Efficient Adsorption of Tetracycline Using Cu+-Modified SBA-15 and Its Adsorption Mechanism

Abstract

Cu+-modified SBA-15 (Cu+/SBA-15, SBA-15 refers well-ordered hexagonal mesoporous silica) was prepared for adsorption of tetracycline (TC) by the pH adjusting reduction method. Some Cu+ species (Cu2O) are aggregated on the outside surface of SBA-15, and the other Cu+ species are highly dispersed in SBA-15 via ≡Si-O-Cu. Its adsorption mechanism was firstly studied via comparison with CuO/SBA-15. The TC adsorption kinetic of Cu+/SBA-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 Qmax of Cu+/SBA-15 for TC calculated from Langmuir model could reach 961.54 mg/g at 313 K. Its good adsorption performance for TC is associated with the high surface area of SBA-15, highly dispersed Cu+ species, and mesoporous structure. The adsorption of Cu+/SBA-15 for TC is an endothermic process, and adsorption heat is 66.88 kJ·mol−1, which implies that the adsorption process of Cu+/SBA-15 for TC is weak chemisorption. The adsorption mechanism of Cu+/SBA-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 Cu+ with NH2 radical of TC, the π-complexation between Cu+, and the benzene ring of TC improved the adsorption capacity of Cu+/SBA-15 for TC comparing with CuO/SBA-15 and showed a dominant role for TC adsorption.