Phosphate desorption kinetics from goethite as induced by arsenate

VIRGINIA PUCCIA; CARINA LUENGO; MARCELO AVENA

COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS

ELSEVIER SCIENCE BV

Año: 2009 vol. 348 p. 221 - 227

0927-7757

The kinetics of the arsenate-induced desorption of phosphate from goethite has been studied with a batch reactor system and ATR-FTIR spectroscopy. The effects of arsenate concentration and temperature between 10ºC and 45ºC were investigated. Arsenate is able to promote the desorption of phosphate because both oxoanions compete for the same surface sites of goethite. The desorption occurs in two steps: a fast step that takes place in less than 5 minutes and a slow step that lasts several hours. In the slow step, arsenate ions exchange adsorbed phosphate ions in a 1:1 stoichiometry, and the reaction is first order with respect to arsenate concentration. The apparent activation energy of 51 kJ mol-1 indicates that the rate-control of the reaction is on the particle. This value, however, does not allow to discriminate between a diffusion-controlled process within the particles or a true ligand exchange reaction between soluble arsenate species and adsorbed phosphate species. ATR-FTIR spectroscopy shows that these adsorbed phosphate species are mainly bidentate inner-sphere surface complexes, and that they are always present at the surface during desorption. There is no spectroscopic evidence for the formation of intermediate species during the reaction.