Trichloroethylene Transformation by Natural ... - ACS Publications

To clarify the deciding role of O2 in the reactions, experiments with different initial concentrations of O2 were conducted. Values of DOi were 0, 0.0...
0 downloads 0 Views 732KB Size
Download PDF
Environ. Sci. Technol. 2008, 42, 7470–7475

Trichloroethylene Transformation by Natural Mineral Pyrite: The Deciding Role of Oxygen H O A T . P H A M , * ,† MASASHI KITSUNEDUKA,‡ JUNKO HARA,§ KOICHI SUTO,† AND CHIHIRO INOUE† Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramaki-Aza, Aoba-ku, Sendai, Miyagi, 980-8579, Japan, Denso Corporation, Syowacyo 1-1 Kariya, Aichi 448-8661, Japan, and Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST) 16-1 Onogawa, 305-8569 Tsukuba, Irabaki, Japan

Received May 12, 2008. Revised manuscript received July 09, 2008. Accepted July 09, 2008.

The transformation of trichloroethylene (TCE) in natural mineral iron disulfide (pyrite) aqueous suspension under different oxygen conditions was investigated in laboratory batch experiments. TCE transformation was pursued by monitoring its disappearance and products released with time. The effect of oxygen was studied by varying the initial dissolved oxygen concentration (DOi) inside each reactor. Transformation rates depended strongly on DOi in the system. In anaerobic pyrite suspension, TCE did not transform as it did under aerobic conditions. The transformation rate increased with an increase in DOi. The TCE transformation kinetics was fitted to a pseudo-first-order reaction with a rate constant k (h-1) varying from 0.004 to 0.013 for closed systems with DOi varying from 0.017 to 0.268 mmol/L under the experimental conditions. In the aerobic systems, TCE transformed to several organic acids including dichloroacetic acid, glyoxylic acid, oxalic acid, formic acid, and finally to CO2 and chloride ion. Dichloroacetic acid was the only chlorinated intermediate found. Both TCE and the pyrite surface were oxidized in the presence of O2. Oxygen consumption profiles showed O2 was the common oxidant in both TCE and pyrite oxidation reactions. Ferric ion cannot be used as an alternative oxidant to oxygen for TCE transformation.

Introduction The aim of this work was to determine the use of natural mineral iron disulfide (pyrite) in the transformation of trichloroethylene (TCE) in aqueous environments and to assess the role of oxygen in this reaction. TCE is a common groundwater pollutant. It was listed as the 16th most common organic pollutant of the 275 most frequently detected in the CERCLA list (1). The widespread occurrence of this compound in the aquatic environment has driven research for remediation of this pollutant in natural and engineered systems. There have been at least three studies reporting the dehalogenation of TCE by pyrite (18, 19, 30), all finding that * Corresponding author phone/fax: +81-(0)22-795-7404, e-mail: [email protected]. † Tohoku University. ‡ Denso Corporation. § National Institute of Advanced Industrial Science and Technology. 7470

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 19, 2008

the transformation of TCE by pyrite occurred under anaerobic conditions. Under anaerobic conditions, aliphatic chlorinated compounds such as carbon tetrachloride (CCl4) and chlorinated ethylenes (tetrachloroethylene, cis-dichloroethylene) were also found to be reductively dechlorinated by pyrite (17-19, 30). In those works, the reaction mechanism for mineralizing pollutants was the reductive dehalogenation of chlorinated compounds by the transfer of electrons from the mineral surface, which was quite similar to the TCE dehalogenation process using zerovalent iron (ZVI) (10, 20). However, the degradation of chlorinated compounds in pyrite suspension in the presence of O2 has been less studied. There was only a study by Kriegman-King and Reinhard (17) that reported the inhibition effect of oxygen on CCl4 transformation by pyrite. Although the role of O2 in the system containing the mineral pyrite and TCE is important from an application point of view, how it affects the transformation reaction has still not been determined. Pyrite-mediated TCE degradation under differing experimental conditions is important from an environmental viewpoint. The specific reasons for conducting this study are (a) pyrite is the most abundant metal sulfide mineral and hence in situ remediation processes are feasible and (b) although pyrite is formed in anaerobic environments, pyrite is often exposed to aerobic conditions upon weathering. The presence of oxygen in pyrite-containing system leads to its well-known and documented weathering process. This process causes acidification of natural waters. Therefore, its oxidation reactivity deserves special consideration. Moreover, the hydroxyl radical, a very strong oxidant, was reported to be induced by pyrite aqueous suspension, which further supports the idea of pyrite having a strong reactivity with organic compounds (3, 5-8). In this study, TCE transformation in pyrite aqueous suspension was studied in a closed system, under a controlled atmosphere, and for an initial dissolved O2 concentration (DOi) varying from 0 to 0.268 mmol/L. The main objectives of this study were to assess the reactivity of pyrite in the transformation of TCE, specifically (1) to evaluate the effect of O2 on the transformation reaction of TCE in pyrite suspension and (2) to investigate the role of O2 in both TCE transformation and pyrite surface oxidation reactions. The transformation of TCE was assessed by monitoring its disappearance and products released with time. The effect of oxygen was studied by varying DOi for each reactor.

Materials and Methods Materials. Standard TCE solution was obtained at more than 97% purity from GL Sciences Inc., Japan. Standard organic acids including high quality oxalic acid, dichloroacetic acid, glyoxylic acid, and formic acid were bought from Wako Co., Japan. Massive natural mineral pyrite was received from Yanahara Mine in Okayama, Japan. The pyrite rock sample was ground with a ceramic mortar and pestle, further ground with a ceramic ball-mill, and wet sieved. The fraction of 20 to 38 µm was retained for use. Prepared pyrite was rinsed several times with distilled water and ultrasonicated for 30 min to remove fine soil minerals from the surface. It was then dehydrated under vacuum until used. After long storage, pyrite powder was washed again with 1 mol/L HCl solution to remove the oxidized layer on the surface. Washing the pyrite with the HCl solution did not affect its reactivity (data not shown). X-ray diffraction (XRD) analysis showed the principal ingredient of the ore was pyrite. Other mineral peaks were not detected by XRD analysis. The metallic compositions of the used pyrite sample were measured by dissolving pyrite 10.1021/es801310y CCC: $40.75

 2008 American Chemical Society

Published on Web 08/21/2008

mineral in the HNO3 solution and quantified using inductively coupled plasma mass spectrometry. Sulfur content was measured by combustion techniques coupled with precipitation as BaSO4. Main elements included iron and sulfur at a molar ratio Fe:S of 1:1.85, which showed that the pyrite was sulfur deficient. Si, Zn, and Cu were present as impurities. The molar percentages for S, Fe, Si, Zn, Cu, which were all the elements present, were 61.7, 33.3, 4.0, 0.7, and 0.2%, respectively. The specific surface area of treated pyrite samples measured using the BET method was 0.2 m2/g. Adjustment of the Initial Oxygen Concentration in Closed Systems. Experiments were conducted in 26 mL glass vials. Anaerobic conditions were attained by deoxygenating both gaseous and aqueous phases using a nitrogen gas stream. The aqueous phase was deoxygenated in 30 min followed by the gaseous phase in 5 min with the cap almost closed, and then the vials were tightly capped immediately. The gaseous phase under anaerobic conditions contained only nitrogen gas, and no residual oxygen was detected by gas chromatography (GC). Aerobic conditions with different O2 conditions were attained by replacing the equivalent amounts of N2 gas in the deoxygenated gas phase with 1, 2, and 5 mL of atmospheric air, which resulted in dissolved oxygen concentrations (DOs) of 0.017, 0.034, and 0.084 mmol/L in 10 mL of aqueous phase in the closed systems, respectively. Experiments with DOi of 0.268 mmol/L were attained by leaving the headspace of the vials equilibrated with the atmosphere. Calculation of DO and Aqueous TCE Concentration ([TCE]). DO and [TCE] were calculated from the total amounts (in µmol) of oxygen and TCE in the closed systems using Henry’s Law. The correlation of partial vapor pressure (Pv) and aqueous concentration (Cw) was expressed as Pv/Cw ) KH. The values of Henry’s constant (KH) for oxygen and TCE at 25 °C are 769.2 and 8.918 (atm L mol-1), respectively (27, 12). Total amounts of oxygen and TCE in the closed systems (including both the gaseous and aqueous phases) were quantified by comparisons of headspace GC peaks with the five-point standards curves. Standard curves of total amounts of oxygen and TCE were made by measuring the responses of GC peaks to known amounts of oxygen and TCE in standard vials. Standard vials were 26 mL vials containing known amounts of pure oxygen or TCE in 10 mL of H2SO4 solution (pH of 3 to 4). Procedure. The vials were first filled with 10 mL of deionized water. Pyrite (1 ( 0.01 g) was then added (each vial contained 100 g/L FeS2, resulting in a specific surface area concentration ([pyrite]) of 20 m2/L), and this was followed by deoxygenation if applicable and crimp-sealing with Teflon-lined septa and aluminum foil caps. The initial oxygen concentrations were adjusted as described above. Experiments were initiated by spiking with concentrated TCE stock using a microsyringe to obtain 10 µmol of TCE in the vial (nTCE,i ) 10 µmol and the initial aqueous concentration ([TCE]i) was 0.62 mmol/L). Control reactions with only TCE, referred to as TCE controls, were conducted to evaluate the loss and absorption of TCE under anaerobic and aerobic conditions using the same procedure without pyrite added. Pyrite controls, under anaerobic and aerobic conditions, were conducted to obtain the dissolution of pyrite in the absence of TCE. Other reactions, referred to as ferric experiments, under anaerobic and aerobic conditions and in the presence and absence of pyrite, were conducted (using 100 mmol/L Fe2(SO4)3) to determine the role of ferric ion as an alternative oxidant to oxygen. After preparation, all vials were placed on a vortex shaker (VR-36D, Taitec) and shaken at approximately 400 rpm in a temperature-controlled incubator (Electric Incubator MIR 153, Sanyo) at 25 °C in darkness to maintain a constant

temperature and eliminate any potential effect of light. No effort was made to maintain constant pH. Transformation of TCE in pyrite suspension was assessed by measuring its disappearance and the concentrations of its transformation intermediates and products released to the gas phase and aqueous phase. The concentration of the gaseous compound, which was CO2, was measured by GC. Concentrations of aqueous compounds including chloride ion and organic acids were measured by high performance liquid chromatography (HPLC). Pyrite dissolution was assessed by measuring sulfate and ferrous ion contents. At each sampling time, one ampule for each experimental condition was removed from the incubator. The gaseous compounds were taken from the headspace by microsyringe for GC analysis of TCE and inorganic gases (O2 and CO2). The ampule was then opened, the solution was filtered with a 0.45 µm membrane filter, and the pH of the solution and amounts of ferrous ion, aqueous organic acids, and inorganic ions (chloride and sulfate) were measured. Chemical Analyses. pH was measured with a pH meter (Beckman Coulter Ø360). Analysis of TCE was carried out by GC-flame ionization detector using the headspace technique (GC-390 (GL Sciences) and capillary column (TC5, 0.32 mm × 30 m, 4 µm, GL Sciences) with a detector temperature of 200 °C, injection temperature of 200 °C, and oven temperature of 50 °C (isothermal)). Inorganic gases (CO2, O2) were analyzed using a GC-thermal conductivity detector (GC-323, GL Sciences) and packed column (Porapak Q 50/80 column, GL Sciences), with an injection temperature of 120 °C, oven temperature of 60 °C, and detector temperature of 100 °C. Chloride and sulfate ions in the supernatant were analyzed by HPLC (L-7300, Hitachi) using a GL-IC-A25 column (4.6 mm × 150 mm) (4 mmol/L Na2CO3 eluent, 1.0 mL/min flow rate, 40 °C column temperature). Organic acids were analyzed by HPLC (L-7200, Hitachi) using a Gelpack GL-C610H-S column (7.8 mm × 300 mm, Hitachi) (0.1% H3PO4 eluent, 0.5 mL/min flow rate, 50 °C oven temperature, UV detector set at a wavelength of 210 nm). Concentrations were quantified by comparisons of GC and HPLC peaks with five-point standard curves. Ferrous ions were colorimetrically quantified using a UV-vis spectrophotometer (U-2001, Hitachi). The ferrous ion content was determined at 510 nm using the 1,10-o-phenanthroline analytical method (11).

Results and Discussion TCE Transformed in Aerobic Pyrite Suspension but Not under Anaerobic Conditions. The experimental results in Figure 1 show the disappearance of TCE in anaerobic and aerobic pyrite suspensions with time. The DOi in aerobic pyrite experiments was 0.268 mmol/L. The total loss and absorption of TCE in the controls under both conditions reached approximately 20% of the initial TCE concentration at the end of the experimental course. In pyrite suspension, the TCE profile under anaerobic conditions was similar to that for the TCE control, while the TCE concentration gradually decreased with time to 0.01 mmol/L (equivalent to 1.7% of 0.62 mmol/L) after a 323 h reaction in the aerobic pyrite system. The amounts of chloride ion released to the aqueous phase under anaerobic and aerobic conditions are shown in Figure 1b. In the TCE controls under both conditions, no chloride was produced. The concentration of chloride ion released under anaerobic conditions was not significant and similar to that for the pyrite control. Conversely, with the presence of O2, chloride ion was rapidly produced and its concentration reached 2.54 mmol/L (equivalent to 85% of the chloride in the initial 10 µmol of TCE in 10 mL solution) after a 323 h reaction, at which time 98% of TCE had degraded. The TCE in the control experiments decreased by 17% after 323 h. VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

7471

FIGURE 1. Transformation of TCE in pyrite suspension ([TCE]i ) 0.62 mmol/L, [pyrite] ) 20 m2/L) under aerobic (DOi ) 0.268 mmol/L) and anaerobic conditions. (a) TCE disappearance with time; (b) chloride production with time. This means that all chloride from the transformed TCE was in the form of chloride ion at the end of the experiment. The transformation of TCE in pyrite suspension was consistent with the finding that pyrite oxidation by O2 follows a Fenton-like mechanism, in which the reduction of O2 at the pyrite surface can induce hydroxyl radical formation (2, 5-8, 21, 25). This induced radical can degrade nucleic acids, and it has been suggested that it reacts nearly instantaneously with most organic compounds. However, this mechanism leaves unsolved the question of pyrite reactivity under anaerobic conditions, which needs further investigation, because pyrite suspension under abiotic conditions has also shown the ability to produce hydroxyl radical at the sulfur-defected site on the pyrite surface (3). Our study is the first research on the reactivity of pyrite with TCE under aerobic conditions. However, we found that the transformation rate of TCE in pyrite suspension under anaerobic conditions was very small and below the detection limit under our experimental conditions. This result is not in agreement with previous works of Weerasooriya and Dharmasena (30) and Lee and Bachelor (18, 19). The degradation pathway of TCE under anaerobic conditions in their research was similar to the degradation pathway of TCE using zerovalent iron (ZVI) (10, 20). However, the degradation pathway found in this study under aerobic conditions, which will be discussed in detail in a later part 7472

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 19, 2008

of this manuscript, was completely different from the degradation pathway of TCE using ZVI. The potential explanation for this difference may be the difference in the surface concentrations, which leads to a different number of reactive sites on the surface of the used pyrites. The high surface area concentration of the pyrite mineral (which was 2340 m2/L) in the study by Lee and Bachelor (18, 19) and the very small particle size (which was under 4 µm) in the study by Weerasooriya and Dharmasena (30) resulted in higher reactivity of pyrite under anaerobic conditions than the reactivity of the pyrite used in our study. The pyrite mineral used in our experiments had a higher particle size (20 to 30 µm) and smaller surface area concentration (20 m2/L), which resulted in the very low reactivity of pyrite under anaerobic conditions compared with reactivities reported in previous research. Effect of Initial Oxygen Concentrations. From the experimental results shown in Figure 1, it is seen that the TCE transformation in pyrite suspension was strongly dependent on the O2 concentration. To clarify the deciding role of O2 in the reactions, experiments with different initial concentrations of O2 were conducted. Values of DOi were 0, 0.017, 0.034, 0.084, and 0.268 mmol/L (the results from the experiment with 0.268 mmol/L DOi conducted previously were also used). All other conditions were kept the same at described above. The concentrations of chloride ion, regarded as the TCE dehalogenated product, were measured. Obtained data show a significant correlation of DOi with the disappearance rate of TCE (Figure 2a). With an increase in DOi, the rate of TCE transformation in pyrite suspension significantly increased. As evidence of the TCE transformation dependence on DOi, chloride ion was not detected in the absence of O2, while more chloride was released with an increase in DOi (Figure 2b). Oxygen quickly became undetectable after approximately 66, 141, and 239 h in the systems with DOi of 0.017, 0.034, and 0.084 mmol/L, respectively. In the system with DOi of 0.268 mmol/L, the dissolved oxygen concentration reduced to 0.1 mmol/L (equivalent to 37% of the DOi) after 323 h (Figure 3). There was a consistency between the TCE degradation (Figure 2a) and O2 consumption profiles (Figure 3). Available O2 promoted the TCE transformation reaction and the exhaustion of O2 stopped the TCE transformation. TCE Transformation Kinetics. To assess the transformation kinetics of TCE, we assumed the reaction was first-order with respect to the TCE concentration, in which the pseudofirst-order rate constant (k) depends on DO. The rate equation can be written as (eq 1). d[TCE]/dt ) -k[TCE]

(1)

The k values were calculated from the linear regression of ln([TCE]/[TCE]i) vs time, where [TCE] and [TCE]i are the concentrations of TCE at time t and time zero, respectively. As previously described, TCE transformed only in the presence of oxygen; therefore, the TCE concentration profiles prior to oxygen being exhausted were used (time durations of 66, 141, 239, and 323 h for the systems with DOi of 0.017, 0.034, 0.084, and 0.268 mmol/L, respectively). The obtained k values were 0.004, 0.005, 0.009, and 0.013 h-1 for DOi of 0.017, 0.034, 0.084, and 0.268 mmol/L with associated R (2) of 0.97, 0.98, 0.99, and 0.99, respectively (Figure 4a). The correlation of k with DOi shown in Figure 4b suggests that the TCE transformation rate constant increased with an increase in the initial oxygen concentration until there was a sufficient oxygen concentration. From these results, it is possible that if a sufficient oxygen concentration is maintained, k will be independent of DO. However, further efforts are still needed to determine the real rate law of the transformation reaction. The oxygen consumption profile for the TCE transformation reaction separated with the

FIGURE 3. DO profiles in the closed systems ([TCE]i ) 0.62 mmol/L, [pyrite] ) 20 m2/L). The profiles displayed here are the total oxygen concentration profiles. It is worth noting that oxygen in these experiments was used for both TCE transformation and pyrite oxidation reactions.

FIGURE 2. Transformation of TCE in pyrite suspension ([TCE]i ) 0.62 mmol/L, [pyrite] ) 20 m2/L) under different O2 conditions (DOi of 0, 0.017, 0.034, and 0.268 mmol/L). (a) TCE degradation with time; (b) chloride ion concentration released to the solution. oxygen profile for the pyrite oxidation reaction is necessary to determine detailed reaction kinetics. Ferric Ion Cannot Be an Alternative Oxidant to Oxygen for TCE Transformation. In aerobic pyrite suspension containing TCE, it is possible that TCE was oxidized by oxygen or by ferric ion produced from surface oxidation reactions (from eq 3, which will be discussed in detail in a later part of this manuscript). Therefore, experiments with ferric ion as the oxidant, in the presence and absence of oxygen and pyrite, were conducted to clarify the role of ferric ion as an alternative oxidant. The experimental results shown in Figure 5a compare the disappearance profiles of TCE for the two oxidants, ferric ion and oxygen. TCE profiles in the systems containing ferric ion under anaerobic and aerobic conditions without pyrite and anaerobic conditions with pyrite were similar to profiles for the TCE controls. This suggests ferric ion is not the oxidant in TCE transformation reactions, even in the presence of pyrite.

FIGURE 4. (a) Experimental data (symbols) and curve fittings (lines) for estimation of k for different DOi values. k values of 0.004, 0.005, 0.009, and 0.013 (h-1) with associated R (2) of 0.97, 0.98, 0.99, and 0.99 were found for DOi of 0.017, 0.034, 0.084, and 0.268 mmol/L, respectively; (b) dependence of k on DOi. Oxygen as a Common Oxidant in Pyrite and TCE Oxidation Reactions. There has been extensive research on VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

7473

The combination of eqs 2 to 4 gives the overall reaction of pyrite oxidation (eq 6). 2FeS2 + 7O2 + 2H2O f 2Fe2+ + 4SO42- + 4H+

FIGURE 5. (a) TCE aqueous concentration profiles in the experiments using ferric ion (Fe2(SO4)3 100 mmol/L) as an oxidant, in the presence and absence of oxygen and pyrite, in comparison with the experiments using oxygen (nO2,i ) 137 µmol) as the oxidant; (b) experimental data and calculated ratios of SO42-/Fe2+, O2 consumed/Fe2+, and O2 consumed/SO42- in the system with nTCI,i ) 10 µmol, nO2,i ) 137 µmol, and [pyrite] ) 20 m2/L. the mechanism and kinetics of pyrite oxidized by oxygen (4, 9, 13-16, 21, 22, 25, 26, 29). The following equations show the generally accepted sequence of pyrite reactions with water and oxygen. 2FeS2 + 7O2 + 2H2O f 2Fe2+ + 4SO42- + 4H+

(2)

4Fe2+ + O2 + 4H+ f 4Fe3+ + 2H2O

(3)

FeS2 + 14Fe3+ + 8H2O f 15Fe2+ + 2SO42- + 16H+

(4)

4Fe3+ + 12H2O f 4Fe(OH)3(s) + 12H+

(5)

The pH in this study was initially 4 and reduced to 3.2 by the end of the experimental course. At this low pH, the presence of solid ferric hydroxide was not expected, or in other words, eq 5 did not occur. 7474

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 19, 2008

(6)

The ferrous and sulfate ions produced in the pyrite oxidation reaction and O2 consumption in closed systems containing TCE were monitored to assess the role of O2. The oxygen consumption here was the total consumption of oxygen in both gas and aqueous phases (in µmol). This study was conducted with a headspace initially equivalent to the atmosphere. The initial amounts of TCE and oxygen were 10 µmol and 137 µmol (equal to DOi of 0.268 mmol/L), respectively. Experimental data are shown in Figure 5b. The production of ferrous and sulfate ions and the consumption of O2 increased with time. The molar ratio of SO42-/Fe2+ experimentally obtained started at 1.71 and decreased to 1.41 by the end of the experimental course. This corresponded to nonstoichiometric dissolution (SO42-/Fe2+ < 2) resulting from a deficit in aqueous sulfur. The low ratio possibly means that a small portion of sulfur precipitated as elemental sulfur. Elementary sulfur together with SO42- has been observed at low pH in other studies (9, 23, 28). Here we intend to determine the maximum amount of oxygen used in the pyrite oxidation reaction. The maximum amount of oxygen in the pyrite oxidation reaction is the amount of oxygen needed to oxidized pyrite to ferrous and sulfate ions following the overall eq 2. The ratios of O2 consumed/ SO42- and O2 consumed /Fe2+ calculated from eq 2 were 1.75 and 3.5, respectively, which were much lower than ratios given by the experimental data (Figure 5b). This shows oxygen in the system was not only used for the oxidation reaction of pyrite but was also used for the transformation of TCE. However, further study is still needed to investigate in detail the oxygen consumption profile for each reaction. TCE Transformation Intermediates and Products under Aerobic Conditions. Under aerobic conditions, with TCE disappearance and chloride production, CO2 and organic acids were detected. Profiles of all intermediates and products together with the TCE disappearance profile with time under aerobic conditions are shown in Figure 6a. The carbon and chloride balance of the transformation reaction reached about 80% by the end of the experimental course (Figures 6a and 6b). The reaction intermediates and products obtained were different to those for the abiotic dehalogenation of TCE by pyrite (18, 19, 30) as well as being different to those for dehalogenation by ZVI (10, 20). These results show the transformation of TCE under aerobic conditions follows its own pathway and is different from the abiotic reductive dehalogenation mechanism. The transformation of TCE to the final products (CO2 and chloride) following eq 7 is thermodynamically favorable (Gibbs free energy of -961.59 kJ/mol). C2HCl3 + H2O + 3/2O2 f 2CO2(g) + 3HCl

(7)

The present experiments have shown that pyrite promotes the transformation of TCE only under aerobic conditions and not under anaerobic conditions. In addition, ferric ion cannot be used as an alternative oxidant to oxygen for TCE transformation. TCE and pyrite were oxidized in the presence of O2; i.e., O2 was the common oxidant in both reactions. The pyrite surface promoted TCE transformation but was itself consumed owing to the surface oxidation reaction. The kinetics of the pyrite surface oxidation reactions in the presence of TCE or other organic compounds was not accounted for. However, this study proposed the strong reactivity of pyrite to organic compounds, which can be considered for natural remediation during the weathering of pyrite or for ex-situ remediation of pollutants. Further

(6)

(7)

(8) (9) (10) (11) (12) (13) (14) (15)

(16) (17) (18) (19)

FIGURE 6. Transformation of TCE in aerobic pyrite suspension (nTCI,i ) 10 µmol, DOi ) 0.268 mmol/L, [pyrite] ) 20 m2/L). (a) TCE disappearance and the intermediates and products produced with time and the carbon balance; (b) chloride balance.

(20)

(21) (22)

experiments providing additional insight into the reaction mechanism are currently underway in our laboratory

Acknowledgments

(23)

This research was supported by Grant-in-Aid for Scientific Research (A) 17206089. (24)

Literature Cited (1) ATSDR (Agency for Toxic Substances and Disease Registry) 2007 CERCLA Priority List of Hazardous Substances, U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, Division of Toxicology and Environmental Medicine, Atlanta, GA, in cooperation with U.S Envrionmental Protection Agency, as updated at URL: http://www.atsdr.cdc.gov/cercla/07list.html, January 2008. (2) Berger, M.; Hazen, M.; Nejjari, A.; Fournier, J.; Guignard, J.; Pezerat, H.; Cadet, J. Radical oxidation reactions of the purine moiety of 2′-deoxyribonucleosides and DNA by iron-containing minerals. Carcinogenesis 1993, 14 (1), 41–46. (3) Borda, M. J.; Elsetinow, A. R.; Strongin, D. R.; Schoonen, M. A. A. A mechanism for the production of hydroxyl radical at surface defect sites on pyrite. Geochim. Cosmochim. Acta 2003, 67 (5), 935–939. (4) Borda, M. J.; Strongin, D. R.; Schoonen, M. A. A. A vibrational spectroscopic study of the oxidation of pyrite by molecular oxygen. Geochim. Cosmochim. Acta 2004, 68 (8), 1807–1813. (5) Cohn, C. A.; Simon, S. R.; Schoonen, M. A. A. Comparison of fluorescence-based techniques for quantification of particle-

(25) (26) (27)

(28)

(29) (30)

induced hydroxyl radicals. Part. Fibre Toxicol. 2008, 5 (2),open assess. Cohn, C. A.; Mueller, S.; Wimmer, E.; Leifer, N.; Greenbaum, S.; Strongin, D. R.; Schoonen, M. A. A. Pyrite-induced hydroxyl radical formation and its effect on nucleic acids. Geochem. Trans. 2006, 7 (3),open assess. Cohn, C. A.; Borda, M. J.; Schoonen, M. A. A. RNA decomposition by pyrite-induced radicals and possible role of lipids during the emergence of life. Earth Planet. Sci. Lett. 2004, 225 (3-4), 271– 278. Cohn, C. A.; Laffers, R.; Schoonen, M. A. A. Using yeast RNA as a probe for generation of hydroxyl radical by earth materials. Environ. Sci. Technol. 2006, 40, 2838–2843. Descostes, M.; Vitorge, P.; Beaucaire, C. Pyrite dissolution in acidic media. Geochim. Cosmochim. Acta 2004, 68 (22), 4559– 4569. Hara, J.; Ito, H.; Suto, K.; Inoue, C.; Chida, T. Kinetics of trichloroethene dechlorination with iron powder. Water Res. 2005, 39, 1165–1173. Harris, D. C. Quantitative Chemical Analysis , 6th ed.; W. H. Freeman and Co.: New York, 2003. Henron, G.; Christensen, T. H.; Enfield, C. G. Henry’s law constant for trichloroethylene between 10 and 95 °C. Environ. Sci. Technol. 1998, 32, 1433–1437. Holmes, P. R.; Crundwell, F. K. The kinetics of the oxidation of pyrite by ferric ions and dissolved oxygen: An electrochemical study. Geochim. Cosmochim. Acta 2000, 64 (2), 263–274. Jerz, J. K.; Rimstidt, J. D. Pyrite oxidation in moist air. Geochim. Cosmochim. Acta 2004, 68 (4), 701–714. Kamei, G.; Ohmoto, H. The kinetics of reactions between pyrite and O2-bearing water revealed from in situ monitoring of DO, Eh and pH in a closed system. Geochim. Cosmochim. Acta 2000, 64 (15), 2585–2601. Kelsall, G. H.; Yin, Q.; Vaughan, D. J.; England, K. E. R; Brandon, N. P. Electrochemical oxidation of pyrite (FeS2) in aqueous electrolytes. J. Electroanal. Chem. 1999, 471, 116–125. Kriegman-King, M. R.; Reinhard, M. Transformation of carbon tetrachloride by pyrite in aqueous solution. Environ. Sci. Technol. 1994, 28, 692–700. Lee, W.; Batchelor, B. Abiotic reductive dechlorination of chlorinated ethylenes by iron-bearing soil minerals. 1. Pyrite and magnetite. Environ. Sci. Technol. 2002, 36, 5147–5154. Lee, W.; Batchelor, B. Reductive capacity of natural reductants. Environ. Sci. Technol. 2003, 37, 535–541. Liu, Y.; Majetich, S. A.; Tilton, R. D.; Sholl, D. S.; Lowry, G. V. TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environ. Sci. Technol. 2005, 39, 1338–1345. Lowson, R. T Aqueous oxidation of pyrite by molecular oxygen. Chem. Rev. 1982, 82 (5), 461–497. Martı´n, F.; Diez, M.; Garcı´a, I.; Simo´n, M.; Dorronsoro, C.; Iriarte, ´ .; Aguilar, J. Weathering of primary minerals and mobility of A major elements in soils affected by an accidental spill of pyrite tailing. Sci. Total Environ. 2007, 378, 49–52. McGuirre, M. M.; Jallad, K. N.; Ben-Amotz, D.; Hamers, R. J. Chemical mapping of elemental sulfur on pyrite and arsenopyrite surfaces using near-infrared Raman imaging microscopy. Appl. Surf. Sci. 2001, 178, 105–115. Park, B.; Dempsey, B. A. Heterogeneous oxidation of Fe(II) on ferric oxide at neutral pH and a low partial pressure of O2. Environ. Sci. Technol. 2005, 39, 6494–6500. Rimstidt, J. D.; Vaughan, D. J. Pyrite oxidation: A-state-of-theart assessment of the reaction mechanism. Geochim. Cosmochim. Acta 2003, 67 (5), 873–880. Rosso, K. M.; Becker, U.; Hochella, M. F. J. The interaction of pyrite {100} surfaces with O2 and H2O: Fundamental oxidation mechanisms. Am. Mineral. 1999, 84, 1549–1561. Sander, R. Compilation of Henry’s Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry (Version 3), 1999. Available at http://www.henryslaw.org. (accessed on 15 March 2008). Sasaki, K.; Tsunekawa, M.; Ohtsuka, T.; Konno, H. Confirmation of a sulfur-rich layer on pyrite after oxidative dissolution by Fe(III) ions around pH 2. Geochim. Cosmochim. Acta 1995, 59, 3155–3158. Singer, P. C.; Stumm, W. Acidic mine drainage: the ratedetermining step. Science 1970, 167, 1121–1123. Weerasooriya, R.; Dharmasena, B. Pyrite-assisted degradation of trichloroethene (TCE). Chemosphere 2001, 42, 389–396.

ES801310Y

VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

7475