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.