Degradation of Poly- and Per-fluoroalkyl Substances (PFASs) Using Photocatalyst Zinc Oxide
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Poly- and per-fluoroalkyl substances (PFASs) in the environment have raised a great public health concern because these compounds are persistent, bioaccumulative and toxic. Degradation of polyfluoroalkyl substances like fluorotelomer alcohols (FTOHs) can produce perfluoroalkyl acids (PFAAs). Previous studies have reported biodegradation of 6:2 FTOH by several FTOH-degrading bacteria to various shorter-chain PFASs. Some transition metals, like zinc oxide (ZnO), have some interesting semiconducting, adsorbing and optical properties in addition to their ability to photocatalyticly degrade contaminants. Accordingly, we hypothesized that an approach that combines biological and chemical treatment will be an effective way to degrade PFASs. This study focused on photodegradation of PFASs using ZnO under environmentally friendly conditions, particularly near neutral pH and room temperature. Two types of ZnO were used: the commercial microscale ZnO and the tetrapod nanoscale ZnO. Results showed that photo-defluorination efficiency for 5:3 polyfluorinated acid was about 14.9% and 13.8% using commercial and tetrapod ZnO, respectively. Either type of ZnO could not successfully degrade any of the three PFAAs used in this study, i.e. PFOA, perfluorohexanoic acid (PFHxA) and perfluorobutyric acid (PFBA). Adding persulfate with tetrapod ZnO improved the defluorination efficiency for PFOA, but it reduced the defluorination of 5:3 acid and 6:2 FTOH. High doses of persulfate (27 mM) without ZnO led to a significant improvement in defluorination of PFOA, PFHxA and PFBA, with PFBA defluorination as high as about 40%. Defluorination of 5:3 acid by ZnO was eliminated when the experiments took place in a growth medium. The competition of these ions with PFASs for adsorption and the high ionic strength were proposed as possible reasons for this elimination. 5:3 Acid defluorination was not much affected by the presence of PFOA in the solution, which indicates that PFOA does not adsorb strongly enough on ZnO to inhibit adsorption of the 5:3 acid. Finally, solutions of the 5:3 acid that had undergone photodegradation with tetrapod ZnO were further examined for their treatability by biological processes. Biodegradation trials using P. flurescens DSM 8341 did not show a significant defluorination improvement, which in part due to the presence of growth medium. This is the first report studying the photodegradation of PFASs using ZnO.
Abada, Bilal Saad Azeez (2016). Degradation of Poly- and Per-fluoroalkyl Substances (PFASs) Using Photocatalyst Zinc Oxide. Master's thesis, Texas A & M University. Available electronically from