Electrochemical oxidation process combined with UV photolysis for the mineralization of nitrophenol in saline wastewater

As a preliminary treatment for the subsequent desalinization process, a combination of heterogeneous electrocatalysis and homogeneous photolysis is developed to mineralize bio-refractory organic pollutants in the wastewater with high salinity. A response surface methodology is used to assess the individual and interactive effects of several operating parameters (initial substrate concentration, pH, current density and Na2SO4 concentration) on the mineralization efficiency (the total organic carbon (TOC) removal rate) of p-nitrophenol. Based on the experimental results, a semi-empirical expression is proposed, allowing to predict and optimize the 5-h TOC removal rate. Both pH and current density are found to be the most significant positive factors that affected the combined process. With respect to the individual electrocatalysis and photolysis process, a synergetic effect on TOC removal rate is found in such a combined process. The oxygen generated by the parasite reaction of electrolysis process is recognized to play a predominant role in the synergetic effect. The degradation kinetics of PNP follows a pseudo-first-order kinetics model and features a higher degradation rate coefficient in alkaline medium and in an appropriate flowrate range. Through the LC–MS analysis of degradation products, the denitration and substitution by hydroxyl radicals on aromatic ring prove to be the first step for the degradation of PNP by the combined process.

Figure optionsDownload as PowerPoint slideResearch highlights▶ This method can effective mineralize the organic pollutants in saline wastewater. ▶ pH and current density were the most significant positive factors. ▶ A synergetic effect was found in this combined process containing Na2SO4 salts. ▶ Oxygen produced by electrolysis played a major role in the synergetic effect. ▶ Polyhydroxylated benzene and nitrated aliphatic compounds were mineralization intermediates of PNP.