Anion exchange of organic carboxylate by soils responsible for positive K_m-f_c relationship from methanol mixture

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 (σ) 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%.