Characterization and Statistical Physics Modeling of Two Coal Hybridized Nanostructures with Two Forms of Polyaniline (Nanofibers Hydrogel and Nanorods) as Adsorbents for Toxic 4-Chlorophenol

Abstract

The present study investigates the hybridization of natural coal flakes with two distinct forms of polyaniline (PANI), namely nanofibers hydrogel (PANI_(NFs)/CA) and nanorods (PANI_(NRs)/CA), to develop advanced hybrid adsorbents with superior performance for 4-chlorophenol (4-CL) removal. The adsorption behaviors were elucidated through a synergetic evaluation of steric and energetic factors using statistical physics equilibrium modeling. A monolayer adsorption model featuring a single energy site (R² > 0.99) was employed, with parameter fitting performed via the Levenberg–Marquardt iterative method coupled with multivariable nonlinear regression. The integration of PANI nanofibers hydrogel resulted in significantly enhanced adsorption performance, achieving a maximum saturation capacity (Q_sat) of 417.5 mg/g, surpassing the capacities of PANI nanorods (Q_sat = 370.9 mg/g) and raw coal (Q_sat = 211 mg/g). The steric analysis further confirmed that PANI_(NFs)/CA exhibited the highest density of active adsorption sites (Nm = 101 mg/g), followed by PANI_(NRs)/CA (Nm = 86.3 mg/g) and raw coal (Nm = 55.9 mg/g). These findings highlight the critical role of the PANI matrix in enhancing the availability of adsorption sites, either through its inherent chemical functionality or by improving interaction interfaces. The crystalline form of PANI nanofibers hydrogel appeared to be of enhanced reactivity and surface area (48.7 m²/g) as compared to semi-crystalline or PANI nanorods. Molecular packing analysis revealed that each adsorption site in both PANI_(NFs)/CA and PANI_(NRs)/CA accommodated four 4-CL molecules, indicative of a vertical stacking arrangement and dominant multi-molecular adsorption mechanisms. These adsorption interactions were predominantly physical, as inferred from adsorption energy values (< 15 kJ/mol), and proceeded spontaneously with exothermic characteristics. The development of PANI(_NFs)/CA as a hybrid adsorbent presents a cost-effective and highly efficient alternative for removing phenolic contaminants, such as 4-chlorophenol (4-CL), from industrial and municipal wastewater.