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 Al13 or Alb in Al-based coagulants over pH 4-9 and increases in the order: PACl2 > PACl1 > AlCl3. As for PACl, the increased B from 0 to 2.4 improves fluoride removal owing to the elevated ratios of Alb. However, at further increased B to 2.8, the formation of colloidal Al species (Alc) 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 AlCl3 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 PACl due to the stronger stability of Al13 or Alb. As for AlCl3 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 Al13 content shows higher removal efficiency towards fluoride than AlCl3 does, especially at high [F]0 or under acidic pH and alkaline pH conditions.