Experimental study on solidification of cadmium-contaminated soil by Sporosarcina pasteurii via MICP method

The immobilization of heavy metals using microbes presents an environmentally friendly approach, utilizing local indigenous microbes for remediating heavy metal-contaminated soils and improving soil environmental quality. Microbially induced carbonate precipitation (MICP), particularly through urea hydrolysis, is a typical biomineralization process in nature that has gained considerable attention. Sporosarcina pasteurii, a popular indigenous urease-producing bacterium, is especially effective in hydrolyzing urea. This study examines the effects of calcium ion (Ca²⁺) addition, cadmium ion (Cd²⁺) concentration, and curing time on the characteristics of cadmium-contaminated soil solidified by acclimatized Sporosarcina pasteurii. Various tests, including toxicity leaching, unconfined compressive strength, soil column leaching, cadmium speciation analysis, and microanalysis, were conducted to evaluate these effects. The results indicate that Ca²⁺ significantly affects the leaching behavior of Cd²⁺ in the contaminated soil after solidification. Without Ca²⁺, the lowest Cd²⁺ leaching concentration (0.42–5.64 mg·L^‒1) was observed at a urea concentration of 0.5 mol·L^‒1. When Ca²⁺ was added, the Cd²⁺ leaching concentration slightly increased, but optimal solidification efficiency was achieved when urea and Ca²⁺ concentrations were 0.5 mol·L^‒1. As curing time increased, the compressive strength of the solidified soil also improved, while Cd²⁺ leaching concentration decreased. The presence of Ca²⁺ further enhanced soil solidification over time. Higher Cd²⁺concentrations led to reduced compressive strength and increased leaching concentration. However, the addition of Ca²⁺ enhanced the solidification effect on cadmium-contaminated soil. At a curing time of 28 d, for a Cd²⁺ concentration of 100 mg·kg^‒1, the unconfined compressive strength of samples with Ca²⁺ reached 312 kPa, a 33.3% increase compared to 234 kPa without Ca²⁺. For a Cd²⁺ concentration of 1600 mg·kg^‒1, the unconfined compressive strength with Ca²⁺ reached 269 kPa, representing a 57.3% increase compared to samples without Ca²⁺, and the leaching concentration decreased by 15.4%. After leaching tests over 15 days using a geoenvironmental osmosis system revealed that the Cd²⁺ concentration in the leachate from samples with Ca²⁺ remained consistently lower than those without Ca²⁺. After solidification, weak acid-extracted cadmium in the contaminated soil transformed into reducible and residual forms. The addition of Ca²⁺ further reduced the proportion of weak acid-extractable cadmium, thereby reducing Cd²⁺ mobility. The calcium carbonate generated by the MICP reaction cements adjacent soil particles, thereby improving overall soil stability. Cadmium ions in the contaminated soil are immobilized by calcium carbonate through coprecipitation, forming stable compounds that reduce Cd²⁺ migration and bioavailability. This effectively alleviates soil heavy metal pollution. Overall, the results of this study will offer significant theoretical insight and technical innovations, holding profound implications for ecosystems, society, and the economy.