In this work the adsorption of pentavalent arsenic on a granular activated carbon (GAC) has been experimentally studied. The effects of arsenic concentration, pH, temperature and salinity on equilibrium adsorption capacity have been investigated. Experimental results show that the adsorption capacity is the highest at neutral pH conditions, low salinity levels and high temperatures. A model for the description of the arsenic adsorption mechanism is reported. This is based on the multicomponent Langmuir adsorption theory applied to the ionic species in solution. The model points out that the adsorption capacity is proportional to the concentration of arsenic anions in solution and decreases by increasing the concentration of competitive ions such as hydroxides and chlorides, allowing a correct interpretation of the pH and salinity effects on the adsorption capacity. Finally, one of the main goals of the proposed model is to preserve the exothermicity of the adsorption phenomena despite the observed trend of experimental results: the increase of adsorption capacity with temperature appears to be related to a higher arsenic dissociation.