Solid solutions between CrO4- and SO4-ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)(1-x)]3*26 H2O.
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 K(CrO4−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 (a0) 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.
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, CH-8600 Duebendorf, Switzerland. sabine.leisinger@eawag.ch
Sabine M
SM
Leisinger
Barbara
B
Lothenbach
Gwenn
G
Le Saout
Ralf
R
Kägi
Bernhard
B
Wehrli
C Annette
CA
Johnson
D000536:Aluminum Hydroxide / Q000737:chemistry; D002840:Chromates / Q000737:chemistry*; D004785:Environmental Pollutants / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008903:Minerals / Q000737:chemistry*; D008956:Models, Chemical; D012037:Refuse Disposal; D012995:Solubility; D012996:Solutions / Q000737:chemistry; D013431:Sulfates / Q000737:chemistry*
D002840:Chromates; D004785:Environmental Pollutants; D008903:Minerals; D012996:Solutions; D013431:Sulfates; C501337:ettringite; D000536:Aluminum Hydroxide