Interaction between ferric hydroxides and dissolved sulfide in anoxic aquifers: Pathway and kinetics of iron and sulfur products formation

The interaction between ferric iron (Fe(III)) and sulfide (S(-II)) is of great importance in permanent or temporary anoxic environments. It is a major component of the biogeochemical cycling of both iron and sulfur and eventually of carbon. Electron transfer between ferric iron and sulfide leads to elemental sulfur and metastable iron sulfide minerals and finally to a thermodynamically more stable iron sulfide, which is pyrite. Pyrite formation, as a significant early diagenesis process, plays an undeniable role as a global sink for the elements Fe and S. Therefore, the interaction of ferric iron and sulfide, especially pyrite formation, has been studied for quite a long time. However, questions remain regarding the role of electron transfer between ferric iron and sulfide on the intermediates and pyrite (end product) formation. The objectives of this dissertation are therefore i) to understand the influence of chemical properties of ferric hydroxides on the kinetics of electron transfer between S(-II) and Fe(III) and eventually on the formation of different intermediated products, ii) to characterize the properties and the fates of intermediate products, iii) to study the pathway and kinetics of the end product, i.e. pyrite formation and, iv) to develop a framework of sulfide reacting with ferric iron at the surface or near-surface regime. We investigated the reactions between aqueous sulfide and ferric hydroxides at neutral pH in an anoxic glove box. The initial ratio of Fe/S was adjusted to be ‘high’ (HR) where excess ferric iron remained after a complete consumption of sulfide and ‘low’ (LR) where excess sulfide remained after a complete consumption of ferric iron. Species were examined with wet chemical analysis as well as solid phase analytic methods including Transmission Electron Microscopy (TEM), Mössbauer spectroscopy and X-ray photoelectron spectroscopy (XPS). Results indicate complex interactions between ferric iron and sulfide. Wet chemical analysis suggests different dynamics in HR and LR experiments. In all experiments sulfide was oxidized within the first 3 h, and a pool of acid extractable ferrous iron (Fe(II)HCl) and methanol extractable sulfur (MES) built up. In HR experiments a decrease of Fe(II)HCl and MES, which was accompanied by pyrite formation, occurred after 24 - 48 h. By contrast, no pyrite formation was observed up to 2160 h in the LR experiments. A significant fraction of generated Fe(II)HCl, could not be recovered as stoichiometric FeS (or mackinawite), which is consistent with previous studies (“excess Fe(II)”) (Hellige et al., 2012; Poulton, 2003; Poulton et al., 2004). The formation of the “excess Fe(II)” seems compete with the formation of FeS/mackinawite. The excess Fe(II) concentration depends on the initial ratio of Fe/S as well as the mineralogical reactivity (represented by mineral types and surface area) of ferric iron. Higher Fe/S ratio and higher reactivity lead to higher excess Fe(II) concentration and less FeS/mackinawite concentration. Furthermore, XPS analysis confirmed that not only elemental sulfur but also polysulfides were the main oxidized sulfur products.. The polysulfides, with predominance of disulfide, accumulated mainly at the mineral surface and could be extracted by methanol with an appropriate pre-treatment with zinc acetate. Therefore, the MES pool comprised elemental sulfur, aqueous polysulfides and surface polysulfides. Rapid pyrite formation in HR experiments is closely linked to the formation of excess Fe(II). The presence of excess Fe(II) and polysulfides at the surface may lead to the potential formation of non-crystalline iron-polysulfide species and a supersaturation with respect to pyrite, thereby inducing rapid pyrite formation in the HR experiment. The rapid pyrite formation has been proposed as a ‘novel’ polysulfide pathway because ferrous iron and disulfide for pyrite formation originate directly from the excess Fe(II) and surface polysulfide. The rapid pyrite formation via this ‘novel’ polysulfide pathway is not kinetically controlled by the FeS dissolution. By contrast, pyrite in LR experiments formed via the normal polysulfide pathway that ferrous iron for pyrite formation is only from FeS dissolution. Pyrite formation in LR experiments is therefore kinetically controlled by FeS dissolution. The formation of iron polysulfide may influence the electromagnetic properties of ferrous iron in FeS/mackinawite, leading to an occurrence of magnetic ordering at 4.2 K. The magnetic ordering is represented by an asymmetric six-line in the Mössbauer spectrum (at 4.2 K). The spectrum of mixed iron sulfide phases generated during Fe(III)-S(-II) interaction is very different from that of pure FeS freshly precipitated from homogeneous solution of ferrous iron and sulfide, which showed no magnetic ordering (a single-line spectrum) at 4.2 K. The pure FeS phase is not stable and tends to transform into the mixed iron sulfide phases. Three key findings from this thesis can be highlighted that help to understand the interaction between iron and sulfur biogeochemistry: 1) The occurrence of surface polysulfide subverts the previous consideration that polysulfide presents only in the solution and may play an overlooked role in both abiotic and biotic sulfur cycling. 2) The Fe/S ratio controlling the kinetics and pathway of pyrite can be applied as an indicator to predict rapid pyrite formation, especially in the temporary anoxic environments. 3) Complex Mössbauer spectra of iron sulfide phases reveal that the properties, especially the electromagnetic property of Fe in mackinawite, can be easily altered by impurities. Results call for the characterization of different iron sulfide minerals (especially mackinawite) with Mössbauer spectroscopy combining a strict synthesis protocol and the investigation of phase transformation among these iron sulfide minerals.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Wan, Moli

Contributors dc:contributor

• Peiffer, Stefan

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2164/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2164