Synthesis of hybrid bio-nanocomposites and their application for the removal of rare earth elements from synthetic wastewater

Abstract

In recent decades, the application of rare earth elements (REEs) has become apparent in numerous technological sectors. The gap in the supply and demand of REEs, as well as the increasing pollution of REEs, has raised the need for the removal and recovery of these elements from both secondary sources and waste streams. Hybrid bio-nanocomposites synthesised using various combinations of organic-inorganic matrices have the potential to remove and recover REEs from aqueous medium. The research focuses on the synthesis of five different bionanocomposites using different organic-inorganic matrices, the characterisation of prepared bionanocomposites, their application for the removal of REEs, the kinetic, isotherm and thermodynamic studies, the determination of possible REEs adsorption mechanism on bionanocomposites and their regeneration abilities.

The bio-nanocomposites, including cellulose intercalated zinc-aluminium layered double hyrdoxides (CL-Zn/Al LDH), sulfuric acid modified cellulose based silica nanocomposite (CLN/SiO2), Gum Arabic grafted polyacrylamide based silica (GA-g-PAM/SiO2), exfoliated biopolymeric-LDH (GA-LDH) and LDH encapsulated in xanthan gum anchored by metal ions (M@XG-ZA) nanocomposites were used to study the adsorptive behaviour towards REEs. The fusion of organic-inorganic matrices combined the advantages of both matrices. The application of CL and GA with LDH for the removal of REEs exhibited promising results compared to LDH encapulation in XG, modification of CL and grafting of PAM chain on GA backbone with SiO2 incorporation. The selection of organic-inorganic matrix and method of synthesis is very important.

The adsorption assays for the removal of REEs were performed in batch mode in order to attain maximum removal. The adsorption of REEs was pH dependent and a fast removal of REEs was indicated by all bio-nanocomposites. The knowledge about surface properties, nature and adsorption mechanism was attained by using different adsorption isotherm and kinetic models. Moreover, the adsorption mechanism, adsorption in a multi-component system with or without competing ions and intra-series adsorption behaviour were also discussed. On the whole, the bio-nanocomposites exhibited the potential for the removal and recovery of REEs.