Effects of fluoride on coagulation performance of aluminum chloride towards Kaolin suspension

Although many studies have investigated the removal of fluoride by aluminum (Al) coagulation, few studies focus on the effects of fluoride on the coagulation behaviors towards colloids such as kaolin suspensions. In this study the effects of fluoride at different molar ratios of fluoride to Al (RF:Al) on turbidity removal, zeta potential, flocs growth, and residual Al levels were investigated by a series of batch experiments, and the Visual MINTEQ software was also used to illustrate the species distribution of Al and fluoride at different RF:Al levels. In insufficient Al dose ranges, fluoride showed adverse effects at RF:Al above 10:10, whereas fluoride exhibited little effect on coagulation at RF:Al below 2:10 at pH < 8. At pH > 8, little effect was observed over wide RF:Al ranges, from 1:10 to 30:30. The adverse effects of fluoride were related to decreased zeta potential, smaller flocs, and elevated levels of residual Al. At pH 6, the fluoride at RF:Al of 10:10 contributed to the decrease of the zeta potential from −1.7 to −6.7 mV, decrease of floc size from 119.7 to 29.0 μm, and increase of residual Al of 0.22 mg/L as Al. For the overdosed dose of 20 mg/L as Al, the presence of fluoride decreased residual turbidity from 22.7 to 20.6 NTU, owing to the lowered zeta potential and the inhibited re-stabilization of particles. Moreover, this study indicates that after adsorbing fluoride, the spent freshly-prepared aluminum hydroxides (in-situ Al2O3·xH2O) may be reclaimed as coagulants for colloid removal after being dissolved by acid solution. This strategy is valuable to minimize the reagent cost for the removal of fluoride by in-situ Al2O3·xH2O.