Bioproduction of cerium-bearing magnetite and application to improve carbon-black supported platinum catalysts.

BACKGROUND: Biogeochemical processing of metals including the fabrication of novel nanomaterials from metal contaminated waste streams by microbial cells is an area of intense interest in the environmental sciences. RESULTS: Here we focus on the fate of Ce during the microbial reduction of a suite of Ce-bearing ferrihydrites with between 0.2 and 4.2 mol% Ce. Cerium K-edge X-ray absorption near edge structure (XANES) analyses showed that trivalent and tetravalent cerium co-existed, with a higher proportion of tetravalent cerium observed with increasing Ce-bearing of the ferrihydrite. The subsurface metal-reducing bacterium Geobacter sulfurreducens was used to bioreduce Ce-bearing ferrihydrite, and with 0.2 mol% and 0.5 mol% Ce, an Fe(II)-bearing mineral, magnetite (Fe(II)(III)2O4), formed alongside a small amount of goethite (FeOOH). At higher Ce-doping (1.4 mol% and 4.2 mol%) Fe(III) bioreduction was inhibited and goethite dominated the final products. During microbial Fe(III) reduction Ce was not released to solution, suggesting Ce remained associated with the Fe minerals during redox cycling, even at high Ce loadings. In addition, Fe L2,3 X-ray magnetic circular dichroism (XMCD) analyses suggested that Ce partially incorporated into the Fe(III) crystallographic sites in the magnetite. The use of Ce-bearing biomagnetite prepared in this study was tested for hydrogen fuel cell catalyst applications. Platinum/carbon black electrodes were fabricated, containing 10% biomagnetite with 0.2 mol% Ce in the catalyst. The addition of bioreduced Ce-magnetite improved the electrode durability when compared to a normal Pt/CB catalyst. CONCLUSION: Different concentrations of Ce can inhibit the bioreduction of Fe(III) minerals, resulting in the formation of different bioreduction products. Bioprocessing of Fe-minerals to form Ce-containing magnetite (potentially from waste sources) offers a sustainable route to the production of fuel cell catalysts with improved performance.

Microbial Reduction of Antimony(V)-Bearing Ferrihydrite by Geobacter sulfurreducens.

The reduction of Sb(V)-bearing ferrihydrite by Geobacter sulfurreducens was studied to determine the fate of the metalloid in Fe-rich systems undergoing redox transformations. Sb(V) added at a range of concentrations adsorbed readily to ferrihydrite, and the loadings had a pronounced impact on the rate and extent of Fe(III) reduction and the products formed. Magnetite dominated at low (0.5 and 1 mol%) Sb(V) concentrations, with crystallite sizes decreasing at higher Sb loadings: 37-, 25-, and 17-nm particles for no-Sb, 0.5% Sb, and 1% Sb samples, respectively. In contrast, goethite was the dominant end product for samples with higher antimony loadings (2 and 5 mol%), with increased goethite grain size in the 5% Sb sample. Inductively coupled mass spectrometry (ICP-MS) analysis confirmed that Sb was not released to solution during the bioreduction process, and X-ray photoelectron spectroscopy (XPS) analyses showed that no Sb(III) was formed throughout the experiments, confirming that the Fe(III)-reducing bacterium Geobacter sulfurreducens cannot reduce Sb(V) enzymatically or via biogenic Fe(II). These findings suggest that Fe (bio)minerals have a potential role in limiting antimony pollution in the environment, even when undergoing redox transformations. IMPORTANCE Antimony is an emerging contaminant that shares chemical characteristics with arsenic. Metal-reducing bacteria (such as Geobacter sulfurreducens) can cause the mobilization of arsenic from Fe(III) minerals under anaerobic conditions, causing widespread contamination of aquifers worldwide. This research explores whether metal-reducing bacteria can drive the mobilization of antimony under similar conditions. In this study, we show that G. sulfurreducens cannot reduce Sb(V) directly or cause Sb release during the bioreduction of the Fe(III) mineral ferrihydrite [although the sorbed Sb(V) did alter the Fe(II) mineral end products formed]. Overall, this study highlights the tight associations between Fe and Sb in environmental systems, suggesting that the microbial reduction of Fe(III)/Sb mineral assemblages may not lead to Sb release (in stark contrast to the mobilization of As in iron-rich systems) and offers potential Fe-based remediation options for Sb-contaminated environments.

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland.

The Rum Eastern Layered Intrusion (ELI; Scotland) is an open-system layered intrusion constructed of 16 macro-rhythmic units. Each of the macro-rhythmic units consists of a peridotite base and a troctolite (± gabbro) top, previously attributed to the fractional crystallisation of a single magma batch. This classic paradigm has been challenged, however, with evidence presented for the emplacement of peridotite sills in Units 9, 10, and 14, such as cross-cutting relationships, upward-oriented apophyses, and lateral discontinuities. To test whether the other major peridotites within the ELI represent sills, we have carried out new field, petrographic, and mineral chemical analyses of the peridotites in Units 7, 8 and 9. The peridotites display large- and small-scale cross-cutting relationships with the overlying troctolite, indicative of an intrusive relationship. The peridotites also show large-scale coalescence and lateral spatial discontinuities such that the ELI unit divisions become arbitrary. Harrisite layers and Cr-spinel seams found throughout Units 7, 8, and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma. Our observations allow for distinct subtypes of peridotite to be defined, separated by intrusive contacts, allowing for their relative chronology to be determined. Older, poikilitic peridotite, rich in clinopyroxene, is truncated by younger, well-layered peridotite, containing abundant harrisite layers. In addition to the new peridotite subtypes defined in this study, we find strong evidence for laterally oriented metasomatism within clinopyroxene-rich wehrlites at the top of the Unit 8 peridotite. The wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene-rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions. The observations presented in this study from the ELI cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre-existing feldspathic cumulate (gabbro or troctolite). The evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism, forming many of the unusual (often clinopyroxene-rich) lithologies that surround the sills. The broad-scale formation of the layered peridotites via incremental sill emplacement, suggested by the occurrence of upward-oriented apophyses, coalescence, and lateral discontinuity, could be applied to much larger ultramafic intrusions, which might have formed by similar mechanisms.

Synthesis, binding and bioactivity of gamma-methylene gamma-lactam ecdysone receptor ligands: advantages of QSAR models for flexible receptors.

Nuclear hormone receptors, such as the ecdysone receptor, often display a large amount of induced fit to ligands. The size and shape of the binding pocket in the EcR subunit changes markedly on ligand binding, making modelling methods such as docking extremely challenging. It is, however, possible to generate excellent 3D QSAR models for a given type of ligand, suggesting that the receptor adopts a relatively restricted number of binding site configurations or 'attractors'. We describe the synthesis, in vitro binding and selected in vivo toxicity data for gamma-methylene gamma-lactams, a new class of high-affinity ligands for ecdysone receptors from Bovicola ovis (Phthiraptera) and Lucilia cuprina (Diptera). The results of a 3D QSAR study of the binding of methylene lactams to recombinant ecdysone receptor protein suggest that this class of ligands is indeed recognised by a single conformation of the EcR binding pocket.