Concentrate and degrade PFOA with a photo-regenerable composite of In-doped TNTs@AC

Iron complex regulated synergistic effect between the current and peroxymonosulfate enhanced ultrafast oxidation of perfluorooctanoic acid via free radical dominant electrochemical reaction

Iron complex regulated electrochemical reaction was triggered for revealing the reaction mechanism, degradation pathway, and applied potential of perfluorooctanoic acid (PFOA). The increased PMS concentrations, electrode spacing, and current density significantly enhanced PFOA elimination, with current density exhibiting a relatively strong interdependency to PFOA complete mineralization. The synergy between PMS and electrochemical reactions greatly accelerated PFOA decomposition by promoting the generation of key reaction sites, such as those for PMS activation and electrochemical processes, under various conditions. Furthermore, density functional theory calculations confirmed that the reciprocal transformation of Fe2+ and Fe3+ complexes was feasible under the electrochemical effect, further promoting the generation of active sites. The developed electrochemical oxidation with PMS reaction (EO/PMS) system can rapidly decompose and mineralize PFOA while maintaining strong tolerance to changing water matrices and organic and inorganic ions. Overall, it holds promise for use in treating and purifying wastewater containing PFOA.

Photocatalysts for chemical-free PFOA degradation - What we know and where we go from here?

Perfluorooctanoic acid (PFOA) is a toxic and recalcitrant perfluoroalkyl substance commonly detected in the environment. Its low concentration challenges the development of effective degradation techniques, which demands intensive chemical and energy consumption. The recent stringent health advisories and the upgrowth and advances in photocatalytic technologies claim the need to evaluate and compare the state-of-the-art. Among these systems, chemical-free photocatalysis emerges as a cost-effective and sustainable solution for PFOA degradation and potentially other perfluorinated carboxylic acids. This review (I) classifies the state-of-the-art of chemical-free photocatalysts for PFOA degradation in families of materials (Ti, Fe, In, Ga, Bi, Si, and BN), (II) describes the evolution of catalysts, identifies and discusses the strategies to enhance their performance, (III) proposes a simplified cost evaluation tool for simple techno-economical analysis of the materials; (IV) compares the features of the catalysts expanding the classic degradation focus to other essential parameters, and (V) identifies current research gaps and future research opportunities to enhance the photocatalyst performance. We aim that this critical review will assist researchers and practitioners to develop rational photocatalyst designs and identify research gaps for green and effective PFAS degradation.

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.

Semiconductor-hydrophobic material interfaces as a new active site paradigm for photocatalytic degradation of perfluorocarboxylic acids

Photocatalytic degradation of long-chain perfluorocarboxylic acid (PFCA) water contaminants has been reported for numerous of semiconductors, including composite TiO₂ particles decorated with graphitic carbon co-catalysts. While pristine TiO₂ degrades PFCAs inefficiently, the carbon components purportedly enhance activity due to their conductive nature and resultant charge separation enhancement. Yet herein, we present evidence that the catalytic activity of a graphene oxide (GO)-TiO₂ composite from the literature arose not due to from charge separation, but to a unique mode of PFCA adsorption occurring at the interface of TiO₂ and hydrophobic GO. Photocatalytic degradation rates by GO-TiO₂ were compared to those of composites containing nonconductive polymer microparticles (polyethylene, polytetrafluoroethylene). Results showed that polymer-TiO₂ composites performed as well as GO-TiO₂ in degrading both perfluorooctanoic acid and oxalate, a common hole scavenger. Thus, the enhanced activity may occur for any TiO₂-hydrophobic interface, regardless of co-catalyst conductivity. Furthermore, compared to an unmodified reference catalyst, chain length dependence of PFCA degradation by a polymer-TiO₂ composite was found to be less severe, with greater activity toward short-chain species indicating enhanced adsorption behavior. Potential adsorption mechanisms are presented, along with broader implications toward improving the applicability of heterogeneous processes toward a wider range of perfluoroalkyl contaminants.

Metal-doped carbon-supported/modified titanate nanotubes for perfluorooctane sulfonate degradation in water: Effects of preparation conditions, mechanisms, and parameter optimization

Metal-doped, activated carbon (AC) supported titanate nanotubes (Me/TNTs@AC) have been shown promising for photocatalytic degradation of per- and polyfluoroalkyl substances (PFAS). However, the preparation recipe of the adsorptive photocatalysts has not yet been optimized in terms of type and content of precursor ACs and the metal dopants as well as synthesizing conditions. In this work, the photocatalytic performance of Me/TNTs@AC was evaluated based on the effectiveness in defluorination of pre-sorbed perfluorooctane sulfonic acid (PFOS) after 4-h UV irradiation. Based on the experimental results, the highest photocatalytic mineralization efficiency (66.2 %) of PFOS was achieved using Ga/TNTs@AC prepared under the following conditions: Filtrosorb-400® = 50 wt%, Ga = 2 wt%, hydrothermal treatment temperature = 130 °C, hydrothermal duration = 72 h, and calcination temperature = 550 °C. To understand the underlying mechanisms, selected materials were characterized via X-ray diffraction, the BET surface area and pore volume, UV-vis diffuse reflectance spectrometry, and photoluminescence. The results revealed that the superior photoactivity of Ga/TNTs@AC is attributed to the Ga-facilitated formation of pure crystallized anatase phase during the calcination, high UV light absorption, formation of microscale hybrid AC-anatase-Ga phases, and oxygen defects induced by Ga³⁺. The information can facilitate preparation and optimization of composite photocatalysts for efficient adsorption and photocatalytic degradation of PFAS in water.