Fractionation and leaching of heavy metals in soils amended with a new biochar nanocomposite

In this study, surface soils of the Bama Pb-Zn mine-impacted area were sampled for an area surrounding the mineral processing plant. After collecting 65 samples and analyzing them for initial Cu, Pb, Zn, and Cd metal contents, the area was zonated based on the concentration distribution using ordinary kriging in R. A single homogenous sample was prepared by mixing equal weights of each sample as being representative of the whole impacted area (S_T). Next, a synthetic model soil (S_M) was prepared according to the mean S_T texture (S_M), divided into two portions, where one portion was amended with a biochar composite (10% w/w) (S_MA), both portions were artificially contaminated with Cu, Pb, Zn, and Cd (S_MAC and S_MC). The mixed soil S_T, and the model soils S_MC and S_MAC, were subjected to soil sequential extraction procedure to determine the variations in fractionation of heavy metals. Results showed that the fractionation in the unamended model soil (S_MC) was very close to the original real soil (S_T). Moreover, in both amended and unamended soils, Cd and Pb had the highest and the lowest mobility, respectively. Zn and Cu showed intermediate mobilities. The performance of the amendment was evaluated using a 150-day column leaching test taking leachate samples at designated time intervals, and Cu, Pb, Zn, and Cd concentrations were analyzed. Results of column leaching were in good agreement with the soil fractionation as Cd and Pb showed the highest and the lowest mobilities, respectively. Leaching through the soil column was also simulated by HP1 model. Results of simulation found in acceptable proximity to the experimental data despite remarkable differences due to limitations in defining soil to the simulation system.