New technology for the extraction of energy critical metals from weee and removal of PFOA from water

Abstract

ABSTRACT Electrical and electronic equipment contains a plethora of energy critical metals which eventually concentrates in the annual 20-50 million metric tonnes of e-wastes produced globally. This has been identified as a threat to the development of sustainable technology and to the environment. In an attempt to reclaim some of these metals, a series of fluorinated extractants, based on the cone and partial cone (PC) calix[4]arene, Tris-(2-Aminoethyl)-Amine (Tren), Trithiacyclononane (9-ane-S3 or TTCN) and Propylenediaminetetraacetic acid (PDTA) scaffolds have been synthesised. These ligands have been employed in extraction of Au3+, Pt2+, Pd2+, Nd3+, Eu3+ and Er3+. For all the amido ligands tested, Au(III) was the most extracted (> 96%) with the partial cone calixarene ligand suggesting selectivity of the amido ligands toward Au(III). The binding constants show that the cone calixarene eventually forms the most stable complexes. Small angle X-ray scattering (SAXS) analysis of the Au(III) complexes showed that the cone calixarene ligand was the most perturbed in terms of size and speciation. Ln3+ were quantitatively extracted by calixaryl phosphine oxide ligand. The synthesis of fluorinated compounds also led to interesting observations in fluorine chemistry. Some compounds wherein fluorine-fluorine interactions are present in the solid state and are not simply due to crystal packing but were shown to be stabilizing and give a significant energy contribution to the structures. Computational studies showed that these Fluorine-Fluorine stabilization energy are of the same order of magnitude as a typical hydrogen bond. These non-covalent interactions have also been harnessed in extraction of Perfluorooctanoic Acid (PFOA) from water. PFOA is a persistent organic pollutant that has been associated with diseases including cancer and kidney diseases. It is has been detected in water, soil, blood serum of animals and humans. PFOA was consistently extracted quantitatively. Formation of weak host-guest complexes and the role of C-F?F-C interaction has also been shown through solution 1D, 2D and solid state NMR spectroscopy, thermogravimetric analysis and computational studies. Physisorption of fluorinated compounds on a modelled fluorinated surface successful have demonstrated the feasibility of physisorption of fluorinated compounds on a fluoropolymer. Physisorption on Teflon was thereby carried out harnessing these stabilising C-F?F-C interaction. Easy adsorption in 11-17% wt and desorption of the fluorinated compounds has been shown and characterised by AFM, SEM, TGA-IR. Metal extractions with the supported extractants showed effective metal extraction and minimal leaching. EXAFS analysis of the metal ligand complexes are consistent with outer sphere ligand interactions. Polymer resins functionalised with PC ligand have been shown to quantitatively extract Au(III).