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This study investigated a novel kinetic model of the adsorption and photocatalytic degradation of methyl orange, an anionic dye, using a commercial ZnO and reduced commercial ZnO photocatalysts. The results were compared and discussed with our previous study on methylene blue, a cationic dye, providing new insights into the interaction of the catalyst with molecules with different charges. The kinetic model was applied to describe adsorption in the dark, followed by photocatalytic degradation under simulated sunlight. In addition, the predicted pK a values of methyl orange were estimated using the kinetic model, which were found to be 4.40 and 9.34. The effect of ZnO oxygen vacancies (created through hydrogen reduction at 500 °C) on the adsorption and photocatalytic degradation of methyl orange was investigated. The existence and effects of oxygen vacancies on ZnO were analyzed using TPR, PL, XPS, XRD, FE-SEM, EDS, BET, Tauc plot, Mott-Schottky, and cyclic Voltammetry. The appearance of oxygen vacancies was clearly observed by the characterization results, while the morphology of the nanoparticles was found to be unchanged. Additionally, the band gap of ZnO was decreased from 3.22 to 3.07 eV, and the maximum valence band of the catalyst was increased from 2.43 to 2.67 eV due to the reduction pretreatment. The findings suggest that the photocatalytic degradation rate of the reduced ZnO substantially increased. However, the adsorption removal of the anionic dye declined, which could be attributed to the more negative zeta potential of ZnO after hydrogen reduction. Finally, the quantum yield of photocatalytic process was calculated and compared with the results of methylene blue. [Display omitted] • The impact of oxygen vacancies on adsorption and photocatalytic degradation of methyl orange by ZnO. • A new approach to modeling the kinetics of continuous adsorption and photocatalytic degradation. • The influence of pH on the speciation of anionic dye and its adsorption and degradation mechanism. • Determination of pK a values by utilizing experimental data on adsorption. [ABSTRACT FROM AUTHOR]