Comparative Study of Four Polyphenols with Enhanced Fe3+/H2O2 Fenton-like Activity for Degrading Phenanthrene over a Wide Initial pH Range

Polyphenol (PP)-enhanced Fe³⁺/H₂O₂ oxidation presents a promising solution to the traditional Fenton process limitations, including acidic pH requirements, restricted Fe³⁺/Fe²⁺ cycling, and low H₂O₂ stability. The type of PP and the initial pH are key factors influencing Fenton-like reactions by affecting iron complexation and reduction. This study evaluates catechol, protocatechuic acid (PCA), gallic acid (GA), and tannic acid (TA) to enhance phenanthrene (PHE) degradation via the Fe³⁺/H₂O₂ process over a wide initial pH range. GA achieved the highest PHE removal rates, which can be attributed to its unique third ortho-hydroxyl group. The significant improvement in PHE degradation observed with adding PPs can be ascribed to their ability to form complexes with Fe³⁺, promote cycling between Fe³⁺ and Fe²⁺, and stabilize H₂O₂, all of which are pH-dependent. Hydroxyl radical (^•OH) and superoxide radical (O₂^•–) were identified as the primary species responsible for PHE degradation in the Fe³⁺/PP/H₂O₂ systems, although their contributions varied with the specific PPs used. The degradation products of both PHE and PPs were characterized by using mass spectrometry, suggesting possible degradation pathways and associated toxicities. Overall, this study demonstrates that the PP-enhanced Fe³⁺/H₂O₂ process holds significant potential for the remediation of polycyclic aromatic hydrocarbon contamination.