UV/TiO2/IO4− Advanced Oxidation of Safranin O: Disentangling Matrix Complexity and Radical-Scavenger Interference

The effectiveness of periodate-assisted photocatalysis in removing the cationic dye Safranin O (SO) was evaluated using a UV/TiO2/IO4− process operated at room temperature under near-neutral pH conditions. Under base conditions ([IO4−] = 0.15 mM, [TiO2] = 0.4 g/L, [SO] = 10 mg/L), the ternary system achieved a pseudo-first-order rate constant of 0.6212 min−1, outperforming the UV/TiO2 and UV/IO4− processes by approximately 21- and 29-fold, respectively. This yielded a synergy ratio of about 12 compared to the sum of the binary processes. Targeted quenching experiments revealed the operative pathways. Strong inhibition by ascorbic acid and phenol indicates that interfacial holes and •OH are key oxidants. Methanol caused a moderate slowdown, consistent with •OH and hole scavenging. Benzoquinone and oxalate suppressed removal by intercepting the electron and O2•− pathways, respectively. Dichromate markedly inhibited the process via optical screening and competition for electrons. Azide had little effect, suggesting a minor role for singlet oxygen. Matrix studies showed progressively slower kinetics from deionized water to mineral water to seawater. This was due to halides, sulfate, alkalinity, and TiO2 aggregation driven by ionic strength. Additional tests confirmed that the dominant modulators of performance were humic acid (site fouling and light screening), chloride and sulfate (radical speciation and surface effects), nitrite (near-diffusion radical quenching), and bicarbonate at pH 8.3 (conversion of •OH to CO3•−). Nonionic surfactants (Tween 80, Triton X-100) also depressed SO removal through micellar sequestration and competitive adsorption on TiO2. The study confirms the potential of UV/TiO2/IO4− as a tunable AOP capable of delivering rapid and reliable dye degradation under a wide range of water quality conditions. The mechanistic mapping unifies two roles for IO4−, an electron acceptor that inhibits recombination and a photochemical precursor of iodine centered and •OH radicals and connect these roles to the observed synergy and to the trend across deionized water, mineral water, and seawater. The scavenger outcomes assign the main oxidant flux to holes and •OH radicals with a contributory electron or O2•− branch from IO4− reduction.