Taking the “forever” out of forever chemicals using zinc sulfide
Mabel L. Day a , Logan C. Myers b, Ian A. Gass a, Rachael K. Matthews a, Harrison J. McAfee a, Mahmoud A. Hamza a, Alexander J. Keltie a, Jhayden Jarman a, Yoichi Kobayashi c, Tak W. Kee a, Christopher P. Higgins b, Ryan M. Richards b, Cameron J. Shearer a*
a Department of Chemistry, School of Physics, Chemistry and Earth Sciences, Adelaide University, SA 5005, Australia. b Department of Chemistry and Civil and Environmental Engineering, Colorado School of Mines, CO 80401, United States. c Department of Applied Chemistry, Ritsumeikan University, Kyoto 603-8577, Japan
Taking the “forever” out of forever chemicals using zinc sulfide.
1. Perfluoroalkyl substances (PFAS) are a group of synthetic chemicals. They are present in everyday items and firefighting foams. PFAS are often called “Forever Chemicals” due to their environmental persistence, arising from the strength of the carbon-fluorine (C-F) bonds. PFAS have been linked to increased risk of cancer, diabetes, mutations, and lowered immune response.
2. There are currently no approved methods for PFAS destruction in Australia.Photocatalysis is a light-driven process whereby the energy from light can drive chemical reactions- in this case, breaking C-F bonds in PFAS. My research utilises zinc sulfide (ZnS), which is made from Earth-abundant, non-toxic, and cheap materials. Sulfur vacancies in ZnS improve activity!
3. Photocatalysis I generally limited to deep UV LEDs, which can be dangerous and energy intensive. The sulfur vacancies present in ZnS improve its visible-light activity. PFAS is mineralized using 365, 395, 420, and 450 nm. This is a promising step toward utilizing sunlight for photocatalysis! ZnS can degrade PFAS under visible-light!
4. A fluorine mass balance allows us to monitor if we are generating toxic byproducts or breaking C-F bonds. No shorter-chain PFAS produced! High fluoride output! Direct e- transfer from ZnS to the PFAS is responsible for C-F bond cleavage. LC-MS analysis identified degradation products (H/F substitution)
5. A circulating continuous flow reactor using ZnS films can treat larger volumes of PFAS-contaminated water.
Within 24 h of continuous circulating flow, PFOS (50 ppm, 12mL) was completely mineralized into fluoride (ideal reaction outcome).
Despite an initial decrease in performance after 24 h, activity was retained between 24 and 72 h of continuous flow.
Activity in flow after 72 h!Aqueous film forming foams (AFFF) is a type of PFAS-containing firefighting foam. This AFFF-contaminated field sample was sources from a contaminated water treatment plant in South Australia and subsequently isolated and concentrated. With increasing UV illumination time, we can see fluoride evolving and PFAS in the AFFF degrading. 66% mineralization of AFFF field sample!
