Effect of sodium and calcium cations on the ion-exchange affinity of organic cations for soil organic matter

Sorption of organic cations to soil organic matter was studied using dynamic column experiments with different compositions of electrolytes in aqueous eluents. The sorption affinity of the tested variety of charged compounds, including primary, secondary, and tertiary amines and quaternary ammonium compounds, all showed the same response to different medium compositions. The sorption affinity to Pahokee peat (i) strongly decreased with increasing electrolyte concentration, up to a factor 250 due to tested electrolyte compositions alone, (ii) was higher in NaCl solutions than in CaCl₂ solutions of similar ionic strength, and (iii) was more sensitive to a decrease in NaCl than to a decrease in CaCl₂, though the selectivity coefficients were not significantly different. For a weak base that was tested in eluent pH either above or below its pK_a, we demonstrated that the sorption affinity of (iv) the neutral base was hardly affected by different electrolyte compositions, comparable to a neutral reference compound, (v) the protonated weak base was strongly affected by different electrolyte compositions, and (vi) the protonated base was in the same range, or stronger, compared to the neutral base. Mass action law equations for ion-exchange reactions predicted similar trends in a qualitative but not in a quantitative way. More complex models are required to fully account for the contributions of ionic interactions to the sorption of organic cations. These results imply that risk assessment models for organic bases should take ion-exchange processes into account when estimating soil sorption coefficients and bioavailability.