Characteristics of cadmium in contaminated soil immobilized by biochar enhanced microbially induced calcite precipitation

The heavy metal pollution in soil caused by human industrial activities is becoming increasingly severe. To alleviate this problem, microbially induced calcite precipitation (MICP) has been extensively investigated for immobile heavy metals. Biochar was applied to improve the immobilization characteristics of cadmium through MICP. The immobilization characteristics are studied through a series of experimental analyses taking account of factors including cadmium concentration, curing time, effect of calcium, and effect of biochar. The results show that the unconfined compressive strength (UCS) increased remarkably and the Cd2+ leaching concentration gradually decreased by extending the curing time. The addition of biochar enhanced the efficiency of Cd2+ immobilization through MICP, resulting in the UCS of the samples 2.51-3.06 times those without biochar and the Cd2+ leaching concentration decreasing by 8.0-52.5 %, leading to the weak acid extractable cadmium decreasing by 6.4 %, and the permeability decreasing by 50.6 %, reducing the toxicity and migration of Cd2+. Moreover, the addition of Ca2+ in cementation solutions further improved the immobilization effectiveness. The cementation of CaCO3 can reduce the porosity of soil and immobilize Cd2+ through the co-precipitation pathway, impeding the release of Cd2+. In addition, the honeycomb structure of biochar with a large surface area and hydroxyl groups can facilitate the adsorption of Cd2+ and confine the migration of Cd2+.