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

## TL;DR

This study examines how the number of iodine atoms in contrast media compounds affects their breakdown in water, finding that fewer iodine atoms lead to faster degradation.

## Contribution

The study reveals how iodination degree influences biodegradation and transformation pathways of iodinated contrast media in different environmental conditions.

## Key findings

- Iopromide derivatives degraded rapidly regardless of iodination degree in Zahn-Wellens tests.
- Diatrizoate and ATIA derivatives showed degradation rates strongly dependent on iodine number and position.
- Mineralization of ATIA derivatives was enhanced for monoiodinated and deiodinated compounds in nitrate-reducing conditions.

## Abstract

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.

The online version contains supplementary material available at 10.1007/s10532-025-10213-6.