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Arsenic is a global pollutant. Recent studies found that Fe(II) can oxidize As(III), but the extent of oxidation with mixed-valent iron minerals and the mechanisms involved are unknown. In this study, we investigated whether As(III) can be oxidized under reducing conditions using green rust sulfate (GR-SO 4 ), an Fe mineral containing both Fe(II) and Fe(III). Batch sorption experiments showed that GR-SO 4 (1 g L -1 ) effectively sorbs environmentally relevant concentrations of As(III) (50-500 μg L -1 ) under anoxic, neutral pH conditions with and without citrate (50 μM). X-ray absorption near-edge structure spectroscopy analysis at the As K-edge demonstrated that approximately 76% of As(III) was oxidized to As(V) by GR-SO 4 . Complete oxidation of As(III) was observed in the presence of citrate. As(III) oxidation can be linked to the phase transformation of GR-SO 4 to goethite, resulting in new reactive Fe(III) species that plausibly drive oxidation. Citrate enhanced this process by stabilizing Fe on the mixed GR-SO 4 /goethite surface, preventing its reduction back to Fe(II) and facilitating further As(III) oxidation without significant Fe loss to the solution. This study highlights the cryptic As(III) oxidation that occurs under reducing conditions, providing new insights into the cycling of arsenic in mixed phases of iron-rich, anoxic environments., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Microbial reduction of Fe(III) minerals is a prominent process in redoximorphic soils and is strongly affected by organic matter (OM). We herein determined the rate and extent of microbial reduction of ferrihydrite (Fh) with either adsorbed or coprecipitated OM by Geobacter sulfurreducens . We focused on OM-mediated effects on electron uptake and alterations in Fh crystallinity. The OM was obtained from anoxic soil columns (effluent OM, efOM) and included-unlike water-extractable OM-compounds released by microbial activity under anoxic conditions. We found that organic molecules in efOM had generally no or only very low electron-accepting capacity and were incorporated into the Fh aggregates when coprecipitated with Fh. Compared to OM-free Fh, adsorption of efOM to Fh decelerated the microbial Fe(III) reduction by passivating the Fh surface toward electron uptake. In contrast, coprecipitation of Fh with efOM accelerated the microbial reduction, likely because efOM disrupted the Fh structure, as noted by Mössbauer spectroscopy. Additionally, the adsorbed and coprecipitated efOM resulted in a more sustained Fe(III) reduction, potentially because efOM could have effectively scavenged biogenic Fe(II) and prevented the passivation of the Fh surface by the adsorbed Fe(II). Fe(III)-OM coprecipitates forming at anoxic-oxic interfaces are thus likely readily reducible by Fe(III)-reducing bacteria in redoximorphic soils.

Neutrophilic microbial pyrite (FeS 2 ) oxidation coupled to denitrification is thought to be an important natural nitrate attenuation pathway in nitrate-contaminated aquifers. However, the poor solubility of pyrite raises questions about its bioavailability and the mechanisms underlying its oxidation. Here, we investigated direct microbial pyrite oxidation by a neutrophilic chemolithoautotrophic nitrate-reducing Fe(II)-oxidizing culture enriched from a pyrite-rich aquifer. We used pyrite with natural abundance (NA) of Fe isotopes ( NA Fe-pyrite) and 57 Fe-labeled siderite to evaluate whether the oxidation of the more soluble Fe(II)-carbonate (FeCO 3 ) can indirectly drive abiotic pyrite oxidation. Our results showed that in setups where only pyrite was incubated with bacteria, direct microbial pyrite oxidation contributed ca. 26% to overall nitrate reduction. The rest was attributed to the oxidation of elemental sulfur (S 0 ), present as a residue from pyrite synthesis. Pyrite oxidation was evidenced in the NA Fe-pyrite/ 57 Fe-siderite setups by maps of 56 FeO and 32 S obtained using a combination of SEM with nanoscale secondary ion MS (NanoSIMS), which showed the presence of 56 Fe(III) (oxyhydr)oxides that could solely originate from 56 FeS 2 . Based on the fit of a reaction model to the geochemical data and the Fe-isotope distributions from NanoSIMS, we conclude that anaerobic oxidation of pyrite by our neutrophilic enrichment culture was mainly driven by direct enzymatic activity of the cells. The contribution of abiotic pyrite oxidation by Fe 3+ appeared to be negligible in our experimental setup.

Uranium (U) in situ bioremediation has been investigated as a cost-effective strategy to tackle U contamination in the subsurface. While uraninite was believed to be the only product of bioreduction, numerous studies have revealed that noncrystalline U(IV) species (NCU(IV)) are dominant. This finding brings into question the effectiveness of bioremediation because NCU(IV) species are expected to be labile and susceptible to oxidation. Thus, understanding the stability of NCU(IV) in the environment is of crucial importance. Fe(II) minerals (such as FeS) are often associated with U(IV) in bioremediated or naturally reduced sediments. Their impact on the stability of NCU(IV) is not well understood. Here, we show that, at high dissolved oxygen concentrations, FeS accelerates NCU(IV) reoxidation. We hypothesize that either highly reactive ferric minerals or radical S species produced by the oxidation of FeS drive this rapid reoxidation of NCU(IV). Furthermore, we found evidence for the contribution of reactive oxygen species to NCU(IV) reoxidation. This work refines our understanding of the role of iron sulfide minerals in the stability of tetravalent uranium in the presence of oxygen in a field setting such as contaminated sites or uranium-bearing naturally reduced zones.

Magnetite (Fe 3 O 4 ) is an environmentally ubiquitous mixed-valent iron (Fe) mineral, which can form via biotic or abiotic transformation of Fe(III) (oxyhydr)oxides such as ferrihydrite (Fh). It is currently unclear whether environmentally relevant biogenic Fh from Fe(II)-oxidizing bacteria, containing cell-derived organic matter, can transform to magnetite. We compared abiotic and biotic transformation: (1) abiogenic Fh (aFh); (2) abiogenic Fh coprecipitated with humic acids (aFh-HA); (3) biogenic Fh produced by phototrophic Fe(II)-oxidizer Rhodobacter ferrooxidans SW2 (bFh); and (4) biogenic Fh treated with bleach to remove biogenic organic matter (bFh-bleach). Abiotic or biotic transformation of Fh was promoted by Fe aq 2+ or Fe(III)-reducing bacteria. Fe aq 2+ -catalyzed abiotic reaction with aFh and bFh-bleach led to complete transformation to magnetite. In contrast, aFh-HA only partially (68%) transformed to magnetite, and bFh (17%) transformed to goethite. We hypothesize that microbial biomass stabilized bFh against reaction with Fe aq 2+ . All four Fh substrates were transformed into magnetite during biotic reduction, suggesting that Fh remains bioavailable even when associated with microbial biomass. Additionally, there were poorly ordered magnetic components detected in the biogenic end products for aFh and aFh-HA. Nevertheless, abiotic transformation was much faster than biotic transformation, implying that initial Fe aq 2+ concentration, passivation of Fh, and/or sequestration of Fe(II) by bacterial cells and associated biomass play major roles in the rate of magnetite formation from Fh. These results improve our understanding of factors influencing secondary mineralization of Fh in the environment.