Applicability of cosolvency model for describing the sorption of organic acids to humic substance was investigated by analyzing dataset of sorption (Km) and solubility (Sm) of selected solutes (benzoic acid, 1-naphthoic acid, 2,4-dichlorophenoxyacetic acid, and 2,4,6-trichlorophenol (2,4,6-TCP)) as a function of pHappCME (apparent pH of liquid phase) and fc (methanol volume fractions). For all solutes, the Km decreased with fc with the Km reduction being less than the Sm-based prediction. The slope of log Km-fc plot in the three organic carboxylic acids was well correlated with their cosolvency power, whereas the data of organic phenolic acid (2,4,6-TCP) was placed above the trend, indicating the different actions of functional groups. The occurrence of Ca2+ bridge between carboxylate and negatively charged humic surface may explain this phenomenon. Normalizing the Km to the corresponding Sm (α′ = Km/Sm) was not in unity over the pHapp-fc range but decreased with fc, indicating a possible structural modification of sorption domain favoring extra sorption. For a given solute, the α′ of neutral species was always greater than that of anionic species, showing that extra interaction will be likely at pHapp <pKa − 2 when both solute and sorbent are uncharged. In short, the knowledge of cosolvent-enhanced solubility is incapable of describing sorption of organic acids by humic substance in methanol/water mixtures. Modification of humic structure and hydrophilic interaction (such as Ca2+ bridge and same-charge repulsion) is considered a relevant process that possibly restricts the applicability of the cosolvency model.
- Humic substance
- Hydrophobic organic acid
ASJC Scopus subject areas
- Environmental Chemistry
- Health, Toxicology and Mutagenesis