Bioremediation of multiple heavy metals through biostimulation of microbial-induced calcite precipitation at varying calcium-to-urea concentrations

Studies on heavy metal bioremediation through microbial-induced calcite precipitation (MICP) typically involve bioaugmentation approaches that use low calcium-to-urea ratios and target single contaminants. We present an investigation on the efficiency of soils' autochthonous ureolytic bacteria to simultaneously remediate multiple heavy metals and sequester carbon through urea hydrolysis and MICP on an urban soil containing excess Pb, Zn, Mn, Sr, Ba and Al. Soils were treated at a fixed urea concentration of 333 mM and increasing calcium content of 0, 50 and 333 mM to provide a range of carbonation potential. Urea hydrolysis (Ca²⁺ = 0 mM) did not produce quantifiable soil carbonation and mobilised Mn into the exchangeable fraction. Ca²⁺ at 50 mM delayed soils' autochthonous ureolytic activity and produced limited carbon and heavy metal mineralisation (CaCO₃ = 0-0.7 %). 333 mM of Ca²⁺ inhibited urea hydrolysis however, if applied following urea hydrolysis, both carbon (CaCO₃ = 4-7 %) and heavy metal (Pb, Zn, Mn, Sr and Ba) mineralisation were maximised. Urea hydrolysis and MICP were most successful in removing Pb and Zn from the exchangeable fraction (>85 %). However, the higher pH induced by urea hydrolysis at Ca²⁺ = 0-50 mM (∼9) compared to 333 mM (∼8.5) favoured partition of Pb into the oxyhydroxide fraction. Instead, partition of Zn, Mn, Sr and Ba into the soil carbonate fraction increased with increasing calcium, whilst there was no evidence of Al carbonation. The results of this study evidence the feasibility of biostimulation approaches to remediate multiple contaminants simultaneously through MICP, provide insights into multiple element's behaviour during urea hydrolysis and MICP and demonstrate carbon and element mineralisation are maximised at equimolar calcium-to-urea ratio of 333 mM.