|Wan, M; Shchukarev, Andrey; Lohmayer, Regina; Planer-Friedrich, Britta; Peiffer, S (2014): Occurrence of surface polysulphides during the interaction between ferric (hydr)oxides and aqueous sulphide, Environmental Science & Technology, 48(9), 5076–5084, DOI: 10.1021/es405612f [Link]|
Polysulphides are often referred to as key reactants in the sulphur cycle, especially during the interaction of ferric (hydr)oxides and sulphide, forming ferrous-sulphide minerals. Despite their potential relevance, the extent of polysulphide formation and its relevance for product formation pathways remains enigmatic. We applied cryogenic X-ray Photoelectron Spectroscopy and wet chemical analysis to study sulphur oxidation products during the reaction of goethite and lepidocrocite with aqueous sulphide at different initial Fe/S molar ratios under anoxic conditions at neutral pH. The higher reactivity of lepidocrocite leads to faster and higher electron turnover compared to goethite. We were able to demonstrate for the first time the occurrence of surface-associated polysulphides being the main oxidation products in the presence of both minerals, with a predominance of disulphide (S22-(surf)), and elemental sulphur. Concentrations of aqueous polysulphide species were negligible (< 1%). With prior sulphide fixation by zinc acetate, the surface-associated polysulphides could be precipitated as zero-valent sulphur (So), which was extracted by methanol thereafter. Of the generated So, 20-34 % were associated with S22-(surf). Varying the Fe/S ratio revealed that surface polysulphide formation only becomes dominant when the remaining aqueous sulphide concentration is low (<0.03 mmol L-1). We hypothesize these novel surface sulphur species, particularly surface disulphide, to act as pyrite precursors. We further propose that these species play an overlooked role in the sulphur cycle.