Soil, solution, and chemical properties attenuating organic acid sorption by variable-charge soils

Chemical-soil interactions in the soil environment have profound impacts on chemical spatial distribution, bioavailability, and persistence in the ecological system. Sorption by soils greatly moderates the mobility, degradation, and ultimate fate of organic acids in aquatic and terrestrial environments. Predicting sorption of organic acid by variable-charge soils is complex and requires that multiple processes such as pH-dependent hydrophilic interactions between organic acids and charged surfaces as well as hydrophobic interactions be considered simultaneously. Sorption of several organic acids (e.g., chlorinated phenols, 2,4-D, and prosulfuron) by variable-charge soils was measured from CaCl₂, KCl, and Ca(H₂PO₄)₂ solutions as a function of pH and ionic strength to identify the primary sorption mechanisms and quantify their contribution to the overall sorption process. Hydrophilic sorption contributed from near zero to 94% to the overall sorption process and was dependent on the primary soil mineral present. For chlorinated phenols and prosulfuron, hydrophilic sorption correlated well with chemical acidity (pK_a) and the ratio of anion exchange capacity (AEC) to cation exchange capacity (CEC), suggesting the dominance of anion exchange. Soils that have significant AEC, but also have high CEC, sorbed organic anions to a lesser extent due to the attenuation by the repulsive charge from cation exchange sites, which was accounted for by normalizing to CEC. For 2,4-D, sorption from cation bridging was observed in addition to simple anion exchange for soils where kaolinite or quartz was the primary soil mineral. In all soils, increasing pH and the addition of phosphate caused a decrease in sorption, which implies that the typical application of lime and phosphate fertilizer for increased crop production may facilitate off-site leaching of soil-applied acidic pesticides. Initial modeling attempts using individually measured hydrophilic and hydrophobic sorption coefficients with easily measured soil and solute properties (pH, pK_a, AEC, and CEC) predicted organic acid sorption by variable-charge soils within a factor of three. This work showed that (1) hydrophilic sorption is an important process for controlling organic acid retention on variable-charge soils; and (2) the trends in sorption can be predicted by chemical acidity, soil surface charge characteristics, mineral type, and solution composition.