Abstract
The world’s largest growing waste stream is that of electronic waste (e-waste), consisting of discarded electrical and electronic equipment. The global production of e-waste was expected to reach 52.2 million tonnes for the year 2021 alone. This global growth in the production of e-waste is unparalleled and unsustainable in the long-term. The development of efficient and sustainable e-waste recycling strategies is therefore critical. Recovery and subsequent recycling of profitable materials within e-waste has become an attractive approach, such as the extraction of precious metals. Various techniques for the recycling of precious metals from e-waste exist, however they have many drawbacks such as the use of large amounts of chemical additives and energy, the production of unwanted by-products and high cost. The goal of this research is therefore the development and fabrication of mesoporous polymer monolith adsorbents for the selective recovery of precious metals from real PCB leach liquors. The desired outcome is that the adsorbents will provide a more eco-friendly, cheaper, and selective alternative to the current existing metal recovery methods. This research project involved three main stages. The first stage included the successful fabrication and characterisation of mesoporous polymer monoliths using neat polyacrylonitrile (PAN), as well as a combination of PAN and poly-4-vinylpyridine (P4VP), with the use of the non-solvent induced phase separation (NIPS) method. Various fabrication and operational parameters employed during the NIPS method were evaluated and optimised. The polymer monoliths were characterised using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and Thermogravimetric Analysis (TGA). The second stage of this project included the application of the polymer monoliths as adsorbents for the recovery of precious metals, Ag(I), Au(III) and Pd(II), from standard solutions. Various adsorption parameters such as pH, contact time, adsorbent type, and monolith mass, were optimised to achieve maximum recovery of the precious metals. Brief desorption studies were carried out to evaluate the reusability of the polymer monoliths as adsorbents. Following optimisation of the adsorption reaction parameters, the monoliths were applied to the recovery of precious metals from simulated e-waste samples, where base metals were also present in solution. The third stage of this project involved the application of the polymer monoliths as adsorbents in real e-waste samples. The polymer monoliths were used for the x removal of precious metals from random access memory (RAM) pins, extracted from a waste printed circuit board (PCB), following hydrometallurgical treatment. The first stage of this research was successfully carried out. The NIPS method was optimised for the fabrication of ten different types of mesoporous polymer monoliths, where six were fabricated from neat PAN and the remaining four from a composite of PAN and P4VP. Analyses carried out with SEM validated the existence of an extensive porous network within all fabricated polymer monoliths. Analyses with TGA showed that the polymer monoliths were thermally stable over an acceptable temperature range and could be reliably used as adsorbents. With regards to the second stage of this project, the polymer monoliths displayed great capacity for precious metal ion adsorption from both standard and simulated e-waste solutions. The optimum adsorption reaction conditions for all three precious metal ions from standard solutions with the use of neat PAN monoliths were found to be as follows: 1) a metal ion solution adjusted to a pH of 4, 2) a contact time of 6 hours, 3) a monolith mass of 0.5 g and 4) the use of neat PAN monolith 480-90 as an adsorbent. Maximum adsorption percentages of 96.8 %, 73.4 % and 98.8 % were achieved for Pd(II), Ag(I) and Au(III) ions, respectively, under the above-mentioned conditions. The composite polymer monoliths were found to achieve complete removal of the precious metal ions. Maximum adsorption percentages of 99.5 %, 97.9 % and 99.6 % of Pd(II), Ag(I) and Au(III) ions, respectively, were achieved. It was therefore concluded that the composite monoliths were the optimal adsorbent for the recovery of precious metals and were therefore used in the subsequent adsorption studies involving simulated and real e-waste samples. The composite monoliths retained a high adsorption capacity for the precious metal ions from simulated e-waste solutions, however low selectivity for the precious metals over the base metals present in solution was seen. All metals in solution were found to be adsorbed by the monoliths...
M.Sc. (Chemistry)