Article pubs.acs.org/est
Occurrence of Surface Polysulfides during the Interaction between Ferric (Hydr)Oxides and Aqueous Sulfide Moli Wan,*,† Andrey Shchukarev,‡ Regina Lohmayer,§ Britta Planer-Friedrich,§ and Stefan Peiffer† †
BayCEER, Department of Hydrology, University of Bayreuth, D-95440, Bayreuth, Germany Environmental and Biogeochemistry, Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden § Environmental Geochemistry, University of Bayreuth, D-95440 Bayreuth, Germany ‡
S Supporting Information *
ABSTRACT: Polysulfides are often referred to as key reactants in the sulfur cycle, especially during the interaction of ferric (hydr)oxides and sulfide, forming ferroussulphide minerals. Despite their potential relevance, the extent of polysulfide formation and its relevance for product formation pathways remains enigmatic. We applied cryogenic X-ray Photoelectron Spectroscopy and wet chemical analysis to study sulfur oxidation products during the reaction of goethite and lepidocrocite with aqueous sulfide 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 polysulfides being the main oxidation products in the presence of both minerals, with a predominance of disulfide (S22−(surf)), and elemental sulfur. Concentrations of aqueous polysulfide species were negligible (Fe IIIOH ⇔ > Fe IIIHS + OH−
It has been postulated that the one-electron transfer between surface-associated sulfide and ferric iron (eq 6) generates sulfide radicals HS·. >Fe IIIHS → > Fe IIHS•
surf
+ (x − y)/8S8 + 4x H 2O
(6)
This species may spontaneously react with an additional HS• radical to form a surface disulfide12 (eq 7), >Fe IIHS• + HS• → > Fe IIS2 + 2H+
(7)
which may tend to further react to form polysulfides with longer chain (Sn2−, n > 2) and to elemental sulfur depending on pH.12 Note that >FeIIIOH, >FeIIIHS, >FeIIHS• and >FeIIS2 are surface species. Surface disulfide species can be regarded to be the precursor required for pyrite formation, which will trigger pyrite formation in the presence of abundant surface-associated Fe(II) either through direct combination (eq 8) or through reaction with FeS (eq 9).
3x H 2S + 2x FeOOH → (2x − 1)FeS + Fe 2 +surf + Sy + 12 −
(5)
aging
(4)
>Fe IIS2 → FeS2,precursor ⎯⎯⎯⎯→ FeS2,pyrite
where Sy+12−surf denotes surface polysulfide and Fe2+surf surfacebound Fe(II). The coefficient x reflects the number of generated S° atoms and y (0 ≤ y ≤ x) is the number of S° atoms associated with surface polysulfides. Note that under conditions where y = 0, we obtain the idealized stoichiometry:
(8) aging
FeS + S2 2 −surface → FeS2,precursor + S2 − ⎯⎯⎯⎯→ FeS2,pyrite + S2 −
(9)
Note that the FeS2, precursor is a noncrystalline form. Reaction 8 leads to FeS dissolution and subsequent reprecipitation as pyrite. HRTEM images discussed in the study of Hellige et al. support this model.3 They observed after 2 h a sulfur-rich rim coating the crystals of lepidocrocite containing domains of nano mackinawite. The coating disintegrated after 72 h of reaction and precipitated as an amorphous phase rich in Fe and S, marking the onset of the formation of pyrite. The S2− released from reaction 8 may be reabsorbed at the surface and react with remaining ferric (hydr)oxides. Moreover, surface associated polysulfides may play an overlooked role in the sulfur cycle. Polysulfides are generally regarded to exert a high reactivity and to be involved in both abiotic and biotic reactions. For instance, polysulfide species were detected as intermediates during microbial sulfur disproportionation and might even be disproportionated themselves.39 They can serve as electron acceptors for specific bacteria such as Deltaproteobacteria from soda lakes.40 Polysulfide pathway is regarded to be the important pathway of biotic oxidation of metal sulfides such as arsenopyrite.41 The chemical bonds between the metal and sulfur are broken by proton attachment, and the sulfur is then liberated as hydrogen sulfide, which could be oxidized in a one-electron step to form polysulfide species.12,41 However, only aqueous polysulfide species have been determined to date.15,16,35,37,38This study clearly demonstrates that a large amount (>50% of generated S°) and previously unknown fraction of the oxidized sulfur is stabilized as polysulfides at the mineral surface. The question arises as to what extent the occurrence of these species may help to decipher unexplained observations, such as the cryptic sulfur cycle driven by iron in the methane zone of a marine sediment.42 Our findings therefore call for a revisiting of the role of polysulfide species in abiotic and biotic sulfur cycling.
3H2S + 2FeOOH → 2FeS + 1/8S8
Implication for Sulfur Biogeochemistry. Polysulfides are regarded to be the key reactants for pyrite formation.9,16 Pyrite occurrence has been demonstrated in solutions either rich in aqueous polysulfides8,9,35 or rich in aqueous S(−II) and S8,8,36 in which aqueous polysulfides can rapidly form.14 Hellige et al. have postulated the contribution of surface bound polysulfides to pyrite formation based on HRTEM measurements and on theoretical considerations.3 They demonstrated that pyrite formation occurred as precipitation of new a phase after 1 week following the reaction between aqueous sulfide and lepidocrocite and disaggregation of iron sulfur associations under experimental conditions comparable to this study. However, they could not further resolve the nature of these species. Our study supports this hypothesis, showing that a large fraction of S can be recovered as polysulfides at the surface of the iron minerals at a low residual concentration of aqueous sulfide (
