Abstract
Competitive adsorption of three anionic orange-type dyes (i.e., methyl orange, orange II, orange G) and three inorganic anions (HPO42–, SO42–, CrO42–) onto Mg–Al layered double hydroxide (LDH) from bisolute aqueous solutions has been studied in view of potential application of the sorption-based technology in dye removal from textile wastewater effluents. The individual solute affinities for the LDH sample were inferred from the calorimetry measurements of the cumulative enthalpy of displacement in single-solute systems, thus leading to the following affinity order: CrO42– ≈ SO42– < OG2– ≈ HPO42– < OII– ≪ MO–. The individual and composite adsorption isotherms from bisolute aqueous solutions were determined by a solution depletion technique and supplemented by XRD studies on the well-defined adsorption systems corresponding to selected points on isotherms. Based on the shape of the individual adsorption isotherms for a dye and inorganic anion, it was possible to distinguish three different competition schemes: when the reduction in adsorption extent concerns mainly one of the solutes (Type I and Type III) or when this reduction applies to both solutes (Type II). The order of decreasing effectiveness of the inorganic anion in depressing the retention capacity of Mg–Al–LDH–NO3 toward a given dye, on a per-equivalent basis, was as follows: HPO42– > SO42– ≈ CrO42–, methyl orange; HPO42– > CrO42– > SO42–, orange II; HPO42– ≈ SO42– > CrO42–, orange G.