BACKGROUND: The performance and mechanism of the biosorptive removal of Ni2+ and Zn2+ from aqueous solution using grapefruit peel (GFP) as a new biosorbent was investigated by using different experimental approaches, such as potentiometric titration, Fourier transform infrared (FTIR) and energy-dispersive X-ray spectroscopy (EDX) analysis, chemical blocking of functional groups and concomitant release of cations (Ca2+, Mg2+, Na+, K+) from GFP with metal (Ni2+, Zn2+) uptake. RESULTS: GFP removed Ni2+ and Zn2+ rapidly, with 84.73% and 92.46% of the equilibrium sorption being reached in 30 min for Ni2+ and Zn2+, respectively. The equilibrium process was described well by the Langmuir isotherm model, with maximum sorption capacity of 1.33 and 1.51 meq g(-1) for Ni2+ and Zn2+, respectively. Release of cations (Ca2+, Mg2+, Na+, K+) and protons H+ from GFP during uptake of Ni2+ and Zn2+ and EDX analysis of GFP before and after metal sorption revealed that the main mechanism of sorption was ion exchange. FTIR spectroscopy showed that carboxyl and hydroxyl groups were involved in the sorption of Ni2+ and Zn2+. Blocking of these groups revealed that carboxylic group was responsible for 78.57% and 73.31% of Ni2+ and Zn2+ removal, respectively whereas 22.63% and 28.54% was due to the hydroxyl group. The GFP could be regenerated using 0.1 mol L-1 HCl, with more than 98% metal recovery and reused for five cycles without any significant loss in its initial sorption capacity. CONCLUSIONS: The study suggests that GFP has promising potential for use as an efficient and cost-effective biosorbent for the removal and recovery of Ni2+ and Zn2+ from aqueous solution. (C) 2009 Society of Chemical Industry