Rapid Metal-Catalyzed Hydrodehalogenation of Iodinated X-Ray Contrast Media

Iodinated X-ray contrast media (ICM) are detected in natural waters at high concentrations relative to other pharmaceuticals due to extensive use in medical diagnostics and high recalcitrance during conventional wastewater treatment. This study examines, for the first time, reductive treatment of ICM in water with hydrogen gas in combination with supported palladium and porous nickel catalysts. Kinetic experiments demonstrate rapid and complete hydrodehalogenation of both ionic (diatrizoate) and neutral (iopromide) ICM. Reaction rates in batch systems (continuous mixing, P<sub>H<sub>2</sub></sub> = 0.1 MPa, 25 °C) appear to be surface-reaction controlled and are dependent upon catalyst identity (e.g., 5 wt % versus 1 wt % Pd/Al<sub>2</sub>O<sub>3</sub>) as well as the concentration of ICM and catalyst. Reaction rates are not markedly affected by ICM structure, pH, or by the presence of many common ions (Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, F<sup>−</sup>, Cl<sup>−</sup>, Br<sup>−</sup>, ClO<sub>4</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup>, HCO<sub>3</sub><sup>−</sup>, and NO<sub>3</sub><sup>−</sup>). In contrast, elevated concentrations of iodide, (bi)sulfide, and dissolved organic matter inhibit hydrodehalogenation of ICM. However, catalyst activity can be regained by washing the catalyst (e.g., with water, NaOCl, or alkaline solution). Catalytic reduction of ICM present in treated wastewater effluent is slower than in deionized water, but similar reaction rates are observed when the effluent is pretreated to reduce the level of dissolved organic matter. The high selectivity of reductive catalytic treatment processes suggest that this is a promising strategy for targeted treatment of ICM present in mixed waste streams and natural water matrices containing much higher concentrations of nontarget constituents.