The adsorption/photocatalytic degradation kinetics of oxygen vacancy-enriched ZnO in relation to surface functional groups of cationic/anionic dyes

This study explores the effects of dye charge and functional groups (e.g., electron-withdrawing and electron-donating groups) on the adsorption and photocatalytic degradation (PCD) of all four target dyes (i.e., two cationic (malachite green and safranin O) vs. two anionic dyes (methyl red and rose bengal)) using oxygen vacancy-enriched ZnO as a catalyst. Oxygen vacancies are introduced by hydrogen reduction using a 10 % H₂/Ar gas mixture at 500 °C. A comprehensive kinetic model has been developed which accounts for reversible adsorption–desorption processes and distinguishes between degradation by hydroxyl radicals (in the solution) and those by electron-holes (on the catalyst surface). Additionally, the model incorporates the speciation of dyes (based on their pK_a values) to account for pH-dependent adsorption behaviors. The results indicate the maximum adsorption removal occurs at pH 3 where all dyes are at their fully protonated state. Density functional theory (DFT) calculations are also performed to generate electrostatic potential (ESP) maps indicating how at acidic conditions most PCD occurs on the catalyst surface while at basic pH occurs in the bulk solution. Accordingly, cationic dyes with electron-donating groups exhibit the highest degradation rates at pH 11, as they readily react with hydroxyl radicals in basic conditions. Conversely, anionic dyes with electron-withdrawing groups reach their maximum degradation rates at pH 3, where they preferentially react with electron-holes on the catalyst surface. Finally, the quantum yield calculations demonstrate that cationic dyes reach their maximum QY of 7.12 × 10⁻⁵ at pH 11 while anionic dyes achieve their highest QY of 3.86 × 10⁻⁵ at pH 3.