Hydroxyl radical induced oxidation of theophylline in water: a kinetic and mechanistic study

Abstract

Oxidative destruction and mineralization of emerging organic pollutants by hydroxyl radicals (˙OH) is a well established area of research. The possibility of generating hazardous by-products in the case of ˙OH reaction demands extensive investigations on the degradation mechanism. A combination of pulse radiolysis and steady state photolysis (H₂O₂/UV photolysis) followed by high resolution mass spectrometric (HRMS) analysis have been employed to explicate the kinetic and mechanistic features of the destruction of theophylline, a model pharmaceutical compound and an identified pollutant, by ˙OH in the present study. The oxidative destruction of this molecule, for intermediate product studies, was initially achieved by H₂O₂/UV photolysis. The transient absorption spectrum corresponding to the reaction of ˙OH with theophylline at pH 6, primarily caused by the generation of (T8-OH)˙, was characterised by an absorption band at 330 nm (k₂ = (8.22 ± 0.03) × 10⁹ dm³ mol⁻¹ s⁻¹). A significantly different spectrum (λ_max: 340 nm) was observed at highly alkaline pH (10.2) due to the deprotonation of this radical (pK_a ∼ 10.0). Specific one electron oxidants such as sulphate radical anions (SO₄˙⁻) and azide radicals (N₃˙) produce the deprotonated form (T(−H)˙) of the radical cation (T˙⁺) of theophylline (pK_a 3.1) with k₂ values of (7.51 ± 0.04) × 10⁹ dm³ mol⁻¹ s⁻¹ and (7.61 ± 0.02) × 10⁹ dm³ mol⁻¹ s⁻¹ respectively. Conversely, oxide radicals (O˙⁻) react with theophylline via a hydrogen abstraction protocol with a rather slow k₂ value of (1.95 ± 0.02) × 10⁹ dm³ mol⁻¹ s⁻¹. The transient spectral studies were complemented by the end product profile acquired by HRMS analysis. Various transformation products of theophylline induced by ˙OH were identified by this technique which include derivatives of uric acids (i, iv & v) and xanthines (ii, iii & vi). Further breakdown of the early formed product due to ˙OH attack leads to ring opened compounds (ix–xiv). The kinetic and mechanistic data furnished in the present study serve as a basic frame work for the construction of ˙OH induced water treatment systems as well as to understand the biological implications of compounds of this kind.