Environmentally Sustainable Hydrous Zirconium Oxide–Inulin as an Efficient Adsorbent for Ciprofloxacin Removal: Mechanistic and Thermodynamic Insights via Statistical Physics and Fractal-like Kinetic Modeling

Pharmaceutical pollutants like ciprofloxacin (CPF) pose environmental risks due to their persistence and limited removal by conventional treatment methods. To address this, a hydrous zirconium oxide-inulin (HZO-inulin) biomaterial was synthesized via wet precipitation and characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction, thermogravimetric analysis-difference thermal analysis, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area analysis, scanning electron microscopy-energy dispersive X-ray spectroscopy, and transmission electron microscopy. HZO-inulin exhibited high chemical stability under various aquatic conditions. CPF adsorption was optimized using the Box-Behnken design with a desirability function approach, achieving 99.28% removal and a maximum adsorption capacity of 181.46 mg/g under optimal conditions (adsorbent dose = 0.01 g, concentration = 85 mg/L, contact time = 35 min). Isotherm analysis using classical and statistical physics models revealed that the Freundlich model best fit the data, suggesting multilayer adsorption, while statistical physics model 2 indicated monolayer adsorption with two energy levels (<i>R</i>² = 0.9994-0.9997). Adsorption energies (<i>E</i>₁: 28.65-38.62 kJ/mol, <i>E</i>₂: 42.58-54.87 kJ/mol) suggested hydrogen bonding and electrostatic interactions. Thermodynamic studies confirmed spontaneous endothermic adsorption. Kinetic studies at 300 K for CPF concentrations (55, 65, 75 mg/L) followed a fractal-like pseudo-second-order model (<i>R</i>² > 0.998), while diffusion modeling identified boundary-layer diffusion dominance, transitioning to intraparticle diffusion. CPF removal from real water samples was 98.9% (tap water), 99.10% (river water), and 99.15% (wastewater). HZO-inulin retained high adsorption efficiency after eight cycles, confirming its reusability. These findings establish HZO-inulin as an efficient, stable, and sustainable material for water purification applications.