Synergistic Electrocoagulation–Electro-Fenton Coupling for Petroleum Refinery Wastewater Mineralization: Statistical Optimization and Cost Analysis

Petroleum refinery wastewaters are highly recalcitrant and recognized as one of the most challenging industrial effluents requiring advanced treatment strategies. This study aims to investigate the synergistic performance of a sequential electrocoagulation (EC) and electro-Fenton (EF) process for the mineralization of this complex effluent. The EC pretreatment was optimized using response surface methodology via Doehlert design, establishing optimal conditions at pH 6.0, 0.8 A, and a 75 min electrolysis time. Under these conditions, 39% of total organic carbon (TOC) and 56% of chemical oxygen demand (COD) were removed. The quadratic polynomial model developed for the EC stage presented an excellent fit with the experimental data (R2 = 0.99, R2adj = 0.97, p < 0.05), confirming its strong predictive robustness. In order to degrade the remaining recalcitrant organic pollutants, the pretreated effluent underwent EF oxidation (0.01 M ferrous ion, 0.8 A, pH 3), leading to TOC and COD removal rates of 68% and 76%, respectively, after a 360 min electrolysis time. The integrated EC-EF process achieved an overall mineralization of 81% and an oxidation efficiency of 89%. Finally, a comprehensive evaluation of the system’s energy consumption and economic viability established a solid techno-economic baseline for this sequential approach, indicating a competitive total operating cost of USD 0.036 per gram of TOC removed.