Solid Solutions between CrO4- and SO4-Ettringite Ca6(Al(OH)6)2[(CrO4) x (SO4)1-x ]326 H2O

• Published in: Environmental science & technology Vol. 44; no. 23; pp. 8983 - 8988
• Main Authors: Leisinger, Sabine M, Lothenbach, Barbara, Saout, Gwenn Le, Kägi, Ralf, Wehrli, Bernhard, Johnson, C. Annette
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01.12.2010
• Subjects: Aluminum Hydroxide - chemistry; Chromates - chemistry; Environmental Modeling; Environmental Pollutants - chemistry; Hydrogen-Ion Concentration; Minerals - chemistry; Models, Chemical; Refuse Disposal; Solubility; Solutions - chemistry; Sulfates - chemistry
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es100554v

Chromate is a toxic contaminant of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment. In cementitous systems, CrO4 2− is thought to be incorporated as a solid solution with SO4 2− in ettringite. The formation of a solid solution (SS) could lower the soluble CrO4 2− concentrations. Ettringite containing SO4 2− or CrO4 2− and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 °C, a solubility product log K S0 of −40.2 ± 0.4 was calculated: log KCrO4−ettringite = 6log{Ca2+} + 2log{Al(OH)4 −} + 3log{CrO4 2−} + 4log{OH−} + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 ≤ XCrO4 ≤ 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a 0) of 2.03. The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO4 2− in cementitious systems.