Fluoride Removal from Low-Temperature and Low-Turbidity Waters by In Situ Flocs Formed by New Integrated Coagulation–Adsorption Technology

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Low-cost and effective F− removal technology and operational parameters were provided. A novel bimetallic fluoride-removing coagulant (BFrC) was synthesized. Simultaneous removal of fluoride (F−) and turbidity were studied using a new integrated coagulation–adsorption technology based on BFrC (BFrC-“Coagulation-driven adsorption” system (BCS)) in treating low-temperature and low-turbidity waters, in which a “Coagulation-driven adsorption” was the key point. Optimal conditions of the BCS were determined, and Zeta potential and meso- and micro-scopic morphologies were characterized. The results showed BFrC was hydrolyzed and hydroxylated to form positively charged flocs, and BCS reduced residual F− concentration to <1 mg/L. The in situ flocs formed in the BCS (BCS-flocs) gave a very large surface area, providing abundant adsorption sites and channels for F−. Neutral water environment (7.5–8) was conducive to remove F−, while neutral and alkaline conditions are beneficial for turbidity removal. The BCS exhibited strong adaptability to varying water temperatures and turbidity levels. Higher initial turbidity in test waters improved F− removal, while elevated initial F− levels linearly reduced F− removal. Divalent anions (SO42−) inhibited defluoridation more significantly than monovalent anions (Cl− or HCO3−). The BCS-flocs effectively removed F− and turbidity simultaneously, in which F- removal was achieved through adsorption on the flocs, maybe including a combination among initial chemical adsorption, subsequent physical adsorption, and complex composite adsorption, and turbidity removal mainly relied on a multi-mechanism process, including double-layer compression, charge neutralization, and sweep flocculation.

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