Dialysis experiments for assessing the pH-dependent sorption of sulfonamides to soil clay fractions

Equilibrium dialysis experiments, a novel approach for conducting soil/water distribution experiments in environmental samples, were found to be applicable for assessing pH-dependent partitioning and to quantify the sorption of three sulfonamides, sulfadiazine, sulfadoxine, and sulfacetamide. Clay fractions from two agricultural soils including both particulate and dissolved soil matter were used in the experiments to achieve a high sorption capacity when varying pH in a relevant environmental range. Stabilizing and controlling pH was done by using organic buffers. In two clay fractions, Kd for sulfadiazine was determined to be 43 and 129 L kg−1, and 1.3 and 4.6 L kg−1 at pH 4.0 and pH 9.0, respectively. This corresponded to Kd for the neutral and ionized form of sulfadiazine, respectively. The difference in sulfadiazine sorption between the two clay fractions could to some extent be related to the difference in the amount of organic carbon. Sorption experiments with sulfacetamide and sulfadoxine also exhibited decreasing sorption when increasing pH. At low pH, maximum Kd for sulfacetamide and sulfadoxine was determined to be 83 and 211 L kg−1, respectively, while at high pH minimum Kd was 4.8 and 1.2 L kg−1, respectively. Hence, compound speciation was important for the quantity of sorbed sulfonamide, which was confirmed by a correlation (R2) close to unity, when using the experimentally obtained Kd values with a simple model weighing the contribution from the neutral and the ionized compound, respectively.