WWRC 93-03
Geochemical Modeling to Predict the Chemistry of Leachates from Alkaline Solid Wastes: Oil Shale Deposits, Western USA
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. EQ3/EQ6, GEOCHEM, MINTEQA2, 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 (CaF2. Contrary to previous findings, powellite (CaMoO4) probably does not control the
concentrations of Mo in OSSW leachates.
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