UV- TiO2 photocatalytic degradation of the x-ray contrast agent diatrizoate: kinetics and mechanisms in oxic and anoxic solutions

摘要

In recent years, triiodinated X-ray contrast agents (XRCs) have been widely detected innatural waters at concentrations that are often relatively high compared to otherpharmaceutical micropollutants due to their extensive use and high biological recalcitrance.Diatrizoate is an ionic XRC that has been found to be especially resistant to conventionalwastewater and drinking water treatment processes. This study examines the aqueousphotocatalytic treatment of diatrizoate using nanophase titanium dioxide (TiO2). Experimentsdemonstrate that diatrizoate is degraded in aqueous TiO2 suspensions illuminated withultraviolet-A (UVA) light under both oxic and anoxic conditions. Both oxidative and reductivetransformation pathways for diatrizoate are initiated by photo-excited charge separation in theTiO2 semiconductor, which generates a strongly oxidizing valence band hole (hvb+) and astrongly reducing conduction band electron (ecb-). In oxic solutions, diatrizoate degradesprincipally by oxidation with hydroxyl radicals (×OH) formed after hvb+ oxidizes adsorbedH2O/OH-. Iodine substituents are readily liberated, but little mineralization of organic carbonand 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 byscavenging ×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 oxidationwith ×OH, so this pathway may be a promising strategy for selectively treating XRCs present inorganic-rich wastewater streams.