Prediction of adsorption properties for ionic and neutral pharmaceuticals and pharmaceutical intermediates on activated charcoal from aqueous solution via LFER model

In this study, we performed modeling analysis based on isothermal experimental data to develop linear free energy relationship (LFER) model, which is a very useful concept for predicting the molecular interaction of an adsorbate in a certain system. Thirty compounds containing cationic, anionic, and neutral were used, and the effect of diverse intermolecular interactions was captured by isotherm fitting of the Langmuir model. The predicted equation of distribution coefficient (K_d), adsorption capacity (q_m), and adsorption affinity (K_ad) of activated charcoal (AC) for compounds was developed. For the K_d values, an activity-dependent LFER model was developed, and could predict log K_d within R² of 0.85. The determined system parameters and their statistical results revealed that the molecular volume (V) of compounds encouraged the adsorption from water to AC, whereas the hydrogen basicity (B) discouraged such adsorption. In the cases of q_m and K_ad, AC had relatively good capacities for neutrals, which were higher than those for cations and anions. Whereas, AC had stronger adsorption affinities with anions, compared to those for neutrals and cations. The phenomena were explained by the developed LFER models, in which the p−/n− electron interaction (E) made a great positive contribution to increase the adsorption capacity and affinity, while Coulombic interactions (J⁺ and J⁻) demonstrated a negative trend. Finally, the developed models can predict the q_m and K_ad in R² of 0.86 and 0.76, respectively.