Study On Stabilization Of Heavy Metals In Polluted Soil

Heavy metal pollution in soil is one of the most important environmental issues today.According to the degree of soil pollution,the commonly used repair techniques include the guest soil method and soil replacement method,deep tillage method,isolation and embedding method,heat recovery method,electric repair method and curing/stabilization.The processing and smelting of Gansu silver non-ferrous metal minerals gradually transformed Dongdagou from the earliest flood drainage channel into the largest sewage channel in Baiyin City,resulting in the composite pollution of various heavy metals in Baidong Dongdagou.It was found to be difficult to reach in the repair stage.Simultaneously repair zinc,cadmium and arsenic.In this paper,the heavy-metal mixed contaminated soil of Dongdagou,Baiyingou,was selected as the research object.The chemical stabilization and repair technology was adopted,using sodium salt（Na₂S,Na₂CO₃）,iron salt（FeS,FeSO₄）,calcium salt（CaO,CaCO₃,Ca₃（PO₄）₂）Three kinds of stabilizers were used as single stabilizers to compare and analyze the effects of these three stabilizers on pH and leaching toxicity of contaminated soil in Dongdagou,and to stabilize their Zn,Cd and As in contaminated soil.Effect and mechanism.It is difficult to achieve the purpose of stabilizing Zn,Cd and As at the same time by a single stabilizer.Therefore,according to the stabilization effect of a single stabilizer,considering the stabilizing effect,soil pH and other factors,the composite experiment of the stabilizer is carried out and the stabilizing effect is explored.The factors affecting the stability of the composite stabilizer during the stabilization process（stabilizer addition sequence,soil moisture content,stabilization time）were analyzed.The experimental results are as follows:（1）With the increase of Na₂S and Na₂CO₃,the pH of soil increased.The pH of soil added with Na₂CO₃ was obviously higher than that of Na₂S.The stability of Zn and Cd increased gradually.The stabilization effect of Na₂CO₃ is stronger than that of Na₂S;the leaching concentration of As is increasing,so the sodium salt has a negative effect on the stabilization process of arsenic.With the increase of FeS and FeSO₄,the soil pH showed a downward trend.The pH of the soil with FeSO₄ decreased significantly faster than the soil with FeS;1%of FeS and heavily polluted soil in slightly polluted soil 2%FeS has a stabilizing effect on Zn.2%to 5%of FeS in heavily polluted soil stabilizes Cd.As the amount of addition continues to increase,the activation is greater than the stabilization effect,resulting in a leaching concentration of Zn and Cd.The leaching concentration of As showed a downward trend,and the stabilization effect of FeS and FeSO₄ on arsenic was significant.With the increase of CaO,CaCO₃ and Ca₃（PO₄）₂,the pH of soil increased,and the pH of soil added with CaO was much higher than that of CaCO₃ and Ca₃（PO₄）₂.The soil pH increase trend of Ca₃（PO₄）₂ is slightly larger than that of CaCO₃.The stabilization efficiency of Zn and Cd is gradually increased.The stabilization of Zn and Cd by these three stabilizers is the strongest,and the CaCO₃ is the weakest.The leaching concentration of As showed an increasing trend,indicating that calcium salt has a negative effect on the stabilization process of arsenic.For the same contaminated soil with the same stabilizer,the concentration of Zn and Cd leached by the horizontal oscillation method is lower than that of the sulfuric acid nitric acid method,and the concentration of As leached by the horizontal oscillation method is higher than that of the sulfuric acid nitric acid method.（2）Through the compound test of stabilizers,the best composite stabilizer suitable for the contaminated soil of silvery Dongdagou complex heavy metals is FeS+CaCO₃.The pH of the mildly and heavily polluted soil stabilized by FeS+CaCO₃ is slightly increased,but within the range of 6-9,it meets the pH limit of the Surface Water Environmental Quality Standard,and is suitable for Zn,Cd and As.There are different degrees of stabilization.Using the Tessier five-step continuous extraction method,the morphological changes of Zn,Cd and As in the soil after the repair of heavy metal contaminated soil and composite stabilizer are analyzed,and the mild and severe pollution after stabilization is carried out.In the soil,the morphological changes of Zn showed a decrease in the exchangeable content,an increase in the carbonate bound state,and an increase in the residual state.The morphological changes of Cd showed a decrease in the exchangeable content and a carbonate bound content.The content of Fe-Mn oxide bound state and organic matter bound state is basically unchanged,and the residual state content is increased.The morphological changes of As are all expressed as exchangeable content decreased,carbonate bound state content increased,Fe-Mn The content of oxide bound state is decreased,the content of organic matter is increased,and the content of residual state is increased.Therefore,the process of stabilizing heavy metal is a process in which the state of heavy metal is converted from unstable to more stable or stable state.（3）The factors affecting the stabilization of Zn,Cd and As in the contaminated soil were studied from the order of stabilizer addition,soil moisture content and stabilization time.The order of addition of the composite stabilizer has no obvious effect on the soil pH after stabilization,and the variation range is less than 0.5.At the same time,the stabilization effect of adding CaCO₃ and FeS heavy metals is better than the other two stepwise addition methods.Considering the leaching concentration and stabilization efficiency of Zn and As,30%water content is adopted for lightly polluted soils;leaching concentration and stabilization efficiency of Zn,Cd and As are considered,and for heavily polluted soil,40%is used.Moisture content.Under the conditions of optimal stabilizer dosage,order of addition and water content,the optimal stabilization time is studied,and the stabilization effect of Zn,Cd and As is considered.In the mild and severely polluted soil,it can be stabilized for 30 days.Achieve optimal stabilization.In this paper,the stabilization and repair of heavy metal contaminated soil in Dongdagou of Baiyingou was carried out.The FeS+CaCO₃ composite stabilizer was used to achieve the simultaneous repair of Zn,Cd and As.It is important for the repair and treatment of heavy metal contaminated soil in Dongdagou of Baiyin reference value.