Influence of a chemical charge on the distribution and degradation of organic chemicals in water-sediment systems

The objective of the present study was to investigate the influence of a chemical charge on the distribution and degradation of organic chemicals in water-sediment systems and in surface water. Three 14C-labelled model substances were used with 4-n-dodecylbenzene sulfonic acid sodium salt (14C-DS-), 4-n-dodecylbenzyltrimethyl ammonium chloride (14C-DA+) and 4-n-dodecylphenol (14C-DP) representing the anionic, cationic and non-ionic model substance, respectively. These model substances were characterised by a high structural similarity of their hydrophobic part, whereas the polar head carried a negative (14C-DS-), positive (14C-DA+) or no charge (14C-DP). Simulation studies following OECD guideline 308 (Aerobic and Anaerobic Transformation in Aquatic Sediment Systems) and OECD guideline 309 (Aerobic Mineralization in Surface Water - Simulation Biodegradation Test) were performed. Natural sediment and surface water were collected from a rainwater retention basin in Aachen, Germany. The behaviour of 14C-DS-, 14C-DA+ and 14C-DP in a water-sediment system was investigated over an incubation period of 120 days. After removing the water phase, the sediment was extracted sequentially using different solvents (aqueous CaCl2 solution, methanol and acetonitrile). The sediment incubated with 14C-DA+ was additionally extracted under Soxhlet conditions. After 120 days of incubation, mineralisation of 14C-DS- and 14C-DP accounted for 68% and 63% of applied radioactivity (AR), respectively. The cationic compound 14C-DA+ was mineralised to a lesser extent (6% AR). The direct bioavailability of the test substances, based on their mineralisation and portions in the CaCl2 fraction, decreased as follows: 14C-DS- > 14C-DP > 14C-DA+. NER formation was highest for 14C-DA+ (33% AR), followed by 14C-DS- (19% AR) and 14C-DP (14% AR). Half-lives (DT50) of the test substances decreased as follows: 14C-DA+ (162 days) > 14C-DS- (22 days) > 14C-DP (14 days). In order to investigate the influence of sediment particle size fractions on the degradation of the test substances, simulation studies following OECD 308 were performed using the sand, silt and clay fraction. After 14 days of incubation and across all particle fractions, higher amounts of 14C-DS- (45-60% AR) and 14C-DP (24-32% AR) were mineralised compared to 14C-DA+ (2-5% AR). The direct bioavailability of the test substances decreased in the same order as in the water-sediment study (14C-DS- > 14C-DP > 14C-DA+). The highest NER formation in the sand and silt fraction was observed for 14C-DS- (16% AR and 18% AR, respectively). In the clay fraction, NER formation was highest for 14C-DA+ (24% AR). The surface water test following OECD 309 was performed as a suspended sediment test over an incubation time of 60 days. Additionally, abiotic degradation of the test substances was examined under sterile conditions using autoclaved surface water treated with sodium azide and γ-irradiated sediment. After 60 days under non-sterile conditions, mineralised portions of 14C-DS- and 14C-DP accounted for 63% and 58% AR, respectively. Mineralisation of 14C-DA+ was considerably lower (7% AR). Under sterile conditions, mineralisation of 14C-DS-, 14C-DA+ and 14C-DP was negligible ( 14C-DP (1.2 days) > 14C-DS- (1 day). The results of the present study demonstrated that the fate of organic chemicals in aquatic sediment systems was influenced by their chemical charge. A positive charge leads to a reduced degradation of a chemical in water-sediment systems and in surface water and increases sorption to sediment particles. The resulting higher persistence of the positively charged test substance was also reflected by higher degradation half-lives compared to the negatively charged and neutral test substance