Coupled reduction of structural Fe(III) in nontronite and oxidation of petroleum hydrocarbons

Fe-bearing clay minerals are frequently present in oil reservoirs as potential electron acceptor. Compounds in crude oil may mediate microbial reduction of structural Fe(III) in clay minerals through their roles as electron donor and shuttle. However, there is limited knowledge about these roles and resulting transformation of oil compounds. In this study, bio-reduction of structural Fe(III) in nontronite (NAu-2, Fe-rich smectite) by the dissimilatory iron-reducing bacterium (DIRB) Shewanella putrefaciens CN32 was investigated in the presence of either crude oil or specific fractions of oil. Lactate was added in some experiments as extra electron donor. The fractions of saturates (nC14-nC20 alkanes and nC8-nC14-alkyl cyclohexanes), aromatics (with two/three benzene rings), and polar compounds (car-boxylic acids) of crude oil were adsorbed to NAu-2 surface. Without CN32, crude oil was able to slightly reduce structural Fe(III) in NAu-2. In the presence of CN32, reduction of Fe(III) in NAu-2 was coupled with oxidation of bulk oil and saturated/aromatic fractions. The reduction extent (21.5 +/- 0.3%) and rate (0.04 +/- 0.00 mM/h) by the saturated oil fraction (nC17-nC30 alkanes and nC10-nC23-alkyl cyclohexanes as electron donors) were slightly lower than those by bulk oil (22.8 +/- 0.4% and 0.05 +/- 0.00 mM/h, respectively), but those by the aromatic fraction with two benzene rings were much higher (30.8 +/- 0.2% and 0.15 +/- 0.00 mM/h, respectively). Fe(III) bio-reduction was coupled with oxidative transformation of oil compounds. Specifically, n-alkanes and alkyl cyclohexanes with shorter carbon chains and aromatic isomers with fewer methyl groups were more easily degraded (i.e., dibenzothio-phene series) and/or desorbed (i.e., phenanthrene series) than n-alkanes and alkyl cyclohexanes with longer carbon chains and aromatic isomers with more methyl groups. These hydrocarbons were degraded to oxygen-containing compounds, mainly saturated fatty acids, monocyclic naphthenic acids, and phenols. In the presence of lactate as extra electron donor, the aromatic fraction significantly promoted the Fe(III) bio-reduction extent and rate, suggesting its role as electron shuttle. The oxidative transforma-tion of oil compounds driven by bio-reduction of clay minerals has broad implications for the coupled biogeochemical cycles of carbon and iron in oil reservoirs and organic-rich sedimentary rocks.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the bio-reduction of structural Fe(III) in clay minerals by the iron-reducing bacterium Shewanella putrefaciens CN32. It finds that compounds in crude oil, particularly the saturated and aromatic fractions, can serve as electron donors and shuttle, facilitating the reduction of Fe(III) in clay minerals. The presence of lactate as an additional electron donor enhances the bio-reduction process, especially for the aromatic fraction. These findings have important implications for the coupled biogeochemical cycles of carbon and iron in oil reservoirs.