Mg-Al oxide obtained by the thermal decomposition of Mg-Al layered double hydroxide (LDH) intercalated with CO32- (CO3·Mg-Al LDH) was found to take up fluoride from aqueous solution. Fluoride was removed by rehydration of Mg-Al oxide accompanied by combination with F-. Using five times the stoichiometric quantity of Mg-Al oxide, the residual concentration of F was decreased from 100 to 6.3mg/L in 480min, which was below the effluent standard in Japan (8mg/L). Removal of F- can be represented by pseudo-second-order reaction kinetics. The apparent rate constants at 10°C, 30°C, and 60°C were 2.3×10-3, 2.2×10-2, and 2.5×10-1gmmol-1min-1, respectively. The apparent activation energy was 73.3kJmol-1. The rate-determining step for F removal by Mg-Al oxide was consistent with chemical adsorption involving intercalation of F- into the reconstructed Mg-Al LDH due to electrostatic attraction. The adsorption of F by Mg-Al oxide follows a Langmuir-type adsorption. The values of the maximum adsorption and the equilibrium adsorption constant were 3.0mmolg-1 and 1.1×103, respectively, for Mg-Al oxide. The F- in the F·Mg-Al LDH thus produced was found to be anion-exchanged with CO32- in solution. The Mg-Al oxide after regeneration treatment had excellent properties for removal of F in aqueous solution. In conclusion, the results of this study indicated that Mg-Al oxide has potential for use in recycling to remove F in aqueous solution.
- Mg-Al oxide