Highly Pure Silica Nanoparticles with High Adsorption Capacity Obtained from Sugarcane Waste Ash

Silica nanoparticles (SiO₂NPs) from renewable sources can be used in very different materials, such as paints, membranes for fuel cells, Li-ion batteries, adsorbents, catalysts, and so on. Brazil is the world's largest producer of sugarcane and generates huge amounts of sugarcane waste ash (SWA), which is a Si-rich source. This study investigates a method to produce highly pure SiO₂NPs from SWA. The SiO₂NPs were characterized by inductively coupled plasma optical emission spectroscopy, scanning and transmission electron microscopy (TEM), X-ray diffraction analyses, specific surface area and pore distribution, UV and Fourier transform infrared spectroscopy, and thermogravimetric analyses and applied as an adsorbent material in the removal of acid orange 8 (AO8) dye from aqueous solution. The SiO₂ content was 88.68 and 99.08 wt % for SWA and SiO₂NPs, respectively. TEM images of SWA and SiO₂NPs exhibit drastic alterations of the material size ranging from several micrometers to less than 20 nm. The SiO₂NPs showed a specific surface area of 131 m² g^-1 and adsorption capacity of around 230 mg g^-1 for acid orange 8 dye. Furthermore, the recycling of the SiO₂NPs adsorbent after AO8 adsorption was very satisfactory, with reuse for up to five cycles being possible. The results indicate that it was possible to obtain highly pure silica in a nanosize from the waste material and produce an adsorbent with high adsorption capacity and the possibility of reuse.