Enhancement of fluoride immobilization in apatite by Al³⁺additives

Immobilization of fluoride (F⁻) in apatite using Ca(OH)₂as a mineralizer in the presence of phosphate is known to be accompanied by a stagnation period. This is caused by the formation of hydroxyapatite and/or fluoroapatite (HAp/FAp) on the surface of Ca(OH)₂, which inhibits the dissolution of Ca(OH)₂. Al³⁺additives effectively eliminated the delay period, leading to the rapid formation of apatites by suppressing the formation of CaCO₃. Zeta potential measurements clearly showed that increasing the quantity of Al³⁺additives caused not only a decrease in the initial surface charge but also a decrease in the rate of the surface charge of the solid residues during the reaction, indicating that Al³⁺additives enhanced the formation of HAp/FAp. 27Al-nuclear magnetic resonance (NMR) studies of the solid residues indicated that the predominant coordination number of Al was always hexagonal ([6]Al) and that the fraction of [6]Al increased with an increase in the molar ratio of F/Al in the solid residues, suggesting that the stable AlF₆ ³⁻complex was easily incorporated into the apatites. In addition, transmission electron microscope- energy dispersive X-ray spectroscopy (TEM-EDX) revealed a uniform distribution of Al in the apatites, which suggests that in the initial stages of the reaction, free Al³⁺ions contribute to the formation of apatite crystal seeds independent of Ca(OH)₂particles, resulting in the efficient growth of apatites containing F⁻. This result is helpful for the treatment of F⁻-bearing industrial wastewaters in practical applications by using an Al-bearing Ca source, such as ground-granulated blast-furnace slag.