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 Ca 5 (PO 4 ) 3 F, CaF 2 , and CaCO 3 , intimately associated with EPS. Quantitative analysis of contributions revealed that microbially mediated mineralization dominated F − immobilization. Calcium (Ca 2+ ) and phosphate (PO 4 3− ) acted as essential mineral precursors, with F − immobilization efficiency increasing from 8.6% (F − alone) to 67.9% and 77.1% (Supplement Ca 2+ and PO 4 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.