Fabrication of a Nanoporous Silica Hydrogel by Cross-Linking of SiO 2 -H 3 BO 3 -Hexadecyltrimethoxysilane for Excellent Adsorption of Azo Dyes from Wastewater.

This study reports a novel cross-linking approach to fabricate the hydrothermally neutralized silica hydrogel of SiO ₂ -H ₃ BO ₃ -hexadecyltrimethoxysilane by grafting alkylsilane groups onto the nanoporous silica. The synthesized silica hydrogel possessed a large specific surface area of 51.3 m ² g ^-1 and showed excellent dye adsorption capability of cationic dyes in neutral (pH 7) and alkaline (pH 9) medium from wastewater. The colloidal electrokinetic potential analysis revealed that the outstanding adsorption efficiency of cationic dyes over anionic dyes strongly relies on the surface charge of the hydrogels. Moreover, the hydrophobic interactions between the dye molecules and the hydrogels were studied, and it was found that the dye adsorption performance can be tuned by altering the concentration of hydrophobic reagents of the hydrogel. The dye adsorption mechanism was established, and the kinetic study suggested that the adsorption is a pseudo-second-order reaction. Adsorption isotherms at various equilibrium conditions fitted well with the Langmuir isotherm. Therefore, this strongly supports the promising and practical application of the prepared silica hydrogel. The recyclability of the hydrogel was studied, and it showed 90% adsorption efficiency by the regenerated gel up to 6 cycles, which has a high potential in wastewater treatment.