My research interests encompass water treatment, environmental chemistry, material science, and catalysis. My PhD work focuses on understanding the reactivity of per- and poly-fluoroalkyl substances (PFAS) in complex systems. Specifically, I study the impact of heterogeneous suspensions on PFAS degradation and the use of UV irradiation techniques as a potential method to mineralize PFAS after concentration and separation from water. PFAS are a large group of fluorochemicals frequently detected in the environment and treated drinking water. Because of their ubiquity and toxicity, six PFAS are currently regulated in drinking water within the nanogram per liter range in the US. Separation techniques such as membrane filtration and adsorption effectively remove some PFAS from water. However, these separation techniques do not destroy PFAS, potentially creating future liability. I studied the role of adsorbent material properties in the degradation and defluorination of perfluorocarboxylic acids (PFCAs) with UV photoreduction. [1] This work established that, in a heterogeneous suspension, PFCAs desorbed in solution can degrade, but PFCAs adsorbed onto the materials are unreactive. Therefore, we concluded that materials that selectively trap and then release PFCAs can achieve higher PFCA mass destruction in heterogeneous photoreduction. PFAS-impacted water often contains many individual types of PFAS molecules with different physicochemical properties, which can hinder destruction techniques. To make PFAS destruction more feasible, we proposed first utilizing homogeneous treatment to simplify the complexity of PFAS mixtures in concentrated streams, as described in our recently accepted Perspective article. [2] As a result, I am currently investigating the oxidative and reductive species generated in 185 nm UV-irradiated water and their reaction mechanisms with common PFAS types found in real waters. With this mechanistic understanding, I plan to use the 185 nm UV redox cycle as a reagent-free method to pretreat and simplify complex PFAS mixtures towards mineralization. Promising results show the oxidative conversion of fluorotelomers to PFCAs, which can then be reductively defluorinated with high efficiency. Revealing the reaction mechanisms of the UV redox cycle can aid the mineralization of complex PFAS mixtures such as those found in AFFF-impacted groundwater.
My future research aims to use my knowledge in water treatment and environmental organic chemistry to provide solutions through advanced separation and catalysis. I plan to continue working toward efficient PFAS treatment by investigating reagent-free mineralization methods, such as heterogeneous catalysis. Current PFAS destruction catalysts have a high energetic cost, making them unfeasible compared to established thermal treatments. One challenge of PFAS catalysis is that defluorination efficiencies decrease with reaction time due to the formation of recalcitrant byproducts and the decreased degradation rate of short-chain PFAS. My research would focus on identifying defluorination mechanisms for recalcitrant products and increasing the affinity of short-chain PFAS to catalyst surfaces to improve mineralization of PFAS mixtures. I am also interested in tackling the challenge of developing real-time PFAS sensors that can detect and quantify low PFAS concentrations in soil and groundwater by taking advantage of PFAS’s surfactant properties and unique fluorine-fluorine interactions. Another research avenue I want to pursue is understanding the environmental fate, transformation, and risk of novel compounds, including pharmaceuticals and polymeric materials. Using techniques such as high-resolution mass spectrometry (HRMS), we can characterize nontarget transformation products of novel compounds and assess their presence in the environment with field monitoring. Using HRMS, it is also possible to design and develop greener replacement compounds that transform into benign intermediates in environmental and engineered systems.
Teaching Interests
As chemical engineers, we are uniquely equipped to solve complex and interdisciplinary environmental challenges. My goal as an educator is to integrate an environmentally focused perspective into the chemical engineering curriculum to prepare students to address these challenges. As a future faculty member, I will teach my students robust fundamental concepts of mass balance, transport phenomena, reaction kinetics, and process design so they can apply them to any number of environmental problems. As an experimentalist, I value an empirical approach to learning through experimental design, data collection, and interpretation. Therefore, I plan to provide both engaging lectures and hands-on activities to cultivate problem-solving skills, critical thought, and environmental awareness. As a PhD student, I created a lecture and designed laboratory activities based on my research on PFAS water treatment for the undergraduate Environmental Engineering course to involve students in the scientific process. To directly engage students with the content and foster their autonomy, I incorporated small group discussions during the lecture portion. The laboratory activities allowed students to test firsthand which treatments were effective and ineffective in removing PFAS from water. A handout including pre- and post-lab questions encouraged students to practice the scientific method by searching for references, formulating a hypothesis, and thinking further about the results. The final lab report stimulated students to think critically and conceive the environmental implications of their results while practicing their scientific writing and communication skills. Through my teaching and mentorship, I intend to equip and inspire my students to follow their careers as environmentally competent engineers and scientists.
References
[1] Santiago-Cruz, H.A.; Lou, Z.; Xu, J; Sullivan, R.C.; Bowers, B.B.; Molé, R.A.; Zhang, W.; Li, J.; Yuan, J.S.; Dai, S.Y.; Lowry, G.V. Carbon Adsorbent Properties Impact Hydrated Electron Activity and Perfluorocarboxylic Acid (PFCA) Destruction. ACS ES&T Engineering 2024 4 (9), 2220-2233.
[2] Glass, S. and Santiago-Cruz, H.A., et.al. Nature Water. In Press.