Shift from heterogeneous to homogeneous catalysis during resorcinol degradation using the solar photo-Fenton process initiated at circumneutral pH

• Heterogeneous to homogenous processes transition is proposed in photo-Fenton system at circumneutral pH.
• The oxidation state and the solubility of different iron forms involved in the photo-Fenton process mechanism, at circumneutral pH, highly depend on pH.
• At pH < 5.7, the homogeneous photo-Fenton process mainly contribute to faster organic matter degradation than at pH > 5.7, where heterogeneous process predominates.

The photo-Fenton process (Fe2+,3+/H2O2/hv) is known to degrade aqueous organic compounds at acidic pHs. Nevertheless, recent studies have shown that this process also occurs at circumneutral pHs. Dihydroxybenzene isomers (i.e., resorcinol, hydroquinone and catechol) were selected as models of degradation by-products of natural organic matter and were degraded in water via the solar photo-Fenton process in a compound parabolic collector (CPC). The experiments were carried out at neutral pH and with 0.7 and 40.0 mg L−1 of iron and H2O2, respectively. Despite sharing a similar chemical structure as the other two isomers, resorcinol was more recalcitrant to degradation under these conditions. Thus, we conducted a detailed study of this compound's transformation through the photo-Fenton process. The effects of initial pH (5.0, 6.0 and 7.4) on resorcinol degradation were investigated, and in all experiments, pH, total organic carbon (TOC), resorcinol, dissolved iron forms and H2O2 were monitored. The results led us to consider that a photo-Fenton system at circumneutral pH follows a different mechanism than the pathway that is generally accepted for acidic pH values. A photo-Fenton process initiated at neutral pH is mainly heterogeneous; however, as the pH progressively decreases due to the generation of acidic by-products, the process becomes more homogeneous, and the degradation rate increases concomitantly. The pH also dramatically drops to approximately 5.7. A mechanism is proposed to explain the key shift from heterogeneous towards homogeneous photo-Fenton degradation processes.

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