Immobilisation of soil cadmium and lead by an integrated approach of SICP and absorbent additives

The combined effects of various soil additives and soybean urease-induced carbonate precipitation (SICP) on soil heavy metal immobilisation remain poorly understood. This study aims to immobilise the soil cadmium (Cd) and lead (Pb) using the SICP technique in combination with various additives, as well as to determine their optimal application dosages. Three additional additives (i.e. porous silicon, coconut shell biochar, and sorbitol) were examined. Among the three additives, coconut shell biochar combined with SICP showed the most significant improvement in the immobilisation of soil cadmium. With an optimal mixing ratio of 10% (by mass), the content of cadmium in the exchangeable state was reduced by 41% compared with the SICP-only group, and the Cd leaching concentration decreased by 45%. In the case of lead-contaminated soils, sorbitol combined with SICP yielded the most significant enhancement. An optimal dosing of 1% (by mass) reduced the lead content in the exchangeable state by 66% and the leaching concentration by 45% compared with the SICP-only group. This was attributed to the ability of sorbitol as a nucleating agent to provide additional carbonate binding sites for the formation of carbonate precipitates with lead ions. This study provides valuable experimental data and guidance for heavy metal immobilisation in contaminated sites.