Highly stable mesoporous silica nanospheres embedded with FeCo/graphitic shell nanocrystals as magnetically recyclable multifunctional adsorbents for wastewater treatment

Abstract

Highly stable and magnetically separable mesoporous silica nanospheres (MSNs) embedded with 4.6 ± 0.8 nm FeCo/graphitic carbon shell nanocrystals (FeCo/GC NCs@MSNs) were synthesized by thermal decomposition of metal precursors in MSNs and subsequent methane CVD. The FeCo/GC NCs@MSNs had a high specific surface area (442 m² g^-1), large pore volume (0.65 cm³ g^-1), and tunable size (65 nm, 130 nm, and 270 nm). Despite the low magnetic metal content (8.35 wt%), the FeCo/GC NCs@MSNs had a sufficiently high saturation magnetization (17.1 emu g^-1). This is due to the superior magnetic properties of the FeCo/GC NCs, which also enable fast magnetic separation of the nanospheres. The graphitic carbon shell on the FeCo NCs not only protects the alloy core against oxidation and acid etching in 35% HCl_(aq), but also facilitates non-covalent, hydrophobic interactions with the hydrocarbon chains of organic dyes such as methyl orange and methylene blue. Surface functionalization of the FeCo/GC NCs@MSNs with thiol groups provides efficient capacity for binding with Hg²⁺ ions. We have shown that the thiol-functionalized FeCo/GC NCs@MSNs (FeCo/GC NCs@MSNs-SH) work as multifunctional adsorbents for organic dyes (target organic pollutants) and Hg²⁺ ions (target inorganic pollutant). We also demonstrated that the FeCo/GC NCs@MSNs-SH are excellent recyclable adsorbents for methyl orange.

Fig. 1. Schematic illustration for the preparation of FeCo/GC NCs@MSNs-SH and the multifunctional adsorption of organic dyes and mercury(ii) ions.

Fig. 2. TEM images of (a, b) 65 nm, (c, d) 130 nm, and (e, f) 270 nm MSNs (a, c, e), and FeCo/GC NCs@MSNs (b, d, f). Insets in Fig. 2b, d, and f are SAED patterns. (g) XRD patterns of 65 nm, 130 nm, and 270 nm FeCo/GC NCs@MSNs.

Fig. 3. (a) Nitrogen sorption isotherms and (b) pore size distributions of 130 nm MSNs and FeCo/GC NCs@MSNs.

Fig. 4. (a) Illustration of thiol functionalization reaction between MPTMS and silanol groups on the silica surface. (b) FT-IR and (c) TGA data of 130 nm FeCo/GC NCs@MSNs and FeCo/GC NCs@MSNs-SH.

Fig. 5. Adsorption isotherms of (a) MB and (c) MO for MSNs (squares), FeCo/GC NCs@MSNs (circles), and FeCo/GC NCs@MSNs-SH (triangles) in water (the solid lines represent the best fit of the experimental data to the Langmuir model). Adsorption curves of (b) MB and (d) MO for MSNs (squares), FeCo/GC NCs@MSNs (circles), and FeCo/GC NCs@MSNs-SH (triangles) in water as a function of contact time.

Fig. 6. Photographs of aqueous solutions of (a) MB and (d) MO and the FeCo/GC NCs@MSNs-SH collected using a magnet in the aqueous solutions after the adsorption of the organic dyes, MB (b) and MO (c).

Fig. 7. Regeneration study of FeCo/GC NCs@MSNs-SH for the adsorption of MO in water at room temperature.