Dissimilatory Fe(III) Reduction Controls on Arsenic Mobilization: A Combined Biogeochemical and NanoSIMS Imaging Approach

Microbial metabolism plays a key role in controlling the fate of toxic groundwater contaminants, such as arsenic. Dissimilatory metal reduction catalyzed by subsurface bacteria can facilitate the mobilization of arsenic via the reductive dissolution of As(V)-bearing Fe(III) mineral assemblages. The mobility of liberated As(V) can then be amplified via reduction to the more soluble As(III) by As(V)-respiring bacteria. This investigation focused on the reductive dissolution of As(V) sorbed onto Fe(III)-(oxyhydr)oxide by model Fe(III)- and As(V)-reducing bacteria, to elucidate the mechanisms underpinning these processes at the single-cell scale. Axenic cultures of Shewanella sp. ANA-3 wild-type (WT) cells [able to respire both Fe(III) and As(V)] were grown using 13C-labeled lactate on an arsenical Fe(III)-(oxyhydr)oxide thin film, and after colonization, the distribution of Fe and As in the solid phase was assessed using nanoscale secondary ion mass spectrometry (NanoSIMS), complemented with aqueous geochemistry analyses. Parallel experiments were conducted using an arrA mutant, able to respire Fe(III) but not As(V). NanoSIMS imaging showed that most metabolically active cells were not in direct contact with the Fe(III) mineral. Flavins were released by both strains, suggesting that these cell-secreted electron shuttles mediated extracellular Fe(III)-(oxyhydr)oxide reduction, but did not facilitate extracellular As(V) reduction, demonstrated by the presence of flavins yet lack of As(III) in the supernatants of the arrA deletion mutant strain. 3D reconstructions of NanoSIMS depth-profiled single cells revealed that As and Fe were associated with the cell surface in the WT cells, whereas for the arrA mutant, only Fe was associated with the biomass. These data were consistent with Shewanella sp. ANA-3 respiring As(V) in a multistep process; first, the reductive dissolution of the Fe(III) mineral released As(V), and once in solution, As(V) was respired by the cells to As(III). As well as highlighting Fe(III) reduction as the primary release mechanism for arsenic, our data also identified unexpected cellular As(III) retention mechanisms that require further investigation.

Distribution and Geochemical Controls of Arsenic and Uranium in Groundwater-Derived Drinking Water in Bihar, India

Chronic exposure to groundwater containing elevated concentrations of geogenic contaminants such as arsenic (As) and uranium (U) can lead to detrimental health impacts. In this study, we have undertaken a groundwater survey of representative sites across all districts of the State of Bihar, in the Middle Gangetic Plain of north-eastern India. The aim is to characterize the inorganic major and trace element aqueous geochemistry in groundwater sources widely used for drinking in Bihar, with a particular focus on the spatial distribution and associated geochemical controls on groundwater As and U. Concentrations of As and U are highly heterogeneous across Bihar, exceeding (provisional) guideline values in ~16% and 7% of samples (n = 273), respectively. The strongly inverse correlation between As and U is consistent with the contrasting redox controls on As and U mobility. High As is associated with Fe, Mn, lower Eh and is depth-dependent; in contrast, high U is associated with HCO3−, NO3− and higher Eh. The improved understanding of the distribution and geochemical controls on As and U in Bihar has important implications on remediation priorities and selection, and may contribute to informing further monitoring and/or representative characterization efforts in Bihar and elsewhere in India.

Crystallisation of REE carbonates from aqueous solutions

<p>Low temperature aqueous synthesis of Rare Earth Element (REE) carbonates show extensive variability in the resulting minerals. Precipitated mineral phases and crystallisation rates vary depending, in part, on the REE used. Indeed, much of the work to date on REE aqueous geochemistry focuses on the individual behaviour of discrete REEs. </p><p><br>We present a low temperature aqueous geochemical investigation of REE carbonate crystallisation pathways, which takes into consideration the influence of multiple REEs in solution. This serves to mimic more realistic conditions that are found in natural geological settings propitious to REE mineralisation. Our experiments focus on the behaviour of La, Ce, Nd, Dy carbonates at 30<sup>o</sup>C.</p><p><br>Concordant with previous studies, our results suggest that the crystallisation process of REE carbonates begins with the formation of an amorphous phase that transitions into a crystalline phase after a lag time that depends on the element and the proportions in the mixture. </p><p><br>This lag time is REE specific and is shorter for lighter REE compared to their heavier counterparts. In particular, the presence of another REE in the system affects the crystallisation timings and the morphology of the resulting crystals. For example, samples of mixed La/Nd carbonates begin their phase transition at lag times in between that of the two end-members (i.e. La and Nd) carbonate compositions. Furthermore, we find that the resulting growth rates and crystal habits are unique to the ratio of the REE mixture, with the underlying ionic potential of the mixture linked to the growth rates. In addition, observations throughout the crystallisation process also show that growth begins with flocculation of nanoparticles followed by crystal growth via Ostwald ripening.</p><p><br>REEs are sought after due to their unique properties and are integral to modern technologies such as lasers, catalytic converters, batteries, electro-magnets and wind turbines. Considering how the crystallisation behaviour with REE mixtures differs from that of discrete REE in solution, this work gives insights into the fundamental chemistry of REEs in aqueous solutions - relevant for studies of REE mineralisation and materials processing. </p>