Effect of biochar on migration of heavy metals in mining soil under leaching conditions

Biochar was prepared from corn straw at 450℃, and the biochar was applied to the heavy metal contaminated soil in mining area with 5% carbon/soil dry weight ratio. The effects of biochar addition on the pH of soil leachate, vertical migration behavior and cumulative release of heavy metals were analyzed by soil column leaching test in laboratory. The results showed that the pH of soil leaching solution in biochar added soil was significantly higher than that of the control treatment, indicating that adding biochar had advantages in alleviating soil acidity. Compared with the control, the addition of biochar reduced the risk of heavy metal migration to the lower layer. The vertical migration of Pb, Cu, Zn and Mn in the soil column after leaching decreased by 65.93%, 50.95%, 49.29% and 30.95%, respectively. The order of inhibition of vertical migration of heavy metals by biochar was Pb>Cu>Zn>Mn, and biochar had the best passivation effect on Pb. With the increase of leaching solution volume, the leaching cumulative releases of Pb, Cu, Zn and Mn in soil appeared to be consisted of two phases involving the initial rapid process followed by a slow continuous process. The cumulative release of the four heavy metals was Pb>Mn>Zn>Cu, and the cumulative release of heavy metals was significantly reduced by adding biochar. The release processes of heavy metals from soil were fitted with mathematical equations. The Elovich equation could better describe the release process of heavy metals, which indicated that the mechanism of leaching and releasing process of these four heavy metals in soil was not a single reaction process, but a complex reaction process with large change of activation energy. After adding biochar, the b value of all heavy metals was lower than that of the control group. The migration rate of heavy metals decreased with the addition of biochar, indicating that the adding biochar could improve the adsorption capacity of soil to heavy metal ions, reduce the migration of heavy metals caused by soil leaching, and could realize the remediation of the soil in heavy metal complex contaminated mining areas. © 2020, Chemical Industry Press. All right reserved.