Removal of an X-Ray contrast chemical from tertiary treated wastewater: Investigation of persulfate-mediated photochemical treatment systems

The degradation of 2.6 μM (2 mg/L) iopamidol (IOPA), a commercially important nonionic, iodinated X-ray contrast chemical and model micropollutant, by UV-A and UV-C photo-assisted persulfate (PS) oxidation in real, tertiary treated municipal wastewater (Organic carbon = 12.4 mg/L; Alkalinity = 130 mg CaCO₃/L; pH = 7.0) was investigated. Preliminary baseline experiments conducted in pure (distilled) water indicated that IOPA could be rapidly and completely removed even by UV-C treatment alone. In the presence of UV-A light, 100% IOPA removal could still be achieved in pure water by PS/UV-A treatment which was studied at varying conditions (0.10–1.00 mM; pH3-11). However, in real wastewater, addition of at least 0.10 mM and 1.00 mM PS was required to achieve high (>90%) IOPA removals by the PS/UV-C and PS/UV-A treatment systems, respectively. PS consumption rates increased under UV-C radiation compared with UV-A radiation and in real wastewater compared to pure water. Organic carbon removals were appreciable for PS/UV-C (48%) and UV-C (40%) treatments in real wastewater. Toxicity assays carried out with the marine photobacteria Vibrio fischeri revealed that the toxicity response did not change significantly after photochemical treatment. According to the bioassay conducted with the freshwater microalga Pseudokirchneriella subcapitata, the toxicity increased after 155 min UV-C photolysis.