Advanced oxidation processes for spent caustic wastewater treatment: A systematic review of efficiency, challenges, and future perspectives

Spent caustic wastewater (SCWW) is a highly alkaline and toxic effluent produced mainly by the refinery and petrochemical industries. It contains elevated levels of sulfides, phenols, and refractory organics, presenting significant challenges for conventional treatment methods. Advanced oxidation processes (AOPs) encompass a range of highly efficient techniques employed to treat SCWW. This systematic review assesses the most commonly applied AOPs for treating SCWW, compares their treatment efficiencies based on existing findings, and provides recommendations to guide future research. Sixteen studies were analyzed in this review, focusing on six AOP techniques: conventional Fenton, electro-Fenton (EF), electro-photo-Fenton (EPF), electro-Peroxone (EP), photocatalysis, and sulfate radical-based AOPs (SR-AOPs). The EF process has been the most frequently studied process, and EPF demonstrated the highest COD removal efficiency. Furthermore, the sulfidic form of SCWW was identified as the most prevalent type. In addition, this review emphasizes the importance of optimizing operational parameters such as pH, reaction time, and oxidant or catalyst dosage to maximize efficiency and prevent the formation of hazardous by-products. • AOPs show strong potential for large-scale SCWW treatment. • Electro-Fenton (EF) process was the most frequently studied for SCWW treatment. • Electro-photo-Fenton yielded the highest COD removal efficiency across the studies. • Combining AOPs with bio-treatment is recommended to improve cost-effectiveness. • AOPs are rated by COD removal, cost, waste generation, pH needs, and scalability.