A Modified Polanyi-based Model for Mechanistic Understanding of Adsorption of Phenolic Compounds onto Polymeric Adsorbents

To obtain mechanistic insight into adsorption of phenolic compounds by two representative polymeric adsorbents, XAD-4 (polystyrene) and XAD-7 (polymethacrylate), a modified Polanyi-based Dubinin-Ashtakhov (D–A) model was developed based on a unique combination of the Polanyi theory, polyparameter linear energy relationships and infinitely dilute solution in n-hexadecane as the reference state. The adsorption potential in the D–A model ε = –RTln­(C_w/C_w^sat) was redefined by replacing the term (C_w/C_w^sat) with the normalized equivalent concentration in n-hexadecane (C_HD), where C_w is the aqueous equilibrium concentration and C_w^sat is the aqueous solubility of the solute. Using the new reference state allows quantitative comparison among various solutes. By fitting adsorption isotherms to the modified model using ε_HD = –RTln­(C_HD/10 000), a new normalizing factor (E_m) was obtained to quantify the contributions of specific interactions (i.e., H-bonding, dipolar/polarizability, etc.) to the overall adsorption energy. Significant linear correlations were established between “A”, the hydrogen-bond acidity, and “E_m” for the investigated compounds, suggesting that, in addition to hydrophobic interactions, hydrogen-bonding is predominantly responsible for the adsorption of phenols by XAD-4 and XAD-7. Additionally, adsorption capacity and affinity of phenolates were significantly less than those of phenols; another model was proposed to accurately predict the effect of pH on the adsorption behavior of phenols.