Arsenic incorporation into FeS₂ pyrite

Scientific problem

Arsenic is recognized as one of the most serious inorganic contaminants in drinking water on a worldwide basis. It is therefore essential to understand the processes involved in the mobility of this element. FeS₂ pyrite, the most abundant of all metal sulphides, plays an important role in the transport of arsenic. In reducing conditions, pyrite can delay the migration of arsenic by adsorption on its surfaces but also by incorporation. Pyrite can host up to about 10 wt % of arsenic. In oxidizing conditions, pyrite dissolves and then controls the arsenic level in solution. Its dissolution also generates acid rock drainage (Fig. 1).

Although being key information, the actual location and speciation of arsenic in pyrite remains a matter of debate. X-ray adsorption spectroscopic studies have been shown that arsenic substitutes for sulphur forming either AsS units or As₂ units. On the other hand, it has also been proposed that arsenic acts like a cation and substitutes for iron. We have then investigated the different arsenic configurations by first-principles calculations.

Methods

We have investigated the different configurations by DFT calculations (Castep code). We have considered 1 or 2 arsenic atoms in a 2 x 2 x 2 supercell of pure pyrite in order to have arsenic concentrations comparable with the natural observations (< 4 wt. % As). The arsenic has been successively placed in iron and sulphur sites. For the later case, we have investigated the three following substitution mechanisms: formation of AsS groups, formation of As₂ groups and substitution of one As atom for one S₂ group (Fig. 2). These different configurations have been compared by considering simple incorporation reactions in both oxidizing and reducing conditions.

Figure 2. Sketch of the 4 arsenic substitutions considered here. Fe, S, As are represented in green, yellow and purple respectively. The Fe-S bonds have been omitted for clarity sake.

Beside accurate geometry optimizations on supercells, many smaller calculations were done for convergence tests and especially for obtaining the total energy of all the different reaction components. This task suits perfectly with the coupled use of the eMinerals minigrid and SRB. The procedure is facilitated by the use of My_condor_submit, which automatically downloads the input files from the SRB, submits the job to the minigrid resources and uploads the output files, working from one and the same computer.

Scientific results

In both redox conditions, it is more energetically favourable to substitute arsenic for sulphur rather than for iron. Moreover the formation of AsS groups is favoured compared to As₂ group (Fig. 3) and arsenic tends to cluster in the pyrite lattice. The very local configuration is then very close to the one in arsenopyrite. During the dissolution of pyrite, the formation of sulphur vacancies will preferentially occur in the neighbouring of arsenic. The presence of this metalloid could hence have an accelerating effect on pyrite dissolution with the environmental consequences that implies.

Figure 3, Relaxed structure of the 2 x 2 x 2 supercell of pyrite containing one AsS group.

Credits

This work was carried out by Marc Blanchard, Kate Wright and Richard Catlow (Royal Institution).