Uptake of fluoride and borate ions from aqueous solution by magnesium-aluminum oxide

Magnesium-aluminum oxide (Mg-Al oxide), which was prepared by the thermal decomposition of a hydrotalcite-like compound, was found to take up fluoride and borate ions in low concentrations in aqueous solution. The degree of fluoride and borate removal increased with the quantity of Mg-Al oxide and temperature. When Mg-Al oxide with a Mg/Al molar ratio of 4.0 was added to 1 mM NaF or H ₃BO₃ solution at five times the stoichiometric amount, the degree of fluoride and borate removal at 60°C for 1 h reached 92.8 and 95.7%, respectively. The fluoride and borate concentrations decreased from 20 and 10 mg L^_1 to 1.4 and 0.5 mg L^_1, respectively. Borate was thought to be removed from the solution via combination of the Mg-Al oxide and orthoborate ion, i.e., H₂BO₃^- produced with increasing pH. The fluoride and borate removal reactions could be described as first-order for the fluoride and borate concentrations. The apparent activation energies for the fluoride and borate removal were 33.8 and 51.2 kJ mol ^-1, respectively, indicating that the reaction proceeded under chemical reaction control. The apparent activation energy for borate removal was higher than that for fluoride removal, indicating that the combination of H₂BO₃^- with Mg-Al oxide was more difficult than that of the fluoride ion. This was attributable to the lower charge density of H₂BO₃^- compared to the fluoride ion.