Green synthesis and characterization of UKMRC-8 rice husk-derived mesoporous silica nanoparticle for agricultural application

Abstract

Agriculture plays a crucial role in safeguarding food security, more so as the world population increases gradually. A productive agricultural system is supported by seed, soil, fertiliser and good management practices. Food productivity directly correlates to the generation of solid wastes and utilization of agrochemicals, both of which negatively impact the environment. The rice and paddy industry significantly adds to the growing menace of waste management. In low and middle-income countries, rice husk (RH) is an underutilized agro-waste discarded in landfills or burned in-situ. RH holds enormous potential in the development of value-added nanomaterials for agricultural applications. In this study, a simple and inexpensive sol-gel method is described to extract mesoporous silica nanoparticles (MSNs) from UKMRC8 RH using the bottom-up approach. RHs treated with hydrochloric acid were calcinated to obtain rice husk ash (RHA) with high silica purity (> 98% wt), as determined by the X-ray fluorescence analysis (XRF). Calcination at 650 °C for four hours in a box furnace yielded RHA that was devoid of metal impurities and organic matter. The X-ray diffraction pattern showed a broad peak at 2θ≈20-22 °C and was free from any other sharp peaks, indicating the amorphous property of the RHA. Scanning electron micrographs (SEM) showed clusters of spherically shaped uniform aggregates of silica nanoparticles (NPs) while transmission electron microscopy analysis indicated an average particle size of < 20 nm. Besides Energy Dispersive X-Ray which validated the chemical constituent of the silica NPs, the Fourier transform infrared (FT-IR) spectra showed peaks at 796.4 cm^-1 and 1052 cm^-1 corresponding to O-Si-O symmetric stretching vibration and O-Si-O asymmetric stretching, respectively. The Brunauer-Emmet-Teller (BET) analysis indicated an average pore size = 8.5 nm while the specific surface area and the pore volume were 300.2015 m²/g and 0.659078 cm³/g, respectively. In conclusion, agrowaste-derived MSN was synthesized using a simple and economical sol-gel method without the addition of surfactant reagents for controlled formation at the structural level. Owing to the MSNs' excellent physical properties, the method established herein, could be used singly (without any modifications) for the functionalization of a myriad of agrochemicals.

Figure 1

X-ray diffraction (XRD) spectra of rice husk ash calcined at four different temperatures.

Figure 2

Physical appearance of the different stages of mesoporous nanosilica synthesis.

Figure 3

(a) FESEM micrograph and (b) EDX spectra of silica nanoparticles synthesized from UKMRC-8 rice husk ash.

Figure 4

Transmission electron micrographs of silica nanoparticles synthesized from UKMRC-8 rice husk ash, viewed at various resolutions.

Figure 5

FT-IR spectrum of silica nanoparticles synthesized from UKMRC-8 rice husk ash.

Figure 6

Nitrogen adsorption–desorption isotherms of silica nanoparticles synthesized from UKMRC-8 rice husk ash.

Figure 7

Pore size distribution of silica nanoparticles synthesized from UKMRC-8 rice husk ash.

Figure 8

Brunauer–Emmet–Teller (BET) surface area distribution of silica nanoparticles synthesized from UKMRC-8 rice husk ash.