Surface Properties and Chemical Constitution as Crucial Parameters for the Sorption Properties of Ionosilicas: The Case of Chromate Adsorption

We report ionosilicas with different chemistries, textures, and morphologies and their use as adsorbents for chromium­(VI). All studied materials are highly efficient anion exchange materials with adsorption capacities between 1.6 and 2.6 mmol/g. The ion exchange capacity of the materials reaches up to 91% of the theoretical value, that is, the molar amount of ionic groups immobilized within the material, indicating a very high accessibility of the organo-ionic groups. Noticeable differences were found regarding the ion exchange properties in terms of capacity and kinetics according to the used material, in particular, its porosity. High specific surface areas favor the adsorption process and result in high adsorption capacity. However, even a nonporous material displays high adsorption capacity of 1.7 mmol/g. This result can be attributed to the high hydrophilicity of ionosilicas that favors diffusion and mass transfer throughout the material. The adsorption kinetics are fast, as 80–90% of the adsorption capacity is reached after ∼10 min. Finally, isotherm titration calorimetry evidences the influence of the constitution of the cationic group on the displacement enthalpy, in relationship with the steric hindrance of the alkyl groups that surround the cationic center.