Fluoride-rich wastewater treatment by ballast-assisted precipitation with the selection of precipitants and discarded or recovered materials as ballast

Abstract

Fluoride is commonly an unwanted by-product in industrial processes becoming highly concentrated at the effluent reaching 1,000 to 50,000 ppm F-. However, most of the defluoridation studies focused on the treatment of low concentrations of fluoride <100 ppm. Thus, traditional precipitation-coagulation by calcium was previously utilized to efficiently defluoridate very high initial concentrations. However, it produced non-settleable solids at initial concentrations between 500 and 5,000 ppm F-. To overcome these drawbacks, the ballast-precipitation was simulated under laboratory scale with a volume of 500-mL containing synthetic wastewater (4,000 ppm F-). The preliminary investigations focused on the comparison of different precipitants previously reported (Al, Ca, Mg, Zr, and Fe). Moreover, the recovered materials from wastewater treatment (CaCO3 and CaHPO4) and discarded industrial solid waste (Al2O3, FeOOH, and SiO2) have been investigated as potential ballast. Optimum defluoridation using different precipitants was also found at different pH. The results showed that Ca remained the most feasible precipitant with the highest defluoridation efficiency (Ca > Fe > Al > Zr > Mg). Meanwhile, among the discarded and recovered materials use for ballast CaCO3 demonstrated the highest defluoridation efficiency (CaCO3 > Al2O3 > CaHPO4 > SiO2 > Fe2O3). Calcite did not only act as ballast but also a precipitant by providing additional calcium ions through leaching. The defluoridation by ballasted precipitation could efficiently remove fluoride by 99% and reduced turbidity >90%. Moreover, the parameter correlation could be fitted in a quadratic equation (r2 from 0.8763 to 0.9999). The coefficients were used to estimate the significance of each parameter (showing pH > Ca/2F ratio > ballast dosage).