The formation of sulfate-green rust through Fe(II) sorption to montmorillonite: Impacts on abiotic nitrate reduction

Green rust (GR) minerals are generally considered to be effective reductants of pollutants and the electron transfer from aqueous Fe(II) to structural Fe(III) in montmorillonite has recently been discovered to be a pathway to GR forma-tion at pH similar to 7.8. In this study, we have further delineated the pH conditions and examined the effect of aqueous sulfate concentrations that allow for the formation of sulfate-GR through this unique pathway. Iron(II) sorption experiments demonstrated that the amount of 'sorbed' Fe(II) on montmorillonite semi-quantitatively transformed into sulfate-GR at pH values >= 7.5 in the presence of environmentally-relevant sulfate concentrations (i.e., 10 mM). However, excess sul-fate concentrations (100 mM) resulted in comparatively less Fe(II) sorption and sulfate-GR was only observed to form at pH 8. As such, it was concluded that the degree of Fe(II) sorption to montmorillonite is critical to GR formation when aqueous Fe(II) and montmorillonite co-exist. In contrast to sulfate-GR minerals formed through other pathways (e.g., co-precipitation of dissolved Fe(II) and Fe(III) species), this montmorillonite-synthesized GR was significantly less reactive towards nitrate reduction, with <2.5 % of 0.2 mM nitrate being reduced over a 6-day period. This behav-iour was correlated to reduction potential and it was, therefore, concluded that the relatively high reduction potential that occurs in the presence of montmorillonite exerts a significant influence on the rate of nitrate reduction by sulfate-GR to the point that it may not be a competitive process to microbiological nitrate denitrification. As such, the environ-mental relevance of green rust to nitrate reduction cannot be inferred simply by its presence, but rather the reduction potential of the environmental system in which it is found.

Automated summary

Green rust minerals are considered effective reductants of pollutants, and electron transfer from aqueous Fe(II) to structural Fe(III) in montmorillonite can lead to the formation of green rust at pH around 7.8. This study investigated the pH conditions and the effect of aqueous sulfate concentrations on the formation of sulfate-green rust via this pathway. It was found that the degree of Fe(II) sorption to montmorillonite is critical to green rust formation in the presence of environmentally-relevant sulfate concentrations.