Electrochemical oxidation characteristics of p-substituted phenols using a boron-doped diamond electrode. Environ. Sci. Technol. 2007, 41 (18), 6541-6546.
8. Podolina, E.A.; Rudakov, O.B.; Khorokhordina, E.A.; Kharitonova, L.A. Use of acetonitrile for the extraction of dihydric phenols from salt aqueous solutions followed by HPLC determination. J. Anal. Chem 2008, 63 (5), 468-471.
9. Burtscher, E.; Binder, H.; Concin, R.; Bobleter, O. Separation of phenols, phenolic aldehydes, ketones and acids by high-performance liquid chromatography. J. Chromatogr. A 1982, 252, 167-176.
10. Xie, Y. Combinative method using HPLC quantitative and qualitative analyses for quality consistency assessment of a herbal medicinal preparation. J. Pharm. Biomed. Anal. 2007, 43 (1), 204.
11. Ross, N.C.; Spackman, R.A.; Hitchman, M.L.; White, P.C. An investigation of the electrochemical reduction of pentachlorophenol with analysis by HPLC. J. Appl. Electrochem. 1997, 27 (1), 51-57.
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12. Motamed, B.; Böhm, J..; Hennequin, D.; Texier, H.; Mosrati, R.; Barillier, D. Development of an HPLC method for the determination of phenolic by-products: Optimisation of the separation by means of the experimental designs methodology. Analusis 2000, 28 (7), 592-599.
13. Park, H.; Vecitis, C.D.; Hoffmann, M.R. Electrochemical Water Splitting Coupled with Organic Compound Oxidation: The Role of Active Chlorine Species. J. Phys. Chem. C 2009, 113 (18), 7935-7945.
14. Harris, D.C. Quantitative Chemical Analysis. 7th ed. W. H. Freeman, New York, 2007.
15. Iniesta, J.; González-Garcıa,́ J.; Expósito, E.; Montiel, V.; Aldaz, A. Influence of chloride ion on electrochemical degradation of phenol in alkaline medium using bismuth doped and pure PbO2 anodes. Water Res. 2001, 35 (14), 3291-3300.
39
Chapter 3 Effect of Chloride-to-Phenol Ratio on Phenol and
Intermediates Conversion during Anodic Oxidation
3.1 Introduction
Using electrochemical technology in wastewater treatment as an alternative has received increasing attention in recent years for its versatility and compatibility especially when it comes to treating bio-refractory organic pollutants (1). Both direct and indirect electrochemical oxidation processes have been considered in previous studies, and there is literature reporting the use of supporting electrolytes with and without the presence of chloride. Chloride-containing electrolytes bring controversial effects. The advantage is that removal efficiency may be increased due to indirect oxidation by free chlorine formed in situ, resulting in higher mineralization of organic pollutants (2, 3). However, the major problem with the presence of chloride is the production of chlorinated organic compounds that may be generally more persistent in water and could potentially increase the toxicity of the effluent (4). Moreover, the formation of undesirable inorganic byproducts, such as chlorate and perchlorate, should not be overlooked, since chlorates are considered disinfection byproducts that are harmful to human health in drinking water systems, according to the WHO Guidelines for Drinking-water Quality (5). Previous studies on electrochemical water disinfection treatment in dilute chloride solutions provide useful information on varying chlorate formation depending on operational
40
conditions such as electrode materials, flow rate and current density (6, 7) Formation of chlorate in electrochemically mediated wastewater treatment has been largely overlooked in previous studies, however.
Phenol was chosen as a model compound in this study because it represents a broad class of organic pollutants found in industrial wastewater streams. Many researchers have studied electrochemical oxidation of phenol and phenolic compounds with non- chloride electrolytes, such as sulfate (8), perchlorate (9) and phosphate (10). They found that polyhydroxybenzenes, quinones (8, 9) and carboxylic acids (8, 10) were the main reaction intermediates. According to the literature, in the presence of chloride, the intermediates produced were mainly chlorophenols and polymers (11). Comninellis et al. reported VOC formation, i.e., chloroform (4). In general, in the absence of chloride, highly reactive hydroxyl radicals generated from water dissociation accomplish phenol destruction, thus hydroxylation is the main reaction pathway. On the other hand, when chloride is present (often at high concentration) and free chlorine is formed in excess, hypochlorination is the dominant reaction followed by polymerization (12). The two pathways more than likely compete when chloride concentration is low, which is often the practical case when dealing with naturally occurring water where the chloride concentration is generally lower than 250 mg L-1 (about 7.04 mM) (7). Sathish et al. proposed combination pathways of the two above-mentioned cases when using a 0.1M
NaCl electrolyte, however, they did not quantify the intermediates or provide time course progression information (13).
The goal of this study was to examine this potential for competition between intermediate pathways in anodic oxidation at different chloride to phenol ratios and to
41
consider the formation of chlorate. The removal of total organic carbon (TOC) was followed as a cross check of reaction completeness and the removal of chemical oxygen demand (COD) was followed as a check of a common pollution loading parameter used in wastewater treatment performance evaluation. The COD also may be used as a means of measuring the instantaneous current efficiency (ICE) for the anodic oxidation of phenol using the relation (14):
>? C ? C H <4= = @AB DE @AB DF∆D IJ (3-1) KL∆M
-1 where ?4NOCM and ?4NOCMP∆M are the COD (in mg O2 L ) at times / and /+ ∆/ (in s), respectively, and < is the current (mA), Q is the Faraday constant (96487 C mol-1), and
5 is the volume of the electrolyte (L).
3.2 Experimental
3.2.1 Chemicals and Reagents
Sodium chloride and sodium sulfate electrolytes were prepared at constant ionic strength of the equivalent of 50 mM sodium chloride. Phenol (Fisher Scientific), benzoquinone, hydroquinone, catechol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,4- dichlorophenol, 2,6-dichlorophenol, 2,3,6-trichlorophenol, 2,4,5-trichlorophenol, 2,4,6- trichlorophenol and pentachlorophenol (Sigma-Aldrich®) were used as standards for
HPLC analysis. All the standards were prepared in 50 mM sodium chloride (Fisher
Scientific) matrix. HPLC-grade acetonitrile, HPLC-grade methanol and acetic acid were used as organic mobile phase. The aqueous mobile phase used in HPLC analysis and all the dilutions were made using Super-Q water (Millipore).
42
3.2.2 Experimental Setup
As shown in Figure 3-1, the experimental apparatus consisted of a glass reactor (Pyrex® with a working volume of 1L and an overall capacity of 1.25L connected to an electrolyte recirculation system and a scanning potentiostat (PAR Model 362, EG & G Instruments).
The reactor was operated as an undivided electrochemical cell. A Teflon-lined screw top plastic lid with openings to accommodate electrodes was used to cover the reactor. PVC holders were used to maintain the two electrodes at the required height. The anode used was BDD, (Adamant Technologies, Switzerland) with dimensions 5.0 × 2.5 × 0.2 cm.
Stainless steel foil (Alfa Aesar®) with dimensions 5.0 × 2.5 × 0.05 cm was used as cathode. The electrodes were immersed into the electrolyte solution to a depth of 12.2 cm. The distance between two electrodes was 3.4 cm. The reference electrode used was a silver/silver chloride (Ag/AgCl) electrode (BASi). A peristaltic pump (Cole-Parmer®) with Masterflex tubing (Cole-Palmer®) was used for continuous mixing. The constant electrolyte flow rate was constant at 26.5 mL s-1. The reactor was operated at room temperature, 22 0.5℃, for all experiments. Throughout the experiments, pH was not controlled but monitored along with sample collection. The measured pH values ranged from 4.7 to 7.1.
43
Figure 3-1. Experimental set-up showing BDD configuration.
3.2.3 Analytical Methods
An Agilent 1100 HPLC with a diode-array detector (DAD) was applied for in-situ
monitoring of phenol and intermediate samples. A Hypersil ODS column (Thermo
Scientific) 150 mm long by 4.6 mm in diameter was used and controlled at a constant
temperature of 30 °4. The mobile phase consisted of Super-Q water and an organic
mixture (90:5:5 acetonitrile:methanol:acetic acid by volume). A time-variant gradient
was used with an initial percentage of water at 90%, and the final percentage of water at
10% in a run time of 15 min. The flow rate was constant at 0.8 mL min-1 and sample
injection volume was 10 µL. UV detection wavelengths were monitored at 270, 280, 290 and 300 nm. A 2-mL portion of the sample was filtered and analyzed for phenol and intermediates. Samples were taken before current was turned on and then at 18-minute
44
(or multiples of 18) intervals to get immediate measurements. Detailed information on
HPLC method development and validation (Chapter 2) is published elsewhere (14).
The free chlorine in the samples was analyzed using an eXact® micro 7+ Photometer
(ITS) at wavelength of 525 nm in conjunction with eXact® Strip Micro DPD-1 test strips
2-4 mLs of sample were diluted before measurements when necessary. Chlorate and chloride ion were measured with a Dionex LC 20 Ion Chromatograph (IC) using an ION
PAC AS 14 column (4 mm ×250 mm), with an ION PAC AG 14 (4mm ×50 mm) guard column. A TOC analyzer (Shimadzu 5050) was used to quantify TOC degradation.
Chemical oxygen demand (COD) was determined by using HACH COD reagent vials
(low range) with 2 mLs sample heated in a digester block at 150 °4 for two hours followed by absorbance measurements at a wavelength of 420 nm in a HACH DR 2000
Spectrophotometer.
3.2.4 Operating Conditions
All experiments were operated at constant current using a BDD anode. The working surface area of BDD was 10 cm2. The applied current density was 12 mA/cm2. The operational parameters considered were initial concentration of chloride (5 mM vs. 50 mM) and initial concentration of phenol (0.25 mM vs. 0.5 mM).
3.3 Results and Discussion
3.3.1 Effect on Phenol Removal and Intermediates Conversion
Run conditions are listed in Table 3-1, as well as first-order phenol removal rates, chlorate production rates, and TOC and COD removal rates (C-1).
45
Table 3-1. First-order rate constants for phenol removal, chlorate production, and TOC and
COD removal
[NaCl] [Phenol] [NaCl] Phenol Rate Chlorate Rate TOC COD (mM) (mM) /[Phenol] Const (C-1) Const (C-1) Rate Const Rate Const (C-1) (C-1) 5 0.5 10 4.92 × 10EW 1.06 × 10EW 1.42 × 10EW 6.20 × 10EW
5 0.25 20 7.15 × 10EW 6.98 × 10E\ 3.35 × 10E\ 5.59 × 10E^
50 0.5 100 3.78 × 10E^ 5.44 × 10EW 3.77 × 10EW 9.59 × 10EW
50 0.25 200 7.87 × 10E^ 7.15 × 10EW 4.24 × 10EW n.a.
Chloride concentration contributes significantly to the removal of phenol and the rate at which byproducts are generated. It appears that at a higher chloride concentration
(same ionic strength), phenol is removed much faster (Figure 3-2). This is because higher chloride concentration increases current efficiency for formation of hypochlorite, which contributes to faster indirect oxidation of phenol in the bulk solution (15). Figure
3-3 shows that phenol removal rate increases linearly as the chloride-to-phenol ratio increases. This indicates that chloride concentration has a greater impact than phenol concentration on phenol removal rate.
46
1.0
0.25 mM Phenol + 50 mM NaCl 0.8 0.50 mM Phenol + 50 mM NaCl 0.25 mM Phenol + 5 mM NaCl 0.50 mM Phenol + 5 mM NaCl 0.6 C/C0
0.4
0.2
0.0 0 1000 2000 3000 4000 5000 6000 Charge (coulombs)
Figure 3-2. First-order phenol removal rates under different treatment conditions.
0.5 mM Phenol + 5 mM NaCl 0.008 0.25 mM Phenol + 5 mM NaCl 0.5 mM Phenol + 50 mM NaCl 0.25mM Phenol + 50 mM NaCl
0.006
0.004
0.002 PhenolRate (C-1)Const Y = (3.91E-05) X - (1.31E-05) R ^2 = 0.9986
0.000 050100150200 [NaCl]/[Phenol]
Figure 3-3. First-order phenol removal rates are proportional to chloride-to-phenol ratio.
47
Figure 3-4 illustrates the impact of chloride and phenol concentrations on phenol degradation and intermediates conversion. The identified chlorophenol intermediates followed an ortho- and para- directed electrophilic substitution progression, which agrees well with findings reported by others (11, 16) that only para and ortho chlorinated phenols were identified during the electrochemical oxidation of phenol. Chlorination proceeds by a stepwise substitution of the 2,4 and 6 positions and TCP was found to be the final chlorine substitution form, which is in accordance with previous research on chlorination of phenol (17). It is generally thought that hydroxylation is more favored than hypochlorination beyond this point. According to Lee, the more highly chlorinated a phenol compound is, the greater its degree of oxidative rupture over additional chlorine substitution(18).
With high initial chloride concentration (50 mM) (Figure 3-4 a, b), in the early stages,
2-chlorophenol was more dominant than 4-chlorophenol, then 2,4-dichlorophenol was formed more dominantly than 2,6-dichlorophenol, followed by a rapid increase in 2,4,6- trichlorophenol formation when the mono- and dichlorophenol concentrations started to drop. The 2,4,6-trichlorophenol concentration reached a maximum at about 40% and
58% of initial phenol concentration for 0.5 mM and 0.25 mM, respectively. The increase of chlorination level of chlorophenol intermediates implies a hypochlorination pathway.
At a lower initial concentration of chloride (5 mM), as shown in Figure 3-4 (c,d), not only chlorophenols but also quinone intermediates were detected, indicating a combination of two competing pathways, i.e., hydroxylation and hypochlorination. Since
2-chlorophenol and 4-chlorophenol were the dominant species, it seems that hypochlorination pathway was more favored but was limited by the amount of chlorine
48
available. To our knowledge, this is the first time this competition between formation of
intermediates in both pathways has been measured and reported.
Figure 3-4. Phenol and intermediates measured (concentrations normalized with respect to
the initial concentration of phenol) when phenol is treated at 12 mA/cm2 constant current density with BDD anode (a) 0.5 mM phenol in 50 mM sodium chloride, (b) 0.25 mM phenol in 50 mM sodium chloride, (c) 0.5 mM phenol in 5 mM sodium chloride and 15 mM sodium sulfate, and (d) 0.25 mM phenol in 5 mM sodium chloride and 15 mM sodium sulfate
(equivalent ionic strengths).
49
Figure 3-5. Chlorine-containing species measured when phenol concentration is treated when phenol is treated at 12 mA/cm2 constant current with BDD anode (a) 0.5 mM phenol in 50 mM sodium chloride, (b) 0.25 mM phenol in 50 mM sodium chloride, (c) 0.5 mM phenol in
5 mM sodium chloride and 15 mM sodium sulfate, and (d) 0.25 mM phenol in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths).
3.3.2 Effect on Undesirable Byproduct
Figure 3-5 shows the time variation of chlorine-containing species, including organic
(chlorophenols) and inorganic (chloride, free chlorine and chlorate), under different conditions. All the organic intermediates were undetected after 1814.4 coulombs of
50
charge have been applied while at this point free chlorine started to increase significantly
(Figure 3-5 a, b). This implies that all the active chlorine species were being used up for the “chlorine demand” caused by phenol oxidation and degradation of organic intermediates in the early stages. Once chlorophenols were no longer measurable, the
“chlorine demand” of the solution was satisfied. The free chlorine production rate after the breakthrough point (about 1800 coulombs) is nearly linear with charge (Figure 3-5 a, b). Other researchers have reported that “breakpoint chlorination” occurs when the chlorine demand has been met during the chlorination treatment process (7, 19). At the breakthrough point, there was still 50% total organic carbon (TOC) remaining (Figure 3-
7) and about 90% of measurable COD had been removed (Figure 3-8), indicating that hard-to-oxidize compounds were formed and thus reaction with free chlorine was going slowly. We hypothesize that these hard-to-oxidize compounds are part of a nonpassivating polymer thin film based on research reported by Gattrell and Kirk (20,
21). Liquid Chromatography Quadruple Time-of-Flight Mass Spectrometry (LC-QTOF
–MS) was used to investigate this hypothesis, and the results are discussed in more detail in Chapter 6.
No breakthrough of free chlorine was observed with low chloride concentration
(Figure 3-5-c&d), because chloride ion concentration was not high enough to form free chlorine. It is interesting to note, however, that there was an approximate 60% drop in total chloride ion, but the measured organic and inorganic chlorine-containing species only accounted for less than 10%. This indicates that significant amounts of chloride ion were converted to unknown chlorine-containing species, potentially chlorine gas, or other more complex chlorine-substituted aromatic compounds. In addition, 40% of the initial
51
TOC remaining at the end of the experiment (Figure 3-7), supported the hypothesis that chlorine-containing organic compounds were formed but not measured.
These data demonstrate that high chloride concentration is beneficial to achieving high phenol removal efficiency, which may be seen as an advantage, given the fact that chloride concentration is already high in many industrial effluents. For example, chloride concentration in the effluent from large direct discharge meat processing plants is as high as 2000 mg/L (about 56.3 mM) (22). However, there are disadvantages when it comes to electrochemistry in the presence of a high chloride concentration. The reported adverse effect of too high chloride concentration has been that formation of a salt film on the electrode surface hinders both hydroxyl radicals and contaminant adsorption on the surface, thus leading to reduced removal rates of contaminants (23). Moreover, it has been reported that high chloride concentration up to 4500 mg/L (about 126.7 mM) helped decrease energy consumption but failed to improve COD removal rate (2). These authors suggested 2500 mg/L (equivalent of 70.4 mM) chloride concentration is an appropriate amount to both increase COD removal and decrease energy consumption. Finally, studies on electrochemical oxidation of phenol in the presence of chloride have shown that undesirable byproduct formation (chlorates) is higher at higher chloride concentration (6, 24). The data reported here confirm that higher chloride concentration correlates wtih higher chlorate production, as can be seen in Figure 3-5. With 10 times higher chloride concentration, chlorate production was more than 20 times higher for the same passage of coulombs. Therefore, a balance in chloride-to-phenol concentration ratio is needed to achieve high removal efficiency while minimizing the formation of undesirable byproduct.
52
3.3.3 Effect on Removal of TOC and COD
First-order TOC removal rates are listed in Table 3-1. Figure 3-6 shows that TOC removal was more efficient at lower chloride-to-phenol ratios. Figure 3-7 shows comparisons between actual measured TOC and calculated TOC, based on concentrations of measured phenol and intermediate compounds. Two conditions were compared when
0.5 mM phenol was treated in 5 mM or 50 mM NaCl. The differences between actual
TOC and calculated TOC indicate that there were more unidentified compounds produced. The increased difference with increased chloride concentration implies that more unknown intermediates were produced when more chloride was present.
1.0 0.25 mM phenol, 50 mM NaCl 0.50 mM phenol, 50 mM NaCl 0.50 mM phenol, 5 mM NaCl
0.8
0.6
0.4 TOC/TOC0
0.2
0.0 0100020003000400050006000 Charge (coulombs)
Figure 3-6. Comparison of TOC removal.
53
3.5 Act TOC Cal TOC, 0.5 mM Phenol + 5 mM NaCl Act TOC Cal TOC, 0.5 mM Phenol + 50 mM NaCl
3.0
2.5
2.0
1.5
1.0
0.5 TotalOrganic Carbon (mM as C)
0.0 0100020003000400050006000 Charge (coulombs)
Figure 3-7. Actual TOC (solid symbol) vs. Calculated TOC (open symbol) degradation.
Figure 3-8 shows that complete COD removal was reached at the end of experiments under all conditions. Figure 3-9 compared measured COD removal with calculated or theoretical COD (ThCOD). ThCOD was calculated based on the theoretical oxygen demand of phenol and identified intermediate compounds measured by HPLC. It quantifies relative oxidation state of each compound. At higher initial chloride concentration (50 mM), ThCOD agreed well with COD degradation, whereas at lower initial chloride concentration (5 mM), ThCOD was higher than measured COD and the difference increased as passing charge increased. The implication of this discrepancy at the lower chloride concentration is that some of the COD that should be measured is not being captured in the analysis. This is intriguing. Three analyses performed for three separate runs done several months apart all agree, so there does not seem to be an isolated problem with reagents or equipment. HPLC samples were measured immediately and
54
COD samples were preserved with acid, refrigerated in closed brown bottles and analyzed the next day. The seeming agreement at the higher chloride concentration may be an artifact of the conversion of the chlorophenols to “other products” that are not included in the ThCOD computation by definition. It is most likely that “other products” are being formed that are not captured in the COD test and, indeed, the measurement of the COD of chlorophenols may also be in question and require further investigation.
1.0
0.50 mM Phenol + 50 mM NaCl 0.50 mM Phenol + 5 mM NaCl 0.8 0.25 mM Phenol + 5 mM NaCl
0.6
0.4 COD/COD0
0.2
0.0 0100020003000400050006000 Charge (coulombs)
Figure 3-8. Comparison of COD removal.
55
3.5 ActCOD, 5 mM NaCl ActCOD, 50 mM NaCl ThCOD (mM), 50 mM NaCl 3.0 ThCOD (mM), 5 mM NaCl
2.5 ) 2
2.0
1.5
COD (mM as O as (mM COD 1.0
0.5
0.0 0100020003000400050006000 Charge (coulombs)
Figure 3-9. Actual COD (solid symbols) vs. Theoretical COD (open symbols).
3.3.4 Effect on Current Efficiency
Figure 3-10 shows the effect of chloride-to-phenol ratio on the instantaneous current efficiency (ICE). In general, ICE is higher when the chloride-to-phenol ratio is higher and in the early stages of treatment when more readily oxidizable material is present. For example, when 0.5 mM phenol was treated, a current efficiency of about 51% was obtained in the presence of 50 mM chloride at 100% phenol removal after ~2600 coulombs, whereas 38% current efficiency was achieved for the 5 mM chloride case at only 70% phenol removal. Note that ionic strength was kept constant throughout using
56
sodium sulfate to adjust, so this difference in efficiency is attributable to the additional
“oxidizing power” of the chlorine species, not a difference in ionic strength.
1.0
0.25 mM Phenol + 5 mM NaCl 0.8 0.50 mM Phenol + 5 mM NaCl 0.50 mM Phenol + 50 mM NaCl
0.6
ICE 0.4
0.2
0.0 0100020003000400050006000 Charge (coulombs)
Figure 3-10. Impact of chloride-to-phenol ratio on ICE.
3.4 Conclusion
Experiments were carried out at two levels of initial chloride concentration and initial phenol concentration. The effects of different chloride-to-phenol concentration ratios were evaluated in terms of phenol removal efficiency, chlorate formation, TOC removal,
COD removal, and current efficiency. The major conclusions are as follows:
The phenol removal rate increased with increasing chloride-to-phenol ratio. When a higher concentration of chloride ion was present, higher amounts but faster disappearance of intermediates were noted in contrast to the lower chloride case. This is related to the free chlorine generation rate, since higher chloride concentration leads to higher current efficiency for hypochlorite production.
57
A combination of two pathways was found with low chloride concentration (5 mM) and the hypochlorination pathway was more dominant. Hypochlorination pathway was found to be the main pathway under high chloride concentration (50 mM) conditions. In
Chapter 5, competition between pathways will be investigated by examining the rate and distribution of products from both pathways in more detail.
Higher current efficiencies were obtained at higher chloride-to-phenol ratio.
Complete COD removal was apparently achieved by the end of experiments but not complete TOC removal, indicating formation of unknown byproducts that were hard to oxidize.
3.5 References
1. Scialdone, O.; Randazzo, S.; Galia, A.; Silvestri, G. Electrochemical oxidation of organics in water: Role of operative parameters in the absence and in the presence of NaCl. Water Res. 2009, 43 (8), 2260-2272.
2. Rajkumar, D.; Kim, J.G.; Palanivelu, K. Indirect electrochemical oxidation of phenol in the presence of chloride for wastewater treatment. Chemical Engineering and Technology 2005, 28 (1), 98-105.
3. Polcaro, A.M.; Vacca, A.; Palmas, S.; Mascia, M. Electrochemical treatment of wastewater containing phenolic compounds: Oxidation at boron-doped diamond electrodes. J. Appl. Electrochem. 2003, 33 (10), 885-892.
4. Comninellis, C. and Nerini, A. Anodic oxidation of phenol in the presence of NaCl for wastewater treatment. J. Appl. Electrochem. 1995, 25 (1), 23-28.
5. World Health Organization Chlorite and Chlorate in Drinking-water. 2005, WHO/SDE/WSH/05.08/86.
6. Polcaro, A.; Vacca, A.; Mascia, M.; Palmas, S.; Rodiguez Ruiz, J. Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides. J. Appl. Electrochem. 2009.
7. Kraft, A.; Stadelmann, M.; Blaschke, M.; Kreysig, D.; Sandt, B.; Schröder, F.; Rennau, J. Electrochemical water disinfection. Part I: Hypochlorite production from very dilute chloride solutions. J. Appl. Electrochem. 1999, 29 (7), 861-868. 58
8. Weiss, E.; Groenen-Serrano, K.; Savall, A. A comparison of electrochemical degradation of phenol on boron doped diamond and lead dioxide anodes. J. Appl. Electrochem. 38 (3), 329-337.
9. Iniesta, J.; Michaud, P.A.; Panizza, M.; Cerisola, G.; Aldaz, A.; Comninellis, C. Electrochemical oxidation of phenol at boron-doped diamond electrode. Electrochimica Acta 2001, 46 (23), 3573-3578.
10. Nasr, B.; Hsen, T.; Abdellatif, G. Electrochemical treatment of aqueous wastes containing pyrogallol by BDD-anodic oxidation. J. Environ. Manage. 90 (1), 523-530.
11. Körbahti, B.K. and Tanyolaç, A. Kinetic modeling of conversion products in the electrochemical treatment of phenolic wastewater with a NaCl electrolyte. Ind. Eng. Chem. Res 2003, 42 (21), 5060-5065.
12. Saylor, G.L.; Chen, L.; Kupferle, M.J. Using toxicity testing to evaluate electrochemical reactor operations. Environmental Toxicology and Chemistry 2012, 31 (3), 494-500.
13. Sathish, M. and Viswanath, R. Electrochemical degradation of aqueous phenols using graphite electrode in a divided electrolytic cell. Korean Journal of Chemical Engineering 2005, 22 (3), 358-363.
14. Comninellis, C. and Pulgarin, C. Anodic oxidation of phenol for waste water treatment. J. Appl. Electrochem. 1991, 21 (8), 703-708.
15. Iniesta, J.; González-Garcıa,́ J.; Expósito, E.; Montiel, V.; Aldaz, A. Influence of chloride ion on electrochemical degradation of phenol in alkaline medium using bismuth doped and pure PbO2 anodes. Water Res. 2001, 35 (14), 3291-3300.
16. Deborde, M. and von Gunten, U. Reactions of chlorine with inorganic and organic compounds during water treatment-Kinetics and mechanisms: A critical review. Water Res. 2008, 42 (1-2), 13-51.
17. Ge, F.; Zhu, L.; Wang, J. Distribution of chlorination products of phenols under various pHs in water disinfection. Desalination 2008, 225 (1-3), 156-166.
18. Lee, G.F. Kinetics of chlorination of phenol-chlorophenolic tastes and odors. Air 1966, 6, 419.
19. Kraft, A.; Blaschke, M.; Kreysig, D.; Sandt, B.; Schröder, F.; Rennau, J. Electrochemical water disinfection. Part II: Hypochlorite production from potable water, chlorine consumption and the problem of calcareous deposits. J. Appl. Electrochem. 1999, 29 (8), 895-902.
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20. Gattrell, M. and Kirk, D.W. Study of the oxidation of phenol at platinum and preoxidized platinum surfaces. J. Electrochem. Soc. 1993, 140 (6), 1534-1540.
21. Gattrell, M. and Kirk, D.W. Fourier transform infrared spectroscopy study of the passive film produced during aqueous acidic phenol electro-oxidation. J. Electrochem. Soc. 1992, 139 (10), 2736-2744.
22. Bodoh, M.J. Reduction of chloride in wastewater Effluent with utilization of Six Sigma. MS Thesis. 2006.
23. Zhao, C.; Feng, C.; Hou, L. Degradation of phenol in the presence of ammonia by electrochemical oxidation in the chloride system. 3rd International Conference on Bioinformatics and Biomedical Engineering, iCBBE, 2009.
24. Polcaro, A.; Vacca, A.; Mascia, M.; Ferrara, F. Product and by-product formation in electrolysis of dilute chloride solutions. J. Appl. Electrochem. 2008, 38 (7), 979-984.
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Chapter 4 Electrochemical Oxidation of Organic Waste:
Statistical Studies and System Optimization
4.1 Introduction
Electrochemical oxidation in water and wastewater treatment has recently attracted attention among researchers around the world (1). It is also common to combine electrochemical pretreatment with other treatment processes such as flocculation or biological treatment in order to achieve better treatment results (2, 3). The electrochemical treatment method requires little or no addition of chemicals and has been reported to achieve high removal efficiencies for organic contaminants that are hard to treat by using traditional biological water treatment methods alone (4). Various types of electrode materials such as graphite (5, 6), glassy carbon (7), Ti/RuO2 (8), platinum (9), boron-doped diamond (10-14) have been widely investigated. Despite a large amount of data that has been reported on electrochemical oxidation treatment efficiencies under various operational conditions, there have not been many studies looking at statistical significance of operational parameters or optimization (15).
The aim of the research reported herein was to use phenol as a target compound to systematically study electrochemical oxidation regarding the effects of various operational conditions (anode material, initial concentration of phenol, initial concentration of chloride and applied current density) on phenol removal rate, chlorate
61
production rate, total organic carbon (TOC) removal rate and chemical oxygen demand
(COD) removal rate. A full 24 factorial design was applied to this work so as to statistically determine the significance of the various factors on the treatment process.
Response surface methodology (RSM) was employed to further investigate the optimal operational conditions for the maximum treatment efficiency and the lowest byproduct formation among the conditions studied.
4.1.1 Identifying Operational Parameters
Operational parameters considered in this paper are listed in Table 1. The parameters were determined based on studies published in the literature and previous work in the authors’ group (1, 16-23). Anode material has a significant impact on the products because of the different surface structures of anodes used (graphite vs. BDD). The difference of oxygen evolution overpotentials between two materials results in different chemistry on the anode surface and side-reactions, which leads to different reaction pathways during advanced oxidation.
Electrolyte composition is an important factor because it provides conductivity for electrolysis to take place. Reactive electrolytes, for example those containing Cl-, may contribute to oxidation as well, leading to a varying reaction pathway and production of undesirable byproducts. NaCl was chosen as the reactive electrolyte in this study. Non- reactive electrolytes (without a reactive ion), do not contribute to oxidation. In this case,
Na2SO4 was chosen because it is commonly used in the literature. Initial phenol and chloride concentration were considered because varying concentrations of both pollutant and electrolyte are factors in industrial waste streams and the chloride-to-waste ratio may control the types of reactions that predonminate. The two levels of chloride
62
concentration values (5 vs. 50 mM) chosen differ by one order of magnitude and represent values of chloride levels in drinking water and the lower end of concentrations found in industrial wastewater, respectively (24). Applied current density was considered since current density controls the rate of electron supply to the surface of the anode, controlling the rates of the surface reactions relative to reaction rates of hypochlorite in the bulk solution.
4.1.2 Analysis of Variance (ANOVA)
Each of the above-mentioned variables is expected to independently impact treatment; response factors that were considered as measures of treatment status included phenol removal rate, chlorate (undesirable byproduct) generation rate, TOC removal rate and
COD removal rate (see Table 4-1). The interaction of the independent variables also may also impact the response factors. Therefore, analysis of variance (ANOVA) was applied as a statistical approach in this work. ANOVA studies were carried out using OriginPro
9 (OriginLab). Each of the four independent variables received two levels of values, as shown in Table 4-2. The raw dataset was based on results from a 24 full factorial design containing 16 experiments with detailed experimental conditions shown in Table 4-3. All the calculations are based on the calculated rate constants, k, as the dependent variable at
95% confidence limit. The probability level, P is compared to 0.05 (95% confidence limit) to evaluate the main effect of operational parameters on k. If the P value is less than 0.05, then it is concluded that the main effect is significant with a 95% confidence limit.
63
Table 4-1. Factors and dependent variables studied in ANOVA
Factor (X) Dependent Vaiable (Y)
Anode Material (X1) Phenol Reaction Rate (Y1)
Initial Phenol Concentration (X2) Chlorate Reaction Rate (Y2)
Initial Chloride Concentration (X3) TOC Removal Rate (Y3)
Applied Current Density (X4) COD Removal Rate (Y4)
Free Chlorine Rate after
Breakthrough (Y5)
Table 4-2. Independent Variables of the 24 Factorial Design of Experiments
Level X1 X2 X3 X4
(mM) (mM) (mA/cm2) 1 (−) BDD 0.25 5 12
2 (+) Graphite 0.5 50 25
64
Table 4-3. Experimental Conditions for the 24 Factorial Design
Run No. Anode Type [Phenol] [NaCl] Current Remarks X X X Density 1 2 3 X (mM) (mM) 4 (mA/cm2) B1 B 0.25 5 12
B2 B 0.25 5 25
B3 B 0.25 50 12 2 duplicates
B4 B 0.25 50 25
B5 B 0.5 5 12 3 duplicates
B6 B 0.5 5 25
B7 B 0.5 50 12 3 duplicates
B8 B 0.5 50 25
G1 G 0.25 5 12
G2 G 0.25 5 25
G3 G 0.25 50 12
G4 G 0.25 50 25
G5 G 0.5 5 12
G6 G 0.5 5 25
G7 G 0.5 50 12 4 duplicates
G8 G 0.5 50 25
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4.1.3 Response Surface Methodology (Optimization)
Response surface methodology (RSM) was first introduced by Box and Wilson in the
1950s (25). In the half a century since then, RSM has been developed and widely used as an important tool in modeling wastewater treatment processes from various industries, such as pharmaceutical (26), textile and dye (27), pulp and paper (28) industries (29).
RSM consists of a combination of mathematical and statistical techniques used for optimizing the processes and evaluating the significance of relative factors. The first step of RSM is to select an appropriate experimental design. There are experimental design techniques available in RSM, including full factorial designs, Box-Behnken designs
(BBD) and central composite design (CCD) (30). Recently, several studies have used
RSM for optimization of process parameters, such as temperature, reaction time, pH and so on (30). For this paper, a 23 full factorial design RSM was used to determine the optimal operational conditions that result in maximum phenol removal rate and minimum chlorate production rate for a given anode type. The data set was divided into two sets by anode type because the RSM method requires use of numerical variables and because this represents the physical constraints of a system as well.
The statistical program “R” (R Development Core Team, 2013) was used for this analysis. In its current form, the RSM method in R only covers optimization for one response variable (31). Therefore, two responses variables, i.e., phenol removal rate and chlorate production rate, were studied separately in this study.
The coded values of the process parameters were determined by equation ( 1):
b`a_daP`a_efg: `aE 3 8_ = (4-1) b`a_daE`a_efg: 3
66
where, 8_ is the coded value of the 0th variable; h_ is the uncoded value of the variable; h__ij and h__k_ are the uncoded values of the 0th variable at the low and high levels, respectively. For this study, the low value for phenol concentration was 0.25 mM and the high value was 0.50 mM, the low value for chloride was 5 mM and the high was
50 mM, and the low applied current density was 12 mA/cm2 and the high was 25 mA/cm2.
An empirical first-order polynomial regression was applied as in equation (4-2): l=mn +mo8o +m383 +m^8^ +mo38o83 +m3^838^ +mo^8^8o +mo3^8o838^ +p (4-2)
where, l is the response (lo is the resonse for phenol removal rate constants, l3 is the response for chlorate production rate); 8o,83 and 8^ are the coded values of independent variables; mn is the intercept coefficient, mo , m3
,m^ , mo3,m3^,mo^ and mo3^are regression coefficients; and p is the error.
Prior to fit into a regression model, the above-mentioned effects are estimated to assess which parameters are significant. This can be achieved by running ANOVA, where F value and P value are used to determine the significance of parameters effects.
4.2 Materials and Methods
4.2.1 Experimental Setup
As shown in Figure 3-1, the experimental apparatus consisted of a glass reactor unit with a working volume of 1L and an overall capacity of 1250 cm3 connected with an electrolyte recirculation system and a scanning potentiostat (PAR Model 362, EG & G
Instruments). The reactor was operated as an undivided electrochemical cell. The anodes used were either a graphite plate or a boron-doped diamond plate (BDD). The
67
graphite plate (GraphiteStore) has dimensions of 5.0 × 2.5 × 0.6 cm. BDD is a p-doped mono-crystalline silicon substrate with boron-doped diamond (Adamant Technologies,
Switzerland) coating (2-3 µm) with dimensions 5.0 × 2.5 × 0.2 cm. A stainless steel foil
(Alfa Aesar) with dimensions 5.0 × 2.5 × 0.05 cm was used as the cathode. The distance between two electrodes was 3.4 cm. The reference electrode used was a silver/silver chloride (Ag/AgCl) electrode. A Teflon-lined screw top plastic lid with openings to accommodate electrodes was used to cover the reactor. PVC holders were used to maintain the two electrodes at the required height. The electrodes were immersed into the electrolyte solution to a depth of 12.2 cm. A peristaltic pump (Cole-Parmer) with
Masterflex tubing (Cole-Palmer) was used for mixing via recirculation of the contents of the reactor. The reactor was operated at room temperature, 22 ± 0.5 ℃, for all experiments. The pH was not buffered but was monitored throughout the experiments, and it ranged between 4.7 - 7.1.
Unless otherwise specified, all chemicals were purchased from Fisher Scientific, including phenol. In all experiments, the ionic strength of electrolytes was kept constant at 50 mM by using either sodium chloride only or combination of sodium chloride and sodium sulfate. Benzoquinone, hydroquinone, catechol, 2-chlorophenol, 3-chlorophenol,
4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 2,3,6-trichlorophenol, 2,4,5- trichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol were purchased from
Sigma-Aldrich for use as standards for HPLC analysis. All the electrolytes and HPLC standards were prepared in Super-Q water (Millipore).
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4.2.2 Analytical Measurements
A high-performance liquid chromatograph (HPLC Agilent 1100) with a diode-array detector (DAD) was applied for in-situ monitoring of phenol and intermediate samples.
A Hypersil ODS column (150 mm × 4.6 mm = L × I.D.) packed with 3µm particles
(Thermo Scientific) was used. Mobile phases were 0.1% formic acid in Super-Q water
(pH was adjusted to 3 with sulfuric acid) and an organic mixture of acetonitrile, acetic acid and methanol in volumetric ratio of 90:5:5. A gradient was used to change the percentage of aqueous phase and organic phase with time. Mobile phase flow rate was set at 0.8 mL/min and UV detection wavelengths were monitored at 270, 280, 290 and
300 nm. Thermostat temperature was controlled at 30 ℃ throughout the sequences. A
2-mL portion of the sample was filtered and analyzed for phenol and intermediates.
Samples were taken before current was turned on and then at 18-minute (or multiples of
18) intervals to get immediate measurements. The free chlorine in the samples was analyzed by using an eXact® micro 7+ Photometer (ITS) at wavelength of 525 nm with eXact® Strip Micro DPD-1 test strips, diluting 2-4 mLs of sample before measurements when necessary for the range. Chlorate and chloride ion were measured with Dionex LC
20 Ion Chromatography (IC) using an ION PAC AS 14 column (4mm × 250 mm), with a guard column ION PAC AG 14 (4mm × 50 mm). A TOC analyzer (Shimadzu 5050) was used to measure total organic carbon (TC) and inorganic carbon (IC) to determine the total organic carbon (TOC) by difference. TOC and IC measurements along with data from HPLC analysis, were used to develop a carbon mass balance to estimate the amounts of intermediates that are not detected by the HPLC. Chemical oxygen demand
(COD) was determined by using a HACH DR 2000 Spectrophotometer with HACH low
69
range COD reagent vials. Sample (2 mL) was immediately added to a HACH COD reagent vial then at the end of the run, all vials in for samples in a run were heated in digester block at 150 °4 for two hours. After cooling down, the vials were analyzed in the HACH spectrophotometer at wavelength 420 nm.
4.3 Results and Discussion
4.3.1 Analysis of Variance
Tables 4-4 shows one-way ANOVA results of individual factors that have significant effects at 95% confidence level. It shows that anode material (ho) has a significant effect on phenol removal rate (Y1), TOC removal rate (Y3) and free chlorine breakthrough (Y5). In general, phenol removal rate is higher with BDD than graphite.
This can be attributed to the higher stability, higher oxidation power and weaker adsorption for chemicals of BDD versus graphite. No free chlorine breakthrough was observed when using graphite or at low chloride concentrations (5 mM) whereas at the higher chloride concentration (50 mM) with a BDD anode, free chlorine started to increase linearly once the measured chlorophenol intermediates were all removed. This indicates that the reactions of free chlorine generation and indirect oxidation were in competition and that free chlorine was consumed as soon as it was generated in the former cases. Table 4-4 also shows that the chloride concentration (X3) has significant impact on phenol removal rate (Y1), chlorate production rate (Y2) and free chlorine generation rate after breakthrough (Y5). Values for these three response factors increase as chloride concentration increases. Figure 4-1 shows plots of the above-mentioned significantly different factors from one-way ANOVA results.
70
Table 4-4. One-way ANOVA Results
X1Y1 DF Sum of Squares Mean Square F Value Prob>F
Model 1 3.87279E-5 3.87279E-5 7.29831 0.01246
Error 24 1.27354E-4 5.30642E-6
Total 25 1.66082E-4
X1Y5 DF Sum of Squares Mean Square F Value Prob>F
Model 1 1.09712E-4 1.09712E-4 10.01183 0.00419
Error 24 2.62998E-4 1.09582E-5
Total 25 3.7271E-4
X3Y1 DF Sum of Squares Mean Square F Value Prob>F
Model 1 6.03216E-5 6.03216E-5 13.68866 0.00112
Error 24 1.0576E-4 4.40668E-6
Total 25 1.66082E-4
X3Y2 DF Sum of Squares Mean Square F Value Prob>F
Model 1 1.0067E-6 1.0067E-6 36.43343 3.722E-6
Error 23 6.3552E-7 2.76313E-8
Total 24 1.64222E-6
X3Y5 DF Sum of Squares Mean Square F Value Prob>F
Model 1 1.11056E-4 1.11056E-4 10.1865 0.00392
Error 24 2.61654E-4 1.09023E-5
Total 25 3.7271E-4
71
) -1
0.006 0.006
0.004 0.004
0.002 0.002
Mean Phenol Removal Rate Constant (C 0.000 ) 0.000 -1 0.010 0.010
0.008 0.008
0.006 0.006
0.004 0.004
No breakthrough No breakthrough 0.002 0.002
0.000
0.000 ) -1 B G 0.0010
X1: Anode Type Mean Free Chlorine Rate After Breakthrough (min Breakthrough After Rate Chlorine Free Mean 0.0008
0.0006
0.0004
0.0002
0.0000
Mean Chlorate Reaction Rate Constant (C 5 50 X : Chloride Concentration (mM) 3
Figure 4-1. One-way ANOVA plots with significant effects.
72
Table 4-5. Two-way ANOVA Results
Factors Dependent F Value P Value Remarks
Variables
X1 · X2 Y1 0.55828 0.46287 Not significant
X1 · X2 Y2 0.05918 0.81016 Not significant
X1 · X2 Y3 0.53889 0.47593 Not significant
X1 · X2 Y4 0.72321 0.41174 Not significant
X1 · X2 Y5 2.8604E-4 0.98666 Not significant
X1 · X3 Y1 23.37532 4.92373E-7 Significant
X1 · X3 Y2 2.86786 0.11614 Not significant
X1 · X3 Y3 n.a. n.a. n.a.
X1 · X3 Y4 2.86786 0.11614 Not significant
X1 · X3 Y5 22.18046 1.0667E-4 Significant
X1 · X4 Y1 2.40534 0.13519 Not significant
X1 · X4 Y2 0.00248 0.96073 Not significant
X1 · X4 Y3 0.43535 0.52089 Not significant
X1 · X4 Y4 2.79519 0.1204 Not significant
X1 · X4 Y5 3.77538 0.02517 Significant
X2 · X3 Y1 2.40534 0.13519 Not significant
X2 · X3 Y2 0.20997 0.65149 Not significant
X2 · X3 Y3 n.a. n.a. n.a.
X2 · X3 Y4 0.00202 0.96487 Not significant
X2 · X3 Y5 0.23874 0.62995 Not significant
73
X2 · X4 Y1 0.12999 0.72189 Not significant
X2 · X4 Y2 0.02758 0.86969 Not significant
X2 · X4 Y3 0.02215 0.88397 Not significant
X2 · X4 Y4 0.07961 0.78263 Not significant
X2 · X4 Y5 0.03028 0.86344 Not significant
X3 · X4 Y1 0.07578 0.78567 Not significant
X3 · X4 Y2 0.01255 0.91186 Not significant
X3 · X4 Y3 n.a. n.a. n.a.
X3 · X4 Y4 0.00662 0.9365 Not significant
X3 · X4 Y5 1.03747 0.31948 Not significant
Tables 4-5 shows two-way ANOVA results for interactive effects between independent variables on the five dependent response factors. It turns out that only interactions of anode type and chloride concentration have significant effects, on either phenol removal rate or free chlorine breakthrough rate.
4.3.2 Surface Response Methodology
To look at optimization of phenol and chloride concentrations for a given reactor configuration, i.e., for either a BBD or graphite anode, a separate 23 full factorial design analysis was done for each case using “R Commander,” a graphical user interface (GUI) for the R programming language developed by John Fox (32). The plug-in package
“RcmdrPlugin.DoE” used for this data analysis was developed by Ulrike Grömping (33).
74
A disadvantage of the factorial 23 design is that it does not have a center point and does not provide the ability to do a model inadequacy check (34).
The BDD case is shown below as an example of the method. The same analysis was conducted for graphite; the detailed data for the graphite case can be found in Appendix
E. The design matrix used for the BDD analysis is shown in Table 4-6, the high values of the two levels are represented by a “+” sign and the low values are indicated by a
“−”sign. The R script for this analysis is provided in Appendix E.
3 Table 4-6. Design matrix of the 2 factorial design (BDD runs) and response factors Y1, Y2
Run [Phenol]/ [Chloride]/ Current Phenol Chlorate Order mM mM Density Rate Rate /mAcm-2 Const/C-1 Const/C-1
X2 X3 X4 Y1 Y2 1 − − − 0.00072 0.00007 2 − − + 0.00037 0.00005 3 − + − 0.00737 0.00004 4 − + − 0.00846 0.00072 5 − + + 0.00669 0.00079 6 + − − 0.00053 0.00005 7 + − + 0.00025 0.00005 8 + + − 0.0038 0.00058 9 + + − 0.00378 0.00055 10 + + − 0.00382 0.00072 11 + + + 0.00289 0.00036 12 + + − 0.00745 0.000372 13 + − − 0.000478 0.000122 14 + + − 0.00745 0.000423 15 + + − 0.00385 0.000620 16 − + − 0.000478 0.000122 17 − + − 0.000471 0.000142
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The full factorial design factors were u, phenol concentration; v, chloride
concentration and 4, applied current density. The goal of design was to achieve simple
models that with the most statistically significant factors.
Prior to the analysis, a half-normal plot was used to determine the significant effects
(Figure 4-2). The important effects for the BDD analysis were phenol concentration (uC,
chloride concentration (vC and the uv interaction. They are distinctly apart from the
remaining ones on the plot (Figure 4-2). The inspection of interaction graph indicates
that changes in chloride concentration produces a much larger change in phenol removal
rate at low phenol concentration than at high phenol concentration (Figure 4-3). The
analysis of variance is summarized in Table 4-7. Based on the w-values shown in the
table, we concluded that for the BDD runs, phenol removal rate (lo), phenol
concentration (u) and chloride concentration (vC are statistically significant effects, so is
the phenol-chloride concentration interaction (uv). Whereas, chloride concentration (v) was found to be the only statistically significant factor for chlorate production rate (l3).
76
* B
* A
* AB half-normal scores *
0.5* 1.0 1.5
*
*
0.000 0.001 0.002 0.003 0.004 0.005 absolute effects A = PhenolConc , B = ChlorideConc , CC= = Current CurrentDensity
Figure 4-2. Half-normal probability plot of effects on response Y1 (phenol removal rate) with
BDD runs.
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Figure 4-3. Chloride - phenol concentration (AB) interaction graph for BDD runs.
The linear regression models for lo and l3, based on the analysis, are
lo = 0.0028275 – 0.000968o + 0.0023683 – 0.00088258o83 (4-3) l3 = 0.0002488 + 0.000248883 (4-4)
where the variables 8o and 83 represent the coded values for independent variables design factors u and v, respectively.
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Figure 4-4. (a) Response surface plot of phenol removal rate; (b) The contour plot. x1 is the coded variable for phenol concentration, x2 is the coded variable for chloride
concentration.
Figure 4-4 shows a response surface plot and contour plot of phenol removal rate as a function of u and v. Both the uv interaction plot and surface plot indicate that phenol removal rate increases as u decreases and v increases, and the best phenol removal rate
79
would be obtained with low level of phenol concentration and high level of chloride concentration.
Table 4-7. ANOVA for the two response factors Y1 and Y2 (BDD runs)
Variables Response: Y1
Sum of Squares Degrees of Freedom F Value Pr (> | t |)
A 7.373E-06 1 39.548 0.00326
B 4.4557E-05 1 239.007 0.0001022
AB 6.23E-06 1 33.421 0.0044473
Residuals 7.46E-07 4
Variables Response: Y2
Sum of Squares Degrees of Freedom F Value Pr (> | t |)
B 3.0031E-07 1 5.8356 0.005217
Residuals 3.0878E-07 6
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* B
*
*
* half-normalscores
0.5 1.0* 1.5 * *
1e-04 2e-04 3e-04 4e-04 5e-04 absolute effects A = PhenolConcentration , B = ChlorideConcentration , CC= = CurrentDensityCurrent
3 Figure 4-5. Half-normal plot of the factor effects from the 2 factorial for response Y2
(chlorate production rate) with BDD runs.
For runs with graphite, from the half-normal plot, only factor B was far from the other remaining points. Therefore, the linear regression model for response l3 is: l3 = 0.000214+ 0.0002125 83 (4-5)
where 83 is the coded values for independent variables that represent factor v, initial chloride concentration (mM). This linear regression model indicates that chlorate production rate increases as chloride concentration increases.
Table 4-8. ANOVA for response Y2 (Graphite runs)
Variables Response: Y2
Sum of Squares Degrees of F Value Pr (> | t |)
Freedom
B 3.6125E-07 1 22.219 0.003279
Residuals 9.755E-08 6
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4.4 Conclusion
The electrochemical oxidation treatment efficiency of phenol with respect to phenol removal rate and chlorate production rate was evaluated, considering the effect of various operational conditions in terms of anode type, initial phenol concentration, initial chloride concentration and applied current density. A 24 factorial design was used and ANOVA was applied. Response surface methodology was used to determine the optimal operational conditions that result in maximum phenol removal rate and minimum chlorate production rate optimize the conditions. The major conclusions are as follows:
(1) Anode type and chloride concentration were the most significant factors either individually or interactively, on phenol removal and free chlorine breakthrough.
(2) Chlorate production rate depended on chloride concentration (rate constant is one order of magnitude higher at higher chloride concentration).
(3) Free chlorine breakthrough rate after “breakpoint chlorination” was dependent on applied current density if free chlorine breakthrough occurred with excess chloride and
BDD.
(4) Linear regression models were developed for experimental runs with a given reactor configuration, either with BDD or graphite anode. Results showed that initial phenol concentration and chloride concentration were significant to treatment efficiency with
BDD, whereas chloride concentration was the only significant factor on runs with graphite. Taken together, the response variable analyses for response variables lo and l3 show that the control strategies are at odds with each other. According to RSM results, phenol removal rate reached the highest at low phenol concentration and high chloride
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concentration, in the range studied, whereas low chloride concentration produced less chlorate.
4.5 References
1. Chen, G.G. Electrochemical wastewater treatment processes in advanced physicochemical treatment technologies. Humana Press: Totowa, NJ, 2007; Vol.5 pp. 57- 106.
2. Walsh, F. and Electrochemical Consultancy A first course in electrochemical engineering. Electrochemical Consultancy: Romsey, 1993; pp. 381.
3. Chen, G. Electrochemical technologies in wastewater treatment. Separation and purification technology 2004, 38 (1), 11.
4. Martínez-Huitle, C.A. and Ferro, S. Electrochemical oxidation of organic pollutants for the wastewater treatment: Direct and indirect processes. Chem. Soc. Rev. 2006, 35 (12), 1324-1340.
5. Sathish, M. and Viswanath, R. Electrochemical degradation of aqueous phenols using graphite electrode in a divided electrolytic cell. Korean Journal of Chemical Engineering 2005, 22 (3), 358-363.
6. Janssen, L.J.J. and Hoogland, J.G. The electrolysis of an acidic NaCl solution with a graphite anode-III. Mechanism of chlorine evolution. Electrochim. Acta 1970, 15 (6), 941-951.
7. Narmadha, M.; Noel, M.; Suryanarayanan, V. Relative deactivation of boron-doped diamond (BDD) and glassy carbon (GC) electrodes in different electrolyte media containing substituted phenols – Voltammetric and surface morphologic studies. J Electroanal Chem 2011, 655 (2), 103-110.
8. Polcaro, A.; Vacca, A.; Mascia, M.; Ferrara, F. Product and by-product formation in electrolysis of dilute chloride solutions. J. Appl. Electrochem. 2008, 38 (7), 979-984.
9. Gattrell, M. and Kirk, D.W. Study of the oxidation of phenol at platinum and preoxidized platinum surfaces. J. Electrochem. Soc. 1993, 140 (6), 1534-1540.
10. Mascia, M.; Vacca, A.; Polcaro, A.M.; Palmas, S.; Ruiz, J.R.; Da Pozzo, A. Electrochemical treatment of phenolic waters in presence of chloride with boron-doped diamond (BDD) anodes: Experimental study and mathematical model. J. Hazard. Mater. 2010, 174 (1–3), 314-322.
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11. Bergmann, M.E.H.; Rollin, J.; Iourtchouk, T. The occurrence of perchlorate during drinking water electrolysis using BDD anodes. Electrochim. Acta 2009, 54 (7), 2102- 2107.
12. Polcaro, A.; Vacca, A.; Mascia, M.; Palmas, S.; Rodiguez Ruiz, J. Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides. J. Appl. Electrochem. 2009.
13. Schmalz, V.; Dittmar, T.; Haaken, D.; Worch, E. Electrochemical disinfection of biologically treated wastewater from small treatment systems by using boron-doped diamond (BDD) electrodes – Contribution for direct reuse of domestic wastewater. Water Res. 2009, 43 (20), 5260-5266.
14. Palmas, S.; Polcaro, A.M.; Vacca, A.; Mascia, M.; Ferrara, F. Influence of the operating conditions on the electrochemical disinfection process of natural waters at BDD electrodes. J. Appl. Electrochem. 2007, 37 (11).
15. Chatzisymeon, E.; Xekoukoulotakis, N.P.; Diamadopoulos, E.; Katsaounis, A.; Mantzavinos, D. Boron-doped diamond anodic treatment of olive mill wastewaters: Statistical analysis, kinetic modeling and biodegradability. Water Res. 2009, 43 (16), 3999-4009.
16. Comninellis, C. and Nerini, A. Anodic oxidation of phenol in the presence of NaCl for wastewater treatment. J. Appl. Electrochem. 1995, 25 (1), 23-28.
17. Iniesta, J.; Michaud, P.A.; Panizza, M.; Cerisola, G.; Aldaz, A.; Comninellis, C. Electrochemical oxidation of phenol at boron-doped diamond electrode. Electrochimica Acta 2001, 46 (23), 3573-3578.
18. Pera-Titus, M.; Garcıá -Molina, V.; Baños, M.A.; Giménez, J.; Esplugas, S. Degradation of chlorophenols by means of advanced oxidation processes: a general review. Applied Catalysis B, Environmental 2004, 47 (4), 219-256.
19. Kapałka, A.; Fóti, G.; Comninellis, C. The importance of electrode material in environmental electrochemistry. Electrochim. Acta 2007, 54 (7), 2018-2023.
20. Kapałka, A.; Fóti, G.; Comninellis, C. Kinetic modelling of the electrochemical mineralization of organic pollutants for wastewater treatment. J. Appl. Electrochem. 2007, 38 (1), 7-16.
21. Panizza, M.; Michaud, P.A.; Cerisola, G.; Comninellis, C. Anodic oxidation of 2- naphthol at boron-doped diamond electrodes. J Electroanal Chem 2001, 507 (1-2), 206- 214.
22. Comninellis, C. and Chen, G. Ebook: Electrochemistry for the environment. 2010.
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23. Sundaram, V. Operation and design impacts on efficiency and toxicity during electrochemical treatment of azo dye-containing wastewater. Master of Science Thesis. University of Cincinnati, 2005, https://etd.ohiolink.edu/.
24. Jung, Y.J.; Baek, K.W.; Oh, B.S.; Kang, J. An investigation of the formation of chlorate and perchlorate during electrolysis using Pt/Ti electrodes: The effects of pH and reactive oxygen species and the results of kinetic studies. Water Res. 2010, 44 (18), 5345- 5355.
25. Myers, R.H.; Montgomery, D.C.; Geoffrey Vining, G.; Borror, C.M.; Kowalski, S.M. Response Surface Methodology: A Retrospective and Literature Survey. Journal of Quality Technology 2004, 36 (1), 53-78.
26. Dhuria, R.S.; Bhatti, R.; Bhatti, M.S.; Singh, P.; Whitcomb, P.J.; Thukral, A.K. Experimental design optimization for electrochemical removal of gentamicin: toxicity evaluation and degradation pathway. Water Science & Technology 2013, 67 (9), 2017.
27. Zarei, M.; Niaei, A.; Salari, D.; Khataee, A. Application of response surface methodology for optimization of peroxi-coagulation of textile dye solution using carbon nanotube-PTFE cathode. J. Hazard. Mater. 2010, 173 (1-3), 544-551.
28. Sridhar, R.; Sivakumar, V.; Immanuel, V.P.; Maran, J.P. Development of model for treatment of pulp and paper industry bleaching effluent using response surface methodology. Environmental Progress and Sustainable Energy 2012, 31 (4), 558-565.
29. Tir, M. and Moulai-Mostefa, N. Optimization of oil removal from oily wastewater by electrocoagulation using response surface method. J. Hazard. Mater. 2008, 158 (1), 107- 115.
30. Sakkas, V.A.; Islam, M.A.; Stalikas, C.; Albanis, T.A. Photocatalytic degradation using design of experiments: A review and example of the Congo red degradation. J. Hazard. Mater. 2010, 175 (1–3), 33-44.
31. Lenth, R.V. Response-surface methods in R, using RSM. J. Stat. Software 2009, 32 (7), 1-17.
32. Fox, J. The R Commander: A basic-statistics graphical user interface to R. Journal of Statistical Software 2005, 14.
33. Ulrike Grömping, F.J. R Commander Plugin for (industrial) Design of Experiments. August 29, 2013, .
34. Myers, R.H.; Montgomery, D.C.; Christine M. Anderson-Cook Response surface methodology: process and product optimization using designed experiments. Wiley: Hoboken, N.J., 2009; pp. 680.
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Chapter 5 Kinetics of the Electrochemical Oxidation of
Phenol and Intermediates in the Presence of Chloride under
Different Reaction Conditions
5.1 Introduction
Many researchers have investigated the oxidation of phenol with non-reactive electrolyte,
2- - such as sulfate (SO4 ), perchlorate (ClO4 ), and there are also many studies that have used chloride ion as well (1-6). Chloride has both positive and negative effects in electrochemical oxidation treatment. Using chloride ion as the electrolyte has been shown to increase removal rate of contaminants due to generation of reactive oxidation agents, i.e., active chlorine species, which add additional oxidizing power. However, at the same time, chloride also creates opportunity for side-reactions during oxidation that
- - result in undesirable byproducts, such as chlorate (ClO3 ) and perchlorate (ClO4 ) (4).
It has been confirmed that during electrooxidation of phenol, reaction intermediate pathways fall into two categories: hydroxylation and hypochlorination pathways (1, 2) as shown in Figure 5-1. While each pathway has been studied independently, very little research has examined the comparison/competition between these pathways with respect to electrolyte composition in the presence (or absence) of chloride.
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In this paper, the phenol and reaction intermediates were measured in real time for both pathways under different reaction conditions and a mathematical model was developed to explore the impact of the reaction conditions on the rates of conversion between organic species with the goal of identifying the rate limiting steps. The intermediate conversions were also compared with mineralization (formation of CO2).
The formation of chlorate was also examined, but was not included in the modeling at this time.
5.2 Experimental Section
5.2.1 Chemicals and Materials
All chemicals were purchased from Sigma-Aldrich. All HPLC standards were prepared in
Super-Q water. In all the experiment conditions examined in this paper, a p-doped mono- crystalline silicon substrate with boron-doped diamond (Adamant Technologies,
Switzerland) coating (2-3 µm) with dimensions 5.0 × 2.5 × 0.2 cm, and a stainless steel foil (Alfa Aesar) with dimensions 5.0 × 2.5 × 0.05 cm were used as anode and cathode, respectively. The reference electrode used was a silver/silver chloride (Ag/AgCl) electrode (BASi).
5.2.2 Experimental Setup
The experimental apparatus was comprised of an undivided Pyrex reactor with a working volume of 1L and an overall capacity of 1250 cm3 mixed with an electrolyte recirculation system controlled with a potentiostat. A Teflon-lined screw top plastic lid with openings
87
to accommodate electrodes was used to cover the reactor. The electrodes were set 3.4 cm apart and immersed into the electrolyte solution about 6 cm deep . PVC holders were used to maintain the electrodes at the required height. A peristaltic pump (Cole-Parmer) with Masterflex tubing (Cole-Palmer) was used for recirculation of the reactor liquids to provide complete mixing. A scanning potentiostat (PAR Model 362, EG & G
Instruments) was applied as an external power supply providing constant current (Figure
3-1). Experiments were conducted at room temperature, 22 ± 0.5℃.
5.2.3 Analytical Methods
During the electrochemical runs, 10-15 mL samples were collected at 18 minute or multiples of 18-min intervals (time for replicate 18-minute HPLC runs). Phenol and organic intermediates were measured simultaneously via HPLC. A Hypersil ODS column (150 mm × 4.6 mm = L × I.D.) (Thermo Scientific) was used. Mobile phase used were solvent A (0.1% formic acid in Super-Q water with pH adjusted to 3 using sulfuric acid) and solvent C (an organic mixture of acetonitrile, acetic acid and methanol in volumetric ratio of 90:5:5). A gradient was used starting at 90 (A):10 (C) and ending at 10 (A):90 (C). Mobile phase flow rate was set at 0.8 mL/min and UV detection wavelengths were monitored at 270, 280, 290 and 300nm. Thermostat temperature was controlled at 30 °C throughout the sequences. Chlorate and chloride ion were measured with a Dionex LC 20 Ion Chromatography (IC) using an ION PAC AS 14 column (4mm
×250 mm), with a guard column ION PAC AG 14 (4mm ×50 mm) using 1-2 mL of sample. Samples were diluted before measurements when necessary. The free chlorine
88
(FAC) in the samples was analyzed by using an eXact ® micro 7+ Photometer (ITS) at wavelength of 525 nm with eXact® Strip Micro DPD-1 test strips.
5.2.4 Experimental Conditions
The experiments examined in this paper were operated with phenol of 0.5 mM initial concentration and 50 mM NaCl (or 17 mM Na2SO4 to compare in the case of absence of chloride) electrolyte. The constant current density was kept at 12 mA/cm2 or 25 mA/cm2.
Solution pH was not controlled but monitored throughout the reaction, pH ranged between 4.7-7.1. There are a total of 16 experiments that are to be discussed below.
Table 5-1. Table of run conditions
Run No. Anode [Phenol] [NaCl] Current Remarks Type (mM) (mM) Density (mA/cm2) B1 B 0.25 5 12 B2 B 0.25 5 25 B3 B 0.25 50 12 2 duplicates B4 B 0.25 50 25 B5 B 0.5 5 12 3 duplicates B6 B 0.5 5 25 B7 B 0.5 50 12 5 duplicates B8 B 0.5 50 25 G1 G 0.25 5 12 G2 G 0.25 5 25 G3 G 0.25 50 12 G4 G 0.25 50 25 G5 G 0.5 5 12 G6 G 0.5 5 25 G7 G 0.5 50 12 4 duplicates G8 G 0.5 50 25
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5.3 Kinetic Modeling on Intermediate Conversion
5.3.1 Modeling Basics
The kinetic mathematical model was based on a nonlinear regression technique using
Mathematica 9.0. The approach was to minimize the sum of the squared difference of measured concentration and the calculated concentration, assuming that all of the reactions are first-order (18). Therefore, for N reactions, the parameter values can be determined: y@z = ∑} % ' ?4C (j = 1, 2,…, N) (5-1) yM |~o | z|
} Ma 4z_ −4zn = ∑|~o %| Ä 'z|>4MHdt (i = 1,2,…,M) (5-2) MÅ
} lz_ ≡4z_ −4zn = ∑|~o %| h_z| (5-3)
3 Ü } 3 É = ∑∑_~o z~oÑlz_?#8"C −lz_?-$*CÖ (5-4)
3 where, É is the least-squares error; lz_ (exp) and lz_ (cal) are the experimentally- measured and the model-calculated values of the dependent variables (concentrations), respectively. Note that a weighing factor was not taken into account. Körbahti et al. (7) used a similar approach in their paper where. during their experimental runs, 2- chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and polymeric and other products were detected as intermediate products in the presence of NaCl.
(Interestingly, they also mentioned that excess replenishment of free chlorine species prevented the polymers from fouling at the electrode surface, which may have happened with our BDD system as well.)
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Figure 5-1. Two proposed pathways: hydroxylation and hypochlorination.
5.3.2 First-Order Reaction Differential Equations
According to identified organic intermediates, a phenol conversion pathway (with and without chloride) is as proposed as shown in Figure 5-1. The rate equations corresponding to this model are developed as: y@ á =−?% +% +% +% +% C4 (5-5) yM o 3 à 3o 3W â y@ ä =% 4 − ?% +% C4 (5-6) yM o â ^ on ã y@ å =% 4 − ?% +% +% C4 (5-7) yM 3 â \ ç oo I y@ é =% 4 +% 4 − ?% +% C4 (5-8) yM ^ ã ç I è o3 ê
91
y@ ë =% 4 − ?% +% C4 (5-9) yM \ I K o^ í y@ ì =% 4 +% 4 − ?% C4 (5-10) yM è ê K í oW L y@ î =% 4 −% 4 (5-11) yM 3^ â 3è ï y@ ñ =% 4 −?% +% C4 (5-12) yM 33 â 3W 3\ @ y@ ó =% 4 +% 4 − ?% C4 (5-13) yM 3o â 3W @ 3ç B y@ ò =% 4 +% 4 +% 4 +% 4 +% 4 +% 4 +% 4 +% 4 +% 4 (5-14) yM à â on ã oo I o3 ê o^ í oW L 3è ï 3\ @ 3ç B
5.4 Results and Discussion
5.4.1 Experimental Results
Experiments were carried out under 16 different conditions (Table 5-1). Figures 5-2 through 5-5 show the progressive formation and conversion of measured organic intermediates as electrooxidation progresses for these cases. Figures 5-6 through 5-9 show changes in the chlorine-containing species. The following observations can be made upon examination of these data: (1) Products from both hydroxylation and hypochlorination pathways were formed and in competition when graphite or a lower chloride concentration was used; (2) A combination of both pathways was found with lower chloride concentration and the hypochlorination pathway was dominant; (3) More types of intermediates were generated with graphite but the peak amounts were higher with BDD; (4) Phenol was removed faster at lower initial phenol concentration, higher chloride concentration and on BDD; (5) No free chlorine breakthrough was reached with
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graphite or with lower initial chloride concentration; (6) Chlorate production rate
increased when chloride concentration increases.
0.5 a b 0.5
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration Concentration (mM)
0.1 0.1
0.0 0100020003000400050006000 0.0 Charge (coulombs) 0100020003000400050006000 Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP Charge (coulombs) phenol 2-cp 4-cp 2,6-dcp 2,4-dcp 2,4,6-tcp
0.5 0.5 c d
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration (mM)
0.1 0.1
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP BQ Phenol 2-cp 4-cp 2,6-dcp 2,4-dcp bq
Figure 5-2. Phenol and intermediates measured when phenol is treated at 12 mA/cm2 constant
current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol;
and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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0.5 0.5 a b
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration (mM) 0.1 0.1
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP
0.5 0.5 c d
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration (mM) 0.1 0.1
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP BQ Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP BQ
Figure 5-3. Phenol and intermediates measured when phenol is treated at 25 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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a b 0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2
Concentration (mM) Concentration (mM) 0.1 0.1
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Phenol BQ CA 2-CP 4-CP 2,6-DCP 2,4-DCP Phenol BQ 2-CP 4-CP 2,6-DCP 2,4-DCP
c d 0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration (mM)
0.1 0.1
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Phenol BQ HQ 2-CP 4-CP 2,6-DCP 2,4-DCP Phenol BQ HQ 2-CP 4-CP 2,6-DCP 2,4-DCP
Figure 5-4. Phenol and intermediates measured when phenol is treated at 12 mA/cm2 constant current density in in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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a b 0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration (mM)
0.1 0.1
0.0 0.0 020004000600080001000012000 020004000600080001000012000 Charge (coulombs) Charge (coulombs) Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP BQ Phenol 2-CP 4-CP 2,6-DCP 2,4-DCP BQ
c d 0.5 0.5
0.4 0.4
0.3 0.3
0.2 0.2 Concentration (mM) Concentration (mM)
0.1 0.1
0.0 0.0 020004000600080001000012000 020004000600080001000012000 Charge (coulombs) Charge (coulombs) Phenol 2-CP 4-CP BQ HQ Phenol 2-CP 4-CP BQ
Figure 5-5. Phenol and intermediates measured when phenol is treated at 25 mA/cm2 constant current density in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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50 50 40 40 a 30 b 30 20 20 10 10
2.0 2.0
1.5 1.5
1.0 1.0
Concentration (mM as Cl) Concentration (mM as Cl) 0.5 0.5
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Free Chloride Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP
50 50 c 40 d 40 30 20 30 10
2.0 2.0
1.5 1.5
1.0 1.0 Concentration (mM Cl) as Concentration (mM as Cl) 0.5 0.5
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP
Figure 5-6. Chlorine-containing species measured when phenol is treated at 12 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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50 50 40 a b 30 20 10 40
2.0 30
1.5
20 1.0 Concentration (mM as Cl) Concentration (mM as Cl) 10 0.5
0.0 0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP
50 50 40 40 d 30 c 30 20 20 10 10
2.0 2.0
1.5 1.5
1.0 1.0 Concentration (mM as Cl) Cl) as (mM Concentration 0.5 0.5
0.0 0.0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP
Figure 5-7. Chlorine-containing species measured when phenol is treated at 25 mA/cm2 constant current density in 50 mM NaCl with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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5 5 b a
4 4
3 3
2 2 Concentration (mM as Cl) as (mM Concentration 1 Cl) as (mM Concentration 1
0 0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP
5 5 c d 4 4
3 3
2 2
Concentration (mM as Cl)as (mM Concentration 1 Cl) as (mM Concentration 1
0 0 0100020003000400050006000 0100020003000400050006000 Charge (coulombs) Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP
Figure 5-8. Chlorine-containing species measured when phenol concentration is treated when phenol is treated at 12 mA/cm2 constant current density in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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5 5 a b
4 4
3 3
2 2 Concentration (mM as Cl) as (mM Concentration Cl) as (mM Concentration 1 1
0 0 020004000600080001000012000 020004000600080001000012000 Charge (coulombs) Charge (coulombs) Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP Free Chlorine Chloride Chlorate 2-CP 4-CP 2,6-DCP 2,4-DCP
5 5 c d
4 4
3 3
2 2
Concentration (mM as Cl) as (mM Concentration Concentration (mM as Cl) as (mM Concentration 1 1
0 0 020004000600080001000012000 020004000600080001000012000 Charge (coulombs) Charge (coulombs) Free Chlorine Chloride 2-CP 4-CP Free Chlorine Chloride Chlorate 2-CP 4-CP Figure 5-9. Chlorine-containing species measured when phenol concentration is treated when phenol is treated at 25 mA/cm2 constant current density in 5 mM sodium chloride and 15 mM sodium sulfate (equivalent ionic strengths) with BDD anode: (a) 0.5 mM phenol (b) 0.25 mM phenol; and with graphite anode: (c) 0.5 mM phenol (d) 0.25 mM phenol.
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5.4.2 Calculatation of Reaction Rate Constants
First-order intermediates conversion rate constant (k) values were calculated by using
Mathematica 9.0. The k values and least-squares errors (LSE) from regression for BDD and graphite experiments are shown in Table 2 and 3, respectively. Regardless of the experimental conditions, rate constants k1 to k8 were the larger values, indicating that stepwise conversions from phenol to monochlorophenols, dichlorophenols and trichlorophenols (hypochlorination pathway) are more rapid than phenol to “other products” including mineralization (CO2 formation). CO2 and the unknown by-products that were not identified by HPLC were included in the term “other products.” The concentration of “other products” was calculated based on subtracting the sum of identified intermediates concentrations from the initial phenol concentration. Insight into what these unknown by-products may be is discussed in Chapter 6.
Table 5-2. Reaction rate constant k values when using BDD anode k (h-1) B1 B2 B3 B4 B5 B6 B7 B8 k1 0.116 0.117 1.252 2.094 2.049 0.071 0.523 1.397 k2 0.141 0.135 2.196 4.192 2.169 0.070 1.384 1.171 k3 0.235 0.166 3.730 3.744 1.718 0.155 1.540 2.228 k5 0.367 0.166 2.544 2.651 0.682 0.204 0.917 1.427 -10 -10 k6 3.137×10 0.014 0.118 2.350 1.772 3.045×10 0.728 0.681 k7 1.618 3.730 2.062 1.022 1.305 -11 k8 1.519 2.347 2.121 0.991 1.060×10 -10 -16 k9 0.038 0.050 0.018 9.251×10 0.445 0.061 4.642×10 0.099 -16 -12 k10 0.053 0.031 0.007 0.153 0.289 0.002 4.642×10 1.598×10 -9 -16 -12 k11 0.071 0.171 1.447 1.510×10 0.452 0.010 4.642×10 1.643×10 -8 -16 k12 0.855 0.605 0.118 1.356×10 0.273 1.052 4.642×10 0.479 -16 k13 2.908 0.540 0.016 0.421 0.479 1.471 4.642×10 2.115 -13 k14 1.337×10 1.270 0.160 0.842 0.308 k21 0.014 0.035 0.013 k22 k23 k24 k25 -11 -12 -11 k26 4.681×10 2.761×10 1.982×10 k27 LSE 8.046×10-4 7.648×10-5 2.431×10-3 5.842×10-3 0.156 3.706×10-4 0.126 0.012 101
Table 5-3. Reaction rate constant k values when using graphite anode k (h-1) G1 G2 G3 G4 G5 G6 G7 G8 k1 0.057 0.042 0.482 0.518 0.040 0.032 2.115 0.370 k2 0.072 0.036 0.826 0.450 0.008 0.028 0.660 0.387 -13 k3 0.063 0.536 0.418 0.154 4.023×10 0.173 0.470 k5 0.059 0.679 0.435 1.158 0.011 0.770 0.492 k6 0.053 0.380 0.599 0.954 0.032 1.773 0.245 k7 1.094 1.282 0.603 0.746 k8 1.571 1.897 2.147 1.467 -10 -11 -13 -16 k9 0.049 4.358×10 1.140×10 0.105 5.863×10 7.673×10 0.142 0.028 -11 -10 k10 0.107 0.144 7.535×10 0.259 0.006 0.132 2.503×10 0.015 -11 -12 -9 k11 0.137 0.144 3.004×10 6.373×10 1.189 0.091 0.786 8.272×10 -9 k12 0.194 0.044 0.410 0.412 1.253×10 0.476 -10 k13 0.115 7.754×10 0.441 0.658 1.748 0.963 k14 1.371 1.607 0.875 1.240 k21 0.026 0.104 0.037 0.093 0.086 0.079 0.066 k22 0.083 0.007 0.013 0.006 k23 -7 -13 k24 1.117 3.613×10 5.903×10 -12 -6 -13 -14 k25 2.329×10 6.628×10 6.074×10 4.525×10 -4 k26 6.913×10 0.013 0.112 0.260 0.006 0.019 0.071 0.074 k27 LSE 7.825×10-3 4.048×10-3 2.198×10-3 4.985×10-4 0.029 0.009 0.033 0.001
Figure 5-10 shows comparison between the rate constants of initial steps of conversion of phenol to intermediates, such as 2-chlorophenol, 4-chlorophenol, hydroquinone, benzoquinone, catechol and other products. Thus, k1, k2, k9, k21, and k22 in all the cases were compared. It can be seen that in most cases, phenol conversion rates to monochlorophenols (k1 and k2) were much faster than the other conversions and k2 values were greater than k1, indicating that hypochlorination pathway was more favored and 2- chlorophenol was the most dominant species. Phenol to other products (k9) was rate- limited in comparison.
The secondary conversion rates are compared in Figure 5-11, i.e., monochlorophenol to dichlorophenol and to trichlorophenol. The reaction route from 4-chlorophenol to 2,4- dichlorophenol (k3) was the most rapid reaction compared to the 2-chlorophenol to 2,4-
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dichlorophenol (k6) and 2,6-dichlorophenol (k5) reactions. The 2,4-dichlorophenol and
2,6-dichlorophenol conversions to 2,4,6-trichlorophenol were competing with comparable rate values (k7 and k8), and were faster than the final conversion of 2,4,6- trichlorophenol to other products (k14).
In Figure 5-12, the reaction rates of dichlrophenols and trichlorophenols conversion to other products (k12, k13, k14) were leading. Comparing k3, k6 and k7 with k12, it implies that monochlorophenols conversions to 2,4-dichlorophenol were faster than 2,4- dichlorophenol conversion to other products (Figure 5-13). Looking at 2,4,6- trichlorophenol as an example, k7 and k8 values were greater than k14 values, thus 2,4,6- trichlorophenol conversion to other products was much slower (Figure 5-14).
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5
) k2
-1 k1 k 1 k2 (h 4 k
3
2
1 Reaction Rate Constant Rate Reaction 0
5 k21
) k22 -1
(h 4 k k21 k22
3
2
1 Reaction Rate Constant Rate Reaction 0 5 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8
) k9 Run No. -1
(h 4 k k9 Other Products
3
2
1 Reaction Rate Constant Rate Reaction 0 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8
Run No.
Figure 5-10. Comparison of phenol conversion rates for formation of initial intermediates. k1 and k2 follow hypochlorination and k21 and k22 follow hydroxylation.
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5 )
-1 k3 k5 k3 k5 (h
k 4
3
2
1 Reaction Rate Constant 0
5 k7 ) -1 k6 k6 k7 (h
k 4
3
2
1 Reaction Rate Constant 0 5
) k8 k14 -1
k8 k14 (h
k 4 Other Products
3
2
1 Reaction Rate Constant Constant Rate Reaction 0 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 Run No. Run No.
Figure 5-11. Comparison of phenol secondary conversion rates in hypochlorination pathway: mono- to dichlorophenols (k3, k5 and k6), di- to trichlorophenol (k7, k8) and trichlorophenol to other products (k14).
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5 k9 k10 4 k9 k ) 10
-1 Other Products Other Products 3 (h k 2
1
0 5 k11 k12 4 k12 k11 Other Products 3 Other Products Reaction Rate Constant Constant Rate Reaction
2
1
0 5
) k13 k14 -1 4 k (h k13 14 k Other Products Other Products 3
2
1
0 k26 5 k25 4 Reaction Rate Constant Constant Rate Reaction k26 k25 Other Products 3 Other Products 2
1
0 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 Run No. Run No.
Figure 5-12. Comparison of predicted reaction rates for all pathways to “other products”: phenol (k9), monochlorophenols (k10, k11), dichlorophenols (k12 and k13), trichlorophenol (k14), and hydroquinone, benzoquinone (k25 and k26).
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) 5 -1 k3 k6
(h k3 k6 k 4
3
2
1
0 Reaction Rate Constant
) 5 -1 k7 k12 (h k7 k k12 4 Other Products
3
2
1
0 Reaction RateConstant B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 Run No. Run No.
Figure 5-13. Comparison between 2,4-dichlorophenol formation and conversion rates (k3, k6, k7 and k12).
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5 k7 k8 ) -1
(h 4 k
3
2
1 Reaction Rate Constant 0 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 5
) Run No. -1 k14 k (h 4 14 k Other Products
3
2
1 Reaction Rate Constant 0 B1 B2 B3 B4 B5 B6 B7 B8 G1 G2 G3 G4 G5 G6 G7 G8 Run No.
Figure 5-14. Comparison between 2,4,6-trichlorophenol formation and conversion rates (k7, k8 and k14).
5.4.3 Evaluation of the Model Fit
The differential equations (5-5) -- (5-14) were solved by Mathematica. The least-squares errors of each regression are provided in Tables 5-1 and 5-2. Figure 5-15 shows the model application to one example experiment (Run G8) that had compounds from both pathways present in measureable amounts. The results obtained from the kinetic modeling are in good agreement with experimental data and show that the hypochlorination pathway was more favored than hydroxylation pathway. Other model 108
results were plotted to compare with experimental results and can be found in Appendix
G.
phenol 4-chlorophenol
2-chlorophenol 2,6-dichlorophenol
2,4-dichlorophenol 2,4,6-trichlorophenol
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benzoquinone “other products”
Figure 5-15. Experimental and model concentration profiles for phenol and intermediates.
Experimental results from G8: 0.5 mM phenol in 50 mM NaCl with graphite anode using 25 mA/cm2 current density.
5.5 Conclusion
The conversion of phenol to intermediate products during electrochemical oxidation in the presence of NaCl under different conditions (anode type, initial phenol concentration, chloride concentration and applied current density) was studied by measuring concentration changes of phenol and intermediates with time. Experimental results showed that hypochlorination and hydroxylation pathways were in competition and hypochlorination was more favored in most cases studied. However, when using graphite with low chloride concentration (5 mM), hydroxylation pathway was more favored.
Phenol was removed faster at lower initial phenol concentration and higher chloride concentration with BDD. Free chlorine breakthrough only occurred with excess chloride and BDD. Chlorate production rate increased when chloride concentration increases.
Rates of conversion between species were estimated using a kinetic model for competition between hypochlorination and hydroxylation pathways. Comparison 110
between the simulated rate constant k values showed that intermediate conversions to
“other products” were the slowest steps. The model may be used to demonstrate which pathway is favored under a specific set of conditions when the two pathways were in competition. For example, hypochlorination pathway was more favored and 2- chlorophenol was the most dominant species in most cases.
A limitation of the model was that the “other products” category may actually represent multiple unknown species, and the values in the proposed model were calculated based on the difference between measured concentrations of known products and the starting phenol concentration. This impacts computation of error when performing the regression.
Consideration of the inorganic chlorine species conversion rates would add another layer of complexity to the model, and would be interesting to pursue. The contribution of chloride ion, free chlorine and chlorate to the chlorine balance in addition to the chlorinated organic species is not captured in the current version of the model.
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5.6 References
1. Janssen, L.J.J. and Hoogland, J.G. The electrolysis of an acidic NaCl solution with a graphite anode-III. Mechanism of chlorine evolution. Electrochim. Acta 1970, 15 (6), 941-951.
2. Park, H.; Vecitis, C.D.; Hoffmann, M.R. Electrochemical Water Splitting Coupled with Organic Compound Oxidation: The Role of Active Chlorine Species. J. Phys. Chem. C 2009, 113 (18), 7935-7945.
3. Polcaro, A.; Vacca, A.; Mascia, M.; Ferrara, F. Product and by-product formation in electrolysis of dilute chloride solutions. J. Appl. Electrochem. 2008, 38 (7), 979-984.
4. Polcaro, A.; Vacca, A.; Mascia, M.; Palmas, S.; Rodiguez Ruiz, J. Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides. J. Appl. Electrochem. 2009.
5. Murata, M.; Ivandini, T.A.; Shibata, M.; Nomura, S.; Fujishima, A.; Einaga, Y. Electrochemical detection of free chlorine at highly boron-doped diamond electrodes. J Electroanal Chem 2008, 612 (1), 29-36.
6. Ferro, S.S. Chlorine evolution at highly boron-doped diamond electrodes. J. Electrochem. Soc. 2000, 147 (7), 2614-2619.
7. Coker, A.K. Modeling of Chemical Kinetics and Reactor Design. Elsevier Gulf: 2001; 1-1136.
8. Körbahti, B.K. and Tanyolaç, A. Kinetic modeling of conversion products in the electrochemical treatment of phenolic wastewater with a NaCl electrolyte. Ind. Eng. Chem. Res 2003, 42 (21), 5060-5065.
9. Vallejo, M.; San Román, M.F.; Ortiz, I. Quantitative assessment of the formation of polychlorinated derivatives, pcdd/fs, in the electrochemical oxidation of 2-chlorophenol as function of the electrolyte type. Environ. Sci. Technol. 2013, 47 (21), 12400-12408.
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Chapter 6 Identification of Intermediate Products during Anodic
Oxidation of Phenol using Liquid Chromatography Quadruple
Time-of-Flight Mass Spectrometry
6.1 Introduction
Electrochemical treatment technology has become more popular in recent decades because it is easy to operate and it can be coupled with renewable energy sources to enhance the treatment efficiencies and reduce the costs (1-3). Anodic oxidation treatment is very efficient when it comes to treating bio-recalcitrant organic wastes that are commonly found in industrial waste effluents. The selection of anode material and the composition of the electrolyte contribute significantly to the treatment efficiency. In this paper, the boron-doped diamond (BDD) was used as the anode material since BDD anodes have been reported to exhibit higher oxygen overpotential than other conventional anodes to yield higher efficiency pertaining to anodic oxidation treatment (4-6). Sodium chloride was selected as the electrolyte because chloride is ubiquitous in both natural water and industrial wastewater and electrochemically generated free available chlorine contributes to improve oxidation efficiency. However, electrooxidation in the presence of chloride brings up the problem of toxic reaction by-products. Potential undesirable by-products include chlorates, chlorinated organic compounds, and endocrine disrupting compounds (EDCs) that are possible carcinogens and can cause adverse effects on human health and the environment (7). Only a few papers have addressed the issue of undesirable by-products formation especially at trace levels in the presence of chloride (7-10).
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Phenol was selected as a model compound in this paper since it represents a large category of organic pollutants from industrial effluents. The mechanism of phenol electrooxidation is complex, including direct electron transfer, indirect oxidation via hydroxyl radicals (11, 12), and indirect oxidation mediated by electrochemically generated oxidizing reagents, such as free available chlorine, if chloride is present in the electrolyte (1, 3, 12). Previous research has reported intermediates distribution under influence of different anode materials and different chloride concentrations (9, 13). It is widely accepted that hydroxylation and hypochlorination are the two initial pathways for phenol electrooxidation in the presence of chloride (14). The former leads to production of hydroquinone, benzoquinone and catechol, and the latter results in chlorophenols including monochlorophenols, dichlorophenols and trichlorophenols (15, 16).
Others have reported a thin polymer film on the surface of electrode formed as a result of dimerization pathway (11, 17-19). In previous studies, the authors were able to simultaneously identify and quantify phenol and its intermediates from both pathways (hydroxylation or hypochlorination) using HPLC; detailed information on the HPLC method used has been published elsewhere (16). However, noticeable discrepancies in the organic and chlorine species mass balances raised questions over what other unmeasured compounds may be forming and reports of polymer formation interested the authors (19, 28). In recent years, the use of LC-
QTOF-MS has become more popular for studying emerging environmental contaminants and their degradates (20-27), so this highly sensitive technique was used to investigate these unknown by-products that were not detected by HPLC. The hypothesis was that chlorinated phenolic dimers with higher molecule weights are being formed.
The aim of this paper is to evaluate the unknown by-products from electrooxidation of phenol in the presence of chloride, especially the hypothesized formation of highly toxic
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chlorinated phenolic dimers. This study analyzed the distribution of intermediate products at two different levels of chlorine concentration. The assessment on by-products emphasized the importance of controlling operational conditions to minimize the production of toxic by- products.
6.2 Experimental Section
6.2.1 Chemicals and Reagents
Sodium chloride and sodium sulfate electrolytes were prepared at constant ionic strength of the equivalent of 50 mM sodium chloride. Phenol (Fisher Scientific), benzoquinone, hydroquinone, catechol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6- dichlorophenol, 2,3,6-trichlorophenol, 2,4,5-trichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol (Sigma-Aldrich®) were used as standards for HPLC analysis. All the standards were prepared in 50 mM sodium chloride (Fisher Scientific) matrix. HPLC-grade acetonitrile, HPLC-grade methanol and acetic acid (Fisher Scientific) were used as the organic mobile phase. Aqueous mobile phase and all the dilutions were made using Super-Q water
(Millipore).
6.2.2 Experimental Setup
The experimental apparatus consisted of a glass reactor (Pyrex®) with a working volume of 1L and an overall capacity of 1.25L connected with an electrolyte recirculation system and a scanning potentiostat (PAR Model 362, EG & G Instruments). The reactor was operated as an undivided electrochemical cell. A Teflon-lined screw top plastic lid with openings to accommodate electrodes was used to cover the reactor. PVC holders were used to maintain the two electrodes at the required height. The anode used was BDD (Adamant Technologies,
Switzerland) with dimensions 5.0 × 2.5 × 0.2 cm. A stainless steel foil (Alfa Aesar®) with
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dimensions 5.0 × 2.5 × 0.05 cm was used as cathode. The electrodes were immersed into the electrolyte solution to a depth of 12.2 cm. The distance between two electrodes was 3.4 cm. The reference electrode used was a silver/silver chloride (Ag/AgCl) electrode (BASi). A peristaltic pump (Cole-Parmer®) with Masterflex tubing (Cole-Palmer®) was used for continuous mixing.
The constant electrolyte flow rate was 26.5 mL s-1. The reactor was operated at room temperature, 22 0.5℃, for all experiments. The pH was not buffered but was monitored throughout the experiments, and it ranged between 4.7 to 7.1.
6.2.3 Analytical Methods
An Agilent 1100 HPLC with a diode-array detector (DAD) was applied for in-situ monitoring of phenol and intermediate samples. A Hypersil ODS column (Thermo Scientific) 150 mm long by
4.6 mm in diameter was used and controlled at a constant temperature of 30 °4. The mobile phase consisted of Super-Q water and an organic mixture (90:5:5 acetonitrile:methanol:acetic acid by volume). A time-variant gradient was used with an initial percentage of water at 90%, and the final percentage of water at10% in a run time of 15 min. The flow rate was constant at
0.8 mL min-1 and sample injection volume was 10 µL. UV detection wavelengths were monitored at 270, 280, 290 and 300 nm. A 2-mL portion of the sample was filtered and analyzed for phenol and intermediates. Samples were taken before current was turned on and then at 18- minute (or multiples of 18) intervals to get immediate measurements.
The free chlorine in the samples was analyzed by using eXact ® micro 7+ Photometer (ITS) at wavelength of 525 nm eXact® Strip Micro DPD-1 test strips. 2-4 mLs of sample was diluted before measurements when necessary. Chlorate and chloride ion were measured with Dionex
LC 20 Ion Chromatography (IC) using an ION PAC AS 14 column (4mm ×250 mm), with a guard column ION PAC AG 14 (4mm ×50 mm). A TOC analyzer (Shimadzu 5050) was used to
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follow TOC degradation. Chemical oxygen demand (COD) was determined by using a HACH
DR 2000 Spectrophotometer. 2 ml samples were first added to HACH COD reagent vials (low range) then were heated in digester block at 150 °4 for two hours. After cooling down, the vials were analyzed in spectrophotometer at wavelength 420 nm.
6.2.4 LC-QTOF-MS Analysis
Liquid chromatography (LC) electrospray-ionization (ESI) quadrupole-time-of-flight (QTOF) mass spectrometry was conducted. The separation was performed on an Agilent 1200 system, equipped with an Agilent reversed phase ZORBAX Eclipse XDB-C18 column (2.1 mm × 50 mm, 3.5 µm particle size). The column temperature was maintained at 30 ℃. Ultrapure water
(solvent A) and methanol (solvent B), both in 5 mM ammonium acetate, were used as mobile phase. The following gradient was used: 0-1 min, 5% B; 15-16 min, 90% B; 16.1 min, 5% B.
The mobile phase was 0.2 mL min-1 and the injection volume was 10 µL.
The HPLC system was connected to an Agilent TOF/Q-TOF mass spectrometer (G6540A).
Ion source was Agilent Jet Stream Technology electrospray ionization (AJS ESI), by using capillary and fragmentor voltages of 4000 V and 125 V, respectively. All samples were analyzed in negative mode (ESI−) using reference masses 112.98558700 and 980.01637500 m/z.
Full MS scan spectra were acquired in the range from 50 to 1000 m/z at a scan rate of 1.5 spectra per second. The full-scan data was recorded and analyzed by Agilent Mass Hunter Workstation
(B.04.00).
Samples were measured via HPLC prior to LC-MS analysis, in order to quantify phenol and chlorophenol intermediates concentration variation along with time. LC-QTOF-MS provides full scan in the TOF mode on the injected samples and generates a report on potential compounds of interest over a nominal m/z range of 50 to 1000 units. Total ion chromatograms
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(TICs) are extracted from background noises and the full scan results are exported into .csv files.
Using the Find by Molecular Feature (FMF) algorithm, Mass Hunter software searches all the ions to associate with chromatographic peaks. The parameters for elemental composition include an error limit of ± 5 ppm and the limits of elements, see Table H-1 in Appendix H. In the further screening step, molecular features extracted from the raw data from the full scan are compared with those of blanks (water and salt matrices).
By using Mass Hunter software (Agilent), accurate m/z values for [M−H]- ions and empirical formulas were calculated based on the “find compounds by molecular feature extraction” (MFE) algorithm. The empirical formula was determined on three parameters: mass accuracy, isotopic distribution and spacing among the ions. The overall score values are given on a scale of 0 to
100, based on the goodness of the match with the theoretical spectra considering three parameters (21, 27). Thereafter, the LC-QTOF-MS system was performed in the MS/MS mode to elucidate chemical structures for candidate by-products. Targeted MS/MS method was used for locating the accurate mass. The MS-MS spectra were recorded using various collision energies (CE) based on the formula: 4= = ô*+"# × ?öõ⁄C⁄100 + Nùù)#/, where slope = 3, offset = 10. MS/MS range from 25 to 600 m/z units using ± 4 m/z units tolerance and ± 0.15 retention time window.
6.3 Results and Discussion
6.3.1 Intermediates Identified by HPLC
When high chloride concentration (50 mM) was used, the identified intermediates produced were monochlorophenols (MCPs): 2-chlorophenol, 4-chlorophenol, dichlorophenols (DCPs): 2,4- dichlorophenol, 2,6-dichlorophenol and trichlorophenol (TCP): 2,4,6-trichlorophenol (Figure 6-
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1a). These findings are in line with previous research (29), in which the authors summarized that the scheme for chlorine reaction with aromatic compounds (phenol was taken as a representative) is mainly electrophilic substitution, and chlorination proceeds by a stepwise substitution of the 2-, 4- and 6- positions. This indicates that ortho/para positions are favored, because the hydroxyl group in phenol is a strong ortho/para director, which lowers the activation energy leading to a resonance-stabilized intermediate (30). TCP was found to be the final chlorine substitution phenol, which is in accordance with previous research on chlorination of phenol (31). It also agrees with results obtained in chemical oxidation of phenol with NaOCl
(15). None of the previous literature we had reviewed had reported that chlorination level would proceed beyond trichlorophenols. According to Lee et al. (32), the higher the number of chlorine substitutions of phenol is, then its degree of oxidative rupture is greater than chlorine substitution. Chlorophenol intermediates showed typical behavior of reaction intermediates that their concentrations first increased then decreased in the range of coulombs studied. 2-CP was preferred over 4-CP, and 2,4-DCP was preferred over 2,6-DCP. 2,4,6-trichlorophenol started to increase rapidly when MCPs and DCPs started to drop, and reached its highest amount at 55% of initial phenol concentration (about 0.24 mM). The increase of chlorination level of chlorophenol intermediates implies a hypochlorination pathway.
When there was low concentration of chloride (5 mM) present (Figure 6-1b), the same types of intermediates were found as in the high chloride condition. 2-CP was the most dominant species, overall at a very low concentration, the highest being 13% of the initial phenol concentration (about 0.06 mM). In the previously reported experiments in the presence of 5 mM
NaCl (at 12 mA/cm2 constant current density or 6 V constant voltage), we had detected catechol and benzoquinone in very low concentrations, and chlorophenols including MCPs and DCPs
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were detected with the exception of TCPs. These findings indicated a competition of both hydroxylation and hypochlorination pathways in the presence of the low chloride condition.
However, the hydroxylation products were not detected in the experimental data reported here although the chlorination level progressed to trichlorophenol along the hypochlorination pathway as previously observed. A brand new BDD anode was used for the current work and this may have shifted the efficiency of surface reactions – an interesting point to pursue in future work.
Results show that TOC and COD removal rates were higher at higher chloride concentration
(Figure 6-1c&d). It took longer to complete the mineralization of phenol (TOC removal) while the majority oxidizable compounds have been removed (COD removal) at the end of each experiment, when comparing the TOC and COD removal rates in both high and low chloride experiments. This indicates that hard-to-oxidize organic compounds were formed accounted for about 40% TOC at the end of the experiments. On the other hand, theoretical TOC values were calculated based on concentrations of identified species on HPLC and compared with experimentally measured TOC (Figure 6-1 e&f). The discrepancies between the calculated TOC and actual TOC values implied formation of unknown by-products. Further investigation on these unknown by-products was carried out by LC-QTOF-MS and results are discussed in the following sections. Figure 6-2 depicts measured chlorine-containing species mass balance, including chloride, free chlorine, chlorate and identified chlorinated intermediates. The discrepancies are attributed to the chlorinated compounds that were not identified by HPLC.
120
50 mM NaCl 5 mM NaCl
1.0 a b Phenol 2-CP 0.8 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP 0.6
C/C0
0.4
0.2
0.0
1.0 c d TOC/TOC0 COD/COD0 0.8
0.6
0.4 TOC/TOC0and COD/COD0 0.2
0.0 e 3.0 f
Act TOC 2.5 Cal TOC
2.0
1.5 TOC(mM)
1.0
0.5
0.0 01002003004005006007000100200300400500600700
Time (min) Time (min)
Figure 6-1. Normalized phenol and intermediates concentration progression with time (a&b),
TOC and COD removal (c&d), and the comparison of actual and calculated TOC (e&f) in the presence of high chloride and low chloride concentration.
121
50 a
40
3
2
1 Concentration (mM as as Cl) (mM Concentration
0 0100200300400500600700800 Time (min) 2-CP 4-CP 2,6-DCP 2,4-DCP 2,4,6-TCP Free Chlorine Chlorate Chloride Total Cl
5 b
4
3
2
1 Concentration (mM as as Cl) (mM Concentration
0 0100200300400500600700800 Time (min) 2-CP 4-CP 2,6-CP 2,4-DCP 2,4,6-TCP FCl Chlorate Chloride Total Cl
Figure 6-2. Chlorine mass balance with high chloride (a) and low chloride concentration (b).
6.3.2 Identification of Unknown By-products
Samples collected at different times then were analyzed by LC-QTOF-MS in order to obtain accurate mass measurements of potential phenol electrooxidation intermediates. Tables H-2 and
H-3 in the Appendix H list the detected intermediates using TOF full scan when the experiments were conducted with high chloride (50 mM) and low chloride (5 mM), respectively. They summarize their retention times, experimental and theoretical m/z values and empirical formulas.
Relative mass differences (MFG, ppm) and overall scores are also provided by the software.
122
Compounds with scores greater than 80 and differences within ± 5 ppm are considered as good candidates, as widely accepted in the literature (21, 24). In most cases, the relative mass errors are below 2 ppm from the accurate mass ion measurement.
Most compounds are chlorine containing C12 compounds, some are C18, indicating formation of dimers or multiples of phenol moieties. A passivating film formation on the surface of the electrode during electrooxidation of phenol have been reported in the literature (17, 34), formed as a result of dimerization of phenol via phenoxy radical coupling reaction. Phenoxy radical coupling reaction during electrochemical oxidation has been studied extensively, because of its importance in many biosynthesis processes (35). The major pathways of phenoxy radical coupling reaction pathways include oxygen-oxygen dimerization, carbon-oxygen dimerization, and carbon-carbon dimerization (17, 35-37).
Note that some compounds share the same empirical formulae but different retention times in chromatograms, indicating possible isomers (labeled as H2a and H2b, for example). More types of intermediates were detected with high chloride concentration. The accurate masses of the candidate compounds were used as mass target in the MS/MS mode in order to confirm their chemical structures. Elucidation of structures is discussed in the following section.
6.3.3 Interpretation of Triclosan Standard MS/MS Spectrum
Prior to injecting samples, triclosan (C12H7Cl3O2) standard was prepared and its MS/MS spectrum was studied as a reference. Triclosan is often used as antibacterial and antifungal agent, found in various types of personal care products, such as toothpaste, mouth rinses and hand soaps (38-43). There is limited number of MS/MS spectra of triclosan available, using ESI method, from the literature we reviewed. Most of the previous studies used derivatization to enhance the chromatographic peaks (41). One review paper mentioned two fragments in
123
negative-ion mode using LC-MS-MS (44). In ESI negative-ion MS−MS, the fragments lost are usually neutral molecules or radicals from the precursor ion or from sequential losses. During the course of method development, the negative mode was found to be most sensitive to the phenol and chlorophenol standards detection, which is probably due to the reason that the hydroxyl group in phenolic compounds can be easily deprotonated (45). Illustration of cleavage sites based on fragmentation information for triclosan is provided in Appendix H.
6.3.4 Structural Confirmation of By-products
Structure elucidation for by-products was carried out in the MS-MS mode. Two samples from each experiment condition with the most abundant full-scan information were re-injected. The
MS/MS spectra and interpretation of fragments are provided in Supporting Information. Since those proposed by-products are not commercially available, we could only compare their fragmentation patterns with triclosan to speculate the possible structures.
By-product H2 (precursor ion at m/z 219) fragmentation ion at m/z 201 indicates the elimination of a water molecule, which suggests that there are two hydroxyl groups in the aromatic ring structure (45). By-product H2 could be formed by carbon-carbon coupling of a phenoxy radical and a monochlorophenoxy radical by ortho-para or ortho-ortho links (Scheme
1) (17). By-product L2 also showed the same fragmentation pattern in the MS/MS spectrum, it is possible that H2 and L2 are isomers.
By-product H5 had the same MS-MS fragmentation pattern as the triclosan standard. As shown in Scheme 2, the ortho-coupling of two dichlorophenols gives rise to an ether-linked dimer structure, as supported in previous literature (45). Therefore, we can confirm that triclosan or a triclosan isomer was formed as one by-product during the electrooxidation of phenol in the presence of chloride. It disappeared after 144 min (Figure 6-3a).
124
The by-product H7 MS/MS spectrum shared some fragments (m/z 179, m/z 219) with those of polychlorinated dibenzo-p-dioxin reported in the literature (25), however, no detailed interpretation information was provided. Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) as by-products from combustion processes have been studied thoroughly (46). Only a small amount of research has reported PCDDs/PCDFs formation in non-combustion processes. Some researchers reported dioxin derivatives as triclosan photodegradation by-products (45). Marta Vallejo et al. reported in the most recent literature the formation of PCDD/Fs from electrochemical oxidation of 2-CP in the presence of chloride using BDD anode (47). They were able to quantify PCDD/Fs concentrations at pg L-1 level, and the USEPA established maximum contamination level (MCL) for 2,3,7,8-TCDD is 30 pg L-1. Previous research had reported phenol and chlorinated phenols as precursors of
PCDD/Fs under various conditions (48-50), including at the room temperature (46). Scheme 3 illustrates the formation pathway of by-product H7. Scheme 3 indicates a PCDD formation pathway. O-C coupling reaction takes place with a monochlorophenoxy radical coupled with a trichlorophenol molecule to release Cl radical. The phenolic oxygen in the monochlorophenoxy radical displaces a chlorine atom from the 2,4,6-trichlorophenol molecule. This results in an intermediate dimer product with an ether linkage. Then the elimination of HCl leads to formation of a dioxin-like structure (by-product H7) (51). As reported in literature that 2,4,6-tcp can be precursor compound to form PCDD.
The by-product H8 spectrum showed a loss of a molecule at m/z 146 indicating a possible ester group, as elucidated in fragmentation pattern from triclosan (Figure S1). Louw reported that radical/molecule bonding to form an ether-linked dimer, therefore, the reaction scheme could be coupling of a dichlorophenoxy and trichlorophenol molecule (Scheme 4) (52).
125
The by-product L4 spectrum reflected an elimination of a CO double bond (m/z 28). It is possible that L4 is result of coupling between a benzoquinone and monochlorophenol (Scheme
5)
126
Scheme 1. Ortho-ortho or ortho-para phenoxy radical coupling pathway.
(ortho-ortho)
(ortho-para)
Scheme 2. 2, 3’, 4-trichloro-2’-hydroxydiphenyl ether (triclosan isomer) formation pathway.
Scheme 3. Polychlorinated dibenzo-p-dioxin (PCDD) formation pathway.
Scheme 4. 2, 3’, 4, 5’-tetrachloro-2’-hydroxydiphenyl ether formation pathway.
Scheme 5. Benzoquinone and monochlorophenol coupling pathway.
127
128
6.3.5 Proposed Reaction Pathways for Electrooxidation of Phenol in the Presence of
Chloride
The proposed reaction pathways are shown in Figure 6-3. The first step is electrophilic substitution, which mainly leads to products from hypochlorination pathway. Chlorine substitution takes place on the ortho- and para- positions on aromatic rings to form 2-CP,
4-CP, 2,4-DCP, 2,6-DCP, and 2,4,6-TCP, respectively. Then oxidative coupling reactions of radicals occur between phenoxy radicals and chlorinated substituted phenoxy radicals to form chlorinated biphenyl and biphenyl ethers, such as triclosan-like and dioxin-like compounds. After the initial steps, the route may continue to cleavage of the ring structure, which results in production of carboxylic acids, as reported in several previous research studies (53-56). Analytical methods are available for detection of carboxylic acid products, however, they were not measured as part of the scope of this paper. At this point, it is not clear whether the detected dimers continue to form higher polymers or degrade to smaller molecules subsequently.
129
Figure 6-3. Proposed pathway from measured by-products.
6.3.6 Influence of Chloride Concentration on By-product Formation
Figure 6-4 shows the comparison of by-product formation with time progression under two different chloride concentrations. Note that by-products found in high chloride experiment (Figure 6-4a) were not found in the last sample taken at the end of the 6-hour experiment, it is possible that they were removed. These by-products account for part of the discrepancy between the measured and calculated TOC values (Figure 1 e&f). Figure
1 e&f also shows that more by-products were formed in high chloride than low chloride, implying more possibly organic acids formation via aromatic ring cleavage, as reported in previous research (47, 54).
130
H1 a 4000000 H2a H2b H3a H3b H3c H4a H4b H5a H5b H5c H5d H5e H6 3000000 H7 H8a H8b H8c H9 H10 H11
H12 2000000 H13a H13b
Counts H14a H14b
1000000
0 0200400600800 Time (min)
b L1a L1b L1c 4000000 L2a L2b L2c L2d L3a L3b L3c L3d L3e L3f L4 L5a L5b L6 3000000
2000000 Counts
1000000
0 0200400600800 Time (min)
Figure 6-4. Time-course progression of by-products formed in high-chloride (a) and low- chloride experiments (b).
6.4 Conclusion
LC-QTOF-MS was used to qualitatively assess the potential by-products formed during anodic electrochemical oxidation of phenol using the BDD anode in the presence of NaCl electrolyte of two different concentrations. Experimental results showed that phenol,
TOC and COD removal rates were higher when higher concentration of NaCl was used.
Chlorophenols formed as a result of electrophilic substitution via electrochemically generated free chlorine were identified as initial intermediates. However, discrepancies
131
between calculated TOC and measured TOC values and gaps in chlorine-containing species mass balance indicated formation of unknown chlorinated by-products that were not identified by HPLC. Accurate mass scans and MS/MS fragmentation spectra suggested that the gaps were mainly dimers, including triclosan-like and polychlorinated dibenzo-p-dioxins (PCDDs) by-products.
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Chapter 7 General Conclusions
7.1 Summary of Conclusions
• In Chapter 2, an HPLC method was developed for simultaneously analyzing
products from both hydroxylation and hypochlorination pathways of phenol
electrooxidation, and this became the stepping-stone for studying the
performance of reactor in the presence of chloride in the following chapters.
The advantage of using the established HPLC method is that trade-offs
between both competing pathways can be examined in the same sample,
which eliminates confounding effects of comparing different samples and
methods and helps to select the optimal treatment conditions that balance
removal efficiency against production of unwanted side reactions and end
products.
• In Chapter 3, the performances of treatment processes were evaluated through
impacts of various operational parameters on: phenol removal efficiency,
TOC removal, COD removal, current efficiency, in addition to unwanted by-
product formation. It was concluded that the removal rate of phenol was
higher at higher initial chloride concentration and lower initial phenol
concentration. Higher current efficiencies were obtained at higher initial
chloride concentration and higher initial phenol concentration, holding the
138
other conditions constant. It was also found out that the impact of initial
chloride concentration on chlorate formation rate was more significant than its
impact on phenol removal rate.
• In Chapter 4, a 24 factorial design was carried out and ANOVA results
showed that anode type and chloride concentration had the most significant
effects either individually or interactively on phenol removal rate, and that
chloride concentration had a considerable effect on chlorate production rate.
Additionally, applied current density had a significant effect on free chlorine
production rate after breakthrough if and when it occurred with BDD in the
presence of excess chloride. A 23 factorial design with a given reactor
configuration with either BDD or graphite anode was optimized using
response surface methodology (RSM) with respect to phenol removal and
control of chlorate production. Linear regression results showed that phenol
removal rate was the highest at low phenol concentration and high chloride
concentration, whereas low chloride concentration minimized chlorate
production.
• In Chapter 5, the intermediate products conversion formed in the
electrooxidation of phenol in the presence of NaCl was studied based on the
24 full factorial design approach. The proposed kinetic model proved to be
sufficient to simulate the intermediate production in the electrochemical
treatment processes. Comparison between the simulated rate constants k
values showed that intermediates conversion to “other products” were rate-
limited. The model clearly demonstrated the more favored pathway and 139
dominant species when two pathways were in competition. The limitation of
the model was that the “other products” may represent multiple unknown
species, and the values were merely calculated based on the difference
between measured concentrations of phenol and known products. This could
impact the errors of computation when performing the regression.
• In Chapter 6, LC-QTOF-MS was used to qualitatively investigate the
unknown by-products formed during phenol electrooxidation in the presence
of chloride at two levels (5 mM and 50 mM) using the BDD anode. Dimers
were found as potential by-products, and data indicate the potential for
formation of polychlorinated dibenzo-p-dioxins (PCDDs).
7.2 Future Study
For future study, it is suggested that the following areas can be further investigated:
• A center point can be added to improve the factorial design. A second or
higher-order regression models could be developed for the response-surface
analysis. In order to conduct the multi-response optimization, more advanced
software packages, such as Design-Expert, can be used for the fitting first- and
second-order response surfaces, and visualizing them. Therefore, the design
objective to simultaneously optimize the operating conditions that maximize
phenol removal efficiency and minimize chlorate production will be achieved
to attain the best system performance.
• The conversion rates of inorganic chlorine-containing species, such as
chloride ion, free chlorine and chlorate, can be included into the kinetic model.
140
This would add another layer of complexity to the model, and would be
interesting to pursue in terms of the chlorine balance.
• Dioxin standards can be purchased for quantification study using the LC-
QTOF-MS. Then the structures of potential formation of dioxin byproducts
can be confirmed by comparing their MS/MS spectra with standards. Also the
concentration of these intermediates can be measured. Therefore, their
contribution to total mass balance can be investigated. Ultimately these
concentration values may be plugged into the kinetic model as an expansion
on the “other products” category in order to make the model prediction more
accurate.
• Bromide could also be added into the matrix since bromate and brominated
disinfection byproducts are of critical concerns in drinking water. Similar
with chloride, bromide is naturally-occurring in the environment and is also
often found to be present in chlorination processes. Preliminary findings in
our lab showed that bromophenol intermediates were more favored than
chlorophenols in the presence of equal molar of chloride and bromide (Figure
7-1). The competition between brominated and chlorinated species adds
another interesting layer to the kinetic model and could lead to more in-depth
study on potential intermediates using LC-QTOF-MS.
141
0.5
0.4 phenol
0.3 4-chlorophenol
4-bromophenol
0.2 2,4,6-tribromophenol Concentration (mM) Concentration
0.1
0 0100200300400 Time (min)
Figure 7-1. Phenol and intermediates conversion with time when 0.5 mM phenol was treated in 25 mM/25 mM NaCl/NaBr using BDD anode at 12 mA/cm2 using the same reactor configuration as in the previous studies.
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