Mobility of platinum and gold in the environment and the roles of microbes
Creators: name: Shar, Sahar
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 310703
subject: Microbial ecology
classificationCode: 4104
subject: Environmental management
classificationCode: 410404
subject: Environmental management

description: Platinum (Pt) and gold (Au) are precious heavy metals that are widely used in the jewellery, automobile, chemical and medical industries. This increased use has resulted in their introduction into the environment at elevated concentrations which can adversely affect both ecosystem and human health. Currently, there is limited information available on the behaviour of Pt and Au in terrestrial environments and their impact on soil microorganisms. There is therefore a need to assess the environmental impacts of increased amounts of Pt and Au contamination. The aims of this study are to study the fate, transport and microbial interactions associated with Pt and Au in Australian soils.<br><br>The first experimental investigation of this thesis was carried out using a bacterium Cupriavidus metallidurans (C. metallidurans), known to significantly biomineralize Au but with limited information available on its interactions with Pt. Therefore, the aim of this research was to assess the ability of C. metallidurans to biomineralize Pt. The results showed that both C. metallidurans and a control microorganism Escherichia coli (E. coli) tolerated platinum (IV) chloride (PtCl4). However, C. metallidurans had a higher tolerance to Pt, with the number of cells surviving at high PtCl4 concentration (10,000 µM) 300-fold more than that of E. coli. Both isolates formed Pt nanoparticles but C. metallidurans showed a higher Pt retention (87%) than E. coli (74%) in sand columns. C. metallidurans was tolerant of Pt and effective at its biomineralization, confirming its suitability for use for further studies on Pt and Au environmental behaviour. <br><br>The interaction of Pt and Au with the biotic and abiotic components of Australian soils is largely unstudied, although metal transformation may lead to the nanoparticles formation. Importantly, little is known about the transportation of Pt and Au nanoparticles in soil. The second scientific investigation evaluated the movement of Pt and Au nanoparticles (AuNPs and PtNPs) in columns containing different matrices (sand only, sand-clay, sand-humic acids and sand-FeO) and soil types, including an organic rich soil, FoxLane (FLN) and an iron rich soil, Jamberoo (JBR). Nanoparticle aggregations with soil particles were observed and Transmission Electron Microscopy (TEM) showed that the nanoparticles formed both homo- and hetero-aggregates in columns. Gold NPs were more mobile (as shown by reduced retention in column) and more reactive (as determined by Fourier-transform infrared spectroscopy (FTIR) analysis) than PtNPs. The rate of movement of nanoparticles based on the breakthrough curves through the various matrices was, in decreasing order of mobility; sand (2 hours) &gt; JBR soil (4 hours) &gt; FLN soil (6 hours) &gt; sand-clay (8-10 hours) &gt;sand-FeO (8-12 hours) &gt; sand-humic acids (12 hours).<br><br>The aim of the third experimental investigation was to assess the effects of different Pt and Au concentrations (1, 25, 100, 500 and 2,000 mg kg-1) on soil respiration and activities of seven key soil enzymes (N-acetyl glucosaminidase, phosphatase, β-D-glucosidase, β-D-cellobiohydrolase, β-D-xylosidase, α-D-glucosidase and arylsulphatase) in a range of Australian soils. The findings from this study showed that the effects of Au and Pt application on the soil microbial activity were related to soil types; (1) a high pH (alkaline) soil (Minnipa, MNP), (2) a low pH (acidic) soil (BrnGrounds, BGR), (3) an iron rich soil (Jamberoo, JBR), (4) an organic matter rich soil (FxLane, FLN), and (5) a high metal/silt soil (Pinpinio, PPN). Platinum or Au at most of the concentrations evaluated (1-2,000 mg kg-1), generally resulted in significant reductions in soil respiration rates in BGR, JBR, MNP and PPN soils. Some reduction in soil respiration was also observed in FLN soils when Pt was applied, while the application of Au increased soil respiration in FLN soils at all concentrations (except 2,000 mg kg-1). The application of Pt inhibited the activities of most of or all the enzymes tested in BRG, FLN, JBR and PPN soils while enhancing the activities of most of the enzymes in MNP soils (1-100 mg kg-1). In contrast, the addition of Au resulted in a significant reduction in most enzyme activities in BGR, JBR and MNP soils while enhancing enzyme activities in FLN and PPN soils. The explanation for these differential results may be a combination of specific enzyme sensitivity, metal concentration and soil type.<br><br>The aim of the final results chapter was to evaluate changes in the bacterial community of four soils (BGR, FLN, PPN and MNP) at three Au and Pt concentrations (1, 25 and 100 mg kg-1) using Next Generation Sequencing tools (NGS). The soil bacterial community was affected by both soil type and metal concentration. While no shift in the dominant groups (Class level) was observed in FLN and MNP soils following addition of Pt and Au with Proteobacteria and Actinobacteria being dominant, other bacterial Phyla/Classes such as Kazan-3B-28, Firmicutes and Caldithrix were selected in BGR and PPN soils. Amendments with Au significantly reduced bacterial community diversity in organic rich BRG soil while amendments with Pt substantially reduced bacterial diversity in the three other soil types. At the Family level, shifts in bacterial community structure were observed in all Pt- and Au- amended samples which were positively correlated with increasing metal concentrations. Bacteria that were selectively enriched only in Pt-amended samples (irrespective of soil type) were identified as belonging to the groups Burkholderiales, Burkholderiaceae, Alicyclobacillaceae, Rubrobacteraceae, Cytophagaceae and Oxalobacteraceae. In Au- amended samples, the bacterial groups that increased in Pt amended samples belonged to Sphingomonadaceae and Rhodospirillaceae. Generally, Au and Pt toxicity was concentration related with Au more toxic to soil bacterial communities at a lower concentration (25 mg kg-1).<br><br>One key aim of this study was to assess the transportation of Pt complexes, Au- and Pt-NPs in different environmental media (soils). Overall, this research has demonstrated that Pt and Au have different rates of transportation in soils which affected their interactions with soil microorganisms and components. This research’s outcomes have important implications in the management (remediation) of Pt and Au-gold contaminated environments. Future research should include the application of NGS to assess Pt and Au effects on microbial function and assays to isolate metal resistant microorganisms for use in bioremediation.<br><br>