Kinetics of the photocatalytic degradation of methylamine: Influence of pH and UV-A/UV-B radiant fluxes

Adsorption, photolysis and photocatalytic degradation of methylamine (MA) were investigated using titanium dioxide (TiO2) as photocatalyst. The influence of various parameters such as concentrations, irradiation time, pH, UV domain and UV-A/UV-B radiant flux has been studied. Optimum conditions for the complete degradation of MA in water have been identified. By investigating the pH-dependent kinetics, it was found that the neutral species CH3NH2 are more rapidly degraded than their protonated form CH3NH3+ because the OH radicals favourably reacted with the lone-pair electron on the nitrogen atom. The nitrogen atom in MA was photoconverted predominantly into NH4+. No nitrite neither nitrate were detected at pH 3.1 and 5.2 by a contrast of a high production at pH 12. The initial disappearance rates r0 were determined at different UV-A radiant fluxes which could be modelized by the Langmuir–Hinshelwood model. It was shown that the rate constant (k) increases with UV-A radiant flux. The efficiency of different UV-A and UV-B radiant fluxes were compared. Whatever the energy of photons (UV-A or UV-B), the same quantum yield equal to 0.033 was obtained considering the ability of TiO2 to absorb UV-A or UV-B radiation.

Graphical abstractAmount of methylamine adsorbed per gram of TiO2 (■, Qe) and initial rate of MA disappearance (, r0) as a function of pH.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Neutral species CH3NH2 are more rapidly degraded than their protonated forms CH3NH3+. ► Nitrogen atom in the MA was photoconverted predominantly into NH4+. ► The rate constant (k) increases with UV-A radiant flux. ► UV-B irradiation is more efficient than UV-A for MA disappearance. ► Whatever the energy of photons (UV-A or UV-B) the same quantum yield was obtained.