This work deals with the preparation of a new sorbent selective and effective for the adsorption of As(III) and As(V)  in waste waters. The sorbent is made of chitosan gel beads impregnated with molybdate ions. This impregnation allows the sorbent to be very selective even with high concentrations of salts and  heavy metals in solution. The adsorption of arsenic species is maximum between pH 2,5 and 4 with uptake capacities equal to  200 mg.g^-1 Mo for As(V) and 70 mg.g^-1 for As(III). The presence of phosphate ions and reducing agents in solution can depressed greatly arsenic sorption.
    ESCA studies carried out on the samples have shown that the oxidation state of Mo and As were not affected during the adsorption process. SEM-EDAX analysis have demonstrated the homogeneous arsenic sorption in the whole volume of the sorbent.                                                                                                                                                                               Complete and selective desorption of As(III) and As(V) is performed with a 0,1 M orthophosphoric acid solution. Hence, more than 10 adsorption-desorption cycles can be carried out with the same quantity of sorbent. The main limiting factor of the adsorption-desorption cycles is the release of molybdate ions during the arsenic sorption step.
    In order to validate the process, the impregnated chitosan gel beads were used for the removal of arsenic in a mine and a semi-conductors waste waters. In spite of the complex composition of these waters, arsenic sorption is only 7 fold decrease.

Keywords : arsenic, chitosan, molybdate ions, impregnated sorbent, kinetic, isotherm, complexation, mechanism, ESCA