Anion exchange of organic carboxylate by soils responsible for positive K-m-f(c) relationship from methanol mixture

Abstract

The cosolvency model was not applicable for predicting the sorption of organic carboxylic acids. The reason of inapplicability was investigated by analyzing the solubility (S-m,) and sorption (K-m) of benzoic acid, 2,4-dichlorophenoxyacetic acid (2,4-D), and 2,4,6-trichlorophenol (2,4,6-TCP). The S-m, and K-m by two iron-rich soils was measured as a function of methanol volume fraction (f(c)), electrolyte compositions, and pH(aPP). For 2,4,6-TCP, the K-m of both neutral and anion species was well-explainable by the cosolvency model, exemplifying the knowledge of cosolvency power (sigma) being sufficient to describe its sorption. However, for benzoic acid and 2,4-D, the K-m of organic anions increased with f(c), illustrating the organic carboxylate to be responsible for the deviation. The S-m of organic anions was not affected by the ionic valence (Ca2+ vs. K+) of liquid phase. Among hydrophilic quantities of the 2,4-D sorption, the fraction of anion exchange increased with f(c) while the fraction of Ca-bridge decreased in the same range. Adding solvent in soil-water system is likely to render soil surface charge more positive, fortifying the anion exchange, but opposing the formation of Ca-bridging. Therefore, it can be concluded that the positive K-m-f(c) relationship is due to the anion exchange of organic carboxylate with positively charged soil surface, whose contribution is >50% of overall sorption at solvent-free system and becomes greater with f(c) up to 82%. (C) 2013 Published by Elsevier Ltd.