Materials Today Communications 32 (2022) 103868 Available online 18 June 2022 2352-4928/© 2022 Elsevier Ltd. All rights reserved. Oxidation behaviour of sperrylite and platarsite (100) surfaces: A DFT study Bradley Nemutudi, Peace P. Mkhonto * , Phuti E. Ngoepe Materials Modelling Centre, University of Limpopo, Private Bag X1106, Sovenga 0727, South Africa A R T I C L E INFO Keywords: DFT Sperrylite Platarsite Oxidation Adsorption energies Electronic structures ABSTRACT The density functional theory (DFT) was employed to investigate the oxidation mechanism and electronic structures of sperrylite (PtAs 2 ) and platarsite (PtAsS) (100) surface. The cluster expansion (CE) and virtual crystal approximation (VCA) methods were used to construct the PtAsS models. The PtAsS constructed model by CE possessed symmetry of P2 1 3, while the VCA maintained the Pa-3 symmetry. The computed surface energies and morphologies for all surface models, depicted the (100) plane the preferred cleavage. The oxidation mechanisms of the (100) surfaces of sperrylite and platarsite favoured the mono atomic oxygen bonding, which resulted from the dissociation of the O 2 molecule on the surfaces. The adsorption energies were more exothermic for PtAs 2 (100) surface oxidation (–315.82 kJ.mol –1 ), suggesting that sperrylite highly oxidises than the platarsite mineral. The PtAsS (CE) (100) surface oxidises stronger than the PtAsS (VCA) (100) surface, which is owed to the less (100) surface stability of the PtAsS (CE) than for PtAsS (VCA). The partial density of states (PDOS) and Bader charges indicated that the adsorbed As, S and As/S atoms donate electrons to the O 2 atoms. These fnding clearly illustrated that the arsenide PGMs primarily oxidises through the As sites for sperrylite and As and S sites for platarsite. Therefore the study has demonstrated the oxidation behaviour of the sperrylite and platarsite plat- inum group minerals (PGMs) and unravelled the surface hindrance for collector interaction during foatation which results in poor foatability of arsenide PGMs. 1. Introduction South Africa is one of the leading countries with high percentage of platinum group minerals (PGMs) in the igneous intrusion of Platreef Bushveld Complex [1]. There are about 21 % of arsenides and 19 % sulphides of PGMs in the Platreef, which are mainly composed of plat- inum (Pt) and palladium (Pd) [2]; and it is clear that the arsenides host the largest amounts of precious minerals. The dominant arsenides PGMs are sperrylite (PtAs 2 ) and platarsite (PtAsS) and their oxidation behav- iour has not been given much attention, in particular from computa- tional aspects. However, there are a few experimental studies that exist on oxidised PGM ores from the Bushveld Igneous Complex of South Africa [3–6]. There are extensive experimental researches reported on the oxidised PGM ores from the Great Dyke of Zimbabwe [7–11]. It has been reported that the oxidised PGM ores typically exist near the surface (i.e. 15–30 m deep), which are altered and oxidised as a result of exposure to atmospheric conditions and acidic ground waters. In the arsenides PGMs, the dissolution of arsenic (As) increase the pollution of the environment and therefore their oxidation is of key importance. The major process occurring during oxidation of PGM ores is thought to be the decomposition of base metal sulphides and forma- tion of iron oxyhydroxides. Consequently, the PGE in oxidised ores exist as: (1) relict primary PGM, (2) secondary PGE alloys, (3) solid solution with relict base metals (BMS), (4) PGM oxides/hydroxides and (5) PGE hosted by secondary oxides/hydroxides or silicates [8,10]. Clearly there is no exact occurrence on the oxidation of the PGMs. The arsenides minerals are generally found to have poor foatability compared to sulphides and their hard to foat behaviour is still not unravelled. The poor foatability of arsenides is worsened in the oxidised PGMs ores and attempts of using conventional fotation methods achieved poor re- coveries (< 50 %) hindering the commercial exploitation of these re- sources [4,6,10]. It has been reported that pyrite containing As is more reactive which may be associated with As p-type conductivity [12,13]. The As competing with Pt atoms in reactivity may be the reason for poor foatability. The platarsite have been reported to easily oxidize at a low pH, similar to arsenopyrite (FeAsS) [14]. As such, oxidation of PGMs, in particular arsenides minerals surface is of outstanding practical impor- tance and requires theoretical fundamental understanding of surface chemistry. The oxidation of arsenopyrite similar to platarsite has slow * Corresponding author. E-mail address: peace.mkhonto@ul.ac.za (P.P. Mkhonto). Contents lists available at ScienceDirect Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm https://doi.org/10.1016/j.mtcomm.2022.103868 Received 25 April 2022; Received in revised form 9 June 2022; Accepted 15 June 2022