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.