Hydrophobic Modification of ZrO₂-SiO₂ Xerogel and Its Adsorption Properties to Rhodamine B

Zirconium nitrate pentahydrate (Zr(NO₃)₄·5H₂O) and tetraethyl orthosilicate (TEOS) are used as the zirconium source and silicon source, respectively, and methyltriethoxysilane (MTES) as the hydrophobic modifier; the hydrophilic and hydrophobic ZrO₂-SiO₂ xerogels were prepared successfully. The xerogels were characterized using Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and N₂ adsorption-desorption measurement. The adsorption mechanism of hydrophobic ZrO₂-SiO₂ xerogels to RhB was described by the kinetic and adsorption isotherms. The results showed that the introduction of Si-CH₃ groups can make the average pore size, BET surface area, and total pore volume of ZrO₂-SiO₂ xerogel increase. The hydrophobic ZrO₂-SiO₂ xerogel displays an adsorption capacity of 169.23 mg·g^-1 for RhB dye at 25 °C and pH = 3. The adsorption process of hydrophobic ZrO₂-SiO₂ xerogel to RhB followed a pseudo-second-order kinetic model. Fitting results from the D-R model of adsorption indicate that the adsorption of RhB onto the hydrophobic ZrO₂-SiO₂ xerogels is mainly physical, accompanied by a spontaneous heat absorption process. The regeneration and recycling properties of hydrophobic xerogels were investigated, and their recoverability and reusability were demonstrated.