Fixação da espécie trivalente de arsênio em oxi-hidróxidos de ferro e de alumínio: avaliação de mecanismos moleculares e suas implicações na mobilidade do arsênio no meio ambiente

Abstract

The mechanisms of aqueous As(III) species immobilization on gibbsite and hematite have been evaluated as a function of pH and coverage level, respectively. For the As(III)- gibbsite system, theoretical calculations and X-ray absorption fine structure spectroscopy (XAFS) were combined to elucidate the structure of arsenite surface complexes on the synthetic oxide. Several adsorption complexes have been evaluated using the selfconsistent charge corrected density-functional based tight-binding (SCC-DFTB) method The bidentate-binuclear/acid-base complex (bb/ab) was found as the most stable geometry for As(III) bonding to gibbsite, showing As-O and As-Al distances of 1.75 and 3.24 Å, respectively. EXAFS results confirmed SCC-DFTB estimates with 3 oxygen atoms in the first shell, at a distance of 1.77 Å, and to 2 aluminum atoms in the second shell, at a distance of 3.21 Å, in a bidentate-binuclear configuration, at all pH evaluated (5.0, 7.0 and 9.0). Regarding the As(III) on hematite, the mechanism was elucidated by combining spectroscopic data (Raman and EXAFS) with theoretical calculations. Raman analyses were first carried out in samples of hematite with different As(III) coverage levels (varying from 0.0045 mmol m-2 to 0.014 mmol m-2) at pH 7.0. Theoretical calculations were then performed considering different possibilities for As(III) linkage on hematite, including monodentate and bidentate sorption complexes and the precipitation of claudetite, a monoclinic As2O3 phase, on the mineral surface. This later was the configuration that best elucidated Raman results, and thus it was used as a model to fit the EXAFS data collected for As(III) immobilized on hematite at the same conditions used in Raman analyses. EXAFS results confirmed the calculations, demonstrating that As(III) may form a claudetite-like precipitated on the surface of hematite with 3 oxygen atoms at a distance of 1.74 Å, 2 arsenic atoms at a distance of 3.28 Å, and 2 iron atoms at a distance of 3.54 Å. Our results demonstrated that: (i) like As(V), As(III) species preferably link to gibbsite by means of inner-sphere complexation, in a pH range of 5-9, supporting the hypothesis that As(III) mobility is related to the reversible character of As(III) surface reactions rather than to the weakness of these interactions. Based on this premise, this work also suggested a mechanism for As(III) remobilization from oxides, when adsorption is the main mechanism of fixation, by means of protonation of the bb/ab adsorbed complex at acidic media, releasing the neutral H3AsO3 molecule; (ii) the feasibility of a claudetite-like precipitation on hematite could explain the similar Raman spectra for As(III) and As(V) species on iron oxy-hydroxides and show that As(III) species may precipitate in the presence of Fe(III) species only. Therefore, the present work improves the knowledge about the mode of As(III) interactions with aluminum and iron oxy-hydroxides. Finally, from our results it was possible to suggest alternatives to control As(III) mobility in Al and Fe-rich soils.