Simultaneous and efficient removal of fluoride and phosphate in phosphogypsum leachate by acid-modified...

The high concentration of fluoride and phosphate in phosphogypsum leachate is harmful to the environment and ecosystem. Thus, there is a need to develop feasible materials or technologies to remove both fluoride and phosphate in acidic phosphogypsum leachate. In this study, sulfoaluminate cement (SC) was used to simultaneously remove fluoride and phosphate in wastewater based on its moderate alkalinity and rich content of metal elements (Ca, Al and Fe, etc). The acidized sulfoaluminate cement (ASC) composite was prepared through modifying SC with hydrochloric acid, which can increase the specific surface areas of the raw SC, as well as the activity of the metal elements in SC. Compared with other coagulants, ASC showed excellent removal performance for fluoride and phosphate, such as higher removal efficiency, better effluent quality, and accelerated settling rate. The fluoride and phosphate removal performances of ASC herein were investigated at different dosages, pH values, coexisting substances, and initial concentrations. As a result, ASC exhibited wide pH adaptability and satisfactory selectivity for fluoride and phosphate. The possible removal mechanisms of fluoride and phosphate by ASC included chemisorption, ion exchange, and precipitation. The main end products associated with fluoride were fluorite (CaF₂), aluminum fluoride (AlF₃), and iron trifluoride (FeF₃). The main final products amid phosphate removal, on the other hand, were brushite (CaHPO₄·2H₂O), aluminophosphate ((H₃O)·AlP₂O₆(OH)₂), silicocarnotite (Ca₂SiO₄·Ca₃(PO₄)₂) and iron phosphate (Fe(H₂PO₄)₃). More importantly, ASC can effectively treat the phosphogypsum leachate at a wide range of concentrations, and the concentrations of phosphate and fluoride in the effluents were lower than 0.5 mg P L^-1 and 4 mg L^-1, respectively. To sum up, ASC is a competitive candidate to treat wastewater with high fluoride and phosphate content, such as phosphogypsum leachate.