A novel, magnetic, strong acid cation nanoadsorbent has been developed by the covalent binding of poly(acrylic acid) on the surface of Fe3O4 nanoparticles followed by sulfonation using sulfanilic acid via carbodiimide activation. The nano-absorbent can be easily recovered or manipulated with an external magnetic field and shows a good capacity for the rapid and efficient adsorption of multivalent metal cations from aqueous solutions.