The degradation of hydroquinone ( \(HQ\) ), a toxic and refractory organic pollutant, was systematically investigated using advanced oxidation processes (AOPs). This study addresses a critical research gap: the lack of quantitative understanding of Mn2+ synergy in Photo-Fenton systems for HQ degradation. The specific objectives were to: (i) compare the efficiency of direct photolysis, single-component catalysis, Fenton, and Photo-Fenton systems; (ii) optimize reagent concentrations; (iii) quantify the synergistic effect of trace \(Mn2+\) ; and (iv) identify degradation intermediates and reactive species. While direct photolysis (300 ppm HQ, \({k}_{obs}\) = 0.0025 min−1) and single-component systems ( \( H_{2} O_{2} /UV \) , \({k}_{obs}\) = 0.008 min−1; Fe2+/UV, \({k}_{obs}\) = 0.006 min−1) showed limited efficiency, the classical Fenton process achieved \({k}_{obs}\) = 0.003 min−1 under optimal conditions ([ \( H_{2} O_{2} \) ] = 0.01 M, [ \(Fe^{2 + }\) ] = 0.005 M, pH 2). The introduction of UV irradiation (Photo-Fenton) dramatically enhanced the degradation kinetics ( \({k}_{obs}\) = 0.025 min−1, \({t}_{1/2}\) = 27.7 min). A breakthrough was achieved by introducing trace Mn2+ (0.5 ppm) as a co-catalyst, yielding a superior rate constant of \({k}_{obs}\) = 0.0693 min−1—a 2.8-fold enhancement over the Mn-free Photo-Fenton system and a 23-fold enhancement over classical Fenton. This reduced the time required for > 99% degradation from > 1500 min (Fenton) to just 60 min ( \(Mn^{2 + }\) -promoted Photo-Fenton). The study elucidated the degradation pathway through identification of key intermediates (p-benzoquinone, RT = 4.2 min; oxalic acid, RT = 6.8 min; acetic acid, RT = 8.1 min; maleic acid, RT = 10.5 min) using HPLC and ion chromatography. Radical quenching experiments confirmed that hydroxyl radicals (·OH) are the dominant reactive species (68% inhibition with tert-butanol). The Electrical Energy per Order ( \(EE/O\) ) decreased from 42.5 kWh/m3 (Fenton) to 15.3 kWh/m3 (Mn2+-promoted Photo-Fenton), demonstrating significant cost reduction. This work establishes a clear hierarchy of efficiency among AOPs, defines optimal conditions, and reveals a novel synergistic strategy for significantly enhancing Fenton-based technologies for HQ-containing wastewater.
