Enhanced Reductive Transformation Of Chloroacetanilide Herbicides In Iron-rich Soils Treated By Dithionite

Chloroacetanilide pesticides are mostly used herbicides which exposing degradation-resistent property in soils. Due to the widely and largely use for a long time, chloroacetanilide pesticides have been found resulting contaminations in soils, groundwater, and even surface runoff, which not only leading to unexpected growth of plant, and also resulting in poisoneous of animals and even human through the food chain. The most important detoxification step of residue chloroacetaminlide herbicides in soils has been reported as the dechlorinaiton, which has became the hot topic and difficulties of research in detoxification processes of the residue pesticides. The previous reported studies showed that the active soil Fe(II) species are important for promoting the dechlorination processes of chlorinated organic contaminants. And also, the soil sulfur speicescan act as the nucleophiles for substituting chlorines in the molecular of chlorinated compounds through nucleophilic reaction process.In order to accelerate the detoxification process of residue chloroacetanilide pesticides in iron-rich soils that widely distributed in South China, in this study, with metolachlor as the target chloroacetanilide pesticides, we conducted a systematically study on changing the physicochemical properties of soils with environment-friendly chemicals, focusing on enhancing the soil reductive potentials with reductant to accerelate the reductive dechlorination processes of chloroacetanilide pesticides.In the study, sodium dithionite, one environmenta-friendly and price-competitively sulfur species was used to treat the iron-rich soils under anaerobic conditions to enhance the reductive potentials of the soils, and the soil physicochemical properties before and after being treated with dithionite were analyzed with electrochemical methods and surface characterization analysis. The redox potentials of soils before and after being treated with dithionite were studied with Cyclic Voltammetry (CV). The results show that the different soils treated with the same concentrations of dithionite obtained different reductive potentials, and the enhancements of reductive potentials of different soils were related with the contents of soils total iron, free iron, complexed iron and also the amorphous iron. The same soil achieved different reductive potentials when treated with different concentrations of dithionite, in which the soil reductive potentials were positively correlated with the concentrations of treating dithionite in a certain concentration range. However, the reductive potentials of soil began to decrease when the treating dithionite increased to an excess high value. The differences of microstructure and phase constitution of the soils were detected by scanning electron microscope-Energy Dispersive X ray Spectrom(SEM-EDX), and discovered aggregation and increase of low-energy state Fe in three-quarter soils afer treated by dithinite. The changes of soil surface bonds were analyzed with Fourier transform infrared spectroscopy (FTIR), and the results show that the signal referring S-S bonds on the surface were strengthen, which indicated that sulfur species, acting as the strong nucleophililes from dithionite, concentrated on the soil particles.The transformation processes of metolachlor were investigated in the dithionite treated soils under anaerobic conditions. The results show that the transformation rates of metolachlor were different in different soils after being treated with the same concentration of dithionite and metolachlor achieved higher transformation rates in soils with higher reductive potentials. In a certain concentration range of dithionite being used to treat the same soil, metolachlor achieved the higher transformation rates when treated with higher dithionite concentrations. However, when with excess higher concentration of dithionite to treat the soil, the transformation rate of dithionite would decrease, which was also consistent with the CV results for the reductive potentials. The pH values of the reaction system significantly affected the transformation of metolachlor, in which the higher transformation rates were achieved in higher pH values. And also, the effects of coexist metal ions on metolachlor transformation in the dithionite treated soil systems were studied, and the results show that the metals of Zn2+and Cu2+both inhibited the trans formation processes.Summary, the soils treated with dithionite would significantly accelerate the transformation processes of residue chloroacetanilide pesticides, and the accelerating mechanism in dithionite treated iron-rich soils were disclosed in the present study. It is expected that the results obtained in this resuts would provide help for understanding the soil geochemical processes of herbicides in iron-rich soils. And also the transformation mechinams disclosed in this study would provide theory suppor for inventing the new soil detoxification technology for contamination of chlorinated pesticides in iron-rich soils.