Ion-Exchange Affinity of Organic Cations to Natural Organic Matter: Influence of Amine Type and Nonionic Interactions at Two Different pHs

Sorption to standard soil organic matter (SOM) has been studied for a wide variety of organic cations using a flow through method with fully aqueous medium as eluent. SOM sorption for weak bases (p<i>K</i>_a 4.5–7) was stronger at pH 4.5 than at pH 7, indicating that the ion-exchange affinity of the cationic species to SOM was higher than the bulk partition coefficient of corresponding neutral species to SOM. In the range of pH 4.5–7, the effect of pH on the sorption coefficients for strong bases with p<i>K</i>_a > 7 was small, within 0.3 log units. For cations with the molecular formula C_<i>x</i>H_<i>y</i>N, sorption was accurately predicted by a model accounting for size (increase with alkyl chain length) and type of charged group (1° amine >4° ammonium of equal size). In addition to the C_<i>x</i>H_<i>y</i>N-model, several empirical correction factors were derived from the data for organic cations with polar functional groups. Models based on <i>K</i>_OW or p<i>K</i>_a fail to explain differences in sorption affinity of the ionic species. Our data on ion-exchange affinities for 80 organic cations show many examples where specific chemical moieties, for example, CH₂-units, aromatic rings or hydroxyl groups, contribute differently to the sorption coefficient as compared to bulk partitioning data of neutral compounds. Other sorption models that were evaluated to explain variation between compounds suffered from outliers of more than one log unit and did not reduce relative log mean standard errors below 0.5. A wider range of sorption coefficients and more sorption data in general are required to improve modeling efforts further