Study on defluorination performance and mechanism of Fe-Al modified oyster shell substrate coupled with microbially induced calcium precipitation for enhanced ecological floating beds.

Fluorine is the most reactive nonmetallic element. Low fluoride concentrations in drinking water prevent dental caries, while excessive fluoride leads to fluorosis. To overcome the low fluoride (F⁻) removal efficiency and high cost of conventional ecological floating beds (EFBs), this study developed two slow-release materials (carbon-releasing and calcium-releasing) and fabricated an Fe-Al modified substrate primarily using oyster shells as the raw material, while integrating microbial-induced calcium precipitation technology to enhance the treatment efficiency of the "substrate-microbe" system for fluoride-contaminated water. The optimal F⁻ removal efficiency (92.0%) and excellent substrate regeneration performance were achieved at 700℃ calcination temperature, under an Fe-Al mass ratio of 2:1 and an oyster shell powder-NH4HCO3-Sodium Alginate mass ratio of 4:0.2:1. Cyclic tests showed that the slow-release materials consistently released carbon (3.02 mg/g) and calcium (1.19 mg/g) over a period of 20 days. Evaluation of five EFB systems indicated EFB-5 with biofilm, slow-release materials, and modified substrate was optimal, achieving 75.2% F⁻ removal (effluent < 1 mg/L), 95.0% NO3⁻ removal, and 75.0% total phosphorus removal. Further analysis of biofilms, the Fe-Al modified substrate, and biogenic precipitates revealed that the substrate enhanced F⁻ removal through adsorption, ion exchange, and co-precipitation. Microbial contributions included increased production of extracellular polymeric substances (EPS), enrichment of fluoride-tolerant bacteria (e.g., Acinetobacter and Pseudomonas), and upregulation of the expression of carbon-nitrogen cycle-related genes (e.g., narG, narH, napA, nirK, nirS, and nosZ). In summary, this work shows great potential for practical in-situ F⁻ removal from slightly polluted river water.