Investigation And Bioremediation Of Agricultural Soil Contaminated With PCBs In E-waste Recycling Area

The crude and unregulated recycling activities of electronic and electric waste have resulted in severe and complex contamination of the surrounding environment by toxic chemicals such as heavy metals as well as persistent organic compounds. Bioremediation is an emerging effective and environmentally friendly mean of remediation for the organic compounds polluted soil. However, most of current bioremediation research works in China were conducted in laboratory. There are few bioremediation cases for the real contaminated soil. Taking a typical e-waste recycling area in Southeast China as an example, we had finished a systematic investigation of heavy metals and organic pollution in agricultural soil. Furthermore, some laboratory studies were carried out to investigate the feasibility of different bioremediation technologies for polychlorinated biphenyls (PCBs). A field study in a PCBs contaminated farmland (4800m2) was also conducted. The main results were summarized as follows.A systematic investigation of heavy metals and organic pollution in agricultural soil was conducted. The results indicated that the e-waste recycling activity had caused critical soil pollution for Town A and B in e-waste recycling area. Cd, Cu, PAHs and PCBs were the main contaminants. The concentration of PCBs was27.1-415.1μg/kg and13.3-242.2μg/kg in soil of Town A and B, respectively. Comparison among the different sampling areas indicated that the soil was highly contaminated in the agricultural area near e-waste recycling workshops. There was significant correlation among Cu, Pb, PAHs, and PCBs, indicating that these contaminants might have a common source.Soil investigation and risk assessment was conducted in typical farmlands in e-waste rccyling area. Furthermore, the remediation target concentration was discussed and the spatial distribution estimation method was established. Six typical polluted farmlands were selected for further heavy metals and organic pollution investigation. Through the health risk assessment, the typical PCBs polluted farmland (4800m2) was observed that had a high level of health risk and was suggested to be remediated immediately. The remediation target concentration was suggested to be90μg/kg for agricultural soil. The spatial distribution estimation of soil pollution data was conducted by the back-propagation neural network analysis. The estimated result was more reasonable than the result estimated by the traditional Kriging analysis.After the investigation and analysis of the field samples in the e-waste recycling area, we found that the residues of PCBs in paddy soil were significantly lower than those from dry land. Then the natural attenuation of PCB congener31or congener mixtures was studied in aged contaminated soil or spiked soil with alternative wet-dry, flooded or dry condition, respectively. It was found that PCBs degradation was more enhanced in the alternative wet-dry treatments as compared to other two treatments. Highly chlorinated PCBs could also be degraded in the alternative wet-dry conditions. The results suggested that the typical anaerobic-aerobic environment in the farmland could enhance the natural attenuation of PCBs.Several composting studies were investigated for the higher concentration of PCBs. With the addition of auxiliary components such as soybean phosphatide, composting without temperature control could remove approximately20percent of PCBs from the real contaminated soil (2.9-3.1mg/kg) within17-24days. After adding earthworms or microorganisms, the removal efficiency increased to about57.1percent after97days of composting for the spiked PCBs soil (224.4-310.8mg/kg). The removal efficiency was increased to nearly70percent when taking some intensive measures such as forced draft ventilation, the addition of soybean phosphatide, earthworms and microorganisms. However, the removal of highly chlorinated PCBs was limited. The concentration of PCBs in the soil was not significantly affected during the anaerobic composting process while the percentage of highly chlorinated PCBs decreased and lower chlorinated congeners increased. On the basis of laboratory-based composting experiments, an anaerobic-aerobic composting technique was presented and a DANO drum composting equipment was designed. Finally, a field composting study was conducted. Results showed that PCBs was removed by16.1percent during the90days of composting treatment, the initial concentration was187μg/kg.For the lower concentration of PCBs, a phytoremediation study was conducted both in greenhouse and field. Greenhouse pot experiment results showed that the combination phytoremedaiton using ryegrass and β-cyclodextrin was an effective means for the PCBs-contaminated agricultural soils. After120days of plant growth, the highest PCB removal percentage (about38%) was observed in the ryegrass planted soil, the initial concentration of which was4.52mg/kg. Addition ofβ-cyclodextrin increased the PCBs bioavailability as well as the biological activities, which might lead to the enhanced degradation of PCBs. Plant uptake contributed about0.08percent of PCBs loss, suggesting that plant uptake was not the main pathway for the PCBs removal in the soil. Furthermore, the phytoremediation efficiency of PCBs was studied in field for110days. Results showed that PCBs could be removed by7.9percent.