Kinetics and equilibrium studies on Mg–Al oxide for removal of fluoride in aqueous solution and its use in recycling

Abstract

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 6.3 mg/L in 480 min, which was below the effluent standard in Japan (8 mg/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 × 10−1 g mmol−1 min−1, respectively. The apparent activation energy was 73.3 kJ mol−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 3.0 mmol g−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.