Extracting phosphogypsum as an alternative calcium source for microbially induced carbonate precipitation: Application of phosphogypsum and insights

The resource utilization of phosphogypsum (PG), a calcium-rich solid waste, still requires urgent attention. Identifying alternative calcium sources to replace traditional ones remains a critical challenge for MICP. This study proposes a novel approach to activate PG as an alternative calcium source for MICP. It systematically analyzes various activators for Ca²⁺ extraction, assesses the feasibility and environmental benefits, and investigates the underlying mechanisms of Ca²⁺ release. The results indicate that the optimal conditions for extracting Ca²⁺ from PG are a 1 mol/L sodium gluconate solution with a solid-to-liquid ratio of 1:20, reacting at 30°C for 30 min. Ammonium acetate and sodium gluconate activators can convert CaSO₄·2H₂O into more easily soluble CaSO₄·0.5H₂O and structurally stable CaSO₄, respectively. The concentrations of heavy metals in the MICP solutions with activated PG calcium sources all comply with the concentration limits. All activators facilitate the formation of calcite precipitates in the bio-mineralization process, with the MICP efficiency of sodium gluconate-extracted PG calcium sources approaching 100 %. The activation mechanisms of PG can be categorized as ion exchange (chloride salts), complexation and ion exchange (ammonium acetate), and chelation and ion exchange (sodium gluconate). This study provides new insights into the application of PG in MICP.