Reductive and oxidative degradation of iopamidol, iodinated X-ray contrast media, by Fe(III)-oxalate under UV and visible light treatment

Iopamidol, widely employed as iodinated X-ray contrast media (ICM), is readily degraded in a Fe(III)-oxalate photochemical system under UV (350 nm) and visible light (450 nm) irradiation. The degradation is nicely modeled by pseudo first order kinetics. The rates of hydroxyl radical ((OH)-O-center dot) production for Fe(III)-oxalate/H2O2/UV (350 nm) and Fe(III)-oxalate/H2O2/visible (450 nm) systems were 1.19 +/- 0.12 and 0.30 +/- 0.01 mu M/min, respectively. The steady-state concentration of hydroxyl radical ((OH)-O-center dot) for the Fe(III)-oxalate/H2O2/UV (350 nm) conditions was 10.88 +/- 1.13 x 10(-14) M and 2.7 +/- 0.1 x 10(-14) M for the Fe(III)-oxalate/H2O2/visible (450 nm). The rate of superoxide anion radical (O-2(-center dot)) production under Fe(III)-oxalate/H2O2/UV (350 nm) was 0.19 +/- 0.02 mu M/min with a steady-state concentration of 5.43 +/- 0.473 x 10(-10) M. Detailed product studies using liquid chromatography coupled to Q-TOF/MS demonstrate both reduction (multiple dehalogenations) and oxidation (aromatic ring and side chains) contribute to the degradation pathways. The reduction processes appear to be initiated by the carbon dioxide anion radical (CO2-center dot) while oxidation processes are consistent with (OH)-O-center dot initiated reaction pathways. Unlike most advanced oxidation processes the Fe(III)-oxalate/H2O2/photochemical system can initiate to both reductive and oxidative degradation processes. The observed reductive dehalogenation is an attractive remediation strategy for halogenated organic compounds as the process can dramatically reduce the formation of the problematic disinfection by-products often associated with oxidative treatment processes. (C) 2014 Elsevier Ltd. All rights reserved.