Fate and behavior of 14C-labelled ionic compounds in a soil simulation test

Highlights: • A chemical charge influences the fate and behavior of organic chemicals in soil. • High sorption of DA⁺ and its degradation products reduces their bioavailability. • Most of DS⁻ and its NER after being released were mineralized. • A positive charge clearly increases the persistency of organic chemicals in soil. 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-¹⁴C(U)] (¹⁴C-DP), negatively charged 4-n-dodecylbenzenesulfonicacid[phenylring-¹⁴C(U)] sodium salt (¹⁴C-DS⁻) and positively charged 4-n-dodecylbenzyltrimethylammonium chloride[phenylring-¹⁴C(U)] (¹⁴C-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 ¹⁴C were examined by means of radio-TLC and -HPLC analyses. Microbial activity of the soil incubated with and without ¹⁴C-DP, ¹⁴C-DS⁻ and ¹⁴C-DA⁺ was determined measuring the reduction of dimethylsulfoxide (DMSO) over time. After 124 days of incubation highest mineralization could be observed for ¹⁴C-DS⁻ (64.5% AR). Except CO₂, no volatile residues were formed over time. Besides the parent compounds, polar (¹⁴C-DP, ¹⁴C-DS⁻ and ¹⁴C-DA⁺) and nonpolar (¹⁴C-DA⁺) transformation products were detected. Highest amounts of ¹⁴C were extracted using methanol and were thus potentially bioavailable for soil microorganisms. Microbial activity was markedly higher in soil incubated with ¹⁴C-DP and ¹⁴C-DS⁻ compared to ¹⁴C-DA⁺ or soil without any treatment. Half-lives (DT_{50 k2}) 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.