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.