Photocatalytic Oxidation of Model Organic Dyes in the Presence of Na+, \({\text{NO}}_{2}^{ - }\), and \({\text{NO}}_{3}^{ - }\) Ions: Theoretical and Applied Aspects

The effect of Na⁺, \({\text{NO}}_{2}^{ - }\), and \({\text{NO}}_{3}^{ - }\) ion concentration on the photocatalytic oxidation rates of model organic dyes, namely, cationic dye methylene blue (MB) and anionic dye methyl orange (MO), is studied. Based on studies of the hydrochemical characteristics of polluted rivers in urban areas (Khabarovsk, Russia) in the period from 1999 to 2019, it is shown that the ion concentration varies in a range of 0.005–0.7, 0.05–15, and 13–180 mg/L for \({\text{NO}}_{2}^{ - }\), \({\text{NO}}_{3}^{ - }\), and Na⁺, respectively. The kinetics of photooxidation of MB and MO is studied by optical spectrophotometry in a concentration ranges of the studied ions of 0–1–10–100–1000–10 000 mg/L using P25 titania as a photocatalyst. Photooxidation time (t) at different conversions (α) of the dyes at the initial (^10%t), middle (^50%t), and final stages (^90%t) of the photocatalytic process is assessed. The effect of absorption of light quanta at wavelengths of 200–350 nm by Na⁺/\({\text{NO}}_{2}^{ - }\) and Na⁺/\({\text{NO}}_{3}^{ - }\) ions as a function of the concentration of these ions in a photocatalytic solution is shown. Recommendations for practical applications of the photocatalytic treatment of real contaminated water are given, while showing the necessity to take into account the concentration of the studied ions. A description of the observed effect of ions on the rate of photocatalytic oxidation of model organic dyes in terms of the band structure of semiconductors, elements of the theory of electrolytic dissociation, and recombination of free radicals in photocatalytic processes is proposed.