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. (C) 2001 Elsevier Science B.V. All rights reserved.