Efficient Adsorption of Tetracycline Using ${\mathrm{Cu}}^{+}$-Modified SBA-15 and Its Adsorption Mechanism

${\mathrm{Cu}}^{+}$-modified SBA-15 (${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$, SBA-15 refers well-ordered hexagonal mesoporous silica) was prepared for adsorption of tetracycline (TC) by the pH adjusting reduction method. Some ${\mathrm{Cu}}^{+}$ species (${\mathrm{Cu}}_2\mathrm{O}$) are aggregated on the outside surface of SBA-15, and the other ${\mathrm{Cu}}^{+}$ species are highly dispersed in SBA-15 via $\equiv \mathrm{Si}\text{-}\mathrm{O}\text{-}\mathrm{Cu}$. Its adsorption mechanism was firstly studied via comparison with CuO/SBA-15. The TC adsorption kinetic of ${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$ fitted the pseudo-second-order model well. The adsorption isotherms at 293, 303, and 313 K were determined and modeled with Langmuir and Freundlich equations, and the $Q_{\max }$ of ${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$ for TC calculated from Langmuir model could reach $961.54\mathrm{mg}/\mathrm{g}$ at 313 K. Its good adsorption performance for TC is associated with the high surface area of SBA-15, highly dispersed ${\mathrm{Cu}}^{+}$ species, and mesoporous structure. The adsorption of ${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$ for TC is an endothermic process, and adsorption heat is $66.88\mathrm{kJ}·{\mathrm{mol}}^{-1}$, which implies that the adsorption process of ${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$ for TC is weak chemisorption. The adsorption mechanism of ${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$ was mainly explored via comparing with CuO/SBA-15 by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) spectroscopy analysis. Besides the coordination complexation of ${\mathrm{Cu}}^{+}$ with ${\mathrm{NH}}_2$ radical of TC, the $\pi$-complexation between ${\mathrm{Cu}}^{+}$, and the benzene ring of TC improved the adsorption capacity of ${\mathrm{Cu}}^{+}/\mathrm{SBA}\text{-}15$ for TC comparing with CuO/SBA-15 and showed a dominant role for TC adsorption.