The discharge of large amounts of oily sludge heat treatment residues constitutes a severe threat to the environment. However, little is known about the toxicity of these heat-treated residues. Current research has mainly focused on the toxic effects of single heavy metals or single hydrocarbons on plants, whereas the phytotoxic effects of hydrocarbon–metal mixtures have remained largely unexplored. In this study, pot experiments were conducted to evaluate the effects of different proportions of heat treatment residues (pyrolysis, heat-washing, and high-temperature oxidation residues) from three kinds of oily sludge on the physiological and biochemical parameters of mung bean plants. Higher proportions of residues decreased the germination rates and enzyme activity of mung beans compared to uncontaminated soil. When pyrolysis residue, hot-washing residue, and high-temperature thermal oxidation residue are used in green planting soil, their content must be lower than 30%, 90%, and 70%, respectively. Additionally, our findings indicated that the accumulation level of pollutants in oily sludge heat treatment residues was not high. However, the three kinds of residues exhibited different degrees of plant toxicity. The pyrolysis residue still exhibited strong ecotoxicity, even at low concentrations. In contrast, the toxicity of the hot-washing residue was much lower than that of the pyrolysis residue and the high-temperature thermal oxidation residue. Our findings indicated that mung bean is highly tolerant of contaminated soil and is therefore well suited for phytoremediation applications.

Supercritical water oxidation (SCWO) technology is efficient and offers significant development potential for hazardous waste treatment. The use of water in a supercritical state with special physical and chemical properties for refractory organic matter processing offers significant advantages. This article summarizes (i) the mechanism of SCWO technology and its research progress for oily sludge treatment; (ii) the effects of reaction temperature, pressure, residence time, oxygen ratio, catalyst, antioxidant, and other factors on oily sludge treatment; and (iii) problems in the current process. Finally, SCWO technology for the future development of oily sludge treatment is discussed.