Carica papaya seeds, an agricultural waste in Nigeria, were defatted to obtain defatted C. papaya seed biosorbent. The Fourier Transformed Infrared spectrum of defatted C. papaya seed biosorbent suggests the presence of C=O, -OH of carboxylic acid, lactonic and amide band functional groups. The adsorption of metal ion onto defatted C. papaya seed biosorbent led to small shifts in the IR bands. The adsorption capacity of defatted C. papaya seed biosorbent was evaluated to be 1666.67 mg/g for Pb2+ and 1000.00 mg/g for Cd2+. In binary metal ion solution, the defatted C. papaya seeds showed decreased adsorption capacity for either metal ion. The influence of different particle sizes was found to have negative impact on the adsorption capacity of C. papaya seed biosorbent in the removal of Pb2+ and Cd2+ from aqueous solution. The adsorption of both metal ions was observed to follow the Freudlich model better than the Langmuir model suggesting that the adsorption of both metal ions was on multisites on the defatted C. papaya seed biosorbent. The adsorption was found to be highly feasible, spontaneous and exothermic in nature. Optimization results suggests 5 m(3) of 100 mg/L of Pb2+ and Cd2+ requires 43.3 and 49.2 kg of defatted C. papaya seeds to remove 95% of the metal ions from aqueous solution. (C) 2011 Published by Elsevier Ltd.