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    High Performance Computing for Pore Scale Design of Flow-Through Electrodes: A Water Desalination Application
    (University of Waterloo, 2026-09-21) McKague, Michael
    A growing population and environmental changes continue to threaten human development and supply of water and energy resources. Electrochemical devices are well positioned to solve many of these challenges due to their ability to store or use energy from renewable sources. Capacitive Deionization (CDI) is an example of such a device that uses electricity to desalinate water. It works by storing ions such as sodium and chloride from brackish water inside the electrical double layers of microporous carbon. Electrochemical devices such as CDI come in a variety of different architectures or flow configurations. Flow-through CDI is one example where water flows through the macropores of the activated carbon or other electrode. The advantage is the fast salt adsorption rates, but the disadvantage is the pumping power required to force fluid through the electrode’s pores. To mitigate pressure losses, hierarchical design of the pore space must be considered carefully. Pore scale models are an excellent route to study and optimize microstructure for flow-through electrodes. One of the challenges with optimization, however, is how computationally demanding it can be. Therefore, much of this thesis is focused on laying the foundation for optimization by applying high-performance computing to the pore scale including GPU accelerated solvers, JIT compilation, and automatic differentiation. The first two works are focused on efficient construction of pore networks from images or experimental data while the final work applies GPU and JIT accelerated solvers to better understand transport around engineered channels for flow-through CDI. This thesis explores two different methods for constructing pore networks that reliably characterize properties important for flow-through electrodes. In the first method, a modern and open-source implementation of the classic medial axis network extraction was written. It featured finding pores along long throats, a parallelized skeleton by chunking, and a fast walker method finding equivalent throat diameter. The new network extraction was tested on a 400³ image of Berea sandstone and the permeability predicted was within 5% of the lattice Boltzmann permeability. A speed-up of 4.2× compared to the watershed segmentation was observed. The second method calibrates a pore network to experimental data using gradient-descent optimization. The accessibility of automatic differentiation in increasingly popular machine learning packages was leveraged to write a fully differentiable pore network, suitable for gradient descent. This approach was used to match porosimetry and permeability data of a Berea sandstone in which a final combined loss of 9.7 × 10⁻⁴ was achieved. The optimization took roughly 20 minutes for a 10³ network of pores. Finally, both methods for constructing a pore network were evaluated and the second method was chosen to construct a pore network of a porous activated carbon electrode because no volumetric image was available. A pair of pore networks were fit to the available experimental data representing the entire range of pore sizes from 50 nm to 10 μm. The pore network was used to simulate ion transport in the pore space surrounding channels engineered for flow-through CDI. The pore network model was written entirely in JAX to access GPU-accelerated and JIT-compiled solvers for electrochemical simulation in pore networks. The computation time of JAX’s iterative implicit solvers were compared to PyPardiso’s direct solvers (after factorization) and results showed a speedup of 23× and 14× for mass and charge transport respectively. The effect of hole size on CDI cell performance was studied and a final recommendation for smaller holes (~30 μm) spaced 100 μm apart was modelled and 17× faster salt adsorption was observed.
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    Groundwork Gallery: A New Materialist Analysis of Arts-based Community Programming for the Building of Sociopolitical Consciousness
    (University of Waterloo, 2026-09-21) Ali, Aisha
    My thesis is an attempt at bridging community programming with broader anti-imperialist pedagogy in addition to academia. I use a New Materialist lens to evaluate my community project, Groundwork Gallery, which provided arts-based anti-imperial education to high school students from marginalized backgrounds. I received a grant from the city of Kitchener and used this to run a six week program biweekly. Students were mentored by six young adults from various artistic fields as they worked on creating their own pieces for an installation held at the Kitchener-Waterloo Art Gallery. As an educator and anti-imperialist, the project was informed by Paolo Freire’s Pedagogy of the Oppressed, to create a site for learning guided by the oppressed and a desire for liberation through the process of praxis: action and reflection in tandem. As both researcher and program lead, I was drawn to the way the multiple networks overlapped at the workshops, that they acted as sites for community development, anti-imperialist education and research. Using a New Materialist lens I address the ways various networks intersect and overlap to position the program as not only informed by the research process but created by the overlap of academia with the broader community. The reflective process created by the data collection required for my thesis informed not only my own experience with the workshops but that of the participants as well, enhancing the action-reflection dialectic required for Freire’s notion of true praxis. I argue that the overlap of the research assemblage and the workshop assemblage created a new site that enhanced the liberatory capacity of the space. I begin with an introduction on the theoretical background of both the project and the New Materialist lens used to assess the project. I then offer three student case studies, presenting their interview responses with my own reflections and their final pieces. I conclude by summarizing participant responses in interviews and focus groups in tandem, again, with my own reflections, before providing insight into the ways community programming and research can both be enhanced by acting as a joint entity rather than separate in community-based research. Programming run by researchers has enhanced capacity for participants as well as researchers by providing stronger connections with participants and subsequently an enhanced capacity for data-collection and insight, rather than the fly-on-the-wall approach that is typically employed.
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    Nutrient Dynamics in Prairie Stream Systems: Patterns, Drivers, and Stoichiometric Implications Across Spatial and Temporal Scales
    (University of Waterloo, 2026-09-21) White, Amy
    Riverine nutrient dynamics influence water quality, ecosystem processes, and nutrient export to downstream ecosystems, yet the processes that shape nitrogen (N) and phosphorus (P) concentrations and their stoichiometry remain poorly understood in many regions. Prairie rivers are particularly understudied, as their strong seasonality, intensive agricultural land use, and episodic hydrologic connectivity may generate stoichiometric patterns that differ from those observed in more studied temperate systems. This thesis examines how nutrient concentrations, loads, and N:P ratios vary across spatial and temporal scales in Prairie stream ecosystems, and how landscape characteristics, hydrologic conditions, and episodic nutrient inputs shape these patterns. To address this goal, I conducted three studies spanning basin-, tributary-, and segment-scales. In Chapter 2, I analyzed 25 years of water quality data from river monitoring stations across the Red-Assiniboine Basin to determine long-term trends in total and dissolved nutrient fractions and their stoichiometry, and to quantify the relative influence of changing catchment characteristics. In Chapter 3, I assessed seasonal spatial patterns in nutrient concentrations and stoichiometry across Red River tributaries, evaluated whether interannual variation in nutrient ratios coincided with shifts in dominant crop types, and examined how long-term seasonal discharge patterns influenced total and dissolved nutrient ratios. In Chapter 4, I quantified the magnitude and distribution of nutrient capture during a 15-day, high concentration wastewater effluent release in Devils Creek, using a mass balance approach to determine the amount of N and P captured and exported along a 17-km stream segment. In Chapter 2, long-term monitoring across the Red-Assiniboine Basin showed that N and P concentrations and stoichiometry were changing through time at most stations, but trends were spatially heterogeneous with no consistent directional shifts across the basin. Localized nutrient sources were important drivers of these patterns, and changes in catchment characteristics contributed to the observed trends to a greater extent than changes in streamflow conditions. In Chapter 3, total N:P ratios were near the Redfield mass ratio across 24 tributaries, whereas dissolved N:P were strongly N-depleted and declined from spring through autumn. Crop type did not consistently influence nutrient ratios, but long-term seasonal discharge was a driver of spring and summer stoichiometry. In Chapter 4, despite elevated nutrient loading from a wastewater effluent release, capture of N and P in Devils Creek was substantial, with 80 % of P captured and 100% of N captured. The findings of this thesis demonstrate that nutrient dynamics in Prairie rivers are influenced by landscape characteristics, hydrologic connectivity, nutrient sources, and episodic inputs. Across multiple spatial and temporal scales, dissolved and total nutrient fractions differed in their absolute values, ratios, and trends, demonstrating that the choice of nutrient fraction is central to interpreting stoichiometric patterns and nutrient depletion in Prairie rivers. The spatial heterogeneity observed across rivers, tributaries, and stream segments indicates that stoichiometric patterns in Prairie rivers emerge from localized nutrient sources, variable connectivity, and strong in-stream processing. Overall, this thesis underscores the need for management approaches that incorporate the sources of variability that govern nutrient dynamics in Prairie rivers.
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    Impact of Sub-Boiling Temperatures on Mass Transfer from Former Manufactured Gas Plant Residuals
    (University of Waterloo, 2026-09-21) Wei, Yunxiao
    The dissolution of polycyclic aromatic hydrocarbons (PAHs) from coal tar residuals at former manufactured gas plant (FMGP) sites constitutes a long-term groundwater contamination challenge. The mass transfer of PAHs from the non-aqueous phase liquid (NAPL) to the aqueous phase is often rate-limited, particularly as weathering increases NAPL viscosity and reduces the availability of contaminants for remediation technologies that rely on aqueous-phase treatment, such as in situ chemical oxidation (ISCO). This research investigated the potential of sub-boiling temperatures (from 25 to 80 °C) to enhance the mass transfer of PAHs from FMGP residuals. The investigation commenced with a series of batch experiments conducted in well-mixed aqueous-phase reactors, where the NAPL was held stationary at the base of the vessel to evaluate the impact of sub-boiling temperatures on the physicochemical properties governing PAH mass transfer. Increasing temperature from 25 to 80 °C elevated the effective solubility of target PAHs by up to an order of magnitude, with the most pronounced enhancement observed for higher molecular weight compounds. The overall mass transfer coefficient increased by factors of up to 45. This dual enhancement of both the driving force (solubility) and the rate coefficient yielded calculated maximum mass fluxes that, based on these measured parameters and assuming zero aqueous-phase concentration, could increase by up to 475 times under the experimental condition. The observed enhancement was greater for an actual FMGP residual relative to a synthetic PAH mixture. This disparity was primarily attributed to a substantial, temperature-driven reduction in the FMGP NAPL viscosity by a factor of ~17, which facilitated a transition from intra-NAPL diffusion-limited to water-phase controlled mass transfer. Screening-level simulations, constrained by the experimental data, illustrated that such enhancements could dramatically increase contaminant mass removal. For instance, after 120 days of heating to 80 °C, the remaining mass of naphthalene (NPH) was projected to be only 6% of that after 120 days of heating at 25 °C, with even greater relative removal achievable at higher groundwater velocities. These results provide clear evidence that heating FMGP residuals to sub-boiling temperatures improves NAPL-water mass transfer. The batch-scale findings were extended through physical flow-through chamber experiments, in which an FMGP residual was emplaced in a fixed reservoir and continuously flushed with temperature-controlled water, with periodic introduction of permanganate pulses to further quantify the impact of temperature on mass transfer. In the control experiments without the periodic addition of permanganate in the injection solution, increasing the temperature from 25 to 80 °C elevated the mass transfer coefficient for NPH by approximately one order of magnitude. Elevated temperatures (80 °C) significantly accelerated the dissolution of soluble PAHs from the NAPL, resulting in an early sharp increase and subsequent rapid decline in effluent concentrations, which was assumed to be a result of the increase in the mass transfer driving force (effective solubility) and the rate coefficient. In the 80 °C experiments, preferential depletion of soluble components reduced the NPH mole fraction from 0.317 to 0.034 within 24 pore volumes, leaving a highly viscous and physically altered residual. This physical transformation coincided with a sharp decline in mass transfer rates, providing direct evidence that the rate-limiting step shifted from interfacial mass transfer to intra-NAPL diffusion. In contrast, the role of the intermittent permanganate addition to the injection solution showed no significant effect on the overall PAH depletion at 25 °C. However, at 80 °C, the addition of permanganate yielded a further enhancement to mass transfer beyond that attributable to temperature alone. This additional effect was consistent with physical disturbance of the NAPL-water interface, potentially from gas generation during oxidation, which likely increased the effective interfacial area available for dissolution. To explore the laboratory mechanistic insights under field conditions, a demonstration project was conducted at an FMGP site in Hefei, China, implementing and comparing thermally enhanced ISCO (TECO), persulfate ISCO alone, and in situ thermal treatment (ISTT) within a sheet-piled 1723 m2 area. The thermal system established a stable sub-boiling temperature field (60 ~ 70 °C) within the TECO demonstration parcels. A dedicated investigation was conducted to quantify temperature-dependent mass transfer rates. Five monitoring wells were installed at increasing distances (0.5 to 2.5 m) from an active heating well, creating a thermal gradient with average temperatures ranging from 60 °C at the closest well to 25 °C at the farthest. After purging each well and replacing native groundwater with deionized water, PAH concentration rebound was monitored daily for 14 days under diffusion-controlled conditions, enabling mass transfer rates to be inferred from the concentration recovery data. Application of a radial diffusion model, incorporating NAPL dissolution as a source term, to these data yielded lumped mass transfer coefficients (K), which increased by an average factor of 5.7 between 30 and 60 °C. A paired t-test comparing the increase in K to the concurrent increase in effective diffusion coefficient (D) between 30 and 60 °C, confirmed that the enhancement in mass transfer was statistically greater (p < 0.05) than could be explained by accelerated aqueous-phase diffusion alone, indicating that the primary benefit of heating extended beyond simple transport enhancement in the water phase. Within the TECO demonstration area where the thermally enhanced treatment was implemented, the TECO parcels achieved >99.9% removal of PAHs from soil, with concentrations reduced to below method detection limits and no observed rebound in groundwater. In the standalone ISCO parcel, removal of higher molecular weight PAHs from soil was limited and groundwater concentration rebounded following treatment, despite a higher oxidant dosage. Consistent with the temperature-dependent mass transfer rates quantified along the thermal gradient, the greater depletion of higher molecular weight PAHs in the TECO parcels reflects the enhanced mass transfer achievable under sub-boiling conditions. This research provides evidence that sub-boiling temperatures can substantially increase mass transfer from FMGP residuals, with the greatest benefit realized for higher molecular weight compounds that are otherwise rate-limited at ambient temperatures. The enhancement observed during systematic laboratory investigations and corroborated by the results of the field study provides a mechanistic basis for improving in situ treatment strategies that rely on aqueous-phase delivery. TECO as evaluated in the field demonstration, represents one example of how sub-boiling temperatures can be leveraged to increase contaminant availability for subsequent treatment.
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    Inside the Crypto Laundering Machine: A Taxonomic Study of Crypto Money Laundering Schemes and Countermeasures
    (University of Waterloo, 2026-09-21) Sarkhosh Sarkendi, Hesam
    The rise of Web3 and Decentralized Finance (DeFi) has enabled borderless access to financial services empowered by smart contracts and blockchain technology. However, the ecosystem’s trustless, permissionless, and borderless nature presents substantial regulatory challenges. The absence of centralized oversight and the technical complexity create fertile ground for financial crimes. Among these, money laundering is particularly concerning, as in the event of successful scams, code exploits, and market manipulations, it facilitates covert movement of illicit gains. Beyond this, there is a growing concern that cryptocurrencies can be leveraged to launder proceeds from drug trafficking, or to transfer funds linked to terrorism financing. Crypto money laundering remains diverse and insufficiently systematized, with prior work fragmented, dated, or limited to individual vectors. We address this through a systematic, interdisciplinary review of academic, regulatory, and industry sources. We first establish the necessary background: how the classical placement, layering and integration stages of laundering adapt to Web3, the pseudonymity that underpins these schemes, and the fact that tracing cryptocurrency transactions requires access to data traces that come in varied forms. Building on this foundation, we consolidate known schemes into a taxonomy of money laundering in Web3, organizing laundering into a set of high-level obfuscation strategies and the intermediary mechanisms through which they are realized, while also identifying emerging schemes not previously documented. We systematize these intermediary mechanisms along several dimensions, including the intent behind their design and whether they are purpose-built privacy tools or exploited infrastructure, their custodial or non-custodial nature, their deployment layer, and their underlying privacy techniques. We then turn to the countermeasures raised against these schemes, spanning both detection, which uncovers illicit activity after it occurs, and prevention, which limits the opportunity for laundering in the first place, as pursued across academia, regulation, and industry. From this synthesis we draw out the open challenges that remain, and chart future research directions toward a more transparent Web3 financial ecosystem, offering insights for researchers, policymakers, and practitioners.