Sulfide is an extremely powerful reductant for ferric (hydr)oxides and it is present at different concentration levels in almost any reducing environment. The interaction with ferric hydroxides is of paramount importance particularly in marine environments but its importance has been highlighted also for terrestrial systems.<br/>The reaction rate has a maximum at pH 7 with a strong interference with Fe(II) and particularly the formation of iron sulfides. Hence, rate constants for its interaction are only available from initial rate experiments. However, there is clear evidence that the reaction is not finished after sulphide is consumed. It does not lead to the instantaneous production of thermodynamically stable products but can induce the formation of a variety of metastable products (Fig. 1) and eventually pyrite. Pyrite formation rate increases with increasing initial Fe(III)/S(-II) ratio.<br/>In this presentation we will propose a novel ferric-hydroxide-surface (FHS) pathway for rapid pyrite formation that is based on the formation of a &gt;Fe(II)-S<sub>2</sub> precursor species at the sulfidized mineral surface. Its formation is competitive to FeS precipitation that occurs at high aqueous sulfide concentrations. We propose that the FHS pathway will be prominent in many aquatic systems with terrestrial influence, i.e. abundance of ferric iron but low sulfide concentrations.

Sulfidization of ferric hydroxides – insights into a dynamic electron transfer process

253rd National Meeting (2017)

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