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

Abstract

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.

Keywords: Carbon dioxide free radical; Denitrification; Nitrite; Photo-induction; Reduction.