Fluoride removal from water by composite Al/Fe/Si/Mg pre-polymerized coagulants: Characterization and application

Fluoride, an anionic pollutant, is possibly to be found in excessive concentrations especially in groundwaters and can show detrimental effects on human health, in concentrations higher than the commonly applied legislation limit of 1.5 mg/L The most commonly applied method for water de-fluoridation is performed by Al-based coagulants, which however presents some important limitations, such as the applied relatively high dosage, producing rather excessive amounts of chemical sludge. In this study, the use of novel pre-polymerized Al-based coagulants was examined, regarding their efficiency towards fluoride removal, as compared with the conventionally applied AlCl₃. The novel coagulants were characterized by measuring the main physico-chemical properties, the aluminum species distribution, the zeta potential, the particles' size distribution and the produced flocs' sizes. The results showed that the Mg-containing coagulant (PSiFAC-Mg_30-10-15) was the most efficient, when applied in pH values relevant to fluoride-containing groundwaters; it was also the only coagulant, which increases its efficiency at pH values > 7. The uptake capacity of coagulants, regarding fluoride, to reach the residual/equilibrium concentration limit of 1.5 mg F/L (Q_1.5-value) at the pH value 7.0 ± 0.1 were found 170, 134 and 94 mg F/g Al for the cases of PSiFAC-Mg_30-10-15, AlCl₃·6H₂O and PSiFAC-Na_1.5-10-15, respectively. Accordingly, at the pH value 7.8 ± 0.2 the Q_1.5-values were found 189, 118 and 41 mg F/g Al for the same coagulants; whereas considering the residual aluminum concentration this was ranged at 15 ± 5, 25 ± 5 and 30 ± 5 μg Al/L, respectively. In addition, (beneficial) increase of residual magnesium concentration, when applying the coagulant PSiFAC-Mg_30-10-15 was 15 ± 5 mg/L.