Semi-Quantitative Determination of Hydroxyl Radicals by Benzoic Acid Hydroxylation: An Analytical Methodology for Photo-Fenton Systems

Advanced oxidation process (AOP) is a widely used technique in environmental remediation of pollutants. Photocatalytic oxidation processes using iron system with H2O2 involve OH radical formation, which aids the degradation of pollutants in the aqueous system. Detection of hydroxyl radical is generally done in ESR spectrometry using DMPO as the spin trapping agent. Here, Methylene Blue (MB) was chosen as the test dye and three iron(III)-complexes, namely iron(III)-salen ferrioxalate, iron(III)-salen and iron(III)-acetylacetonate with H2O2, were chosen as photo-Fenton agents. All the systems were effective on the degradation of MB dye. However, the iron(III)-salen complex with H2O2 did not show its corresponding DMPO-OH signal in ESR. Hence, the present work was initiated to develop an alternative methodology for the determination of hydroxyl radicals in any remediation process (classical and non-classical photo- Fenton systems). Benzoic acid, here used as a chemical probe, gets hydroxylated to salicylic acid (SA) by the hydroxyl radicals, and the complex formed by SA with Fe3+ ions was measured spectroscopically. The formation of SA had increased with increase in the irradiation time, which indicates the progress of OH radicals. Interference study reveals that the detection of SA is unaffected in the presence of different electrolytes (c = 5 × 10-5 M). Repeatability of the developed method was verified, and it has significant precision (≤ 1.4% RSD, n = 5). Hence, this methodology could be used as a semi-quantitative tool for the determination of OH radicals in the environmental remediation by AOP. Keywords: Hydroxyl radicals, Photo-fenton, Spectrophotometry, Salicylic acid, Complexation, Advanced oxidation process, Photocatalytic oxidation processes, ESR spectrometry, Benzoic acid, Photochemical reactions