A rapid synthesis method for magnetic nanoparticles CoxNi1-xFe2O4 (x = 0, 0.3, 0.7, 0.9, 1.0) (Co-Ni) using the colloid mill and hydrothermal technique is investigated. The structure and magnetic performance of as-synthesized samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectrum, BET surface area method and vibrating sample magnetometer (VSM). The adsorption capability of Co-Ni ferrite to remove Congo Red (CR) dye from aqueous solution is examined carefully by UV spectrophotometer at room temperature. Research indicates that the substitution of Co2+ has a marked effect on the magnetic properties and CR adsorption of the Co-Ni ferrite nanoparticles. VSM shows that all samples, synthesized at 140 degrees C for a short reaction time 2 h, have excellent magnetic properties. Also, all samples show high adsorption capacity for CR dye. The CR adsorption isotherms and kinetics are studied. It is worth noticing that the largest adsorption capacity of the as-synthesized Co-Ni ferrite nanoparticles occurs at x = 0.3, and the quickest adsorption appears at x = 0.5. The corresponding adsorption mechanisms are analyzed. (C) 2013 Elsevier B.V. All rights reserved.