In this study, the biosorption of nickel(II) ion on deactivated protonated yeast was investigated as a function of temperature at different initial metal ion concentrations. The effect of temperature on the sorption was more significant at lower nickel(II) ion concentrations compared to higher concentrations. The protonated yeast biomass exhibited the highest nickel(II) ion uptake capacity at 27 degrees C at an initial nickel(II) ion concentration of 400 mg/l and an initial pH of 6.75. The biosorption capacity decreased from 9.8 to 9.3 mg/g at an initial nickel(II) ion concentration of 400 mg/l, while at a lower initial concentration of 100 mg/l, it decreased from 8.2 to 4.9 mg/g, as the temperature was increased from 27 degrees C to 60 degrees C. The equilibrium data fit better to the Freundlich and Redlich-Peterson isotherm models compared to the Langmuir model in the concentration range studied (10-400 mg/l). Kinetic models applied to the sorption data at different temperatures showed that nickel(II) ion uptake process followed the pseudo-second order rate model and the adsorption rate constants decreased with increasing temperature. The activation energy of biosorption (E-a) was determined to be - 13.3 kJ/mol using the pseudo-second order rate constants. The results indicated that the biosorption of nickel(II) ion on to baker's yeast was spontaneous and exothermic in nature. Desorption studies revealed that the protonated yeast biomass can be regenerated using 0.1 N HCI and reused. (c) 2007 Elsevier Ltd. All rights reserved.