Environ. Sci. Technol. 2005, 39, 5837-5843
Effect of Current Density on Enhanced Transformation of Naphthalene AKRAM N. ALSHAWABKEH* AND HUSSAM SARAHNEY Department of Civil and Environmental Engineering, 360 Huntington Avenue, Boston, Massachusetts 02115
The effect of current density on electrochemically enhanced transformation of naphthalene is evaluated. Electrochemical reactors, composed of an anode and a cathode separated by a Nafion membrane, were used to evaluate the effect of three current densities (1, 9, and 18 mA/ L) on the transformation of naphthalene at two concentration levels (13 and 25 mg/L). Transformation rates varied based on the concentration and current density. Almost 88% of the 13 mg/L naphthalene is degraded after 8 h of treatment under 18.2 mA/L. At the same time, more than 90 h was required to degrade the same amount under 9 mA/ L. The results show that most of the naphthalene degradation occurred in the first 4 h under transformation rates of 2.24 and 1.11 mg/L h under applied currents of 18.2 and 9 mA/L, respectively. Increasing the naphthalene concentration to 25 mg/L produced similar results. Under 18.2 mA/L, the redox potential increased significantly at the anolyte in the first 8 h to about 900 mV. After that, the redox potential continued to increase, but at a lower rate, until it reached 1380 mV at the end of processing. Similar behavior is noted for the anolyte pH, which decreased significantly in the first 8 h to less than 2.5 and continued to decrease until it reached a pH value of 1.86 at the end of testing. Naphthalene transformation can be attributed to electrochemically enhanced oxidation at the anolyte by chlorine gas produced by electrolysis.
Introduction Biodegradation of naphthalene as a model PAH is welldocumented (1-5). Generally, low bioavailability of PAHs and the slow transformation rates are the limiting factors for biological treatments, and complete biodegradation of PAHs is often limited (i.e., it is rare to obtain carbon dioxide as a final product). Abiotic water and wastewater treatment methods such as ozonation, UV treatment, and chlorination show different efficiencies in degrading PAHs. In photooxidation, single oxygen (O•) and other oxidants such as ozone (O3) and hydroxyl radicals (OH•) induce the PAH reaction (6). The main products of PAH photooxidation are endoperoxides that produce many other products such as diones. While complete disappearance of naphthalene is demonstrated (7) by injection of free chlorine and without formation of any chlorinated naphthalene byproducts, studies (8) reported that quinines and polychlorinated aromatic compounds are the main products of chlorine and ozone reaction with PAHs. Oxidation of PAHs by ozonation and chlorination * Corresponding author phone: (617) 373-3994; fax: (617) 3734419; e-mail:
[email protected]. 10.1021/es049645f CCC: $30.25 Published on Web 06/30/2005
2005 American Chemical Society
is dependent on the PAH type, pH, and water temperature. For example, 60% of PAHs is removed by chlorination in filtered water at 17 °C and a pH of 7.5, and reduction in PAH levels increase with decreasing pH due to hypochlorous dissociation (9, 10). Furthermore, the selectivity of abiotic transformation of PAHs has been reported. For example, ozone oxidation of three- to five-ring PAHs dissolved in oil/ water emulsions showed that high PAHs were not oxidized without first removing low-level PAHs and that the reaction was dependent upon dissolved ozone concentration (11). Electrochemical redox methods depend on passing a direct current (dc) across electrodes in an aqueous medium. If the electrolytes are separated to prevent mixing but allow charge flow, then two main pH and redox conditions may develop (12, 13): at the anolyte (anode solution), protons and oxygen are produced (oxidizing, acidic conditions develop), and at the cathode, hydroxyl anions and hydrogen gas are produced (reducing, alkaline conditions develop):
2H2O f O2 + 4H+ + 4e- E° ) +1.229 V 4H2O + 4e- f 2H2 + 4OH- E° ) -0.827 V Other electrolysis products may result depending on the availability of ions and their electrochemical redox potentials. The presence of chloride ions may result in the formation of chlorine gas instead of oxygen gas at the anode
2Cl- f Cl2 + 2eAlthough some secondary reactions might be favored at the cathode because of their lower electrochemical potential, the water reduction is dominant. Within the first few days of processing, electrolysis reactions drop the pH at the anode to below 2 and increase it at the cathode to above 10. The characteristics of oxygen generation and side reactions were evaluated (14) using an electrolytic cell assembly with a 1.4 cm anode-cathode spacing (the setup does not allow separation of electrolysis products at the cathode and anode). Removal of up to 98% of naphthalene by electrochemically enhanced oxidation is reported using this setup (14) in the presence of 35 mg/L chlorine in the electrolyte at pH ) 4. The removal efficiency was orders of magnitude higher than that at neutral pH. Other studies (15) investigated the electrochemical aeration for naphthalene and the effects of anodic oxidation of chloride, formation of HOCL, and cathodic reduction of oxygen, producing hydrogen peroxide for direct oxidation of naphthalene. Direct electrochemical oxidation at the anode may result in the breakdown of some organic contaminants, depending upon electrode type and the electrochemical redox potential of the contaminant. Electrochemical treatment of wastewater containing naphthalene sulfonic acid and anthraquinonesulfonic acid resulted in more than 95% COD reduction using Ti/Pt electrode at 5 g/L sodium chloride (16). Electrochemical transformation of methyl-parathion (MeP) pesticide was demonstrated using a Ti/Pt anode and a stainless steel cathode and was more effective when the pH of the brine solution was acidic (17). Electrooxidation of 1,4-benzoquinone in a water solution was shown to depend on the type of electrode (18). Primary oxidation (i.e., the breakdown of a benzene ring) was attained at the IrO2 anode, resulting in an accumulation of carboxylic acid formation as a final nontoxic product. Using the SnO2 anode resulted in a faster rate of formation and then oxidation of carboxylic producing only CO2 as the final product (18). VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic demonstrating the concept of an electrochemical redox barrier.
While contaminants, such as naphthalene, can be transformed by several biological and chemical methods, there are always challenges. For example, biological methods tend to require a relatively long time (on the order of weeks) for degradation, and chemical methods require injection of significant volumes of additives, such as hydrogen peroxide or chlorine gas. Electrochemical enhancement can accelerate chemical transformation without the injection of additives. The techniques may not be limited to organics but also for heavy metals and other contaminants that can be affected by redox changes. For example, studies (19) showed that electrochemical redox can be adopted for enhanced reduction of Cr(VI) in contaminated clay. A technology for electrochemically enhanced redox can be developed for in situ or ex situ treatment of contaminants, such as naphthalene and Cr(VI), in groundwater. Permeable electrodes can be inserted in aquifers to develop electrochemical redox barriers (ERB) for enhanced oxidation and/ or reduction of target contaminants. Figure 1 shows a schematic where the flowing, contaminated groundwater is intercepted by an electrochemical redox barrier. Anodes are placed first, producing an oxidizing environment, followed by cathodes. The separation between the electrodes by soil is critical because it minimizes mixing of the oxidizing environment at the anode and reducing environment at the cathode. The spacing between anode and cathode will depend on groundwater flow and kinetics of electrochemical reactions and transport. Other configurations are also possible based on the kinetics of contaminant transformation. However, it is necessary to evaluate electrochemically enhanced transformation of contaminated groundwater prior to engineering in situ electrochemical redox barriers. In other words, patch experiments on the feasibility, kinetics, and transformation rates of contaminants are necessary to identify engineering parameters. This study focuses on batch reactors to demonstrate electrochemically enhanced transformation of naphthalene in water. This will facilitate identification of optimum conditions required for implementation in soils. This paper evaluates naphthalene, as a representative PAH, breakdown by electrochemically enhanced oxidation. Naphthalene is selected because it has similar chemical and physical properties as other PAHs, except that it is more soluble in water, a property that makes naphthalene more available in solution and more exposed to reactivity than most other PAHs. The specific objectives for this study are to evaluate naphthalene transformation under different electric current densities, to assess the impacts of different naphthalene concentrations on electrochemical degradation, 5838
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FIGURE 2. Schematic of the experimental setup. and to study the correlation/effect of redox potential, pH, and Cl2 on naphthalene transformation.
Materials and Methods The electrochemical transformation experiments were conducted using a glass electrolysis reactor to minimize adsorption of organics on the cell walls. Each reactor consists of two 550 mL bottle-shaped compartments that were connected by a glass tube at the bottom (Figure 2). A protonpermeable membrane (Nafion-112) (Nafion is a Dupont registered trademark for its brand of perfluorosulfonic acid polymer products, made and sold by E. I. du Pont de Nemours and Company) was used as a cell junction that physically separates the electrolytes in the two compartments. Nafion membranes perform as electrically conductive barriers that selectively transport cations across the electrochemical cell. They are chemically resistant and durable. The use of the membranes is not proposed for field implementation but only in the laboratory experimental setup to separate the electrolytes. In practice, the anodes and cathodes will be separated by soil, which serves as an ion exchange medium and minimizes electrolyte mixing. Each compartment of the setup has one port that is used for fitting the electrodes into the solution and for sampling. The electrodes, made of a mesh of titanium core with mixed metal oxide (MMO) coating, are 10.2 cm × 1.3 cm × 0.12 cm (4 in. × 0.5 in. × 1/16 in.). A lab bench DC power supply (HPE3612A) was used to deliver direct electric current.
TABLE 1. Electrochemical Effect Experiments concentration of naphthalene (in 0.2 M NaCl) (mg/L)
applied current (mA)
current density (mA/L)
13
1 5 10 0.0
1.8 9 18.2 control
25
1 5 10 0.0
1.8 9 18.2 control
35
5 10 0.0
9 18.2 control
sampling time (h) 0, 4, 8, 24, 48, 72, 96
0, 4, 8, 24, 48, 72, 96
0, 2, 8, 48, 72
total no. of experiments
Table 1 lists the experimental parameters, which include three current densities (1.8, 9, and 18.2 mA/L) and two concentration levels (13 and 25 mg/L). The current densities are presented per volume to measure the electric charge applied per specific volume of electrolyte, which is useful for evaluating the electrochemical changes. This is different than electrokinetic or electrophoresis applications in which current densities are measured per cross-sectional area, an important parameter for species transport rates. The concentrations reflect levels at or below saturation for naphthalene. The initial concentrations in Table 1 represent the average of duplicate test cells immediately after naphthalene injection and after 2 min of manually shaking the reactors. Another cell was used as a control (no electric current applied) to account for physical losses through volatilization, the losses through adsorption onto the cell glass, and any losses into the Teflon coated port cover. The solutions used in electrochemical redox were prepared by injecting a specific amount of the 20 000 mg/L stock solution of naphthalene in deionized water to produce the desired concentration. Fifty milliliters of NaCl (0.2 M) was used to increase the conductivity of the electrolyte. To assess the effects of chloride, tests were also conducted using similar concentrations of sodium nitrate solution as an electrolyte. All electrochemical redox tests, including the control, were conducted inside dark boxes to prevent any potential photooxidation. Samples were collected and analyzed after 0, 2, 4, 8, 24, 48, 72, and 96 h. Replicate experiments were also conducted to assess the effect of the anolyte (as a medium) on the naphthalene degradation. A current density of 18 mA/L was applied for 3 days on a 0.2 M NaCl solution in deionized water without injecting naphthalene. The anolyte was transferred to 100 mL serum bottles, and naphthalene was injected into the solution to form three different concentrations (15, 30, and 60 mg/L). The samples and controls were analyzed directly by high performance liquid chromatography (HPLC). Measurements of pH were conducted using ThermoOrion combination electrode with a VWR Scientific Model 200 m. The pH electrodes were standardized using a commercial buffer of pH 7 and an appropriate buffer of either pH 4 or 10. Redox potential (Eh) of the electrolytes was determined using a Cole-Parmer combination Ag/Ag/Cl reference electrode with a platinum (Pt) band and an Accumet Basic meter. The electrode was standardized by a YSI Zobell ORP calibration solution. Millivolt readings were converted to Eh, using the electrode readings plus the standard potential of a reference electrode at a given temperature. Conductivity measurements were conducted using a Cole-Parmer conductivity cell, Model 19550-60, and a Cole-Parmer conductivity meter, Model 30. Chemical injection and liquid
replicates 2 2 2 1 2 2 2 1 1 1 1 17
sampling were conducted using Hamilton series 1700 Gastight Syringes equipped with Teflon fluorocarbon Resin Luer Look (TLL). Naphthalene was purchased from Sigma-Aldrich Co. with 99% or greater purity. Acetonitrile, super-gradient HPLC grade with a purity of 99.9% or greater, was purchased from Alfa Aesar. Ultrapure water (deionized water) was used to prepare the stock solution for the reactors with 18 MΩ cm using a Milli-Q RG water purification system with a Purification Pak from Millipore. The stock solution was prepared by mixing 1 g of pure naphthalene into 50 mL of acetonitrile to produce a 20 000 mg/L solution concentration. Standard solution concentrations of 5, 10, 30, 50, 70, and 100 mg/L were prepared from the stock solution. Naphthalene was analyzed by a Hewlett-Packard Series 1050 HPLC equipped with UV detector at 254 nm. Acetonitrile with water (35:65) was used as a mobile phase, while a ChromoSep SS column (5µm, 250 mm * 4.6 mm, Varian, Inc.) at ambient temperature was used as the stationary phase. Helium was used as a degasser at a pressure of 62 kPa (9 psi). Chrompack HPLC ChromSpher PAH columns are packed with a derivatized silica material, which is considered to be a reversed phase material designed specifically for the analysis of PAHs. To condition this type of column, it is necessary to first rinse it with acetonirile and then equilibrate it with the eluent of choice. Analysis of naphthalene was conducted following the Environmental Protection Agency procedure (20), also stated by the North Dakota Department of Public Health (21). Direct measurements were conducted by injecting samples collected from the reactor directly into HPLC, without extracting naphthalene from the water. This procedure minimizes potential interferences from reagents, solvents, and glassware required for the extraction method. Before (and sometime during) each use of HPLC, a flushing procedure was required. The column was flushed with a 50:50 (volume/volume or v/v) mixture of water and acetonitrile, followed by a 10:90 (v/v) mixture and then another brief flushing with 50:50 (v/ v) water and acetonitrile. Finally, the column was flushed with a 90:10 (v/v) mixture of reagent water and acetonitrile.
Results and Discussion Enhanced Electrochemical Transformation. Figure 3 shows the change in the naphthalene concentration in the electrochemical cell under different current densities. The figure summarizes average values achieved in two duplicate tests for each case. Transformation of naphthalene occurred at variable rates that depend on the concentration and current density. For the case of an initial naphthalene concentration of 13 mg/L, 88% was transformed after 8 h of treatment under 18.2 mA/L (10 mA current). In comparison, more than 90 h VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mation was achieved after 72 h for both current densities. However, application of lower current density of 1.8 mA/L requires a much longer treatment time than the higher current densities to achieve comparable transformation. The higher current densities accelerate degradation rates and shorten treatment time, but they may result in higher energy expenditure. The losses in the control cell were below 10%, which may be attributed to volatilization, potential sorption to the cell components, and diffusion through the anolyte solution or adsorption to the proto-permeable membrane. Assessment of transformation rates per charge (or energy) is necessary for evaluation of the efficiency of the process. Naphthalene breakdown rates per charge, expressed in milligrams of transformed energy per Coulomb passed through the system, are calculated by
rate (in mg/C) )
FIGURE 3. Average naphthalene concentrations during electrochemical treatment; initial concentration is 13 mg/L.
FIGURE 4. Average naphthalene concentrations during electrochemical treatment; initial concentration is 25 mg/L. were required to transform the same amount under 9 mA/L (5 mA current). Most of the naphthalene transformation occurred in the first 4 h at rates of 2.24 and 1.11 mg/L h under 18.2 and 9 mA/L, respectively. More than 90% transformation was achieved in tests under 18.2 and 9 mA/L after 96 h of processing. The rate of transformation was much lower under 1.8 mA/L (1 mA current). More than 48 h was required to achieve 71% degradation. When the experiments were terminated after 96 h, 80% of naphthalene had been transformed. Thus, the fastest transformation occurred at the highest current density of 18.2 mA/L, while the lowest rate occurred at the lowest current density of 1.8 mA/L. However, the same degradation percentage was ultimately achieved under different currents under different transformation rates, but the time required varied inversely with current density. Increasing the naphthalene concentration to 25 mg/L in the anolyte produced similar results to those with a lower concentration (13 mg/L), as indicated in Figure 4. More than 75% of the naphthalene was transformed in the first 4.5 h under 18.2 mA/L, at a rate of 2.2 mg/L h. About 48 h was required to transform the same percentage under 9 mA/L. In general, the degradation rate decreases in time due to the decrease in naphthalene concentration, and 90% transfor5840
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∆c/∆t Id
(1)
where ∆c is the decrease in naphthalene concentration (mg/ L), ∆t is the duration (hours), and Id is the current density (amp/L) or the current per unit volume of electrolyte treated. This equation is an experimental measure of incremental changes, assuming linear concentration changes for each time step. While the rate is expected to be linear initially, it is expected to decrease in time due to many limiting factors, such as naphthalene concentration. For the case with an initial concentration of 13 mg/L, high current densities (9 and 18.2 mA/L) resulted in higher rates of transformation per charge (0.0343 mg/C) in the first 4 h but then decreased sharply. Reasonable transformation rates can still occur under 9 mA/L after 8 h. However, the low current density of 1.8 mA/L resulted in continuous efficient transformation rates (approximately 0.025 mg/C) during the first 48 h. The same behavior appears for the case of a higher initial concentration, 25 mg/L, but transformation rates per charge are higher than in the case of a lower concentration (13 mg/L). Comparisons of transformation rates between the low and high concentrations in time (measured in mg/L h) and per charge (in mg/C) are presented in Figure 5, which shows that increasing the current density accelerates the transformation of naphthalene. As previously indicated, the increase in transformation rates appears to change linearly with increasing current density, especially for the lower concentration case. At the same time, higher naphthalene concentrations result in a significant increase in transformation rates. To assess the role of the electrolyte type, experiments were conducted using sodium nitrate as an electrolyte instead of sodium chloride. The goal was to assess the role of Cl2 gas generation on the process by using an anion different than chloride. Figure 6 shows that application of 9 and 18.2 mA/L currents did not produce any significant changes on the naphthalene concentration as compared to the control. The initial naphthalene concentration was 35 mg/L, and the final concentration after 72 h was about 25 mg/L in all samples, including the control. Since the losses are similar to the losses in the control, it is concluded that breakdown of naphthalene did not occur and that its breakdown in the NaCl electrolyte was due to formation of the chlorine gas. Further tests were conducted to assess the effect of acidification and electrolysis on naphthalene. These tests were conducted to evaluate chemical changes produced by electrolysis and if transformation can occur in a separate compartments using the anolyte produced by electrolysis. Electrochemical Effects. The effects of electrolysis on anolyte pH and redox potential were evaluated for 5 days. Under 5 mA (9 mA/L), the redox potential increased significantly at the anolyte in the first 8 h from less than 500 mV to about 900 mV (Figure 7). After that, the redox potential
FIGURE 6. Changes in naphthalene concentration as a result of electrochemical treatment using sodium nitrate electrolyte.
FIGURE 7. Redox potential and pH changes at the anolyte under a constant current density of 9 mA/L.
FIGURE 5. Average degradation rates during the first 8 h of treatment. (a) Degradation rates per time and (b) degradation rates per charge passed through the system. continued to increase but at a lower rate until it reached 1380 mV after 5 days. Similar behavior is noted for the anolyte pH, which decreased significantly in the first 8 h to less than 2.5 and continued to decrease until it reached an acidity of 1.86 at the end of testing (Figure 7). The figure shows that the pH and Eh rates of change are relatively logarithmic. At the anode, electrolysis produces protons by extracting electrons from water molecules, resulting in an increase in proton concentration and a decrease in electron concentration. Theoretically, the rate of proton generation by electrolysis should be equal to the rate of proton flux through the Nafion membrane to maintain electrical neutrality of the anolyte (i.e., net change in charge in the anolyte should be zero). The buildup of proton concentrations in the anolyte indicates that the rate of proton generation by electrolysis is higher than the rate of proton transport through the membrane toward the catholyte. This can only be possible if other cations, in this case sodium, are migrating through the membrane to maintain the electric current and to preserve electrical neutrality of the anolyte and catholyte. It should also be noted that the pH and Eh changes in the anolyte
are affected by the acid/base and redox equilibrium chemistry of the solution, which can be complex depending on the type and constituents of the electrolytes. The trend was similar under 18.2 mA/L current density (10 mA current) and at the end of treatment, with both current densities producing similar effects. The transformation of naphthalene at the anode as a result of electrolysis can be attributed to limited factors, mainly electrochemical oxidation at the anode surface or oxidation by the anolyte. Experiments were conducted to assess the role of the electrolyte environment (pH, Eh, and Cl2(g)). The tests involved mixing the naphthalene with the anolyte solution after processing the solution under 10 mA (18.2 mA/ L) for 3 days without injecting naphthalene. The anolyte was then extracted and divided into 100 mL solutions and placed into serum bottles. The pH and redox potential of the anolyte were measured to be 1.94 and 1296 mV, respectively. Naphthalene was injected to produce initial concentrations of 15, 30, and 45 mg/L into serum bottles. The control consisted of similar concentrations in the same solution prior to electrolysis, and injections were conducted in duplicates for quality assurance. Figure 8 shows that naphthalene degradation did occur in these solutions directly after injection; however, no change occurred in the control serum bottles. VOL. 39, NO. 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Naphthalene degradation (three different concentrations) in serum bottles after mixing with anolyte (anolyte redox potential ) 1296 mV and pH ) 1.94; control redox potential ) 350 mV and pH ) 6.5). The transformation of naphthalene during the three different concentrations mixed with the anolyte solutions can be attributed to the electrolysis effects in the solutions, emphasizing the importance of the solution oxidizing potential in the presence of chlorine gas. The solutions’ high redox potential (about 1300 mV), combined with an abundance of chlorine gas, shows that there is an extreme lack of electrons in the anolyte and a highly oxidizing environment. The Cl2 generation, lack of electrons, and oxidation potential are proportional to the combined effect of current density and duration. This is the reason that naphthalene transformation under three different current densities produced similar final concentrations. While measurements and analysis demonstrated the naphthalene transformation, the oxidation process was easily detected visually as the anolyte solution developed a cloudy appearance during processing. From previous experiments, it was noted that a similar cloudiness appeared after running the electric field and it was always in combination with a reduction in the naphthalene concentration. While the redox potential increased significantly and was clearly the reason for the transformation of naphthalene, the anolyte pH decreased to less than 2, and its effect was not addressed. Therefore, it was necessary to evaluate the role of acidification on the process. Sulfuric acid (95.9% with a normality of 36), phosphoric acid (85.8%), hydrochloric acid (37.1% with normality of 12.1), and nitric acid (69.5% with normality of 15.8) were used to assess the effect of acids on naphthalene. Acid solutions were prepared by mixing 0.01 mL of concentrated acids with 0.09 mL of deionized water (10% acid solution). Measurements showed that all of the acid solutions had pH values of less than 2. A 0.5 mL naphthalene stock solution was added to each acid solution and to a control. The concentrations after adding naphthalene to the acid were measured to be 63.4, 69.2, 56.8, and 72.6 mg/L. The results show that while acidification may produce a loss of naphthalene, the effect is limited since the final concentrations are significant. Further, no cloudiness was observed in these solutions, indicating that breakdown of naphthalene did not occur. Transformation of naphthalene at the anolyte produced four compounds with small peaks that had a retention time of 12.0-12.5 min. Buildup of such compounds can be a hindering factor, as they might be toxic and their transformation will require more energy. Fortunately, the concentrations of these byproducts were small, and the results demonstrated their continuous transformation in time. 5842
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Samples were analyzed by an off-campus, independent laboratory to assess the results and identify some of the potential products. The results from the independent laboratory did not show significant concentrations of naphthalene or any of its byproducts. In addition, the effect of polarity reversal was addressed to evaluate the potential for complete transformation by oxidation first followed by reduction. After 96 h of treatment, the electrode polarity was reversed, and the treatment was continued for another 48 h (under 5 and 10 mA currents) to assess the effect of electrochemical reduction on the solution and byproducts. The pH increased to above 11 with yellow and orange colors appearing, respectively. The results showed that the four daughter compounds that were produced due to oxidation (retention time in the range of 12.0-12.5) completely disappeared. While toxic byproducts (including any potential chlorinated compounds) may form at small concentrations, electrochemically mediated reductions appear to result in complete transformation of these byproducts.
Acknowledgments This material is based upon work supported by the National Science Foundation under Grant 0093752.
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(16) Panizza, M.; Bocca, C.; Cerisola, G. Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res. 2000, 34 (9), 2601-2605. (17) Arapoglou, D.; Vlyssides, A.; Israilides, C.; Zorpas, A.; Karlis, P. Detoxification of methyl-parathion pesticide in aqueous solutions by electrochemical oxidation. J. Hazard. Mater. 2003, 98 (1-3), 191-199. (18) Pulgarin, C.; Adler, N.; Pe´ringer, P.; Comninellis, C. Electrochemical detoxification of a 1,4-benzoquinone solution in wastewater treatment. Water Res. 2003, 28 (4), 887-893. (19) Pamukcu, S.; Weeks, A.; ad Wittle, J. K. Enhanced Reduction of Cr by Direct Current in Contaminated Clay. Environ. Sci. Technol. 2004, 38 (4), 1236-1241. (20) Environmental Protection Agency. Determination of Polycyclic Aromatic Hydrocarbons in Drinking Water by Liquid-Liquid Extraction and HPLC with Coupled Ultraviolet and Fluorescence
Detection; U.S. EPA Drinking Water Methods for Chemical Parameters: 1990; Method 550; http://www.ultrasci.com/Docs/ 500/550.pdf. (21) NDDOH-North Dakota Department of Health. Standard Operating Procedure for the Determination of Polycyclic Aromatic Hydrocarbons (PAHs) by Liquid-Liquid Extraction and High Performance Liquid Chromatography. Division of Chemistry, Standard Operating Procedure (SOP) I-2-38: 2003; http:// www.health.state.nd.us/lab/Methods/I-2-37.pdf.
Received for review March 7, 2004. Revised manuscript received May 13, 2005. Accepted June 1, 2005. ES049645F
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