Overlooked fluoride release from modified phosphogypsum and its effective immobilization by Nocardia sp

The application of modified phosphogypsum (MPG) as a soil amendment has emerged as a promising strategy for its large-scale utilization, yet the potential environmental risk of fluoride (F^-) release during this process has been largely overlooked. This study systematically demonstrated that MPG-amended soils pose a persistent F⁻ leaching hazard, with leachate contents reaching up to 3.0 mg L^-1, which was three times the Class III limit of Chinese Groundwater Quality Standards. To address this risk, Nocardia sp. X10 was isolated and its efficient F^- immobilization mechanisms were elucidated. Direct quantification of extracellular polymeric substances (EPS) embedded within the biominerals (30.7-67.7 mg L^-1) provided evidence that EPS served as nucleation templates for fluorine-containing minerals. Comprehensive characterization identified the biominerals as a co-precipitated assemblage of Ca₅(PO₄)₃F, CaF₂, and CaCO₃, intimately associated with EPS. Quantitative analysis of contributions revealed that microbially mediated mineralization dominated F^- immobilization. Calcium (Ca²⁺) and phosphate (PO₄^3-) acted as essential mineral precursors, with F^- immobilization efficiency increasing from 8.6% (F^- alone) to 67.9% and 77.1% (Supplement Ca²⁺ and PO₄^3- successively). Soil column validation confirmed that strain X10 achieved superior F^- immobilization (65.7%) compared to chemical passivators (57.2% and 61.8%), consistently maintaining leachate F^- below regulatory limits. The findings will help reveal the potential environmental risks and the biological immobilization mechanisms involved in the resource utilization of industrial by-products.