The solidification of heavy metal Pb2+-contaminated soil by enzyme-induced calcium carbonate precipitation combined with biochar

The remediation of heavy metal Pb²⁺-contaminated soil by enzyme (urease)-induced calcium carbonate precipitation (EICP) combined with biochar was studied. The solidification efficiency of Pb²⁺ reached 98.41 % when the mass ratio of CaCl₂/urea was 1:1 using EICP technology to remedy Pb²⁺-contaminated water. However, the formed precipitate was accompanied by unstable vaterite, and Pb²⁺ had the risk of secondary leaching. When the biochar of 5 wt% was added to the Pb²⁺-contaminated soil, the soil structure tended to be dense and the toxic leaching concentration of Pb²⁺ was less than 5 mg/L, which met the national standard of China. The addition of biochar increased the pH of the contaminated soil and changed the free Pb²⁺ into insoluble Pb(OH)₂. The biochar provided more nucleation sites for urease, and part of Pb²⁺ were adsorbed on its surface or diffused into the pores of biochar, which effectively solidified Pb²⁺ in the soil.