Retention of heavy metal ions on SBA-15 mesoporous silica functionalised with carboxylic groups

Three samples of SBA-15 functionalised with -(CH(2))(3)COOH groups have been prepared by co-condensation, starting from solutions of TEOS and 4-(triethoxysilyl)butyronitrile, acting as -(CH(2))(3)COOH precursor, of different molar compositions. Materials were characterised by X-ray diffraction, nitrogen adsorption, and FT-IR spectroscopy. The pK(a) and the acidic capacity were measured for all samples by potentiometric titration. The acidic capacity increases with increasing amount of -COOH precursor in the synthesis mixture only up to 10% molar of total alkoxysilane. The value for the pK(a)(4.75) is independent of the acidic capacity of the material. The sample prepared starting from an amount of -COOH precursor equal to 10% molar of total alkoxysilane was chosen to test selective interactions with heavy metals of environmental importance (Ag(+), Cd(2+), Fe(3+), Cu(2+), Zn(2+)) at different pH values and ionic strengths. The significant and selective adsorption exhibited by the material has been exploited in a preliminary cation-exchange chromatographic application showing the possibility of eluting the metal ions at different retention times.