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

Three samples of SBA-15 functionalised with –(CH2)3COOH groups have been prepared by co-condensation, starting from solutions of TEOS and 4-(triethoxysilyl)butyronitrile, acting as –(CH2)3COOH precursor, of different molar compositions. Materials were characterised by X-ray diffraction, nitrogen adsorption, and FT-IR spectroscopy. The pKa 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 pKa (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+, Cd2+, Fe3+, Cu2+, Zn2+) 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.