Adsorption Characteristics of Sulfur-Functionalized Silica Microspheres with Respect to the Removal of Hg(II) from Aqueous Solutions

This paper presents the study of the adsorption characteristics of sulfur-functionalized silica microspheres (S-SMs), synthesized through co-condensation of tetraethyl orthosilicate with 3-mercaptopropyl trimethoxysilane (MPTMS) and bis(triethoxysilylpropyl) tetrasulfide (BTESPT) as sulfur ligands, with respect to the removal of Hg(II) from aqueous solutions. The synthesized adsorbents were characterized using a scanning electron microscope, an X-ray diffractometer, a nitrogen adsorption-desorption analyzer, a Fourier transform infrared spectrophotometer, and an energy dispersive X-ray diffractometer. The effects of pH, concentration, temperature, stirring time, and adsorbent reusability were studied via batch adsorption experiments. It was found that the optimal adsorption pH values for all synthesized adsorbents were between 5.8 and 8.2. The adsorption capacity of SMs was 20.0 mg/g and increased to 37.0 and 62.3 mg/g for BTESPT-SMs and MPTMS-SMs, respectively. Hg(II) adsorption was found to be exothermic in nature and followed the chemisorption mechanism. The Langmuir isotherm model was found to be the best fitted model for describing the isotherm data, while the kinetic data obeyed the pseudo-second-order kinetic model, in which film diffusion was found to be the rate-controlling step. The regeneration study using potassium iodide as a regeneration agent showed high reusability, up to five-cycle activity.