Structural model of arsenic(III) adsorbed on gibbsite based on DFT calculations Augusto F. Oliveira a,d , Ana Cla ´udia Q. Ladeira b,c , Virgı ´nia S.T. Ciminelli b , Thomas Heine d , He ´lio A. Duarte a, * a Departamento de Quı ´mica, Universidade Federal de Minas Gerais, 31.270-901 Belo Horizonte, MG, Brazil b Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais, 31.270-901 Belo Horizonte, MG, Brazil c Centro de Desenvolvimento da Tecnologia Nuclear-CDTN, 31.270-010 Belo Horizonte, MG, Brazil d Institut fu ¨r Physikalische Chemie und Elektrochemie, TU Dresden, D-01062 Dresden, Germany Received 14 July 2005; accepted 11 August 2005 Available online 9 January 2006 This paper is dedicated to the 60th birthday of Annick Goursot. Abstract It is well known that As(III) has a higher mobility compared to As(V) in the environment. This fact is explained by the high reversibility of As(III) adsorption on minerals such as Al (hydr)oxides. Nevertheless, experimental results pointed out that adsorption of As(III) leads to inner- sphere complexes, which appears to be in conflict with the high mobility of these species as inner-sphere complexes should be strongly attached. In this work we used density functional methods and cluster models to study two different mechanisms for the H 3 AsO 3 adsorption on gibbsite-g- Al(OH) 3 , one of the most abundant aluminum hydroxide minerals. Our results show that, differently from the As(V) case, As(III) is not adsorbed via an acid/base, but by a non-dissociative mechanism in which O–H bonds are not being broken. This non-dissociative mechanism also conciliates the high remobilization of As(III) with the apparently inconsistent formation of inner-sphere adsorption complexes. q 2005 Elsevier B.V. All rights reserved. Keywords: Gibbsite; Arsenic; As(III); H 3 AsO 3 ; Adsorption; DFT; Cluster model 1. Introduction Arsenic contamination is a matter of concern over the world, especially after the severe human contamination in India and Bangladesh [1,2]. The predominant water-soluble species are the As(III) and As(V) derivatives of the arsenous (H 3 AsO 3 ) and arsenic (H 3 AsO 4 ) acids, respectively. The trivalent species is of great environmental concern not only due to its relatively high toxicity, but also in view of its higher mobility in soils. In addition to that, even under oxidative conditions both arsenic species occur together due to the fact that As(III) oxidation to As(V) is a kinetically slow process. Surface adsorption reactions, mainly on ferric and aluminum oxides, may retain As in a relatively immobile form. Therefore, together with iron hydroxides, aluminum oxides in nature are ubiquitous and greatly responsible for trace element mobility in natural systems. It is important to recognize that the environmental impacts caused by As(III) mobility are not related to the lack of adsorption on soil constituents but mainly to the reversible nature of the reaction. Previous work [3] demonstrated a significant uptake of arsenic by gibbsite: 4.6 mg g K1 for As(V) and 3.3 mg g K1 for As(III). Desorption of arsenic was shown to vary drastically with arsenic speciation. While only 1–2% of As(V) was leached from samples, leaching the As(III) reached up to 32% in the presence of sulfate ions. Except for the sulfate ions, no significant differences were observed with respect to the As(III) leached by water or concentrated chloride and nitrate salt solutions. A clear lack of significant effect of ionic strength of the aforementioned leaching solutions favors the hypotheses of inner-sphere complexes. Recent reports on As(III) adsorption on different Al derived mineral phases have presented contrasting results. Goldberg and Johnston [4] postulated that A(III) exhibits only a weak affinity for amorphous H 3 AsO 3 , which can not be discerned by IR or Raman spectroscopic methods, thus forming an outer- sphere complex. Weerasooriya et al. [5] proposed that As(III) Journal of Molecular Structure: THEOCHEM 762 (2006) 17–23 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2005.08.038 * Corresponding author. Tel.: C55 31 3499 5748; fax: C55 31 3499 5700. E-mail address: duarteh@ufmg.br (H.A. Duarte).