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SOIL CHEMISTRY

Impact of Structural Perturbation of Aluminum Hydroxides by Tannate on Arsenate Adsorption

^a Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Dr. Saskatoon, SK Canada S7N 5A8
^b Università di Torino, DI.VA.P.R.A.– Chimica Agraria, Via L. da Vinci 44, 10095 Grugliasco (TO), Italy
^c Institute of Soil Science, Chinese Academy of Sciences, 210008 Nanjing, China
^d Università di Napoli Federico II, Dipartimento di Scienze del Suolo, della Pianta, dell'Ambiente e delle Produzioni Animali, Via Università, 100- 80055 Portici (NA), Italy

^* Corresponding author (pmh936{at}mail.usask.ca).

The impacts of the biomolecule-induced structural perturbation of Al hydroxides and the resultant alteration of their surface reactivity toward the adsorption of nutrients and contaminants have received, to date, scant attention, in spite of their significance in determining the mineralogy and surface chemistry of these mineral colloids. This study investigated the equilibria and kinetics of As(V) adsorption on a crystalline Al hydroxide, a pure amorphous Al hydroxide and a short-range ordered Al–tannate coprecipitate. Isotherms and kinetics of As(V) adsorption were conducted at pH 6.5; the kinetic experiments (0.083–24 h) were performed at 288, 298, 308, and 318 K. The adsorption data followed multiple second-order kinetics, with an initial fast reaction step, followed by a slow reaction. While As(V) adsorption on the crystalline Al hydroxide was a rapid process, the poorly ordered minerals required longer contact intervals and greater activation energies. Compared with the pure amorphous Al hydroxide, the incorporation of tannate into the structural network of Al hydroxide decreased the adsorption rate, capacity, and affinity for As(V). These effects were attributable to the blocking of part of the adsorption sites by tannate, to the electrostatic repulsion induced by the net negative charge caused by the deprotonated organic molecules exposed on the surface of the Al–tannate coprecipitate, and to the steric hindrance of tannate, hampering access of the adsorbate to the micropores. These findings are of fundamental significance in understanding the sorption behavior and mobility of As as influenced by biomolecule-induced structural perturbation of Al hydroxides in the environment.

Abbreviations: EGME, ethylene glycol monoethyl ether • PZC point of zero charge