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 TiO2 during plasma treatment, the degradation of the dye AO7 has been studied by gliding arc discharge in the presence of a TiO2 catalyst (CGAD). The results revealed that the adsorption of the dye on TiO2 is a physical adsorption in accordance with Langmuir isotherm, with a constant of adsorption KL = 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: ΔH < 0), and spontaneous (free enthalpy: ΔG < 0). The negative entropy (ΔS < 0) confirms the affinity of the dye molecules for TiO2. 20 min of CGAD treatment in the presence of an optimal quantity of TiO2 (2 g/L enabled us to bleach the solution of AO7 (100 μM) completely. The discoloration rate with and without the addition of TiO2 was 100% and 28%, respectively. 40 additional minutes of treatment allowed a total abatement of the chemical oxygen 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 kr = 14.97 mg L−1 min−1 and the adsorption coefficient is KL—H = 0.010 L/mg. The latter being negligible compared to kr, adsorption is therefore weakly performed during the plasma treatment.