Fate of C-labelled ionic organic chemicals in a water-sediment system and surface water.

The persistence assessment of organic chemicals is based on neutral reference substances. Therefore, our study aimed at investigating the influence of a chemical charge on the degradation of organic compounds in a water-sediment system (OECD 308) and surface water (OECD 309). We used radiolabelled 4-n-dodecylbenzenesulfonic acid sodium salt (C-DS, anionic), 4-n-dodecylbenzyltrimethylammonium chloride (C-DA, cationic) and 4-n-dodecylphenol (C-DP, non-ionic) which are structurally similar except their charges. After 120 days of incubation in a water-sediment system, 68% (C-DS), 6% (C-DA) and 63% (C-DP) of the applied radioactivity (AR) were mineralized. The formation of non-extractable residues (NER) after 120 days was highest for C-DA (33% AR), followed by C-DS (19% AR) and C-DP (14% AR). Dissipation half-lives (DT) at 12 °C decreased as follows: C-DA (346 days) ≫ C-DS (47 days) > C-DP (30 days). After 60 days of incubation in surface water with suspended sediment, mineralization of C-DS, C-DA and C-DP accounted for 63%, 7% and 58% AR, respectively. Highest NER formation was observed for C-DP (21% AR), followed by C-DA (14% AR) and C-DS (9% AR). DT (12 °C) decreased as follows: C-DA (45 days) > C-DP (3 days) > C-DS (2 days). We showed that a positive charge reduces the degradability of organic chemicals in both test systems. From a scientific point of view, simulation studies following OECD 309 should always be complimented by tests with high sorption capacity, e.g. OECD 308 and OECD 307 tests in order to assess the degradation of a compound, especially in case of cationic organic compounds.