Effects of UV/PMS oxidation on the degradation of zidovudine: kinetics, degradation products, and reaction pathways

Abstract

Zidovudine (AZT), a persistent pharmaceutical contaminant detected in diverse biological and environmental matrices, raised significant concerns due to its ecological and health risks. This study systematically investigates the degradation kinetics, mechanisms, and toxicity evolution of AZT in a UV/peroxymonosulfate (UV/PMS) system. The UV/PMS process demonstrated superior performance with a degradation rate constant of 0.0384 min⁻¹, surpassing UV/H₂O₂ (0.0138 min⁻¹) and UV/NaClO (0.0300 min⁻¹), achieving 84.44% removal efficiency. Radical quenching experiments and kinetic modeling revealed synergistic contributions from direct photolysis (51.0%), hydroxyl radicals (18.1%), and sulfate radicals (30.9%). Degradation exhibited strong pH dependence, with optimal efficiency at pH 5.2–6.1 (k = 0.0486 min⁻¹, >92% removal), while alkaline conditions significantly inhibited the process. Coexisting substances differentially influenced degradation: HCO₃⁻ (10 mM) reduced efficiency to 68.6% (k = 0.0194 min⁻¹), NO₃⁻ (3 mM) slightly enhanced removal to 90.85% (k = 0.0414 min⁻¹), and NO₂⁻ (3 mM) and humic acid (10 mg L⁻¹) caused severe suppression (46.2% and 36.84% removal, respectively) through radical quenching and UV absorption. In real water matrices, Yellow River source reservoir water inhibits AZT degradation: under identical oxidant concentrations, UV/PMS, UV/NaClO, and UV/H₂O₂ systems showed 26.85%, 31.2%, and 32.9% lower efficiencies than in ultrapure water. Increasing PMS to 15 and 25 mg L⁻¹ enhanced UV/PMS removal to 70.04% and 81.03%. Inhibition is linked to inorganic ions, scavenging radicals, alkaline pH (8.27), high turbidity interfering with UV absorption, and organics competing for radicals. Three primary degradation pathways were identified, involving thymine formation, azide group elimination, demethylation, and double bond addition. Toxicity assessments using Vibrio fischeri bioluminescence indicated an initial increase followed by partial reduction in acute toxicity, though residual toxicity persistently exceeded baseline levels.