Effects of Bentonite Addition on the Speciation and Mobility of Cu and Ni in Soils from Old Mine Tailings

Bentonite has important applications in curbing soil heavy metal pollution. Jinchang city is an important industrial city in western China, and the mining activities in this area inevitably lead to the heavy metal pollution of soil; in particular, the maximum concentrations of Ni and Cu in the soil exceeded the standard by 30 and 25 times, respectively. In this study, we conducted soil cultivation experiments to study the effects of bentonite addition (BA) on the fractions of the heavy metals Ni and Cu in an old tailings area of Jinchang city. Surface soil (0-20 cm) samples were collected, and Ni and Cu fractionation was performed using the Tessier sequential extraction method. The results showed that residual (R) was the main fraction of Ni and Cu, which accounted for 53% and 57% of their total amounts, respectively. The fraction bound to organic matter (BM), bound to Fe-Mn oxides (BO), bound to carbonates (BC), and the exchangeable (E) accounted for 20% and 16%, 18% and 12%, 6%, and 7%, 3% and 4% of the total amounts, respectively. Their contents ranked in the order: R > BM > BO > BC > E. Each fraction of Ni and Cu decreased with increasing levels of BA. The proportion of E of Ni and Cu was considerably reduced, while the proportion of BO and R increased significantly following the BA. BA can significantly reduce the mobility of Ni and Cu. Our findings indicated that BA can significantly reduce the biological toxicity and mobilization of heavy metals in polluted soil, which can be recommended as a safe stabilizer for heavy metal pollution in soil.