Effects and behaviors of Microcystis aeruginosa in defluorination by two Al-based coagulants, AlCl3 and Al13.

Abstract The complexity of natural water made it difficult to remove fluoride. Based on the environmental problems found in the investigation, the fluoride removal research in the water containing algal cells was carried out. In this study, AlCl3 and [AlO4Al12(OH)24(H2O)12]7+ (Al13) were used to remove fluoride. Additionally, the role of aluminum speciation in fluoride removal and the effect of Microcystis aeruginosa on the fluoride removal by different aluminum species coagulants were elucidated. The results showed that AlCl3 mainly removed fluoride by physical interactions, surface adsorption and enmeshment. When algal cells were added to the system, the fluoride removal rate increased from 22.75 % to 72.99 % at a dosage of 40.0 mg/L. This was because algal cells greatly increased the distribution of Al(OH)3 in the flocs. In particular, the specific surface area of the flocs containing algal cells reached 160.77 m2/g, which allowed more fluoride to be adsorbed. However, excessive Al3+ led to serious damage to algal cells and release of intracellular organic matter (IOM), worsening the effect of defluoridation. F− and Al3+ formed AlF2+ and AlF2+ via complexation in water. These compounds were not conducive to defluoridation. Al13 removed fluorine mainly through ion exchange, substitution and hydrogen bonding. Algal cells had an inhibitory effect on defluorination, which was observed in the process of coagulation by different Al dosages. Al13 achieved agglomeration of algal cells and generated small and dense flocs through charge neutralization and electrostatic patch mechanism. Once Al13 combined with algal cells and algae organic matter (AOM), the reaction between Al13 and fluoride would be weakened. Al13 not only maintained the defluoridation performance, but also did not damage the integrity of algal cells, even at high dosages.