This work reported the rate of nickel biosorption using acid-washed barley straw (AWBS) at different initial nickel concentration, AWBS particle size, solution pH, and temperature. The biosorption process was rapid and the equilibrium was reached in about 100 min with initial nickel concentrations from 250 to 1000 mg/L. AWBS with particle size of <0.425 mm exhibited a greater adsorption rate and reached equilibrium faster than particle sizes of 0.425-1.18 mm. An increase in pH from 3.0 +/- 0.1 to 7.0 +/- 0.1 increased the rate of adsorption and resulted in a higher equilibrium nickel uptake. Nickel adsorption was more favourable at 23 +/- 1 degrees C compared to 5 +/- 1 degrees C and 40 +/- 1 degrees C. The external mass transfer model was able to fit the dynamic nickel biosorption data and provided acceptable overall volumetric mass transfer coefficients.