Detection of volatile hydroperfluoroalkanes during hydrothermal liquefaction of perfluoroalkyl carboxylic acids at circumneutral pH.

Hydrothermal liquefaction (HTL) is a promising technology for converting wet organic waste such as sewage sludge into biocrude oil while simultaneously destroying per- and polyfluoroalkyl substances (PFAS). This study tracked the fate and degradation of six representative PFAS in water to address the effect of perfluoroalkyl chain length on degradation rates and the formation of volatile transformation products at 300-350 °C. While perfluorosulfonic acids were recalcitrant, perfluoroalkyl carboxylic acids (PFCAs) were rapidly and completely decarboxylated to hydroperfluoroalkanes (1 H-perfluoroheptane in the case of perfluorooctanoic acid). The volatile hydroperfluoroalkane was subsequently defluorinated without detectable fluorocarbon intermediates yielding 30-60 % defluorination for ammonium perfluoro(2-methyl-3-oxahexanoate), perfluorobutanoic acid and perfluorooctanoic acid after 2 h at 350 °C. Increasing temperature (especially at 350 °C) and longer perfluoroalkyl chains substantially enhanced the defluorination. This is the first study to report volatile hydroperfluoroalkanes from PFCAs in HTL, raising concern about the potential emission of long-lived greenhouse gasses into the atmosphere, but also opening new avenues for PFAS destruction through HTL.

Comparison of the photocatalytic degradability of PFOA, PFOS and GenX using Fe-zeolite in water.

Knowledge on the photocatalytic degradability of the emerging poly- and perfluorinated alkyl substances (PFAS) in water, specifically GenX, is limited. GenX has been detected globally in river water and is considered potentially more toxic than legacy PFAS. In this study, we compared the photocatalytic degradability of GenX with the legacy compounds perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) using Fe-zeolite photocatalysts. After 7 h of irradiation, GenX showed lower removal (79%) and defluorination (33%) as compared to PFOA (100% removal and 69% defluorination) and PFOS (100% removal and 51% defluorination). The quasi-first-order degradation rate of GenX (1.5 h1) was 12 and 1.2 times lower than PFOA (18.4 h-1) and PFOS (1.8 h-1), respectively. Additionally, PFOA's defluorination rate (0.9 h-1) was approximately 2.6 and 9 times higher than GenX (0.35 h-1) and PFOS (0.1 h-1), respectively. These outcomes correlate with GenX's lower hydrophobicity, leading to reduced adsorption (40%) compared to PFOA (99%) and PFOS (87%). Based on identified transformation products, we proposed a GenX degradation pathway, resulting in ultra-short-chain PFASs with a chain length of 2 and 3 carbon atoms, while PFOA and PFOS degraded stepwise, losing 1 carbon-fluorine bond at a time, leading to gradually shorter chain lengths (from 7 to 2 carbon atoms). In conclusion, GenX is more challenging to remove and degrade due to its lower adsorption on the photocatalyst, potential steric hindrance, and higher production of persistent ultra-short-chain transformation products through photocatalysis.

Nano Metal-Containing Photocatalysts for the Removal of Volatile Organic Compounds: Doping, Performance, and Mechanisms.

Volatile organic compounds (VOCs) in indoor air are considered a major threat to human health and environmental safety. The development of applicable technologies for the removal of VOCs is urgently needed. Nowadays, photocatalytic oxidation (PCO) based on metal-containing photocatalysts has been regarded as a promising method. However, unmodified photocatalysts are generally limited in applications because of the narrow light response range and high recombination rate of photo-generated carriers. As a result, nano metal-containing photocatalysts doped with elements or other materials have attracted much attention from researchers and has developed over the past few decades. In addition, different doping types cause different levels of catalyst performance, and the mechanism for performance improving is also different. However, there are few reviews focusing on this aspect, which is really important for catalyst design and application. This work aims to give a comprehensive overview of nano metal-containing photocatalysts with different doping types for the removal of VOCs in an indoor environment. First, the undoped photocatalysts and the basic mechanism of PCO is introduced. Then, the application of metal doping, non-metal doping, co-doping, and other material doping in synthetic metal-containing photocatalysts are discussed and compared, respectively, and the synthesis methods, removal efficiency, and mechanisms are further investigated. Finally, a development trend for using nano metal-containing photocatalysts for the removal of VOCs in the future is proposed. This work provides a meaningful reference for selecting effective strategies to develop novel photocatalysts for the removal of VOCs in the future.