Two of the most basic environmental pollutants discharged to water or wastewaters are metals and dyes. Amongst the numerous techniques regarding the removal of environmental pollutants, adsorption has been found to be a superior technique as compared to other methods of wastewater treatment in terms of cost, simplicity of design and operation, availability, effectiveness, and insensitivity to toxic substances. In the current work, the co–existence of different–type pollutants (i.e. both dye and metal) in binary aqueous systems is under consideration. As adsorbent material, chitosan powder grafted with poly(ethylene imine) and cross–linked with epichlorohydrin (denoted hereafter as CSPEI) was used given its high adsorption capacity showed in our previous work. As heavy metal (ion), Cr(VI) was selected to be used because of its toxicity and widespread existence in aqueous media (effluents etc). As dye molecule, Remazol Red 3BS (reactive dye) was selected given its well–known toxicity and its already report to other previous works. Based on the results, the multi–component adsorption studied was found to be of competitive nature. The data clearly suggests deviation from the usual approach of thermodynamic equilibrium dictating the adsorption isotherm. Having identified an asymmetry to the interaction on adsorption degree between the dye and the metal the new concept of irreversible equilibrium is introduced. The corresponding model is developed and it is shown that no one of the two extreme scenarios of thermodynamic equilibrium and irreversible equilibrium can fully describe the experimental data. A combination of the two phenomena is likely to occur but the identification of such a mechanism requires more sophisticated experimental campaigns including sequential adsorption experiments. It was shown that the Cr(VI) adsorption isotherms can be considered to be independent from the dye concentration, whereas the kinetics depends on it. Based on the findings of the analysis semi–empirical relations are derived for the dye adsorption isotherms in the presence of Cr(VI) and for the diffusion coefficients of each component in the presence of the other.

Multi–component adsorption, Heavy metal, Reactive dye, Modeling, Chitosan adsorbents