Micro-spectroscopic investigation of selenium-bearing minerals from the Western US Phosphate Resource Area

Abstract

Mining activities in the US Western Phosphate Resource Area ͑WPRA͒ have released Se into the environment. Selenium has several different oxidation states and species, each having varying degrees of solubility, reactivity, and bioavailability. In this study we are investigating the speciation of Se in mine-waste rocks. Selenium speciation was determined using bulk and micro-x-ray absorption spectroscopy ͑XAS͒, as well as micro-x-ray fluorescence mapping. Rocks used for bulk-XAS were ground into fine powders. Shale used for micro-XAS was broken along depositional planes to expose unweathered surfaces. The near edge region of the XAS spectra ͑XANES͒ for the bulk rock samples revealed multiple oxidation states, with peaks indicative of Se͑ϪII͒, Se͑IV͒, and Se͑ϩVI͒ species. Micro-XANES analysis of the shale indicated that three unique Se-bearing species were present. Using the XANES data together with ab initio fitting of the extended x-ray absorption fine structure region of the micro-XAS data ͑micro-EXAFS͒ the three Se-bearing species were identified as dzharkenite, a di-selenide carbon compound, and Se-substituted pyrite. Results from this research will allow for a better understanding of the biogeochemical cycling of Se in the WPRA.

Selenium Speciation in Framboidal and Euhedral Pyrites in Shales

“…Sample spectra exhibited noisier and more complex oscillations compared to the standards (Figure 3 and in Figure S7 in the SI) due to the lower concentration of Se (max 670 ppm in euhedral grains; Table 2). The fingerprints in k-space between both POI and the synthetic ferroselite 29 showed that both spectra were comparable in phase but with smaller oscillatory amplitudes at low wavenumbers. The sample spectra were clearly out of phase with the seleno-L-cysteine, Se sulfide, and elemental Se standards (Figures S7 and S10 in the SI).…”

“…Se K-edge μ-XANES spectra from POIa and POIb and selected standards. We also included the Se(-I) ferroselite standard from Ryser et al 29 but note that the spectral position in this spectrum is offset, yet the general shape and amplitude of the spectrum can be compared with our data. Se Chemical Environment and Bonding Characteristics.…”

“…In the enlarged image and the X-ray maps for POIb the dotted lines are just to guide the eye and show the distortion. variable numbers of shells at close proximity to Se, similar to the approach of Ryser et al 29 Specifically, we trial fitted our spectra using the models of pyrite 30 and marcasite 30 (FeS 2 ) and also ferroselite 31 (FeSe 2 ) but fits to marcasite were not successful. We did not fit our spectra to dzharkenite (a ferroselite polymorph FeS 2 ), 29 because this model has Se−Se distances of 2.28 Å and our very first tests showed no fit to Se−Se distances for the first shell.…”

“…We did not align the Ryser et al 29 spectrum with our spectra as the details of beamline set up were unavailable, although, to the best of our knowledge, the ferroselite spectrum was collected with a step size of 0.7 eV. 29 Despite the similarities between POIa and POIb and Se(-II) sulfide, Se(-I) ferroselite, and elemental Se(0), it was not possible to determine the oxidation state, or the local coordination environment, of Se in our samples as the uncertainty of the data at the XANES region was 0.4 eV and the XANES spectra for the Se(-II) sulfide, Se(-I) ferroselite, and elemental Se(0) standards are all visually similar. Based on our comparisons we concluded from the μ-XANES data that the possible Se oxidation states at both POI were Se(-II), Se(-I), or Se(0).…”

“…Although Se concentrations varied greatly between the two morphologies or within the same morphological type in a single sample, overall Se concentrations tended to be higher in euhedral grains (Table 2 and Table S8 in the SI). Because of the close link between shale pyrite weathering and Se release, 8,12,22,23,29 hereafter we focused our Se μ-XAS analyses solely on samples UK-1 and UK-2. These represent typical shales containing pyrite and organic matter (Table S6 in the SI) and the highest bulk Se concentrations among the samples studied.…”