Activation of dissolved molecular oxygen by ascorbic acid-mediated circulation of copper(II): Applications and limitations

The environmental redox processes involving the circulation of common transition metals are of great interest to environmental scientists for water/soil remediation applications. In this study, a novel oxidation system using the combination of Cu(II) with ascorbic acid was investigated. Benzoic acid was used to quantify the reactive species via the detection of p-hydroxybenzoic acid. The generation of p-hydroxybenzoic acid showed that ascorbic acid was highly effective in the promotion of the activation of dissolved molecular oxygen (O-2) by Cu(II) to generate strong oxidizing reactive species and the amount of reactive species generated was positively correlated with the dose of ascorbic acid. During the activation of O-2, ascorbic acid showed much higher reactivity than the commonly known reductants. Meanwhile, ascorbic acid-Cu(II) was found to work effectively when the solution pH value was increased from 3.0 to 9.0, which suggests that this oxidative combination had a wide effective pH range. The mechanistic investigations using electron paramagnetic resonance spectroscopy, radical scavenging experiments, and UV-Vis spectroscopy showed that hydroxyl radical (center dot OH), instead of Cu(III), was the primary reactive species. Based on these results, a mechanism for the generation of center dot OH was proposed. However, due to the no selectivity of center dot OH, ascorbic acid-Cu(II) showed limited efficiency in the degradation of sulfamethoxazole and inactivation of Escherichia coli, which suggests that this combination may not suitable for the removal of contaminants that exist at relatively low concentrations. The results from this study may shed new light on the treatment of wastewater that simultaneously contains Cu(II) and organic contaminants.