| Hunan province is a famous "town of non-ferrous metals", and the long-term non-ferrous metal mining has produced a large number of non-ferrous mineral tailings. Tailings were usually disposed by damming at the valley. Tailings contain a large number of heavy metal pollutants, which have significant impacts on the regional ecological environment because of lack of tailings seepage treatment. In this paper, this environmental problem was systematically studied. A comprehensive survey of heavy metal contamination and evaluation of10large-scale mine tailings reservoirs in Hunan province was conducted, then the mechanism of chemical extraction techniques, small-scale soil column experiments and chemical modification techniques were carried out. The results were expected to offer technical and theoretical references for the development of tailings treatment and management. The results show that:The heavy metal contents of the10non-ferrous metal mine tailings surveyed in this study all exceeded the standard. The main heavy metal pollutants were lead, zinc, copper and cadmium. The contents of Zn in samples from Tongshanling, Lengshuijiang and Huangshaping were particularly high (10475.63mg/kg,151500.00mg/kg and12922.50mg/kg, respectively). In Yongzhou, Baoshan, Lengshuijiang and Huangshaping, the Pb contents were particularly high (5846.42mg/kg,4742.30mg/kg,6470.00mg/kg and6850.17mg/kg, respectively). For the speciation analysis of heavy metal, there were a variety of main different forms of heavy metals in the tailings:Pb was mainly in the form of Fe/Mn oxide bound and residual state, and Pb content existed in the form of exchangeable and carbonate bound was very few. However, the Pb content of exchangeable bound in Yongzhou samples was as high as69%. Cd mainly existed in residual state, and there was no obviously regularity of the other forms. Most of Cu was in the sulfide/organic bound, followed by the residual state. Zn was in the main existing forms of sulfide and organic matter bound and residual state. According to the analysis of PI values and DI values of soil samples, the mining sites which needed treatment and remediation as soon as possible were:Yongzhou, Baoshan, Xiangtan manganese and Lengshuijiang.The mechanism of chemical extractant on the tailings is very complex, involving a series of complex physical and chemical processes such as precipitation/dissolution, complexation/dissociation, adsorption/readsorption, etc. All the physical and chemical properties of extractant species, dosage of extractant, pH, extraction time, the mineral tailings type, types of heavy metals and the fraction of heavy metal speciation are influence factors of chemical extractant. EDTA was a high-quality chemical extractant in our study, which showed a good adaptability and high efficiency on different types of tailings. Of the four kinds of extractants in this study, the best extractant for Yongzhou lead-zinc mine tailings was EDTA; the best extractant for Xiangtan manganese tailings required further verification except that adipic acid extractant was not suitable; the best extraction agent needs further proof for Chejiang copper tailings except that the extractant citric acid was not suitable. Selecting a suitable extractant was a prerequisite for the success of chemical extraction technology. Generally, EDTA was a good choice. However, if the heavy metal pollutant was mainly in oxidable form, oxidative extractant was suitable, such as nitric acid, potassium permanganate, etc. If the heavy metal pollutant was mainly in reducible form, reductive extractant was suitable, such as oxalic acid. If the heavy metal pollutant was mainly in acid extractable form, EDTA or inorganic acid (such as hydrochloric acid, nitric acid and sulfuric acid and so on) was suitable extractant. Rhamnolipid can enhance the efficiency of extraction, but its application should not be considered because it would add the cost of repair. Humus can influence the repair efficiency, and the impact is likely to be both positive and negative. In this experiment adding humus in the chemical extraction of tailings should not to be considered in terms of technology. The optimal conditions for Yongzhou lead-zinc tailings chemical extraction were:EDTA concentration of0.05mol/L, pH5, solid-liquid ratio (m/v)1:20, and extraction time was more than12h. Citric acid showed better extraction efficiency for the manganese, lead and cadmium in Xiangtan manganese tailings remediation. Compared with hydrochloric acid and EDTA, it was the best extraction agent, but for the removal of lead, EDTA was better.In soil column experiment, three extractants were ordered by the performance of the extraction efficency as EDTA>HNC>3>CaCl2. EDTA was a good extractant for the chemical extraction of Pb, Zn, Cu and Cd. The extraction efficiency of EDTA for four kinds of metal was in the order:Cd> Zn> Cu> Pb. During extraction process, Pb, Cu and Cd polluants were easy to be readsorbed by soil particles, so that the effect of extraction efficency also reduced significantly with the increase of soil depth, especially Cu. There was no correlation between the effect of Zn extraction efficency and the soil depth. The extraction efficiency was greatly affected by the speciation of heavy metals, but not heavy metal with form of strong activity can be completely extracted by extractant, and the form of inert heavy metal can not be extracted. The results indicated the four kinds of metal speciation can be extracted by extractants, and the form of acid soluable increased after extracted by HNO3and CaCl2. After extraction, with the increase in the depth of the soil column, the concentrations of metal in all forms were increased, particularly in the acid soluable. In soil column experiment, the heavy metal content of arbitrary soil cross-section was a three-dimensional variable with time and soil column depth, and its content was increased gradually to the maximum then dropped in the process of remediation. The most appropriate time of soil column experiments was about12hours.Humic acid can be used in improvement of heavy metal contaminated soils, it was reported that there were different effects on different heavy metals by adding humic acid in soil. The speciation of Pb and Cd in soil was strongly affected by humic acid, but the speciation of Cu and Zn can hardly be affected by humic acid. Accordingly, the soil polluted by Pb and Cd can be remedied by humic acid to decrease its bioavailability. |
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