Factors influencing inapplicability of cosolvency-induced model on organic acid sorption onto humic substance from methanol mixture

Abstract

Applicability of cosolvency model for describing the sorption of organic acids to humic substance was investigated by analyzing dataset of sorption (K-m) and solubility (S-m) 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(appCME) (apparent pH of liquid phase) and f(c) (methanol volume fractions). For all solutes, the Km decreased with fc with the Km reduction being less than the S-m-based prediction. The slope of log K-m-f(c) 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 (alpha'= K-m/S-m) was not in unity over the pH(app)-f(c) range but decreased with f(c), indicating a possible structural modification of sorption domain favoring extra sorption. For a given solute, the alpha' of neutral species was always greater than that of anionic species, showing that extra interaction will be likely at pH(app) <pK(a)-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.