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 (Al) and that the fraction of 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.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Chemical Engineering(all)
- Industrial and Manufacturing Engineering