Environ. Sci. Technol. 2006, 40, 3043-3049
Dechlorinating Chloroacetanilide Herbicides by Dithionite-Treated Aquifer Sediment and Surface Soil HARDILJEET K. BOPARAI, PATRICK J. SHEA,* STEVE D. COMFORT, AND DANIEL D. SNOW School of Natural Resources, University of Nebraska-Lincoln, Lincoln, Nebraska 68583-0915
The prevalent use of chloroacetanilide herbicides has resulted in nonpoint contamination of some groundwater and surface water. We determined the efficacy of dithionitetreated sediment and soils to transform chloroacetanilides. When used alone, dithionite rapidly dechlorinates chloroacetanilides in water, with the following order of reactivity: propachlor > alachlor > acetochlor > metolachlor. Stoichiometric release of chloride occurs during reaction with dithionite. and thiosulfate herbicide derivatives are produced. Treating aquifer sediment with dithionite reduces native Fe(III), creating a redox barrier of Fe(II)-bearing minerals and surface-bound Fe(II). Washing the reduced sediment (buffered with citrate-bicarbonate) with oxygenfree water removed Fe(II) and excess dithionite and no alachlor transformation was observed. In contrast, a dithionitetreated surface soil, rich in clay and iron, effectively dechlorinated alachlor after washing. Exposing alachlor to aquifer sediment treated with dithionite in potassium carbonate buffer (pH 8.5-9.0) produced dechlorinated alachlor as the major degradation product. Our results provide proof-of-concept that dechlorination of chloroacetanilide herbicides by dithionite and dithionite-treated aquifer sediment and soil is a remediation option in natural environments where iron-bearing minerals are abundant.
Introduction Chloroacetanilide herbicides are among the most popular and commonly used pesticides in agriculture. About 15.9 million kg of acetochlor was used in the United States in 1999, followed by metolachlor (13.6 million kg), alachlor (4.5 million kg), and propachlor (0.9 million kg) (1). The prevalent use of these herbicides has resulted in nonpoint contamination of some groundwater and surface water. The National Alachlor Well Water Survey of approximately 6 million wells (1988-1989) found metolachlor in about 1% of the wells (g0.03 µg L-1) and alachlor in 0.02% of the wells (g0.02 µg L-1) (2). Surface waters have also been contaminated by metolachlor in at least 14 states (3). Acetochlor, registered in the United States in 1994, was detected in shallow groundwater within one year of use (4). Chloroacetanilides can be attenuated naturally in the soilwater system but degradation may be slow or negligible in aquifer environments. Although various biological and chemical approaches have been used for remediation in the * Corresponding author e-mail:
[email protected]; phone: (402) 472-1533; fax: (402) 472-7904. 10.1021/es051915m CCC: $33.50 Published on Web 03/30/2006
2006 American Chemical Society
past (5-8), a more recent alternative is to use naturally occurring redox-active soil components such as iron-bearing clay minerals, iron oxides, sulfides, and organic matter. In soils and sediment, iron is the most abundant redox active element and contributes most toward redox buffering capacity. Chemical reductants include structural Fe(II) present as iron oxides and hydroxides and Fe(II) sorbed to iron (hydr)oxides, as well as iron sulfides, green rusts, and Fe-bearing clay minerals. Previous studies have shown that mineral phases containing structural Fe(II), including phyllosilicates (9-14), green rusts (15, 16), iron sulfides (13, 17, 18), magnetite (19, 20), and Fe(II) sorbed at iron (hydr)oxides (21-23) can reduce uranium (IV), chromates, nitrates, nitroaromatics, and chlorinated organic compounds. Although extensive research has been conducted on the reductive degradation of pollutants in natural environments by microorganisms and specific natural reductants, comparatively little research has investigated the reactivity of aquifer sediment and whole soils. It has been reported that chromates were degraded by creating a subsurface-permeable treatment zone of anoxic sediment on the Hanford Site in Washington (24), and abiotic degradation of halogenated alkanes and alkenes by reduced soil has also been observed (25, 26). To our knowledge, however, the abiotic degradation of chloroacetanilide herbicides by reduced aquifer sediment or soil has not been characterized. This information is needed to assess the costs and benefits of this technology for remediating groundwaters contaminated with chloroacetanilides. Various biological and chemical approaches can be adopted to create subsurface permeable treatment zones for remediating halogenated organic compounds. Chemically reducing Fe(III) in clay minerals is likely to be faster than natural (biological) mechanisms (27, 28), but the reductant must be relatively nontoxic in its original and reacted forms. Gan et al. (29) compared several reducing agents (including dithionite, sulfide, thiosulfate, hydrazine, and ascorbic acid) and found dithionite most effective in reducing structural iron in smectite. Dithionite reduces structural iron in clays (9, 10) as well as amorphous and crystalline Fe(III) oxides in natural sediment (30), producing various Fe(II) species. Reduction of iron by dithionite results from dissociation to sulfoxyl radicals which reduce Fe(III) to Fe(II) (30):
S2O42- S 2SO2•-
(1)
SO2•- + Fe3+ + H2O S Fe2+ + SO32- + 2H+
(2)
Dithionite and its reaction products (thiosulfate, sulfite, and sulfate) are relatively nontoxic (10). Thus, dithionite can be used as an environment-friendly reductant in aquifers containing iron-bearing clay minerals (27). Once these minerals are reduced, their subsequent oxidation can be coupled to the reduction of contaminants in groundwater. Our objective was to determine the transformation kinetics and products of chloroacetanilide herbicides treated by dithionite and dithionite-treated aquifer sediment and an iron-rich surface soil.
Experimental Section Chemicals, Soil, and Sediment. Alachlor [2-chloro-2′,6′diethyl-N-(methoxymethyl) acetanilide; 99%), propachlor [2-chloro-N-isopropylacetanilide; 99.5%], metolachlor [2-chloro-2′-ethyl-6′-methyl-N-(1-methyl-2-methoxyethyl) acetanilide; 96.5%], and acetochlor [2-chloro-N-ethoxymethylVOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Physicochemical Characteristics of Pantex Sediment and Sharpsburg Soil property
units
pH (1:1 H2O) organic matter cation exchange capacity DTPA-extractable Fe sand silt clay
% meq 100 g-1 mg kg-1 % % %
Pantex Sharpsburg sediment soil 8.97 0.1 8.7 16.0 90.7 3.3 6.0
5.5 3.2 15.9 90 14 58 28
N-(2-ethyl-6-methylphenyl) acetanilide; 98%] were obtained from Chem Service (West Chester, PA). Reagent-grade sodium dithionite (Na2S2O4) and sodium citrate dihydrate crystals (Na3C6H5O7‚2H2O) were purchased from Aldrich Chemical Co. (Milwaukee, WI), and sodium bicarbonate (NaHCO3) was obtained from J. T. Baker (Phillipsburg, NJ). Ferrous sulfate (FeSO4‚7H2O) was obtained from Mallinckrodt Baker Inc. (Paris, KY), and potassium carbonate (K2CO3) and Optimagrade acetonitrile were purchased from Fisher Scientific (Fair Lawn, NJ). Deionized deoxygenated water (prepared by sparging with nitrogen) was used to prepare the aqueous solutions. Nitrogen (N2, oxygen-free) and N2 + H2 (oxygenfree) gases were also required for the study. An O2-free environment was maintained within an anaerobic chamber (Coy Laboratory Products, Grass Lake, MI) by purging with O2-free N2 (95% N2/5% H2). The alachlor (k ) 0.053 h-1) > acetochlor (k ) 0.026 h-1) > metolachor (k ) 0.005 h-1) (significantly different at the 1% level; Figure S2, Supporting Information). Kinetics of Alachlor Reaction with Reduced Sediment and Soil. In experiments conducted with 10-200 mM dithionite solution and 2 g of sediment, the alachlor transformation rate increased with increasing dithionite concentrations (Figure 2). This could be attributed to reduction of more Fe(III) to Fe(II) (Table 2) or more dithionite remaining entrained in the sediment after discarding the supernatant. Because the sediment was not washed following reduction by dithionite, alachlor thiosulfate was the pre-
Results and Discussion
4 SO2•- a
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TABLE 2. Iron Phases during Reduction of Pantex Sediment and Sharpsburg Soil with Dithionite dithionite concn mM
sorbed FeII
untreated 10 20 50 100 200 200 (washed)
0.1 ( 0.0 5.9 ( 0.8 6.9 ( 1.2 12.8 ( 1.1 17.9 ( 1.8 25.5 ( 1.4 13.1( 0.8
untreated 200 200 (washed)
0.8 ( 0.1 59.9 ( 2.9 41.2 ( 1.1
total FeIII
FeII in supernatant mg L-1
Pantex Sediment 0.12 ( 0.0 24.2 ( 1.3 11.8 ( 0.6 53 ( 4.5 22.1 ( 1.5 73 ( 3.7 25.4 ( 2.1 85 ( 3.9 41.4 ( 1.9 139 ( 3.3 55.2 ( 3.4 217 ( 8.9 54.3( 2.3 203( 6.4
314 ( 11 285 ( 14 267 ( 10 251( 8.9 191 ( 6.8 116 ( 2.3 102 ( 1.9
0.0 ( 0.0 25 ( 0.9 43 ( 0.2 61 ( 1.1 73 ( 2.1 109 ( 0.7 108 ( 1.3
Sharpsburg Soil 6.2 ( 0.9 137 ( 2.2 315 ( 4.0 581 ( 19 287 ( 7.9 519 ( 13
787 ( 5.4 242 ( 11 203 ( 10
0.0 ( 0.0 201 ( 0.7 203 ( 0.3
µmol g-1 FeIICO3/S
dominant degradation product. The insignificant change in alachlor concentration in 18 days (0.24 to 0.23 mM) in the control treatment indicated no significant adsorption to the sediment. In subsequent experiments to eliminate the possibility that residual thiosulfate and/or sulfoxyl radicals were responsible for the alachlor transformation, the sediment was washed twice with deoxygenated water. Very little alachlor loss was observed in the dithionite-treated Pantex sediment that was washed prior to reacting with alachlor (Figure 3A). Nzengung et al. (10) similarly found no degradation of PCE by washed, dithionite-treated clays. Because large amounts of Fe(II) are washed away with the supernatant (108 mg Fe(II) L-1 after centrifugation, and 11 and 6 mg L-1 after first and second washings), the lack of alachlor transformation was probably due to insufficient Fe(II) redox capacity
FIGURE 3. (A) Comparison of alachlor (C0 ) 0.2 mM) transformation by dithionite (200 mM)-reduced and washed Pantex sediment and Sharpsburg soil (buffered with citrate-bicarbonate at pH 8.5). (B) Transformation of alachlor in unbuffered, citrate-bicarbonate (pH 8.5) and K2CO3 buffered Pantex sediment reduced with 200 mM dithionite. Bars on symbols represent standard deviations; where absent, bars fall within symbols. 3046
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total FeII
remaining in the system. In contrast, the reduced Sharpsburg soil still transformed alachlor after washing (Figure 3A). The difference between the efficacies of washed, reduced Pantex sediment and Sharpsburg soil can be explained by differences in clay and Fe contents. After washing, sorbed Fe(II) decreased from 25.5 to 13.1 µmol g-1 in the Pantex sediment, which may not be sufficient to transform alachlor, but was 41.2 µmol g-1 in the Sharpsburg soil and provided the redox capacity needed for the reaction (Table 2). The control treatment showed little change in alachlor concentration in 18 days (0.20 to 0.195 mM), indicating insignificant adsorption to the soil. Protons are generated during iron reduction by dithionite (eqs 1 and 2), lowering the pH of the sediment to near neutral. Thus, alachlor transformation was compared under buffered and unbuffered conditions. Alachlor was dechlorinated by dithionite-treated, washed sediment in potassium carbonate buffer at pH 8.5-9.0 (Figure 3B). The sediment turned black, likely due to formation of magnetite (Fe3O4), which contains both Fe(II) and Fe(III). Magnetite formation is favored at neutral to alkaline pH and low Eh (34) in the presence of Fe(II) and Fe(III) ions. Formation of iron sulfides (also black in color) should be considered but was unlikely because in the absence of buffer (pH ) 7.2) the reduced sediment was light green. Moreover, if significant amounts of sulfides were present, they would react with alachlor to form mercaptoalachlor (8), which was not detected in our LC-MS analysis. The pH of the reduced sediment decreased to 7.2 in the absence of buffer and alachlor transformation was hindered. This indicates that alachlor transformation not only requires Fe(II) but certain oxides must be present for this surfacemediated reaction at neutral to alkaline pH. Citratebicarbonate buffer in the presence of dithionite reduces Fe(III) but also dissolves various amorphous and crystalline oxides, which are then washed away with supernatant, leaving less Fe(II) and Fe(II)-containing oxide surfaces for alachlor degradation. Using the same reduced sediment, reaction rates decreased in the following order: propachlor (k ) 0.243 d-1) > alachlor (k ) 0.120 d-1) > acetochlor (k ) 0.069 d-1) > metolachlor (k ) 0.030 d-1) and were significantly different at the 1% level (Figure 4A). The rate constant for alachlor in this experiment was the same as reported for reaction with Fe0 (5) and the relative reactivities of the chloroacetanilides parallel reactions with thiosulfate salts (33). Loch et al. (8) similarly found that propachlor was more reactive with HSthan alachlor, followed by metolachlor. The order of reactivity was consistent with calculated activation energy barriers (8) and attributed to structural differences (8, 33), which may likewise explain our results. IC analysis showed stoichiometric release of chloride with alachlor loss at each sampling time (Figure 4B). The first-
FIGURE 4. (A) Dechlorination of chloroacetanilide herbicides in K2CO3 (400 mM) buffered Pantex sediment reduced with 200 mM dithionite. (B) Liberation of chloride ions after dechlorination of alachlor. Bars on symbols represent standard deviations; where absent, bars fall within symbols. order expressions (eqs 3 and 4) provided a good fit to the observed decreases in alachlor concentration (r2 ) 0.986, k ) 0.149 d-1) and production of chloride (r2 ) 0.988, k ) 0.105 d-1). The primary product of treatment with dithionite-reduced Pantex sediment in K2CO3 buffer is dechlorinated alachlor, with small amounts of alachlor thiosulfate, alachlor ESA, and hydroxyalachlor also detected (Figure S3, Supporting Information). Dechlorinated alachlor (molecular formula of C14H21NO2 and nominal mass of 235) was identified in the positive ion mode with [M + H]+ ion at m/z 236. The reaction of alachlor with iron metal results in the formation of dechlorinated alachlor (5), whereas dithionite, thiosulfate, and sulfides transform alachlor to sulfur-containing derivatives (8, 33). The fact that dechlorinated alachlor is the major product indicates that washing the reduced sediment twice removes most of the dithionite and its degradation products and thus Fe(II) is mainly responsible for alachlor transformation. Removal of chlorine often increases biodegradability (35). Comfort et al. (6) found that dechlorinated metolachlor (a closely related chloroacetanilide) was 5-fold more biodegradable than the parent compound. Alachlor Transformation by Fe(II) in Dithionite-Treated Pantex Sediment. Washing the reduced Pantex sediment removed sorbed Fe(II) and no herbicide transformation was observed (Figure 5A). Degradation of nitroaromatic and chlorinated organic compounds on several mineral surfaces in the presence of structural or adsorbed Fe(II) has been reported (23, 36-40). Hypothesizing that dithionite treatment facilitates formation of various iron oxides, we investigated alachlor transformation after adding Fe(II) to suspensions of Pantex sediment. Satapanajaru et al. (40) reported maximum metolachlor transformation by Fe(II) in the presence of goethite (R-FeOOH) at pH 8.0. After adding Fe-
FIGURE 5. (A) Changes in alachlor concentration following treatment with washed and unwashed Pantex sediment reduced with 100 mM dithionite, and washed Pantex sediment reduced with 100 mM dithionite + 200 mg Fe2+ L-1. (B) Effects of multiple Fe2+ additions (100 mg L-1 at 24, 48, and 72 h) on alachlor loss and chloride release following treatment with washed dithionite-treated Pantex sediment. The reduced sediments were buffered with citrate-bicarbonate (pH 8.5). Bars on symbols represent standard deviations; where absent, bars fall within symbols. (II) (as FeSO4) to the washed, reduced Pantex sediment at pH 8.5, alachlor was rapidly tranformed but the reaction stopped within 48 h (Figure 5A). When Fe(II) was added to washed, reduced sediment without adjusting pH (pH ) 6.9), no alachlor transformation occurred, which indicated that the reaction was highly pH-dependent. Lee and Batchelor (41) similarly reported an increase in the rate of TCE dechlorination by green rust as pH was increased from 6.8 to 8.1. This was attributed to conversion of unreactive sites to reactive Fe(II) sites and the higher electron density of unprotonated tFeIIIOFeIIOH0 than protonated tFeIIIOFeIIOH2+ surface groups (42). Aqueous solutions of Fe(II) alone (pH ) 8.5) were unreactive (not shown). No dechlorination was observed with untreated Pantex sediment, even after adding Fe(II) as the pH decreased to 6.8. These observations indicate that both the oxides and soluble Fe(II) are necessary for alachlor dechlorination. When we repeated the experiment and added more FeSO4 (100 mg Fe2+ L-1) to the reduced Pantex sediment at 24, 48, and 72 h, alachlor concentration continued to decrease, with stoichiometric release of chloride (Figure 5B). These observations indicate that a lack of available Fe(II) was the reason chloroacetanilide concentration remained essentially constant after washing the reduced Pantex sediment. Satapanajaru et al. (40) similarly reported a continued decrease in metolachlor concentrations when Fe(II) was added to magnetite-metolachlor suspensions. Dechlorinated alachlor was the primary product of alachlor transformation by washed, dithionite-treated Pantex sediment amended with Fe(II). Potential Use of Dithionite in Remediation. Chloroacetanilide herbicides are rapidly dechlorinated in water by dithionite, producing thiosulfate-substituted products, as VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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confirmed by LC/MS. Moreover, chloroacetanilides can be rapidly transformed by iron-rich sediment and soils following treatment with dithionite. Reaction kinetics and products depend on pH and the buffering matrix. Our results indicate the potential of dithionite to remediate environments where iron-bearing minerals are abundant in sediment and soils. Because various iron oxides and Fe(II) species may form during reduction of sediment and soil by dithionite, identification of those forms responsible for chloroacetanilide transformation would help guide further applications of this technology. There is also the potential to combine dithionite treatment with zerovalent iron (Fe0) technology to remediate contaminated soils. Soils amended with Fe0 will have very high concentrations of Fe(III) that can be reduced with dithionite to Fe(II) to continue the treatment process.
Acknowledgments This research was supported by EPA-EPSCoR and the Nebraska Research Initiative and is a contribution of University of Nebraska-Lincoln Agricultural Research Division projects NEB 40-002 and 019, J. Series No.14617.
Supporting Information Available LC-MS data and a figure showing the kinetics of transformation of several chloroacetanilide herbicides in citratebicarbonate buffered dithionite solution. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review September 27, 2005. Revised manuscript received February 23, 2006. Accepted February 24, 2006. ES051915M
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