The synergetic effect of a structure-engineered mesoporous SiO2-ZnO composite for doxycycline adsorption

Abstract

The design and synthesis of an efficient adsorbent for antibiotics-based pollutants is challenging due to the unique physicochemical properties of antibiotics. The development of a mesoporous SiO₂-ZnO composite is a novel way to achieve excellent adsorption efficiency for doxycycline hydrochloride (DOX) in aqueous solutions due to the engineered highly open mesoporous structure and the ZnO-modified framework. Unlike the traditional method of obtaining mesoporous composites by post-synthesis techniques, the novel one-step method developed in this study is both effective and environment-friendly. The adsorption mechanism based on the novel synergetic effect between SiO₂ and ZnO was demonstrated through several experiments. SiO₂ led to the creation of a 3D open framework structure that provides sufficient space and rapid transport channels for adsorption, ensuring rapid adsorption kinetics. A higher number of active sites and enhanced affinity of the contaminants are provided by ZnO, ensuring high adsorption capacity. The mesoporous SiO₂-ZnO could be easily regenerated without a significant decrease in its adsorption efficiency. These results indicate that the developed strategy afforded a simple approach for synthesizing the novel mesoporous composites, and that mesoporous SiO₂-ZnO is a possible alternative adsorbent for the removal of DOX from wastewater.

Fig. 1. TEM image of mesoporous SiO₂–ZnO. Inset describes the size distribution histogram of mesoporous SiO₂–ZnO.

Fig. 2. XRD patterns of mesoporous SiO₂ (a) and SiO₂–ZnO (b).

Fig. 3. N₂ adsorption–desorption isotherms of mesoporous SiO₂ (a) and SiO₂–ZnO (b). Inset describes the pore size distribution.

Fig. 4. FTIR spectra (A) of mesoporous SiO₂ (a) and SiO₂–ZnO (b) and XPS spectra (B–D) of mesoporous SiO₂–ZnO.

Fig. 5. Effect of the solution pH on adsorption efficiency of DOX by mesoporous SiO₂–ZnO. (m_sorbent = 50 mg, V_(DOX) = 100 mL, C_0(DOX) = 100 mg L⁻¹, t = 24 h, T = 298 K).

Fig. 6. Fitting of the experimental adsorption data into the Langmuir (a); Freundlich (b); Tempkin (c) and D–R (d) models.

Fig. 7. Kinetics of DOX sorption onto mesoporous SiO₂–ZnO: pseudo-first-order and pseudo-second-order kinetics (a); intra-particle diffusion kinetics (b). (m_sorbent = 100 mg, V_(DOX) = 200 mL, C_0(DOX) = 100 mg L⁻¹, initial pH, t = 24 h, T = 298 K).

Fig. 8. Effect of ionic strength for DOX adsorption onto mesoporous SiO₂–ZnO (a) and sorption–desorption cycles of mesoporous SiO₂–ZnO (b). (m_sorbent = 10 mg, V = 20 mL, C_0(DOX) = 100 mg L⁻¹, T = 298 K, initial pH).