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We investigated the chemistry of Hg(II) during exposure of exponentially growing bacteria ( Escherichia coli, Bacillus subtilis, and Geobacter sulfurreducens) to 50 nM, 500 nM, and 5 μM total Hg(II) with and without added cysteine. With X-ray absorption spectroscopy, we provide direct evidence of the formation of cell-associated HgS for all tested bacteria. The addition of cysteine (100-1000 μM) promotes HgS formation (>70% of total cell-associated Hg(II)) as a result of the biodegradation of added cysteine to sulfide. Cell-associated HgS species are also detected when cysteine is not added as a sulfide source. Two phases of HgS, cinnabar (α-HgS) and metacinnabar (β-HgS), form depending on the total concentration of Hg(II) and sulfide in the exposure medium. However, α-HgS exclusively forms in assays that contain an excess of cysteine. Scanning transmission electron microscopy images reveal that nanoparticulate HgS is primarily located at the cell surface/extracellular matrix of Gram-negative E. coli and G. sulfurreducens and in the cytoplasm/cell membrane of Gram-positive B. subtilis. Intracellular Hg(II) was detected even when the predominant cell-associated species was HgS. This study shows that HgS species can form from exogenous thiol-containing ligands and endogenous sulfide in Hg(II) biouptake assays under nondissimilatory sulfate reducing conditions, providing new considerations for the interpretation of Hg(II) biouptake results.

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Redox flow batteries (RFBs) are attractive long-duration energy storage solutions, especially if system costs can be further reduced. One approach to lowering the system cost is to increase the energy and power densities through the use of higher voltage redox chemistries. To date, most techno-economic studies have considered the maximum open-circuit voltage (OCV) of aqueous RFBs to be ≤1.5 V. We provide a simple techno-economic analysis to illustrate the impact of cell voltage on capital cost and note the secondary benefits that arise from the operational flexibility enabled by aqueous RFBs with OCVs >1.5 V. We subsequently discuss potential pathways to achieving higher voltage chemistries. The goal of this Perspective is to inspire the research community to re-examine the perceived upper limits of aqueous RFB voltages and to explore options for achieving high-voltage systems, as well as identify the need for fundamental research on a variety of topics.

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