Biochemical Mechanisms And Kinetics Of Reductivetransformationof DDT In Soils

DDT(1,1-trichloro-2,2-bis(4-chlorophenyl)ethane, DDT) is a persistent organic pollutant existing widely in the soil of coastal wetland in Southern China and that contaminated by pesticides. How to eliminate the DDT in soil has become an environmental problem to be solved urgently. Dechlorination is the key step to transform DDT in soil. Iron reduction driven by micro-organisms plays a role in the process of DDT’s dechlorination.By the contaminated soil with high concentration of DDT, this study builds a system of Dechlorinating bacteria-soil-AQDS and ZVI-soil-AQDS. Combine with the system of DDT added soil in coastal wetland, we take the anaerobic culturing. By GC-MS analysis, we study the way of DDT’s dechlorinating and the kinetics of reduction and transformation in the 3 systems. By T-RFLP and clone library, we study the micro-organisms related to DDT’s dechlorination to analyze the relationship between DDT’s dechlorination and the physical and chemical properties, microbial community structures, functional and microbial abundance and other surrounding factors in the soil. Then we explain the dechlorination is affected by DDT’s environment behavior and the carbon source, AQDS, ZVI, the characteristics of anoxia, slightly alkaline and high salinity in coastal wetland to find those micro-organisms who answer DDT’s dechlorination. We deeply discuss the mechanism of iron reduction coupled with DDT reduction dechlorination in the soil of coastal wetland with the characteristics of anoxia, slightly alkaline and high salinity. The main contents and conclusions are followed:(1)The reductive transformation and its biochemical mechanism of DDT in the “Dechlorinating bacteria-soil-AQDS”system. DDT in non-bacteria system was hardly reduced. DDT without non-bacteria system was only reduced by 18.4% due to the micro-organisms after 42 days. DDT added carbon source of sodium lactate was reduced by 28.4%, whose rate is nearly 100% higher than before. The rate was raised by humus model-AQDS, and reached to 45.3% after 42 days.DDT became DDD first by dechlorination, and then became DDMU by reducing HCl. DDMU finally became DBP under anaerobic condition by sophisticated redox reaction. Adding carbon source and electron shuttle--AQDS can improve the abundance of dechlorine bacteria in the soil, that are Desulfitobacterium, Pyrinomonas and Herbaspirillum. Carbon source of sodium lactate can also strengthen the growth and enrichment of dechlorine bacteria and other microorganisms, to promote DDT’s reduction and transformation.(2)The reductive transformation and dechlorination of DDT in ZVI-soil-AQDS system. DDT in the system added ZVI was reduced obviously by 70.1% in 7 days. When carbon source of sodium lactate and AQDS was added respectively, DDT was reduced by 74.1% and 75.8% after 7 days. In the ZVI-sodium lactate-AQDS system, DDT was reduced by 78.4% after 7 days. The change of DDT and its first reduction product--DDD is: ZVI+AQDS+ C3H5 Na O3>ZVI+ C3H5 Na O3>ZVI+AQDS>ZVI. C3H5 Na O3 can promote the enrichment of dechlorine bacteria, iron-reductive bacteria and other microorganisms. Herbaspirillum and Desulfitobacterium is the main speciesin thereduction and transformation.(3) The reductive transformation system of DDT in the soil of coastal wetland. DDT’s concentration in non-bacteria soil hardly changed. DDT added artificially without non-bacteria soil were reduced by 93.8% after 56 days. DDT added carbon source lactate was reduced by 95.3% after 56 days. DDD, DDE, DDMU and DBP produced by the reduction were detected. Adding carbon source of sodium lactate into the soil can promote the generation of adsorbed Fe(II). It can also improve the abundance of dechlorination bacteria Dehalococcoidaceae and Rhodobacteraceae, so as to promote the conversion of DDT and its middle products.