Chitosan-modified nickel ferrite nanocomposite for ciprofloxacin adsorption: experimental and density functional theory study

<title>Abstract</title> <p> Antibiotic residues in water motivate the development of accessible adsorbents. A chitosan-modified nickel ferrite nanocomposite (NiFe₂O₄@CS) was prepared by <italic>Foeniculum vulgare</italic> seed-extract-assisted synthesis followed by ex situ chitosan incorporation and evaluated for ciprofloxacin (CIP) adsorption. XRD, XPS, SEM–EDS, TEM, and FTIR supported spinel NiFe₂O₄ formation and association of chitosan-derived functional groups; Scherrer analysis gave an apparent coherent-domain size of 16.5 nm, and TEM showed ferrite domains of approximately 16–21 nm. Adsorption measurements were conducted at pH 7.0, close to the measured point of zero charge (pH _pzc = 6.97). The contact-time experiment yielded <italic>q</italic> _{<italic>e,exp</italic>} = 16.82 mg g⁻¹, whereas a separate six-point concentration series gave a maximum directly measured 120-min uptake of approximately 47–48 mg g⁻¹. The Sips equation gave the highest isotherm R² (0.989), although its fitted qₘ was treated as extrapolated rather than a measured saturation capacity. The pseudo-second-order equation best described kinetics (R² = 0.994) without identifying a unique rate-limiting mechanism. Finite-cluster DFT yielded <italic>E</italic> _{<italic>ads</italic>}  = − 0.6069 eV and supported possible hydrogen-bonding, electrostatic and ferrite metal-site contributions. NiFe₂O₄@CS therefore adsorbed CIP under single-solute batch conditions, but regeneration, magnetic separability, leaching and real-water performance require experimental validation. </p>