Correlation between photolysis quantum yields and efficiency of hydroxyl radicals generation upon excitation of natural iron(III) carboxylates.

Fe(III) complexes with simple carboxylic acids play an important role in environmental photochemistry of atmosphere and natural waters and are widely studied as potential photoactive agents for advanced oxidation processes (AOPs). The paper presents approaches to the correct determination of quantum yields of complexes photolysis (φphot) and •OH generation (φOH) upon excitation of natural Fe3+ carboxylates by UV irradiation. The first approach is based on the application of optical spectroscopy to argon-saturated solutions. This method is less time-consuming compared to standard Fe2+-phenanthroline technique and gives reproducible and correct results. The second approach is based on the use of [FeOH]2+ hydroxocomplex as a reference system with a well-known φOH value and benzene as a selective trap for the •OH radical. Particular attention is paid to the thermal reactions of primary photogenerated species leading to a decrease of φphot and a significant increase of φOH in post-irradiation period in air-equilibrated solutions. For the first time, the quantum yields of the •OH radical have been determined for a wide range of Fe3+ complexes (1:1 composition) with natural carboxylic acids and their correlation with φphot was established. The results allow to overcome the existing discrepancy of known literature data on φphot and φOH appeared due to using of different approaches to determining these values. The developed approaches will be also important for the correct determination of the efficiency of •OH radical generation in the systems used in environmental photochemistry and AOPs.