Removal of arsenite from aqueous solutions by anionic clays

Layered double hydroxides are antitype 2/1 clay minerals that can be synthesized rapidly under laboratory conditions. Due to their high anion exchange capacities, layered double hydroxides have been investigated as potential adsorbents for removal of anionic contaminants from aqueous systems. In this study, uncalcined and calcined layered double hydroxides were prepared and characterized by X-ray diffraction, with the products evaluated for their ability to adsorb As(III) in aqueous solutions. Results indicated that As(III) could be adsorbed on chloride layered double hydroxide and calcined layered double hydroxide, but no adsorption occurred for carbonate layered double hydroxide. The adsorption isotherms of As(III) on chloride layered double hydroxide and calcined layered double hydroxide were typical L and H-type curves, respectively. The adsorption of As(III) on calcined layered double hydroxide was a slow process and reached a quasi-equilibrium after a 20 hr reaction time. The layered double hydroxides had high pH buffering capacities and the As(III) adsorption on calcined layered double hydroxide was a function of pH. Competing anions strongly affected adsorption, with As(III) adsorption increasing in the order: HPO(2-)4 < SO(2-)4 < CO(2-)3 < F- < Cl- < Br- approximately equals I- < NO(-)3. Adsorbed As(III) on calcined layered double hydroxide could be desorbed by different anions, but there was no systematic relationship between As(III) desorption and anion affinities for the calcined layered double hydroxide. Calcination immobilized the As(III) adsorbed on calcined layered double hydroxides. Although layered double hydroxides could be recycled and used as an adsorbent, its adsorption efficiency was reduced with successive treatments.