Brian Beukema, Pincus Lab & Mary Elias, Cahill Group, Graduate Students, GW Department of Chemistry
Friday, April 17, 202610:00 am - 11:00 am
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Brian Beukema will discuss: Environmental Degradation of Plastics: Influence of Weathering on Low-Density Polyethylene
An estimated 52 million metric tons (Mt) of plastics are released into the environment yearly through mismanaged waste streams resulting in plastics being among the most widespread pollutants globally. Given the ubiquity of plastic pollution, a systematic and predictive understanding of their environmental fate is critical. Many laboratory-based studies have examined how singular environmental variables (i.e. pH, salinity, UV exposure) affect plastic degradation but fail to completely encapsulate the effects of natural systems. Field-based observations of plastic pollution offer key insights quantifying fluxes of microplastic pollution between different environments, but lack of knowledge of the age of plastic pollution makes determination of degradation rates and mechanisms challenging. Here we present data from a two-year field investigation into the effects of different environmental parameters (e.g. pH, salinity, and oxygen availability) for one of the most ubiquitous types of plastic pollution, low-density polyethylene (LDPE). Spectroscopic techniques including ATR-FTIR was used to examine photo-oxidative weathering as a function of environmental parameters (e.g. pH, salinity, terrestrial vs aqueous conditions). X-ray absorption near edge structure (XANES) and micro X- ray fluorescence microscopy (µ-XRF) were used to examine accumulation of inorganic species on the surface of weathered LDPE. This work provides novel mechanistic and kinetic insights into how LDPE photo-oxidizes and degrades as a function of natural environmental conditions.
BIO
Brian earned his B.S. in Geology at Western Michigan University, and his M.S. in Environmental Metrology and Policy at Georgetown University focusing in per- and polyfluoroalkyl substance (PFAS) detection at part per trillion concentrations using HPLC-MS. Before coming to GWU he was a member of the Trace Element Analysis Core at Dartmouth College where he utilized ICP-MS (and other hyphenated ICP-MS techniques) for elemental analysis of environmental and biological samples. Since joining the Pincus lab in Fall 2024, he has been using spectroscopic techniques to understand the surface chemistry of environmentally weathered plastics and their interactions with inorganics.
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Mary Elias will discuss: Exploring the Structure and Photoreactivity of Uranyl-Bearing Materials
Uranyl (UO22+) containing materials typically exhibit a green luminescence profile resulting from ligand-to-metal charge transfer transitions between uranyl bonding and non-bonding molecular orbitals. Moreover, vibronic coupling with Raman-active uranyl symmetric stretching mode (ν1) gives rise to characteristic ‘fingered’ spectra, the energies of which are quite sensitive to coordination environment, making luminesce spectroscopy a useful probe of electronic structure and bonding. Beyond the relevance to bonding studies and spectroscopic utility, uranyl photoexcitation promotes photooxidation reactions in both solution and the solid-state, enabling applications in catalysis, degradation of dyes/organic pollutants, and radiation detection devices. In solution, photoexcited uranyl is well known to act as a strong oxidizing species capable of hydrogen abstraction and radical generation, while being reduced, but uranyl photoreacitivity in the solid-state remains far less understood. More recently, the solid-state photoreactivity of uranyl complexes has been studied, with luminescence experiments revealing an unusual mechanism in which upon photoexcitation, the uranyl tetrachloride anion [UO2Cl4]2- is oxidized rather than reduced, showcasing a fundamental difference between solution and solid-state photochemistry. Building upon this, we investigate how noncovalent interactions (NCIs) influence uranyl photophysics and reactivity by engineering halogen bonding interactions between halogenated organic cations and uranyl tetrachloride anions. We have prepared a series of novel uranyl-viologen compounds featuring X…Cl interactions as characterized by single crystal X-ray diffraction. Upon UV irradiation, all the materials exhibit time-dependent quenching of uranyl luminescence, consistent with radicalization and photoinduced electron transfer processes. Our results show a correlation between the extent of quenching and halogen bond characteristics: increased van der Waals overlap corresponds to reduced luminescence quenching, suggesting that stronger halogen bonding modulates electronic coupling and inhibits charge transfer pathways. These findings establish halogen bonding as a tunable handle for modulating solid-state uranyl photoreactivity, and this behavior is further supported by related (UO2Cl4)(halo-pyridine)2 systems, where NCIs similarly direct solid-state assembly and influence photoreactivity trends. In parallel, direct coordination of various redox-active organic ligands to the uranyl chloride unit also displays photoreactivity in the form of luminescence quenching over time upon UV irradiation, and the energy/electron transfer pathways in these compounds are currently being explored. Overall, these results demonstrate that bonding motifs and second-sphere interactions – particularly halogen bonding – provide a tunable strategy for controlling solid-state uranyl photoreactivity and a platform for learning more about these photoinduced processes.
BIO
Mary earned her B.S. in Chemistry at Seton Hall University in May 2024. Since joining the Cahill Research Group in Fall of 2024, she has been working on synthesizing uranyl-bearing materials for exploring photoreactivity and structure-property relationships.
