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Lepidocrocite, as a ubiquitous iron mineral, is widely detected in different morphologies in natural environments, controlling the mobility and availability of heavy metal ions (HMIs). These different morphologies of lepidocrocite commonly exhibit different surface reactivities due to its exposed facets. However, the adsorption mechanism of HMIs onto lepidocrocite facets remains ambiguous. In this study, Cr(vi) adsorption on lepidocrocite with different exposed facets was investigated viamacroscopic batch experiments, synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy, in situATR-FTIR spectroscopy, and density functional theory (DFT) calculations. Macroscopic experimental results showed that the maximum Cr(vi) adsorption capacity for rod-like lepidocrocite with a higher proportion of {001}/{010} (R-LEP) was 0.137 mg m−2, 2.21 times higher than that of plate-like lepidocrocite (P-LEP). Meanwhile, the Cr(vi) site density on the {001} facet was estimated to be 4.30 #Cr nm−2, much higher than that on the {010} facets (0.42 #Cr nm−2). The fitting results of EXAFS spectroscopy revealed that Cr(vi) coordinated to the surfaces of P-LEP and R-LEP viathe formation of both bidentate and monodentate inner-sphere complexes. In situATR-FTIR spectroscopy combined with theoretical frequency calculations showed that two Cr(vi) species formed in different relative proportions on P-LEP and R-LEP, the bidentate binuclear complex was the dominant species on the {001} facets, and the protonated monodentate binuclear configuration formed on the {010} facets. DFT+U calculations further confirmed that the two Cr(vi) surface structures on their corresponding facets both exhibited high thermodynamic stability. This study will help to understand the roles of lepidocrocite in mobilization of HMIs in environments and provide guidance to design an efficient adsorbent for environmental remediation.

The photochemical reduction of Hg(II) is an important pathway in the environmental Hg cycle because it competes with Hg methylation and potentially limits the formation of bioaccumulative methylmercury. Hg stable isotope systematics have proven to be an effective tool for investigating the transport, transformation, and bioaccumulation of Hg. The dominant cause of mass independent isotope fractionation (MIF) of Hg in nature is the photochemical reduction of various species of Hg(II). However, it is difficult to fully interpret Hg stable isotope signatures due to the lack of mechanistic information about which Hg compounds are susceptible to MIF and why. This study investigates Hg isotope fractionation during the photochemical reduction of Hg(II) complexed to organic ligands, which are representative of the available binding sites in natural dissolved organic matter. The photochemical reduction of Hg(II) in the presence of cysteine resulted in both negative and positive MIF in residual Hg(II), where the sign depended on pH and dissolved oxygen level. In the presence of serine, either nuclear volume or magnetic isotope effects were observed depending on the wavelength of light and the extent of Hg(II) complexation by serine. In the presence of ethylenediamine, MIF was negative. Our Hg stable isotope results suggest that MDF and MIF are induced at different steps in the overall photochemical reduction reaction and that MIF does not depend on the rate-determining step but instead depends on photophysical aspects of the reaction such as intersystem crossing and hyperfine coupling. The behavior of Hg isotopes reported here will allow for a better understanding of the underlying reaction mechanisms controlling the Hg isotope signatures recorded in natural samples.

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensisMR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (<3000 Da) soluble EOCs produced by MR-1 to washed cells restored Cr(VI) reduction though Cr(VI) reduction was fastest when both size fractions were added together. Spectral and electrochemical characterization of EOCs indicated the presence of flavins and c-type cytochromes (c-Cyts). A model of the kinetics of individual elementary reactions between cells, flavins, released c-Cyts, and Cr(VI), including the direct reduction of flavins, released c-Cyts, and Cr(VI) by cells and the indirect reduction of Cr(VI) by reduced forms of flavins and released c-Cyts, was developed. Model results suggest that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins. Hence, extracellular c-Cyts produced by MR-1 likely play a role in extracellular electron transfer under electron acceptor-limited conditions. These findings provide new insights into extracellular electron shuttling and the metabolic strategy of metal-reducing bacteria under electron acceptor-limited conditions.

Although it has been established that electron mediators substantially promote extracellular electron transfer (EET), electron shuttling pathways are not fully understood. Here, a new electron shuttling pathway was found in the EET process by Shewanella oneidensisMR-1 with resazurin, a lipophilic electron mediator. With resazurin, the genes encoding outer-membrane cytochromes (mtrCBAand omcA) were downregulated. Although cytochrome deletion substantially reduced biocurrent generation to 1–12% of that of wild-type (WT) cells, the presence of resazurin restored biocurrent generation to 168 μA·cm–2(ΔmtrA/omcA/mtrC), nearly equivalent to that of WT cells (194 μA·cm–2), indicating that resazurin-mediated electron transfer was not dependent on the Mtr pathway. Biocurrent generation by resazurin was much lower in ΔcymAand ΔmtrA/omcA/mtrC/fccA/cctAmutants (4 and 6 μA·cm–2) than in WT cells, indicating a key role of FccA, CctA, and CymA in this process. The effectiveness of resazurin in EET of Mtr cytochrome mutants is also supported by cyclic voltammetry, resazurin reduction kinetics, and in situ c-type cytochrome spectroscopy results. The findings demonstrated that low molecular weight, lipophilic electron acceptors, such as phenoxazine and phenazine, may facilitate electron transfer directly from periplasmic and inner membrane proteins, thus providing new insight into the roles of exogenous electron mediators in electron shuttling in natural and engineered biogeochemical systems.