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Title:UV- TiO2 photocatalytic degradation of the x-ray contrast agent diatrizoate: kinetics and mechanisms in oxic and anoxic solutions
Author(s):Sugihara, Matthew N.
Advisor(s):Strathmann, Timothy J.
Department / Program:Civil & Environmental Eng
Discipline:Environ Engr in Civil Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
x-ray contrast media
titanium dioxide
Abstract:In recent years, triiodinated X-ray contrast agents (XRCs) have been widely detected in natural waters at concentrations that are often relatively high compared to other pharmaceutical micropollutants due to their extensive use and high biological recalcitrance. Diatrizoate is an ionic XRC that has been found to be especially resistant to conventional wastewater and drinking water treatment processes. This study examines the aqueous photocatalytic treatment of diatrizoate using nanophase titanium dioxide (TiO2). Experiments demonstrate that diatrizoate is degraded in aqueous TiO2 suspensions illuminated with ultraviolet-A (UVA) light under both oxic and anoxic conditions. Both oxidative and reductive transformation pathways for diatrizoate are initiated by photo-excited charge separation in the TiO2 semiconductor, which generates a strongly oxidizing valence band hole (hvb +) and a strongly reducing conduction band electron (ecb -). In oxic solutions, diatrizoate degrades principally by oxidation with hydroxyl radicals (×OH) formed after hvb + oxidizes adsorbed H2O/OH-. Iodine substituents are readily liberated, but little mineralization of organic carbon and nitrogen occurs at short reaction times. Rates of photocatalytic oxidation decrease with increasing pH, attributed to unfavorable electrostatic interactions between anionic diatrizoate and the negatively charged TiO2 surface at elevated pH conditions. High concentrations of bicarbonate (HCO3-), a common natural water constituent, inhibit diatrizoate oxidation by scavenging ×OH and forming less reactive carbonate radicals (CO3 ×-). In anoxic solutions, experiments demonstrate that in the absence of another exogenous electron acceptor (e.g., BrO3-), diatrizoate can also be reductively deiodinated by reaction with ecb-, and this pathwaydominates if high concentrations of other ×OH-reactive chemicals (e.g., methanol) are present. Photocatalytic reduction is a much more chemical-selective treatment process than oxidation with ×OH, so this pathway may be a promising strategy for selectively treating XRCs present in organic-rich wastewater streams.
Issue Date:2010-08-20
Rights Information:Copyright 2010 Matthew N. Sugihara
Date Available in IDEALS:2010-08-20
Date Deposited:2010-08

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