Remediation Of Organochlorine Pesticides Contaminated Soils By Soil Washing

Site contamination associated with organochlorine pesticides (OCPs) has been a severe problem in China, which poses great threat to both ecosystem and public health. As the urbanization proceeds rapidly in China, most of the OCPs contaminated sites face a secondary development, making remediation of these sites extremely urgent. Soil washing as a cost-efficient remedial alternative has been extensively investigated for soils containing high contents of heavy metals or organic pollutants. Unfortunately, insufficient works have been carried out concerning soil washing of OCPs-contaminated soils, and moreover, limited efforts have been paid to the post-treatment of soil washing effluent and the recovery of solubilizing agents. In addition, few present investigations have emphasized the contribution and effects of soil organic matter (SOM) on the soil washing process.In the present study we chose hexachlorobenzene (HCB) as the representative OCP, and simulated or real HCB contaminated soils as the tested soils, to systematically investigate the soil washing remediation of OCPs contaminated soils. The main contents included (1) selecting solubilizing agents that are environmental friendly for HCB contaminated soils; (2) exploring technologies for effluent treatmtent, especially for solubilizing agents recovery/reuse; (3) elucidating the effects of SOM on soil washing of HCB from soils by surfactants; (4) investigating the combined effect of electrokinetic enhanced washing (EK)-zero valent iron permeable reactive barrier (ZVI PRB) to remediate HCB contaminated soils. In general, main observations and conclusions can be summed up as follows:(1) The combination of methyl-β-cyclodextrin (MCD) and ethanol exhibited significant synergy in HCB solubilization compared with MCD and ethanol alone. The synergy increased with the increase of either the volume fraction of ethanol or the mass concentration of MCD in mixed MCD/ethanol systems. The synergistic effect of HCB removal from soils by MCD/ethanol washing was heavily depended on the fraction of ethanol, with the optimal ethanol fraction of 30%. The synergistic effect of washing associated with MCD/ethanol system implies less reagent needed and thereby, a reliable reduction of reagent cost. Our results suggest that rhamnolipids biosurfactants also showed considerable solubilization and enhanced washing effects for HCB.(2) Microscale Pd/Fe bimetal particles showed an extremely high reactivity of reductive dechlorination to HCB, much higher than that did by microscale Cu/Fe. The presence of nonionic surfactant Triton X-100 (TX-100) promoted the degradation of HCB by microscale Pd/Fe. Nevertheless, no appreciable HCB reduction was recorded by the same Pd/Fe particles when rhamnolipid was selected as the solubilizing agent. Wood-based granular activated carbon (GAC) could selectively adsorb HCB from TX-100 solutions, therefore regenerating the solubilizing agent. Compared with GAC, PAC was more effective to remove HCB from rhamnolipid solution via selective adsorption. When 10 g/L PAC was applied,99% removal of HCB from rhamnolipid solution could be reached, and the corresponding loss of rhamnolipid was 17%. In addition, further research indicated that the combined rhamnolipid enhanced soil washing-PAC selective adsorption processes could be successfully applied to the successive remediation of HCB contaminated soils, which could benefit the reduction of reagent cost of surfactants.(4) The combination of EK enhanced soil washing with Cu/Fe or Pd/Fe PRB could greatly promote the removal of HCB from soils, with HCB removal of 59-63%, while conventional EK exhibited no apparent HCB removal. Complete degradation of HCB passing by was obtained by Pd/Fe PRB, while degradation efficiency of HCB by Cu/Fe PRB section was estimated as 82%. Generally, degradation of HCB via ZVI PRB reduction played a primary role in HCB removal from soils. It is suggested that EK enhanced washing coupled with ZVI PRB could be an cost-efficient remedial alternative for clayed soils contaminated with OCPs.