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

2012-06-19T00:00:00Z (GMT) by Bingjun Pan Huichun Zhang
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 <i>n</i>-hexadecane as the reference state. The adsorption potential in the D–A model ε = –<i>RT</i>ln­(<i>C</i><sub>w</sub>/<i>C</i><sub>w</sub><sup>sat</sup>) was redefined by replacing the term (<i>C</i><sub>w</sub>/<i>C</i><sub>w</sub><sup>sat</sup>) with the normalized equivalent concentration in <i>n</i>-hexadecane (<i>C</i><sub>HD</sub>), where <i>C</i><sub>w</sub> is the aqueous equilibrium concentration and <i>C</i><sub>w</sub><sup>sat</sup> 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<sub>HD</sub> = –<i>RT</i>ln­(<i>C</i><sub>HD</sub>/10 000), a new normalizing factor (<i>E</i><sub>m</sub>) 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 “<i><b>A</b></i>”, the hydrogen-bond acidity, and “<i>E</i><sub>m</sub>” 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.