Fabrication and Characterisation of a Waste-Derived Magnetic PA6/CNCs–Fe₃O₄ Composite for Cd(II) Adsorption from Aqueous Solution

Cadmium contamination of water poses serious environmental and public-health risks, creating a need for efficient, reusable, and readily recoverable treatment materials. This study developed a magnetic PA6/CNCs–Fe₃O₄ composite from waste polyamide 6 (PA6), cellulose nanocrystals (CNCs), and Fe₃O₄ for Cd(II) removal from aqueous solution. The composite was prepared via in situ co-precipitation and characterised using SEM–EDS, FTIR, XRD, BET, zeta-potential analysis, and VSM. Characterisation confirmed composite formation, retention of the crystalline magnetite phase, mesoporosity, and the presence of Fe₃O₄ nanoparticles, with sufficient magnetic responsiveness for effective separation. Batch experiments evaluated the effects of pH, contact time, initial Cd(II) concentration, adsorbent dosage, and temperature. Under selected conditions of pH 5.0, an initial Cd(II) concentration of 18.97 mg L⁻ 1 , a contact time of 120 min, an adsorbent dosage of 1.0 g L⁻ 1 , and 298 K, the composite achieved an adsorption capacity of 18.62 mg g⁻ 1 and a removal efficiency of 98.15%. The pseudo-second-order model best described the kinetic data. The linearised Langmuir model yielded a maximum adsorption capacity of 39.68 mg g⁻ 1 , whereas the nonlinear Sips model provided the closest fit on the untransformed scale. Apparent Gibbs free-energy changes ranged from − 21.75 to − 28.38 kJ mol⁻ 1 , indicating favourable adsorption. The composite retained 86.30% removal efficiency after five adsorption–desorption cycles, demonstrating its potential as a reusable, magnetically recoverable adsorbent for Cd(II)-contaminated water.