Enhanced Adsorptive Removal of Cadmium from Water by Immobilized Hydrophobic Ionic Liquids on Nano-Silica Sorbents

Excellent adsorptive removal of cadmium was accomplished from a pH 1.0 adjusted acidic aqueous solution by using two physically loaded ionic liquids with various hydrophobicity, [${\mathrm{Emim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$] and [${\mathrm{Omim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$], on nano-silica sorbents, [$\mathrm{Nano}\text{-}\mathrm{Si}\text{-}\mathrm{OH}\text{-}{\mathrm{Emim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$] and [$\mathrm{Nano}\text{-}\mathrm{Si}\text{-}\mathrm{OH}\text{-}{\mathrm{Omim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$], respectively. Adjusted solutions to a pH 1.0 were found to produce the highest cadmium adsorption capacity values ($1.200–1.300\mathrm{mmol}{\mathrm{g}}^{-1}$). An anion exchange mechanism was proposed for such high adsorption of cadmium via the exchange of anionic cadmium species by [${\mathrm{Tf}}_2{\mathrm{N}}^{-}$] anion. Several operational controlling factors were monitored and optimized, including the reaction pH, reaction time, sorbent dose, initial cadmium concentration, interfering ions, and hydrophobicity of ionic liquids. The more hydrophobic ionic liquid, [${\mathrm{Omim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$], was found to enhance the adsorption efficiency of the modified sorbent in all evaluated controlling factors. Adsorption equilibrium modeling data were fitted to Brauner-Emmet-Teller and Langmuir models. The potential implementations of [$\mathrm{Nano}\text{-}\mathrm{Si}\text{-}\mathrm{OH}\text{-}{\mathrm{Emim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$] and [$\mathrm{Nano}\text{-}\mathrm{Si}\text{-}\mathrm{OH}\text{-}{\mathrm{Omim}}^{+}{\mathrm{Tf}}_2{\mathrm{N}}^{-}$] were successfully established for cadmium removal. The percentage extraction values of cadmium from industrial waste water and drinking tap water samples, both adjusted to a pH 1.0, were found to be $97.25–99.30\pm 2.00–4.00\%$. The determined percentage extraction values ($94.11–98.47\pm 2.00–4.00\%$) were also identified for the two water samples adjusted to pH 7.0 after three stages of microcolumn elution.