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Effect of oxygen pressure on the photoelectrochemical oxidation of C.I. Direct Black 22 azo dye on Ti/TiO 2 /RuO 2 and Ti/TiO 2 electrodes was studied. The degree of solution decoloration in its treatment by various methods: electrochemical, photocatalytic, and photoelectrochemical, was estimated.The photoelectrochemical oxidation of azo dyes has been extensively studied [1][2][3][4][5][6][7][8], which is due to prospects for their use in development of power-saving and energy-independent techniques for detoxication of dye-containing wastewater. In [9], data on photoelectrochemical oxidation of Active Blue 17 dye on Ti/Ru 0.3 Ti 0.7 O 2 electrode were reported. It was shown that treatment for 120 min leads to a 95% solution discoloration and 52% decrease in the total organic carbon (TOC).In [10], the role of a photoanode in photoelectrochemical oxidation of Active Blue 4 in a 0.1 M Na 2 SO 4 solution at pH 12 was played by titanium coated with titanium dioxide. A 1-h exposure of a dye solution to UV light leads to full discoloration of the solution and 37% mineralization of dye molecules to give CO 2 and H 2 O. It has been shown that the discoloration effi ciency increases and the TOC decreases in photoelectrochemical oxidation on a Ti/Ru 0.3 Ti 0.7 O 2 electrode for Active Brilliant Orange dye in a 0.05 M NaCl solution [11].The possibility of photoelectrochemical discoloration of Active Orange 16 (AO) textile azo dye in an aqueous solution on a TiO 2 fi lm produced by the sol-gel technique was considered in [12]. The optimal parameters for maximum destruction of AO are pH > 10 and pH < 6 for Na 2 SO 4 and NaCl solutions, respectively. In both cases, a low dye concentration and a +1.0-V electrode potential are recommended for full discoloration of a solution during 20 min of photoelectrolysis. Oxalic acid was identifi ed by high-performance liquid chromatography and UV spectroscopy as the main product formed in dye oxidation.A solution of C.I. Direct Black 22 azo dye (DB) is discolored under photoirradiation and electrochemical and photoelectrochemical treatments as a result of destruction of azo dye molecules both at conjugated bonds and aromatic moieties, with the highest degree of destruction obtained under a photoelectrochemical treatment [13]. However, spectrophotometric data indicate that the aromatic moieties of the molecule are partly preserved in solution.In the present study, we examined, in order to intensify the process, the effect of the oxygen pressure on the photoelectrochemical discoloration of an aqueous solution of DB, which exhibits sensitizing properties and favors excitation of titanium dioxide, under exposure to daylight in a 0.1 M Na 2 SO 4 solution on Ti/TiO 2 /RuO 2 and Ti/TiO 2 electrodes. EXPERIMENTALThe photoelectrochemical oxidation of the DB dye (see structural formula) was performed as follows. A dye solution of a certain concentration was poured into a cell described in [14], a 200-ml fl uoroplastic vessel 1

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