Synthesis, Crystal Chemistry and Stability of Ettringite , A Material with Potential Applications in Hazardous Waste Imm
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SYNTHESIS, CRYSTAL CHEMISTRY AND STABILITY OF ETfINGITE, A MATERIAL WITH POTENTIAL APPLICATIONS IN HAZARDOUS WASTE IMMOBILIZATION GREGORY J. McCARTHY*, DAVID J. HASSETIT** and JASON A. BENDER* * Department of Chemistry, North Dakota State University, Fargo, ND 58105 U.S.A. **Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202 U.S.A.
ABSTRACT Recent results on synthesis, crystal chemistry, structure and stability of ettringite*, with specific reference to use of this material in hazardous waste immobilization, are presented. Ettringites with selenate, chromate, borate, sulfite and carbonate replacing sulfate have been synthesized and characterized by XRD for phase purity and structure. In X-ray powder diffractograms, selenate and chromate etringiteshave the normal trigonal P3 Ic ettringite structure, whereas the borate, sulfite and carbonate ettringitescan be indexed on a hexagonal unit cell with a halved c parameter. In excess of 95% of the Se and B in solutions spiked with various concentrations of these elements has been removed by precipitation of etringite. Ettringitesappear to be unstable in solutions outside of the approximate pH range 11.0-12.5, and there is a potential for some oxyanion exchange between ettringitesand oxyanion-containing solutions. Buffering of pH, and dissolved Ca, Al and sulfate activities by other components of ettringite-basedhazardous waste forms, combined with low-reactivity C-S-H matrices, could largely mitigate these concerns. INTRODUCTION Previously in this symposium series, Hassett and Pflughoeft-Hassett [1,21 and Kumarathasan et al. [3] reported reductions in concentrations of several dissolved oxyanions (selenate, borate, arsenate) in leaching solutions over an ettringite*-forming composite material composed of gasification ash, coal ash and SO, scrubber residues. This potential connection between reductions in leachate hazardous element concentrations and ettringite formation led to a literature review of oxyanion substituted ettringites, and prompted an ongoing research program with the following objectives: (1) to investigate the possibility of oxyanion incorporation by synthesizing and characterizing partially and fully substituted ettringites; (2) to understand the crystal chemistry, structure, solid solution behavior, structure and thermal behavior of such ettringites; (3) to investigate and then optimize selenium and boron removal from solution by ettringite precipitation; (4) to investigate and then optimize immobilization of hazardous elements in solid wastes and sludges after solidification with ettringite-forming cementitious coal residuals or commercially-available cements; (5) to evaluate the long-term stability and hazardous element leachability of the etringite-containing products. For a detailed discussion of the literature review, the synthesis and characterization (elemental composition determined by ICAP, oxyanions by FTIR, phase identification and structure by XRD, thermal analysis by LOI and DSC) in the earl
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