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Electrochemical reduction of haloacetic acids and exploration of their removal by electrochemical treatment
Electrochimica Acta 47 (2001) 747– 751 www.elsevier.com/locate/electacta
Electrochemical reduction of haloacetic acids and exploration of their removal by electrochemical treatment Gregory V. Korshin *, Mark D. Jensen Department of Ci6il and En6ironmental Engineering, Box 352700, Uni6ersity of Washington, Seattle, WA 98195 -2700, USA Received 26 October 2000; received in revised form 26 March 2001
Abstract The electrochemical reduction of chlorine- and bromine-containing haloacetic acids (HAAs) was quantified using copper and gold electrodes. All HAAs (except monochloroacetate) were shown to be electrochemically active. The rate of reduction increased with the number of halogen atoms in the organic molecule and was higher for copper. It was determined that the electrochemical treatment could lead to a virtually complete dehalogenation of brominated HAAs. However, the electrochemical treatment of chlorinated acetic acids was accompanied by the formation of monochloroacetate, whose direct reduction is difficult. Although the yield of the latter species decreases at increasingly cathodic potentials, the formation of monochloroacetic acid is likely to constitute a limitation for the total dehalogenation of chlorinated HAAs. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Chloroacetic and bromoacetic acids; Cathodic reduction; Electrochemical dehalogenation; Copper
1. Introduction Halogen-containing disinfection by-products (DBPs) formed in chlorinated drinking water [1 – 8] are ubiquitous and have been associated with adverse health effects in exposed populations [9,10]. Volatile trihalomethanes (THMs), haloacetic acids (HAAs) and other identified species (e.g. haloacetonitriles, chloral hydrate) reportedly contribute ca. 25, 20 and 5%, respectively, of the organic halogen incorporated into DBPs [2,3,7,8]. Compounds of unknown chemical nature constitute the remaining ca. 50%. Individual HAA species include mono-, di- and trichloroacetic acids (MCAA, DCAA and TCAA, respectively). The formation of MCAA in chlorinated water is typically low, while DCAA and TCAA predominate and have roughly comparable concentrations reaching up to 10 − 6 M [2,3,5,11,12]. In addition to chlorine-containing HAAs, mono-, di- and tribromoacetic acids (MBAA, DBAA and TBAA, respectively) and mixed species (e.g. bromochloroacetic acid, BCAA) have been found in the
* Corresponding author. Tel.: + 1-206-543-2374; fax: +1-206-6859185. E-mail address: [email protected] (G.V. Korshin).
presence of bromide [3,5 –8]. Of brominated HAAs, MBAA, BCAA and DBAA occur most notably. The control of HAAs is a challenge and a necessity because of the uncertainty of mechanisms of their generation upon halogenation of natural organic matter (NOM) in drinking water, high hydrophilicity, high yield at pHB 7 and health effects putatively associated with them [13 –15]. Because accepted treatment technologies (e.g. adsorption on activated carbon) may not perform well with HAAs due to their very high hydrophilicity, it is relevant to evaluate alternative techniques to decrease the concentration of these compounds in drinking water. Electrochemical treatment appears to represent an alternative method to achieve this goal. Indeed, the literature indicates that HAAs [16 –24] and other important DBPs such as THMs [25 –27] can undergo reductive dehalogenation in electrochemically controlled conditions. Although well documented, the utility of this process to decrease the concentration of HAAs in drinking water has not been documented, while the yield and identities of the HAA reduction products have not been adequately ascertained. Providing more data on the electrochemical HAA reduction constitutes the goal of this communication.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 7 5 5 - 1
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Fig. 1. Dependence of current density vs. electrode potential for 10 − 3 M mono-, di- and tribromoacetic and bromochloroacetic acids. Gold rotating disc electrode, rotation speed 400 rpm. Na2SO4 (0.01 M) background electrolyte, pH 7.0.
2. Experimental Reductive dehalogenation was studied for MCAA, DCAA, TCAA, MBAA, DBAA, TBAA and BCAA. All measurements were performed at 20 °C. Electrochemical experiments were carried out using a Cypress1080 computer-controlled potentiostat and a conventional electrochemical cell. A Pine ASR rotator was used for experiments with copper and gold rotating disk electrodes (RDE). The nominal surface area of the RDE was 0.6 cm2. All reported rotation speeds are quoted in revolutions per minute (rpm). Prior to the measurements, the electrodes were rinsed with 0.1 M HCl, methanol and high purity water and polarized for 3 min at E = − 0.2 V in the background electrolyte. All potentials were measured using an Ag/AgCl reference electrode and are reported versus this standard. All reported currents densities are normalized using the nominal surface area of the RDE and expressed in A/m2 units. The HAA dehalogenation and speciation of the reduction products were also studied using a flowthrough electrochemical reactor. The reactor consisted of a section of copper tubing employed as a cathode and a coaxially positioned stainless steel anode. The
Fig. 2. Dependence of current density vs. electrode potential for 10 − 3 M tribromoacetic acid. Gold rotating disc electrode, varying rotation speeds (as shown in figure). Na2SO4 (0.01 M) background electrolyte, pH 7.0.
diameters of the anode and cathode were 4.7 and 9.5 mm, respectively. The nominal volume of the reactor was 8.8 ml. The residence time in the reactor was varied from 3.5 to ca. 25 min. Prior to experiments, the cell was flushed with several bed volumes of 0.1 M HCl followed by extensive rinsing with deionized water. A double-junction Ag/AgCl reference electrode was utilized to control the potential of the cathode. Measurements of the effluent composition were made at three potential settings, including no electrochemical polarization and potentials −0.8 and − 1.0 V. A LPS-163A Leader power supply was used to maintain the potential of the cathode of the flow-trough reactor. All solutions were prepared using analytical grade reagents and Milli-Q water (specific conductivity of 18.2 MV cm − 1). The background electrolyte was either 0.01 M Na2SO4 or 0.01 M K2HPO4 for RDE and flow-through experiments, respectively. The phosphate salt was used in the latter case to prevent possible peak overlap in anionic chromatograms. The pH of the solutions was adjusted to 7.0. In the case of RDE experiments, oxygen was stripped from solutions by argon. The concentrations of HAAs and other anions in the effluent from the electrochemical reactor were determined using a Dionex DX500 ion chromatograph equipped with an AS-11A anion-exchange column. The precision of the acetate mass balance in these measurements was ca. 90–110% of the nominal value.
3. Results and discussion The potentiodynamic scans showed notable electroactivity of all bromine-containing HAAs (Fig. 1), TCAA and DCAA at the gold and copper electrodes. As illustrated by the results for TBAA (Fig. 2), the reduction of all HAAs was affected by mass transfer and the electrode potential. The behavior of highly brominated species had specific features. Namely, the current for TBAA had a ‘hump’ in the range of potentials + 0.3 to − 0.2 V. The intensity of this feature did not significantly change with rotation speed. The nature of this hump remains to be ascertained. For potentials between −0.3 and − 0.7 V, the current sharply increased for both TBAA and DBAA (Fig. 1). The onset of the cathodic reduction wave for MBAA was observed at potentials B −0.8 V. The potentiodynamic curve for BCAA was located between those for DBAA and MCAA. At EB − 1.0 V, the current was approaching its maximum for all brominated HAAs. In this range of potentials, the current density of HAA reduction exhibited a strong correlation with the number of bromine atoms in the organic molecule (Fig. 3). The chlorinated HAAs were also electroactive, but their behavior was more difficult to quantify using the
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Fig. 3. Dependence of the current density on the type of brominecontaining haloacetic acids. Gold electrode, potential − 1.2 V, HAA concentration 10 − 3 M, rotation speed 400 rpm.
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Fig. 6. Effects of mass transfer on the reduction of dibromoacetic acid. Gold and copper rotating disc electrodes. Potential − 1.0 V, pH 7.0, background electrolyte 0.01 M Na2SO4.
gold electrode since its surface was apparently fouled during successive potentiodynamic scans in solutions of mono- and dichloroacetic acids (MCAA and DCAA), although this was not observed for TCAA. No fouling was seen for copper, but its use was restricted to EB − 0.20 V since the surface of this metal was oxidized at more anodic potentials. A representative set of the potentiodynamic scans for the reduction of 10 − 3 M DCAA at the copper surface is shown in Fig. 4. As opposed to DCAA and TCAA, MCAA exhibited little
electroactivity at either gold or copper surfaces cathodically polarized up to − 1.3 V. However, the quantification of the MCAA reduction may be hindered by the onset of hydrogen evolution at potentials more cathodic than − 1.1 V. RDE experiments showed that the rate of electrochemical reduction of brominated HAAs was markedly different for copper and gold. The current corresponding to the reduction of MBAA was almost an order of magnitude higher for copper at medium to high rotation speeds (Fig. 5). It increased quasi-linearly with the square root of the rotation speed indicating mass transfer limitations, while the reduction of MBAA at the gold surface was apparently kinetically limited. The rate of DBAA and TBAA reduction at the copper surface was also higher than that for gold (Figs. 6 and 7). The higher HAA reduction rates associated with the use of copper indicated that this material could perform well in continuous flow electrochemical experiments. Direct measurements of the composition of effluents from the flow-through reactor demonstrated a notable decrease of HAA concentrations and accumulation of the associated reduction products following the electrochemical treatment at the copper cathode. This conclu-
Fig. 5. Effects of mass transfer on the reduction of monobromoacetic acid. Gold and copper rotating disc electrodes. Potential − 1.0 V, pH 7.0, background electrolyte 0.01 M Na2SO4.
Fig. 7. Effects of mass transfer on the reduction of tribromoacetic acid. Gold and copper rotating disc electrodes. Potential −1.0 V, pH 7.0, background electrolyte 0.01 M Na2SO4.
Fig. 4. Dependence of current density vs. electrode potential for 10 − 3 M dichloroacetic acid. Copper rotating disc electrode, varying rotation speeds (as shown in figure). Na2SO4 (0.01 M) background electrolyte, pH 7.0.
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Fig. 8. Composition of the effluent from the flow-through electrochemical reactor as a function of residence time. Influent contained 0.01 K2HPO4 and 6.3 ×10 − 4 M trichloroacetic acid. Potential − 1.0 V, pH 7.0.
Fig. 9. Speciation of reduced acetate species in effluents containing trichloroacetic, dichloroacetic, tribromoacetic or dibromoacetic acids following treatment in the flow-through reactor at E= − 0.8 V.
sion is exemplified by the data for TCAA (Fig. 8). This figure shows that the concentration of TCAA decreased with increase of the residence time, while reaction products such as chloride-ion, acetate, MCAA and DCAA were released. Similar data were obtained for DCAA, all brominated HAAs and BCAA. Comparison of the effluent composition with and without polarization of the cathode showed that all HAAs were stable in the
Fig. 10. Speciation of acetate species in effluents containing trichloroacetic, dichloroacetic, tribromoacetic or dibromoacetic acids following treatment in the flow-through reactor at E= − 1.0 V.
absence of electrochemical control. Only TBAA exhibited some spontaneous reduction. To identify the predominant reduction products, calculations of the speciation of the reduced acetate were carried out. In these calculations, DCAA, MCAA and acetate proper were considered as the products of TCAA reduction. For DCAA, they included MCAA and acetate. Analogous bromine-containing species were accounted for in experiments with MBAA, DBAA, TBAA and BCAA. It was determined that in all cases, the speciation of the reduction products was generally independent on the reactor residence time but sensitive to the electrode potential and the HAA nature. At E= − 0.8 V, DCAA was the major product of reduction of TCAA with notable presence of MCAA and a much lower concentration of acetate (ca. 65, 25 and 10% of the total reduced acetate, respectively) (Fig. 9). For DCAA, the distribution of reduced acetate at E= − 0.8 V was ca. 80 and 20% for MCAA and acetate, respectively. As opposed to chlorinated HAAs, the reduction of TBAA and DBAA at − 0.8 V was accompanied by the predominant formation of acetate and only a minor presence of MBAA (ca. 95 and 5%, respectively), while no DBAA was detected even in the case of TBAA reduction. The decrease of the electrode potential to −1.0 V significantly affected the distribution of the reduced acetate species (Fig. 10). For TCAA, the contribution of DCAA decreased from 65 to 15%, the contribution of MCAA changed from 25 to 20%, while that of acetate increased from 10 to 65%. For DCAA, the contribution of MCAA decreased from 80 to 25% with the corresponding increase of the acetate yield. The distribution of the reduced species for brominated HAAs at − 1.0 V was virtually the same compared to that at − 0.8 V. The data for BCAA are shown separately (Fig. 11). For this compound, no MBAA was found at either − 0.8 or − 1.0 V, and almost all reduced acetate was seen as MCAA or acetate. The contribution of MCAA
Fig. 11. Comparison of speciation of reduced acetate species in bromochloroacetic acid-containing effluents following treatment in the flow-through reactor at E = −0.8 and −1.0 V.
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was ca. 70% at − 0.8 V and decreased to 40% at − 1.0 V, while the contribution of acetate simultaneously increased from ca. 30 to 60%.
4. Conclusions The electrochemical behavior of six HAA species was examined in this study. All HAAs except monochloroacetic acid were determined to be active in electrochemical reduction. The reduction rates were significantly higher for copper compared with those for gold. Therefore, the nature of the electrode may be of major importance for technological applications. It was shown that brominated HAAs could be virtually completely dehalogenated through electrochemical treatment. Their reduction was accompanied by the release of free bromide and reduced organic species, which was shown to be overwhelmingly acetate. In contrast to bromine-containing HAAs, the electrochemical dehalogenation of chlorinated acetic acids was accompanied by the notable formation of monochloroacetic acid, whose direct reduction is difficult. The formation of this product may constitute a bottleneck for the total dehalogenation of chlorinated HAAs. Whether the release of MCAA caused by electrochemical reduction of DCAA and TCAA represents a tolerable scenario or this pathway has to be eliminated or circumvented constitutes an issue of further research. Despite the limitation associated with the formation of MCAA, our results indicate that electrochemical treatment may help achieve the goal of decreasing the concentration of HAAs (most notably, all brominated species) in drinking water. Implementation of this approach requires further elucidation of the HAA reduction rates for a variety of important electrode materials. Detailed exploration of the nature and occurrence of reaction by-products and intermediates is also necessary.
Acknowledgements This study was supported by the Royalty Research Fund of University of Washington (grant c 1986). The authors would like to thank Paul Kawamoto and Dr Stuart Strand (University of Washington) for their
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participation in the experimental activities and discussion of the data.
References [1] R.A. Larson, E.J. Weber, Reaction Mechanisms in Environmental Organic Chemistry, Lewis Publishers, Boca Raton, FL, 1994. [2] S.W. Krasner, M.J. McQuire, J.C. Jacangelo, N.L. Patania, K.M. Reagan, E.M. Aieta, J. Am. Water Works Assoc. 81 (8) (1989) 41. [3] P. Pourmoghaddas, A.A. Stevens, Water Res. 29 (1995) 2059. [4] A.G.I. Dalvi, R. Al-Rasheed, M.A. Javeed, Desalination 129 (2000) 261. [5] G.A. Cowman, P.C. Singer, Environ. Sci. Technol. 30 (1995) 16. [6] J.M. Symons, P.L.K. Fu, R.C. Dressman, A.A. Stevens, J. Am. Water Works Assoc. 79 (12) (1987) 114. [7] J. Nawrocki, S. Bilozor, J. Water Services Res. Aqua 46 (1997) 304. [8] L. Heller-Grossman, J. Manka, B. Limoni-Relis, M. Rebhun, Water Res. 27 (1993) 1323. [9] K. Waller, S.H. Swan, G. De Lorenze, B. Hopkins, Epidemiology 9 (1998) 134. [10] R.J. Bull, L.S. Birnbaum, K.P. Cantor, et al., Fundam. Appl. Toxicol. 28 (1995) 155. [11] P.C. Uden, J.W. Miller, J. Am. Water Works Assoc. 75 (1983) 524. [12] D.A. Reckhow, P.C. Singer, J. Am. Water Works Assoc. 82 (1989) 173. [13] J.M. Law, L. Lopez, A.B. De Angelo, Toxicol. Lett. 94 (1998) 19. [14] R.E. Linder, G.R. Klinefelter, L.F. Strader, et al., Reprod. Toxicol. 11 (1997) 681. [15] M.J. Nieuwenhuijsen, M.B. Toledano, N.E. Eaton, et al., Occup. Environ. Med. 57 (2000) 73. [16] P.J. Elving, C.-S. Tang, J. Am. Chem. Soc. 71 (1950) 3244. [17] P.J. Elving, I. Rosenthal, M.K. Kramer, J. Am. Chem. Soc. 73 (1951) 1717. [18] R. Horze, U. Fette, J. Electroanal. Chem. 339 (1992) 247. [19] N. Hu, J.F. Rustling, Anal. Chem. 63 (1991) 2163. [20] G.N. Kamau, J.F. Rustling, Langmuir 12 (1996) 2645. [21] H. Carrero, J.F. Rustling, Talanta 48 (1999) 711. [22] P. Akhtar, C.O. Too, G.G. Wallace, Anal. Chim. Acta 341 (1997) 141. [23] A.N. Zhuravlev, S.D. Shamshinov, I.A. Avrutskaya, Sov. Electrochem. 28 (1992) 709. [24] A.N. Zhuravlev, I.A. Avrutskaya, Sov. Electrochem. 25 (1989) 505. [25] C.S. Criddle, P.L. McCarthy, Environ. Sci. Technol. 25 (1991) 973. [26] L.J. Matheson, P.G. Tratnyek, Environ. Sci. Technol. 28 (1994) 2045. [27] N. Sonoyama, T. Sakata, Environm. Sci. Technol. 33 (1999) 3438.
Electrochemical reduction of haloacetic acids and exploration of their removal by electrochemical treatment Gregory V. Korshin *, Mark D. Jensen Department of Ci6il and En6ironmental Engineering, Box 352700, Uni6ersity of Washington, Seattle, WA 98195 -2700, USA Received 26 October 2000; received in revised form 26 March 2001
Abstract The electrochemical reduction of chlorine- and bromine-containing haloacetic acids (HAAs) was quantified using copper and gold electrodes. All HAAs (except monochloroacetate) were shown to be electrochemically active. The rate of reduction increased with the number of halogen atoms in the organic molecule and was higher for copper. It was determined that the electrochemical treatment could lead to a virtually complete dehalogenation of brominated HAAs. However, the electrochemical treatment of chlorinated acetic acids was accompanied by the formation of monochloroacetate, whose direct reduction is difficult. Although the yield of the latter species decreases at increasingly cathodic potentials, the formation of monochloroacetic acid is likely to constitute a limitation for the total dehalogenation of chlorinated HAAs. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Chloroacetic and bromoacetic acids; Cathodic reduction; Electrochemical dehalogenation; Copper
1. Introduction Halogen-containing disinfection by-products (DBPs) formed in chlorinated drinking water [1 – 8] are ubiquitous and have been associated with adverse health effects in exposed populations [9,10]. Volatile trihalomethanes (THMs), haloacetic acids (HAAs) and other identified species (e.g. haloacetonitriles, chloral hydrate) reportedly contribute ca. 25, 20 and 5%, respectively, of the organic halogen incorporated into DBPs [2,3,7,8]. Compounds of unknown chemical nature constitute the remaining ca. 50%. Individual HAA species include mono-, di- and trichloroacetic acids (MCAA, DCAA and TCAA, respectively). The formation of MCAA in chlorinated water is typically low, while DCAA and TCAA predominate and have roughly comparable concentrations reaching up to 10 − 6 M [2,3,5,11,12]. In addition to chlorine-containing HAAs, mono-, di- and tribromoacetic acids (MBAA, DBAA and TBAA, respectively) and mixed species (e.g. bromochloroacetic acid, BCAA) have been found in the
* Corresponding author. Tel.: + 1-206-543-2374; fax: +1-206-6859185. E-mail address: [email protected] (G.V. Korshin).
presence of bromide [3,5 –8]. Of brominated HAAs, MBAA, BCAA and DBAA occur most notably. The control of HAAs is a challenge and a necessity because of the uncertainty of mechanisms of their generation upon halogenation of natural organic matter (NOM) in drinking water, high hydrophilicity, high yield at pHB 7 and health effects putatively associated with them [13 –15]. Because accepted treatment technologies (e.g. adsorption on activated carbon) may not perform well with HAAs due to their very high hydrophilicity, it is relevant to evaluate alternative techniques to decrease the concentration of these compounds in drinking water. Electrochemical treatment appears to represent an alternative method to achieve this goal. Indeed, the literature indicates that HAAs [16 –24] and other important DBPs such as THMs [25 –27] can undergo reductive dehalogenation in electrochemically controlled conditions. Although well documented, the utility of this process to decrease the concentration of HAAs in drinking water has not been documented, while the yield and identities of the HAA reduction products have not been adequately ascertained. Providing more data on the electrochemical HAA reduction constitutes the goal of this communication.
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 7 5 5 - 1
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Fig. 1. Dependence of current density vs. electrode potential for 10 − 3 M mono-, di- and tribromoacetic and bromochloroacetic acids. Gold rotating disc electrode, rotation speed 400 rpm. Na2SO4 (0.01 M) background electrolyte, pH 7.0.
2. Experimental Reductive dehalogenation was studied for MCAA, DCAA, TCAA, MBAA, DBAA, TBAA and BCAA. All measurements were performed at 20 °C. Electrochemical experiments were carried out using a Cypress1080 computer-controlled potentiostat and a conventional electrochemical cell. A Pine ASR rotator was used for experiments with copper and gold rotating disk electrodes (RDE). The nominal surface area of the RDE was 0.6 cm2. All reported rotation speeds are quoted in revolutions per minute (rpm). Prior to the measurements, the electrodes were rinsed with 0.1 M HCl, methanol and high purity water and polarized for 3 min at E = − 0.2 V in the background electrolyte. All potentials were measured using an Ag/AgCl reference electrode and are reported versus this standard. All reported currents densities are normalized using the nominal surface area of the RDE and expressed in A/m2 units. The HAA dehalogenation and speciation of the reduction products were also studied using a flowthrough electrochemical reactor. The reactor consisted of a section of copper tubing employed as a cathode and a coaxially positioned stainless steel anode. The
Fig. 2. Dependence of current density vs. electrode potential for 10 − 3 M tribromoacetic acid. Gold rotating disc electrode, varying rotation speeds (as shown in figure). Na2SO4 (0.01 M) background electrolyte, pH 7.0.
diameters of the anode and cathode were 4.7 and 9.5 mm, respectively. The nominal volume of the reactor was 8.8 ml. The residence time in the reactor was varied from 3.5 to ca. 25 min. Prior to experiments, the cell was flushed with several bed volumes of 0.1 M HCl followed by extensive rinsing with deionized water. A double-junction Ag/AgCl reference electrode was utilized to control the potential of the cathode. Measurements of the effluent composition were made at three potential settings, including no electrochemical polarization and potentials −0.8 and − 1.0 V. A LPS-163A Leader power supply was used to maintain the potential of the cathode of the flow-trough reactor. All solutions were prepared using analytical grade reagents and Milli-Q water (specific conductivity of 18.2 MV cm − 1). The background electrolyte was either 0.01 M Na2SO4 or 0.01 M K2HPO4 for RDE and flow-through experiments, respectively. The phosphate salt was used in the latter case to prevent possible peak overlap in anionic chromatograms. The pH of the solutions was adjusted to 7.0. In the case of RDE experiments, oxygen was stripped from solutions by argon. The concentrations of HAAs and other anions in the effluent from the electrochemical reactor were determined using a Dionex DX500 ion chromatograph equipped with an AS-11A anion-exchange column. The precision of the acetate mass balance in these measurements was ca. 90–110% of the nominal value.
3. Results and discussion The potentiodynamic scans showed notable electroactivity of all bromine-containing HAAs (Fig. 1), TCAA and DCAA at the gold and copper electrodes. As illustrated by the results for TBAA (Fig. 2), the reduction of all HAAs was affected by mass transfer and the electrode potential. The behavior of highly brominated species had specific features. Namely, the current for TBAA had a ‘hump’ in the range of potentials + 0.3 to − 0.2 V. The intensity of this feature did not significantly change with rotation speed. The nature of this hump remains to be ascertained. For potentials between −0.3 and − 0.7 V, the current sharply increased for both TBAA and DBAA (Fig. 1). The onset of the cathodic reduction wave for MBAA was observed at potentials B −0.8 V. The potentiodynamic curve for BCAA was located between those for DBAA and MCAA. At EB − 1.0 V, the current was approaching its maximum for all brominated HAAs. In this range of potentials, the current density of HAA reduction exhibited a strong correlation with the number of bromine atoms in the organic molecule (Fig. 3). The chlorinated HAAs were also electroactive, but their behavior was more difficult to quantify using the
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Fig. 3. Dependence of the current density on the type of brominecontaining haloacetic acids. Gold electrode, potential − 1.2 V, HAA concentration 10 − 3 M, rotation speed 400 rpm.
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Fig. 6. Effects of mass transfer on the reduction of dibromoacetic acid. Gold and copper rotating disc electrodes. Potential − 1.0 V, pH 7.0, background electrolyte 0.01 M Na2SO4.
gold electrode since its surface was apparently fouled during successive potentiodynamic scans in solutions of mono- and dichloroacetic acids (MCAA and DCAA), although this was not observed for TCAA. No fouling was seen for copper, but its use was restricted to EB − 0.20 V since the surface of this metal was oxidized at more anodic potentials. A representative set of the potentiodynamic scans for the reduction of 10 − 3 M DCAA at the copper surface is shown in Fig. 4. As opposed to DCAA and TCAA, MCAA exhibited little
electroactivity at either gold or copper surfaces cathodically polarized up to − 1.3 V. However, the quantification of the MCAA reduction may be hindered by the onset of hydrogen evolution at potentials more cathodic than − 1.1 V. RDE experiments showed that the rate of electrochemical reduction of brominated HAAs was markedly different for copper and gold. The current corresponding to the reduction of MBAA was almost an order of magnitude higher for copper at medium to high rotation speeds (Fig. 5). It increased quasi-linearly with the square root of the rotation speed indicating mass transfer limitations, while the reduction of MBAA at the gold surface was apparently kinetically limited. The rate of DBAA and TBAA reduction at the copper surface was also higher than that for gold (Figs. 6 and 7). The higher HAA reduction rates associated with the use of copper indicated that this material could perform well in continuous flow electrochemical experiments. Direct measurements of the composition of effluents from the flow-through reactor demonstrated a notable decrease of HAA concentrations and accumulation of the associated reduction products following the electrochemical treatment at the copper cathode. This conclu-
Fig. 5. Effects of mass transfer on the reduction of monobromoacetic acid. Gold and copper rotating disc electrodes. Potential − 1.0 V, pH 7.0, background electrolyte 0.01 M Na2SO4.
Fig. 7. Effects of mass transfer on the reduction of tribromoacetic acid. Gold and copper rotating disc electrodes. Potential −1.0 V, pH 7.0, background electrolyte 0.01 M Na2SO4.
Fig. 4. Dependence of current density vs. electrode potential for 10 − 3 M dichloroacetic acid. Copper rotating disc electrode, varying rotation speeds (as shown in figure). Na2SO4 (0.01 M) background electrolyte, pH 7.0.
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Fig. 8. Composition of the effluent from the flow-through electrochemical reactor as a function of residence time. Influent contained 0.01 K2HPO4 and 6.3 ×10 − 4 M trichloroacetic acid. Potential − 1.0 V, pH 7.0.
Fig. 9. Speciation of reduced acetate species in effluents containing trichloroacetic, dichloroacetic, tribromoacetic or dibromoacetic acids following treatment in the flow-through reactor at E= − 0.8 V.
sion is exemplified by the data for TCAA (Fig. 8). This figure shows that the concentration of TCAA decreased with increase of the residence time, while reaction products such as chloride-ion, acetate, MCAA and DCAA were released. Similar data were obtained for DCAA, all brominated HAAs and BCAA. Comparison of the effluent composition with and without polarization of the cathode showed that all HAAs were stable in the
Fig. 10. Speciation of acetate species in effluents containing trichloroacetic, dichloroacetic, tribromoacetic or dibromoacetic acids following treatment in the flow-through reactor at E= − 1.0 V.
absence of electrochemical control. Only TBAA exhibited some spontaneous reduction. To identify the predominant reduction products, calculations of the speciation of the reduced acetate were carried out. In these calculations, DCAA, MCAA and acetate proper were considered as the products of TCAA reduction. For DCAA, they included MCAA and acetate. Analogous bromine-containing species were accounted for in experiments with MBAA, DBAA, TBAA and BCAA. It was determined that in all cases, the speciation of the reduction products was generally independent on the reactor residence time but sensitive to the electrode potential and the HAA nature. At E= − 0.8 V, DCAA was the major product of reduction of TCAA with notable presence of MCAA and a much lower concentration of acetate (ca. 65, 25 and 10% of the total reduced acetate, respectively) (Fig. 9). For DCAA, the distribution of reduced acetate at E= − 0.8 V was ca. 80 and 20% for MCAA and acetate, respectively. As opposed to chlorinated HAAs, the reduction of TBAA and DBAA at − 0.8 V was accompanied by the predominant formation of acetate and only a minor presence of MBAA (ca. 95 and 5%, respectively), while no DBAA was detected even in the case of TBAA reduction. The decrease of the electrode potential to −1.0 V significantly affected the distribution of the reduced acetate species (Fig. 10). For TCAA, the contribution of DCAA decreased from 65 to 15%, the contribution of MCAA changed from 25 to 20%, while that of acetate increased from 10 to 65%. For DCAA, the contribution of MCAA decreased from 80 to 25% with the corresponding increase of the acetate yield. The distribution of the reduced species for brominated HAAs at − 1.0 V was virtually the same compared to that at − 0.8 V. The data for BCAA are shown separately (Fig. 11). For this compound, no MBAA was found at either − 0.8 or − 1.0 V, and almost all reduced acetate was seen as MCAA or acetate. The contribution of MCAA
Fig. 11. Comparison of speciation of reduced acetate species in bromochloroacetic acid-containing effluents following treatment in the flow-through reactor at E = −0.8 and −1.0 V.
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was ca. 70% at − 0.8 V and decreased to 40% at − 1.0 V, while the contribution of acetate simultaneously increased from ca. 30 to 60%.
4. Conclusions The electrochemical behavior of six HAA species was examined in this study. All HAAs except monochloroacetic acid were determined to be active in electrochemical reduction. The reduction rates were significantly higher for copper compared with those for gold. Therefore, the nature of the electrode may be of major importance for technological applications. It was shown that brominated HAAs could be virtually completely dehalogenated through electrochemical treatment. Their reduction was accompanied by the release of free bromide and reduced organic species, which was shown to be overwhelmingly acetate. In contrast to bromine-containing HAAs, the electrochemical dehalogenation of chlorinated acetic acids was accompanied by the notable formation of monochloroacetic acid, whose direct reduction is difficult. The formation of this product may constitute a bottleneck for the total dehalogenation of chlorinated HAAs. Whether the release of MCAA caused by electrochemical reduction of DCAA and TCAA represents a tolerable scenario or this pathway has to be eliminated or circumvented constitutes an issue of further research. Despite the limitation associated with the formation of MCAA, our results indicate that electrochemical treatment may help achieve the goal of decreasing the concentration of HAAs (most notably, all brominated species) in drinking water. Implementation of this approach requires further elucidation of the HAA reduction rates for a variety of important electrode materials. Detailed exploration of the nature and occurrence of reaction by-products and intermediates is also necessary.
Acknowledgements This study was supported by the Royalty Research Fund of University of Washington (grant c 1986). The authors would like to thank Paul Kawamoto and Dr Stuart Strand (University of Washington) for their
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participation in the experimental activities and discussion of the data.
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