New Insights into the In Situ Fenton-like Process by Copper(II)-hydroxylamine Coupling: Reactive Species, Applications, and Limitations

Abstract This work studied the effect of pH, [O2], H2O2 external addition, chelating agents, common anions, transition metals, and produced toxicity, in the process Cu­(II)/NH2OH (employing benzoate as a probe). It was observed that (i) acidification by NH2OH·HCl addition hindered the process significantly as the Cu­(NH2OH)2+ is principally formed at pH = 6–8, (ii) high HCO3– concentrations (≥1 mM) accelerated the NH2OH consumption (>95% in 5 min) and pollutant oxidation (kobs ≈ 2 × 10–2 min–1 with initial benzoate concentration 50 μM) due to the buffer effect and the formation of a Fenton-like active species, CuCO3(aq), (iii) the Mo­(VI)-peroxo species formed by Mo­(VI) reaction with the generated H2O2 consumed the NH2OH, being a strong interference, and (iv) NH2OH, although mainly decomposed into gaseous products, remained in trace concentrations, exhibiting toxicity. Results with selective scavenger addition, electron paramagnetic resonance, NO(g) kinetics, and high resolution mass spectrometry suggested that NH2OH not only acts as a Cu­(II) reducing agent (responsible for starting the Fenton reaction due to the in situ formation of H2O2 by Cu­(I)/O2 reaction), but it also changes the Fenton mechanism of Cu­(I, II)/H2O2, suggesting that the formed reactive nitrogen species are responsible for pollutant abatement rather than HO• or Cu­(III).