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Fe(III):S(-II) Concentration Ratio Controls the Pathway and the Kinetics of Pyrite Formation during Sulfidation of Ferric Hydroxides

Title data

Wan, Moli ; Schröder, Christian ; Peiffer, Stefan:
Fe(III):S(-II) Concentration Ratio Controls the Pathway and the Kinetics of Pyrite Formation during Sulfidation of Ferric Hydroxides.
In: Geochimica et Cosmochimica Acta. Vol. 217 (November 2017) . - pp. 334-348.
ISSN 0016-7037
DOI: https://doi.org/10.1016/j.gca.2017.08.036

Abstract in another language

The formation of pyrite has been extensively studied because of its abundance in many anoxic environments. Yet, there is no consensus on the underlying pathways and kinetics of its formation. We studied the formation of pyrite during the reaction between reactive ferric hydroxides (goethite and lepidocrocite) and aqueous sulfide in an anoxic glove box at neutral pH. The formation of pyrite was monitored with Mössbauer spectroscopy using 57Fe isotope-enriched ferric (hydr)oxides. The initial molar ratios of Fe(III):S(-II) were adjusted to be ‘high’ with Fe(III) concentrations in excess of sulfide (HR) and ‘low’ (LR) with excess of sulfide. Approximately the same surface area was applied in all HR runs in order to compare the mineral reactivity of ferric hydroxides. Electron transfer between aqueous sulfide and ferric hydroxides in the first 2 hours led to the formation of ferrous iron and methanol-extractable oxidized sulfur (MES). Metastable FeSx formed in all of the experiments. Pyrite formed at a different rate in HR and LR runs although the MES and ferrous iron concentrations were rather similar. In all HR runs, pyrite formation started after 48 hours and achieved a maximum concentration after 1 week. In contrast, pyrite started to form only after 2 months in LR runs (Fe(III):S(-II) ~ 0.2) with goethite and no pyrite formation was observed in LR with lepidocrocite after 6 months. Rates in LR runs were at least 2-3 orders of magnitude slower than in HR runs. Sulfide oxidation rates were higher with lepidocrocite than with goethite, but no influence of the mineral type on pyrite formation rates in HR runs could be observed. Pyrite formation rates in HR runs could not be predicted by the classical model of Rickard (1975). We therefore propose a novel ferric-hydroxide-surface (FHS) pathway for rapid pyrite formation that is based on the formation of a precursor species >FeIIS2-. Its formation is competitive to FeSx precipitation at high aqueous sulfide concentrations and requires that a fraction of the ferric hydroxide surface not be covered by a surface precipitate of FeSx. Hence, pyrite formation rate decreases with decreasing Fe(III):S(-II)aq ratio. In LR runs, pyrite formation appears to follow the model of Rickard (1975) and to be kinetically controlled by the dissolution of FeS. The FHS-pathway will be prominent in many aquatic systems with terrestrial influence, i.e. abundance of ferric iron. We propose that the Fe(III):S(-II)aq ratio can be used as an indicator for rapid pyrite formation during early diagenesis in anoxic/suboxic aquatic systems.

Further data

Item Type: Article in a journal
Refereed: Yes
Additional notes: BAYCEER142720
Institutions of the University: Faculties > Faculty of Biology, Chemistry and Earth Sciences > Department of Earth Sciences > Chair Hydrology
Faculties > Faculty of Biology, Chemistry and Earth Sciences > Department of Earth Sciences > Chair Hydrology > Chair Hydrology - Univ.-Prof. Dr. Stefan Peiffer
Research Institutions
Research Institutions > Research Centres
Research Institutions > Research Centres > Bayreuth Center of Ecology and Environmental Research- BayCEER
Faculties
Faculties > Faculty of Biology, Chemistry and Earth Sciences
Faculties > Faculty of Biology, Chemistry and Earth Sciences > Department of Earth Sciences
Result of work at the UBT: Yes
DDC Subjects: 500 Science
Date Deposited: 11 Jan 2018 13:05
Last Modified: 11 Jan 2018 13:05
URI: https://eref.uni-bayreuth.de/id/eprint/41283