报告人： Boya Xiong
The use of non-toxic drag reducing polyacrylamide (~107 Da) as a friction reducer in water
intensive hydraulic fracturing has made massive gas extraction possible in shale plays nationwide and globally. Yet little attention has been paid to the mechanism of chemical and mechanical degradation of polymer under conditions that are unique to deep subsurface. We simulated chemical stress by mixing polymer with mineral shale at elevated temperatures and pressures, with mechanical stresses imposed by passing the polymer through a narrow capillary at high shear rates. Chemically, the generation of reactive oxygen species from Fe2+ in the anoxic shale auto-oxidized by the dissolved oxygen in the fresh fluid lead to molecular size reduction of a factor of 100. The degree of degradation was controlled by temperature and the Fe2+ content relative to alkaline minerals. Oxidants such as persulfate, often added during fracturing, could generate additional radicals that cleave the polymer backbone. In addition to these novel chemical transformation pathways, simulated mechanical shear stress resulted in polymer chain rupture as well, representing a new paradigm for macromolecular degradation in the environment (i.e., mechanochemistry). The final MW of the mechanically degraded polymer decreases with increasing shear rates (γ), roughly following a scaling relationship of MW∝γ-0.77. These previously unappreciated processes can potentially contribute up to 4-5 orders of magnitude reduction in mean molecular weight in deep shales, enhancing the likelihood of releasing neurotoxin acrylamide. In addition to impact on water quality, we also demonstrated the degradation pathways largely influence the fouling index of a microfiltration membrane during wastewater treatment, as polyacrylamide was found to be the dominant contributor of membrane fouling compared to all other chemical additives. These mechanistic findings establish the groundwork for further investigation of mechanisms and conditions that give rise to carcinogenic acrylamide occurrence in high volume wastewater from the energy sector.
Boya Xiong will start her tenure-track Assistant Professor position at the University of Minnesota in 2020 fall, and is currently a Postdoctoral Associate at MIT. Her research centers at the interface of water and polymers, to better evaluate the potential toxic pollutant release from polymer degradation, and to better design an environmentally benign peptide-enabled polymeric material for water treatment. With polymer characterization and environmental analytical chemistry tools, Boya Xiong explores complex polymer degradation science space that remains as a large knowledge gap in the field of environmental organic chemistry. The goal is to enable sustainable design, use and manage of polymer chemicals, materials, and wastes. Boya Xiong received her Ph.D. in 2018 from the Department of Civil and Environmental Engineering at the Pennsylvania State University, where she also received her M.Sc. in the Department of Agricultural and Biological Engineering. She holds a B.Sc. in Biotechnology from the East China University of Science and Technology.