31 research outputs found
An integrated study of uranyl mineral dissolution processes: etch pit formation, effects of cations in solution, and secondary precipitation
Understanding the mechanism(s) of uranium-mineral dissolution is crucial for predictive modeling of U mobility in the subsurface. In order to understand how pH and type of cation in solution may affect dissolution, experiments were performed on mainly single crystals of curite, Pb2+3(H2O)2[(UO2)4O4(OH)3]2, becquerelite, Ca(H2O)8[(UO2)6O4(OH)6], billietite, Ba(H2O)7[(UO2)6O4(OH)6], fourmarierite Pb2+1−x(H2O)4[(UO2)4O3−2x(OH)4+2x] (x= 0.00-0.50), uranophane, Ca(H2O)5[(UO2)(SiO3OH)]2, zippeite, K3(H2O)3[(UO2)4(SO4)2O3(OH)], and Na-substituted metaschoepite, Na1−x[(UO2)4O2−x(OH)5+x] (H2O)n. Solutions included: deionized water; aqueous HCl solutions at pH 3.5 and 2; 0.5mol L−1 Pb(II)-, Ba-, Sr-, Ca-, Mg-, HCl solutions at pH 2; 1.0mol L−1 Na- and K-HCl solutions at pH 2; and a 0.1mol L−1 Na2CO3 solution at pH 10.5. Uranyl mineral basal surface microtopography, micromorphology, and composition were examined prior to, and after dissolution experiments on micrometer scale specimens using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Evolution of etch pit depth at different pH values and experimental durations can be explained using a stepwave dissolution model. Effects of the cation in solution on etch pit symmetry and morphology can be explained using an adsorption model involving specific surface sites. Surface precipitation of the following phases was observed: (a) a highly-hydrated uranyl-hydroxy-hydrate in ultrapure water (on all minerals), (b) a Na-uranyl-hydroxy-hydrate in Na2CO3 solution of pH 10.5 (on uranyl-hydroxy-hydrate minerals), (c) a Na-uranyl-carbonate on zippeite, (d) Ba- and Pb-uranyl-hydroxy-hydrates in Ba-HCl and Pb-HCl solutions of pH 2 (on uranophane), (e) a (SiOx(OH)4−2x) phase in solutions of pH 2 (uranophane), and (f) sulfate-bearing phases in solutions of pH 2 and 3.5 (on zippeite
An integrated study of uranyl mineral dissolution processes: Etch pit formation, effects of cations in solution, and secondary precipitation
ISSN:0033-8230ISSN:2193-340
New Clues to the Local Atomic Structure of Short-Range Ordered Ferric Arsenate from Extended X‑ray Absorption Fine Structure Spectroscopy
Short-range
ordered ferric arsenate (FeAsO<sub>4</sub>·<i>x</i>H<sub>2</sub>O) is a secondary As precipitate frequently
encountered in acid mine waste environments. Two distinct structural
models have recently been proposed for this phase. The first model
is based on the structure of scorodite (FeAsO<sub>4</sub>·2H<sub>2</sub>O) where isolated FeO<sub>6</sub> octahedra share corners
with four adjacent arsenate (AsO<sub>4</sub>) tetrahedra in a three-dimensional
framework (framework model). The second model consists of single chains
of corner-sharing FeO<sub>6</sub> octahedra being bridged by AsO<sub>4</sub> bound in a monodentate binuclear <sup>2</sup>C complex (chain
model). In order to rigorously test the accuracy of both structural
models, we synthesized ferric arsenates and analyzed their local (<6
Ã…) structure by As and Fe K-edge extended X-ray absorption fine
structure (EXAFS) spectroscopy. We found that both As and Fe K-edge
EXAFS spectra were most compatible with isolated FeO<sub>6</sub> octahedra
being bridged by AsO<sub>4</sub> tetrahedra (<i>R</i><sub>Fe–As</sub> = 3.33 ± 0.01 Å). Our shell-fit results
further indicated a lack of evidence for single corner-sharing FeO<sub>6</sub> linkages in ferric arsenate. Wavelet-transform analyses of
the Fe K-edge EXAFS spectra of ferric arsenates complemented by shell
fitting confirmed Fe atoms at an average distance of ∼5.3 Å,
consistent with crystallographic data of scorodite and in disagreement
with the chain model. A scorodite-type local structure of short-range
ordered ferric arsenates provides a plausible explanation for their
rapid transformation into scorodite in acid mining environments
Speciation of Zn in Blast Furnace Sludge from Former Sedimentation Ponds Using Synchrotron X-ray Diffraction, Fluorescence, and Absorption Spectroscopy
Blast furnace sludge (BFS), an industrial waste generated in pig iron production, typically contains high contents of iron and various trace metals of environmental concern, including Zn, Pb, and Cd. The chemical speciation of these metals in BPS is largely unknown. Here, we used a combination of synchrotron X-ray diffraction, micro-X-ray fluorescence, and X-ray absorption spectroscopy at the Zn K-edge for solid-phase Zn speciation in 12 BPS samples collected on a former BFS sedimentation pond site. Additionally, one fresh BPS was analyzed for comparison. We identified five major types of Zn species in the BFS, which occurred in variable amounts: (1) Zn in the octahedral sheets of phyllosilicates, (2) Zn sulfide minerals (ZnS, sphalerite, or wurtzite), (3) Zn in a KZn-ferrocyanide phase (K2Zn3[Fe-(CN)(6)](2)center dot 9H(2)O), (4) hydrozincite (Zn-5(OH)(6)(CO3)(2)), and (5) tetrahedrally coordinated adsorbed Zn. The minerals franklinite (ZnFe2O4) and smithsonite (ZnCO3) were not detected, and zincite (ZnO) was detected only in traces. The contents of ZnS were positively correlated with the total S contents of the BPS. Similarly, the abundance of the KZn-ferrocyanide phase was closely correlated with the total CN contents, with the stoichiometry suggesting this as cyanide phase. This study provides the first quantitative Zn speciation in BFS deposits, which is of great relevance environmental risk assessment, the development of new methods for recovering Zn and Fe from BPS, and potential applications of BFS as sorbent materials in wastewater treatment