Sulfhydryl-modified SiO2 cryogel: A pH-insensitive and selective adsorbent for efficient removal of mercury in waters

In this study, water glass was used as the precursor, functionalized with sulfhydryl groups, to prepare SiO2 cryogel through the sol–gel method and freeze-drying process. The optimal preparation conditions were obtained by the response surface methodology. The SiO2–SH cryogel was composed of spherical particles, which had high surface area, abundant internal pores, and sulfhydryl groups. The four divalent heavy metal ions had minimal influences on the adsorption of Hg(II) on the SiO2–SH cryogel, and different concentrations of NaCl showed negligible impacts on the adsorption properties. The theoretical maximum adsorption capacity from the Langmuir isotherm model was 709.2 mg/g, with the removal rate able to reach 99.99% in one minute. Regeneration of the material showed the SiO2–SH cryogel conserved a good Hg(II) adsorption capacity even after five cycles. The adsorption mechanism was traced predominantly to the chemical adsorption between sulfhydryl groups and mercury, partly attributed to the electrostatic attraction and physical adsorption of the high surface area. Most importantly, the proposed cryogel could maintain high activity at wider pH (1–9), even at neutral or relatively alkaline pH, the H+ produced from the reaction between sulfhydryl groups and Hg(II), which reduced the pH to an acidic condition, promoted Hg(II) dissociation into ions, and hence the cryogel could maintain the high removal rate at approximately 99.98%. Overall, the SiO2–SH cryogel exhibited high capacity, rapid absorption and high selectivity, and pH-insensitivity, which meant it offered considerable potential application in the removal of wastewater containing high Hg content and multiple heavy metals, or highly saline wastewater.