In this study, a new magnetic nanocomposite was synthesized via radical polymerization of methyl acrylate onto modified magnetic nanoparticles followed by the functionalization of the methyl ester groups with ethylenediamine and sodium chloroacetate. The generated magnetic nanocomposite was characterized by FT-IR, TEM, SEM, TGA, VSM, XRD and elemental analysis. Its key role as an adsorbent for the removal of typical cationic dyes, methyl violet and malachite green was investigated in terms of pH, contact time and initial dye concentration. The resulted adsorbent displays excellent adsorption capacities for cationic dyes which are more effective than most of the adsorbents reported so far. Study of the kinetic and isotherm of adsorption indicated that dyes adsorption process is well-described by pseudo-second-order-kinetic and Langmuir isotherm models, respectively. Furthermore, to find an accurate correlation between the adsorption enthalpy (Delta H) and experimental maximum adsorption capacity (q(m)), calculation of the adsorption enthalpy of a series of cationic dyes onto MNC-EDDA adsorbent was investigated using density functional theory (DFT). Interestingly, an excellent agreement between experimental q(m) and calculated Delta H (kcal/mol) was found which is able to predict the maximum adsorption capacities onto the prepared adsorbent for the other cationic dyes. (C) 2016 Elsevier B.V. All rights reserved.