Assessing the role of iodination degree on biodegradation kinetics and transformation pathways of iodinated contrast media and derivatives

Triiodinated aromatic compounds used as iodinated contrast media in medical imaging are poorly biodegradable. Reductive dehalogenation enhances biotransformation, but the specific influence of iodination degree on biodegradation remains unclear. This study investigated the biodegradation of twelve model substances: iopromide and diatrizoate (both iodinated contrast media), 5-amino-2,4,6-triiodoisophthalic acid (ATIA, a precursor and transformation product), and their diiodinated, monoiodinated, and deiodinated derivatives. Biodegradation kinetics were assessed in Zahn-Wellens tests and nitrate-reducing aquifer material-water batch tests. DT₅₀ values were calculated to compare degradation rates. Transformation pathways were reconstructed based on literature data and detected transformation products, and evaluated for dependencies on iodination degree. In Zahn-Wellens tests, the iopromide derivatives degraded rapidly (DT₅₀: 0.7-1.2 d) regardless of iodination degree. In contrast, degradation of the diatrizoate derivatives (DT₅₀: 0.9-65 d) and the ATIA derivatives (DT₅₀: 0.3-44 d) was strongly influenced by iodine number and position. For iopromide, transformation pathways were consistent across derivatives and included several novel transformation products beyond the previously assumed final product DDPI. In nitrate-reducing aquifer suspensions, aerobic pathways also occurred for the iopromide derivatives (DT₅₀: 38.7-42.3 d). In contrast, only the monoiodinated and deiodinated diatrizoate were transformed (DT₅₀: 5.6-8.2 d). Mineralization of the ATIA derivatives, measured via dissolved organic carbon, was significantly enhanced for the monoiodinated and deiodinated compound. The findings underline the importance of iodination degree for biotransformation and mineralization. This is particularly relevant for bank filtration, where (partial) deiodination to iodinated aromatics occurs before these compounds enter aerobic drinking water treatment.