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Competition between Electrochemical Advanced Oxidation and Electrochemical Hypochlorination of Sulfamethoxazole at a Boron-Doped Diamond Anode Jordache Boudreau, Dorin Bejan, Shuhuan Li, and Nigel J. Bunce* Electrochemical Technology Centre, Chemistry Department, UniVersity of Guelph, 50 Stone Road East, Guelph, Ontario, Canada N1G 2W1
Sulfamethoxazole (SMX) was used as a model substrate for electrochemical oxidation at a boron-doped diamond anode in the presence of chloride ion, which is present in many waste streams. In the absence of chloride, oxidation of SMX involves mineralization, an electrochemical advanced oxidation process (EAOP) that is initiated by attack of anode-derived hydroxyl radicals. The rate of disappearance of SMX increased monotonically upon addition of chloride ion but without inhibiting mineralization in the early stages of oxidation. This demonstrated that electrochemical hypochlorination (EH) and EAOP are not mutually exclusive reaction pathways; products of EH can undergo EAOP and vice versa. Persistent chlorinated byproducts were formed in the presence of chloride ion, indicating that chloride is a significant detriment to the success of EAOP. No mineralization was observed upon chemical hypochlorination of SMX with sodium hypochlorite. Introduction Research into electrochemical methods for the treatment of aqueous wastes that contain organic contaminants has focused on anodes with a high overpotential for oxygen evolution, notably β-PbO2, Ti/SnO2, and boron-doped diamond (BDD).1-4 These “inactive” anodes are especially effective at producing sorbed hydroxyl radicals, also called “physisorbed active oxygen”.5 Hydroxyl radicals are key intermediates in a variety of remediation methods known as “advanced oxidation processes” (AOPs) because of their high reactivity toward most organic compounds through addition and/or hydrogen abstraction reactions.6,7 Some recalcitrant organics can be mineralized completely and with high apparent current efficiency at BDD anodes, in what has been termed an electrochemical advanced oxidation process (EAOP).8-13 Hydroxyl radicals do not form efficiently at “active” dimensionally stable anodes (DSAs) based on Ti/IrO2 and Ti/RuO2, which promote two-electron oxidation through a “higher oxide” mechanism (“chemisorbed active oxygen”).5 Active anodes promote efficient conversion of chloride (which is present in many waste streams) to hypochlorous acid (or, in alkaline solution, hypochlorite ion) in the same potential range as water oxidation. Hypochlorous acid and hypochlorite ion, known as free available chlorine (FAC), are chlorinating agents as well as oxidizing agents, and can produce chlorinated byproducts that are more toxic or more recalcitrant to remediation than the original contaminants. Electrochemical treatment of a chloridecontaining waste stream at anodes based on Ti/IrO2 and Ti/ RuO2 might therefore be expected to parallel conventional chemical hypochlorination. When FAC is generated electrochemically in the presence of substrate, electrochemical hypochlorination (EH) is a mediated electrolysis in which FAC reacts with the substrate with regeneration of chloride ion. There is also a trivial case of EH known as on-site generation of hypochlorite, in which FAC is produced electrochemically but reacts separately with the substrate. In the context of wastewater treatment, chloride ion commonly increases the loss of substrate at both active and inactive anodes, accompanied by the formation of chlorinated byproducts * To whom correspondence should be addressed. E-mail: nbunce@ uoguelph.ca.
due to hypochlorination.14-17 The electrochemical oxidation of oxalate in the presence of chloride18,19 is an exceptional case because no chlorinated byproducts are possible. In this work we examined the effect of chloride on the mineralization of sulfamethoxazole (SMX) at a BDD anode. Environmental sources of SMX include sewage, through excretion in human urine and improper disposal of unused material to drains,20,21 animal husbandry,22,23 and point discharges from pharmaceutical manufacturing plants.22,24-26 SMX is refractory toward conventional sewage water treatment,27-29 and various chemical oxidants have been studied for its post-treatment oxidation.30-34
Sulfonamides have been degraded by FAC in several matrixes.35-38 SMX reacts with FAC with second-order rate constants of 1.1 × 103 M-1 s-1 for neutral SMX and 2.4 × 103 M-1 s-1 for anionic SMX.39 These rather slow rate constants, compared with the high rate constant for the reaction of OH radicals with SMX (5.5 × 109 M-1 s-1),30 encouraged the belief that mineralization might be possible at a BDD anode even in the presence of chloride ion. Our hypothesis was that EH and EAOP would interact mechanistically at inactive anodes such as BDD, because the initial products of hypochlorination should be susceptible to hydroxyl radical attack, and vice versa. In addition, the oxidation of chloride is inefficient at BDD compared with active DSAs.40-42 Materials and Methods Materials. Sulfamethoxazole, 4-amino-N-(5-methyl-3-isoxazolyl)benzenesulfonamide (SMX), potassium iodide 99%+, and sodium perchlorate 99%+ were supplied by Sigma-Aldrich (Oakville, ON). Sodium sulfate, minimum 99%, and HPLC grade acetonitrile were obtained from Caledon Laboratories (Georgetown, ON); barium chloride dihydrate, sodium thiosulfate pentahydrate, sodium chloride, sodium phosphate monoba-
10.1021/ie900614d 2010 American Chemical Society Published on Web 02/18/2010
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sic monohydrate, and sodium hydroxide were supplied by Fisher Scientific Co. (Toronto, ON). “Ultra” bleach (6%) was purchased from No-Frills, Guelph ON, subsidiary of Loblaws Inc., Toronto, ON. Solutions were prepared using a Millipore Milli-Q Reagent Water System with water having a resistivity of about 18.2 MΩ cm. Oxygen and argon gases were supplied by BOC Canada Ltd. (Mississauga, ON). The Si/BDD anodes were obtained from Swiss Center for Electronics and Microtechnology, Inc., Neuchaˆtel; the cathode material was stainless steel (chemistry machine shop, University of Guelph). The Hg/Hg2SO4 reference electrode, Ered ) +0.65 V vs NHE, was supplied by Radiometer Analytical SAS, Lyon, France. Apparatus and Experimental Procedures. Batch electrolyses were performed with a custom-made Plexiglas undivided reactor, volume 65 mL, internal dimensions 3.8 × 3.8 × 4.6 cm. The immersed areas of both the stainless steel cathode and the Si/BDD anode were 3.75 cm2 in a total volume of solution of 45 mL. Some electrolyses with a solution volume of 90 mL were performed in 150 mL beaker. Solutions were stirred during electrolyses using a Thermix stirrer (Fisher Scientific Model 120 MR). Voltage, electrode potential, and current were monitored using a Wavetek DM5XL voltmeter and a Mastercraft 052-0060-2. Power to the electrochemical reactor was supplied by an EG&G Model 363 potentiostat/galvanostat. Chemical hypochlorination was performed in 250 mL beaker with a solution volume of 100 mL of 1 mM SMX and by adding 40 µL of bleach every 10 min. The bleach was standardized by iodometry: nominal concentration 6% w/v; actual 6.4%, 0.86 M. Cyclic voltammetry measurements were carried out at room temperature in the identical batch cell (single compartment, three-electrode Plexiglas reactor) as used for electrolysis, with a volume of 40 mL. Boron-doped diamond (Si/BDD) plate (1 cm2) was used as the working electrode, and a platinum plate was used as the counter electrode. The SMX solutions had concentrations of 0.01, 0.05, or 1.0 mM and contained 0.050 M Na2SO4 as the supporting electrolyte. Cyclic voltammetry was performed with an EG&G Model 273 potentiostat/galvanostat with a fixed sweep rate of 50 mV s-1. Data were recorded using Power Suite version 2.12.1 software. Analytical Measurements. The progress of the reactions was followed by HPLC, which employed an Agilent 1200 series system, equipped with a 1200 Series variable wavelength detector set at 254 nm or a Waters 600E system, equipped with a Waters 2487 dual λ absorbance detector set at 254 nm and a Supelco Discovery C18 column 150 × 4.6 mm (5 µm), equipped with a silica precolumn guard. A new HPLC method was developed by using a mobile solvent of acetonitrile:water (50: 50) (filtered through a 0.2 µm filter) at a flow rate of 1.0 mL min-1 (retention time of SMX was ∼2.8 min). Samples were manually injected into the 20 µL sample loop of a Rheodyne injector. Total organic carbon (TOC) was analyzed with a Shimadzu TOC analyzer, Model TOC-VCPN. Adsorbable organic halogen (AOX) was determined by ALS Laboratory Group, Thunder Bay, ON. Sulfate was determined by turbidimetry using barium chloride with a Pharmacia LKB Novaspec II UV/vis Spectrometer set to 420 nm to measure the apparent absorbance of the samples.43 Results and Discussion In principle, electrochemical oxidation of a substrate at an inactive anode could involve a combination of direct electron
Figure 1. Consecutive (no cleaning in between) cyclic voltammograms (0 to +1.2 V and back to 0 V) for 1 mM SMX at Si/BDD electrode. Sweep rate 50 mV s-1; supporting electrolyte 0.05 mM Na2SO4; working electrode area 1 cm2. (A) First sweep; (B) second sweep; (C) third sweep; (D) solvent.
transfer at the anode, reaction with hydroxyl radicals, and (in the presence of chloride ion) hypochlorination. Preechaworapun et al.44 observed oxidation peaks at ∼+1.1 V vs Ag/AgCl for several 4-aminosulfonamides by cyclic voltammetry, indicating that it should be possible to examine the direct oxidation of SMX at potentials less positive than for the oxidation of water, and hence in the absence of hydroxyl radicals. Direct oxidation of SMX to the corresponding azo compound had previously been observed at carbon anodes, through a series of direct oneelectron oxidations.45 Potentiostatic Oxidation of SMX. We studied the direct electrooxidation of 1 mM SMX at Si/BDD within the range of water stability in the absence of chloride by potentiostatic electrolysis at +0.85 V vs Hg/Hg2SO4 (+1.5 V vs NHE), using 0.05 M Na2SO4 as the supporting electrolyte. No gas bubbles were observed, showing that there was no discharge of water and hence no production of hydroxyl radicals at this potential. The solution remained colorless at pH 5-6; hence no azo compound was formed. A small but irreproducible loss of SMX was observed, suggesting that polymeric organic material had deposited on the anode, as is known in the electrooxidation of other aromatic amines.46,47 We confirmed this postulate by cyclic voltammetry. Figure 1 shows a well-defined irreversible oxidation current at ∼0.85 V vs Hg/Hg2SO4 (+1.5 V vs NHE) on a first sweep from 0 to +1.2 V vs Hg/Hg2SO4, with diminishing oxidation current upon repeated cycles. When conventional linear sweep voltammograms (50 mV s-1) were run out to +2.5 V vs Hg/Hg2SO4, the oxidation peak at +0.85 V could be seen reproducibly without cleaning the electrode between scans. The current increased with concentration of SMX, reaching maximal values at g0.75 mM (Figure 1 in the Supporting Information). We concluded that the hydroxyl radicals generated at the BDD anode at +2.5 V “clean off” the material deposited on the anode and restore its activity. When electrolyses were carried out using 0.05 M Na2SO4 as the supporting electrolyte, with and without 1 mM SMX, and measuring the current as the potential vs Hg/Hg2SO4 increased point by point from 0 to 2.5 V, the induced current was lower in the presence of SMX, notwithstanding the oxidation peak at
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Figure 2. Comparison of 1.0 mM SMX potentiostatic electrolysis at +2.5 V vs Hg/Hg2SO4 (A, ×) and amperostatic electrolysis at 10 mA (B, ]) at Si/BDD in an undivided batch cell using the half-order kinetic model. Supporting electrolyte 0.050 M Na2SO4.
Figure 3. Rate of SMX loss as a function of chloride concentration. Total ionic strength 100 mM; initial SMX concentration 1 mM; current 20 mA.
+0.85 V seen in Figure 1 (see Figure 2 in the Supporting Information). In essence, this experiment constituted a linear sweep voltammogram, in which the electrode was deactivated by SMX during the experiment due to the very slow “sweep rate”. In potentiostatic oxidation of 1 mM SMX at +2.5 V vs Hg/ Hg2SO4 (+3.15 V vs NHE), gas was evolved at the anode, the pH remained constant at 5-6, and the concentration of SMX decreased continuously throughout the electrolysis (Figure 3 in the Supporting Information). The current flowing through the cell remained constant at approximately 11 mA. After 195 min, the loss of SMX by HPLC was 80% and the reduction of TOC was 55% (both triplicate measurements), compared with TOC loss of only 9% in a potentiostatic electrolysis for 120 min at 0.85 V. This experiment closely paralleled an amperostatic electrolysis at 10 mA of a similar 1.0 mM solution of SMX for which E(anode) was +2.26 V vs Hg/Hg2SO4. Figure 2 compares the two sets of data using the half-order kinetic model developed in our previous work.48 We supported the hypothesis that OH radicals continuously “cleaned” the anode surface of polymeric material at the more positive anode potential by carrying out SMX potentiostatic electrolysis under alternating low (+0.85 vs Hg/Hg2SO4) and high (+2.5 V vs Hg/Hg2SO4) potential. No reaction occurred at the lower potential (symbol [), but at the higher potential (symbol O) copious gas was evolved at the cathode (slight gas evolution at the anode) with steady degradation of SMX (Figure 4 in the Supporting Information). The potentiostatic experiments demonstrate that the remediation of SMX occurs through the EAOP/hydroxyl radical mechanism rather than by direct electron transfer from SMX at the anode. The need for concomitant hydroxyl radical formation to avoid anode fouling explains why we consistently observed only minimal loss of SMX upon electrolysis at a Ti/IrO2-Ta2O5 anode, at which physisorbed hydroxyl radicals are not produced.5 Amperostatic Oxidation of SMX in the Presence of Chloride Ion. Amperostatic electrolyses of 40 mL of sulfamethoxazole (0.25-1.0 mM) were run in an undivided batch cell using a Si/BDD anode and stainless steel cathode in the
presence of 1-50 mM NaCl. The solutions remained neutral in the range pH 6-7. The loss of SMX was followed by HPLC with UV detection at 254 nm; as the reaction proceeded, the HPLC traces showed the formation of additional products (see below). Because of near overlap of some of these peaks with residual SMX, the analyses in the presence of chloride were made on the basis of peak heights rather than peak areas, and SMX could not realistically be quantitated beyond about 70% conversion. The kinetics of disappearance of SMX in the presence of chloride ion followed a partial order kinetic model with respect to substrate (rate ) k[SMX]0.5), showing contributions from both current control and mass transport control (Figure 5A,B in the Supporting Information), similar to what was previously observed in the absence of chloride.48 Unlike the previous work, where the rate varied with (current density)0.4, the rate of loss of SMX increased strongly with current (Figure 6 in the Supporting Information). This indicates that attack by FAC, whose formation will be current controlled at these concentrations of chloride, contributes significantly to the disappearance of SMX. Full data are given in Table 1 in the Supporting Information. The rate of loss of SMX increased gradually with the concentration of chloride ion in the absence of other supporting electrolyte (Table 1 in the Supporting Information). At constant 100 mM total ionic strength in mixtures of NaCl and NaClO4, the loss of SMX increased about 3-fold as the concentration of chloride increased from 0 to 100 mM (Figure 3). In the absence of chloride the HPLC traces showed only minor byproduct peaks, all with retention times less than that of SMX (more polar than SMX), but when chloride was present there were numerous byproducts, mostly at longer retention times than that of SMX (Figure 7A-C in the Supporting Information). Adsorbable organic chloride (AOX) analysis showed that they were chlorination byproducts. Although their specific identification was not pursued, the work of Dodd and Huang39 on the reaction of SMX with FAC suggests mechanisms involving N-chlorination of the aniline ring, followed by either Orton rearrangement to 2-chloro-SMX or cleavage of the molecule to give a quinonemethide.
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Figure 5. Chemical hypochlorination of 0.5 mM SMX. [, SMX (µmol); 9, TOC (ppm C).
Figure 4. (A) TOC loss in the early stages of electrolysis (SMX still present) for different concentrations of chloride: O, no chloride; ×, 25 mM; 0, 100 mM. Total ionic strength 100 mM; initial SMX concentration 1 mM; current 20 mA. (B) Electrolysis of SMX 1 mM at 100 mA in the presence of chloride (100 mM NaCl): b, SMX; 2, TOC; 9, AOX; O, TOC in the absence of chloride (100 mM NaClO4).
Early in the reaction, before most of the SMX had disappeared, mineralization was proportional to charge passed through the solution (current controlled loss of TOC), about 4 times slower than the disappearance of substrate, and independent of the chloride concentration (Figure 4A). Beyond the disappearance of SMX, mineralization was inhibited by chloride and AOX analysis showed that persistent chlorinated byproducts were present in solution (Figure 4B). This is a significant limitation on the use of EAOP for wastes that contain chloride ion, although almost complete mineralization was possible, in both the presence and absence of chloride at sufficiently long electrolysis times (Figure 8 in the Supporting Information). We emphasize that successful “remediation” of a waste is not achieved once the original contaminant has disappeared if the reaction byproducts are more toxic or (as in this case) more persistent than the original contaminant. For example, Zaviska et al.49 at a DSA and Wu et al.16 at BDD showed that chloride
accelerated the electrochemical bleaching of various dyes, but they did not study the reaction products. Chloride ion accelerated both substrate removal and mineralization of the azo dye Acid Red 29 at an inactive anode based on Ti/SnO2.50 Costa et al.15 found that mineralization of Acid Black 210 was incomplete and that AOX persisted in solution beyond the disappearance of substrate at a BDD anode in the presence of chloride. Chloride ion had little effect on the rate of disappearance of 2-naphthol at Ti/(Ru,Ir)O2 but significantly retarded mineralization.51 Unexpectedly, chloride ion accelerated the loss of phenol at Ti/IrO2, due to hypochlorite formation, but not at Ti/SnO2, an “inactive” anode.14 As a control reaction, we studied the chemical hypochlorination of SMX using NaOCl solution in buffered solution at pH 6. The loss of SMX was efficient on the criterion of moles of SMX lost per mole of NaOCl added (Figure 5), but unlike EAOP in the presence of chloride, chemical hypochlorination was not accompanied by mineralization. Chlorinated byproducts were formed, but their pattern was different from that observed in EAOP in the presence of chloride ion (Compare Figures 7C and 7D in the Supporting Information). The near constancy of the initial loss of TOC with varying [Cl-] indicates that the early products of EH attack on SMX are susceptible to further reaction by the EAOP route, and the different patterns of byproducts confirm our original hypothesis of “crossover” between the EH and EAOP pathways. The key difference between chemical hypochlorination and chloride-assisted electrolysis under EAOP conditions is that chemical hypochlorination does not lead to mineralization. We could not study the chloride-assisted degradation of SMX at Ti/IrO2-Ta2O5, where electrochemical hypochlorination (EH) should dominate, because of anode fouling as discussed earlier. Equation 1 represents the mineralization of SMX, assuming that all the sulfur and nitrogen atoms in SMX were oxidized to sulfate and nitrate. C10H11N3O3S + 30H2O f 10CO2 + 3NO3- + SO42- + 71H+ + 66e- (1) Although Dodd and Huang39 had reported qualitatively the formation of sulfate in the reaction of SMX with FAC,
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turbidimetric analysis revealed that the yield of sulfate in this work was only ∼25%, even when mineralization was almost complete. Copious gas was evolved at the cathode, but little gas evolution occurred at the cathode; i.e., little charge was “wasted” by oxidizing water to O2. This implies that the current efficiency for mineralization of SMX was high, but a quantitative estimate was not possible without identification and quantification of all products of incomplete mineralization. In conclusion, we have demonstrated that the rate of disappearance of a model aromatic substrate (SMX) is accelerated by chloride ion under EAOP conditions at a BDD anode, confirming our initial hypothesis that the products of EH are susceptible to EAOP. Although the rate of mineralization of SMX is initially indifferent to the presence of chloride, competing electrochemical hypochlorination yields chlorinated byproducts that are substantially more resistant to mineralization than the starting material. Simply electrolyzing until the original contaminant has disappeared from solution is not a sufficient criterion for remediation, but almost complete mineralization is possible for sufficiently prolonged electrolysis. An unresolved issue in the context of electrochemical waste remediation is whether the oxidation of chloride ion gives worrisome amounts of chlorate and perchlorate, a concern that has been expressed in the context of drinking water disinfection.52-56 Overoxidation of hypochlorite is inevitable in those systems because of the requirement for an FAC residual, but it may not be the case for wastewater treatment. Acknowledgment This work was supported financially by the Natural Sciences and Engineering Research Council of Canada. Supporting Information Available: Table 1 listing degradation rates of SMX in electrochemical oxidation at Si/ BDD. Figures 1-8 showing voltammetry, electrolysis and hypochlorination results involving SMX and Si/BDD. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Martinez-Huitle, C. A.; Brillas, E. Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review. Appl. Catal., B 2009, 87, 105. (2) Martinez-Huitle, C. A.; De Battisti, A.; Ferro, S.; Reyna, S.; CerroLopez, M.; Quiro, M. A. Removal of the pesticide methamidophos from aqueous solutions by electrooxidation using Pb/PbO2, Ti/SnO2, and Si/BDD electrodes. EnViron. Sci. Technol. 2008, 42, 6929. (3) Oturan, M. A.; Brillas, E. Electrochemical advanced oxidation processes (EAOPs) for environmental applications. Port. Electrochim. Acta 2007, 25, 1. (4) Kraft, A. Doped diamond: a compact review on a new, versatile electrode material. Int. J. Electrochem. Sci. 2007, 2, 355. (5) Kapalka, A.; Foti, G.; Comninellis, C. Kinetic modelling of the electrochemical mineralization of organic pollutants for wastewater treatment. J. Appl. Electrochem. 2008, 38, 7. (6) Kapalka, A.; Foti, G.; Comninellis, C. The importance of electrode material in environmental electrochemistry. Electrochim. Acta 2009, 54, 2018. (7) Comninellis, C.; Kapalka, A.; Malato, S.; Parsons, S. A.; Poulios, I.; Mantzavinos, D. Advanced oxidation processes for water treatment: advances and trends for R&D. J. Chem. Technol. Biotechnol. 2008, 83, 769. (8) Rodrigo, M. A.; Michaud, P. A.; Duo, I.; Panizza, M.; Cerisola, G.; Comninellis, C. Oxidation of 4-chlorophenol at boron-doped diamond electrode for wastewater treatment. J. Electrochem. Soc. 2001, 148, D60.
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ReceiVed for reView April 21, 2009 ReVised manuscript receiVed September 21, 2009 Accepted February 5, 2010 IE900614D
