Tramadol Degradation by high-energy ionizing radiation: A study on radical reactions and matrix effects

Tramadol, a widely used opioid analgesic, has been detected as an emerging contaminant in both surface waters and wastewaters. Its removal during conventional wastewater treatments is incomplete resulting at concentrations from the ng dm 3 range up to the low μg dm 3 range in municipal wastewater treatment plant effluents in many countries. This study provides a detailed investigation of tramadol degradation under high-energy ionizing radiation conditions, with emphasis on hydroxyl radical ( • OH) reactions and the impact of matrix on removal efficiency. The p K a of Tramadol (9.41) plays a major role in • OH reactions. Below p K a the predominant pathway is • OH addition to the aromatic ring, whereas above p K a , the reactions at the amino group have also high yield via H -abstraction from the C -atoms next to N . For the interpretation of matrix effect on the removal efficiency of Tramadol, degradation in ultrapure water, tap water, synthetic wastewater, and treated wastewater matrices were investigated. Oxidation was clearly observed, while mineralization was more limited, and the biodegradability continuously increased with the absorbed doses. In the standard Vibrio fischeri bioassays the fluorescence inhibition of both the parent compound and its degradation products remained below 20% in pure water. The matrix constituents like organic and inorganic matters, metal ions acted as radical scavengers and reduced the efficiency of irradiation. From the technological perspective, high-energy ionizing radiation proved effective to disrupt the molecular structure of Tramadol, thereby decreasing its environmental impact.