| In order to improve the crop yield, organophosphorus pesticides (OPPs) as effective insecticide to inhibit the function of acetylcholine esterase (AChE) in nervous system of insects have been widely used in the process of agricultural production. Although possessing the weak ecological risk for aquatics and humans due to the shorter half-life in the environment, the OPPs existed ubiquitously in environmental matrix derived from the extensive usage in agricultural acitivities when the OPPs had gradually substituted for the higher toxicity of organochlorine pesticide. Currently, various residues of OPPs had been detected in groundwater system. Long-term exposure of living organisms to OPPs, particularly for the developing children, has been concluded to modulate reproductive, carcinogenic and immunosuppressive effects.In this study, the distribution characteristics and possible source of OPPs in water and soil environment has been initially investigated in Shahu field, Jianghan Plain. Subsequently, the research of oxidation and reduction transformation mechanisms of the selected OPPs in sensitive mediums exsited in sediments and its pore waters, including the manganese dioxide mineral, the reducing sulfur species and the organic matters, had been conducted to elucidate the abiotic attenuation in natural environment, which offer useful information related to the rational use of these pollutants in farmland system.1. The distribution and source of OPPs in study areaThe analysis results showed that the residues of selected monitoring of organophosphorus pesticides were ubiquitous in most samples. Of which the omethoate and dizinone were the main ingredients. And the total contents of OPPs in all samples were not above the standard value of the Groundwater Environment Quality Standards.Moreover, the concentrations of OPPs in most monitoring wells decreased with the increasing depth, which indicated that the migration risk to groundwater system derived from the residual OPPs in surface water exsited in our study area. Additionally, the difference of the concentration of OPPs in soil samples was significant. The main ingredients of the detected OPPs in soil samples were dichlorvos, methyl parathion, dimethoate and quinophos, respectively. Due to the similar characteristics of the content of OPPs in depth between groundwater and profile soil samples, the OPPs were also apt to migrate to deep soil in our study area.2. The oxidation behavior of OPPs by manganese dioxide (MnO2)Based on the batch experiments in laboratory, the degradation behavior and transformation pathway of methyl parathion and dimethoate by a-MnO2was studied profoundly. The results demonstrated that less than 6% of methyl parathion was adsorbed to oxide surface during the entire degradation process. The removal efficiency of methyl parathion and dimethoate depended strongly on the loading ofa-MnO2 and pH.The coexisting metal ion (Ca2+, Mg2+ and Mn2+) in reactionsinhibited the hydrolysis and oxidation reaction.HA-Naexhibited a significant promotion effect on the hydrolysis of methyl parathion by a-MnO2 while presented an inhibitive effect on the oxidation rate of methyl parathion by a-MnO2. The hydrolysis of methyl parathion by a-MnO2 was significantly higher than its oxidation rate by a-MnO2.According to the degradation products identified by GC-MS and LC/HRMS,nucleophilic substitutions at the P atom (SN2@P) and the carbon atom (SN2@C) were proven to be the main mechanism in the hydrolysis of methyl parathion by a-MnO2. The oxidation behaviors of methyl parathion and 4-nitrophenol by a-MnO2involving the radical coupling reaction were also proposed. Meanwhile, the further transformation pathway of 4-nitrophenol on oxide surface, the main hydrolysis products of methyl parathion by a-MnO2, was also depicted in our study involving the mineralization, hydroxylation and radical coupling processes.3. The reduction behavior of OPPs in hydrogen sulfide and dissolved organic matterThe kinetics of the transformation of methyl parathion have been investigated in aqueous solution containingreduced sulfur species and dissolved organic matter, such as Jianghan Plain sediment humic acids and commercial anthraquinone. It was shown that the degradation behavior of methyl parathion was mediated significantly by sediment humic acids and anthraquinone in aqueous solution containing hydrogen sulfide. The degradation rate of methyl parathion increased gradually with increasing amount of humic acids in reaction solution. Similarity, its degradation rate depended strongly on the concentration of anthraquinone and hydrogen sulfide. Moreover, the solution pH influenced remarkably the rate of degradation of methyl parathion. The results indicated that the degradation rate with anthraquinone has a maximum value at pH around neutral.Additionally, under the condition of the hydrogen sulfide and sediment humic acid, the concentration of element sulfur and thiosulfate increased gradually in aqueous solution. The generated element sulfur furtherly transformed to polysulfide as nucleophilic reagent could enhance the removal efficiency of methyl parathion.It had no substantial effect on the degradation rate of methyl parathion by the two other transformation species of hydrogen sulfide in reaction solution, sulfite (SO32-) and thiosulfate (S2O32-),respectively. Moreover, the organic sulfur compounds with thiol functional group (such as cysteine) as an electron shuttle could increase the degradation rate of methyl parathion.The degradation behavior of the main degradation products of methyl parathion in environment matrix,4-nitrophenol and methyl parathion, had remarkable difference in hydrogen sulfide containing humic acids, which was manily ascribed to the distinct structure of the above compounds. Similarily, the structural differences of other organophosphorus pesticides including dichlorovos and dimethoate resulted in the different degradation rate in the hydrogen sulfide solution.Based on the degradation products identified by GC-MS and LC/HRMS,the reduction degradation mechanism of methyl parathion was proposed in hydrogen sulfide containing dissolved organic matter.On the one hand, nucleophilic substitutions at the P atom (SN2@P) and the carbon atom (SN2@C)were proven to be the hydrolysis mechanism in the degradation of methyl parathion. Moreover, the special compounds in the dissolved organic matter, such as the quinone moieties, as an electron shuttle enhanced the rate of reduction for the nitro functional groups of methyl parathion.4. The reduction behavior of OPPs in hydrogen sulfide and paticular organic matterThe degradation behaviors of methyl parathion have been investigated in aqueous solution containing reduced sulfur species and particular organic matter, including graphite carbon and biochar. It was shown that the methyl parathion sorbed on the particular organic matter could degrade rapidly in the aqueous hydrogen sulfide. Both the concentration of particular organic matter and hydrogen sulfide determined the degradation rate of methyl parathion. With the increase of the amount of particular organic matter and hydrogen sulfide, the degradarion rate of methyl pararathion increased correspondingly. The solution pH influenced remarkably the rate of degradation of methyl parathion, which the maximum value of degradation rate of methyl parathion was around neutral. The presence of the sediment humic acid was found to inhibit the degradation activity, which the inhibitory effect was more obvious with the increase of the amount of humic acid in the reaction solution. Similarity, the sulfite (SO32-) and thiosulfate (S2O32-) did not enhance the degradation rate, but the degradation rate of methyl parathion increased by the organic sulfur compound (cysteine). Moreover, there was a significance difference for the degradation behavior of methyl parathion on biochar with different pyrolysis temperature. The degradation mechanism of methyl parathion sorbed on the particular organic matter was also proposed in aqueous hydrogen sulfide solution according to the identified degradation products. |
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