Effect of surface coordination on 2,4-D sorption by kaolinite from methanol/water mixture

- The solubility of 2,4-D anion does not invoke the positive Km-fc relationship.
- The charge-related hydrophilic interaction is the primary cause of the inapplicability of cosolvency sorption model.
- The bidentate coordination is formed on the positively charged mineral in the hydrophilic sorption.
- The positive relationship of Km-fc is a result of bidentate coordination of 2,4-D anion on the positively charged surface.

The application of the cosolvency sorption model to predict the sorption of carboxylic acids in cosolvent systems was found to be problematic. The cause of the discrepancy was investigated by analyzing the solubility (Sm) and sorption (Km, linear sorption coefficient) of the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) on metal-saturated kaolinites, as a function of methanol volume fraction (fc) and apparent solution pH. The possible formation of a coordination compound during the hydrophilic interaction of 2,4-dichlorophenoxyacetate in methanol-water mixture was identified using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The Sm of 2,4-dichlorophenoxyacetate increased with increasing fc. Thus, the shift of activity of solute occurred in the liquid phase was not significant for determining the cause of the positive Km-fc relationship. However, Km value of 2,4-dichlorophenoxyacetate on the metal-saturated kaolinite increased log-linearly with fc. The ATR-FTIR results on surface coordination of 2,4-dichlorophenoxyacetate on the metal-saturated kaolinite surface showed that bidentate coordination is more likely to occur instead of a bridging coordination with increasing fc. Therefore, it can be reasonably concluded that the positive Km-fc relationship is due to the charge-related hydrophilic interaction through bidentate coordination of 2,4-dichlorophenoxyacetate, and it increases with increasing positive charge density on the kaolinites, as a function of fc.