Computational simulation of the pyrite/arsenopyrite interface- galvanic effects in chemical reactivity of minerals = Simulação computacional da interface pirita/arsenopirita- efeitos galvânicos na reatividade química de minerais

Abstract

Galvanic effects play an important role in the oxidation mechanism of sulfide minerals. Pyrite (FeS2) is the most common sulfide mineral in the Earth, and in the presence of arsenopyrite (FeAsS) its oxidation is delayed and the oxidation rate of arsenopyrite is increased, releasing As(III) and As(V) species in the medium. These arsenic ions are anenvironment hazards and become health problem in the vicinities of the mining regions. We report a DFT/plane waves study of the pyrite/arsenopyrite galvanic cell (Chapter 3). To build the interface models we tested the commensurability of the pyrite (100) surface (the pyrite surface with highest occurrence in the nature) with the twelve arsenopyrite surfaces reported in the literature. Among all evaluated surfaces the most stable interfaces were built, and their structure, stability, and electronic properties were evaluated. As pyrite and arsenopyrite have different (i) bond distances and (ii) crystal lattice parameters, it is not possible to perfectly match the phases, and distorted octahedral sites are formed in the interfacial region. Consequently, strained structures are observed. This structural observation agrees with the word of adhesion and the formation energy analysis. Both parameters suggest that the formation of FeS2/FeAsS interface is not favourable. This is in agreement with what is observed in natural samples. The local reactivity of the interfaces was evaluated by water adsorption energies (Chapter 4). Comparable adsorption values were found among the interfaces (FeS2/FeAsS) and isolated phases (FeS2 and FeAsS). This suggests that the possible interface formation would occur without modifying the Lewis acid-base properties of the active sites in the event of material rupture. Band offset (Chapter 5) calculations for the most stable interfaces showed that their band gap values are at least three times lower than the values found for the pure pyrite or arsenopyrite. Since sulfide minerals are semiconductor materials that can participate in electrochemical reactions, the decrease in the band gap thus facilitates electron transfer at the pyrite/arsenopyrite interface. This fact contributes to explain why the galvanic interactions increase the oxidation of arsenopyrite in the presence of pyrite. The present study underlines the importance of galvanic effects to understand the oxidation mechanism of arsenopyrite in the presence of pyrite. It was developed a methodology capable of simulating the water/pyrite interface (chapter 6). It is known in the literature that the DFTB2 method does not describe well static and dynamic properties of liquid water. Therefore, the DFTB2 parameters were reparametrized from an empirical correction of the DFTB2 repulsion energy by constructing the parameters labelled in this thesis as water-matsci. Radial functions of type gO-O and gO-H were described in agreement with experimental data. In addition, the new parameters are capable of describing the tetrahedrality of the water molecules at temperatures 298 and 254 K. Finally, it was observed that the modification of the parameters does not modify the water adsorption energy at the pyrite surface (100). All calculations of a new route for the adsorbed SO2 oxidation extend the knowlegment about the pyrite oxidation mechanism (Chapter 7) As far as we know, only the initial stages of the pyrite oxidation process have been made from theoretical calculations. According to our calculations, it is observed that the dissociation of SO2 species can occurs on the surface. The activation energy (0.44 eV) was estimated to be approximately two times the activation energy of the limiting step of the global reaction (0.80 eV), suggesting that this species can be formed on the pyrite surface. Moreover, the SO2 species leads to the formation of adsorbed SO3 and SO4 species on the surface with activation energies lower than 0.16 eV (this value is 5 times less than the rate limiting step). Another important aspect of the mechanics proposed by our calculations is the fact that oxygen molecule is capable to react with the SO2 species on the surface as it was suggested by experimental analyses, but up to date, it was not theoretically calculated. The investigated route for the adsorbed SO2 oxidation by the molecular oxygen was labelled as Type IIIreactions that occur on the pyrite surface.