Kinetics and Mechanism of the Degradation of Methyl Parathion in

Dec 22, 2005 - The kinetics of the transformation of methyl parathion have been investigated in aqueous solution containing reduced sulfur species and...
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Environ. Sci. Technol. 2006, 40, 900-906

Kinetics and Mechanism of the Degradation of Methyl Parathion in Aqueous Hydrogen Sulfide Solution: Investigation of Natural Organic Matter Effects XIAOFEN GUO AND URS JANS* Department of Chemistry, The City College of New York, CUNY, New York, New York 10031

The kinetics of the transformation of methyl parathion have been investigated in aqueous solution containing reduced sulfur species and small concentrations of natural organic matter (NOM) from different sources such as soil, river, and peat. It was shown that NOM mediates the degradation of methyl parathion in aqueous solutions containing hydrogen sulfide. After evaluating and quantifying the effect of the NOM concentration on the degradation kinetics of methyl parathion in the presence of hydrogen sulfide, it was found that the observed pseudo-first-order reaction rate constants (kobs) were proportional to NOM concentrations. The influence of pH on the degradation of methyl parathion in the aqueous solutions containing hydrogen sulfide and NOM has been studied. The rate of degradation of methyl parathion was strongly pH dependent. The results indicate kobs with a commercially available humic acid has a maximum value at approximately pH 8.3. Two main reaction mechanisms are identified to dominate the degradation of methyl parathion in aqueous solution containing hydrogen sulfide and NOM based on the products aminomethyl parathion and desmethyl methyl parathion. The two mechanisms are nitro-group reduction and nucleophilic attack at the methoxy-carbon. The reduction of the nitro-group is only observed in the presence of NOM. The results of this study form an important base for the evaluation and interpretation of transformation processes of methyl parathion in the environment.

Introduction Organophosphorus pesticides (OPs) are a group of closely related pesticides that affect the nervous system (1). However, residues of OPs have been reported in food (2-5) and other environmental matrixes such as groundwater and surface water (6, 7). Methyl parathion is widely used as an organophosphorus insecticide and acaricide to control many insect pests of agricultural crops, primarily on cotton and vegetables (8). Methyl parathion is a highly toxic insecticide and is classified by the EPA (U.S. Environmental Protection Agency) as a “toxin” of class I. It is highly toxic to birds, aquatic invertebrates, and bees. There is serious concern about the toxicological and environmental risks associated with methyl parathion residues (9). Methyl parathion primarily affects the nervous system. It can cause cholinesterase inhibition in humans, which leads to an overstimulation of the nervous * Corresponding author phone: (212)650-8369; fax: (212)650-6107; e-mail: [email protected]. 900

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system, causing nausea, confusion, and at high exposures respiratory paralysis and death (8). To predict the environmental fate of methyl parathion, it is necessary to understand its mobility and rate of degradation under typical environmental conditions. Methyl parathion can be transported into the aqueous environment via runoff or spray drift. Aquatic natural organic matter (NOM) consists of biogenic substances and is ubiquitous to all natural waters: concentrations range from 1 to 70 mg of C/L in uncontaminated streams with a worldwide average of 5.8 mg of C/L (10-13). It is a common generalization that NOM is a reducing agent, and NOM will reduce metals that can, in turn, reduce organic pollutants (14). Fulvic acid and humic acid are fractions of NOM that have been shown to bind hydrophobic compounds (15). Humic acid molecules generally contain more carbonyl groups and aliphatic moieties than fulvic acid molecules. It has been shown that NOM is influencing the degradation of organic contaminants (organophosphorus and organonitrogen pesticides) present in different natural waters (16-18). Peijnenburg et al. found that the rate of halogenated hydrocarbon reduction increases with increasing NOM content of the sediment in sediment-water systems (19). Sediments in general are anoxic. Anoxic environments are favorable for the development of microorganisms implied in the formation of inorganic reduced sulfur species, such as hydrogen sulfide, elemental sulfur, or related polysulfides. These inorganic sulfur species can react with organic matter and produce low molecular weight organosulfur compounds and macromolecules (20-23). Reactions of inorganic sulfur with NOM may occur at points of unsaturated moieties or at points of attachment of any functional group in organic molecules (24). Dunnivant et al. have shown that NOM increased the rate of reduction of nitroaromatic compounds in the presence of bisulfide, and it was proposed that the hydroquinone/quinonetype redox couple which is known to be present in NOM is acting as an electron-transfer mediator (25). Nurmi and Tratnyek studied NOM from different sources and quinone model compounds by cyclic voltammetry and found that some NOMs showed similar cyclic voltammograms to some of the model compounds (28). However, Perlinger and co-workers showed that inorganic forms of S(-II) act as nucleophiles and electrophiles in addition reactions to the R,β-conjugated moieties of quinones, which are present in trace quantities in NOM (26, 27). It is also reported that semiquinones are formed from reactive quinones during the reduction of NOM by microorganisms (29). It is possible that the produced semiquinone radicals are important in controlling the transformation of organic pollutants. In all these studies, the presence of NOM can affect the observed empirical pesticide degradation rate in the environment. It is important to understand the relative influence of various environmental and experimental factors which can affect the observed methyl parathion degradation rates. In this paper we focused on the effect of NOM on the reaction kinetics of the methyl parathion in different aqueous solutions. The major goals of this investigation were (1) to determine the effect of NOM from different sources on the degradation kinetic of methyl parathion in different aqueous solutions, (2) to evaluate and quantify the effect of NOM concentration on the degradation kinetics of methyl parathion in solutions containing hydrogen sulfide, (3) to elucidate the influence of pH on the degradation of methyl parathion in the aqueous solution containing hydrogen 10.1021/es051453c CCC: $33.50

 2006 American Chemical Society Published on Web 12/22/2005

sulfide and NOM. In addition, the degradation mechanism of methyl parathion in aqueous systems containing hydrogen sulfide and NOM is discussed.

TABLE 1. Reaction Rate Constants for the Reaction of Methyl Parathion with Hydrogen Sulfide with NOM or without at Various pH Values (25 °C)

Experimental Methods Chemicals. Methyl parathion (98.7%, CAS registry no.29800-0) was obtained from Chem Service (West Chester, PA). 4-Nitrophenol (99%, CAS registry no.24-132-6) was obtained from Aldrich (Milwaukee, WI). Ethyl acetate and methanol were HPLC grade, obtained from Fisher Scientific (Pittsburgh, PA) and used without further purification. Solutions. All solutions were prepared in a controlledatmosphere glovebox (95% N2, 5% H2) using deionized water (DW) (Milli-Q gradient system, Millipore, Bedford, MA). DW was transferred into the glovebox immediately after being purged with ultrahigh-purity argon (99.999%) for 1 h. All glassware was soaked in 1 M HNO3, rinsed several times with DW, and dried at 200 °C prior to use. Glassware used for preparing bisulfide solutions was washed with a methanol/NaOH mixture to remove traces of sulfur impurities prior to acid washing. Solutions containing sulfur nucleophiles were prepared in deoxygenated pH buffer and were handled within the glovebox. Na2S solutions were prepared from Na2S‚9H2O (98%, EM Science) and polysulfide solutions from Na2S4 (90%, Alfa Aesar) (31, 35). Thiophenol solutions were prepared from thiophenol (99%, CAS no. 108-98-5, Lancaster) with deoxygenated buffer solution in the glovebox. The concentration of the total hydrogen sulfide/bisulfide solutions, polysulfide, and thiophenol were determined by iodometric titration. The sodium thiosulfate solution for the titration was standardized against potassium iodate daily. All reaction solutions were prepared in the glovebox. The reaction solutions were prepared in volumetric flasks and then transferred to 20 mL glass syringes equipped with a polycarbonate stopcock and a PTFE needle tubing. The syringes contained five glass rings to facilitate mixing. All reaction solutions contained 5% methanol, 100 mM NaCl, and 50 mM buffer (sodium phosphate or sodium tetraborate). The glassware including syringes for reaction experiments was autoclaved 30 min at 120 °C to inhibit biological growth. In addition, the buffer solutions were filtered (0.2 µm, Anotop 25-sterile, Whatman Ltd., Maidstone, England). The spike solution of methyl parathion was prepared by dissolving methyl parathion in deoxygenated methanol. Experiments conducted at varying methanol concentrations (0-15%) indicated that these levels of methanol did not influence the degradation rates. Three types of pH buffers were used in this experiment. These included phosphate buffer (prepared from NaH2PO4‚7H2O and Na2HPO4‚H2O), borate buffer (prepared from sodium tetraborate), and HEPES (N-(2hydroxyethyl) piperazine-N′-2-ethanesulfonic acid). An Accumet pH meter (Fisher Scientific) with a Ross combination pH electrode (ThermoOrion, Beverly, MA) was used to measure solution pH. Characterization of Natural Organic Matter. Various types (water, soil, and peat) of NOMs were evaluated for their ability to promote the degradation of the methyl parathion. One NOM used was humic acid (AAHA), which was purchased from Alfa Aesar (Woodhill, MA, stock no. 41747, lot no. D16N23, CAS no. 1415-93-6). The others were purchased from the International Humic Substances Society (IHSS). They are Suwannee River humic acid standard II (SRHA), Elliott soil humic acid standard (ESHA), Pahokee peat humic acid standard (PPHA), Leonardite humic acid standard (LEHA), and Suwannee River fulvic acid standard (SRFA). A summary of the total organic carbon (TOC) concentrations and selected transition metal (Cu, Zn, Ni, and Fe) content of some of these solutions is given in Table 1 in the Supporting Information. Both organic carbon and

NOM AAHA SRHA LEHA PPHA SRFV ESHA AAHA SRHA LEHA PPHA SRFV ESHA a

kobs k2 k2/kobs kNu (h-1 [H2S]T TOC pH (mM) (mg/L) (10-2 h-1)a (10-2 h-1) (%) (M[H2 S]T)-1)b 7.4 7.2 7.2 7.2 7.2 7.3 7.3 8.4 8.7 8.6 8.6 8.5 8.6 8.6

3.3 4.4 3.7 3.9 3.7 3.9 4.0 5.9 5.8 4.5 3.7 3.7 4.2 3.7

0 28 47 49 52 49 51 0 30 43 46 48 49 51

95% confidence level.

1.0 ( 0.1 6.9 ( 0.1 3.1 ( 0.1 9.5 ( 0.1 9.3 ( 0.1 2.4 ( 0.1 12.0 ( 0.1 2.2 ( 0.1 2.4 ( 0.1 1.9 ( 0.1 2.6 ( 0.1 2.6 ( 0.2 1.9 ( 0.1 4.4 ( 0.2 b

0 5.4 2.0 8.1 8.3 1.3 10.8 0 0 0.5 1.2 1.1 0.2 3.1

0 79 67 86 89 52 90 0 0 26 46 42 11 70

3.0 3.2 2.6 3.4 2.7 2.8 3.0 3.7 4.1 3.1 3.5 3.8 4.1 3.5

kNu ) k1/[H2S]T

metal concentrations were determined on filtered solutions (0.2 µm pore diameter membrane filter, Anotop 25 plus). The NOM stock solutions were prepared under argon from NOM using deoxygenated DW. The stock solutions were stored in the glovebox. The total organic carbon concentrations were performed on a SHIMADZU TOC-VCPH TOC analyzer. Dissolved metal ion concentration was measured in the solution by flame atomic absorption spectrophotometry (SHIMADZU AA-6200). Experimental Setup. The deoxygenated reaction solutions were prepared by initially mixing together pH buffer, electrolyte (NaCl), deoxygenated methanol, and NOM solutions. The pH was controlled by 50 mM sodium phosphate buffer for the pH values of 6-9 and was adjusted with deoxygenated hydrochloric acid. The solutions were equilibrated overnight and then transferred to an autoclaved syringe via a syringe and sterile 0.2 µm filter. An appropriate amount of methyl parathion stock solution in methanol was then added to the syringe to initiate the reaction yielding an initial concentration of 100 µM. All these operations were carried out in the glovebox. Reactors were vigorously mixed for 30 s in the glovebox, taken out from the glovebox, and were incubated in a water bath at 25.0 ( 0.1 °C. Aliquots (1 mL) were periodically removed, followed by the addition of 2 drops of 6 M HCl. The mixture was extracted into ethyl acetate, followed by analysis of the organic fraction via HPLC. The acidification ensures the extraction of p-nitrophenol and desmethyl methyl parathion. Control solutions were prepared to determine individual components or multicomponent effects. These solutions contained either distilled water and pH buffer (no sodium sulfide, no NOM), or only distilled water, pH buffer, and sodium sulfide (no NOM), or distilled water, NOM, and pH buffer (no sodium sulfide). Analytical Methods. The extent of pesticide degradation was determined by the disappearance of the parent compound using reversed-phase HPLC (2690 separation module, Waters, Milford, MA) equipped with a photodiode array detector (996, Waters) (30). The stationary phase was an Xterra MS C18, 3.9 mm × 150 mm, 5 µm column (Waters) with a guard column (3.9 mm × 20 mm) of the same material. The mobile phase was a water-methanol gradient at a flow rate of 0.7 mL/min. The water was 1 mM H3PO4 in DW. The gradient started at 42% methanol and increased to 70% at 13 min. The system then returned to 42% methanol at 15 min and was kept under this condition for 3 min in order to reequilibrate. The eluent was monitored at a wavelength of 273 nm for methyl parathion, 4-nitrophenol, and desmethyl methyl parathion. VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Degradation of methyl parathion in 5.0 mM aqueous hydrogen sulfide solution containing NOM (AAHA, TOC ) 31 mg of C/L) at pH 5.8 and 25 °C. The initial concentration of methyl parathion was 90-110 µM. The degradation products of methyl parathion were also analyzed by a GC-17A gas chromatograph (Shimadzu) with an Econo-Cap EC-5 30 m × 0.25 mm i.d. × 0.25 µm column (Alltech) and a QP-5000 mass spectrometer (Shimadzu) detector. Product Quantitation. One product (4-nitrophenol) and the parent compound (methyl parathion) could be quantified using reference materials that were commercially available for this study. The quantification of desmethyl methyl parathion by HPLC is based on experiments of methyl parathion with thiophenolate in oxygen-free phosphate buffer solutions. Under this condition the predominant reaction for the reaction of methyl parathion with thiophenolate is the nucleophilic attack of thiophenolate at a methoxy group resulting in the formation of desmethyl methyl parathion and methylthio-benzene. The formed methylthiobenzene concentration corresponds well with the concentration of the reacted methyl parathion. In addition the peak area of the desmethyl methyl parathion is increasing along with the methylthio-benzene peak and is stable once all the methyl parathion has reacted for the reaction time observed. Based on these observations we assumed that the concentration of desmethyl methyl parathion formed is equal to the concentration of methylthio-benzene formed. The exact same approach was used for chlorpyrifos-methyl reacting with thiophenolate (31). Chlorpyrifos-methyl has a very similar reactivity toward sulfur nucleophiles as that of methyl parathion. However, in the case of chlorpyrifos-methyl standards for desmethyl-chlorpyrifos-methyl were available, and it was shown that the concentration of desmethylchlorpyrifos-methyl formed is equal to the concentration of methylthiol-benzene formed when chlorpyrifos-methyl is reacting with thiophenolate.

Results and Discussion Degradation of Methyl Parathion in NOM Solutions. The degradation of methyl parathion in aqueous solution containing hydrogen sulfide was studied. The results show that the degradation rate of methyl parathion in the presence of hydrogen sulfide was slow but increased significantly by the addition of NOM. The results clearly demonstrate that NOM mediates the degradation of methyl parathion in aqueous solution containing hydrogen sulfide. Similar results were observed for all other NOMs at pH 7.4. The good fit to the semilogarithmic plot for degradation of methyl parathion in Figure 1 illustrates that the kinetics of AAHAmediated degradation was pseudo-first-order in methyl parathion. 902

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Experiments with polysulfide and thiophenol instead of bisulfide were also performed. The results (Figure S-1, parts a and b, in the Supporting Information) show that degradation rates of methyl parathion do not change significantly when the AAHA is added to thiophenol or polysulfide solutions. No significant difference between the two rate constants at the 95% confidence level indicates that NOM seems not to significantly affect the degradation of methyl parathion in the aqueous thiophenol or polysulfide solutions. However, it should be noted that the reactions with thiophenol and polysulfide are much faster than with bisulfide alone. This can be attributed to the fact that thiophenol and polysulfide are much better nucleophiles than bisulfide that can attack at the carbon atom of the methoxy groups (31, 36). Influence of Reaction Time of Hydrogen Sulfide with NOM on the Degradation of Methyl Parathion. To investigate the influence of reaction time between hydrogen sulfide and NOM to form the reactive intermediate (21-23), experiments were performed with different reaction times (0.2 and 16 h) between NOM and HS- before methyl parathion was added. The results (Figure S-2 in the Supporting Information) demonstrate that a different reaction time between HS- and NOM does not influence the degradation of methyl parathion. There are two possible explanations: (a) The reaction between hydrogen sulfide and NOM is very fast. (b) There may be no reaction between hydrogen sulfide and NOM; the increasing degradation rate of methyl parathion is the result from the two compounds coeffect (e.g., NOM acts as catalyst, possibly by binding to methyl parathion and making it more susceptible to reactions). Reaction Rate as a Function of Organic Carbon Concentration. The dependence of the degradation rate of methyl parathion on the NOM concentration was investigated. The results show that with increasing NOM (AAHA) concentration, the degradation rate of methyl parathion is increasing. As illustrated in Figure 2, at fixed pH and hydrogen sulfide concentration, pseudo-first-order rate constants were found to be a linear function of the NOM concentration (TOC). Degradation Rates of Methyl Parathion as a Function of pH Value. The effect of pH on the degradation rate of methyl parathion in the presence of NOM was investigated in solutions containing 31 mg of C/L of AAHA and 5.0 mM hydrogen sulfide. The pH was varied from 5.5 to 9.5, and the results are shown in Figure 3. This series of experiments illustrates that kobs increases with increasing pH in the range of pH 5.5-8.3. It has a maximum value at pH 8.3. For pH >

FIGURE 2. Degradation of methyl parathion in 4.0 mM aqueous hydrogen sulfide solution containing different concentrations of NOM (AAHA) at pH 6.4 and 25 °C. The initial concentration of methyl parathion was 90-100 µM.

FIGURE 3. kobs value of methyl parathion as a function of pH in aqueous 5.0 mM hydrogen sulfide with NOM (AAHA, TOC ) 31 mg of C/L) and 25 °C. 8.3, the kobs value drops abruptly and shows no obvious change with the further increasing pH. Experiments between pH ) 8.3 and 9.5 were performed with borate, phosphate, and HEPES buffer. Statistically identical (within the 95% confidence level) rate constants were determined for the three different types of buffer. Since the complexing properties of the three different buffers toward Fe(II)/Fe(III) are very different, it is quite likely that dissolved Fe(II)/Fe(III) is not involved in the reaction. The observed pH dependence indicates that at higher pH values the reactive intermediate is not formed to the same extent. Tratnyek and Macalady (32) investigated the reduction of methyl parathion by model compounds for NOM (reduced indigodisulfonate and anthraquinonedisulfonate). They reported a similar pH dependence of the observed reaction rate constants and explain it with the expected concentration of the monophenolate species of the hydroquinone. It was concluded that the monophenolate is the only reactive species in the quinone/hydroquinone redox system that is responsible for the fast reduction of methyl parathion. It is possible that a similar situation is encountered with possible quinone groups present in NOM. When the observed degradation rate constant is compared with the reaction rate constant of hydrogen sulfide with methyl parathion without AAHA, it can be seen that the rate constants with and without AAHA are in the same range at higher pH (pH > 8.3). An example of a time course can be found in the Supporting Information (Figure S-3). It can be seen that the reaction rate of methyl parathion in the hydrogen sulfide system with or without NOM at pH ) 9.0 is 0.016 h-1 and 0.017 h-1, respectively.

FIGURE 4. Reaction of methyl parathion with hydrogen sulfide and the formation of 4-nitrophenol and desmethyl methyl parathion in an aqueous system containing NOM (AAHA, TOC ) 31 mg of C/L) at 25 °C: [H2S]T ) 4.0 mM and pH ) 8.5. Mechanism of Degradation of Methyl Parathion in Aqueous of Hydrogen Sulfide Solution Containing NOM. Figure 4 shows that two reaction products are produced in a reaction of methyl parathion with hydrogen sulfide in an aqueous system containing NOM. Desmethyl methyl parathion is the predominant product detected while smaller concentrations of 4-nitrophenol are also detected. It can be seen that the concentration of 4-nitrophenol is not increasing with reaction time in a solution containing NOM (AAHA, 31 mg of C/L), 4.0 mM hydrogen sulfide at pH ) 8.5, 25°C. The reactivity of 4-nitrophenol toward hydrogen sulfide in the aqueous system containing NOM was tested in separate experiments, showing that there is no reaction between nitrophenol and bisulfide over the time interval relevant for the experiments presented here. It has been reported that nitrophenol is the major product of the hydrolysis of methyl parathion (34). In our experiments, 4-nitrophenol concentrations were small. They were the same as the concentrations in the blank. Therefore, hydrolysis can be neglected as an important reaction in our experiments. The observed 4-nitrophenol in our experiments originates very likely from impurities in the purchased pesticide. Moreover, the observed results can be explained by hydrogen sulfide reacting with methyl parathion predominantly via a nucleophilic attack at the carbon of methoxy group which is supported by the observed formation of desmethyl methyl parathion. Tratnyek and Macalady (32) reported that the abiotic reduction of nitro aromatic pesticides occurs in homogeneous solutions of quinone redox couples. The kinetics of methyl parathion disappearance and aminomethyl parathion appearance was established for these model systems. On the basis of their work in model systems, it may be assumed that reduction of methyl parathion occurs also in the solutions of hydrogen sulfide and NOM. To verify this assumption, the products were tentatively identified by GC/MS. The results are presented in the Supporting Information (Figure S-4 and S-5). The gas chromatogram of the extracted reaction mixture at pH 8.1 after a reaction time of 3.8 h shows that two peaks are detected at the beginning of the reaction. From the mass spectrum and from the retention time of standards, it was concluded that one peak is methyl parathion; the other peak might be an unknown compound from NOM. At increased reaction time (27 h), two additional peaks were detected. From the MS spectrum (Figure S-5 in the Supporting Information), one peak was identified as amino methyl parathion according to the analysis of its mass spectra. No conclusive identification of the second additional peak was possible VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Plot of ln(C/C0) vs time for the reaction of methyl parathion with 4.0 mM hydrogen sulfide in an aqueous solution containing different sources of NOM at pH 7.3, 25 °C. The initial concentration of methyl parathion was 80-110 µM.

SCHEME 1

based on the MS spectrum (Figure S-6 in the Supporting Information). In contrast, in experiments conducted at pH 9.3 (Figure S-7 in the Supporting Information) no peak for amino methyl parathion was observed during the degradation of methyl parathion. Therefore, it can be concluded that no amino methyl parathion was formed at detectable concentrations at pH 9.3. From these results, it can be concluded that nitro reduction is a reaction pathway in the degradation of methyl parathion with hydrogen sulfide and AAHA at pH < 8.3. When the pH is higher than 8.3, the nitro reduction is no longer occurring at a significant rate. On the basis of our experiments and previous publications (25, 32, 33) it can be postulated that nitro reduction of methyl parathion by NOM/bisulfide might occur with quinone moieties acting as electron-transfer mediators. Influence of the Different NOM on the Degradation of Methyl Parathion with Hydrogen Sulfide. Figure 5 shows time courses for methyl parathion and bisulfide in the presence of NOM from different sources at pH 7.3, 4.0 mM [H2S]T, and 50 mg of C/L NOM. The reaction is the fastest for ESHA (0.12 h-1) while it is the slowest for SRFA (0.024 h-1). The two postulated reaction mechanisms for the degradation of methyl parathion with hydrogen sulfide in aqueous solution containing NOM are nucleophilic substitution at the methoxy-carbon and nitro-group reduction (Scheme 1). Assuming that both reactions obeyed first-order kinetics, the overall degradation rate constant kobs can be expressed as the sum of the nucleophilic substitution rate constant k1 and the nitro-group reduction rate constant k2.

kobs ) k1 + k2 k1 and kobs were determined by simultaneously fitting the data for the formation of desmethyl methyl parathion and the degradation of methyl parathion using the program Scientist for Windows. The nitro-group reduction rate constant k2 was calculated by subtracting k1 from kobs. The 904

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FIGURE 6. (a) Degradation of methyl parathion in aqueous 4.0 mM hydrogen sulfide solution containing NOM (PPHA, TOC ) 52 mg of C/L) at 25 °C, pH 7.5. The solid lines represent model fits. (b) Degradation of methyl parathion in aqueous 4.0 mM hydrogen sulfide solution containing NOM (PPHA, TOC) 48 mg of C/L) at 25 °C, pH 8.5. The solid lines represent model fits. relative contribution of nucleophilic substitution or nitrogroup reduction to the overall degradation may be estimated from the ratio of k2 over kobs. The result for different NOMs is shown in Table 1. The results show that the observed reaction rate constant is higher at pH 7.4 than pH 8.5 for all six different NOMs. For the aqueous hydrogen sulfide solution without NOM at pH 7.4, it was found that nucleophilic substitution is predominant, contributing 100% to the overall degradation. For the aqueous hydrogen sulfide solution containing NOM (PPHA), nucleophilic substitution accounted for only 11% of the total degradation, whereas nitrogroup reduction is calculated to contribute for 89% (Figure 6a). For the other NOMs the contribution of the nucleophilic substitution to the overall degradation rate of methyl parathion at pH 7.4 varies from 10-48%. Such observations could be explained by assuming that different NOM contain different concentrations of active moieties involved in the reduction. NOM quite likely acts as an electron-transfer mediator enabling the reduction reaction and resulting in a higher overall reaction rate constant for the methyl parathion degradation. This would explain the relatively small contribution from nucleophilic substitution in the aqueous hydrogen sulfide solution containing NOM (pH 6-8), because the added NOM is increasing kobs significantly. For the aqueous hydrogen sulfide without NOM at pH 8.4, it was found that nucleophilic substitution was predominant; desmethyl methyl parathion was the product that accounted for 100% of the methyl parathion loss. For the aqueous hydrogen sulfide solution containing AAHA at pH ) 8.7, the result is similar to the solution without NOM. For the other NOMs (SRHA, LEHA, PPHA, SRFA), nitro-group reduction accounts for 11-46% of the total reaction rate constant, whereas nucleophilic substitution contributes 54-89% (Figure 6b). For ESHA the estimated nitro-group reduction accounted still for 70% at pH 8.6 which is higher

than for the other NOMs; however, it is smaller than for the comparable experiment at pH 7.4. The smaller contribution of nitro-group reduction at pH 8.5 versus 7.4 can be explained by the decreased influence of NOM at pH 8.5. It can be seen from Table 1 that the overall reaction rate kobs is changing with different NOM at different pH. For all NOMs, the overall reaction rate kobs at pH 7.4 is faster than at pH 8.5. However, the determined contribution of the nucleophilic substitution at the methoxy-carbon kNu is the same for all experiments with NOM as for the reaction of methyl parathion with hydrogen sulfide without NOM. For pH ) 7.4, kNu is about 3.0 h-1 (M[H2S]T)-1 and kNu is around 3.5 h-1 (M[H2S]T)-1 at pH ) 8.6. It can be concluded that the presence of NOM is enabling the nitro-group reduction; however, it is not altering the competing nucleophilic substitution. It can also be seen that the influence of pH on the degradation of methyl parathion with hydrogen sulfide is different for different NOM. Therefore, it can be concluded that for the aqueous hydrogen sulfide solution containing NOM, the nitro-group reduction can be the dominant mechanism. However, this reaction is pH dependent, and at pH higher than 8.6, the dominant mechanism is the nucleophilic substitution at the methoxy-carbon for all sources of NOM used except ESHA. With this study, it has been demonstrated that NOM is an effective mediator for the reduction of methyl parathion in homogeneous aqueous solution containing bisulfide as a bulk electron donor. Since NOM is likely to play an important role in abiotic reduction of methyl parathion in natural systems, the results of this study are significant from an environmental point of view. As has been illustrated, the reactivity of NOM may depend in a rather complex way on pH. The type of NOM and the pH of the solution play an important role for the relative importance of the observed reduction reaction in relation to the overall degradation of methyl parathion. Consequently, considering the large diversity of NOMs and the rather complex pH dependence of the reactivities of the NOM moieties involved in the electron-transfer reactions, it may be concluded that prediction of rates of NOM-mediated reduction of methyl parathion from a natural system is a very difficult task at this point. Further work is required to elucidate in detail which kind of NOM moieties are involved in the reduction of methyl parathion.

Acknowledgments The authors thank the reviewer for many thoughtful comments. This material is based upon work supported by the National Science Foundation under Grand No. 0134759, the Herman Frasch Foundation, and the Petroleum Research Fund. The authors thank Tong Wu for her help quantifying desmethyl methyl parathion.

Supporting Information Available Composition of the solution containing natural organic matter, degradation of methyl parathion in 1.0 mM aqueous thiophenol solution or polysulfide solution [S(II)T ) 1.44 mM] containing NOM, degradation of methyl parathion in 5.0 mM aqueous hydrogen sulfide solution with NOM or without, and the GC/MS spectrum of methyl parathion degradation in the aqueous 5.0 mM hydrogen sulfide containing NOM. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review July 25, 2005. Revised manuscript received November 11, 2005. Accepted November 18, 2005. ES051453C