Comparison of UV, chlorination, UV-hydrogen peroxide and UV-chlorine processes for tramadol removal: Kinetics study and transformation products identification

Tramadol (TRA) is a poorly biodegradable pharmaceutical, ubiquitously distributed in the aquatic environment. Herein, we present a thorough comparison of two conventional (UV and chlorination) and two advanced oxidative processes (UV-H2O2 and UV-chlorine), in terms of TRA removal efficiency and transformation products (TPs) formation. The performance of these processes was evaluated in different complexity water samples, previously adjusted at neutral pH. Using 1 mg L−1 as initial concentration of oxidant, UV-chlorine was the most effective process followed by UV-H2O2, direct photolysis and chlorination. The efficiency of the investigated treatments decreased for surface and wastewater samples compared to model solutions. Despite this limitation, the UV-chlorine treatment removed more than 70% of TRA measured in wastewater samples after 5 min of UV irradiation, in presence of 10 mg L−1 of chlorine. UV-chlorine involved additional degradation routes to those described for the rest of treatments, including the formation of tramadol-N-oxide, several chlorinated derivatives, and the volatile species chloroform, 1,1-dichloroacetone and anisole. Some of these specific TPs might pose a similar toxicity to that associated to the parent compound; however, their maximum formation yields remained below 2.5%, decreasing steady for reaction times longer than 5 min