Environ. Sci. Technol. 2008, 42, 577–583
Rapid Metal-Catalyzed Hydrodehalogenation of Iodinated X-Ray Contrast Media L I N D S A Y E . K N I T T , †,§ J O H N R . S H A P L E Y , ‡,§ A N D T I M O T H Y J . S T R A T H M A N N * ,†,§ Department of Civil and Environmental Engineering, and Department of Chemistry, University of Illinois at Urbana– Champaign, Urbana, Illinois 61801, and Center of Advanced Materials for the Purification of Water with Systems, Urbana, Illinois 61801
Received July 24, 2007. Revised manuscript received October 15, 2007. Accepted October 22, 2007.
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, PH2 ) 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/ Al2O3) 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+, Ca2+, Mg2+, F-, Cl-, Br-, ClO4-, SO42-, HCO3-, and NO3-). 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.
Introduction In recent years, a growing number of pharmaceuticals and associated metabolites have been detected in natural water bodies around the world (1, 2). Conventional wastewater treatment is ineffective for many pharmaceuticals, and a significant fraction of these compounds is discharged to receiving waters (2, 3). Continuous release of pharmaceuticals * Corresponding author email:
[email protected], phone: 217244-4679, fax: 217-333-6968. † Department of Civil and Environmental Engineering. § Center of Advanced Materials for the Purification of Water with Systems. ‡ Department of Chemistry. 10.1021/es071837y CCC: $40.75
Published on Web 12/07/2007
2008 American Chemical Society
and other wastewater-derived micropollutants (e.g., personal care products, disinfection byproducts) into aquatic environments raises a number of ecological and public health concerns (4, 5). Iodinated X-ray contrast media (ICM) are a class of radiopaque agents used for brain and body imaging. Diatrizoate and iopromide are representative ionic and neutral ICM, respectively.
Worldwide use of ICM in medical diagnostic procedures is in the thousands of tonnes/yr (6). Hospital and domestic wastewater often contains significant ICM loadings because the compounds exhibit high biochemical stability and are excreted mainly in their unmetabolized form (7). Recent reports indicate that ICM are poorly removed during wastewater treatment and are highly persistent once released into receiving waters (8, 9). Their recalcitrance is attributed to low biodegradability by aerobic heterotrophs (10, 11) and hydrophilic properties which limit removal with biosolids (7). Although the acute toxicities of ICM are believed to be minimal (7), and environmental concentrations are usually found to be low (typically
