Preparation of silica gel supported amidoxime adsorbents for selective adsorption of Hg(II) from aqueous solution

Two types of silica gel supported amidoxime (SG-HE-AO and SG-HO-AO) adsorbents were prepared via the reactions of silica gel supported acrylonitrile functional group (SG-AN) with hydroxylamine under alkaline conditions. The SG-AN was obtained by so-called “heterogeneous” and “homogeneous” methods and named SG-HE-AN and SG-HO-AN, respectively. All the structures of adsorbents and intermediate were characterized by FT-IR, SEM, porous structure analysis and elemental analysis. The amidoxime group content of SG-HE-AO and SG-HO-AO were 1.28 and 0.77 mmol g−1, respectively, indicating that the heterogeneous method is more effective than the homogeneous method to anchored functional groups onto the surface of silica gel in this study. In terms of Hg(II) adsorption, SG-HE-AO exhibited better capability than SG-HO-AO. The adsorption kinetics of the two adsorbents followed the pseudo second-order process in the temperature range of 5–35 °C. Increased temperature was beneficial to adsorption. Langmuir and Freundlich isotherm models were both applied to experimental data analysis, and the former proved to be a better fit. Finally, in Hg(II)–Pb(II), Hg(II)–Ag(I), Hg(II)–Cu(II) and Hg(II)–Ni(II) binary systems, both SG-HE-AO and SG-HO-AO exhibited excellent adsorption selectivity for Hg(II).

► Two novel silica gel supported amidoxime adsorbents are prepared by heterogeneous and homogeneous methods.
► The heterogeneous method is proven to be a more effective method to introduce functional groups on the silica gel.
► Two types of silica gel supported amidoxime adsorbents exhibit excellent adsorption selectivity for Hg(II).