Trace element mineral transformations associated with hydration and recarbonation of retorted oil shale

A laboratory study was conducted to evaluate the influence of hydration and recarbonation on the solidphase distribution of trace elements in retorted oil shale. The oil shale samples were retorted by the Paraho direct heating process and equilibrated with deionized—distilled water under controlled carbon dioxide conditions. A sequential extraction technique was then used to fractionate trace elements into soluble, KNO₃-extractable (easily exchangeable), H₂O-extractable (easily adsorbed), NaOh-extractable (organic), EDTA-extractable (carbonate), HNO₃-extractable (sulfide), and residual (nonextractable silicate) phases. The chemical fractions present in retorted oil shale and hydrated and recarbonated retorted oil shale were compared to identify trace element mineralogical changes that may occur in retorted oil shale disposal environments.Trace elements examined in this study were found to reside predominantly in the HNO₃-extractable and residual fractions. Hydration of retorted oil shale resulted in a shift in the majority of trace elements from residual to extractable forms. Cobalt, nickel, and zinc extractabilities were not significantly influenced by hydration, whereas antimony increased in the residual fraction. Subjecting retorted oil shale to atmospheric (0.033%) and 10% CO₂(g) levels over a nine-month equilibration period resulted in partial and full recarbonation, respectively. As the influence of recarbonation increased, trace elements reverted to residual forms. Vanadium, choromium, copper, zinc, antimony, and molybdenum in the 10% CO₂(g) recarbonated material were more resistant to sequential extraction than in retorted oil shale, whereas strontium, barium, and manganese were less resistant to sequential extraction. The extractabilities of cobalt, nickel, and lead were not affected by recarbonation. Recarbonation did not result in a predicted increase in EDTA-extractable trace elements. In general, the amounts of trace elements extracted by EDTA (and correlated to carbonate forms) were invariant with respect to equilibrium CO₂(g) levels.A significant result of this study was that the mineralogical residencies of trace elements in retorted oil shale were altered in response to conditions that may be present in a disposal environment. Thus, the long-term release of trace elements in retorted oil shale disposal environments may not be adequately predicted by applying the toxicity characteristic leaching procedure (TCLP).