The differential adsorption mechanism of hexahydrated iron and hydroxyl irons on a pyrite (1 0 0) surface: A DFT study and XPS characterization

Abstract

Abstract In this paper, we have modeled a fractured surface of pyrite (1\u202f0\u202f0). From the viewpoint of bond lengths, we inferred that the chemical adsorption capacity of Fe(III) complexes follows the order [Fe(III)(OH)]2+\u202f>\u202f[Fe(III)(OH)3]\u202f>\u202f[Fe(III)(OH)5]2−\u202f>\u202f[Fe(III)(OH)2]+\u202f>\u202f[Fe(III)(H2O)6]3+\u202f>\u202f[Fe(III)(OH)4]−\u202f>\u202f[Fe(III)(OH)6]3−. The Mulliken population analysis of the quantum chemical model reveals that bonding between the Fe(III) of complexes and the S of the pyrite surface occurs during the adsorption process. The interaction between the S of pyrite (1\u202f0\u202f0) and Fe of [Fe(III)(OH)]2+ most tends to be a covalent bonding, while the interactions between the S of pyrite (1\u202f0\u202f0) and Fe of [Fe(III)(OH)4]−/[Fe(III)(OH)5]2− most tend to be the ionic bonding and are confirmed by projected density of states (PDOS) analysis. The X-ray photoelectron spectroscopy (XPS) study of the S 2p core line reveals distinct contributions of the surface, which is determined to have contributions due to surface S2− monomers, S22− dimers, Sn2−, S0 and surface sulphate. The charge accumulation on the surface S2− monomers is conjectured to be achieved via a transfer of charge from the surface Fe2+ to the surface S2− monomers. The XPS spectra of the Fe 2p, C 1\u202fs, and O 1s also prove that the addition of organic iron compounds (Fe(III)-EDTA and Fe(III)-citrate) and an inorganic iron compound (Fe2(SO4)3) leads to oxidation of Fe2+ and sulfide species of the ruptured pyrite surface, resulting in a higher ratio of Fe3+ to Fe2+ and an increase of surface C O and COOH groups and oxides.