Removal of Cr(III, VI) by quaternary ammonium and quaternary phosphonium ionic liquids functionalized silica materials

A series of silica-based organic-inorganic hybrid materials were prepared by the sol-gel process for Cr(III) and Cr(VI) adsorption. These silica materials generally had high surface areas, good physical-chemical stability and high thermal stability. Trialkylmethylammonium bis 2,4,4-trimethylpentylphosphinate ([A336][C272]) and trihexyl(tetradecyl)phosphonium bis 2,4,4-trimethylpentylphosphinate (Cyphos IL 104) were explored as porogens to prepare porous silica and as extractants to extract chromium ions. Cyphos IL 104 and [A336][C272] functionalized silica sorbents (SG-2, SG-5) can be effectively used for the removal of Cr(III) and Cr(VI) from aqueous solutions by adjusting pH values, whereas trialkylmethylammonium chloride (Aliquat 336) and bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) functionalized silica sorbents (SG-3, SG-4) can only be used for the removal of the single chromium species, Cr(VI) or Cr(III). The maximum adsorption amounts of Cr(III) and Cr(VI) were 2.14 and 19.31 mg g^-1 for SG-2 and 2.32 and 15.29 mg g^-1 for SG-5. Langmuir and Freundlich isotherm models were used to evaluate the adsorption of Cr(III) and Cr(VI) on sol-gel sorbents. The adsorption kinetics of Cr(III) and Cr(VI) on SG-2 and SG-5 could be well described by pseudo-second-order kinetic model. In terms of Cr(III) and Cr(VI) uptake capacities and kinetics, SG-2 and SG-5 appeared to be more suitable for Cr(III) and Cr(VI) removal than SG-3 and SG-4.