Adaptive fluoride removal across concentration scales: Potential roles of microbial and acicular gypsum interactions...

Fluoride contamination in groundwater is a critical environmental and public health concern. Traditional remediation methods, including chemical precipitation and adsorption, are hindered by low nucleation efficiency at low fluoride concentrations and severe microbial inhibition under high fluoride stress. This study introduces an adaptive two-stage remediation system that synergistically integrates abiotic and biotic mechanisms to achieve effective fluoride removal across concentration scales. In Stage I, under elevated fluoride stress (100 mg/L), acicular gypsum (AG) facilitated abiotic calcium precipitation, effectively reducing fluoride toxicity and creating favorable conditions for microbial activity. Residual calcium released from AG further supported microbial-induced calcium precipitation (MICP) in Stage II under lower fluoride stress (10 mg/L). The system achieved a fluoride removal efficiency of 98.85 % under high fluoride conditions and demonstrated consistent performance across a broad concentration range. This integrated approach, combining abiotic and biotic mechanisms, offers a promising strategy for addressing diverse fluoride contamination scenarios. Here, phosphate (P)-mediated mineralization and microbial denitrification drive pH modulation, stabilizing fluoride as fluorapatite [Ca₅(PO₄)₃F]. Microbial community and network analysis revealed key taxa, including Cupriavidus and Ralstonia, which facilitated P cycling and fluorapatite formation. Additionally, Bradyrhizobium enhanced nitrogen (N) cycling and supported early microbial adaptation, emphasizing the interplay of microbial interactions in driving system functionality. Functional predictions using PICRUSt2 identified genes associated with N and P cycling, highlighting the capacity of the system for nutrient adaptation under complex environmental conditions.