| Mining activities produce waste tailings which pose serious environmental impacts to aquatic and terrestrial ecosystems. In most mining activities, these waste tailings are left without proper management. The negative impact of these mining activities on the surroundings is mainly due to the presence of high volumes of tailings. These tailings are usually unfavorable to the surrounding environment, contributing to the contamination of soil substrates, destruction of soil texture, short of nutrient, destruction of ecological diversity. Heavy metals are considered highly hazardous to ecosystems and humans, not only because of their direct toxicity to organisms but also due to their potential for bioaccumulation along the food chain. This is a global concern. Studies of natural and man-made environmental interaction of metals after mining have become increasingly important to society.Xiushan County is located in southwest China and is rich in Mn ore. Rongxi Manganese mine is one of the biggest mines in Xiushan County. This region is economically underdeveloped where mining is the most important pillar industry. Mining activities in this region has deleterious effects on the local environment due to deposition of large volumes of wastes on the soil. In this paper, the method of combining field sampling, laboratory analysis with pot experiments was taken to provide scientific basis for the ecological restoration and revegetation of Rongxi Manganese mine. The main results are as follows:1. The contents of heavy metals (Mn, Cd, Cu, Zn and Pb) in soils and vegetables were measured in Rongxi Manganese mine. Geo-accumulation index and Hakanson’s ecological risk index were employed to evaluate the heavy metal pollutions in soils. The results showed that the average contents of Mn, Cd, Cu, Zn and Pb in soils collected from Manganese mining tailing were48382.5,3.91,79.97,131.23and80.68mg/kg, respectively. Geo-accumulation index indicated that Mn and Cd posed extreme contaminations, Cu and Pb posed slight to moderate contamination, while Zn posed nearly no contamination. Hakanson’s ecological risk index showed that Mn posed slight ecological risk in Qianzhimen, and strong or very strong ecological risk in the other four tailing stacking areas, including Xiaochayuan, Gaoloucun, Shatianwan and Yuhewan. Cd posed greatly strong ecological risk in the five tailing stacking areas, while Cu, Zn and Pb posed slight potentially ecological risk. The surrounding farmland soils were also mainly polluted by Cd, Hakanson’s ecological risk index showed that Cd posed very strong to greatly strong ecological risk. Moreover, the contents of Cd and Pb in vegetables far exceeded food standards which were unfit for consumption.2. The contents of Mn and Cd in the dry shoots of most dominant plants exceeded the upper limit of the normal range, while the contents of Cu, Zn and Pb fell within the normal range. According to the plant-soil relationships, plants were classified into three groups:accumulators, excluders and compartments. Accumulators such as Phytolacca americana L. and Polygonum lapathifolium Linn. absorbing high concentrations of heavy metals in the shoots are suitable to clean up the compound pollution of heavy metals. Compartments like Miscanthus sinensis Anderss. and Stenoloma chusanum Ching store high levels of heavy metals in the roots and transfer little to shoots. Excluders including Artemisia annua L., Desmodium sequax Wall., and Artemisia annua are the plants accumulating little heavy metals in their bodies though they grow on the severely polluted soil. The latter two can be used to remediate mine soils with higher heavy metals levels and low use value, and excluders had an important value in the study of plant heavy metal exclusion mechanism.3. The dry weight, plant height and root length of Phytolacca americana L. were increased when added20%soils or20%sludges, especially the adding of sludges, which increased the dry weight significantly (P<0.05). Compared with the control, substrate modifications made the heavy metal contents (Mn, Cd, Cu, Zn and Pb) in roots, stems, leaves and seeds of Phytolacca americana L. decreasing significantly (P<0.05). But the adding of sludges increased the accumulation amounts of heavy metals in both roots and shoots of Phytolacca americana L.. The whole Mn accumulation amount was29.31mg/plant, while the Cd, Cu, Zn and Pb were17.58,154.22,537.25,133.72μg/plant, respectively. The increase rang was from40%to250%compared with the control. But the whole accumulation amounts of Mn, Cd, Cu and Zn in Phytolacca americana L.were reduced under the adding of soils. 4. After the application of chelators (CA, EDTA and NTA), there were no toxic symptoms in Phytolacca americana L., and the shoot dry weights increased to some degree. The contents of heavy metals (Mn, Cd, Cu, Zn and Pb) were highest in leaves. The implementation of EDTA can improve the Mn and Cd accumulating in Phytolacca americana L.. When the concentration of EDTA was5.0mmol/kg, the whole plant absorption of Mn reached the maximum (52.28mg), increasing78.4percents compared with the control (29.31mg), the absorption of Cd also reached the maximum (78.43μg),. The citric acid can increase the absorption of Mn, Cd, Cu and Zn when its concentration was5.0mmol/kg, and it was also conducive to the extraction of Pb when the concentration citric acid was7.5mmol/kg. The implementation of NTA improved the accumulation of Mn, Cd and Cu, especially Pb, in the root of Phytolacca americana L. as well as the accumulation of Pb in the shoot.5. The orthogonal design, L9(34), was adopted to study the effects of sludge (A), phosphate (B), pH value (C) and EDTA (D) on the uptake and accumulation in Mn and Cd of Phytolacca americana L.. The results showed that T9treatment(A3B3C3D1, A20%+B480mg/kg+C6g/kg slaked lime+D2.5mmol/kg) was most conducive to the increase of Phytolacca americana L. biomass. The pH value, accounting for46.51%, was the primary factor that affected the accumulation of Mn, the addition of hydrated lime can increase the pH value of soils, reduce the available Mn content of the soils, and then limit the absorption and accumulation of Mn in the shoot, but the accumulation of Cd in the shoot was affected by the combined various factors. The Mn accumulation in the shoot of Phytolacca americana L. reached the maximum210.78mg/plant under T7treatment (A1B3C1D3, A5%+B480mg/kg+C0.2mol/L H2SO4+D7.5mmol/kg) which was4.8times of the control treatment. T7was also conducive to the accumulation of Cd (39.67μg/plant). The Cd accumulation in the shoot of Phytolacca americana L. reached the maximum53.39μg/plant under T2treatment (A2B1C1D1, A10%+B120mg/kg+C0.2mol/L H2SO4+D2.5mmol/kg), which was3.0times of the control treatment. T2was also conducive to the accumulation of Mn(149.04mg/plant). Therefore, T7and T2were most conducive to the accumulation of Mn and Cd in the shoot of Phvtolacca americana L. |
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