Impact of M. aeruginosa on fluoride removal efficiency of AlCl3 and FeCl3 coagulants and the mechanism

In our investigation, the fluoride content of surface water in northern China exceeds the standard, and accompanied by the problem of eutrophication. At present, there have been many studies on the coagulation treatment of pure fluoride-polluted waters, but the removal of fluoride in the waters containing algae has received little attention. In this paper, a typical type of cyanobacteria, M. aeruginosa, was chosen, and two coagulants (AlCl3 and FeCl3) were selected to explore the difference and mechanism of algae on the coagulation removal of fluoride. Results indicated that when AlCl3 dosage was between 1.0 mmol/L and 1.5 mmol/L, M. aeruginosa contributed to efficient fluoride removal because algae promoted the adsorption of F by increasing the distribution of Al(OH)3. In particular, M. aeruginosa increased the F removal rate by 50.22% at 1.5 mmol/L AlCl3 dosage. However, FeCl3 showed unsatisfactory effects on fluoride removal. The residual Al/Fe in the coagulated effluent showed that M. aeruginosa fixed large quantities of Fe, same with AlCl3, manifesting no selectivity in adhering metals. Besides, FeCl3 had appeared to be more advantageous in removing organic matters. In particular, the high complexation capacity between HA and FeCl3 caused the blockage and competition of the adsorption sites, which was why the presence of algal cells had no obvious promoting effect on fluoride removal by FeCl3. In addition, the HA flocs formed by using AlCl3 also had an excellent ability to adsorb F-, suggesting that other substances in waters could promote the fluoride removal, not just M. aeruginosa.