Study On Stabilization Of Heavy Metals In Soil Using Inorganic Stabilizers

Soil is one of the important natural resources for human survival.However,with the continuous development of industry and agriculture,heavy metal pollution in the soil has become more and more serious.Heavy metals in contaminated soil have caused serious damage to the survival of animals,plants and humans through the food chain.Therefore,it is necessary to deal with the soil contaminated by heavy metals to reduce the harm of heavy metals in the soil.In this paper,rice husk ash and lime were used as stabilizer to investigate the influence of different parameters,such as adding proportion of rice husk ash and time of natural curing on the stabilization of heavy metals in the soil by natural curing or intermediate-temperature thermal treatment.Based on the experimental results,the modified rice husk ash was proposed to study its effects on the stabilization of heavy metals in the soil.The changes in chemical speciation of heavy metals and the crystal structure before and after treatment were tested by European Community Bureau of Reference（BCR）sequential extraction experiment,and X-ray diffraction spectroscopy to explore the mechanisms of the stabilization of heavy metals by natural curing and intermediate-temperature thermal treatment.The main conclusions are as follows:The leaching concentrations of Cu,Zn,Pb and Cd in brown soil treated with 9%rice husk ash or 6%lime at 450℃for 60 min decreased significantly,and the stability effect was better than that of natural curing samples.Therefore,it is not recommended to add rice husk ash or quicklime to stabilize heavy metals in soil with serious Cr pollution.With the addition of rice husk from 9%to 12%,the leaching concentration of Cu,Zn,Pb,Cr and Cd in brown soil or black soil could be effectively reduced after natural curing.The results show that the effect of adding lime on the stability of heavy metals is lower than that of adding rice husk by nature curing,and the addition of lime led to the oxidation of Cr³⁺to unstable Cr⁶⁺,resulting in the increase of Cr leaching concentration.Black soil and brown soil have almost the same composition except humus content.The non-specific adsorption of heavy metal cations by humus with higher content in black soil or the specific adsorption of Cu,Pb and Cd by functional groups such as carboxyl and phenol hydroxyl made the leaching concentrations of Cu,Zn,Pb,Cr and Cd in black soil samples lower than those in brown soil samples.Rice husk ash or lime as stabilizer can promote the formation of complex aluminosilicates in the process of natural curing or intermediate-temperature thermal treatment of heavy metal contaminated soil,such as Ca₄Al₂（SO₄）（OH）₁₂,albite,nepheline,Ca Al（Si₃Al）O₈（OH）₄·H₂O,and these substances can make the heavy metals change from unstable acid soluble and reducible state to stable oxidizable and residual state by coating and ion replacement,resulting in a significant reduction of the leaching concentration.However,the intermediate-temperature thermal treatment and lime can make Cr change from oxidizable state to unstable reducible state,resulting in the increase of Cr leaching concentration.After alkali modification of rice husk ash with lime,the surface pore and pore volume of rice husk ash increased,and the reactivity of rice husk ash further increased.The leaching concentrations of Cu,Zn,Pb,Cr and Cd in brown soil and black soil mixed with a certain amount of modified rice husk ash were further reduced after natural curing and intermediate-temperature thermal treatment,and the stability rate was increased by 0.8%～51.1%.The p H of brown soil and black soil was 7.91and 7.84 respectively after 5 weeks of natural curing with 9%modified rice husk ash,and the p H of brown soil was 8.12 after 60 minutes of intermediate-temperature thermal treatment at 500℃with 9%modified rice husk ash,and all the three kinds of soils were weak alkaline soil.The use of modified rice husk ash to the stabilization of heavy metals in the soil not only improves the stability of heavy metals,but also makes the treated soil be used in green belt and other occasions.