Hybrid water treatment flow plant using hydrogen peroxide-based electro-activated persulfate and photoelectro-Fenton processes: The combustion of Reactive Orange 16 dye

This paper implements an innovative hybrid water treatment process in a flow plant using hydrogen peroxide-based electro-activated persulfate (EPS-H₂O₂) combined with photoelectro-Fenton (PEF) method. The Reactive Orange 16 (RO16) azo dye was used as a persistent organic pollutant (POP) model. A filter-press reactor utilized a Ti|RuO₂ plate and a gas diffusion electrode (GDE) as the anode and cathode. In EPS-H₂O₂, persulfate (PS) reacts with H₂O₂ yielding hydroxyl (^•OH) and sulfate (${SO}_4^{·-}$) radicals. In PEF, H₂O₂ reacts with Fe²⁺ producing more ^•OH, completing the TOC removal in the presence of UVA light. The influence of pH (3–9), initial PS concentration (0.5–1.5 mM), initial RO16 dye content (30–50 mg L^-1 TOC), applied current density (j, 5–30 mA cm⁻²), and mean linear flow velocity (u, 7.3–29.1 cm s⁻¹) on the EPS-H₂O₂ process efficiency was examined. The best trial for EPS-H₂O₂ achieved 37% TOC removal in 360 min electrolysis at j = 10 mA cm⁻¹, u = 14.6 cm s⁻¹, and P_GDE = 3 psi. Subsequently, at t ≥ 360 min, the synergistic effect between the EPS-H₂O₂ and PEF processes was carried out by adding 0.5 mM Fe²⁺ and UVA light. The best hybrid (EPS-H₂O₂ + PEF) process test reached 98% mineralization with 30% current efficiency and 0.060 kWh (g TOC)^-1 electrolytic energy consumption. Six carboxylic acids were determined as the main intermediaries, whereas ammonium was the main nitrogen compound throughout the EPS-H₂O₂ and PEF processes.