Fabrication of carbon nanotubes reinforced silica composites with improved rare earth elements adsorption performance

The continuous exploration of carbon nanotubes (CNTs) in terms of their chemical functionalization attributes, has engrossed the members of both the research and industrial community for scientific advancements alongside a variety of potential applications. Hence, in this article, we explored the different modification schemes to chemically tailor the surface properties of CNT by hybridizing them in the organic-inorganic network for the selective sorption of rare earth elements (REEs). We utilized both single-walled- (SWNT) and multi-walled carbon nanotubes (MWNT) for functionalization with nano-silica through the silanization process, with (3-aminopropyl)triethoxy silane (APTES) acting as a coupling agent. In addition, a coordination ligand, 1-(2-pyridylazo)-2-naphthol (PAN), was loaded onto these nanocomposites to enhance the selectivity via two different grafting routes, i.e. one-pot synthesis and step-by-step telescopic synthesis process. The comprehensive physicochemical characterization of these CNT-silica nanocomposites using the current state of techniques ascertained its promise in the direction of REE selective sorption (all REEs), whereby this achievement was not possible by pristine CNT or bare silica (except scandium). Furthermore, in terms of REE sorption, CNT reinforced silica composite materials (mere CNT loading of 10%) have shown to be far more advantageous over the PAN grafted onto APTES-silica. The findings from this work also revealed the relevance of different synthetic strategies needed to be adopted for SWNT and MWNT, providing a deeper insight into the implications of the ligand grafting technique towards REE affinity. Moreover, the selective REE recovery from AMD demonstrated by these adsorbents (no affinity towards water hardness ions such as Ca2+ and Mg2+) can attract many industrial applications.