Removing sulfide from spent caustic petrochemical wastewater with electro-Fenton treatment

Spent caustic is a particularly harmful wastewater from petrochemical industries. It has a complex mixture of components, causing problems and difficulties for conventional treatment processes. In this study, electro-Fenton treatment was investigated as an advanced oxidation process (AOP) for the removal of sulfide from synthetic spent caustic. The effects of contributing factors such as pH, electrode distance, voltage, hydrogen peroxide (H2O2) concentration and time on sulfide removal were experimentally scrutinized. It was shown that under the optimum conditions, sulfide removal reached 97% and that the most influential factors were reaction duration and peroxide concentration. Interestingly, when adequate reaction times (above 30 min) were provided, changes in electrode distance and voltage were shown to be inconsequential. A pH of 5 and peroxide dosage above 0.5 mL/L were most effective. In order to illustrate the interaction of the independent variables on sulfide removal, statistical modeling was also carried out followed by optimization and prediction analysis. Highlights The electro-Fenton process was used for spent caustic treatment. Effects of pH, electrode distance, voltage, H2O2 concentration and time were studied. Importantly, at t > 30 min, electrode distance and voltage became insignificant. At 4.5–6.5 pH and 0.45–0.65 mL/L H2O2, optimum results are obtained. Under optimum condition 97% removal of sulfide was achieved.