Assessment of the potential of oily sludge char for removing organics from petroleum refinery wastewater

• ZnCl 2 -activated-oily sludge char (SB-Zn-400) has heterogeneous surface supporting multilayer adsorption. • SB-Zn-400 achieved rapid COD removal, reaching equilibrium within 2 h with a 28 % reduction in COD. • >97 % of various organic compounds in PR-WW were effectively removed by SB-Zn-400. • Hydrogen bonding and π-π interactions were the main adsorption mechanism. • Thermal regeneration of SB-Zn-400 allows for reuse, maintaining higher adsorption efficiency compared to pristine-OSC. Oil sludge (OS), a by-product of petroleum refinery (PR) waste, contains various toxic organic and inorganic compounds. Improper handling of OS poses serious environmental risks, highlighting the need for an effective and sustainable solution to mitigate these hazards and transform OS into a valuable product. This study converted OS into char-(OSC), including both pristine-OSC and ZnCl 2 -activated-OSC, for the removal of organic pollutants from refinery wastewater (WW) with a high COD concentration (89,233 mg/L). ZnCl 2 -activated-OSC produced at 400 °C (SB-Zn-400) showed superior adsorption capacity compared to pristine-OSC, due to enhanced oxygen-containing functional groups, crystallinity, thermostability, and superior degradation (OS). The adsorption process demonstrated rapid COD removal, reaching equilibrium within 2 h and achieving a 28 % reduction in COD. The adsorption capacity was found to be 420.5 mg-COD/g-OSC. SB-Zn-400 exhibited heterogeneous surface properties and supported multi-layer adsorption, with hydrogen-bonding and π–π interactions likely adsorption mechanisms. Moreover, the total concentration of organic compounds in PR-WW was >2700 mg/L, and SB-Zn-400 reduced this concentration to 34 mg/L, achieving >98 % removal. Although the adsorption treatment reduced the inorganic parameters of PR-WW, leaching of Mn, Ni, and Zn was observed, likely due to the nature of OS and the ZnCl 2 -activation process. Thermal regeneration of spent SB-Zn-400 allowed the reuse of OSC, with adsorption efficiency remaining higher than that of pristine-OSC, indicating that SB-Zn-400 has potential to be reused. These findings highlight the effectiveness of OSC in treating PR-WW, supporting a circular economy approach to enhance resource efficiency and minimize the environmental impact of OS from PR industries.