School of Chemistry - Theses
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ItemPhytoextraction of mercury (Hg) and gold (Au) from contaminated mine tailings and biosolids( 2018)The present study is based on a long-term goal of using phytotechnologies to rehabilitate Hg-contaminated substrates like mine tailings and biosolids with the additional value of recovering Au residues. To address this goal, this research aimed to (1) to develop a Hg-Au phytoextraction system employing the best substrate-plant combination by examining the growth and metals uptake of selected plant species on substrates consisting of biosolids, Au mine tailings, or different combinations of both, and (2) to provide in-depth information on the spatial distribution and localization of Hg and Au in the plant root tissues in order to present mechanistic hypotheses for their phytoextraction potential. An extensive survey of literature has been conducted on Hg phytoremediation and Au phytoextraction, as well as the challenges and limitations in this field. The reviewed studies demonstrate that no naturally-occurring Hg or Au hyperaccumulators have been recorded to date. A glasshouse-based screening study was done to examine the growth of candidate plant species, known for their ability to phytoextract Hg and/or Au using chemical amendments, which can grow on substrates consisting of biosolids-amended mine tailings. The germination and establishment of plants over 8-12 weeks were monitored for Brassica juncea (Indian mustard), Daucus carota (carrot), Lupinus albus (white lupin), Beta vulgaris (sugar beet), Solanum tuberosum (potato), and Manihot esculenta (cassava). The most suitable biosolids-mine tailings combination was determined to be 75% biosolids – 25% mine tailings. Of the 3 successfully established plant species—mustard, carrot, and cassava—the latter showed the most promise in terms of ease in propagation and its cyanogenic capability. The potential of M. esculenta to phytoextract Hg and Au was successfully demonstrated for the first time. Metals uptake was found to be greatest in the fibrous roots of plant cuttings grown in Hg- and/or Au-amended hydroponics solutions. A plausible competitive metal effect was observed in plants grown in equal Hg and Au concentrations, where less Hg was taken up by the plants as compared to when the plants were exposed to Hg only. Increasing the Hg concentration while keeping the Au concentration constant substantially increased Hg uptake whilst decreasing Au uptake. The micro-PIXE analysis affirmed these results and gave a clear insight into the distribution of the metals in the roots. Though Hg and Au were found in all parts of the root cross-section they were mainly localized in the vascular bundle when plants were treated with each metal individually. Exposure to equal Hg and Au concentrations revealed both elements to be localized only in the epidermis, thereby suggesting a competition between Hg and Au. High-resolution transmission electron microscopy and X-ray diffraction measurements revealed that Au nanoparticles were formed inside cassava root tissues. Results also indicate that the presence of Hg increases the size of the AuNPs formed. As of date, there have been no studies linking cyanogenesis with the ability of cassava to hyperaccumulate Hg and Au. Preliminary data from Matrix Assisted Laser Desorption Ionisation Mass Spectrometry Imaging suggests that the differential localization of the cyanogenic glucoside linamarin in the root tissue sections from controls and metal treatments might be involved in Hg and Au uptake.
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ItemSurface plasmon spectroscopy of redox processes on single gold nanocrystals( 2016)Gold nanocrystals (Au NCs) are important materials for catalysis, sensing and photonics. Au NCs exhibit strong scattering signals in the visible and near infrared range due to localised surface plasmon resonances. Due to statistical averaging, when measurements are made on ensembles of particles, the precise determination of the effect of gold nanocrystal size, shape and local environment on specific application performance is not feasible. This ensemble problem is overcome by applying a combination of dark field imaging with surface plasmon spectroscopy, enabling the scattering spectra of individual nanocrystals to be measured. This approach allows changes in the electron concentration of a single Au NC to be observed via localised surface plasmon resonance shifts. In this thesis, the dark field microscopy technique has been expanded to study single gold nanocrystal electrodeposition, gas-phase adsorption, photoreduction, and solid-state charging. The first key objective was to understand how nanocrystal morphology and surface properties influence underpotential deposition. The second key objective was to understand how metal oxide supports influence charge transport during hydrogen adsorption and photoexcitation. These studies clearly demonstrate that the above parameters are crucial to the electrochemical and catalytic properties of Au NCs. By performing the measurements on single nanocrystals, the chemical kinetics and charging rates could be uncovered with detail never before achieved at this scale. In addition to using standard dark field microscopy techniques in this work, an upgraded laser illuminated dark field system was developed to optically reveal single particle charging rates by measuring electron transfer in real-time. The added sensitivity of this new approach has enabled the optical detection of fewer than 150 electrons as they are transferred to a single gold nanorod. In order to characterise the performance of the laser system, a reliable and reproducible method to rapidly charge single gold nanocrystals was developed. Au NCs were integrated in an ion gel capacitor, enabling them to be charged in a solid, transparent and highly capacitive device, ideal for transmission microscopy.
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ItemThe use of polymer inclusion membranes (PIMs) for the recovery of gold(III) from highly acid solutions and the preparation of monolayers of precious metal nanoparticles( 2014)Polymer inclusion membranes (PIMs) have recently gained importance in solid phase extraction of both metallic and non-metallic species. The suitability of PIMs for gold recovery from acid digestion of electronic scrap was studied. PIMs containing Aliquat 336 or Cyphos® IL 104 incorporated in different polymer matrixes were studied for Au(III) extraction and back-extraction. The membranes containing 30 wt% Cyphos® IL 104 and 70 wt% PVDF-HFP has shown high Au(III) extraction rate and stability in aqua regia solution used for the digestion of electronic scrap. It was found that Na2SO3 could strip Au(III) efficiently from these PIMs. PIMs have been used as templates to prepare gold nanoparticles (Au NPs), silver nanoparticles (Ag NPs) and palladium nanoparticles (Pd NPs). [AuCl4]- was extracted into the membrane (20 wt%) Aliquat 336, 10 wt% 1-dodecanol and 70 wt% PVC) and subsequently reduced by L-ascorbic acid, tri-sodium citrate, NaBH4 or EDTA to form Au NPs. EDTA at pH 6.0 has been shown to be an effective reducing agent capable of forming a uniform monolayer of Au NPs of average size of 20 nm on the surface of the membrane. Similarly Ag+ or Pd2+ were extracted into PVC-based PIMs containing 45 wt% or 30 wt% D2EHPA, respectively, and subsequently reduced with different reducing agents. L-ascorbic was found to form nanoparticles on the surface with relatively uniform size of 360 nm for Ag and 38 nm for Pd. One hypothesis views PIMs as incorporating a network of nanosized channels. However, the use of surface imaging techniques has not clearly identified surface pores. A novel approach using the preparation of Au NPs on the membrane surface was proposed to study surface pore distribution of PIMs. Scanning Electron Microscopy (SEM) has been used to map the surface distribution of the Au NPs assuming that it is identical to the pore distribution.