Enhanced defluoridation using covalently bonded organosilicon-bimetallic composite coagulants: Al-Mg and Al-Fe driven divergence in hydrolysis speciation and active site substitution pathways.

Fluoride-containing wastewater posed significant risks to both ecological and human health, with the advanced removal of low-level fluoride (≤ 20 mg/L) remaining particularly challenging. In this study, covalently bonded organosilicon-bimetallic composite coagulants (Al-Mg-based CSAM and Al-Fe-based CSAF) were constructed via silane coupling agents to address structural instability, poor fluoride removal efficiency and pH buffering, high metal residues, and weak floc settleability for conventional coagulants. Compared to commercial coagulants (polyaluminum chloride PAC and polyaluminum ferric chloride PAFC), the covalent types exhibited 6.06-28.97 % higher F⁻ removal, 30.51-83.94 % lower metal residues, superior floc performance and especially stable effluent pH (6.5-8.5). Polymetallic complexation, high-polymeric coprecipitation, and adsorption-bridging flocculation in the process of CSAM and CSAF coagulation improved F⁻ fixation, and meanwhile active-site connectivity enhanced which would strengthen floc stability. Furthermore, differential mechanisms between CSAM and CSAF revealed magnesium-enhanced Al-based coagulant hydrolysis, lower Mg-F solubility affinity enabled complete coprecipitation and Mg-Al polymerization elevated Al-site coordination capacity. In photovoltaic wastewater, CSAM achieved equivalent F⁻ removal at half the commercial coagulants dosage, further reduced F⁻ to 1.04 mg/L with increased dosing, eliminated alkali adjustment and simplified treatment. This work would provide a strategic approach for ensuring drinking water safety and sustainable industrial wastewater purification.