Exceptional 3D magnetite nanoparticles (Fe3O4) with high surface area, flower-like morphology, and suitable interaction with the natural polymeric matrices have been selected as inorganic nano-filler in preparation of magnetite/κ-carrageenan nanocomposites (MCNCs). Chemical and structural properties of MCNCs were studied and characterized by ATR-FTIR, X-ray powder diffraction (XRPD), scanning and transmission electron microscopies (SEM and TEM), and thermogravimetric analyses (TGA). The MCNCs are considered as a magnetic adsorbent for adsorptive removal of contaminations such as Cu(II) from aqueous solutions. Based on adsorption data, MCNC 10 wt.% was selected for adsorption studies and different parameters including pH, contact time, and initial concentration of Cu(II) ions were optimized. The batch sorption mechanism and kinetics were estimated using three reaction kinetic models including pseudo-second-order, Elovich, and intra-particle diffusion. Besides, the adsorbent performance was evaluated by two common isotherm models: Langmuir and Freundlich. More significantly, kinetics and isotherm equilibrium data showed a major fitting with the intra-particle diffusion and Langmuir model, respectively. The maximum value of adsorption capacity to Cu(II) ions was found to be 22.57 mg g−1 (pH = 6, adsorbent dose 0.005g (1 g L-1), 25 °C, 180 rpm, and 80 min). The relative standard deviations (RSDs) for sorbent-to-sorbent reproducibility was 7.5 % (n = 3). The MCNC 10 wt.% was separated easily by a supermagnet and recycled 4 times easily with the adsorption efficiency of 84 % in the final cycle.