Characterization of water extracts of oil shale retorting residues form gaseous and solid heat carrier processes

• Content of trace elements and organics in the retorting ash and semi-coke and their leachates is presented.
• Release of elements and organics from retorting ash and semi-coke is minimal.
• Leachate of retorting ash is toxic to Daphnia magna at material's own pH.
• All water extracts are toxic to tested organisms at lower pH.
• The toxicity of retorting ash and semi-coke correlates with the release of anions.

The present work focuses on the environmental effect of oil shale retorting residues originating from gaseous (GHC) and solid heat carrier (SHC) technologies. Samples were taken from Viru Keemia Grupp Oil factory (VKG Oil Ltd, Estonia). The water extracts of semi-coke samples from GHC retorting plants were non-toxic. The leachate water of SHC retorting ash was acutely toxic to Daphnia magna among tested organisms. The water extracts of semi-coke and retorting ash were acutely toxic at lower pH values and the release of anions and molybdenum increased significantly. The results show strong positive correlation between the toxicity and salinity/electric conductivity of the leachates. Release of anions in turn is related to the degree of decomposition of mineral phases during the retorting process.