Methylamine and dimethylamine photocatalytic degradation—Adsorption isotherms and kinetics

The photocatalytic degradation of two nitrogenous organic compounds, methylamine CH3NH2 and dimethylamine (CH3)2NH, used in pharmaceutical and chemical industries was investigated in the presence of UV-irradiated TiO2 aqueous suspensions. Different parameters were studied: adsorption under dark and UV-A conditions, photolysis, kinetics of degradation, and chemical pathway of methylamine (MA) and dimethylamine (DMA) degradation. The percentage of covered OH in the dark was equal for different concentrations of MA and DMA. The adsorption isotherms of these two amine compounds MA and DMA follow the Langmuir model. The photocatalytic oxidation kinetics of MA and DMA are described by the Langmuir–Hinshelwood model with a first order kinetics for concentrations below 0.5 mmol L−1, and then reach a plateau.For both MA and DMA, the coverage rates of the TiO2 surface in the dark and under illuminated conditions were different. The nitrogen atoms were decomposed mainly to ammonium (NH4+). Nitrite (NO2−) was also formed but was rapidly oxidized to nitrate (NO3−). MA was detected as an intermediate product during the degradation of DMA. Formic acid (HCOOH), and two other products not identified were detected. These non-identified products do not correspond to formamide. Total Organic Carbon (TOC) analysis shows the presence of final slightly mineralised intermediate compounds.

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► Same area density (molecule nm−2) of adsorbed MA and DMA are obtained.
► The higher reactivity of MA than DMA is due to the higher reactivity of the cationic form.
► The breaking of C–N bond is the main initial step in the degradation of DMA.
► For MA and DMA, the coverage rates of the TiO2 in dark and under UV were different.