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 (CHD), where Cw 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 εHD = -RTln(CHD/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.