Electrochemical Reduction of X-ray Contrast Iohexol at Mixed Metal Oxide Electrodes: Process Optimization and By-product Identification

The reduction of commonly used X-ray contrast iohexol (IOX) by the electrooxidation process is presented in this study. To begin with, the effect of anode material was examined, and different mixed metal oxide electrodes (MMOs) such as Ti/RuO2, Ti/Pt, Ti/IrO2-RuO2, Ti/IrO2-Ta2O5, Ti/Ta2O5-SnO2-IrO2, and Pt/SnO2 were used. To assess experimental conditions at Ti/RuO2 anode, provided the highest removal efficiency, the response surface method was applied and the key influencing parameter was the process time. The determined optimal conditions were triplicated with real wastewater samples, and the average degradation efficiency of IOX was found to be 99%. By-products of the IOX degradation on the Ti/RuO2 anode have been identified using density functional theory and LC/MS-MS analysis. The results showed that IOX degradation opened with OH group detachment and resulted in the formation of a by-product with a molecular mass of 804 g mol(-1). Further degradation mechanism took place due to the breakup of C-4-C-10 and C-5-I-7 bonds with a by-product formed as 603 g mol(-1). Iodide atom replacement by OH groups caused the formation of a molecular fragment with 375 g mol(-1) molecular weight. The further disintegration of C-2-C-11 and C-6-N-16 sigma- bonds led to the formation of molecular masses of 133, 126, and 119 g mol(-1), respectively.