Fate and behavior of C-labelled ionic compounds in a soil simulation test.

The influence of an ionic functional group on the fate and behavior of chemicals in the environment has so far not been systematically investigated. This study, therefore, examines the following three substances with high structural similarity but differing charge: non-charged 4-n-dodecylphenol[phenylring-C(U)] (C-DP), negatively charged 4-n-dodecylbenzenesulfonicacid[phenylring-C(U)] sodium salt (C-DS) and positively charged 4-n-dodecylbenzyltrimethylammonium chloride[phenylring-C(U)] (C-DA). They were investigated in a soil simulation study according to the OECD 307 test guideline by measuring the distribution of the applied radioactivity (AR) among volatile, mineralized, extractable and non-extractable residues (NER) in one soil after 0, 1, 7, 14, 49, 84 and 124 days of incubation. Extractable portions of C were examined by means of radio-TLC and -HPLC analyses. Microbial activity of the soil incubated with and without C-DP, C-DS and C-DA was determined measuring the reduction of dimethylsulfoxide (DMSO) over time. After 124 days of incubation highest mineralization could be observed for C-DS (64.5% AR). Except CO, no volatile residues were formed over time. Besides the parent compounds, polar (C-DP, C-DS and C-DA) and nonpolar (C-DA) transformation products were detected. Highest amounts of C were extracted using methanol and were thus potentially bioavailable for soil microorganisms. Microbial activity was markedly higher in soil incubated with C-DP and C-DS compared to C-DA or soil without any treatment. Half-lives (DT) at 18 °C were as follows: DA (61.8 days) > DS (18.2 days) > DP (10.0 days). In case of the cationic compound and its transformation products we conclude that a higher sorption affinity to soil particles leads to reduced bioavailability for microorganisms and thus reduced mineralization resulting in a higher persistence compared to anionic and non-charged organic compounds in soil. The impact of our findings on the persistence assessment of chemicals when performing OECD guideline tests in soil, water-sediment and surface water is discussed.