Measurement of the hydroxyl radical formation from H₂O₂, NO₃^-, and Fe(III) using a continuous flow injection analysis

Production of hydroxyl radical ({radical dot}OH) is of significant concern in engineered and natural environment. A simple in situ method was developed to measure {radical dot}OH formation in UV/H₂O₂, UV/Fe(III), and UV/NO₃^- systems using trapping of {radical dot}OH by benzoic acid (BA) and measuring fluorescence signals from hydroxylated products of BA. Method development included characterization of {radical dot}OH trapping mechanism and measurement of quantum yields (Φ_{{radical dot}OH}) for {radical dot}OH. The distribution of OHBA isomers was in the order of o-OHBA > p-OHBA > m-OHBA, although it changed with the H₂O₂ concentration and light intensity. This supports that {radical dot}OH attacks dominantly on the benzene rings. The quantum yields for {radical dot}OH formation in the UV/H₂O₂ process were 1.02 and 0.59 at 254 and 313 nm, which were in good agreement with the literature values, confirming that the method is suitable for the measurement of {radical dot}OH production from UV/H₂O₂ processes. Using the continuous flow method developed, quantum yields for {radical dot}OH in UV/H₂O₂, UV/Fe(III), and UV/NO₃^- systems were measured varying the initial concentration of {radical dot}OH precursors. The Φ_{{radical dot}OH} values increased with increasing concentrations of H₂O₂, Fe(III), and NO₃^- and approached constant values as the concentration increased. The Φ_{{radical dot}OH} values were 0.009 for H₂O₂ at 365 nm, showing that {radical dot}OH production is not negligible at such high wavelength. The Φ_{{radical dot}OH} values during the photolysis of Fe(OH)²⁺ (pH 3.0) and Fe(OH)₂⁺ (pH 6.0) at 254 nm were 0.34 and 0.037, respectively. The Φ_{{radical dot}OH} values for NO₃^- approached a constant value of 0.045 at 254 nm at the initial concentration of 10 mM.