Crystal Chemistry of Vanadium-Bearing Ellestadite Waste Forms

Vanadate ellestadites Ca₁₀(SiO₄)_<i>x</i>(VO₄)_{6–2<i>x</i>}(SO₄)_<i>x</i>Cl₂, 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 < <i>x</i> < 3 adopt the <i>P</i>6₃/<i>m</i> structure, while the vanadate endmember Ca₁₀(VO₄)₆Cl₂ is triclinic with space group <i>P</i>1̅. A miscibility gap exists for 0.77 < <i>x</i> < 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