Kinetic and mechanistic investigation of azathioprine degradation in water by UV, UV/H2O2 and UV/persulfate

• Degradation of azathioprine (AZA) was studied by UV and UV-AOPs.
• UV/persulfate is the most efficient and economical for azathioprine removal.
• AZA degradation efficiency in both UV-AOPs reduces with increasing pH.
• Presence of NOM and inorganic anions inhibits azathioprine degradation.
• The possible azathioprine degradation pathways are proposed.

This study investigated the removal of azathioprine (AZA), an immunosuppressant xenobiotic found in hospital effluent, using direct UV-254 nm photolysis, UV/H2O2 and UV/persulfate (UV/PS). AZA cannot be effectively degraded by direct UV photolysis, while the photodegradation efficiency of AZA significantly increases with addition of H2O2 or PS, due to the generation of HO and SO4−, respectively. Compared with AZA removal of 10% by direct UV photolysis at UV dose of 510 mJ cm−2, UV/H2O2 and UV/PS can remove 68% and 87% of AZA, respectively, at an oxidant dose of 100 μM. The second-order rate constants of AZA with HO and SO4− are 1.86 × 109 M−1 s−1 and 2.16 × 109 M−1 s−1, respectively. There is a proportional increase of AZA degradation efficiency with the increasing oxidant dose in the range of 10–100 μM, beyond which radical scavenging effect can negate the radical generation process. The effects of water quality, including pH, NOM and inorganic anions, were also investigated. The removal efficiency of AZA in the UV/H2O2 and UV/PS processes constantly decreases with the increasing pH. NOM significantly reduces the degradation efficiency of AZA through radical scavenging and UV absorption. The inhibition of AZA photodegradation is also affected by the presence of inorganic anions, following the order of HCO3− > Cl− > NO3− ≈ SO42−. Real water samples of treated water from a water treatment plant and secondary effluent from a wastewater treatment plant were used for the assessment of the UV treatment performances. Various AZA transformation by-products were identified to investigate the degradation mechanism of AZA in the UV-AOP systems. The addition of 100 μM oxidant significantly decreases the treatment cost from 0.844 (for UV photolysis) to 0.078 (for UV/H2O2) and 0.067 US$ m−3 order−1 (for UV/PS), respectively, indicating that UV/PS is the most cost-effective process for AZA degradation.