Applicability of cosolvency model for describing the sorption of organic acids to humic substance was investigated by analyzing dataset of sorption (K<inf>m</inf>) and solubility (S<inf>m</inf>) 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 pH<sup>appCME</sup> (apparent pH of liquid phase) and f<inf>c</inf> (methanol volume fractions). For all solutes, the K<inf>m</inf> decreased with f<inf>c</inf> with the K<inf>m</inf> reduction being less than the S<inf>m</inf>-based prediction. The slope of log K<inf>m</inf>-f<inf>c</inf> 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 Ca<sup>2+</sup> bridge between carboxylate and negatively charged humic surface may explain this phenomenon. Normalizing the K<inf>m</inf> to the corresponding S<inf>m</inf> (α′ = K<inf>m</inf>/S<inf>m</inf>) was not in unity over the pH<sup>app</sup>-f<inf>c</inf> range but decreased with f<inf>c</inf>, 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 pH<sup>app</sup> <pK<inf>a</inf> − 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 Ca<sup>2+</sup> 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