The biosorption potential of chemically modified onion skin (CMOS) for cadmium (II) ions (Cd 2+ ) from its aqueous solutions at different conditions of initial Cd 2+ concentration, contact time, pH and temperature was investigated under batch mode. The results showed that biosorption of Cd 2+ were influenced by these different conditions. The equilibrium biosorption data were analyzed by two-parameter (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R)), three-parameter (Redlich-Peterson, Sips, Toth, and Khan) and four-parameter (Baudu and Fritz-Schlunder) isotherm models, respectively, using the linear and non-linear regression method. The best fitting isotherms to describe the biosorption data of Cd 2+ -CMOS system were found to be Freundlich, Sips, and Fritz-Schluender among the two-, three-, and four-parameter models, respectively. A maximum biosorption capacity ( max Q ) of 18.34 mg/g was obtained forCMOS as compared to 11.90 mg/g for raw onion skin (OS). The biosorption kinetics followed pseudo-second-order kinetic model and the biosorption mechanisms controlled by boundary layer surface diffusion. Thermodynamic parameters of Gibbs free energy ( o G ∆ ), enthalpy ( o H ∆ ), entropy change of biosorption ( o S ∆ ), and activation energy Downloaded by [Erciyes University] at 22:55 25 December 2014 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2( a E ) have been evaluated and was found that the biosorption process was spontaneous, feasible, endothermic and predominantly of physisorption process, however, with some element of chemisorption. A single-stage batch adsorber was designed for different biosorbent dose-to-effluent volume ratios using the Fritz-Schluender equation. The CMOS could be regenerated and reused till five cycles without any significant change of efficiency. Therefore, CMOS has potential for application as an effective biosorbent for the removal of Cd 2+ from aqueous solution.