Selenium adsorption on Mg–Al and Zn–Al layered double hydroxides

Layered double hydroxides (LDHs) have high anion exchange capacities that enhances their potential to remove anionic contaminants from aqueous systems. In this study, different Mg–Al and Zn–Al LDHs were synthesized by a coprecipitation method, with the products evaluated for their ability to adsorb selenite (SeO32−) and selenate (SeO42−). Results indicated the adsorption isotherm for SeO32− retention by Mg–Al and Zn–Al LDHs could be fitted to a simple Langmuir equation with the affinity of SeO32− on Zn–Al LDH higher than that on Mg–Al LDH. The adsorption trends for both SeO32− and SeO42− on LDHs were similar under the experimental conditions. The SeO32− adsorption was rapid and was affected by the initial SeO32− concentration. The quasi-equilibrium for 0.063 and 0.63 cmol/l SeO32− solutions was obtained within the first 30 and 60 min of adsorption, respectively. The maximum adsorption of SeO32− on Mg–Al LDH was higher than that of Zn–Al LDH and decreased with an increase in the LDH mole ratio of Mg/Al. The high pH buffering capacities and the SeO32− adsorption for Mg–Al and Zn–Al LDHs was a function of pH. Competing anions strongly affected the adsorption behavior of SeO32− with SeO32− adsorption increasing in the order: HPO42−<SO42−<CO32−<NO3−. The release of adsorbed SeO32− depended upon the type of competing anion in the aqueous solution. For example, with CO32− the adsorbed SeO32− could be desorbed completely from Mg–Al LDH. X-ray diffraction patterns indicated that d-spacing increased when SeO42− was adsorbed, but not with SeO32− adsorption.