Biochar Facilitated Hydroxyapatite/Calcium Silicate Hydrate for Remediation of Heavy Metals Contaminated Soils

High levels of heavy metals in soil pose considerable threats to the ecosystem. The in situ remediation technology is obtaining increasing global concerns as a sustainable remediation strategy. In this study, the hydroxyapatite/calcium silicate hydrate (HAP/CSH) was recovered from waste water to evaluate the effects on heavy metal immobilization by coupling with biochar. Five mixtures of HAP/CSH and biochar with different weight ratios (10:0, 0:10, 1:9, 2:8, 4:6 w/w) were prepared to remediate two heavy metal-contaminated soils planted with water spinach (Ipomoea aquatica Forsk.). The mixture of HAP/CSH and biochar in the ratio of 4:6 shows the best immobilization effect assessed by toxicity characteristic extraction procedure (TCLP) and BCR sequence extraction procedure. After remediation, the immobilization efficiency decreased 83%, and content of heavy metals in plants decreased 72.8%. The conversion efficiency of heavy metal residual fraction was 3.95 times higher than that of the control group. At the same time, soil pH, water-soluble organic carbon (WSOC), and soil microbial biomass (SMB) all showed an increasing trend, indicating the improvement of soil conditions. The combined application of HAP/C-S-H and biochar changed soil bacterial community structure, leading to an increase in soil bacterial diversity. The results of redundancy analysis (RDA) suggested that pH and the concentration of heavy metals were the main factors affecting microbial community. Therefore, the mixture of HAP/C-S-H and biochar can be considered as an effective, feasible, and environmentally friendly amendment for the remediation of Cd-contaminated soil and multi-metal-contaminated soil.