Enhanced treatment of tannery wastewater using the electrocoagulation process combined with UVC/VUV photoreactor: Parametric and mechanistic evaluation

Abstract Accelerated treatment of tannery wastewater was conducted using the electrocoagulation process combined with UVC/VUV photoreactor. In order to better determine the mechanisms, synthetic solutions were studied before the tannery wastewater. For synthetic solutions (50 mg/L Cr(VI) and sulfide), almost complete removals of Cr(VI) and sulfide were obtained in pH levels ≤7 in 30 min. For real tannery wastewater, the highlighted parameters were chemical oxygen demand (COD)  = 20600 mg/L, total chromium (Cr(T)) 30.11 mg/L, Cr(VI) = 6.95 mg/L and sulfide = 66.13 mg/L. The combined system exhibited excellent performance by which the final COD of the effluent reached 96.88 mg/L (99.52% removal efficiency). Cr(T) and sulfide removal efficiencies were 100% and 98.27%, respectively. An interesting phenomenon was observed in UVC/VUV process which was reflected as appearance of white flocs along with complete removal of Cr(T). Energy dispersive X-ray (EDX) analysis and Cr(T) tracing of the flocs as well as mass balance analysis indicated that a specific condition (typically in case of solution pH) had led to the generation of Al(OH)3-based flocs from the aluminate ion. Actually, using the proposed combined system brought about advanced oxidation of oxidisable inorganic (S2−) and organic (COD) contents in simultaneous with the reduction of Cr(VI) through a multi-lateral mechanism including advanced reduction (H2O photoionization-generated electrons) which was boosted by sulfide and organic content of tannery wastewater. Co-precipitation of Cr(OH)3 and Al(OH)3 was determined as the main mechanism for complete removals of Cr(T) and sulfide. It can be concluded that the selected combination is an efficient and promising method for the treatment of real industrial tannery wastewaters.