Kinetic Study and Modeling of the Degradation of Aqueous Ammonium/Ammonia Solutions by Heterogeneous Photocatalysis with TiO₂ in a UV-C Pilot Photoreactor

The degradation mechanism of NH₄⁺/NH₃ in aqueous solutions by heterogeneous photocatalysis (TiO₂/SiO₂ ) and photolysis in UV-C pilot photoreactor has been studied. Under the conditions used, NH₄⁺/NH₃ can be decomposed both by photolytically and photocatalytically, without disregarding stripping processes. The greatest degradation is achieved at the highest pH studied (pH 11.0) and at higher lamp irradiation power used (25 W) with degradation performances of 44.1% (photolysis) and 59.7% (photocatalysis). The experimental kinetic data fit well with a two parallel reactions mechanism. A low affinity of ammonia for adsorption and surface reaction on the photocatalytic fiber was observed (coverage not higher than 10%), indicating a low influence of surface phenomena on the reaction rate, the homogeneous phase being predominant over the heterogeneous phase. The proposed reaction mechanism was validated, confirming that it is consistent with the photocatalytic and photolytic formation of nitrogen gas, on the one hand, and the formation of nitrate, on the other hand. At the optimal conditions, the rate constants were k₃ = 0.154 h⁻¹ for the disappearance of ammonia and k₁ = 3.3 ± 0.2 10⁻⁵ h⁻¹ and k₂ = 1.54 ± 0.07 10⁻¹ h⁻¹ for the appearance of nitrate and nitrogen gas, respectively.