CALCIUM MOLYBDATE SOLUBILITY IN SPENT OIL-SHALE AND A PRELIMINARY EVALUATION OF THE ASSOCIATION CONSTANTS FOR THE FORMATION OF CAMOO40(AQ), KMOO4-(AQ), AND NAMOO4-(AQ)

In order to better understand the fate and behavior of molybdenum in spent oil shale disposal environments, the equilibrium solubility of CaMoO4(c, powellite) in deionized-distilled water, potassium nitrate solutions, and spent oil shale leachates was determined. The calculated log Ksp for the dissolution of CaMoO4(c) is −8.05. The ion association constants for the formation of CaMoO40(aq) and KMoO4−(aq) were also determined by using an iterative estimation technique. The estimate of log Q for the formation of CaMoO40(aq) is 3.09. Similarly, an estimate of log Q for the formation of KMoO4−(aq) is 1.29. Calcium molybdate solubility in CaMoO4(c)-spiked and nonspiked spent oil shale equilibrium solutions was evaluated. The CaMoO4(c)-spiked spent oil shale solutions were initially evaluated as supersaturated with respect to CaMoO4(c). Evaluation of the data indicated that the NaMoO4−(aq) ion pair may significantly contribute to total soluble molybdenum. The spiked spent oil shale solution data are used to generate a log Q of 1.66 for the formation of NaMoO4−(aq). Inclusion of this species in the geochemical model analyses of the spiked spent oil shale solutions suggests that the solutions are in equilibrium with CaMoO4(c). Geochemical model analyses show that CaMoO4(c) does not control molybdenum concentrations in the spent oil shale (nonspiked) equilibrium solutions. Further, these data suggest that CaMoO4(c) may not occur in spent oil shales. © 1990, American Chemical Society. All rights reserved.