Abstract
Adsorption of three anionic Orange-type dyes (i.e., Methyl Orange, Orange II, Orange G) and divalent chromate oxyanion onto Amberlite® IRN-78 from single- and bi-solute aqueous solutions have been investigated in view of testing the removal technologies for dye-contaminated wastewater. The research methodology was based on a combination of equilibrium adsorption, isothermal titration calorimetry, and 13C CP MAS NMR studies. The adsorption followed the pathway of anion exchange between the pristine OH− ions and the oncoming anionic species. The supernatant solution thus became more alkaline, which, in some cases, gave rise to new dye species with an additionally ionized R-N-NH-R moiety, e.g., OII2− or even OG3−. In the single-solute systems, the retention capacity of the resin towards various anionic solutes, on a per-mole basis, decreased in the following order: CrO42− > OG > OII ≈ MO. The retention of dyes was irreversible even in strongly alkaline media contrary to CrO42−, which was completely desorbed from the resin by a 1 mol L−1 NaOH solution. The competition between dye and chromate anions from their equimolar solutions mainly caused a marked decrease in the uptake of CrO42−, thereby pointing to preferential dye retention by Amberlite® IRN-78.