Sol-gel materials for the solid phase extraction of metals from aqueous solution

Silica that was prepared by sol-gel chemistry to have pore widths in the microporous and in the mesoporous domains was evaluated as the host for performing solid phase extraction (SPE). Selective SPE of NiII was accomplished with dimethylglyoxime (DMG)-doped silica, but the pore width was demonstrated to influence the chemistry of the material. With microporous silica as the host, the stoichiometry of the NiII-DMG complex was 1:1 rather than 1:2, which is the value observed in aqueous solution. A green shift in the visible absorption spectrum was the primary evidence for the difference in stoichiometry; the alternative explanation of a rigidochromic effect on the spectrum was eliminated. The capacity of the DMG-doped mesoporous silica was only 9 μmol Ni g−1 because of leaching of the complexing agent. The microporous material showed no loss of DMG, but low permeability lowered the capacity. An alternative, albeit not selective, approach was to employ a mesoporous host to which a complexing agent, diethylenetriamine (DTA), was covalently bound. In this case, a capacity of 0.156 mmol Cu g−1, was achieved.