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Crystal Chemistry of Vanadium-Bearing Ellestadite Waste Forms

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journal contribution
posted on 16.07.2018, 13:10 authored by Yanan Fang, Sam J. Page, Gregory J. Rees, Maxim Avdeev, John V. Hanna, Tim J. White
Vanadate ellestadites Ca10(SiO4)x(VO4)6–2x(SO4)xCl2, serving as prototype crystalline matrices for the fixation of pentavalent toxic metals (V, Cr, As), were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance (SS-NMR). The ellestadites 0.19 < x < 3 adopt the P63/m structure, while the vanadate endmember Ca10(VO4)6Cl2 is triclinic with space group P1̅. A miscibility gap exists for 0.77 < x < 2.44. The deficiency of Cl in the structure leads to short-range disorder in the tunnel. Toxicity characteristic leaching testing (TCLP) showed the incorporation of vanadium increases ellestadite solubility, and defined a waste loading limit that should not exceed 25 atom % V to ensure small release levels.