Enhanced pollutant removal from real petrochemical wastewater by the combination of solar-powered electrocoagulation and plant-based coagulants: Optimization and mechanistic study

Petrochemical and petroleum extraction industries are discharging increasing amounts of oily wastewater, posing a serious threat to the ecological environment, water supplies, and human health. Therefore, developing efficient, green technologies for oily wastewater treatment is particularly urgent. Traditional wastewater treatment technologies have limitations in terms of efficiency and cost-effectiveness for treating oily wastewater with low chemical oxygen demand (COD). This study reports a novel hybrid system, combining solar-powered electrocoagulation (SPEC) with natural plant-based coagulants (PBCs), derived from Hami melon seeds, to treat petrochemical wastewater. Using the removal rates of COD, turbidity, color, and suspended solids as evaluation indicators, the effects of factors, including coagulant dosage, pH value, stirring speed, and stirring time, on the treatment efficiency were deeply explored through single-factor experiments. The experimental results show that under the optimized conditions of a coagulant dosage of 5 mg/L, a pH value of 2, a stirring speed of 150 rpm, and a stirring time of 30 min, the coagulation treatment achieves the highest efficiency. Furthermore, the efficiency of the combined SPEC-PBCs was investigated, and the response surface methodology was adopted to optimize the key operating parameters of the coupled system. With the help of the Box-Behnken experimental design and taking the COD removal rate as the response value, the effect of pH, dosage, and stirring speed on the treatment efficiency were systematically studied. The results show that under the optimal conditions of pH=5, dosage=3.2 mg/L, and stirring speed=246 rpm, the COD removal rate is as high as 62.18%. Moreover, the application of solar energy significantly reduced the dominant operational costs, thereby, enhancing the economic feasibility of the overall process. In conclusion, the obtained results proved that combining both technologies is a feasible approach that paves the way for green growth and ecological balance in treating oily wastewater. • A hybrid system using novel PBCs from melon seeds effectively treated low-COD oily wastewater. • A 62.18% COD removal efficiency was achieved via RSM-BBD optimization of the coupled system. • The optimal conditions were identified: 5 mg/L dosage, pH=2, 150 rpm, 30 min reaction time. • Melon seeds waste was upcycled into PBCs for wastewater treatment, supporting a circular economy. • This green, low-cost SPEC-PBCs wastewater treatment approach was highly feasible.