Crystal Chemistry of Vanadium-Bearing Ellestadite Waste Forms

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.