Mechanistic Study of the ZEA Organic Pollutant Degradation System: Evidence for H₂O₂, HO•, and the Homogeneous Activation of O₂ by Fe^IIEDTA

Abstract

The ZEA ( z ero valent iron, e thylenediaminetetraacetic acid (EDTA), and a ir) organic pollutant degradation system has been previously shown to degrade a variety of organic pollutants and chemical warfare agent surrogates; however, mechanistic details and reactive intermediates formed in this system have not been identified. It is hypothesized that the ZEA system produces reactive oxygen species (H 2 O 2, HO•) by the reduction of oxygen by Fe II EDTA(aq). This hypothesis is examined through an electrochemical model of the ZEA system. A carbon basket electrode is used as the reducing agent in place of Fe(0). The Fe III EDTA complex (0.5 mM) is electrochemically reduced to Fe II EDTA at an applied potential of −120 mV (vs Ag/AgCl) under aerobic conditions. Hydrogen peroxide was observed to form in the presence of the metal complex with a maximum concentration reaching 0.139 mM H 2 O 2 after 3 h of electrolysis. In the absence of FeEDTA, 0.04 mM H 2 O 2 is obtained by the direct reduction of O 2 at the electrode surface. Electron resonance spectroscopy (ESR), along with 5,5-dimethyl-1-pyrroline- N -oxide (DMPO) as a spin trap and methanol as a radical scavenger confirms the formation of HO• produced via the Fenton reaction in the electrochemical system. Hydroxyl radical attack on EDTA caused the degradation of Fe II/III EDTA to a steady-state concentration of 0.14 mM from 0.5 mM as observed by HPLC. The pH of the electrolysis solution increased from 2.64 to 9.25 during 6 h of reductive electrolysis which is indicative of the consumption of H + during the reduction of O 2 to form H 2 O 2 . These experiments provide evidence that the ZEA system uses atmospheric O 2 to produce reactive oxygen species including those that deeply oxidize organics under room temperature and pressure conditions.