Effect of urea concentration on the combined pollution of Cd and Ni in microbiologically induced calcite precipitation (MICP) treatment

Soil heavy metal contamination is one of the most severe global environmental challenges today. Microbiologically induced calcite precipitation (MICP), as an environmentally friendly bioremediation technique, demonstrates significant potential in addressing such pollution. To optimize the MICP process, the research systematically investigated the influence of urea concentration on the remediation of soils co-contaminated with cadmium (Cd) and nickel (Ni). A highly efficient urease-producing strain, SX4 (Enterobacter sp.), was isolated from mining areas, showing the highest urease activity (conductivity change: 22.14 mS·cm⁻¹) among all isolates. Under optimal growth conditions (pH = 7, urea concentration 20 g·L⁻¹, OD₆₀₀ = 1.76), the remediation cycles for Cd– and Ni-contaminated soils were 120 h and 132 h, respectively. Evaluation of different urea concentrations (0, 10, 20, 40 g·L⁻¹) confirmed that the 20 g·L⁻¹ group was the most effective. It achieved effective removal rates of 45.71% and 43.34% for Cd and Ni, respectively, in single-pollutant contamination, and 32.44% for Cd and 38.75% for Ni, in co-polluted conditions. The findings elucidate the pivotal role of urea concentration in the MICP remediation process, providing crucial scientific evidence for optimizing the practical engineering parameters of this technology.