Arsenate adsorption and desorption kinetics on a Fe(III)-modified montmorillonite

CARINA LUENGO; VIRGINIA PUCCIA; MARCELO AVENA

JOURNAL OF HAZARDOUS MATERIALS.

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2010 vol. 186 p. 1713 - 1719

0304-3894

The adsorption-desorption kinetics of arsenate on a Fe(III)-modified montmorillonite (Fe-M) was studied at different arsenate concentrations, pH and stirring rates. The general characterization of Fe-M by chemical analyses, XRD, TG/DTA and N2 adsorption revealed that the synthesized solid was a porous sample where Fe(III) was present as a mix of monomeric and polymeric Fe(III) species in the interlayer and on the external surface. The Fe(III) species were the responsible for arsenate adsorption. The adsorption took place in a two-step mechanism. There was an initial fast binding of arsenate to Fe(III) species at the external surface (with half-lives of 1 min or shorter) followed by a slower binding to less accessible Fe(III) species in pores and the interlayer (with half-lives of around 1 h). Desorption experiments with arsenate adsorbed at different times also reflected the presence of externally and internally adsorbed arsenate. Arsenate adsorbed at the external surface desorbed quickly, whereas arsenate adsorbed at internal surfaces desorbed more slowly. At pH 6 the maximum adsorbed arsenate was 52 mol/g, a value that is rather low as compared to adsorption on ferrihydrite (700 mol/g) and goethite (192-220 mol/g). However, since the Fe(III) content of Fe-M is much lower than that of ferrihydrite and goethite, Fe(III) species in Fe-M are more efficient in binding arsenate than in ferrihydrite or goethite. One As atom is attached every 8.95 atoms of iron in Fe-M. This high binding efficiency indicates that Fe(III) species are well spread over the montmorillonite surface, forming small oligomeric species or small surface clusters containing just a few iron atoms