Abstract
Fluorochemicals are a persistent environmental contaminant that require specialized techniques for degradation and capture. In particular, recent attention on per- and poly-fluoroalkyl substances (PFAS) has led to numerous explorations of different techniques for degrading the super-strong C-F bonds found in these fluorochemicals. In this study, we investigated the hydrodefluorination mechanism using silylium-carborane salts for the degradation of PFAS at the density functional theory (DFT) level. We find that the degradation process involves both a cationic silylium (Et3Si+) and a hydridic silylium (Et3SiH) to facilitate the defluorination and hydride-addition events. Additionally, the role of carborane ([HCB11H5F6]−) is to force unoccupied anti-bonding orbitals to be partially occupied, weakening the C-F bond. We also show that changing the substituents on carborane from fluorine to other halogens weakens the C-F bond even further, with iodic carborane ([HCB11H5I6]−) having the greatest weakening effect. Moreover, our calculations reveal why the C-F bonds are resistant to degradation, and how the silylium-carborane chemistry is able to chemically transform these bonds into C-H bonds. We believe that our results are further applicable to other halocarbons, and can be used to treat either our existing stocks of these chemicals or to treat concentrated solutions following filtration and capture.
| Original language | English |
|---|---|
| Pages (from-to) | 2085-2099 |
| Number of pages | 15 |
| Journal | Environmental Science: Processes and Impacts |
| Volume | 24 |
| Issue number | 11 |
| DOIs | |
| State | Published - Sep 12 2022 |
| Externally published | Yes |
Funding
Special thanks to Dr Manoj Kolel-Veetil of the Naval Research Lab (NRL) for introducing us to the topic and for introducing us to Prof. Oleg Ozerov and Olivia Gunther from Texas A&M University, who also receive our thanks for conversations regarding PFAS and silylium, and we would also like to thank Drs Michael Roth, Timothy C. Schutt, and Caitlin G. Bresnahan for their comments and criticisms of the current manuscript. The use of trade, product or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. The tests described and the resulting data presented herein were obtained from research conducted under the Installations and Operational Environments Program of the United States Army Corps of Engineers by the USAERDC under the work supported by the United States Office of the Undersecretary of Defense (Research and Engineering) (OUSD(R&E)) through the Applied Research for the Advancement of Science and Technology Priorities (ARAP) program. Permission was granted by the Chief of Engineers to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. Authors also acknowledge grant of computer time from the DOD High Performance Computing Modernization Program at ERDC, Vicksburg MS.