Simultaneous adsorption of oxytetracycline and arsenic onto hydrous zirconium-modified chitosan/β-cyclodextrin composite: Mechanistic insights via statistical physics modeling, fractal-like kinetics, thermodynamic evaluation, and site energy distribution analysis

Abstract

A multifunctional composite, hydrous zirconium hydroxide cross-linked chitosan grafted with β-cyclodextrin (Zr-CCS-g-β-CD), was synthesized via a wet precipitation method and thoroughly characterized using FT-IR, XRD, BET, TGA-DTA, XPS, SEM-EDS, and TEM analyses to elucidate its physicochemical, morphological, and surface attributes. The composite exhibited excellent aqueous stability, making it viable for environmental remediation. Optimization of oxytetracycline (OTC) adsorption using Central Composite Design (CCD) integrated with a desirability function revealed a high adsorption capacity (187.52 mg g -1 ) and removal efficiency (98.68%) under optimized conditions (adsorbent dose = 0.01 g, OTC concentration = 90 mg L -1 , pH = 6.5, contact time = 45 min). Mechanistic insights were probed using classical isotherms and statistical physics modeling across 298-318 K. The Freundlich model suggested multilayer adsorption on heterogeneous surfaces, while statistical physics Model 2 revealed a dominant monolayer adsorption governed by dual energy site interactions (R² = 0.9993-0.9998). Energy distribution analysis indicated two interaction regimes: (Ea₁ = 21.72-34.87 kJ mol -1 ) attributed to weak forces such as hydrogen bonding and van der Waals interactions, and (Ea₂ = 41.33-52.76 kJ mol -1 ) associated with stronger electrostatic and coordination forces. Notably, the statistical physics model revealed that the number of bound molecules per site (n) remained below unity at all temperatures, supporting a multi-docking interaction regime where single OTC molecules simultaneously interact with multiple active sites. This configuration facilitates horizontal alignment of OTC on the surface, stabilized via cooperative weak interactions. Fractal-like pseudo-second-order (FL-PSO) kinetics exhibited superior fitting (R² > 0.999), capturing the heterogeneity of the adsorption process, while Weber-Morris analysis confirmed a dual diffusion mechanism: initial film diffusion followed by intraparticle transport. Thermodynamic parameters indicated a spontaneous, endothermic process with positive entropy changes, confirming enhanced affinity and structural reorganization at the solid-liquid interface. Practical utility was validated in real water matrices, achieving high removal efficiencies: 98.20% (tap), 97.10% (river), and 96.95% (wastewater). Remarkably, OTC-loaded Zr-CCS-g-β-CD demonstrated secondary functionality by efficiently removing As(III) from aqueous media (>96%) and acid-digested food matrices (91.84%-96.26%). The composite retained over 96% efficiency across eleven adsorption-desorption cycles, establishing its reusability and environmental sustainability.