| Groundwater pollution caused by mine tailings has been one of the world’s major problems of environmental pollution. Today in the field of contaminated sites remediation, the treatment and remediation of groundwater pollution in mining areas are still a hot and tough problem. PRB(Pemeable Reactive Barrier) is a latest and efficient in situ remediation technology of groundwater contamination, and has been widely used domestically and internationally. In this paper, we took the groundwater polluted by a lead-zinc tailings in Hunan for the example to study the PRB technology systematically. The main contents included the preliminary site reconnaissance, the investigation and analysis of pollution status, the assessment of health risk, the screening and mixing of PRB materials, the affecting factors and mechanism of heavy metal removing, the PRB column simulation experiments, the design of pilot PRB structure and parameters, and the implementation of practical PRB,etc. The main conclusions are summarized as follows:(1) The pollution investigation and risk assessment of groundwater and soil were conducted in four sites:a the arsenic factory in Wenshan, a coal gangue landfill in Shuangfeng, a tungsten tailing in TanAo ridge,Chenzhou, and a lead-zinc mine in Yueyang. The results showed that the four sites were polluted in different degrees. As contents in soil, groundwater, surface water and plants in arsenic factory surpassed the national limiting standards; In tungsten ore field (Huang hole, Tan Ao ridge, Chen zhou), the arsenic contents of groundwater were more than the national standard, while other heavy metals content did not, but the flow of wastewater was strongly basic; The Cd, As, Zn content in groundwater and soil were all over the national standards in gangue landfill which located in Shuangfeng; In the lead-zinc mine of Yueyang, the Pb, Zn, Cd, As content in soil, surface water and groundwater exceeded the related national standards in different degrees, the risk assessment results of this site showed that there were serious cancer risk and the area was not suitable for farming and a source of drinking water.(2) In view of the kinds and concentrations of heavy metal pollution in soil and groundwater, the batch experiments were conducted to study the ability of different materials to remove heavy metals, in order to find out the best PRB materials. The experimental results showed that there were six kinds of materials could be used to remove Pb at 90% removal rate, and they were iron manganese (existing), fly ash (Shijiazhuang), steel slag, the chemical activated carbon, sponge iron, and iron needles. The materials which removed Zn at rate over 90% were iron manganese (existing), fly ash (Shijiazhuang), steel slag, coconut shell activated carbon, the second reduced iron, chemical iron powder, bentonite, and red mud. The materials which removed As at 90% remove rate were steel slag, iron needles, chemical iron powder, and iron manganese (HSR). According to the results of screening experiments for Cd-removal materials, there were 13 kinds of materials had the better effect. All of the results showed that iron manganese and steel slag were the best materials to treat Cd, Pb, Zn, and As compound pollution in the groundwater.(3) The conditions and influence factors of different kinds of heavy metals removal by steel slag were studied experimentally. The results showed that the optimum pH of the adsorption of zinc, arsenic, lead and cadmium by steel slag was 7.0. The temperature did not affected the adsorption of zinc, but not the adsorption of lead, cadmium and arsenic. The optimum time of cadmium adsorption by steel slag was 16h. The optimum time of arsenic and zinc adsorption by steel slag was 12h.(4) The adsorption and desorption isotherms of several kinds of heavy metals by steel slag were studied. The results showed that Langmuir and Freundlich equation could well fit the lead, zinc, cadmium and arsenic adsorption behavior of steel slag. The maximum adsorption amount of lead, zinc, cadmium and arsenic calculated from Langmuir isothermal adsorption equation were 894.04 mg/g,454.07 mg/g,2424.90 mg/g and 63.23 mg/g respectively. The desorption rate of lead, zinc, cadmium by three adsorbents were less than 1%, indicating that the desorption rate of the three heavy metals were very low, and the material was not suitable for reuse. And the desorption rate of arsenic by NaOH was best, with the desorption rate of 35%, but the effect of other adsorbents was not good.(5) The adsorption kinetics of several heavy metals by steel slag was studied. The results suggested that the pseudo-second-order rate equation could well fit the lead, zinc, cadmium and arsenic adsorption behavior of steel slag, and their correlation coefficients were all more than 0.97. It also indicated that the absorption rates of several heavy metals removal by steel slag were very high, and that steel slag was suitable for heavy metals removal rapidly from groundwater.(6) The influences of accompanying ions were investigated in lead or zinc or cadmium or arsenic removing from water by steel slag. The results showed that the accompanying ions of Ca2+, NO3-, SO42-, SiO32-, H2PO4- had no effects on the removing percentages of lead and cadmium of steel slag. The accompanying ions of Ca2+ almost had no effect on the percentages of zinc removal. The accompanying ions of SiO32-, H2PO4- could restrain significantly the zinc removal. With the increase of concentration of Ca2+, the percentages of arsenic (Ⅲ) removal was increased. The accompanying ions of NO3-, H2PO4- increased arsenic (Ⅲ) removal. With the concentration of SiO32- increasing, the percentage of arsenic (Ⅲ) removal was changed from great to low. When the concentration of SO42- was high, the ability of arsenic (Ⅲ) removal by steel slag was reduced.(7) The X-ray diffraction patterns were analyzed before and after several heavy metals removing from water by steel slag. The study showed that many new diffraction peaks were formed on steel slag after adsorption, and several new crystal compounds were identified. They were Zn5 (OH) 8Cl2·H2O, CdCl2·H2O, Cd (OH) Cl, Fe2As4O12, Pb2SiO3·xH2O. At the same time, the results revealed that the adsorption processes belonged to the surface adsorption, and redox, complexation reactions or redox reactions were happened on the adsorption process.(8) The dynamic adsorption of heavy metals removal by materials was studied in laboratory simulation penetration test for PRB to provide the parameters of designing PRB. When the concentration of Pb was 1.0 mg/L, zinc was 40 mg/L, the Cd was 1.0 mg/L, pH 6.0 approximately, and the simulation flow was 0.169 ml/min, the sewage treatment was 228L by 1 kg 20-40 mesh slag (excluding activated carbon). And after running a period of time, the liquid became neutral. |
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