Mechanistic insights into paracetamol adsorption from water using ZnO nanoparticle-immobilized chitosan-inulin composites: fractal kinetics, statistical physics, thermodynamic analysis, and application to real water samples

Abstract

A novel ZnO/chitosan-inulin (ZnO/CS-In) nanocomposite was prepared and evaluated for its potential to remove paracetamol (PCM) from aqueous solutions. Comprehensive characterization of the material was conducted using techniques such as FT-IR, XRD, TGA-DTA, SEM-EDX, BET, DLS, and UV-visible spectroscopy to assess its structural and functional attributes. RSM was employed to optimized the operational parameters to achieve highest elimination efficiency of PCM. At the ideal conditions (55 minutes of contact time, 10 mg adsorbent dose and 65 mg L⁻¹ initial PCM concentration) 99.58% PCM was extracted. All experiments were performed at pH 7.2. The Langmuir isotherm model provided the best fit for the adsorption data, with high R² values (0.9988–0.9996) and exhibited demonstrated a significant monolayer sorption capacity of 327.28 mg g⁻¹ at 298 K. Statistical physics model-2 (M₂) provided the best fit (R² > 0.997) and revealed that PCM molecules bind to two distinct receptor sites (n₁ and n₂). Furthermore, the values of n₁ and n₂ at all studied (n₁ < 1 and n₂ > 1) suggesting that adsorption mechanisms involved both multidocking and multimolecular interactions on the first and second receptor sites, respectively. Adsorption energies (E₁ = 28.35–35.36 kJ mol⁻¹; E₂ = 11.97–13.03 kJ mol⁻¹) along with isosteric heat of adsorption suggest that physical interactions primarily govern the adsorption of PCM onto the nanocomposite. Kinetic studies revealed that the adsorption process followed a fractal-like pseudo-first-order model (R² > 0.9952) and deduced that PCM anchored on an energetically heterogeneous surface of ZnO/Cs-In. Intra-particle diffusion analysis revealed that both film and particle mechanisms were found to contribute to the adsorption process. The material demonstrated high reusability, maintaining effective performance for up to 13 cycles, affirming its potential for repeated applications in removing PCM from aqueous systems.