Noninvasive monitoring of photocatalytic degradation of X-ray contrast media using Raman spectrometry Sabina Salkic, a,b Logan H. Eckler a and Matthew J. Nee a * The photocatalytic degradation of the X-ray contrast agents iohexol and diatrizoate are monitored by in situ Raman spectrometry measurements in aqueous solution with a TiO 2 photocatalyst. Spectral features and changes are interpreted with the use of density functional theory calculations. While we observe similar results to those published previously, significant changes seen in the Raman spectrum allow us to better identify the mechanisms without the need for deductive approaches or complicated sample preparation. Diatrizoate spectra are consistent with hydroxyl radical attack leading to the loss of CO 2 . Iohexol is more difficult to interpret, representing a current challenge to the use of Raman spectrometry for real-time monitoring of photocatalytic degradation experiments. We explore internal standards that can be added to gauge overall degradation rates to some extent. Potential extensions of the work are described, particularly in terms of increased rate of data acquisition and more general application. Copyright © 2013 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: photocatalytic degradation; X-ray contrast media; wastewater; environmental chemistry Introduction An increasing number of organic pollutants have been identified that are not well treated by waste water management systems. [1] Highly stable compounds, particularly those with aromatic and other cyclic structures, can be difficult to remove in a membrane bioreactor, with over 90% remaining in the water supply follow- ing treatment. [2,3] Studies showing the residence of polycyclic aromatic hydrocarbons, [4] hormones, [5] prescription and over the counter drugs, [6] and other medical wastes [7–9] have led to many alternative approaches to treatment. Iodinated X-ray contrast media such as diatrizoate and iohexol represent unique challenges to waste management systems. [3,10] These compounds are admin- istered to patients in very high dosages (therapeutic levels are near molar concentrations), are not metabolized by the body and are therefore excreted, largely unmodified, into the waste water stream. [11,12] As a result, waste management systems near medical facilities are often faced with large spikes in the concentrations of these compounds. [10,13] One of the most promising methods under consideration is photocatalytic degradation, which has been shown to be broadly effective for many classes of compounds. [14–19] In photocatalytic degradation, waste contaminants are cycled into a reactor with a photocatalyst such as TiO 2 and exposed to ultraviolet (UV) radi- ation of sufficient photon energy to inject electrons into the aqueous phase. [20] Electrons and holes at the surface of the photocatalyst react with water and dissolved oxygen to produce a variety of oxidizing species, which subsequently decompose the organic pollutant, particularly at aromatic and alkene sites. Alternatively, the holes can oxidize target compounds directly by the generation of organic radicals. While most studies have centered on the rates of photocatalytic degradation of organic compounds in solution, some studies have been carried out to identify the mechanisms and kinetic proper- ties of reaction networks initiated by charge separation in the photocatalyst. Photocatalytic degradation has shown mixed success in removing iodinated X-ray contrast media compounds from the waste stream. [21] Separate studies by Doll et al. showed that approximately 15% of iopremol (an iodinated contrast com- pound very similar to iohexol) was degraded by simple UV expo- sure but that the addition of photocatalyst could improve the rate of degradation by as much as tenfold. [6,21] Building on this, Jeong et al. used a series of experiments (including γ-radiolysis to generate hydrated electrons and OH radicals) to show that iohexol behaves similarly but that diatrizoate was significantly less suscep- tible to photocatalytic effects. [22] By restricting oxygen content, the effect of molecular oxygen (and thus, superoxide anion) in the pro- cess can be identified, while the addition of radical scavengers and electron hole scavengers has clarified the role of hydroxyl radical. The general themes that have emerged from these studies include an acknowledgment that different catalysts and different classes of compounds appear to behave differently. To arrive at a more coherent set of predictive principles, further work is still needed. Chromatographic analysis techniques have been most com- monly used for photocatalytic degradation mechanism studies be- cause of their high sensitivity and chemical selectivity, particularly when coupled with mass analysis of the solution components. [13] * Correspondence to: Matthew J. Nee, Department of Chemistry, 1906 College Heights Blvd, Western Kentucky University, Bowling Green, KY 42101, USA E-mail: matthew.nee@wku.edu a Department of Chemistry, Western Kentucky University, 1906 College Heights Blvd, Bowling Green, KY 42101, USA b Ticona Engineering Polymers, 8040 Dixie Hwy., Florence, KY 41042, USA J. Raman Spectrosc. 2013, 44, 1746–1752 Copyright © 2013 John Wiley & Sons, Ltd. Research article Received: 14 May 2013 Revised: 29 August 2013 Accepted: 29 August 2013 Published online in Wiley Online Library: 20 September 2013 (wileyonlinelibrary.com) DOI 10.1002/jrs.4389 1746