The Effect Of Freeze Thaw On The Stability Of Heavy Metals In Contaminated Soil

Contaminated soil with Pb, Zn and Cd was treated by solidification technique using red mud and sodium sulfide as additive. Through the simulation of freeze-thaw process, the leaching toxicity, bioaccessibility, pH and structure of the soil were studied for investigating the long-term influence of freeze-thaw on solidification treatment of soil contaminated by Pb, Zn and Cd. The results showed that the leaching toxicities of Pb, Zn and Cd were reduced 98.5%?99.8% and 99.7% than the original soil samples after the stabilizing treatment. The bioavailability of Zn and Cd were reduced from 28.8% and 49.5% to 11.5% and 21.8% after 30d curing time. The bioavailability of Pb rose from 52.9% to 57.9%. The stability varieties of the three kinds of heavy metals in soil was analyzed after 60 freeze-thaw cycles for forecasting the release characteristics of Pb, Zn and Cd. The leaching toxicities of Pb, Zn and Cd increased by 4.27,2.47 and 89.7mg/L, and the bioavailability were slightly higher than the soil samples without freeze-thaw treatment. The pH of soil has no significant changes after freeze-thaw cycles. Sulfide and hydroxide of Pb, Zn and Cd precipitation were detected by X-ray diffraction analysis. It can be seen clearly that the soil gap become bigger, and the soil structure looser after the continuous freeze-thaw cycle from the scanning electron microscopy images. The results of qualitative and quantitative analysis indicated that the freezing-thaw process increases the release risk of Zn, Pb and Cd in the soil. Mechanical penetration results showed that the freezing and thawing soil of permeability coefficient greater than the magnitude of the others.