Alternanthera philoxeroides biomass, a type of freshwater macrophyte, was used for the sorptive removal of Ni(II), Zn(II) and Cr(VI) from aqueous solutions. Variables of the batch experiments include solution pH, contact time, particle size and temperature. The biosorption capacities are significantly affected by solution pH. Higher pH favors higher Ni(II), Zn(II) removal, whereas higher uptake of Cr(VI) is observed as the pH is decreased. A two-stage kinetic behavior is observed in the biosorption of Ni(II), Zn(II) and Cr(VI): very rapid initial biosorption in a few minutes, followed by a long period of a slower uptake. It is noted that an increase in temperature results in a higher Ni(II), Zn(II) and Cr(VI) loading per unit weight of the sorbent. Decreasing the particle sizes of the Alternanthera philoxeroides biomass leads to an increase in the Ni(II), Zn(II) and Cr(VI) uptake per unit weight of the sorbent. All isothermal data are fairly well fitted with Langmuir equations. The thermodynamic parameter, Delta G degrees, were calculated. The negative Delta G degrees values of Cr(VI), Ni(II) and Zn(II) at various temperatures confirm the adsorption processes are spontaneous. (c) 2006 Elsevier B.V. All rights reserved.