Phosphate desorption kinetics from goethite as induced by arsenate

PUCCIA, V.; LUENGO, C.; AVENA, M.

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 fromgoethite has been studied with a batch reactor system and ATR-FTIR spectroscopy. The effects of arsenate concentration, adsorbed phosphate, pH and temperature between 10 and 45 ◦Cwere investigated. Arsenate is able to promote phosphate desorption 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 min and a slowstep that lasts several hours. In the slow step, arsenate ions exchange adsorbed phosphate ions in a 1:1 stoichiometry. The reaction is first order with respect to arsenate concentration and is independent of adsorbed phosphate under the experimental conditions of thiswork. The rate lawis then r = kr[As], where r is the desorption rate, kr is the rate constant and [As] is the arsenate concentration in solution. The values of kr at pH 7 are 1.87×10−5 Lm−2 min−1 at 25 ◦C and 7.95×10−5 Lm−2 min−1 at 45 ◦C. The apparent activation energy of the desorption process is 51 kJ mol−1. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surfacecomplexes. The identity of thesecomplexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reaction◦Cwere investigated. Arsenate is able to promote phosphate desorption 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 min and a slowstep that lasts several hours. In the slow step, arsenate ions exchange adsorbed phosphate ions in a 1:1 stoichiometry. The reaction is first order with respect to arsenate concentration and is independent of adsorbed phosphate under the experimental conditions of thiswork. The rate lawis then r = kr[As], where r is the desorption rate, kr is the rate constant and [As] is the arsenate concentration in solution. The values of kr at pH 7 are 1.87×10−5 Lm−2 min−1 at 25 ◦C and 7.95×10−5 Lm−2 min−1 at 45 ◦C. The apparent activation energy of the desorption process is 51 kJ mol−1. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surfacecomplexes. The identity of thesecomplexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reactionr = kr[As], where r is the desorption rate, kr is the rate constant and [As] is the arsenate concentration in solution. The values of kr at pH 7 are 1.87×10−5 Lm−2 min−1 at 25 ◦C and 7.95×10−5 Lm−2 min−1 at 45 ◦C. The apparent activation energy of the desorption process is 51 kJ mol−1. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surfacecomplexes. The identity of thesecomplexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reactionAs] is the arsenate concentration in solution. The values of kr at pH 7 are 1.87×10−5 Lm−2 min−1 at 25 ◦C and 7.95×10−5 Lm−2 min−1 at 45 ◦C. The apparent activation energy of the desorption process is 51 kJ mol−1. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surfacecomplexes. The identity of thesecomplexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reaction◦C and 7.95×10−5 Lm−2 min−1 at 45 ◦C. The apparent activation energy of the desorption process is 51 kJ mol−1. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surfacecomplexes. The identity of thesecomplexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reaction−1. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surfacecomplexes. The identity of thesecomplexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reaction