A biodegradable adsorbent prepared from long-root Eichhornia crassipes has been tested for aqueous adsorption of As(III) and As(V). The surface properties and morphology of the root powder have been characterized by means of SEM, zeta-sizer, potentiometric titrimeter, FTIR, and XPS. Chemical composition analysis confirmed that the hydroxyl, amino, and carboxyl groups on the surface of the root powder facilitated the adsorption of As(III) and As(V) by either electron sharing or electron transfer. The optimal pH values for the adsorption of As(III) and As(V) were determined as 7.5 and 3.0, respectively. The effects of co-ions and counter-ions on the adsorption of arsenite and arsenate have also been examined. It was shown that the adsorption efficiencies of As(III) and As(V) remained unchanged in the presence of Cl-, SO42-, and NO3-. The presence of PO43-, however, which has a similar stereochemical structure to that of the arsenic species, reduced the adsorption capacity substantially. The adsorption of As(III) or As(V) could be well described by the pseudo-second-order dynamic model at different initial concentrations and the Langmuir model at different temperatures. (C) 2012 Elsevier B.V. All rights reserved.