Competitive Contribution of Catalyst and Adsorption Roles of TiO ₂ on the Degradation of AO7 Dye During Plasma Treatment

In order to investigate the role of TiO 2 during plasma treatment, the degradation of the dye AO7 has been studied by gliding arc discharge in the presence of a TiO 2 catalyst (CGAD). The results revealed that the adsorption of the dye on TiO ₂ is a physical adsorption in accordance with Langmuir isotherm, with a constant of adsorption K _L = 0.52 mg/L and a maximum adsorption capacity b = 18.1 mg/g. The temperature variation of the reaction medium made it possible to consider thermodynamic parameters. Indeed, the adsorption is exothermic (enthalpy: ?H < 0), and spontaneous (free enthalpy: ?G < 0). The negative entropy (?S < 0) confirms the affinity of the dye molecules for TiO ₂ . 20 min of CGAD treatment in the pres- ence of an optimal quantity of TiO ₂ (2 g/L enabled us to bleach the solution of AO7 (100 µM) completely. The discoloration rate with and without the addition of TiO 2 was 100% and 28%, respectively. 40 additional minutes of treatment allowed a total abatement of the chemical oxy- gen demand. The elimination of AO7 molecules during the plasma-catalytic treatment follows Langmuir-Hinshelwood (L-H) model kinetics. According to this model, the speed constant is k r = 14.97 mg·L ⁻¹ ·min ⁻¹ and the adsorption coefficient is K _L−H = 0.010 L/mg. The latter being negligible compared to k r, adsorption is therefore weakly performed during the plasma treatment.