Catalytic hydrodehalogenation of iodinated contrast agents in drinking water: A kinetic study

The occurrence of iodinated contrast media (ICM) in water bodies represents a significant threat for public health due to their strong biochemical stability and recalcitrance to conventional water treatment systems. In this work, the feasibility of catalytic hydrodehalogenation (HDH) for the degradation of a representative group of ICMs (iohexol (IHX), iopromide (IPR), iopamidol (IPM) and diatrizoic acid (DIA)) is investigated. Complete degradation and deiodination of the isolated ICMs (1.6·10 −2 mmol L −1 ) was achieved in less than 30 min in the presence of 0.25 g L −1 of different precious metal catalysts (Pd/Al 2 O 3 , Rh/Al 2 O 3 and Pt/Al 2 O 3 ) under ambient conditions, closing iodine balance by >95 %. Based on the identified reaction intermediates and final products, reaction mechanisms were accordingly proposed. A sequential reaction pathway was proposed for DIA removal, while reaction mechanisms encompassing both sequential and concerted reaction steps were proposed for the non-ionic ICMs (IHX, IPR, and IPM). Subsequent kinetic models, which successfully described experimental data, were developed from the elucidated reaction mechanisms, considering all species involved in the reaction. The versatility of the process was finally demonstrated in the treatment of real aqueous matrices (surface water and tap water) spiked with the ICMs mixture, except in the case of Pt/Al 2 O 3 which suffered deactivation due to surface poisoning by halides. For this catalyst, kinetic constants ranged from 0.31 to 0.22 min −1 in deionized water, and from 0.04 to 0.08 min −1 in surface water as the reaction matrix. Among the tested catalysts, Pd/Al 2 O 3 showed the highest activity and stability for the removal of ICMs. • HDH is an effective technology for the removal of iodinated contrast media. • Reaction mechanisms were proposed based on the intermediates and products identified. • The process feasibility was proved in real aqueous matrices (surface and tap water). • The Pd/Al 2 O 3 catalyst has shown high stability upon three consecutive applications.