An Pots Experimental Study On Historical Pb-Zn Smelting Slag Land Reclamation In Northwestern Guizhou Province

The environmental problem resulted from ore minerals development is a global concern and have accepted much attention in the world. So far, a lot of researches focused on the release of hazardous substances (such as heavy metals) in mine tailings during the weathering process, the production of acid mine drainage and its threat to environment. Nevertheless, the research for environmental problems caused by Pb, Zn smelting (especially historical Pb-Zn smelting) slag is less. The Northwestern Guizhou Province is a famous place for its long history in Pb-Zn smelting. The Mack dust omitted by Pb-Zn smelting contains large amounts of lead, zinc, nickel, cadmium, sulfur and carbon monoxide, which has a potential risk to human beings and crops. The historical Pb-Zn smelting slag has caused an extremely serious pollution to the local ecological environment without any treatment adopted. On the basis of field investigation, sampling, and analyses for heavy metals contents in contaminated soils, pollution situations in soils contaminated by Pb-Zn smelting were presented and maize pots experiment were conducted with phosphorus fertilizer as remediation media in different slag to soil ratios. According to the maize pots experiment results, some concluding remarks have been made as are as follows :1. Historical Pb-Zn smelting slag land in Northwestern Guizhou Province have been contaminated by heavy metals in varying content., which soil Zn between 261.39 to 1647.29 mg/kg; Cd between 3.47 to 18.32 mg/kg;Pb between 128.72 to 941.75 mg/kg; Cu between 19.09 to 267.59 mg/kg;As between 13.73 to 39.32 mg/kg. According to Environmental quality standard for soils (GB15618-1995), heavy metals in the soil around the Pb-Zn smelter have greatly exceeded soil back -ground values in Guizhou Province, contents of layer A in average Chinese soil, and contents of average soils over the world. Soils around the historical Pb-Zn smelters has been seriously polluted by heavy metal.2.The results of environmental impact assessment indicate that soils and vegetables grown on the soils have been seriously polluted by heavy metals, especially Cd, except the soil in Yemachuan of Hezhang County being moderately polluted. This has a potential threat to health of human beings and plants around the Pb-Zn smelters.3. Mixing soil with slag in the ratio of 1:1 in pots did not affected the maize germination rate too much, but greatly inhibited growth of the maize by lowering plant height and weight.4. The maize grows in pure slag have high content of heavy metals in its leaves, and significantly exceeded the national food hygiene standards. In accordance with the ratios conducted, the contents of heavy metals in maize leaves has declined, Zn, Cd, and Cu in leaves of maize have been lowest concentrations in the ratio of K₄ (15 mg/kg P), and Pb in K₅ (17.5 mg/kg P) ratio. On the contrary, the lowest concentrations for roots of various heavy metals is in K₁ (5 mg/kg) ratio, but have the high concentration of value in K₄ (15 mg/kg P) and K₅ (17.5 mg/kg P) ratio. Considering four heavy metals in the cumulative maize situation, phosphorus ratio in 15--17.5 mg/kg is suitable to immobilizing heavy metal transported from soils to plants.5. Different growth periods have greatly affected the heavy metals contents of maize. The contents of heavy metals in maize leaves in growth period of 50 days were lower than 20 days.6. Amounts of phosphorus fertilizer adding to pots have significant impact on the content of available heavy metals in soils. The available Zn, Cd, and Cu contents show the trend of ascending first, descending second, and ascending again along with increased phosphorus fertilizer addition. This provides a reliable basis for further study. But Pb showed the opposite trend. Therefore, it is difficult to control heavy metal composite pollution by simply adding soluble phosphate to contaminated soil.7. In general, the phosphorus ratio between 10ppm to 17.5ppm is better to reclaim soils contaminated by heavy metals.