Contrasting effects of historically and freshly contaminated soils by biochar on the immobilization of cd under combined aging

Abstract

Most current research focuses on the remediation of heavy metal contamination by biochar in one type of soil. However, there are few comparative studies on the remediation effects in fresh and historically contaminated soils. To simulate the eight-year natural aging process, we measured the transformation of Cd in both freshly contaminated soil (NS) and historically contaminated soil (OS) treated with rice straw biochar made at 400 °C (BC400) and 700 °C (BC700) during combined dry-wet and freeze-thaw cycles for 72 days. Standing comparison with CK, the availability of Cd in NS-BC400 and NS-BC700 reduced from 0.16 mg·kg^− 1 to nearly 0, and OS-BC400 and OS-BC700 reduced from 1.7 mg·kg^− 1 to 0.6 and 0.2 mg·kg^− 1 at day 72. During the 72-day aging process, the average Tessier exchangeable Cd percent of OS, OS-BC400, and OS-BC700 were 70%, 59%, and 52%; and those of NS, NS-BC400, and NS-BC700 were 65%, 62%, and 51%. Compared with CK, the Tessier exchangeable Cd (F1) of NS-BC700, OS-BC400 and OS-BC700 significantly reduced by 20.3%, 10.7%, and 22.2% at day 72, respectively, but that of NS-BC400 had no obvious change. The contribution of biochar and soil to the reduction of F1 was different in OS and NS with different biochars. For OS treated with biochar, the relative percent contribution of biochar to the reduction of F1 was nearly 100%, and that of OS was almost no contribution. However, for NS treated with biochar, the relative percent contribution of BC400 and BC700 to the reduction of F1 decreased from 86.1% to 89.5% at day 3 to 21.2% and 49% at day 72.