From Rice Husk Ash to Silica-Supported Carbon Nanomaterials: Characterization and Analytical Application for Pre-Concentration of Steroid Hormones from Environmental Waters

Abstract

Rice husk (RH) in the rice industry is often air-burnt to obtain energy in the form of heat and RH ash (RHA) residue. In this work, RHA was applied as a starting material to obtain silica-supported carbon nanomaterials, resulting in a new reuse of a globally produced industrial waste product, in a circular economy approach. The preparation involves ultrasound-assisted one-pot oxidation with a sulfonitric mixture followed by wet oven treatment in a closed vessel. A study of oxidation times and RHA amount/acid volume ratio led to a solid material (nC-RHA@SiO2) and a solution containing silica-supported carbon quantum dots (CQD-RHA@SiO2). TEM analyses evidenced that nC-RHA@SiO2 consists of nanoparticle aggregates, while CQD-RHA@SiO2 are carbon-coated spherical silica nanoparticles. The presence of oxygenated carbon functional groups, highlighted by XPS analyses, makes these materials suitable for a wide range of analytical applications. As the main product, nC-RHA@SiO2 was tested for its affinity towards steroid hormones. Solid-phase extractions were carried out on environmental waters for the determination of target analytes at different concentrations (10, 50, and 200 ng L−1), achieving quantitative adsorption and recoveries (RSD < 20%, n = 3). The method was successfully employed for monitoring lake, river, and wastewater treatment plant water samples collected in Northern Italy.

Comparative Study of Different Pretreatment and Combustion Methods on the Grindability of Rice-Husk-Based SiO2

“…The broadband around 2500-3600 cm −1 was attributed to O-H stretching in RH-based silica, which confirms the presence of hydroxyl group at the silica nanoparticles' surface and the water absorption in samples [41,42]. The FTIR spectra of RH-based silica samples treated in different ways showed continuous absorption peaks at 1003, 802, and 462 cm −1 [39,43]. In brief, the vibration signal at 1003 cm −1 was the asymmetric stretching of Si-O-Si.…”

“…The absence of other distinct crystallographic peaks and the ge ilarity across patterns suggest that the primary crystal structure of RHA rema cantly unchanged across different treatment methods [38]. Second, these patte mirror the standard spectrum of amorphous SiO2, confirming the primary con the resultant product as amorphous SiO2 [39,40]. Third, the diffraction intensi ples treated at a combustion temperature of 800 °C was more pronounced tha terparts at lower temperatures.…”

A Comparative Study on Rice Husk, as Agricultural Waste, in the Production of Silica Nanoparticles via Different Methods

“…The presence of these amorphous silica characteristic groups proves that there is an amorphous silica component in all the samples. Additionally, the vibrational peaks around 3400 cm −1 and 1640 cm −1 are ascribed to chemically bound water, represented by O-H and H-O-H bond vibrations [39,40]. Figure 3a delineates the FTIR spectra of the silica derived from RH through various methods.…”

Improving the grindability of rice husk-based green silica through pyrolysis process optimization employing the Taguchi method and response surface methodology

“…This is basically consistent with the standard profile of amorphous SiO 2 , indicating the amorphous SiO 2 constitution of the resultant product. 59 Furthermore, this underscores that varying pretreatment modalities, along with alterations in torrefaction and pyrolysis atmospheres, remains ineffectual in modulating the crystalline form of the ultimate product, thus preserving the inherent amorphous state of SiO 2 . 60 3.4 Effect of different treatment methods on crystal structure and particle size distribution of RHA-SiO 2 Fig.…”