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(2021)ThesisCellulose is the most abundant polymer on earth. Its use within the bioenergy and bio-materials sector to provide raw feedstock molecules is critical to supplant petrochemical-derived resources. The alkaline carbohydrate fuel cell has recently been of growing interest as it has been shown to produce power directly from monosaccharides without further breakdown and combustion steps that introduce substantial energetic losses. The development of the direct alkaline carbohydrate fuel cell is currently in its infancy, with developments needed to refine understandings of cell geometry, charge mediation and industrial application pathways. The findings of this thesis represent contributions to these areas, demonstrating greater mechanistic understanding of these devices and new pathways forward. Mechanistic insights were gained through examining simple geometric design considerations that proved capable of significantly improving fuel cell performance. Decreasing the distance between electrodes from 20 to 6 mm increased power outputs by ~35 % and increasing the density of the nickel foam anode from 250 mg cm3 to 1000 mg cm3 increased power outputs by ~30 %. Further, indigo carmine was found to be unstable in highly alkaline solutions. Breakdown of the dye produced significant amounts of current without any carbohydrate present, calling into question the previously reported relationship between the indigo carmine concentration and power generation within an alkaline carbohydrate fuel cell. As pure carbohydrate fuels may raise ethical concerns about food security, new organic fuels were proposed and new cellulose to energy pathways examined. III A novel low-temperature hydrothermal cellulose degradation process was developed to create new cellulose-derived alternatives. The process converted over 61 % of microcrystalline cellulose to soluble aldaric acids whilst simultaneously producing value-added magnetic nanoparticles. Finally, a new novel fuelling approach was reported in which the selected fuel was reverse-engineered from the desired oxidation products. Within the fuel cell, the oxidation pathway between 5-HMF and FDCA was exploited to generate 6.5 x the power of the equivalent glucose fuel cell, whilst synthesising high-value molecules known to be useful for bioplastics. As the scale of bio-molecule production increases, this passive energy generation scheme presents an opportunity to produce significant power from an otherwise untapped chemical process, increasing the green chemical industry’s energy efficiency. Overall, this thesis entails optimisations towards enhancing energy production within the alkaline organic fuel cell and explores new novel applications which broaden the scope of these green electricity-generating devices.
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(2021)ThesisDrought is one of the most complex hydroclimatic extremes, with enormous effects on the environment, economy, and society. In many locations, climate change is expected to increase the severity and frequency of droughts. However, most climate change impacts to date focus on relatively short-duration droughts (e.g., 12 months) and do not consider the evolution of droughts in terms of time or space. This means that evaluation of climate models and projections based on those same climate models are limited to point-by-point analyses with no information on drought evolution and decay and the future changes in these attributes. The multi-scale nature of droughts which can last over multiple years is also not considered in many climate change impact assessments which use fixed temporal windows for analysis. This thesis proposes new methods to address these issues and applies the new methods to analyse future drought across Australia with a specific focus on the spatial and temporal evolution of drought historically and in the future in the Murray Darling Basin (MDB). The main methodological contribution of the thesis is a new multi-time scale drought index termed the Residual Mass Severity Index (RMSI). The RMSI improves on the existing drought indices because it does not require a temporal window to be specified to characterize extreme deficit periods and these periods can be automatically identified. The RMSI is then used to evaluate the performance of general circulation models (GCMs) in characterizing historical drought. There is a reasonable consistency in GCMs in terms of representing drought frequency but only a few GCMs exhibited acceptable skill in capturing drought recovery and peak magnitude of drought. Spatiotemporal drought evolution was then characterized in the MDB. The areas worst affected by drought tend to experience faster build-up and slower recession than the basin as a whole. Finally, spatiotemporal changes in drought characteristics in MDB represented by GCMs in the current and future climate were assessed. Overall, an increase in drought duration, severity, and time to recede are projected for the future.
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(2021)ThesisDue to the high toxicity and ubiquitous presence of inorganic arsenic (As) in groundwaters, there have been a number of studies investigating the removal of inorganic arsenic though most of the existing technologies are constrained by low energy efficiency and ineffectiveness in As(III) removal under circumneutral pH conditions. In this work we present proof-of-concept of a modified double potential step chronoamperometry (DPSC) method which achieves in-situ As(III) oxidation and concomitant electro-sorption of As(V) onto the electrode. The use of both carbon cloth and redox active polyvinylferrocene (PVF) functionalized carbon nanotube electrodes are investigated in this work. Our work with the carbon cloth electrode showed that in-situ anodic As(III) oxidation and concomitant sorption of As(V) formed was achieved at an applied voltage of 1.2 V. Our results further showed that the sorbed As(V) was effectively electro-desorbed by reversing the polarity thereby regenerating the electrode. The in-situ anodic As(III) oxidation, sorption of As(V) and desorption of As(V) are affected by aqueous pH with high oxidation and sorption/desorption rates observed at elevated pH. The increase in As(III) oxidation and concomitant As(V) adsorption at higher pHs is related to (i) the rapid oxidation of the deprotonated species compared to the protonated species and (ii) stronger electrochemical interaction between the multi-charged As(V) species and the electrodes. At 1.2 V and an electrical energy consumption of 0.06 kWh m−3, the total As concentration can be reduced from 150 to 15 µg L−1 using an electrochemical cell with electrode area of 10 cm × 8 cm and electro-sorption time of 120-min. Based on the experimental results, we have developed a mathematical model to describe the kinetics and mechanism of arsenic removal by the modified DPSC method using the carbon cloth electrode with this model of use in predicting, and potentially optimising, process performance under various conditions. While the carbon cloth electrode was effective in As removal in the absence of competing anions, in the presence of anions such as Cl−, SO42− and NO3−, the As(III) removal efficiency by the carbon cloth electrode was very low. In order to improve the selectivity of As over competing anions that are typically present in groundwaters, we investigated the sorption of arsenic from simulated groundwaters by a redox active PVF functionalized electrode. Our results show that effective and sustainable As(III) removal was achieved even at 0 V once the electrode is activated via anodic polarization. During activation, ferrocene (Fc) in PVF is oxidized to the ferrocenium ion (Fc+) with the latter facilitating As(III) sorption and subsequent oxidation as well as As(V) sorption. The high affinity of Fc+ to As and weak attraction to competing anions at 0 V ensure high selectivity of As over Cl−, SO42− and NO3− at concentrations typical of groundwaters. Following the removal process, efficient regeneration of the electrode is achieved at −1.2 V wherein Fc+ is reduced to Fc thereby facilitating As desorption from the electrode surface. Our results further show that O2 and associated generation of hydrogen peroxide (H2O2) during the electrode regeneration step drives the oxidation of Fc to Fc+, thereby maintaining the constant generation of Fc+ required to achieve As(III) removal in subsequent cycles. Our results show that 91.8% 0.6% of As(III) could be selectively removed from a simulated groundwater over 10 cycles at an ultralow energy consumption of 0.12 kWh m−3. We also investigated the influence of divalent cations (i.e. Ca2+and Mg2+) on the removal of As(III) by a PVF functionalized electrode. Our results show that in the absence of divalent cations, nearly 90% ± 0.9% of As(III) removal is achieved over ten continuous cycles by single-pass DPSC, even in the presence of competing anions (5 HCO3−, 3 mM Cl−, 0.5 mM NO3−, 2 mM SO42−) however the presence of divalent cations (Ca2+ and Mg2+) significantly inhibits electrode regeneration. While the presence of Ca2+ and Mg2+ facilitates As(III) removal in the 1st step of the DPSC, the regeneration of the electrode in the 2nd step of DPSC is significantly inhibited, thereby decreasing the As(III) removal in the subsequent cycles. Based on the results from various control experiments and surface characterization, it appears that Ca2+/Mg2+ either acts as a bridge between the electrode surface and As anions or the sorption of Ca2+/Mg2+ increases the positive charge on the electrode surface thereby facilitating the sorption of As. The Ca2+/Mg2+ assisted sorption occurs even during the electrode regeneration process, thereby decreasing the electrode regeneration efficiency and As(III) removal in subsequent cycles. Overall, the results show that the DPSC method with either carbon cloth or PVF functionalized electrodes achieves in-situ As(III) oxidation, concomitant sorption of As(V) and desorption of As(V). However, the adverse impact of the presence of divalent cations (Ca2+ and Mg2+) on electrode regeneration and thus the efficacy of the As removal process must be taken into account. Future work should focus on i) optimization of the configuration of the system in order to minimize the influence of divalent cations on regeneration of the electrodes and ii) scale up of the unit to a size that is meaningful for practical application. Additionally, development of more stable PVF electrodes and/or alternate electrode materials for better removal efficiency is also required.