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Title: Uptake of nickel by synthetic mackinawite

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Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
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Resource Type:
Journal Article
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Journal Name: Chemical Geology; Journal Volume: 462
Country of Publication:
United States

Citation Formats

Wilkin, Richard T., and Beak, Douglas G. Uptake of nickel by synthetic mackinawite. United States: N. p., 2017. Web. doi:10.1016/j.chemgeo.2017.04.023.
Wilkin, Richard T., & Beak, Douglas G. Uptake of nickel by synthetic mackinawite. United States. doi:10.1016/j.chemgeo.2017.04.023.
Wilkin, Richard T., and Beak, Douglas G. Thu . "Uptake of nickel by synthetic mackinawite". United States. doi:10.1016/j.chemgeo.2017.04.023.
title = {Uptake of nickel by synthetic mackinawite},
author = {Wilkin, Richard T. and Beak, Douglas G.},
abstractNote = {},
doi = {10.1016/j.chemgeo.2017.04.023},
journal = {Chemical Geology},
number = ,
volume = 462,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
  • As(III) uptake from solution by synthetic mackinawite is examined as a function of pH and initial As(III) concentration using X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). XAS data indicate that when mackinawite is reacted at pH 5, 7, and 9 with 5 x 10{sup -4} M As(III), arsenic is reduced from its original +3 valence state and is primarily coordinated as As-S ({approx}2.26 {angstrom}) and As-As ({approx}2.54 {angstrom}), which is consistent with the formation of a realgar-like phase in agreement with XRD data. At 5 x 10{sup -5} M As(III), samples are markedly different from those collected at anmore » order of magnitude higher concentration and differ at each pH value. The XAS analysis of mackinawite samples reacted with 5 x 10{sup -5} M As(III) shows a transition from As-O coordination to As-S coordination as pH decreases, with the sample reacted at pH 5 resembling realgar. Under alkaline conditions, arsenic retains its original valence state of +3 and is primarily coordinated to oxygen at a distance of 1.75 {angstrom}. This may be attributed to uptake by adsorption as an As(III) oxyanion. These results provide the basis for selecting the reactions needed for modeling and are beneficial in understanding the mechanisms of arsenite uptake by mackinawite under anoxic sulfidic conditions.« less
  • The retention of radionuclides by interaction with mineral phases has significant consequences for the planning of their short- and long-term disposal to geological systems. An understanding of binding mechanisms is important in determining the ultimate fate of radionuclides following release into natural systems and will give increased confidence in predictive models. X-ray absorption spectroscopy (XAS) has been used to study the local environment of uranium taken up from aqueous solution by the surfaces of goethite, lepidocrocite, muscovite, and mackinawite. On both iron hydroxides uranium uptake occurs by surface complexation and ceases when the surface is saturated. The muscovite surface doesmore » not become saturated and uptake increases linearly suggesting formation of a uranium phase on the surface. Uranium uptake on mackinawite also suggests a replacement or precipitation process. XAS indicates that bidentate inner-sphere surface complexes are formed on the iron hydroxides by coordination of two surface oxygens from an iron octahedron in the equatorial plane of the complex. Uranium uptake on muscovite may occur through surface precipitation, the first layer of uranium atoms binding through equatorial coordination of two adjacent surface oxygens from a silicate tetrahedron, with the axial oxygens of the uranyl unit aligned across the hexagonal cavities created by the rings of tetrahedra. At low concentrations, uptake on mackinawite occurs at locally oxidized regions on the surface via a similar mechanism to that on iron hydroxides. At the highest concentrations, equatorial oxygen bond distances around 2.0--2.1 {angstrom} are observed, inconsistent with the presence of uranyl species. The average number of axial oxygens also decreases with increasing concentration, and these results suggest partial reduction or uranium. The nature of these different surface reactions plays an important role in assessing the geochemical behavior of uranium in natural systems, particularly under reducing conditions.« less
  • Iron sulfide was synthesized by reacting aqueous solutions of sodium sulfide and ferrous chloride for 3 days. By X-ray powder diffraction (XRPD), the resultant phase was determined to be primarily nanocrystalline mackinawite (space group: P4/ nmm) with unit cell parameters a = b = 3.67 {angstrom} and c = 5.20 {angstrom}. Iron K-edge XAS analysis also indicated the dominance of mackinawite. Lattice expansion of synthetic mackinawite was observed along the c-axis relative to well-crystalline mackinawite. Compared with relatively short-aged phase, the mackinawite prepared here was composed of larger crystallites with less elongated lattice spacings. The direct observation of lattice fringesmore » by HR-TEM verified the applicability of Bragg diffraction in determining the lattice parameters of nanocrystalline mackinawite from XRPD patterns. Estimated particle size and external specific surface area (SSA{sub ext}) of nanocrystalline mackinawite varied significantly with the methods used. The use of Scherrer equation for measuring crystallite size based on XRPD patterns is limited by uncertainty of the Scherrer constant (K) due to the presence of polydisperse particles. The presence of polycrystalline particles may also lead to inaccurate particle size estimation by Scherrer equation, given that crystallite and particle sizes are not equivalent. The TEM observation yielded the smallest SSA{sub ext} of 103 m{sup 2}/g. This measurement was not representative of dispersed particles due to particle aggregation from drying during sample preparation. In contrast, EGME method and PCS measurement yielded higher SSA{sub ext} (276-345 m{sup 2}/g by EGME and 424 {+-} 130 m{sup 2}/g by PCS). These were in reasonable agreement with those previously measured by the methods insensitive to particle aggregation.« less
  • The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) under oxic conditions. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(VI) solid phases by oxygen and Fe(III). Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO2) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO2 in the presence and absence of synthetic mackinawite (FeS) under simulated groundwatermore » conditions of pH = 7, PO2 = 0.02 atm, and PCO2 = 0.05 atm (equivalent to total dissolved carbonate of 0.01 M). The kinetic profiles of dissolved uranium indicate that FeS inhibited UO2 dissolution for 51 hr by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mössbauer and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO2 oxidative dissolution occurred at an initial rate of rm = 1.2 ± 0.4 ×10-8 mol•g-1•s-1, higher than rm = 5.4 ± 0.3 ×10-9 mol•g-1•s-1 in the control experiment where FeS was absent. Soluble U(VI) products were adsorbed by iron oxyhydroxides (i.e. nanogoethite and ferrihydrite) formed from FeS oxidation, which facilitated the detachment of U(VI) surface complexes and more rapid dissolution of UO2. XAS analysis confirmed the adsorption of U(VI) species, and also showed that U(VI) was not significantly incorporated into iron oxyhydroxide structure. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events.« less