Photocatalytic Degradation of Safranin O: Unraveling the Roles of Dissolved Gases, Environmental Matrices, and Reactive Species

This study investigates the impacts of the gas environment, water matrix, and reactive species on the TiO2-mediated photocatalytic degradation of safranin O (SO), a dye commonly found in wastewater. A slurry reactor (UVA, 365 nm) was used to quantify SO oxidation while systematically varying the SO concentration (5–40 mg/L), the TiO2 loading (0–3 g/L), the temperature (15–45 °C), and the pH (2–12). The dissolved gases (air, nitrogen, and argon) and matrices (deionized water, mineral water, and seawater) were also examined. Eight mechanistic probes (ascorbic acid, methanol, azide, nitrite, benzoquinone, oxalate, sucrose, and phenol) were used to identify active oxidants. UVA/TiO2 achieved rapid decolorization in approximately 90 min at 10 mg/L of SO and 0.4 g/L of TiO2. Decolorization rates decreased with increasing SO concentration due to active-site competition and inner-filter effects. Rates also exhibited a bell-shaped dependence on TiO2 loading due to light scattering and aggregation at high solids concentrations. Temperature exhibited a non-monotonic profile with an optimum around 25 °C, and the pH displayed an optimum range with maximal removal occurring around pH 10 and declining at pH 12. Air saturation outperformed N2 and Ar, indicating that O2 is the terminal electron acceptor. Photocatalytic performance decreased in the order deionized water > mineral water > seawater, owing to bicarbonate/chloride scavenging and ionic-strength effects. Scavenger tests converged on •OH dominance, with measurable contributions from superoxide/hydrogen peroxide (O2•−/H2O2) and valence-band holes (h+); singlet oxygen (1O2) played a minor role. These findings underscore the critical interplay between operational and environmental factors and offer a practical framework for scaling TiO2-based SO abatement in real waters.