Removal of fluoride ions from wastewater via simple low-temperature thermal decomposition-modified phosphogypsum

Modified phosphogypsum (PG) was prepared via low-temperature thermal decomposition to remove fluoride ions from wastewater. This study discussed the effect of heat treatment (pyrolysis temperature and time) and operating conditions (initial fluoride ions concentration, dosage, solution temperature, and initial pH) on fluoride ions removal efficiency. The results showed that PG was most successful in removing fluoride at 140 ℃ for 5 h. At an initial fluoride ions concentration of 100 mg/L, a dosage of 10 g/L, a solution temperature of 20 ℃, and strongly acidic (pH=2) and neutral (pH=7) environments, the fluoride removal efficiency of the PG reached 93.9 % and 66.7 %, respectively. In addition, when the initial fluoride ion concentration was reduced (25 mg/L), the heat-treated modified PG exhibited inadequate fluoride removal (<40 %). Increasing the solution temperature and initial pH was not conducive to fluoride ions removal. SEM-EDS, TG-DTG, FTIR, XRD and XPS analyses showed that thermal decomposition at a low temperature transformed the calcium sulfate dihydrate (CaSO₄·2H₂O) in the PG into calcium sulfate hemihydrate (CaSO₄·0.5H₂O) with higher solubility, increasing the Ca²⁺ ions concentration in the solution and further removing the F^- ions via chemical precipitation. Quantitative X-ray diffraction (QXRD) analysis revealed that the concentration of CaSO₄·0.5H₂O in PG is highest when subjected to optimal heat treatment conditions (140 ℃ for 5 h). Therefore, simple low-temperature thermal decomposition treatment (140 ℃) reduced PG energy consumption and effectively removed fluoride ions from wastewater, achieving the purpose of “ waste-treating-waste”.