Saturday, 15 July 2006

Structural Perturbations of Aluminum Hydroxides and the Impact on their Kinetics of Arsenate Adsorption.

Maria Martin1, Guifen Yu2, Elisabetta Barberis1, Antonio Violante3, Leonard M. Kozak4, and P. M. Huang5. (1) DIVAPRA, Univ of Turin, Grugliasco (Torino), via L. da Vinci 44, Grugliasco, 10095, Italy, (2) Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Street, Nanjing, 210008, China, (3) Univ of Naples FedericoII, Portici (Napoli), Via UniversitÓ 100, Naples, 80055, Italy, (4) Dep. of Soil Science, University of Saskatchewan, Saskatoon, SK Canada, 51 campus drive, saskatoon, SK S7N 5A8, Canada, (5) Dept of Soil Science, Univ of Saskatchewan, 51 campus drive, Saskatoon, SK S7N 5A8, Canada

Structural perturbations of Al hydroxides have been well documented. However, little is known on the impact of these structurally perturbed Al hydroxides on their adsorption behaviour of arsenate, which is a toxic compound common in natural environments. The objective of this study was to investigate the kinetics of arsenate adsorption on Al hydroxides with varying degrees of crystallinity and formed under the influence of tannate. The adsorption isotherms show that the arsenate adsorption maximum values at pH 6.5 and temperature of 298 K after 24 h of equilibration followed the order: pure amorphous Al hydroxide (290.8 Ámol As g-1) > amorphous Al hydroxide formed at a tannate/Al molar ratio of 0.1 (203.7 Ámol As g-1) > crystalline Al hydroxide (140.5 Ámol As g-1). The reaction between 0 and 5 min was too fast to be investigated by conventional kinetic methods. During the first 5 min reaction period, however, more than 50% of the arsenate adsorbed at equilibrium was adsorbed by the pure amorphous Al hydroxide and by the crystalline Al hydroxide; by contrast, only 14% of the arsenate was adsorbed by the amorphous Al hydroxide formed under tannate perturbation. Six kinetic models, i.e., the zero-order, first-order, second-order, Elovich, parabolic diffusion and power law equations were used to fit the experimental data. Based on the r2, P, and standard error values, the first-order equation was selected to determine the rate constants of arsenate adsorption by these oxides at 288, 298, 308 and 318 K. The adsorption reactions were a multi step process involving an initial fast reaction (5-30 min) followed by a slow reaction (40-720 min). The rate constants for the arsenate adsorption followed the order: pure amorphous Al hydroxide > amorphous Al hydroxide structurally perturbed by tannate ≥ crystalline Al hydroxide. The results are related to steric hindrance in relation to nanoporosity development and electrostatic repulsion caused by the exposed carboxylate groups, and to the enhancement of the exposure of reactive sites through structural perturbation. The rate constant of arsenate adsorption in both the fast and slow reactions can be interpreted in terms of activation energy and collision frequency. The impacts of organic-induced structural perturbation of Al hydroxides on the transformation and transport of As in the terrestrial environment warrant in-depth study for years to come.

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