H2O2-activated anthracite impregnated with chitosan as a novel composite for Cr(VI) and methyl orange adsorption in single-compound and binary systems: Modeling and mechanism interpretation

A novel composite was prepared from chitosan and hydrogen peroxide-treated anthracite sheets and it was applied for the single and simultaneous adsorption of Cr(VI) and methyl orange (MO) at different temperature (25-45 degrees C) and pH (2-10). Experimental results showed that Cr(VI) as HCrO4- species was mainly removed by electrostatic interactions, while the reduced form Cr(III) was adsorbed via ion exchange and complexation reactions, thus concluding that Cr(VI) uptake was governed by adsorption-reduction coupled mechanism. Adsorption of MO on this composite was controlled by hydrogen bonding, n-pi and also electrostatic attractions. Experimental data were fitted satisfactorily with Langmuir and Freundlich equations indicating that the composite was more selective for MO adsorption. Langmuir-based monolayer adsorption capacities (q(max)) ranged from 188.01 to 201.77 mg/g for Cr (VI) and from 297.28 to 295.75 mg/g for MO in the single and binary systems, respectively, at 25 degrees C. Physicochemical parameters of statistical physics models were estimated and utilized to understand the adsorption mechanism of both adsorbates. A multi-docking and multi-molecular mechanisms could be present in the adsorption of Cr(VI) and MO, respectively. Density of receptor sites (N-M) proved to be an essential factor that was associated to the adsorption capacities of the studied adsorbates and its values confirming the high selectivity of anthracite/chitosan active sites for MO uptake. The values of the adsorbed capacity ratio (Q(b)/Q(s)) revealed that MO enhanced uptake of Cr(VI) due to the newly formed active sites (i.e., synergism interaction), while MO adsorption was slightly decreased in the binary system at all temperatures reflecting a weak antagonism interaction. The adsorption energies corresponded to an exothermic process governed by physical forces for both pollutants.