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1. Electrochemically induced metal- vs. ligand-based redox changes in mackinawite: identification of a Fe ³⁺ - and polysulfide-containing intermediate

   https://doi.org/10.1039/d1dt01684a  

   Sanden, Sebastian A.; Szilagyi, Robert K.; Li, Yamei; Kitadai, Norio; Webb, Samuel M.; Yano, Takaaki; Nakamura, Ryuhei; Hara, Masahiko; McGlynn, Shawn E. (August 2021, Dalton Transactions)

   Under anaerobic conditions, ferrous iron reacts with sulfide producing FeS, which can then undergo a temperature, redox potential, and pH dependent maturation process resulting in the formation of oxidized mineral phases, such as greigite or pyrite. A greater understanding of this maturation process holds promise for the development of iron-sulfide catalysts, which are known to promote diverse chemical reactions, such as H + , CO 2 and NO 3 − reduction processes. Hampering the full realization of the catalytic potential of FeS, however, is an incomplete knowledge of the molecular and redox processess ocurring between mineral and nanoparticulate phases. Here, we investigated the chemical properties of iron-sulfide by cyclic voltammetry, Raman and X-ray absorption spectroscopic techniques. Tracing oxidative maturation pathways by varying electrode potential, nanoparticulate n (Fe 2+ S 2− ) (s) was found to oxidize to a Fe 3+ containing FeS phase at −0.5 V vs. Ag/AgCl (pH = 7). In a subsequent oxidation, polysulfides are proposed to give a material that is composed of Fe 2+ , Fe 3+ , S 2− and polysulfide (S n 2− ) species, with its composition described as Fe 2+ 1−3 x Fe 3+ 2 x S 2− 1− y (S n 2− ) y . Thermodynamic properties of model compounds calculated by density functional theory indicate that ligand oxidation occurs in conjunction with structural rearrangements, whereas metal oxidation may occur prior to structural rearrangement. These findings together point to the existence of a metastable FeS phase located at the junction of a metal-based oxidation path between FeS and greigite (Fe 2+ Fe 3+ 2 S 2− 4 ) and a ligand-based oxidation path between FeS and pyrite (Fe 2+ (S 2 ) 2− ). 

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2. Abiotic Reduction of Mercury(II) in the Presence of Sulfidic Mineral Suspensions

   https://doi.org/10.3389/fenvc.2021.660058  

   Coulibaly, Mariame; Mazrui, Nashaat M.; Jonsson, Sofi; Mason, Robert P. (June 2021, Frontiers in Environmental Chemistry)

   null (Ed.)

   Monomethylmercury (CH 3 Hg) is a neurotoxic pollutant that biomagnifies in aquatic food webs. In sediments, the production of CH 3 Hg depends on the bacterial activity of mercury (Hg) methylating bacteria and the amount of bioavailable inorganic divalent mercury (Hg II ). Biotic and abiotic reduction of Hg II to elemental mercury (Hg 0 ) may limit the pool of Hg II available for methylation in sediments, and thus the amount of CH 3 Hg produced. Knowledge about the transformation of Hg II is therefore primordial to the understanding of the production of toxic and bioaccumulative CH 3 Hg. Here, we examined the reduction of Hg II by sulfidic minerals (FeS (s) and CdS (s) ) in the presence of dissolved iron and dissolved organic matter (DOM) using low, environmentally relevant concentrations of Hg and ratio of Hg II :FeS (s) . Our results show that the reduction of Hg II by Mackinawite (FeS (s) ) was lower (<15% of the Hg II was reduced after 24 h) than when Hg II was reacted with DOM or dissolved iron. We did not observe any formation of Hg 0 when Hg II was reacted with CdS (s) (experiments done under both acidic and basic conditions for up to four days). While reactions in solution were favorable under the experimental conditions, Hg was rapidly removed from solution by co-precipitation. Thermodynamic calculations suggest that in the presence of FeS (s) , reduction of the precipitated Hg II is surface catalyzed and likely involves S −II as the electron donor. The lack of reaction with CdS may be due to its stronger M-S bond relative to FeS, and the lower concentrations of sulfide in solution. We conclude that the reaction of Hg with FeS (s) proceeds via a different mechanism from that of Hg with DOM or dissolved iron, and that it is not a major environmental pathway for the formation of Hg 0 in anoxic environments. 

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3. Discovery and potential ramifications of reduced iron-bearing nanoparticles—magnetite, wüstite, and zero-valent iron—in wildland–urban interface fire ashes

   https://doi.org/10.1039/d2en00439a  

   Baalousha, Mohammed; Desmau, Morgane; Singerling, Sheryl A.; Webster, Jackson P.; Matiasek, Sandrine J.; Stern, Michelle A.; Alpers, Charles N. (November 2022, Environmental Science: Nano)

   The increase in fires at the wildland–urban interface has raised concerns about the potential environmental impact of ash remaining after burning. Here, we examined the concentrations and speciation of iron-bearing nanoparticles in wildland–urban interface ash. Total iron concentrations in ash varied between 4 and 66 mg g −1 . Synchrotron X-ray absorption near-edge structure (XANES) spectroscopy of bulk ash samples was used to quantify the relative abundance of major Fe phases, which were corroborated by transmission electron microscopy measurements. Maghemite (γ-(Fe 3+ ) 2 O 3 ) and magnetite (γ-Fe 2+ (Fe 3+ ) 2 O 4 ) were detected in most ashes and accounted for 0–90 and 0–81% of the spectral weight, respectively. Ferrihydrite (amorphous Fe( iii )–hydroxide, (Fe 3+ ) 5 HO 8 ·4H 2 O), goethite (α-Fe 3+ OOH), and hematite (α-Fe 3+ 2 O 3 ) were identified less frequently in ashes than maghemite and magnetite and accounted for 0–65, 0–54, and 0–50% of spectral weight, respectively. Other iron phases identified in ashes include wüstite (Fe 2+ O), zerovalent iron, FeS, FeCl 2 , FeCl 3 , FeSO 4 , Fe 2 (SO 4 ) 3 , and Fe(NO 3 ) 3 . Our findings demonstrate the impact of fires at the wildland–urban interface on iron speciation; that is, fires can convert iron oxides ( e.g. , maghemite, hematite, and goethite) to reduced iron phases such as magnetite, wüstite, and zerovalent iron. Magnetite concentrations ( e.g. , up to 25 mg g −1 ) decreased from black to gray to white ashes. Based on transmission electron microscopy (TEM) analyses, most of the magnetite nanoparticles were less than 500 nm in size, although larger particles were identified. Magnetite nanoparticles have been linked to neurodegenerative diseases as well as climate change. This study provides important information for understanding the potential environmental impacts of fires at the wildland–urban interface, which are currently poorly understood. 

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4. Seagrass Recovery Following Marine Heat Wave Influences Sediment Carbon Stocks

   https://doi.org/10.3389/fmars.2020.576784  

   Aoki, Lillian R.; McGlathery, Karen J.; Wiberg, Patricia L.; Oreska, Matthew P.; Berger, Amelie C.; Berg, Peter; Orth, Robert J. (January 2021, Frontiers in Marine Science)

   null (Ed.)

   Worldwide, seagrass meadows accumulate significant stocks of organic carbon (C), known as “blue” carbon, which can remain buried for decades to centuries. However, when seagrass meadows are disturbed, these C stocks may be remineralized, leading to significant CO 2 emissions. Increasing ocean temperatures, and increasing frequency and severity of heat waves, threaten seagrass meadows and their sediment blue C. To date, no study has directly measured the impact of seagrass declines from high temperatures on sediment C stocks. Here, we use a long-term record of sediment C stocks from a 7-km 2 , restored eelgrass ( Zostera marina ) meadow to show that seagrass dieback following a single marine heat wave (MHW) led to significant losses of sediment C. Patterns of sediment C loss and re-accumulation lagged patterns of seagrass recovery. Sediment C losses were concentrated within the central area of the meadow, where sites experienced extreme shoot density declines of 90% during the MHW and net losses of 20% of sediment C over the following 3 years. However, this effect was not uniform; outer meadow sites showed little evidence of shoot declines during the MHW and had net increases of 60% of sediment C over the following 3 years. Overall, sites with higher seagrass recovery maintained 1.7x as much C compared to sites with lower recovery. Our study demonstrates that while seagrass blue C is vulnerable to MHWs, localization of seagrass loss can prevent meadow-wide C losses. Long-term (decadal and beyond) stability of seagrass blue C depends on seagrass resilience to short-term disturbance events. 

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5. Effects of Hypoxia on Coupled Carbon and Iron Cycling Differ Between Weekly and Multiannual Timescales in Two Freshwater Reservoirs

   https://doi.org/10.1029/2022JG007071  

   Lewis, Abigail S. L.; Schreiber, Madeline E.; Niederlehner, B. R.; Das, Arpita; Hammond, Nicholas W.; Lofton, Mary E.; Wander, Heather L.; Carey, Cayelan C. (January 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Freshwater lakes and reservoirs play a disproportionate role in the global organic carbon (OC) budget, as active sites for carbon processing and burial. Associations between OC and iron (Fe) are hypothesized to contribute substantially to the stabilization of OC in sediment, but the magnitude of freshwater Fe‐OC complexation remains unresolved. Moreover, global declines in bottom‐water oxygen concentrations have the potential to alter OC and Fe cycles in multiple ways, and the net effects of low‐oxygen (hypoxic) conditions on OC and Fe are poorly characterized. Here, we measured the pool of Fe‐bound OC (Fe‐OC) in surficial sediments from two eutrophic reservoirs, and we paired whole‐ecosystem experiments with sediment incubations to determine the effects of hypoxia on OC and Fe cycling over multiple timescales. Our experiments demonstrated that short periods (2–4 weeks) of hypoxia can increase aqueous Fe and OC concentrations while decreasing OC and Fe‐OC in surficial sediment by 30%. However, exposure to seasonal hypoxia over multiple years was associated with a 57% increase in sediment OC and no change in sediment Fe‐OC. These results suggest that the large sediment Fe‐OC pool (∼30% of sediment OC in both reservoirs) contains both oxygen‐sensitive and oxygen‐insensitive fractions, and over multiannual timescales OC respiration rates may play a more important role in determining the effect of hypoxia on sediment OC than Fe‐OC dissociation. Consequently, we anticipate that global declines in oxygen concentrations will alter OC and Fe cycling, with the direction and magnitude of effects dependent upon the duration of hypoxia. 

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