Highly efficient arsenic adsorption onto Mg/Al/Fe layered double hydroxides: Kinetics, isotherm, XPS and Mossbauer spectroscopies

The adsorption of arsenate onto a Mg/Al/Fe layered double hydroxide (Fe-LDH) was studied at different anion concentration, pH, temperature and stirring rate. Different characterization techniques such as FT-IR, X-ray diffraction, XPS and Mo center dot ssbauer spectroscopies, and zeta potential measurements were used for determining the mechanism of arsenate adsorption onto the studied solid. Two mechanisms are postulated: a) anion exchange reactions with chloride and hydroxyl ions of the interlayer space, and b) ligand exchange reactions with formation of inner-sphere complexes on the Fe-LDH surfaces, with participation of Fe(III) ions from the structure. Arsenate adsorption on Fe-LDH was dependent on the initial pH and the concentration, but was independent of the stirring rate and temperature. Fe-LDH exhibited excellent As(V) removal capacity, much higher than other adsorbents reported in the literature. A dose of 10 g L-1 Fe-LDH was sufficient to reach a percentage removal higher than 95 % for a concentration of arsenic of 1 x 10-4 M found in certain places in Argentina, obtaining residual arsenate values below the recommended World Health Organization limit (0.01 mg L-1). Although the material dissolves at low pH, its high arsenate removal efficiency, the low cost and the green and sustainable characteristics of the material make it an effective adsorbent for the treatment of arsenic contaminated waters.