Preparation of a novel bimetallic fluoride-removing coagulant and its defluorination performance in an integrated coagulation-adsorption process

Using jar tests, a novel bimetallic fluoride-removing coagulant (BFrC) composed of iron and aluminum was prepared by a two-stage progressive orthogonal test in treating a synthesized water with low-temperature and low-turbidity containing fluoride, and the impact of dilution degree on its particle size and charge characteristics were investigated, respectively. Compared with poly-Si-Fe coagulant, the fluoride and turbidity removal by both liquid BFrC and its solid sample were evaluated in an integrated coagulation-adsorption process, along with the comparison of their mesoscopic morphology. The results indicated that the optimal iron/aluminum, iron/polyaluminum and copolymerization temperature for BFrC preparation were 0.5, 0.8, and 20 ℃, respectively. When BFrC was diluted to 0.012 5 mg/L (as “iron+ aluminum”), moderate primary hydrolysis was achieved, forming flocs with a reasonable size distribution and strong adsorption capacity. When BFrC dosage was 18.5 mg/L, fluoride concentration was reduced to<1 mg/L, corresponding to the removal rate of 58.4%, while the greatest fluoride removal by poly-Si-Fe coagulant was only 11.3%, though giving a higher turbidity removal than BFrC. Liquid BFrC gave similar fluoride removal to solid BFrC, but the former posed a higher turbidity removal due to its larger flocs having better settling performance. The larger flocs formed by liquid BFrC exhibited strong aggregative properties, but the size of smaller flocs in liquid BFrC was significantly larger than that in solid BFrC, but having lower number. The filtration almost gave no further removal of fluoride, but showed a higher turbidity removal.