Heavy Metal Fugitive State Transformation and Environmental Risk Study of In Situ Oil Shale Conversion Process

In situ conversion of shale oil is currently a more environmentally friendly mining method. In this study, the SCW method is used to simulate the in situ conversion of shale oil in the laboratory. After the simulation, the wastewater and residues produced by the subcritical water (SCW) method were collected, and the changes in the concentration of heavy metals in the wastewater and the appearance of heavy metals in the residues were analyzed in combination with infrared spectroscopy, pH value, and pore changes. The wastewater of supercritical water is acidic, and FT-IR analysis shows that there are many kinds of minerals in oil shale and its residues, among which pyrite is the main cause of wastewater acidification. With the prolongation of the reaction time of sub-critical water, the concentration of heavy metals in wastewater showed a downward trend. From the heavy metal enrichment experiment in the residue, it was found that some heavy metal ions in the wastewater were readsorbed by the residue. Combined with the calculation results of adsorption kinetics, the enrichment of heavy metals in the residue is related to the output of organic matter and the development of pores. The enrichment of arsenic mainly depends on the chemisorption of organic matter, while the enrichment of lead depends on the characteristics of lead itself, which mainly changes from physical adsorption to chemical adsorption at different temperatures. According to the kinetic law, even if the residue is soaked in water again. The outward release concentration also does not increase too much. This study revealed the potential heavy metal pollution that may occur during the in-situ conversion of subcritical water into oil shale. The results can provide reference data for the prevention and control of heavy metal risks in groundwater caused by in-situ oil shale conversion.