Experimental study and quantum chemical calculation of free radical reactions in p-nitrophenol degradation during electrochemical oxidation process

Electrochemical oxidation process (EOP) has received attentions in wastewater management in recent years. However, high electrodes cost and low current efficiency have challenged the industrial application. Therefore, dimensional stable anodes (DSA) are developed due to relative low price and high efficiency of chlorinated active species generation. However, with low concentration of mediums, the direct oxidation still plays the main role in the organic pollutants degradation, resulting in low current efficiency of process. Thus, in this study, the impact of general operating conditions including type of electrolyte, active species addition, pH and current density on proportion of direct and indirect oxidation during p-nitrophenol degradation in DSA system have been studied, trying to enhance the contribution of indirect oxidation. The contribution was measured by scavenging tests. It concluded that low pH and adding NaClO can improve the contribution of indirect oxidation, but extra H(2)O(2 )significantly hindered the PNP degradation. Furthermore, we have studied the behaviors of free radical reactions by quantum chemical calculation. Three mechanisms including hydrogen atom transfer (HAT), radical adduct formation (RAF) and electron transfer (ET) were discussed in hydroxyl free radical and chlorine-derived radicals reactions. According to calculated thermodynamic and kinetic parameters, it indicated that active sites in primary reactions initiated by 'OH are significantly more than that in chlorine-derived radical reactions, corresponding to the differences of experimental phenomenon under different mediums. Degradation pathways and main byproducts formation were also proposed. We provided an insight to study oxidation mechanisms of EOP by means of experiments combining quantum chemical calculation.