Propriedades termodinâmicas, estruturais e eletrônicas da hematita, goethita e ferridrita e o efeito das substituições isomórficas por alumínio nos processos de adsorção de água e arsenato revelados por cálculos DFT.

Abstract

Hematite (Hem, α-Fe2O3), goethite (Goe, α-FeOOH), and their aluminum isomorphous corundom (Cor, α-Al2O3) and diaspore (Dia, α-AlOOH), besides ferrihydrite (Fh, Fe5O8H), are oxyhydroxides present in latosols and responsible for several processes of environmental and geochemical interest. These materials are also used as adsorbents for the removal and immobilization of arsenic present in the tailings of gold and other noble metal ores. Computational calculations of electronic structure based on Density Functional (DFT) and Plane waves (PW) were performed for these materials to estimate the electronic and thermodynamical properties and the effect of the aluminum isomorphic substitutions involved in water and arsenate adsorption. Hem, Goe, and Cor were revisited and compared with the calculations for Dia. The results demonstrated that the adsorption energy of water follows the tendency Cor > Hem > Goe > Dia, occurring preferentially through a dissociative mechanism. Dia and Goe present lower water adsorption energies compared to the correlated Cor and Hem. The thermodynamic process of isomorphic substitution of iron by aluminum is not favorable in the used level of theory. These results indicate that the substitution process is kinetically controlled, and this explain the large ratios of isomorphic substitution reported in the literature. The phase stability of the substituted system was investigated by varying the chemical potential of water to show that the formation of Hem-Cor from the Goe-Dia phase is favorable. Fh is a nanometric mineral of difficult analyses due to its high degree of disorder. An adequate computational model was proposed, and it is coherent with the 6L phase available in the crystallographic database. The thermodynamic analysis of the isomorphic substitution of iron by aluminum in Fh is favorable up to molar ratio 8.5%. The As(V), H2AsO4 - e HAsO4 2-, adsorption process in the mineral surfaces was investigated. The bidentate-binuclear mechanism for the adsorption is the most stable for As(V), with an estimated value of 18 kJ mol-1 more stable than other adsorption sites. The occurrence of aluminum substituting the iron favors the adsorption of As(V). The results demonstrated that the water and As(V) adsorption energies are in the same order of magnitude as Hem, Cor, and Fh. The adsorption of As(V) in Goe and Dia is more favorable than the water adsorption. This thesis represents an important advance for the understanding at a molecular level of the oxyhydroxides of iron and aluminum and their chemical reactivity concerning the water and As(V) adsorption.