Defluorination Of Perfluorooctanoic Acid Research Articles

Co-Removal of Perfluorooctanoic Acid and Nitrate from Water by Coupling Pd Catalysis with Enzymatic Biotransformation.

PFAS (poly- and per-fluorinated alkyl substances) represent a large family of recalcitrant organic compounds that are widely used and pose serious threats to human and ecosystem health. Here, palladium (Pd0)-catalyzed defluorination and microbiological mineralization were combined in a denitrifying H2-based membrane biofilm reactor to remove co-occurring perfluorooctanoic acid (PFOA) and nitrate. The combined process, i.e., Pd-biofilm, enabled continuous removal of ∼4 mmol/L nitrate and ∼1 mg/L PFOA, with 81% defluorination of PFOA. Metagenome analysis identified bacteria likely responsible for biodegradation of partially defluorinated PFOA: Dechloromonas sp. CZR5, Kaistella koreensis, Ochrobacterum anthropic, and Azospira sp. I13. High-performance liquid chromatography-quadrupole time-of-flight mass spectrometry and metagenome analyses revealed that the presence of nitrate promoted microbiological oxidation of partially defluorinated PFOA. Taken together, the results point to PFOA-oxidation pathways that began with PFOA adsorption to Pd0, which enabled catalytic generation of partially or fully defluorinated fatty acids and stepwise oxidation and defluorination by the bacteria. This study documents how combining catalysis and microbiological transformation enables the simultaneous removal of PFOA and nitrate.

Novel enhanced defluorination of perfluorooctanoic acids by biochar-assisted ultrasound coupling ferrate: Performance and mechanism

An ultrasound (US)/biochar (BC)/ferrate (Fe (VI)) system was firstly proposed to enhance perfluorooctanoic acid (PFOA) defluorination. It achieved 93 % defluorination optimally, higher than the sum of 77 % (28 % and 49 % for US/BC and US/Fe (VI) respectively), implying synergistic effect. Besides, the mechanism study confirmed that, this system can not only increase the specific surface area of BC and the generation of reactive oxidant species (ROS), enriching the active sites and forming new oxygen-containing functional groups, but also promote the formation of intermediate iron species. The PFOA degradation in the US/BC/Fe (VI) was probably an adsorption-degradation process, both ROS and electron transfer promoted the defluorination. Additionally, its sustainability was also demonstrated with 14 % reduced defluorination percentage after five cycles of BC. Overall, the synergistic effect of the US/BC/Fe (VI) and its enhancing mechanism for PFOA defluorination were clarified firstly, which contributes to the development of biochar for assisting polyfluoroalkyl substances degradation.

UV/Sulfite reduction kinetics of perfluorobutane sulfonic acid (PFBS), perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA)

Reduction of persistent perfluorinated compounds (PFCs) by UV-activated sulfite (SF) has been recognized as one of the most effective advanced reduction processes (ARPs). With the UV/SF ARP, PFBS degradation reached 66.6 % at 25 °C after 6 h, while PFOS and PFOA were completely degraded within 80 and 20 mins, respectively. PFOA has the largest defluorination efficiency of 77.5 %, greater than 57.8 % for PFOS and 42.7 % for PFBS after 6 h. With the UV/SF system operated at 15, 25, 35 and 45 °C, activation energy (Ea) of PFBS defluorination was determined to be 360.8 kJ/mol, larger than those of PFOS and PFOA (319.4 and 67.3 kJ/mol, respectively), which implies that PFBS is more persistent and difficult to be degraded in the natural environment. Based on the consumption of SF, secondary-order kinetics models were developed to describe the degradation and defluorination of PFBS, PFOS, and PFOA at various reaction times. Concentrations of various intermediate species were determined by liquid chromatography quadrupole time-of-flight mass spectrometry (LC/QTOF-MS) during the PFBS degradation to investigate the reaction pathways in the UV/SF ARP. The proposed reaction mechanisms for PFBS degradation include desulfonation, H/F exchange, and chain shortening via direct C − C bond cleavage.

Electrochemical Reduction of Perfluorooctanoic Acid (PFOA): An Experimental and Theoretical Approach.

Perfluorooctanoic acid (PFOA) is an artificial chemical of global concern due to its high environmental persistence and potential human health risk. Electrochemical methods are promising technologies for water treatment because they are efficient, cheap, and scalable. The electrochemical reduction of PFOA is one of the current methodologies. This process leads to defluorination of the carbon chain to hydrogenated products. Here, we describe a mechanistic study of the electrochemical reduction of PFOA in gold electrodes. By using linear sweep voltammetry (LSV), an E0' of -1.80 V vs Ag/AgCl was estimated. Using a scan rate diagnosis, we determined an electron-transfer coefficient (αexp) of 0.37, corresponding to a concerted mechanism. The strong adsorption of PFOA into the gold surface is confirmed by the Langmuir-like isotherm in the absence (KA = 1.89 × 1012 cm3 mol-1) and presence of a negative potential (KA = 3.94 × 107 cm3 mol-1, at -1.40 V vs Ag/AgCl). Based on Marcus-Hush's theory, calculations show a solvent reorganization energy (λ0) of 0.9 eV, suggesting a large electrostatic repulsion between the perfluorinated chain and water. The estimated free energy of the transition state of the electron transfer (ΔG‡ = 2.42 eV) suggests that it is thermodynamically the reaction-limiting step. 19F - 1H NMR, UV-vis, and mass spectrometry studies confirm the displacement of fluorine atoms by hydrogen. Density functional theory (DFT) calculations also support the concerted mechanism for the reductive defluorination of PFOA, in agreement with the experimental values.

Experimental and numerical investigation on the effect of frequency combinations on PFOA defluorination by dual-frequency ultrasound coupling persulfate

Due to the synergistic contribution, dual-frequency ultrasound can significantly enhance the degradation of organic contaminants, but the effects of different frequency combinations have not been clarified and need to be explored. In this study, several frequency combinations were designed firstly both in experimental exploration and numerical simulation within the dual-frequency ultrasound coupling persulfate (PS) system to investigate the effect of frequency combinations on the degradation of perfluorooctanoic acid (PFOA). The monitoring parameters included the defluorination percentage and the simulated maximum expansion ratio (Rmax/R0). The simulated maximum expansion ratios in different frequency combinations showed a high correlation with their defluorination results, indicating that the discrepancy in defluorination effects of different frequency combinations was mostly attributed to their various maximum expansion ratios. Additionally, various maximum expansion ratios could result in different maximum solution temperatures, and the increase in temperature was found positive for PFOA defluorination in this study. In addition, the synergistic parameters that affected the maximum expansion ratio were analyzed, including the phase difference, frequency ratio, and frequency value. The obtained results showed that the maximum expansion ratio varied with different phase differences, and the frequency ratio could affect the variation trend. For the same fundamental frequency, the larger the frequency ratio, the smaller the maximum expansion ratio. Under the same phase difference and frequency ratio, the larger the frequency value, the lower the cavitation.

Biochar and surfactant synergistically enhanced PFAS destruction in UV/sulfite system at neutral pH

The UV/sulfite-based advanced reduction process (ARP) emerges as an effective strategy to combat per- and polyfluoroalkyl substances (PFAS) pollution in water. Yet, the UV/sulfite-ARP typically operates at highly alkaline conditions (e.g., pH > 9 or even higher) since the generated reductive radicals for PFAS degradation can be quickly sequestered by protons (H+). To overcome the associated challenges, we prototyped a biochar-surfactant-system (BSS) to synergistically enhance PFAS sorption and degradation by UV/sulfite-ARP. The degradation and defluorination efficiencies of perfluorooctanoic acid (PFOA) depended on solution pH, and concentrations of surfactant (cetyltrimethylammonium bromide; CTAB), sulfite, and biochar. At high pH (8–10), adding biochar and BSS showed no or even small inhibitory effect on PFOA degradation, since the degradation efficiencies were already high enough that cannot be differentiated. However, at acidic and neutral pH (6–7), an evident enhancement of PFOA degradation and defluorination efficiencies occurred. This is due to the synergies between biochar and CTAB that create favorable microenvironments for enhanced PFOA sorption and deeper destruction by prolonging the longevity of reductive radicals (e.g., SO3•−), which is less affected by ambient pH conditions. The performance of UV/sulfite/BSS was further optimized and used for the degradation of four PFAS. At the optimal experimental condition, the UV/sulfite/BSS system can completely degrade PFOA with >30% defluorination efficiency for up to five continuous cycles (n = 5). Overall, our BSS provides a cost-effective and sustainable technique to effectively degrade PFAS in water under environmentally relevant pH conditions. The BSS-enabled ARP technique can be easily tied into PFAS treatment train technology (e.g., advanced oxidation process) for more efficient and deeper defluorination of various PFAS in water.

Defluorination of perfluorooctanoic acid and perfluorooctane sulfonic acid by heterogeneous catalytic system of Fe-Al2O3/O3: Synergistic oxidation effects and defluorination mechanism

In this study, catalytic ozonation by Fe-Al2O3 was used to investigate the defluorination of PFOA and PFOS, assessing the effects of different experimental conditions on the defluorination efficiency of the system. The oxidation mechanism of the Fe-Al2O3/O3 system and the specific degradation and defluorination mechanisms for PFOA and PFOS were determined. Results showed that compared to the single O3 system, the defluorination rates of PFOA and PFOS increased by 2.32- and 5.92-fold using the Fe-Al2O3/O3 system under optimal experimental conditions. Mechanistic analysis indicated that in Fe-Al2O3, the variable valence iron (Fe) and functional groups containing C and O served as important reaction sites during the catalytic process. The co-existence of 1O2, OH, O2− and high-valence Fe(IV) constituted a synergistic oxidation system consisting of free radicals and non-radicals, promoting the degradation and defluorination of PFOA and PFOS. DFT theoretical calculations and the analysis of intermediate degradation products suggested that the degradation pathways of PFOA and PFOS involved Kolbe decarboxylation, desulfonation, alcoholization and intramolecular cyclization reactions. The degradation and defluorination pathways of PFOA and PFOS consisted of the stepwise removal of -CF2-, with PFOS exhibiting a higher defluorination rate than PFOA due to its susceptibility to electrophilic attack. This study provides a theoretical basis for the development of heterogeneous catalytic ozonation systems for PFOA and PFOS treatment.

Method of H2 Transfer Is Vital for Catalytic Hydrodefluorination of Perfluorooctanoic Acid (PFOA).

The efficient transfer of H2 plays a critical role in catalytic hydrogenation, particularly for the removal of recalcitrant contaminants from water. One of the most persistent contaminants, perfluorooctanoic acid (PFOA), was used to investigate how the method of H2 transfer affected the catalytic hydrodefluorination ability of elemental palladium nanoparticles (Pd0NPs). Pd0NPs were synthesized through an in situ autocatalytic reduction of Pd2+ driven by H2 from the membrane. The Pd0 nanoparticles were directly deposited onto the membrane fibers to form the catalyst film. Direct delivery of H2 to Pd0NPs through the walls of nonporous gas transfer membranes enhanced the hydrodefluorination of PFOA, compared to delivering H2 through the headspace. A higher H2 lumen pressure (20 vs 5 psig) also significantly increased the defluorination rate, although 5 psig H2 flux was sufficient for full reductive defluorination of PFOA. Calculations made using density functional theory (DFT) suggest that subsurface hydrogen delivered directly from the membrane increases and accelerates hydrodefluorination by creating a higher coverage of reactive hydrogen species on the Pd0NP catalyst compared to H2 delivery through the headspace. This study documents the crucial role of the H2 transfer method in the catalytic hydrogenation of PFOA and provides mechanistic insights into how membrane delivery accelerates hydrodefluorination.

Microbial defluorination of TFA, PFOA, and HFPO-DA by a native microbial consortium under anoxic conditions

In this study, the biodegradability of trifluoroacetate (TFA), perfluorooctanoic acid (PFOA), and perfluoro-2-methyl-3-oxahexanoic acid (HFPO-DA) by a native microbial community was evaluated over a 10-month incubation period. The observed microbial defluorination ratios and removal efficiency were 3.46 ( ± 2.73) % and 8.03 ( ± 3.03) %, 8.44 ( ± 1.88) % and 13.52 ( ± 4.96) %, 3.02 ( ± 0.62) % and 5.45 ( ± 2.99) % for TFA, PFOA and HFPO-DA, respectively. The biodegradation intermediate products, TFA and pentafluoropropionic acid (PFA), of PFOA and HFPO-DA were detected in their biodegradation treatment groups. Furthermore, the concentrations of the PFOA metabolites, perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA), in the aqueous solutions after incubation were quantified to be 0.21 and 4.14 µg/L. TFA, PFOA and HFPO-DA significantly reduced the microbial diversity and changed the structure of the community. The co-occurrence network analysis showed that low abundance species, such as Flexilinea flocculi, Bacteriovorax stolpii, and g_Sphingomonas, are positively correlated with the generation of fluoride ion, implying their potential collaborative functions contributing to the observed biodefluorination. The findings in this study can provide insights for the biodegradation of perfluoroalkyl carboxylic acids and their emerging alternatives by indigenous microorganisms in the environment.

Reductive degradation mechanism of perfluorooctanoic acid (PFOA) during vacuum ultraviolet (VUV) reactions combining with sulfite and iodide

In this study, PFOA removal and defluorination were examined during vacuum ultraviolet (VUV) photolysis in the presence of sulfite and sulfite/iodide conditions. PFOA (24 μM) degradation rate constant (kobs) and defluorination amount in VUV photolysis, and VUV/sulfite, and VUV/sulfite/iodide reactions under nitrogen-purging condition were 5.50 × 10−3, 7.26 × 10−2, 1.60 × 10−1 min−1, and 34.6, 72.7, 73.9% in 6 h, respectively. When tert-butanol (t-BuOH), NO2−, and NO3− ions were added as radical scavengers, hydrated electrons (eaq−) was confirmed as the main species responsible for degrading PFOA and mediating defluorination in VUV-based reactions. While, during VUV photolysis, short-chain perfluoroalkyl carboxylic acids (PFCAs), such as PFHpA, PFHxA, PFPeA, and PFBA, were mainly produced as transformation products (TPs) by the chain-shortening mechanism, additional 14 and 15 TPs were identified in the VUV/sulfite and VUV/sulfite/iodide reactions by LC-QTOF/MS, respectively. The main degradation mechanisms in these reactions are H–F exchange (e.g., TP395 (m/z = 394.9739) and TP377 (m/z = 376.9838)), •SO3−–F exchange (TP474, m/z = 474.9323), carbon double bond formation by defluorination (e.g., TP392 (m/z = 392.9455), TP410 (m/z = 410.9355), and TP436 (m/z = 436.9347)), and H–F exchange followed by hydration reaction (TP393, m/z = 392.9773), respectively. PFOA degradation pathways were proposed for these VUV-based reactions based on the identified TPs, their time profiles, and the density functional theory (DFT). Finally, the toxicity of PFOA and its TPs produced during three reactions were assessed using ECOSAR simulation.

Enhanced piezoelectric catalysis of BaTiO3 by ZVAl for mechanochemical defluorination of PFOA: Promotion of electron transfer

Mechanochemical (MC) destruction of pollutants is effective; however, the emerging electron transfer mechanism is ambiguous owing to a lack of systematic evaluation. Therefore, this study aims to evaluate the contribution of electrons to perfluorooctanoic acid (PFOA) defluorination during MC process. A synergistic effect was obtained by activating BaTiO3 to generate piezoelectrons and applying zero-valence aluminum (ZVAl) to facilitate electron transfer, with 95.66% PFOA defluorination and reaction time decreasing from 6 h to 3 h. The mechanism of piezoelectric catalysis of the BaTiO3/ZVAl system was further investigated through kinetic analyses and intersystem comparisons. The major contribution of piezo-excited electrons was revealed through probe detection and quantitative determination. A positive correlation between electron generation and PFOA defluorination was ascertained, and the calculation of the electron utilization ratio revealed an electron transfer mechanism. The detached fluorides were confirmed to be bonded directly to the additives. Furthermore, PFOA decomposition was identified as a cyclical process with constant dissociation of the CF2 groups.

Investigating the effect of polarity reversal of the applied current on electrochemical degradation of per-and polyfluoroalkyl substances

Advanced electrochemical oxidation methods have demonstrated significant potential in degrading per- and polyfluoroalkyl substances (PFAS). However, the widespread application of these techniques is impeded by the long term performance and the limited service life of electrodes. This study introduces a novel approach by investigating the impact of polarity reversal (PR) of the applied current on the electrochemical degradation of perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), 6:2 fluorotelomer sulfonate (6:2 F TS), and aqueous film-forming foam (AFFF) samples. A laboratory-scale electrochemical cell equipped with two titanium sub-oxide (Ti4O7) electrodes is used for the experimental investigation. The results indicate 6-h direct current (DC) operation causes fouling and ageing of the electrode (anode) such as the formation of titanium oxide (TiO2). The application of PR effectively mitigates fouling and aging, leading to an enhanced mass transfer and an improved long-term defluorination of PFOA, PFOS, 6:2 FTS, and AFFF samples. The investigation also suggests that PR can help to reduce the energy consumption by ∼13%. Overall, the application of PR emerges as a promising and practical strategy to augment the electrochemical degradation of PFAS, addressing critical issues related to electrode stability and longevity.

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.

Efficient defluorination of perfluorooctanoic acid enabled by single-atom Cu/reduced graphene oxide electrocatalytic anode and peroxymonosulfate activation

In this study, an electrocatalytic system using reduced graphene oxide supported single-atom copper (SA-Cu/rGO) anode was constructed for perfluorooctanoic acid (PFOA) defluorination. The successful doping of SA-Cu elevated the oxygen evolution potential and electrochemical active surface area of the anode and provided high electrocatalytic properties. Under optimum initial conditions with a peroxymonosulfate (PMS) concentration of 10 mM, a current density of 15 mA·cm−2, and an unadjusted pH of 7.75, the SA-Cu/rGO system achieved 98.8% removal ratio, 92.5% defluorination, and 94.4% mineralization of 20 mg·L-1 PFOA in 120 min. Kinetic studies indicate PFOA degradation following a pseudo-first-order model, with a rate constant of 4.7 × 10−2 min−1. The presence of PMS in the electrocatalytic system significantly improved the degradation rate and defluorination ratio. Radical trapping and quenching experiments confirmed that both ·OH and ·SO4- radicals contributed to PFOA degradation, with ·SO4- playing a more important role in defluorination. The system show high flexibility in initial PFOA concentrations (0.2–20 mg·L−1) and environmental pH values (3.0–10.0), as well as good durability. The electrocatalytic degradation mechanism of PFOA was proposed through determination of intermediates and density functional theory (DFT) calculation. The total short-chain perfluorocarboxylic acids (PFCAs) intermediates generated throughout the degradation were far from sufficient recovery of the PFOA loss, indicating that apart from the ·OH mediated decarboxylation-hydroxylation-elimination-hydrolysis (DHEH) pathway, the ·SO4- mediated decarboxylation-hydroxylation-oxidation-decarbonyl fluoride (DHOD) pathway may play a significant role in the near-complete defluorination and mineralization of PFOA. Moreover, DFT calculations suggest that the DHOD pathway is energetically favorable over DHEH. This study provides a further understanding of the defluorination mechanism of PFOA in electrocatalytic systems and demonstrates that electrochemical degradation with single-atom copper catalyst is an promising method for PFOA remediation in aqueous environment.

Differences between reductive defluorination of perfluorooctanoic acid by chlorination, bromination, and iodization in the vacuum-ultraviolet/sulfite process

The introduction of iodide (I-) has broad perspectives on the decomposition of perfluorocarboxylates (PFCAs, CnF2n+1COO-). However, the iodinated substances produced are highly toxic synthetic chemicals, hence, it is urgent to find a similar alternative with less toxicity. In this work, the defluorination of perfluorooctanoic acid (PFOA) by I-, bromide (Br-) and chlorine (Cl-) was systematically compared in the VUV/sulfite process. Results indicated that the PFOA defluorination rates increased with increasing nucleophilicity of halogens (I > Br > Cl). Meanwhile, the introduction of I-, Br-, and Cl- reduced the interference of the coexisting water matrix on the degrading influence of PFOA. The in situ produced eaq-, SO3•-, H•, and HO• were recognized, among the addition of I- maximized the relative contribution of eaq- but Br- and Cl- decreased that of H• and other radicals. Additionally, HPLC/MS analysis revealed the presence of I-, Br-, and Cl- had a vital impact on the difference in product concentrations, while they had a negligible effect on the change in the pathway of degradation. Overall, this study demonstrated the similarities and differences between I-, Br-, and Cl-, which has significant implications for further understanding VUV/sulfite degradation.

Efficient defluorination of PFOA by microwave discharge plasma in liquid: Influence of actual water environment factors

Perfluorooctanoic acid (PFOA) is a persistent and refractory organic pollutant. In recent years, how to remove PFOA efficiently and thoroughly is a research hotspot. In this work, a new method with high mass transfer efficiency, microwave discharge plasma in liquid (MDPL) is used to study the defluorination and degradation characteristics of PFOA. Combined with the actual water environmental factors, the effects of initial concentration, power, conductivity, pH, coexisting ions and dissolved organic matter (DOM) on the defluorination of PFOA are studied. The highest removal of PFOA (4 mg/L) in 100 min is 100 %, and the defluorination is 96.0 %. Under actual water environment factors, the defluorination of PFOA can reach 87.8 %. The removal of TOC can reach 62.9 %. The high defluorination means that the MDPL technique can completely degrade PFOA and reduce the secondary contamination of the intermediates. It is confirmed that eaq− is the key reactive species to initiate the decomposition of PFOA, and the contribution rate can reach 58.3 %. The results show that MDPL has a good effect on the defluorination of PFOA, and it also has a significant advantage in the defluorination of PFOA under actual water environment factors.

Amine-Functionalized A-Center Sphalerite for Selective and Efficient Destruction of Perfluorooctanoic Acid.

Perfluorochemicals (PFCs), especially perfluorooctanoic acid (PFOA), have contaminated the ground and surface waters throughout the world. Efficient removal of PFCs from contaminated waters has been a major challenge. This study developed a novel UV-based reaction system to achieve fast PFOA adsorption and decomposition without addition of sacrificial chemicals by using synthetic photocatalyst sphalerite (ZnS-[N]) with sufficient surface amination and defects. The obtained ZnS-[N] has the capability of both reduction and oxidation due to the suitable band gap and photo-generated hole-trapping properties created by surface defects. The cooperated organic amine functional groups on the surface of ZnS-[N] play a crucial role in the selective adsorption of PFOA, which guarantee the efficient destruction of PFOA subsequently, and 1 μg L-1 PFOA could be degraded to <70 ng L-1 after 3 h in the presence of 0.75 g L-1 ZnS-[N] under 500 W UV irradiation. In this process, the photogenerated electrons (reduction) and holes (oxidation) on the ZnS-[N] surface work in a synergistic manner to achieve complete defluorination of PFOA. This study not only provides promising green technology for PFC-pollution remediation but also highlights the significance of developing a target system capable of both reduction and oxidation for PFC degradation.

A bifunctional single-atom catalyst assisted by Fe2O3 for efficiently electrocatalytic perfluorooctanoic acid degradation by integrating reductive and oxidative processes

Concerns about the environmentally persistent and bioaccumulative per/polyfluoroalkyl substances (PFAS) in water are emerging since their potentially harmful effects on humans. However, the rapid mineralization and defluorination of PFAS are rarely achieved although various techniques have been developed. Hereby, a bifunctional single-atom catalyst (SAC) characterizing a Co-CN2 configuration is constructed, which can simultaneously induce the degradation of PFOA and reduce O2 to H2O2. Oxidative and reductive degradation are intergraded within one system for perfluorooctanoic acid (PFOA) degradation with the immobilized Fe2O3 as a Fenton catalyst. Consequently, 96.1% of PFOA is degraded over Co-CN2-Fe2O3 at −0.06 V in an H-cell equipped with a gas diffusion cathode. The defluorination efficiency reaches 95.9% in 120 min. The Co site shielding experiment combined with the ·OH quench experiment validates that both reductive and oxidative degradation processes are indispensable in rapid and complete PFOA mannerization and defluorination. A reduction and oxidation cooperative chain shortening mechanism is proposed for PFOA degradation based on the intermediates analysis and shielding experiment.

Water hyacinth powder-assisted in-situ fabrication of visible light responsive CQDs/BiOCl heterojunctions with exceptional photocatalytic detoxification performance

As a typical Bi-based photocatalyst, BiOCl has been received increasing attention because of its many advantages. However, poor visible light response ability and high recombination rate of photoexcited charge pairs significantly inhibit the intense application of BiOCl, therefore, it is considerable to develop a strategy to overcome the inherent shortcomings of BiOCl to boost its photocatalytic properties. The former works have strongly affirmed that coupling carbon quantum dots (CQDs) with BiOCl to construct heterojunctions is a practical method to boost the photoactivity of BiOCl. In this demonstration, water hyacinth powder was used to provide raw material for CQDs, and CQDs/OVs-BiOCl heterostructures were in-situ fabricated through a solvothermal synthesis. The successful modification of CQDs on BiOCl and the formation of abundant oxygen vacancies (OVs) were confirmed by several analytical methods. The test results of photoelectric property reveal that CQDs/OVs-BiOCl has faster photogenerated charge separation rate than the reference BiOCl. Photoactivity of all the samples was examined by destruction of rhodamine B (RhB) and defluorination of perfluorooctanoic acid (PFOA). The CQDs/OVs-BiOCl heterostructures exhibit the ascendant photocatalytic destruction activity. Among them, the degradation efficiency of RhB over 1%CQDs/OVs-BiOCl under visible light irradiation is 2.6 folds of that of over the reference BiOCl. Besides, the defluorination rate of PFOA on 1%CQDs/OVs-BiOCl under UV light irradiation is 1.4 times of that of on BiOCl. The cyclic experimental results solidly prove that the CQDs/OVs-BiOCl samples have outstanding stability and good application prospect. Free radical detection experiments reveal that 1%CQDs/OVs-BiOCl reaction system produces more active species than the reference BiOCl system, and ·O2- is the most important active substance. Additionally, the total organic carbon (TOC) test strongly demonstrate that RhB can be effectively mineralized into inorganic small substances on 1%CQDs/OVs-BiOCl under light shine. Based on the results of all the characterizations, a photocatalytic improvement mechanism for CQDs/OVs-BiOCl was reasonably elaborated. This work provides a new approach to obtain CQDs to modify BiOCl, which not only improves the photocatalytic degradation activity of BiOCl, but also achieves the purpose of use waste to treat waste.

Impact of supporting electrolyte on electrochemical performance of borophene-functionalized graphene sponge anode and degradation of per- and polyfluoroalkyl substances (PFAS)

Graphene sponge anode functionalized with two-dimensional (2D) boron, i.e., borophene, was applied for electrochemical oxidation of C4-C8 per- and polyfluoroalkyl substances (PFASs). Borophene-doped graphene sponge outperformed boron-doped graphene sponge anode in terms of PFASs removal efficiencies and their electrochemical degradation; whereas at the boron-doped graphene sponge anode up to 35% of the removed PFASs was recovered after the current was switched off, the switch to a 2D boron enabled further degradation of the electrosorbed PFASs. Borophene-doped graphene sponge anode achieved 32–77% removal of C4-C8 PFASs in one-pass flow-through mode from a 10 mM phosphate buffer at 230 A m−2 of anodic current density. Higher molarity phosphate buffer (100 mM) resulted in lower PFASs removal efficiencies (11–60%) due to the higher resistance of the graphene sponge electrode in the presence of phosphate ions, as demonstrated by the electrochemical impedance spectroscopy (EIS) analyses. Electro-oxidation of PFASs was more efficient in landfill leachate despite its high organic loading, with up to 95% and 75% removal obtained for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), versus 77% and 57% removal in the 10 mM phosphate buffer, respectively. Defluorination efficiencies as determined relative to the electrooxidized fraction of PFASs indicated up to 69% and 82% of defluorination of PFOS and PFOA in 10 mM phosphate buffer, which was decreased to 16 and 29% defluorination, respectively, for higher buffer molarity (100 mM) due to the worsened electrochemical performance of the sponge. In landfill leachate, relative defluorination efficiencies of PFOS and PFOA were 33% and 45%, respectively, indicating the inhibiting effect of complex organic and inorganic matrix of landfill leachate on the C-F bond breakage. This study demonstrates that electrochemical degradation of PFASs is possible to achieve in complex and brackish streams using a low-cost graphene sponge anode, without forming toxic chlorinated byproducts even in the presence of >7 g L−1 of chloride.