Sorption of acidic organic solute onto kaolinitic soils from methanol-water mixtures

Abstract

The fate of the acidic organic solute from the soil-water-solvent system is not well-understood. In this study, the effect of the acidic functional group of organic solute in the sorption from cosolvent system was evaluated. The sorption of naphthalene (NAP) and 1-naphthoic acid (1-NAPA) by three kaolinitic soils and two model sorbents (kaolinite and humic acid) were measured as functions of the methanol volume fractions (f(c) <= 0.4) and ionic compositions (CaCl2 and KCl). The solubility of 1-NAPA was also measured in various ionic compositions. The sorption data were interpreted using the cosolvency-induced sorption model. The K-m values (= the linear sorption coefficient) of NAP with kaolinitic soil for both ionic compositions was log linearly decreased with f(c). However, the Km values of 1-NAPA with both ionic compositions remained relatively constant over the f(c) range. For the model sorbent, the Km values of 1-NAPA with kaolinite for the KCl system and with humic acid for both ionic compositions decreased with f(c), while the sorption of 1-NAPA with kaolinite for the CaCl2 system was increased with f(c). From the solubility data of 1-NAPA with f(c), no significant difference was observed with the different ionic compositions, indicating an insignificant change in the aqueous activity of the liquid phase. In conclusion, the enhanced 1-NAPA sorption, greater than that predicted from the cosolvency-induced model, was due to an untraceable interaction between the carboxylate and hydrophilic soil domain in the methanol-water system. Therefore, in order to accurately predict the environmental fate of acidic pesticides and organic solutes, an effort to quantitatively incorporate the enhanced hydrophilic sorption into the current cosolvency-induced sorption model is required.