Application of Classic Model Equations in Describing Aqueous-Phase Adsorption Isotherm for Activated Carbon–Aromatic Compound System

The method of isotherm simulation by approximating mathematical expressions that are valid for a small experimental section of the isotherm over the entire range of equilibrium concentrations is an important and useful tool in adsorption practice. Nine adsorption equilibrium systems of activated carbon (AC)–derivatives of aniline and phenol were examined. The adsorbent used was activated carbon obtained from fruit pits. The results of isothermal adsorption of aromatic substances with relatively small molecules on a microporous carbon adsorbent indicate the simultaneous adequacy of the Freundlich and Langmuir models to the experimental data in moderate concentration ranges ((0.1–100)n mg/dm³, where n = 1, 2, 3, 4, 5). Thus, the results of adsorption from the aqueous phase do not demonstrate the energy differences of the AC surface that are incorporated into the theoretical classical model isotherms. The isotherm reflects the adsorption process on both homogeneous and heterogeneous surfaces simultaneously. This may be due to the displacement nature of adsorption from the aqueous phase, where initial surface screening by water molecules occurs during adsorbent wetting, leading to the leveling of its energetic heterogeneity. In the next stage—displacement of some water molecules by the organic adsorbate—the energetic differences of the adsorption sites are not as clearly manifested as in gas-phase adsorption. Therefore, theorizing the nature of the AC surface based on the best model simulation of experimental isotherms using classical equations becomes questionable. Three-parameter equations (Langmuir–Freundlich, Redlich–Peterson) demonstrated a finer simulation of the experiment compared to the classical two-parameter models. The study emphasizes that the mathematical description of the isotherm is a convenient method for the efficient storage and use of information about the adsorption properties of the system. It serves to compare the effectiveness of new materials with commercial analogs and to predict the performance of real purification systems under dynamic conditions.