Nanosized MCM-41 silica from rice husk and its application for the removal of organic dyes from water

Abstract

A novel synthesis of a nanometric MCM-41 from biogenic silica obtained from rice husk is here presented. CTABr and Pluronic F127 surfactants were employed as templating agents to promote the formation of a long-range ordered 2D-hexagonal structure with cylindrical pores and to limit the particle growth at the nanoscale level thus resulting in a material with uniform particle size of 20-30 nm. The physico-chemical properties of this sample (RH-nanoMCM) were investigated through a multi-technique approach, including PXRD, ²⁹Si MAS NMR, TEM, Z-potential and N₂ physisorption analysis at 77 K. The results were compared to those of a nanometric MCM-41 synthesized from a silicon alkoxide precursor. The adsorption capacity of RH-nanoMCM towards the cationic dye rhodamine B from aqueous phase was investigated at different initial dye concentrations by means of UV-vis spectroscopy. Insight into the non-covalent interactions between the dye molecules and the adsorbent surface was gained by means of ¹H and ¹³C MAS NMR spectroscopy and FT-IR spectroscopy.

Fig. 1. Graphical representation of the main steps of the synthesis of RH-nanoMCM from rice husk.

Fig. 2. Low-angle PXRD patterns of MCM-41 nanometric (a) and RH-nanoMCM (b).

Fig. 3. TEM micrographs of RH-nanoMCM (frames A, A′) and of MCM-41 nano (frames B, B′) at magnifications of 800 00× (top images) and 1 000 00× (bottom images).

Fig. 4. ²⁹Si MAS NMR spectra of RH-nanoMCM and MCM-41 nano.

Fig. 5. N₂ adsorption–desorption isotherms at 77 K (frame A) and NLDF pore size (frame B) of RH-nanoMCM (triangles) and MCM-41 nano (circles).

Fig. 6. Rhodamine B removal percentages after a contact time of 20 min with RH-nanoMCM (triangles) and MCM-41 nano (circles). The initial RhB concentrations are 2.0 × 10⁻², 1.0 × 10⁻², 5.0 × 10⁻³, 2.5 × 10⁻³ and 1.2 × 10⁻³ mmol L⁻¹. The error bars represent the standard deviations over three measurements.

Fig. 7. ¹H MAS NMR (A) spectra of RH-nanoMCM after dehydration (a) and of RH-nanoMCM + RhB before (b) and after (c) dehydration. ¹³C CPMAS NMR (B) spectrum of RH-nanoMCM + RhB. Inset shows the molecular structure of RhB. * spinning sidebands.

Fig. 8. FT-IR spectrum of RH-nanoMCM (a) and of RH-nanoMCM after the adsorption of rhodamine B (b). Prior to the adsorption measurements, the pellets were treated under vacuum (residual pressure below 10⁻³ mbar) at beam temperature (35 °C) for 3 hours.