Selective reduction of nitrite to nitrogen gas by CO2 anion radical from the activation of oxalate

Nitrite (NO2−) reduction by carbon dioxide anion radical (CO2•−) from the activation of small molecule carboxylic acid was investigated to selectively reduce nitrite to nitrogen gas (N2). However, the CO2•− generation efficiency from the activation of small molecule carboxylic acid needs to be enhanced to increase the NO2− reduction efficiency. In this study, a novel and promising process for selective NO2− reduction was proposed based on activation of oxalic acid (OA) by UV radiation coupled with Fe3+. In Fe(III)/OA/UV system, the activation of OA by photo-induced electron transfer process of Fe(III)-oxalate complex and by •OH radical from the photolysis of Fe3+ or NO2− could promote the generation of CO2•− radical, which enhanced the NO2− reduction. The 100% removal efficiency of NO2−, 94.72% of total nitrogen (TN) and 94.72% of selectivity for N2 were achieved in Fe(III)/OA/UV/NO2− system, at the Fe3+ dosage of 8 mmol/L, initial pH of 1.70, OA dosage of 16 mmol/L, initial NO2− concentration of 30 mg N/L, and reaction time of 180 min. CO2•− radical played a significant role in the reduction of NO2− by Fe(III)/OA/UV system based on the inhibition experiments in which methyl violet was used as a quenching agent of CO2•− radical. Based on the results from batch experiments and FTIR analysis, the activation mechanism of OA and selective reduction mechanism of NO2− in Fe(III)/OA/UV system was proposed. [Display omitted] •Removal efficiency of nitrite could be 100% in Fe(III)/oxalate/UV system.•CO2.•− produced from Fe(III)/oxalate/UV system played a main role in nitrite removal.•The selectivity of nitrite reduction to nitrogen gas was over 94%.