<p>Spent caustic is considered among the most hazardous pollutants in industrial wastewater produced from refining industries. Extensive studies have been conducted based on various treatment techniques and materials to achieve the highest removal efficiency. Fenton and electro-Fenton techniques are among the methods regarded with great interest for this purpose in recent years. In this study, the electro-Fenton technique was implemented as a subset of advanced oxidation processes for COD removal from spent caustic effluents of the 8th refinery of Iran's South Pars gas complex using solar panels as a renewable energy source. Influential parameters such as H₂O₂/COD, current density, pH, and reaction time were selected, and response surface methodology was implemented using Design Expert software for experimental design. The optimal operating conditions were H₂O₂/COD = 1.264 mL/L, pH = 5.865, current density = 20.144 mA/cm², and reaction time = 35 min, resulting in 94.43% COD removal. In addition, by synthesizing the MWCNT-Fe₃O₄ nanocomposite as a nano-absorbent, the electro-Fenton sludge was treated, and the effluent quality was improved. Based on the results, electro-Fenton technology is proposed as an environmentally friendly and efficient technique for caustic treatment, while the hybrid technique improves wastewater quality by removing the remaining pollutants.</p>