TiO₂-photocatalyzed transformation of the recalcitrant X-ray contrast agent diatrizoate

Iodinated X-ray contrast media (ICM) are biologically recalcitrant chemicals that are often detected in wastewater-impacted environments at higher concentrations than other pharmaceutical micropollutants of concern. Diatrizoate is an anionic ICM that is especially resistant to conventional wastewater and drinking water treatment processes. This study examined the aqueous photocatalytic treatment of diatrizoate using nanophase titanium dioxide (TiO₂). Experiments demonstrated that diatrizoate can be degraded in aqueous TiO₂ suspensions illuminated with ultraviolet-A (UVA) light. In oxic solutions, diatrizoate degraded principally via oxidation by adsorbed hydroxyl radicals (OH), releasing iodine substituents stoichiometrically, but causing little mineralization of organic carbon and nitrogen. Introduction of OH scavengers significantly slowed the rate of photocatalytic degradation. At circumneutral pH, diatrizoate was oxidized much more slowly than iopromide, a nonionic ICM, in part because of unfavorable electrostatic interactions with the negatively charged TiO₂ surface; increased rates of oxidation observed at lower pH conditions can be attributed to more favorable diatrizoate-TiO₂ sorptive interactions. Experiments also demonstrated that diatrizoate was degraded at appreciable rates in anoxic solutions, and reductive dehalogenation products are identified by liquid chromatography-mass spectrometry. The higher selectivity of reductive processes suggests a promising strategy for targeted treatment of recalcitrant ICM in organic-rich matrices like wastewater effluent.