Geochemical modeling to predict the chemistry of leachates from alkaline solid wastes: oil shale deposits, western U.S.A.

Abstract-Oil shale processing at elevated temperatures to extract oil results in large amounts of alkaline oil shale solid wastes (OSSW). The objective of this study was to use a geochemical model to help predict the chemistry of leachates, including toxic chemicals, from OSSW. Several geochemical models were evaluated (e.g. EQ3EQ6, GEOCHEM, MINTEQAZ, PHREEQE, SOLMINEQ, WATEQFC); the model GEOCHEM was selected based on its more comprehensive capabilities. The OSSW samples were subjected to solubility and XRD studies. Element concentrations and pH of OSSW leachates were used as input to GEOCHEM to predict their chemistry. Ion activity products were used to infer the likely solid phases controlling the concentration of toxic elements (e.g. F and Mo) in these leachates. The model predicted that silicate phases produced during the heating process buffered the pH and controlled concentrations of major cations. The F concentrations in OSSW leachates appeared to be controlled by fluorite (CaF3. Contrary to previous findings, powellite (CaMo04) probably does not control the concentrations of Mo in OSSW leachates