In this study, magnetic oxidized multiwalled carbon nanotube (OMWCNT)-kappa-carrageenan-Fe3O4 nano composite was prepared as a new adsorbent and characterized by specific surface area, Fourier transform infrared (FTIR), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), thermal gravimetric analysis (TGA), scanning electron microscope (SEM) and transmission electron microscope (TEM) measurements. Magnetic adsorbent displayed a super-paramagnetic property. Cationic Crystal Violet (CV) and anionic Reactive Black 5 (RB5) dyes were used to evaluate the adsorption ability of magnetic OMWCNT-kappa-carrageenan-Fe3O4 nanocomposite. The effect of contact time, initial dye concentration, pH and temperature on the adsorption process was investigated. The equilibrium time was determined to be 300 min. An increase in the initial dye concentrations and the temperature of dye solutions led to an increase in the adsorption amounts of magnetic adsorbents. A decrease in the adsorption amount of CV dye was observed at low pH values, on the contrary to RB5 dye. For the adsorption of CV and RB5 dyes onto magnetic adsorbent, the adsorption kinetics followed the pseudo second-order model. Langmuir isotherm model exhibited the best fitting with the experimental isotherm data obtained at 25, 35 and 45 degrees C. Thermodynamic parameters showed that adsorption processes are spontaneous and endothermic. On the contrary to anionic RB5 dye, the adsorption capacity of magnetic OMWCNT-kappa-carrageenan-Fe3O4 nanocomposite for cationic CV dye is higher than that of magnetic OMWCNT-Fe3O4 nanocomposite. Therefore, magnetic OMWCNT-kappa-carrageenan-Fe3O4 nanocomposite may be used as a potential adsorbent to remove the cationic dyes from aqueous solution. (C) 2016 Elsevier B.V. All rights reserved.