Study On Environmental Hazards Of Heavy Metals In Ancient Lead And Zinc Smelting Slag Yard

Metal smelting slag yard has seriously threatened to ecological safety of surrounding environment, and brought some negative impacts on the ecological environment and socio-economic life.This dissertation chooses Fengdu ancient lead and zinc smelting slag yard, located in Chongqing, as a case study object. Through scene investigation, systematic waste slag sampling and point sampling of surrounding impacted sites, the experiment test on heavy metals content in waste slag and farmland soil under the mountain is conducted by using micro-wave digestion, fluorescence spectroscopy and the instruments. Then the dissertation has completed risk assessments on heavy metals pollution in waste slag and farmland soil under the mountain by adopting"soil and groundwater pollution site risk assessment system"similar to RBCA. The author further studies on the waste slag in the slag yard by acute toxicity test, corrosion and leaching toxicity analysis, and then judge the nature and pollution status of the waste slag. Finally, it proposes control measures and suggestions to reduce heavy metal contamination in this area.According to the test results of total analysis, the content of Zn and Pb metal in the waste slag is relatively high. The average content of Zn + Pb in the waste slag highly reaches 68,818 mg / kg, and the slag still has a higher value of re-use. More than 3.67% of Zn + Pb have been released to the surrounding environment and caused the pollution by computing. The content of As,Cd,Cr in the waste slag is also higher with heavy toxicity, which imposed a threat to surrounding environment. From the total analysis on farmland soil down the mountain, the pollution has spread to the farmlands.Through the assessment by"soil and groundwater pollution site risk evaluation system", the non-carcinogenic risk indexes of human activities of the slag yard and farmland reach 55 and 8 respectively, higher than the alarm value"1".In acute toxicity experiment, the LD50 of mouse acute oral toxicity in slag water extraction is all lager than 20ml/kg.bw. In the cross of the experiment, no little white rats died but declined markedly in rats'body weight. The lixivium of the waste slag water in the slag yard has chronic toxicity.The whole slag yard was in weak alkaline environment through corrosion analysis on the waste slag. The pH value on the surface of the slag yard is much higher than the middle and bottom layers and the middle and bottom layers are relatively low, approximated to neuter. It follows that the surface of the waste slag yard impacted by rain leaking and other outside factors is more serious than the middle and bottom layer of the slag yard. As the slag heaping time is relative later, the pH value is thus quite lower. With the increase of slag heaping time, the pH value gets up.An experimental research on the leaching toxicity of slag samples was carried out. It is found that the leaching content of Zn and Pb in the waste slag is very low with just a little amount of Zn leaching content. And the leaching amount of Pb is even lower than the detection limit. On the contrary, there is a little leaching amount of Cr and Cd. It is illustrated that, under the normal conditions of rainfall leaking, the leaching time of Cr and Cd content is longer than Zn and Pb leaching amount in Fengdu lead and zinc metal smelting slag yard. It is because the waste slag in weak alkaline environment has the buffer capacity of the weak acid leaching, which thus results in lower leaching toxicity of the waste slag. The results suggest that, after several hundred years of rainfall leaching, the waste slag still possesses a certain leaching toxicity, which has a bigger threat to the surrounding environment.Finally, the thesis puts forward some measures and proposals to reduce and control heavy metal pollution in this region. Meanwhile, safety control and prevention programs are proposed to solve the problem of heavy metals migration in waste slag, as well as the safety of natural environment, residents in the lower reaches, farmland and livestock food, and drinking water.