Show Abstract ▼

Abstract: This study investigated proton adsorption to an extracellular polymeric substance (EPS) producing bacterial strain, Bacillus licheniformis S-86, in order to characterise and quantify the contribution made by EPS to cell surface reactivity. Potentiometric titrations were conducted using both untreated cells and cells from which the EPS layer had been extracted. Surface-complexation modelling indicated the presence of four different functional groups in both untreated and EPS-free cells. These sites are assigned to phosphodiester, (pKa 3.3–3.4), carboxylic (pKa 5.3–5.4), phosphoryl/ (pKa 7.4–7.5) and hydroxyl/amine (pKa 9.9–10.1) type groups. The pKa values for the four groups were very similar for untreated and EPS-free cells, indicating no qualitative difference in composition, but site concentrations in the untreated cells were statistically found to be significantly higher than those in the EPS-free cells for the pKa 3.3–3.4 and pKa 9.9–10.1 sites. Infrared analysis provided supporting evidence that site 2 is carboxylic in nature but did not reveal any difference in IR absorption between the native and EPS-free cells. Dissolved organic carbon (DOC) analysis conducted during this study indicated that DOC release by cells is significant, and that the EPS layer is the major contributor.

Show Abstract ▼

Abstract: Ce LIII-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of synthetic glasses prepared over a range of oxygen fugacities (fO2s, from ? 10 to + 11 in logarithmic units relative to the quartz–fayalite–magnetite, QFM, buffer), temperatures (1300–1500 °C), and pressures (1 atm and 1 GPa). The oxidation state ratio of Ce, Ce4 +/? Ce (where ? Ce = Ce3 ++ Ce4 +), was determined from the spectra allowing the relationships between Ce4 +/? Ce and fO2, temperature, pressure and melt composition to be determined. Ce4 +/? Ce varied systematically with fO2 from 0 to ~ 0.8 over the range of conditions studied. Ce4 + is stabilised relative to Ce3 + by less polymerised compositions and lower temperatures, while pressure appears to have almost no effect (possibly stabilising Ce4 +). Ce4 + in an Fe2 +-bearing melt is not preserved on cooling to a glass due to the reaction Ce4 ++ Fe2 + = Ce3 ++ Fe3 +. Ce4 +/? Ce in natural melts is exceedingly small but may be recorded in the mineral zircon as an increased abundance of Ce relative to the other rare earth elements, which occur exclusively in the trivalent state. The magnitude of this Ce anomaly has considerable potential as an oxy-barometer.

Show Abstract ▼

Abstract: Iron formations (IF) are iron- and silica-rich chemical precipitates that were deposited during the Precambrian. Several recent studies have demonstrated how the trace metal abundances in IF can be used as proxies for the bioavailability of trace metals in ancient seawater; with the ultimate goal being to understand first-order controls on the composition of the ancient biosphere. However, the utility of IF as proxies depends on the immobilization of trace metals during diagenesis. Here, we assess the mobility of Zn and Ni from ferric oxyhydroxides (ferrihydrite) in the absence and presence of organic matter (glucose) during simulated diagenesis (170 °C, 1.2 kbar); similar to what some Precambrian IF experienced. Quantitative concentration data, coupled with X-ray diffraction analysis and electron microprobe element mapping, demonstrate that both metals are relatively immobile during simulated diagenesis. Additionally, the mechanism for initial Ni sorption is examined using X-ray adsorption spectroscopy. For the initial sorption of trace elements in abiotic ferrihydrite experiments, 93.38% Zn and 65.95% Ni were initially sorbed. In experiments utilizing biogenic ferrihydrite, 97.03% of Zn and 93.38% of Ni were initially sorbed. Following the diagenetic capsule treatments, more than 99% of Zn and more than 91.9% of Ni were retained under the varied conditions considered here. Capsule experiments suggest the strong retention of Zn and Ni following the diagenesis of either abiotic or biogenic ferrihydrite. Overall, our results indicate that paleomarine Zn and Ni concentrations are likely to be faithfully recorded in well-preserved IF deposits.

Show Abstract ▼

Abstract: Organo-mineral composites formed by the association of iron (hydr)oxides and organic matter are widespread in natural environments and play an important role as scavengers of bioessential elements and contaminants. To better understand the mobility and fate of Cu in natural soils and sediments we precipitated ferrihydrite and goethite organo-mineral composites using humic acid as an analogue for natural organic matter, with organic carbon content in the composites ranging from 2 to 16 wt% C. We then measured the adsorption of Cu to the end-member mineral and organic phases and the composites as a function of pH and Cu concentration. We determined the molecular mechanisms of Cu adsorption to the end-member phases and the composites, and used this information to develop molecularly constrained thermodynamic surface complexation models to quantify Cu adsorption. By combining our work here with previous work on the adsorption of Cu to ferrihydrite-bacteria composites, we provide insight into the predominance of Cu-carboxyl binding for Cu adsorption to iron (hydr)oxide organo-mineral composites, and the nature of Cu adsorption behaviour across a range of iron (hydr)oxide composites composed of different minerals and different types of organic matter. Taken as a whole our results show that Cu adsorption to the carboxyl group present in organic matter coatings on iron (hydr)oxides is likely common to most iron (hydr)oxide composites, such that Cu-carboxyl binding provides a key control on the fate and mobility of Cu in soils and sediments. Our work also suggests there is a universal adsorption behaviour for Cu adsorption to ferrihydrite organo-mineral composites, in which the mineral:organic mass ratio is a crucial parameter for determining Cu uptake. Overall we show that ferrihydrite composites composed of different types of organic matter and containing a wide range of organic mass ratios, but where the mineral is the dominant composite fraction, possess additive Cu adsorption behaviour which can be predicted using a component additivity surface complexation model.

Show Abstract ▼

Abstract: Groundwater at legacy nuclear facilities around the world is contaminated with radionuclides including strontium-90 and technetium-99, which are often present as co-contaminants. Here we investigated whether biostimulation of indigenous microbial communities by glycerol phosphate can co-treat 90Sr through incorporation into phosphate biominerals, and 99Tc through microbially-induced reduction of the sediment to form less mobile Tc(IV) phases via reaction with reduced species (e.g. Fe(II)). Results showed that 95% of Sr was removed from solution in sediment microcosms treated with glycerol phosphate, and sequential extraction showed that ~18% of the Sr in the resulting solid phase was associated with the pH 5 Na-acetate fraction and 75% was in the ion exchangeable fraction. This removal and partitioning to recalcitrant phases during glycerol phosphate treatment was greater than in the untreated controls, where only 60% of Sr was removed from solution, and of thatsolid-associated Sr, 95% was present in the exchangeable fraction. Fitting of Sr K-edge EXAFS spectra confirmed these findings, with shell by shell fitting suggesting ~30% of sediment-associated Sr was present in a coordination environment consistent with phosphate biominerals following glycerol phosphate treatment, whilst Sr was present only as outer-sphere complexes in the controls. In addition,16S rRNA sequencing of sediments stimulated with glycerol phosphate demonstrated the growth of potential phosphate-solubilising species such as Chryseobacterium and Serratia spp. Finally, glycerol phosphate treatment stimulated bioreduction via addition of electron donor in the form of glycerol to the system, in turn this stimulated the removal of 99Tc from solution concomitant with microbial Fe(III) reduction to form poorly soluble hydrous Tc(IV)O2 like phases. In sediments amended with an electron donor, the microbial community also reflected the onset of bioreduction with an increased relative abundance of Fe(III)- and sulfate-reducing bacteria such as Geothrix, Geobacter and Desulfobulbus spp. Overall these results suggest application of glycerol phosphate offers a promising bioremediation strategy to co-treat both 90Sr and 99Tc contaminated groundwaters, and promotes the formation of Sr-phosphate and Tc(IV) bearing biominerals when reducing conditions are maintained. Combined with past work which shows the scavenging of uranium from solution following addition of glycerol phosphate, this extends the scope for glycerol phosphate as a treatment for radioactive contamination in groundwaters.