Degradation mechanisms and kinetic studies for the treatment of X-ray contrast media compounds by advanced oxidation/reduction processes

The presence of iodinated X-ray contrast media compounds (ICM) in surface and ground waters has been reported. This is likely due to their biological inertness and incomplete removal in wastewater treatment processes. The present study reports partial degradation mechanisms based on elucidating the structures of major reaction by-products using γ-irradiation and LC-MS. Studies conducted at concentrations higher than observed in natural waters is necessary to elucidate the reaction by-product structures and to develop destruction mechanisms. To support these mechanistic studies, the bimolecular rate constants for the reaction of OH and e−aq with one ionic ICM (diatrizoate), four non-ionic ICM (iohexol, iopromide, iopamidol, and iomeprol), and the several analogues of diatrizoate were determined. The absolute bimolecular reaction rate constants for diatrizoate, iohexol, iopromide, iopamidol, and iomeprol with OH were (9.58 ± 0.23)×108, (3.20 ± 0.13)×109, (3.34 ± 0.14)×109, (3.42 ± 0.28)×109, and (2.03 ± 0.13) × 109 M−1 s−1, and with e−aq were (2.13 ± 0.03)×1010, (3.35 ± 0.03)×1010, (3.25 ± 0.05)×1010, (3.37 ± 0.05)×1010, and (3.47 ± 0.02) × 1010 M−1 s−1, respectively. Transient spectra for the intermediates formed by the reaction of OH were also measured over the time period of 1–100 μs to better understand the stability of the radicals and for evaluation of reaction rate constants. Degradation efficiencies for the OH and e−aq reactions with the five ICM were determined using steady-state γ-radiolysis. Collectively, these data will form the basis of kinetic models for application of advanced oxidation/reduction processes for treating water containing these compounds.