Photocatalytic Degradation of Perfluoroalkyl Substances in Water by Using a Duo-Functional Tri-Metallic-Oxide Hybrid Catalyst

<b>Abstract</b><br/>Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are the most typical poly- and perfluoroalkyl substances (PFASs) compounds. PFOA is detected in surface water, groundwater, sediment, sludge, and municipal wastewater. In recent years, various adsorbents such as activated carbon, polymer-supported carbon, anion exchange metal-organic framework, and beta-cyclodextrin have been used for the removal of PFASs. The recalcitrant nature of PFASs urges scientists to discover solutions to permanently remove PFAS contaminations from water with less energy in contrast to incineration. Here we present a duo-functional tri-metallic-oxide (<i>f</i>-TMO) hybrid photocatalyst, which displayed both high adsorption capacity and high defluorination rate of a series of PFASs including PFOA, PFOS, PFHpA, PFHxA and PFBA due to the generated holes/electrons (h⁺/e^-) and multi-radicals (Samuel, Shang, Niu. <b><i>Chemosphere</i></b>, 293 (2022) 133568). Particularly the Langmuir adsorption capacities up to 827.8 and 714.5 mg g^-1 along with the adsorption efficiency of 99.8% and 99.4% for PFOS and PFOA were respectively achieved, which are more than two times higher than the most used activated carbons. A defluorination ratio of as high as 74.8% with PFOA and a ratio up to 67.6% with PFOS were respectively achieved. Over 98% PFOA molecules were degraded within as fast as 15 min, which demonstrates an excellent degradation kinetics. After degradation, the PFOA residuals were as low as 10 ng L^-1 in industrial wastewater with an initial concentration of 1 ppb. As for the more recalcitrant sulfonic acid of PFOS, an as high as 95.5% degradation efficiency was obtained within 300 min. It was found the most high-F molecules such as C₇F₁₅COOH were gradually degraded into low-F molecules such as CF₃COOH within 60 min before they were completely decomposed into non-toxic F^-. In parallel, the <i>f</i>-TMO photocatalyst still exhibited a &gt;96.2% degradation efficiency after eight regeneration cycles. The high physical adsorption capacity and high defluorination rate make this <i>f-</i>TMO catalyst a promising candidate in removing various PFASs from a broad range of residential and industrial water systems.<br/><br/><b>Keywords:</b> Adsorption; Perfluoroalkyl substances (PFASs); Photocatalytic degradation; Water treatment