Influence of chemical charge on the fate of organic chemicals in sediment particle size fractions.

In order to investigate the influence of differently sized particle fractions on the fate of ionic chemicals in water-sediment systems, we performed simulation studies following OECD guideline 308. We used C-labelled anionic (4-n-dodecylbenzenesulfonic acid sodium salt, 'C-DS'), cationic (4-n-dodecylbenzyltrimethyl ammonium chloride, 'C-DA') and non-ionic (4-n-dodecylphenol, 'C-DP') organic chemicals. The sediment was subjected to particle size fractionation. For each particle fraction and test compound, water-sediment systems were prepared and incubated for 14 days. Across all particle fractions, higher amounts of applied radioactivity (AR) of C-DS (in sand 60.1%, in silt 45.1%, in clay 57.0%) and of C-DP (sand: 31.8%, silt: 24.4%, clay: 29.2%) were mineralised compared to C-DA (sand: 5.1% AR, silt: 3.5% AR, clay: 2.4% AR). The highest bioavailability was observed for C-DS followed by C-DP and C-DA across all particle fractions. Formation of non-extractable residues (NER) of C-DS did not substantially differ between the particle fractions, whereas NER formation of C-DA was higher in the clay fraction (24.3% AR) than in silt (15.9% AR) and sand (8.4% AR). The same trend was observed for C-DP. We showed that differently sized particle fractions have an influence on the fate of ionic chemicals in water-sediment systems and conclude that this should be considered when simulation studies in soils and sediments with different textural compositions are performed. Since a positive charge of organic chemicals tends to form higher portions of NER in the clay fraction of sediments, these NER should be further investigated in terms of their nature and types of binding.