Research On Geochemical Characters And Environmental Geochemical Effects Of Representative Tailings Impoundments In The Region Of Tongling, Anhui Province, China

The metal mine tailings impoundments are a kind of land resource which should be rehabilitated and utilized urgently, and they are also a potential contamination source. How to rehabilitate and utilize the tailings impoundments and reduce environmental contamination is a problem which attracts people's attention all over the world. In this paper, the geochemical characters of several representative tailings impoundments in the region of Tongling are studied. The heavy metal transfer rule and environmental effect are researched by sample analyzing and making simulation laboratory experiment, and technological measures for utilizing of tailings impoundments are put forward. The study results provide a theoretic basis for utilizing of tailings impoundments. The conclusions are as follows.In the study area, acid-neutralization capacity of copper tailings is stronger. The initiatory pH of water exerts little effect on the lixiviation of the elements. The initial lixiviation rate of metallic elements were different, commonly, Zn>Pb>Mn>Cu>Ni>Cr>As>Hg. For the reason that acid in water was neutralized rapidly, heavy metal ions lixiviated were able to take part in the second reaction, and metal hydroxides produced were deposited. When lixiviating was in dynamic equilibrium with depositing, the quantity of total ions became a constant. The mathematic models for describing the lixiviation of different heavy metals were obtained in this work.The contents of remaining metals in tailings weathered are in direct proportion to accumulating time. When metallic minerals of tailings have been weathered, metal ions were eluviated down slowly by rainfall. Because the oxygen content in different layers of the tailings dump is not same, the degree of weathering of tailings is different. The transfer rate of different elements in a layer of tailings was dissimilar. Some heavy metal elements might get secondarily enriched in 50. The transfer rates of Cu, As, Hg, Cd, and Pb were faster comparatively. After eluviating over a 20-year period, the contents of these elements in the shallow tailings have decreased obviously. The eluviating rates of Zn and Cr were slower than the above elements. The contents of Co and Ni have changed simultaneously. The content of Mn was steady relatively. It is noteworthy that the transfer rate of Pb was faster in tailings layers than in soils though pH of the pore water was alkalescent.The distribution of remaining heavy metal contents in the space is different due to a long period of weathering. The metal ions transferred not only in the perpendicular direction but also in the horizontal direction with the flow of surface runoff and underflow. The quantity of metal ions and their transfer rates are related to the weathering process, precipitation time, and rainfall, and also related closely to the inundating time. At the route-way of runoff, the metal contents in surface tailings decreased obviously. At points in the edge of tailings impoundments, tailings particle sizes were bigger relatively, and although it has been a long time of weathering, the heavy metal contents in each layer of tailings were greater than other points obviously. At the low-lying area, Zn, As, and Cd deposited at the surface of tailings secondly, resulting in metal ions enrichment; and the peak values of some elements due to secondary transfer also appeared at some positions of lower layers.Through weathering and eluviating over a 20-year period, the easily eluviating fraction of most heavy metals in the tailings above 110 cm in depth have eluviated in the Yangshanchong impoundment. The major existing state of Cu, Pb, Mn, As, Hg, Ni, Co, and Cr was the residual state with the fraction being greater than 35%, and the second was the oxidation state of both iron and manganese. The major existing state of Zn and Cd was the oxidation state of both iron and manganese. In the surface tailings, the contents of the metals in the oxidation state of both iron and manganese were higher, while the fraction of the organic-bound state that Ni and Co existed was bigger. Comparatively, the copper tailings of the Linchong impoundment experienced weathering, eluviating and reclaiming with plants for a much longer time, the contents of metal elements have been changed obviously. Except that the contents of minority elements have risen due to plants enrichment and some elements got secondarily enriched at the 60 cm layer, most metal contents of tailings above 150 cm became steady. In the Xiangsigu impoundment which is being used, metal elements contents in the tailings have changed in perpendicular. It is obvious that heavy metal contents in tailings can be reduced evidently after the tailings were piled and rehabilitated over a long period of time. The existing states of remaining metal elements were the residual state, the oxidation state of both iron and manganese, and the organic-bound state, and the harm to environment was reduced obviously.Impoundment rehabilitation influences the distribution and transfer of heavy metals obviously. For the Yangshanchong impoundment where vegetation came back only, the effect depth is that grass roots can arrive at. Some elements could be absorbed by grasses, and remained on the surface of tailings as grasses perished, so the contents of some elements in surface tailings rose singularly. For the Linchong impoundment which has been rehabilitated and cultured in full scale for years, the oxygen content in the tailings layers has increased, the content of organic substance has been increased, the activity of microorganism is enhanced, and the rate of tailings weathering has increased. Therefore heavy metal contents in tailings were reduced clearly except that the content of one or two elements rose slightly as compared with the lower layer, which may be a result of defoliation. Thus full scale culturing is important for improving the characters of tailings, while surface planting of grass can just decrease dust flying upwards, but the characters of tailings can not be improved fleetly, and even some elements may get enriched again in the surface of tailings, which may produce bad influence on the rehabilitation of tailings impoundments.The investigation proved that leaked water and tailings did not influence obviously the quality of water in the Xiangsi river. The quality of water in the Xiangsi river satisfies the requirement of IV class water in the environmental quality standard of surface water (GB3838-88), and the water can be used to irrigate, but should be clarified. A quantity of tailings depositing at the bottom of the river may influence the soil in the low-lying area along the river. The soil in the area bounded with 500 m leeward of the Xiangsigu impoundment has been influenced by flying dust. The less the distance, the greater the influence, that is, the heavy metal contents in the soil is higher than others.For the area influenced by both leaking water of tailings impoundments and flying dust of opencast working, the emission source's contribution to increasing metal contents in soils can be simulated and separated according to its diffusion rule. Investigation proved that the area bound influenced by flying dust of opencast working was 300 m. Each factor's contribution to every metal element was different. The area has been submerged during the raining season. Flooding and the tailings carried by the river water could influence the soil in that area. By analyzing the existing state of metal elements in soil samples of different points, it was found that the contents of metal elements in the exchangeable state decreased with lessening of the distance. Because metals in the ion state could be easily transferred, the remaining contents in soil were less. The contents of metal elements in the humic acid combined state were also lower. At all points, the contents of metal elements in the oxidation state of both iron and manganese were major.The results of eluviating experiment prove that in the soil which was covered with tailings, the metals leached from tailings could influence 70 cm in depth under the continuous acid rainfall condition. Different elements have different rates of transfer. Some elements were adsorbed by soils in the course of transfer, and these elements had less influence on groundwater in a short term, but it is important that the effect of Hg and Pb should not neglected. By comparing the metal contents in the eluviating water sampled at different time with those in the soils at different sections, it is found that the quantity of metal elements leached and adsorbed was different distinctively. The Cu the content of which is higher in tailings eluviated and transfered less. The transferring ability of Pb and Hg in soil was biggish, and most of them eluviated, so attention should be paid to that Pb and Hg may cause groundwater contamination. The transferring ability of Cr was feeblish, and it can not easily move to groundwater, but can cause soil contamination. The transferring ability of Zn in soil was biggish, and the sorption of Zn by soil was also bigger, so the Zn content in soils and eluviating water had risen.The result of experiment on the toxic effect of metal elements on seed germination proved that metal elements in tailings have less influence on the germination rate of greenstuffs. It is possible to grow trees and economic crops on copper tailings. After the tailings were mixed with soil, EDTA, or PAM, the structure of tailings layers would be improved, and their ability of keeping water and fertilizer was raised. After the tailings were mixed with EDTA or PAM, plants grew well. Comparative analysis shows that both EDTA and PAM can restrain plants from absorbing certain metal elements. Comparatively, the capability of PAM was better, and it could restrain plants from absorbing three metal elements availably, and its capability of keeping water and fertilizer was better.The work of tailings impoundment rehabilitating should be carried out step by step. First some technological measures can be used to strengthen oxidating and eluviating of heavy metal elements, and to accelerate improving of soil in tailings impoundment. Second, initial vegetation rehabilitating can be made progressively from the edge to the center of tailings impoundments. Trees and grasses can be planted mainly to restrain flying dust and improve tailings during the initial 5-10 years. According to the change of heavy metal contents in tailings, economic crops can be planted after 10 years. The tailings particle sizes are bigger in the edge of tailings impoundments for the rate of metallic mineral weathering and oxidating is slower, so it is necessary that tailings should be mixed with soils for improving. If the technological measures are appropriate and just in time, the time of rehabilitating can be shortened.