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

Production of hydroxyl radical (OH) is of significant concern in engineered and natural environment. A simple in situ method was developed to measure OH formation in UV/H2O2, UV/Fe(III), and UV/NO3− systems using trapping of OH by benzoic acid (BA) and measuring fluorescence signals from hydroxylated products of BA. Method development included characterization of OH trapping mechanism and measurement of quantum yields (ΦOH) for 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 OH attacks dominantly on the benzene rings. The quantum yields for 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 OH production from UV/H2O2 processes. Using the continuous flow method developed, quantum yields for OH in UV/H2O2, UV/Fe(III), and UV/NO3− systems were measured varying the initial concentration of OH precursors. The ΦOH values increased with increasing concentrations of H2O2, Fe(III), and NO3− and approached constant values as the concentration increased. The ΦOH values were 0.009 for H2O2 at 365 nm, showing that OH production is not negligible at such high wavelength. The ΦOH 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 ΦOH values for NO3− approached a constant value of 0.045 at 254 nm at the initial concentration of 10 mM.