Remediation Of Polychlorinated Biphenyls Contaminated Soil By Nanoscale Zerovalent Iron Or NZVI-microorganism Integrated Treatment

Polychlorinated biphenyls (PCBs) are typical representatives of chlorinated organic pollutants. Soil and sediment pollution by PCBs has attracted great globle concern. Highly chlorinated biphenyls must be decblorinated to lower-substituted counterparts or biphenyl which can be father degraded by microorganism thoroughly. Nanoscale zerovalent iron (nZVI) has been shown to successfully promote the dechlorination of PCBs. In the present study, nZVI and Pd/Fe bimetallic nanoparticles were used to remove2,2’,4,4’,5,5’-hexachlorobiphenyl (PCB153) from deionized water to explore hydrodechlorination efficiencies and pathways, and then applied to PCBs contaminated soil collected from an e-waste recycling area. Removal efficiencies and the effect of soil properties on dechlorination were discussed. Finally, degradation of PCBs in contaminated soil by a sequential treatment using nZVI and an aerobic bacterium was investigated. The main results of this research are as follows:Characterization results indicated the success of nanoparticle preparation. The initial concentration of PCB153in deionized water was2mg·L-1, and the removal efficiencies by nZVI and Pd/Fe bimetallic nanoparticles were74.9%and93.8%after20h, respectively. The hydrodechlorination process followed pseudo-first-order kinetics and the obtained rate constants were0.110h-1and0.120h-1. When PCB153was absorbed on the surfaces of nZVI or Pd/Fe nanoparticles, it was reduced to its lower-substituted counterparts, which were subsequently hydrodechlorinated to biphenyl. The diversity of hydrodechlorination pathways was observed in the presence of Pd/Fe nanoparticles.nZVI and Pd/Fe nanoparticles were used to remove PCBs in contaminated soil collected from the e-waste recycling area. The PCBs removal efficiencies in sandy loam soil (initial concentration12.90mg·kg-1) were67.4%and81.5%after12d, and those were48.3%and53.4%in silty clay soil (initial concentration6.57mg·kg-1). The residual concentrations of trichlorobiphenyl, tetrachlorobiphenyl, pentachlorobiphenyl, hexachlorobiphenyl, and heptachlorobiphenyl in soil treated with Pd/Fe nanoparticles were lower than those with nZVI. It is concluded that the removal efficiencies and reaction rates were increased by adding Pd as a coating on zerovalent iron. A comparison of PCBs dechlorination in two soil samples showed that the PCBs removal efficiencies from sandy loam soil were higher than those from silty clay soil, which might be affected by the soil properties, such as organic matter and texture. Furthermore, the catalytic activity of Pd was also affected by the soil properties.Compared with nZVI or microorganism single treatment, higher PCBs removal efficiencies from contaminated soil were achieved with nZVI-microorganism integrated treatment. The initial concentration of PCBs in soil was3.60mg·kg-1. PCBs removal efficiency was59.4%with a sequential treatment using nZVI for8d and microorganism for24d. Nevertheless, the removal efficiencies were46.1%and34.4%by nZVI and microorganism, respectively. The residual concentrations of PCBs homologs in soil were lowest with nZVI-microorganism treatment. Highly chlorinated biphenyls could be hydrodechlorinated to lower-substituted counterparts and biphenylby nZVI, which were easily degraded by aerobic bacterium.These results suggest that nZVI and Pd/Fe bimetallic nanoparticles were effective to dechlorinate PCBs in contaminated soil. Furthermore, hydrodechlorination by nZVI and biodegradation by an aerobic bacterium could be combined to achieve higher removal efficiency of PCBs.