A novel magnetic nano-adsorbent was developed by treating Fe 3O 4 nanoparticles with gum arabic to remove copper ions from aqueous solutions. Gum arabic was attached to Fe 3O 4 via the interaction between the carboxylic groups of gum arabic and the surface hydroxyl groups of Fe 3O 4. The surface modification did not result in the phase change of Fe 3O 4, while led to the formation of secondary particles with diameter in the range of 13–67 nm and the shift of isoelectric point from 6.78 to 3.6. The amount of gum arabic in the final product was about 5.1 wt%. Both the naked magnetic nanoparticles (MNP) and gum arabic modified magnetic nanoparticles (GA-MNP) could be used for the adsorption of copper ions via the complexation with the surface hydroxyl groups of Fe 3O 4 and the complexation with the amine groups of gum arabic, respectively. The adsorption rate was so fast that the equilibrium was achieved within 2 min due to the absence of internal diffusion resistance and the adsorption capacities for both MNP and GA-MNP increased with increasing the solution pH. However, the latter was significantly higher than the former. Also, both the adsorption data obeyed the Langmuir isotherm equation. The maximum adsorption capacities were 17.6 and 38.5 mg/g for MNP and GA-MNP, respectively, and the Langmuir adsorption constants were 0.013 and 0.012 L/mg for MNP and GA-MNP, respectively. Furthermore, both the adsorption processes were endothermic due to the dehydration of hydrated metal ions. The enthalpy changes were 11.5 and 9.1 kJ/mol for MNP and GA-MNP, respectively. In addition, the copper ions could desorb from GA-MNP by using acid solution and the GA-MNP exhibited good reusability.