Biosourced adsorbent prepared with rice husk part 1: A complete understanding of the structure of materials, the major role of mineral impurities for metal extraction

Rice husk is a global agricultural co-product and is already the subject of several studies, notably for wastewater treatment. Rice husk is composed of 3 types of components: amorphous silica, organic biopolymers and others (salts, oxides…). Depending on the treatment, rice husk becomes either a carbon-free material or a mixed carbon/silica biochar. In this paper, a thorough characterisation of rice husk was carried out by SEM, TEM, TGA, XRF, XRD, XPS, FTIR, SAXS, SANS, NMR and N2 adsorption-desorption. The results show that silica can be present as dense silica or silica nanoparticles and that native organic matter can be converted to turbostratic carbon. This carbon “drowns” the silica nanoparticles and prevents them from sintering. Particular attention is paid to impurities which play a crucial role in several properties. They are present in different forms, such as CaCO3, KCl or Al2O3 or in the silica lattice. They can be removed, but if retained, they induce sintering and crystallisation of the silica nanoparticles, resulting in a decrease in specific surface area from 330 m2.g−1 to 15 m2.g−1. Moreover, the sorption efficiency of the materials is strongly dependent on the presence of impurities since the extraction rate drops drastically from 99% to <0.5% when the impurities are removed. The maximum capacity reached for nickel is 11.7 mg.g−1.