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Interactions between aqueous ferrous iron (Fe(II)) and secondary Fe oxyhydroxides catalyze mineral recrystallization and/or transformation processes in anoxic soils and sediments, where oxyanions, such as silicate, are abundant. However, the effect and the fate of silicate during Fe mineral recrystallization and transformation are not entirely understood and especially remain unclear for lepidocrocite. In this study, we reacted (Si-)ferrihydrite (Si/Fe = 0, 0.05, and 0.18) and (Si-)lepidocrocite (Si/Fe = 0 and 0.08) with isotopically labeled 57Fe(II) (Fe(II)/Fe(III) = 0.02 and 0.2) at pH 7 for up to 4 weeks. We followed Fe mineral transformations with X-ray diffraction and tracked Fe atom exchange by measuring aqueous and solid phase Fe isotope fractions. Our results show that the extent of ferrihydrite transformation in the presence of Fe(II) was strongly influenced by the solid phase Si/Fe ratio, while increasing the Fe(II)/Fe(III) ratio (from 0.02 to 0.2) had only a minor effect. The presence of silicate increased the thickness of newly formed lepidocrocite crystallites, and elemental distribution maps of Fe(II)-reacted Si-ferrihydrites revealed that much more Si was associated with the remaining ferrihydrite than with the newly formed lepidocrocite. Pure lepidocrocite underwent recrystallization in the low Fe(II) treatment and transformed to magnetite at the high Fe(II)/Fe(III) ratio. Adsorbed silicate inactivated the lepidocrocite surfaces, which strongly reduced Fe atom exchange and inhibited mineral transformation. Collectively, the results of this study demonstrate that Fe(II)-catalyzed Si-ferrihydrite transformation leads to the redistribution of silicate in the solid phase and the formation of thicker lepidocrocite platelets, while lepidocrocite transformation can be completely inhibited by adsorbed silicate. Therefore, silicate is an important factor to include when considering Fe mineral dynamics in soils under reducing conditions., Environmental Science & Technology, 56 (9), ISSN:0013-936X, ISSN:1520-5851

Mercury (Hg) is a toxic trace element of global environmental concern which has been increasingly dispersed into the environment since the industrial revolution. In aquatic and terrestrial systems, Hg can be reduced to elemental Hg (Hg0) and escape to the atmosphere or converted to methylmercury (MeHg), a potent neurotoxin that accumulates in food webs. FeII-bearing minerals such as magnetite, green rusts, siderite, and mackinawite are recognized HgII reducers. Another potentially Hg-reducing mineral, which commonly occurs in Fe- and organic/P-rich sediments and soils, is the ferrous iron phosphate mineral vivianite (FeII3(PO4)2·8H2O), but its reaction with HgII has not been studied to date. Here, nanoparticulate vivianite (particle size ∼ 50 nm; FeII content > 98%) was chemically synthesized and characterized by a combination of chemical, spectroscopic, and microscopic analyses. Its ability to reduce HgII was investigated at circumneutral pH under anoxic conditions over a range of FeII/HgII ratios (0.1–1000). For FeII/HgII ratios ≥1, which are representative of natural environments, HgII was very quickly and efficiently reduced to Hg0. The ability of vivianite to reduce HgII was found to be similar to those of carbonate green rust and siderite, two of the most effective Hg-reducing minerals. Our results suggest that vivianite may be involved in abiotic HgII reduction in Fe and organic/P-rich soils and sediments, potentially contributing to Hg evasion while also limiting MeHg formation in these ecosystems., Environmental Science & Technology, 55 (5), ISSN:0013-936X, ISSN:1520-5851

Under anoxic conditions, the interactions between As-bearing ferrihydrite (Fh) and As(V)-reducing bacteria are known to cause Fh transformations and As mobilization. However, the impact of different types of organic matter (OM) on microbial As/Fe transformation in As-bearing Fh-organic associations remains unclear. In our study, we therefore exposed arsenate-adsorbed ferrihydrite, ferrihydrite-PGA (polygalacturonic acid), and ferrihydrite-HA (humic acid) complexes to two typical Fe(III)- and As(V)-reducing bacteria, and followed the fate of Fe and As in the solid and aqueous phases. Results show that PGA and HA promoted the reductive dissolution of Fh, resulting in 0.7-1.6 and 0.8-1.9 times more As release than in the OM-free Fh, respectively. This was achieved by higher cell numbers in the presence of PGA, and through Fe-reduction via electron-shuttling facilitated by HA. Arsenic-XAS results showed that the solid-phase arsenite fraction in Fh-PGA and Fh-HA was 15-19% and 27-28% higher than in pure Fh, respectively. The solid-associated arsenite fraction likely increased because PGA promoted cell growth and As(V) reduction, while HA provided electron shuttling compounds for direct microbial As(V)-reduction. Collectively, our findings demonstrate that As speciation and partitioning during microbial reduction of Fh-organic associations are strongly influenced by PGA and HA, as well as the strains' abilities to utilize electron-shuttling compounds.

In redox-affected soil environments, electron transfer between aqueous Fe(II) and solid-phase Fe(III) catalyzes mineral transformation and recrystallization processes. While these processes have been studied extensively as independent systems, the coexistence of iron minerals is common in nature. Yet it remains unclear how coexisting goethite influences ferrihydrite transformation. Here, we reacted ferrihydrite and goethite mixtures with Fe(II) for 24 h. Our results demonstrate that with more goethite initially present in the mixture more ferrihydrite turned into goethite. We further used stable Fe isotopes to label different Fe pools and probed ferrihydrite transformation in the presence of goethite using 57Fe Mössbauer spectroscopy and changes in the isotopic composition of solid and aqueous phases. When ferrihydrite alone underwent Fe(II)-catalyzed transformation, Fe atoms initially in the aqueous phase mostly formed lepidocrocite, while those from ferrihydrite mostly formed goethite. When goethite was initially present, more goethite was formed from atoms initially in the aqueous phase, and nanogoethite formed from atoms initially in ferrihydrite. Our results suggest that coexisting goethite promotes formation of more goethite via Fe(II)-goethite electron transfer and template-directed nucleation and growth. We further hypothesize that electron transfer onto goethite followed by electron hopping onto ferrihydrite is another possible pathway to goethite formation. Our findings demonstrate that mineral transformation is strongly influenced by the composition of soil solid phases., Environmental Science & Technology, 56 (17), ISSN:0013-936X, ISSN:1520-5851

Complex interactions between redox-driven element cycles in soils influence iron mineral transformation processes. The rates and pathways of iron mineral transformation processes have been studied intensely in model systems such as mixed suspensions, but transformation in complex heterogeneous porous media is not well understood. Here, mesh bags containing 0.5 g of ferrihydrite were incubated in five water-saturated paddy soils with contrasting microbial iron-reduction potential for up to twelve weeks. Using X-ray diffraction analysis, we show near-complete transformation of the ferrihydrite to lepidocrocite and goethite within six weeks in the soil with the highest iron(ii) release, and slower transformation with higher ratios of goethite to lepidocrocite in soils with lower iron(ii) release. In the least reduced soil, no mineral transformations were observed. In soils where ferrihydrite transformation occurred, the transformation rate was one to three orders of magnitude slower than transformation in comparable mixed-suspension studies. To interpret the spatial distribution of ferrihydrite and its transformation products, we developed a novel application of confocal micro-Raman spectroscopy in which we identified and mapped minerals on selected cross sections of mesh bag contents. After two weeks of flooded incubation, ferrihydrite was still abundant in the core of some mesh bags, and as a rim at the mineral-soil interface. The reacted outer core contained unevenly mixed ferrihydrite, goethite and lepidocrocite on the micrometre scale. The slower rate of transformation and uneven distribution of product minerals highlight the influence of biogeochemically complex matrices and diffusion processes on the transformation of minerals, and the importance of studying iron mineral transformation in environmental media., Environmental Science: Processes & Impacts, 24 (10), ISSN:2050-7887, ISSN:2050-7895

Ferrihydrite (Fh) is a poorly crystalline Fe(III)-oxyhydroxide found in abundance in soils and sediments. With a high specific surface area and sorption capacity at circumneutral pH, ferrihydrite is an important player in the biogeochemical cycling of nutrients and trace elements in redox-dynamic environments. Under reducing conditions, exposure to Fe(II) induces mineral transformations in ferrihydrite; the extent and trajectory of which may be greatly influenced by organic matter (OM). However, natural OM is heterogeneous and comprises a range of molecular weights (MWs) and varied functional group compositions. To date, the impact that the chemical composition of the associated OM has on Fe(II)-catalyzed mineral transformations is not clear. To address this knowledge gap, we coprecipitated ferrihydrite with model organic ligands selected to cover a range of MWs (25 000-50 000 vs200 Da) as well as carboxyl content (polygalacturonic acid (PGA)citric acid (CA)galacturonic acid (GA)). Coprecipitates (C:Fe ≈ 0.6) were reacted with 1 mM

In freshwater wetlands, redox interfaces characterized by circumneutral pH, steep gradients in O2, and a continual supply of Fe(II) form ecological niches favorable to microaerophilic iron(II) oxidizing bacteria (FeOB) and the formation of flocs; associations of (a)biotic mineral phases, microorganisms, and (microbially-derived) organic matter. On the volcanic island of Iceland, wetlands are replenished with Fe-rich surface-, ground- and springwater. Combined with extensive drainage of lowland wetlands, which forms artificial redox gradients, accumulations of bright orange (a)biotically-derived Fe-rich flocs are common features of Icelandic wetlands. These loosely consolidated flocs are easily mobilized, and, considering the proximity of Iceland's lowland wetlands to the coast, are likely to contribute to the suspended sediment load transported to coastal waters. To date, however, little is known regarding (Fe) mineral and elemental composition of the flocs. In this study, flocs from wetlands (n = 16) across Iceland were analyzed using X-ray diffraction and spectroscopic techniques (X-ray absorption and 57Fe Mössbauer) combined with chemical extractions and (electron) microscopy to comprehensively characterize floc mineral, elemental, and structural composition. All flocs were rich in Fe (229–414 mg/g), and floc Fe minerals comprised primarily ferrihydrite and nano-crystalline lepidocrocite, with a single floc sample containing nano-crystalline goethite. Floc mineralogy also included Fe in clay minerals and appreciable poorly-crystalline aluminosilicates, most likely allophane and/or imogolite. Microscopy images revealed that floc (bio)organics largely comprised mineral encrusted microbially-derived components (i.e. sheaths, stalks, and EPS) indicative of common FeOB Leptothrix spp. and Gallionella spp. Trace element contents in the flocs were in the low μg/g range, however nearly all trace elements were extracted with hydroxylamine hydrochloride. This finding suggests that the (a)biotic reductive dissolution of floc Fe minerals, plausibly driven by exposure to the varied geochemical conditions of coastal waters following floc mobilization, could lead to the release of associated trace elements. Thus, the flocs should be considered vectors for transport of Fe, organic carbon, and trace elements from Icelandic wetlands to coastal waters., Science of The Total Environment, 816, ISSN:0048-9697, ISSN:1879-1026

Phylogenetically diverse species of bacteria can mediate anaerobic oxidation of ferrous iron [Fe(II)] and/or arsenite [As(III)] coupled to nitrate reduction, impacting the biogeochemical cycles of Fe and As. However, the mechanisms for nitrate-dependent anaerobic oxidation of Fe(II) and As(III) remain unclear. In this study, we isolated two bacterial strains from arsenic-contaminated paddy soils, Ensifer sp. ST2 and Paracoccus sp. QY30. Both strains were capable of anaerobic As(III) oxidation, but only QY30 could oxidize Fe(II) under nitrate-reducing conditions. Both strains contain the As(III) oxidase gene aioA, whose expression was induced greatly by As(III) exposure. Both strains contain the whole suite of genes for complete denitrification, but the nitrite reductase gene nirK was not expressed in QY30 and nitrite accumulated under nitrate-reducing conditions. When the functional nirK gene was knocked out in strain ST2, its nitrite reduction ability was completely abolished and nitrite accumulated in the medium. Moreover, the ST2ΔnirK mutant gained the ability to oxidize Fe(II). When nirK gene from ST2 was introduced to QY30, the recombinant strain QY30::nirK gained the ability to reduce nitrite but lost the ability to oxidize Fe(II). These genetic manipulations did not affect the ability of both strains to oxidize As(III). Our results indicate that nitrite accumulation is required for anaerobic oxidation of Fe(II) but not for As(III) oxidation in these strains. The results suggest that anaerobic Fe(II) oxidation in the two bacterial strains is primarily driven by abiotic reaction of Fe(II) with nitrite, while reduction of nitrate to nitrite is sufficient for redox coupling with anaerobic As(III) oxidation catalyzed by Aio. Deletion of nirK gene in denitrifiers can enhance anaerobic Fe(II) oxidation.

In freshwater wetlands, organic flocs are often found enriched in trace metal(loid)s associated with poorly crystalline Fe(III)-(oxyhydr)oxides. Under reducing conditions, flocs may become exposed to aqueous Fe(II), triggering Fe(II)-catalyzed mineral transformations and trace metal(loid) release. In this study, pure ferrihydrite, a synthetic ferrihydrite-polygalacturonic acid coprecipitate (16.7 wt % C), and As- (1280 and 1230 mg/kg) and organic matter (OM)-rich (18.1 and 21.8 wt % C) freshwater flocs dominated by ferrihydrite and nanocrystalline lepidocrocite were reacted with an isotopically enriched 57Fe(II) solution (0.1 or 1.0 mM Fe(II)) at pH 5.5 and 7. Using a combination of wet chemistry, Fe isotope analysis, X-ray absorption spectroscopy (XAS), 57Fe Mossbauer spectroscopy and X-ray diffraction, we followed the Fe atom exchange kinetics and secondary mineral formation over 1 week. When reacted with Fe(II) at pH 7, pure ferrihydrite exhibited rapid Fe atom exchange at both Fe(II) concentrations, r...

In submerged soils and sediments, clay minerals are often exposed to anoxic waters containing ferrous iron (Fe2+). Here, we investigated the sorption of Fe2+ onto a synthetic montmorillonite (Syn-1) low in structural Fe (

Poorly crystalline Fe(III) (oxyhydr)oxides like ferrihydrite are abundant in soils and sediments and are often associated with organic matter (OM) in the form of mineral-organic aggregates. Under anoxic conditions, interactions between aqueous Fe(II) and ferrihydrite lead to the formation of crystalline secondary minerals, like lepidocrocite, goethite, or magnetite. However, the extent to which Fe(II)-catalyzed mineral transformations are influenced by ferrihydrite-associated OM is not well understood. We therefore reacted ferrihydrite-PGA coprecipitates (PGA = polygalacturonic acid, C:Fe molar ratios = 0–2.5) and natural Fe-rich organic flocs (C:Fe molar ratio = 2.2) with 0.5–5.0 mM isotopically labeled 57Fe(II) at pH 7 for 5 weeks. Relying on the combination of stable Fe isotope tracers, a novel application of the PONKCS method to Rietveld fitting of X-ray diffraction (XRD) patterns, and 57Fe Mossbauer spectroscopy, we sought to follow the temporal evolution in Fe mineralogy and elucidate the fate of ad...

Iron rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. Under low flow stream conditions flocs may settle become buried and eventually be subjected to reducing conditions facilitating trace metal(loid) release. In this study we reacted freshwater flocs (704 1280 mg As/kg) from a minerotrophic peatland (Gola di Lago Switzerland) with sulfide (5.2 mM S( II)(spike)/Fe = 0.75 1.62 mol/mol) at neutral pH and studied the speciation changes of Fe S and As at 25 +/ 1 degrees C over 1 week through a combination of synchrotron X ray techniques and wet chemical analyses. Sulfidization of floc ferrihydrite and nanocrystalline lepidocrocite caused the rapid formation of mackinawite (52 81 of Fe solid at day 7) as well as solid phase associated S(0) and polysulfides. Ferrihydrite was preferentially reduced over lepidocrocite although neoformation of lepidocrocite from ferrihydrite could not be excluded. Sulfide ieacted flocs contained primarily arsenate (47 72) which preferentially adsorbed to Fe(III) (oxyhydr)oxides despite abundant mackinawite precipitation. At higher S( II)(spike)/Fe molar ratios (>= 1.0) the formation of an orpiment like phase accounted for up to 35 of solid phase As. Despite Fe and As sulfide precipitation and the presence of residual Fe(III) (oxyhydr)wddes mobilization of As was recorded in all samples (As aq = 0.45 7.0 mu M at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33 of As aq. Our findings show that freshwater flocs from the Gola di Lago peatland may become a source of As under sulfate reducing conditions and emphasize the pivotal role Fe rich organic freshwater flocs play in trace metal(loid) cycling in S rich wetlands characterized by oscillating redox conditions.