Chemical forms of heavy metals in agricultural soils affected by coal mining in the Linhuan subsidence of Huaibei Coalfield, Anhui Province, China

Total concentrations of heavy metals in soils may not be enough to understand their mobility and bioavailability. It is important to evaluate the degree of association of heavy metals with different chemical forms of soil. The sequential extraction method was applied to evaluate the mobile behavior of Cd, Cr, Cu, Ni, Pb, and Zn in 42 representative soil samples from the Linhuan subsidence of Huaibei Coalfield, Anhui Province, China. The results showed that mean concentrations of heavy metals were higher than background values of Huaibei City surface soil by a factor of 1.16 to 3.21 (Cd, 3.21; Cr, 1.19; Cu, 1.16; Ni, 1.23; Zn, 1.85) except Pb (0.89). Most of the total Cr, Cu, Ni, Pb, and Zn were present in the residual forms (above 70 %), while Cd was dominated by the exchangeable forms (42 %). The correlations analysis showed that the mobility of Cd, Cu, Pb, and Zn in soil was affected by both physicochemical properties and total metal concentrations. In contrast, the moblity of Cr and Ni of soil was mainly affected by their total metal concentrations. According to assessments by the potential ecological risk index (RI) and the risk assessment code (RAC), Cr, Cu, Ni, Pb, and Zn posed no or low risk. However, Cd presents high to very high risk, due to its higher exchangeable and carbonate-bound fractions.