Enabling metal-free photocatalytic self-Fenton mineralization of refractory pollutants via a quinone/hydroquinone redox cycle

A quinone/hydroquinone redox loop drives metal-free photocatalytic self-Fenton reactions. • Semiquinone radicals on THQ carbonyl sites activate H 2 O 2 into •OH radicals. • Accelerates CIP degradation by 4.98-fold and mineralizes distillery wastewater. Photocatalytic self-Fenton catalysts synergistically remove organic pollutants by coupling photosynthesis of H 2 O 2 with the Fenton reaction. However, metal-based self-Fenton systems inevitably generate secondary pollution associated with transition metal sludge disposal. Here, we rationally designed a metal-free graphitic carbon nitride (g-C 3 N 4 ) catalyst covalently grafted with tetrahydroxy-1,4-benzoquinone (THQ) to construct an efficient, sustainable photocatalytic self-Fenton system. The integrated THQ not only enhances the visible-light harvesting and directional separation of photogenerated carriers of g-C 3 N 4 , but also serves as a dual-functional active center. Specifically, THQ is proposed to facilitate the two-electron oxygen reduction reaction (2e - ORR) for in-situ H 2 O 2 generation and subsequently promote the activation of the generated H 2 O 2 into highly reactive hydroxyl radicals (•OH) through its quinone/hydroquinone (Q/QH 2 ) redox cycle. Consequently, the THQ/g-C 3 N 4 composite exhibits exceptional catalytic performance, achieving a 4.98-fold enhancement in the degradation rate constant of ciprofloxacin (CIP) within 2 h compared to pristine g-C 3 N 4 . Furthermore, the catalyst demonstrates remarkable practical applicability, achieving a 50 % total organic carbon (TOC) removal efficiency in Chinese Baijiu distillery wastewater within 5 h. This study establishes metal-free, photocatalytic self-Fenton catalysis as a promising strategy for sustainable environmental remediation. Download: Download high-res image (264KB) Download: Download full-size image Metal-free graphitic carbon nitride covalently grafted with tetrahydroxy-1,4-benzoquinone (THQ/g-C 3 N 4 ) enhances visible-light absorption and directional charge separation, significantly prolonging the charge carrier lifetime. Quinone active sites promote two-electron ORR toward in-situ H 2 O 2 production; meanwhile, hydroquinone sites activate the yielded H 2 O 2 into reactive •OH via rapid Q/QH 2 redox cycling for efficient pollutant degradation.