Photocatalytic oxidation of model organic dyes in the presence of Na+, NO2–, NO3– ions: theoretical and applied aspects

The influence of the concentration of Na+, NO2–, NO3– ions on the rate of photocatalytic oxidation of model organic dyes was studied in the article: cationic – methylene blue (MB); anionic – methyl orange (MO). Based on studies of hydrochemical indicators of polluted rivers in urban areas (Khabarovsk, Russia) in the period from 1999 to 2019, it was shown that the concentration of ions varies in the ranges: 0.005–0.7 mg/l for NO2–; 0.05–15 mg/l for NO3–; 13–180 mg/l for Na+. Optical spectrophotometry was used to study the kinetics of photooxidation of MC and MO in the concentration ranges of the studied ions: 0–1–10–100–1000–10000 mg/l using P25 titanium oxide as a photocatalyst. The photooxidation time (t) was estimated at various values of the degree of transformation (α) of dyes at: initial – 10 %t; medium – 50 %t; final – 90 %t stages of the photocatalytic process. The effect of absorption of light quanta with wavelengths of 200–350 nm by Na+/NO2– and Na+/NO3– ions is shown depending on the concentrations of these ions in the photocatalytic solution. Recommendations for practical applications of the method of photocatalytic water purification of real polluted waters are given, showing the need 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 is proposed from the point of view of the band structure of semiconductors, elements of the theory of electrolytic dissociation and recombination of free radicals in photocatalytic processes.