Role of Ester Sulfate and Organic Disulfide in Mercury Methylation in Peatland Soils.

We examined the composition and spatial correlation of sulfur and mercury pools in peatland soil profiles by measuring sulfur speciation by 1s X-ray absorption near-edge structure spectrocopy and mercury concentrations by cold vapor atomic fluorescence spectroscopy. Also investigated were the methylation/demethylation rate constants and the presence of hgcAB genes with depth. Methylmercury (MeHg) concentration and organic disulfide were spatially correlated and had a significant positive correlation (p < 0.05). This finding is consistent with these species being products of dissimilatory sulfate reduction. Conversely, a significant negative correlation between organic monosulfides and MeHg was observed, which is consistent with a reduction in Hg(II) bioavailability via complexation reactions. Finally, a significant positive correlation between ester sulfate and instantaneous methylation rate constants was observed, which is consistent with ester sulfate being a substrate for mercury methylation via dissimilatory sulfate reduction. Our findings point to the importance of organic sulfur species in mercury methylation processes, as substrates and products, as well as potential inhibitors of Hg(II) bioavailability. For a peatland system with sub-µmol L-1 porewater concentrations of sulfate and hydrogen sulfide, our findings indicate that the solid-phase sulfur pools, which have a much larger sulfur concentration range, may be accessible to microbial activity or exchanging with the porewater.

Chemical state mapping of simulant Chernobyl lava-like fuel containing material using micro-focused synchrotron X-ray spectroscopy.

Uranium speciation and redox behaviour is of critical importance in the nuclear fuel cycle. X-ray absorption near-edge spectroscopy (XANES) is commonly used to probe the oxidation state and speciation of uranium, and other elements, at the macroscopic and microscopic scale, within nuclear materials. Two-dimensional (2D) speciation maps, derived from microfocus X-ray fluorescence and XANES data, provide essential information on the spatial variation and gradients of the oxidation state of redox active elements such as uranium. In the present work, we elaborate and evaluate approaches to the construction of 2D speciation maps, in an effort to maximize sensitivity to the U oxidation state at the U L3-edge, applied to a suite of synthetic Chernobyl lava specimens. Our analysis shows that calibration of speciation maps can be improved by determination of the normalized X-ray absorption at excitation energies selected to maximize oxidation state contrast. The maps are calibrated to the normalized absorption of U L3 XANES spectra of relevant reference compounds, modelled using a combination of arctangent and pseudo-Voigt functions (to represent the photoelectric absorption and multiple-scattering contributions). We validate this approach by microfocus X-ray diffraction and XANES analysis of points of interest, which afford average U oxidation states in excellent agreement with those estimated from the chemical state maps. This simple and easy-to-implement approach is general and transferrable, and will assist in the future analysis of real lava-like fuel-containing materials to understand their environmental degradation, which is a source of radioactive dust production within the Chernobyl shelter.