Enhanced immobilization of cadmium and lead in contaminated soil using calcium alginate-modified HAP biochar: Improvements in soil health and microbial diversity

A novel adsorbent, calcium alginate-modified HAP (Hydroxyapatite)-wood ear mushroom sticks biochar (CA-HAPMB), was synthesized to enhance the immobilization of Cd and Pb in soil. Over 150 days, applying CA-HAPMB at concentrations of 0%–3% in contaminated soils from Chenzhou City in Hunan Province (CZ) and Shenyang City in Liaoning Province (SY) resulted in decreased effective concentrations of Cd and Pb. Specifically, in CZ soil, Cd and Pb decreased by 30.9%–69.3% and 31.9%–78.6%, respectively, while in SY soil, they decreased by 27.5%–53.7% and 26.4%–62.3%, respectively. Characterization results, obtained after separating CA-HAPMB from the soil, indicate that complexation, co-precipitation, and ion exchange play crucial roles in the efficient immobilization of Cd and Pb by CA-HAPMB. Additionally, adjusting the amount of CA-HAPMB added allows modulation of soil pH, leading to increased soil organic matter and nutrient content. Following treatment with CA-HAPMB for immobilizing Cd and Pb, soil bacteria abundance and diversity increased, further promoting heavy-metal immobilization. [Display omitted] •A new type of Granular adsorbent with biochar as substrate (CA-HAPMB) was designed.•CA-HAPMB enhanced immobilization of Cd and Pb in soil.•Available nutrient (N, P and K), CEC and SOM increased.•Bacterial richness and diversity in soil were restored after passivation of Cd and Pb.