In present study a biomass derived from the stem of Acacia nilotica has been investigated to remove As ions from surface water samples of different origins (lake, canal and river). The effects of various parameters viz. pH, biosorbent dosage, contact time and temperature on the biosorption processes were systematically studied. Experimental data were modeled by Langmuir. Freundlich and Dubinin-Radushkevich (D-R) isotherms. It was observed that As biosorption best fitted to the Langmuir and Freundlich isotherms. The mean sorption energy (E) calculated from D-R model, indicated physico-chemical biosorption. Study of thermodynamic parameters revealed the endothermic, spontaneous and feasible nature of biosorption process. The pseudo-second-order rate equation described better the kinetics of As biosorption with good correlation coefficients than pseudo-first-order equation. The biomass of A. nilotica was found to be effective for the removal of As with 95% sorption efficiency at a concentration of <200 mu g/L of As solution, and thus uptake capacity is 50.8 mg As/g of biomass. The A. nilotica biomass could be used as a low-cost biosorbent for As ion removal. (C) 2010 Elsevier B.V. All rights reserved.