An evaluation of microbially induced carbonate precipitation for bioremediation of complex, real-world slag materials

Mining and the metalliferous industry create solid waste byproducts hazardous to the environment. The current study characterized the hazardous nature of 2 complex, real-world slag materials according to: (i) US EPA Identification of Listing of Hazardous Waste (40 CFR 261.24) and Land Disposal Restrictions (40 CFR 268.48) according to TCLP regulations; (ii) Québec's Soil Protection and Rehabilitation of Contaminated Sites Policy (SPRCSP), Québec's Regulation Respecting Hazardous Materials (Q-2, r. 32), and Québec's surface water quality criteria (SWQC). Microbial induced carbonate precipitation (MICP) was used as a solidification/stabilization (S/S) technique for the hazardous, leachable materials. S1 exhibited biologically induced calcium carbonate (CaCO₃) precipitation at the surface of slag particles but insufficient precipitation occurred to bind the particles for development of a biocement matrix. In contrast, S2 developed a biocement matrix that withstood slaking and water absorption. The Sporosarcina pasteurii treated samples precipitated CaCO₃, dolomite (CaMg(CO₃)₂), and magnesite (MgCO₃). Uniquely, CaCO₃ precipitated uniformly around the slag particles causing particle-particle binding, but MgCO₃ precipitated at the surface creating a crust. Formation of a densely compacted biocement matrix occurred that resisted physical degradation over 168 h. Eleven metal(loid)s exhibited a reduction in leaching in the bacteria treated sample in contrast to an untreated sample. The results suggest MICP efficacy is case specific influenced by initial magnesium concentration, pH, and pore structure, whereby higher values are suggested to have improved carbonate precipitation and biocementation. The results from this study successfully illustrate the unique mechanisms of S/S via MICP to better understand the field of biochemical remediation to complex, real-world slag materials.