Treatment of sulfidic spent caustic by integrated system of Fenton oxidation and activated carbon absorption

Abstract

Spent caustic is a wastewater that generated from the refinery activities. Due to the hazardous and toxic nature of this waste, it must be treated well before being discharged to the water bodies. The purpose of this study was to investigate the treatability of Fenton oxidation as pretreatment and activated carbon (AC) adsorption as the post treatment of spent caustic. In this research, synthetic spent caustic and industrial spent caustic were employed. The pH adjustment step was meant to prepare an acidic condition for the subsequent Fenton oxidation. Whereas, the optimum conditions for this Fenton oxidation was determined by the aid of response surface methodology (RSM). Besides, the AC adsorption acted as the post treatment of synthetic spent caustic. The equilibrium isotherm study was done in order to determine the best correlation of the isotherm model for this adsorption process. Finally, the performance of the integrated system of Fenton oxidation-AC adsorption to treat industrial spent caustic was determined by analyzing the chemical oxygen demand (COD), sulfide, iron and sulfate concentrations after each treatment step. The pH adjustment step caused the COD and sulfide to remove by 24% and 68%, respectively. By using RSM, the optimal conditions for Fenton oxidation were found to be at H2O2/COD = 0.09, Fe/H2O2 = 1.44 and reaction time = 24 minutes. The best process conditions of AC adsorption were found to be at adsorbent dosage = 10 g/l, pH = 7, temperature = 30oC and contact time = 60 minutes. The equilibrium isotherm study showed that the AC adsorption fitted well with the Freundlich model. After the integrated system, the COD was reduced from 10920 mg/L to 283 mg/L. The final concentration of sulfide, iron and sulfate of the industrial spent caustic were 0.72, 8.4 and 454 mg/L, respectively. In conclusion, the integrated system has potential to effectively treat synthetic as well as industrial spent caustic.