Processes And Mechanisms Of Agronomic Factors For Enhancing Phytoremediation Of Cd-Zn/DDTs Co-contaminated Soils

The use intensity of farming soil was high and it was strongly affected by human activities in our country. The soils are faced with exogenous pollutants brought by industrial and mining enterprises, traffic, urbanization process and public health defense, and also highly intensive agriculture pollution from itself. These cause pollutants from multiple sources into the soil environment for human survival. Soil-crop system was showing the complex situation as coexistence of old and new pollutants, organic and inorganic co-compounbined pollution. Especially soil pollution caused by heavy metals and chlorinated persistent organic pollutants were widespread concerned. In recent years, the constantly outbreak of environmental pollution accidents between the human body and soil-water system put forward urgent demand for contaminated soil remediation and restoration.In this research, the screening of new energy plant varieties and agronomic measures regulation to enhance phytoremediation efficiency were carried out under the mild farmland contaminated soil with heavy metal co-contamination and heavy metal-organochlorine pesticide co-combined pollution. Through the process of phytoremediation strengthening technology and mechanism investigation, primary results were summarized as follows:1. Improving the efficacy of phytoextraction is critical for its successful application in metal contaminated soils. Mineral nutrition affects plant growth and metal absorption and subsequently the accumulation of heavy metal through hyper-accumulator plants. This study assessed the effects of di-hydrogen phosphates (KH2PO4, Ca(H2PO4)2, NaH2P04and NH4H2PO4) application at three levels (22,88and352mg P kg-1soil) on Sedum alfredii growth and metal uptake by three consecutive harvests on aged and Zn-Cd combined contaminated paddy soil. The shoot P and Zn concentrations are enhanced significantly (P<0.05), while the Cd concentration is decreased with the increase of phosphate levels.The addition of phosphates (P) significantly increased the amount of Zn taken up by S. alfredii due to increased shoot Zn concentration and dry matter yield (DMY)(P<0.05). The phosphate application reduces N, the secondary nutrients (Ca, Mg and S) and micronutrients (Mn, Fe and Cu) concentrations, while the K concentration has no significant effect. The total carbon concentration of shoots is increased at the high P level. The amount of Zn removed by phytoextraction increased in the order of Clipping1<2nd clipping<3rd clipping, and for Cd extraction the order was2nd clipping<1st clipping<3rd clipping. The highest phytoextraction of Zn and Cd was observed in KH2PO4and NH4H2PO4treatment at352mg P kg-1soil. These results indicate that application of P fertilizers coupled with multiple cuttings can enhance the removal of Zn and Cd from contaminated soils by S. alfredii, thus shortening the time needed for accomplishing remediation goals.2. Microcosm experiments were conducted in greenhouse to examine the effect of different phosphorus (P) sources on zinc (Zn) phytoextraction by Sedum alfredii in aged Zn contaminated paddy soil. The Zn accumulation, soil pH, microbial biomass and enzyme activity, available Zn changes and soil Zn phytoremediation efficiency after plant harvest were determined. Upon addition of P, Zn and Cd uptake of S, alfredii significantly increased. Mehlich-3extractable or the fractions of exchangeable and carbonate bound soil Zn were significantly increased at higher P applications. But Mehlich-3extractable or the fractions of exchangeable Cd significantly increased, and carbonate and Fe-Mn oxides bound Cd were significantly increased as P applications. Soil pH significantly decreased with increasing P application rates. Soil microbial biomass in the P-treated soils was significantly higher (P﹤0.05) than those in the control. Shoot Zn concentration was positively correlated with Mehlich-3extractable P (P﹤0.0001) or exchangeable/carbonate-bound Zn (P﹤0.001), but negatively related to soil pH (P﹤0.0001). These results indicate that application of P fertilizers has the potential to enhance Zn bioavaiability and uptake by hyperaccumulating plant S. alfredii, thus increasing phytoremediation efficiency of Zn contaminated soils.3. The objective of this study was to compare the ability of23genotypes of Ricinus communis in mobilizing and uptake of Cd and DDTs (p,p’-DDT, o,p’-DDT, p,p’-DDD and p,p’-DDE) in the co-contaminated soil. The pot experiments under field conditions and laboratory analysis was done. The plant genotypes varied largely in the uptake and accumulation of DDTs and Cd, with mean concentrations of0.37,0.43and70.51for DDTs, and1.22,2.27and37.63mg kg-1dw for Cd in leaf, stem and root, respectively. The total uptake of DDTs and Cd varied from83.1(B09032) to267.8(B09053) and66.0(ZJ3) to155.1(B09053)μg per pot, respectively. These results indicate that R. communis has great potential for removing DDTs and Cd from contaminated soils attributed to its fast growth, high biomass, strong absorption and accumulation for both DDTs and Cd.4. The pot experiment was done to evaluate the combined effecf of zero-valent iron (Fe0) and DDTs accumulating castor variety to remediate Cd and DDTs co-contaminated soil. No sgnificant (P<0.05) differences in root and shoot biomass were observed among the Fe0, nonionic surfactants Brij35and citric acid amendments. TheDTs content in castor shoot was highest under Brij35treatment with5.79mg kg-1, significantly higher than Fe0or citric acid, but no significant difference with CK; While shoot ΣDDTs content was significantly lower than CK. Fe0treatment dramaticly increase soil and plant root (6.7mg kg-1dw) p,p’-DDD content. Chemical amendments significantly decrease the shoot Cd content, an average reduction of28.6-64.6%, especially for Fe0and citric acid; they were significant lower than Brij35. The total DDTs and Cd uptake by castor were144.6-193.6μg pot-1, and52.78-116.28μg pot-1, the increase effect of the Brij35was the most obvious. Soil ΣDDTs have significantly dissipated after Fe0-castor planting; the reduction amount was1.85fold of CK, but no citric acid and Brij35treatments. Fe0abiotic dechlorination was accompanied by Ricinus communis uptake of the DDT and its intermediate metabolites such as DDE and DDD provides a promising alternative to the traditional techniques of elimination also from the economic point of view.5. Effects of soil additives, in particular of oil tea seed residue (T3), vermicompost (T4), biosurfactant tea saponin (T5) and reduction elemental sulfur (T6) on DDTs-Cd combined soil remediation using R. communis under field pot experiment were investigated. Meanwhile, we compared the difference of DDT degradation bacteria (Pseudomonas sp. DDT-1) inoculation and indigenous microorganisms on soil pollutant removal process or plant uptake. Exogenous organic waste adding (T3/T4) improve plant root and shoot growth, the increase of root was22.1%and15.5%, and shoot was16.5-17.4%, but the increment was no significant. Soil additives singnicatly decreased shoot Cd content, an average reduction of31.0-45.9%%. Root Cd content also has some increase except for T4, T5was a substantial increase (42.5%) in the plant root Cd uptake, but the difference was not significant. Inoculation bacteria rapidly increase the castor root Cd content, the increase was8.6folds than CK, the other treatments increased by1.2-7.3folds. Whether it is indigenous microorganisms or inoculation bacteria conditions, all exogenous soil amendments increase castor shoot DDTs content, the highest increase of48.0%. The soil microbial biomass carbon, alkaline phosphatase, catalase and dehydrogenase activity of T3treatment was significantly higher than other treatments. All soil additives significantly increased the degradation rate of soil DDTs, and the most obvious effect was T3treatments. These results indicate that oil tea seed residue containing the composition of tea saponin is suitable for use on enhancing DDTs-Cd co-contaminated remediation by R. communis.