Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation

This study investigates the effects of aluminum (Al) and F interactions on the coagulation behaviors of Al salts, i.e., AlCl3 and polymer aluminum chloride (PACl) at different basicity (B) towards fluoride. These coagulants achieve the optimum fluoride removal over pH 6-7, and PACl exhibits higher removal efficiency than AlCl3 does over pH 4-9. The removal of fluoride is positively correlated with the content of either Al-13 or Al-b in Al-based coagulants over pH 4-9 and increases in the order: PACl(2) > PACl(1) > AlC13. As for PACl, the increased B from 0 to 2.4 improves fluoride removal owing to the elevated ratios of Al-b. However, at further increased B to 2.8, the formation of colloidal Al species (Al-c) occurs and this effect adversely inhibits fluoride removal thereafter. The removal of fluoride by these coagulants increases with elevated initial fluoride concentrations ([F](0)) from 0 to 20 mg/L. At high [F](0) of 80 mg/L, fluoride removal by AlC13 decreases to as low as 13.1% owing to the formation of soluble Al-F complexes, as indicated from the high residual Al concentrations of 16 mg/L. Interestingly, this adverse effect can hardly be observed for PACT due to the stronger stability of Al-13 or Al-b. As for AlC13 and PACl, the introduction of fluoride changes the Al species distribution to some extent owing to the Al-F complexation, as indicated by the analysis of electrospray ionization mass spectrometry. The possible Al-F complexes formed fluoride and different Al species are proposed thereafter. PACl with diverse Al species and high Alb or Al-13 content shows higher removal efficiency towards fluoride than AlC13 does, especially at high [F](0) or under acidic pH and alkaline pH conditions. (C) 2016 Elsevier B.V. All rights reserved.