Degradation kinetics and mechanism of pentoxifylline by ultraviolet activated peroxydisulfate

Abstract

Degradation of pentoxifylline (PTX) by sodium peroxydisulfate (SPDS) assisted by UV irradiation has been investigated in deionized water. The treatment was more favorable over direct photolysis or peroxydisulfate oxidation alone. The effects of various parameters, including different dosage of oxidant agent, PTX concentration, initial solution pH levels, and the presence of inorganic ions like chloride, nitrate and carbonate have been evaluated. The rate of PTX decomposition depends on the oxidant agent dose. The highest degradation was determined at pH 10.5, which can be explained by the generation of additional hydroxyl radicals (HO˙) in the reaction between sulfate radicals and hydroxide ions. The presence of inorganic ions, especially the carbonate ions quench valuable sulfate radicals and have successfully retarded the PTX decomposition. Six PTX oxidation products were identified using UPLC-QTOF-MS for trials in a basic environment. The main degradation product (3,7-dimethyl-6-(5-oxohexyloxy)-3,7-dihydro-2H-purin-2-one) was isolated by column chromatography and identified by ¹HNMR and LC MS analyzes.

Fig. 1. Molecular structure and atom numbering of PTX.

Fig. 2. Photoreactor for degrading PTX by UV/S₂O₈²⁻ process.

Fig. 3. Comparison of PTX degradation extent by direct photolysis, reaction with SPDS alone and SPDS activated by UV irradiation. PTX 0.2 mM; [SPDS]₀/[PTX]₀ = 20; pH = 5.2; UV_max 253.7 nm; the light intensity 63.7 W m⁻².

Fig. 4. (A) Effect of different [SPDS]₀/[PTX]₀ molar ratio on PTX degradation; (B) effect of different [SPDS]₀/[PTX]₀ molar ratio on TOC removal after 10 min of the reaction. PTX 0.2 mM; [SPDS]₀/[PTX]₀ = (5, 10, 15 or 20); pH = 5.2; UV_max 253.7 nm; the light intensity 63.7 W m⁻².

Fig. 5. (A) The influence of the initial pH of the solution on PTX degradation; (B) effect of different pH solution on TOC removal after 5 min of the reaction. PTX 0.2 mM; [SPDS]₀/[PTX]₀ = 10; UV_max 253.7 nm; the light intensity 63.7 W m⁻².

Fig. 6. Effect of the addition of (A) chloride (B) nitrate and (C) carbonate ions on the degradation of PTX. PTX 0.2 mM; [SPDS]₀/[PTX]₀ = 10; [inorganic salts]₀=(0, 1, 10 or 100) mM; UV_max 253.7 nm; the light intensity 63.7 W m⁻².

Fig. 7. The EE/O values at different initial SPDS concentrations for PTX oxidation by SPDS/UV system.

Fig. 8. Proposed mechanism of formation of the PTX degradation products.

Fig. 9. HPLC-MS chromatogram of an aliquot of the mixture after 10 min of SPDS/UV degradation (PTX 0.2 mM; [SPDS]₀/[PTX]₀ = 20; UV_max 253.7 nm).

Fig. 10. ¹H NMR (δ, 400 MHz, DCCl₃) of PTX and its main degradation product 3,7-dimethyl-6-(5-oxohexyloxy)-3,7-dihydro-2H-purin-2-one.

Fig. 11. Fragmentation pathways of compounds 1–6.