Enhanced degradation of perfluorooctanoic acid using dielectric barrier discharge with La/Ce-doped TiO2

Current state of knowledge of environmental occurrence, toxic effects, and advanced treatment of PFOS and PFOA

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic synthetic compounds, with high chemical and thermal stability and a persistent, stable and bioaccumulative nature that renders them a potential hazard for the environment, its organisms, and humans alike. Perfluorooctane sulfonic acid (PFOS) and Perfluorooctanoic acid (PFOA) are the most well-known substances of this category and even though they are phased out from production they are still highly detectable in several environmental matrices. As a result, they have been spread globally in water sources, soil and biota exerting toxic and detrimental effects. Therefore, up and coming technologies, namely advanced oxidation processes (AOPs) and advanced reduction processes (ARPs) are being tested for their implementation in the degradation of these pollutants. Thus, the present review compiles the current knowledge on the occurrence of PFOS and PFOA in the environment, the various toxic effects they have induced in different organisms as well as the ability of AOPs and ARPs to diminish and/or eliminate them from the environment.

Subnanoscale spatially confined heterogeneous Fenton reaction enables mineralization of perfluorooctanoic acid

Superoxide radical (•O2-) is capable of degrading perfluorinated compounds that are persistent in nature and cannot be removed by biological or advanced oxidation treatments, but the inherent drawback is the negligible reactivity of •O2-in aqueous phases due to the hydration effect. Here, we explored an innovative way to make use of •O2- by modulating a partial hydration state through spatial confinement control. We demonstrated this idea by conducting heterogeneous Fenton reaction with layered iron oxychloride (FeOCl) catalyst, wherein •O2-radicals produced and confined within the catalyst structure (interlayer spacing of 7.92 Å) showed defluorination effect dealing with perfluorooctanoic acid (PFOA) as model compound. The defluorination combined with advanced oxidation achieved mineralization. Mechanism study revealed that the confinement frustrated the hydration shell of •O2-with coordination number reduced from 3.3 (for bulk phase) to 1.89, and thereby changed its orbital electron properties and enhanced the nucleophilic ability. We further demonstrated a compact FeOCl membrane reactor with highly efficient degradation of PFOA (kobs up to 1.2 min-1) and cost-effective mineralization (2 × 10-6 $ per mgC), operated under ultrafiltration reaction mode. Our findings highlight the great interest of developing spatial confinement technology to modulate •O2--based reactions, as well as the feasibility of combining confinement catalyst structures with heterogeneous Fenton reaction to achieve the mineralization treatment goal.

A Review of PFAS Destruction Technologies

Per- and polyfluoroalkyl substances (PFASs) are a family of highly toxic emerging contaminants that have caught the attention of both the public and private sectors due to their adverse health impacts on society. The scientific community has been laboriously working on two fronts: (1) adapting already existing and effective technologies in destroying organic contaminants for PFAS remediation and (2) developing new technologies to remediate PFAS. A common characteristic in both areas is the separation/removal of PFASs from other contaminants or media, followed by destruction. The widely adopted separation technologies can remove PFASs from being in contact with humans; however, they remain in the environment and continue to pose health risks. On the other hand, the destructive technologies discussed here can effectively destroy PFAS compounds and fully address society's urgent need to remediate this harmful family of chemical compounds. This review reports and compare widely accepted as well as emerging PFAS destruction technologies. Some of the technologies presented in this review are still under development at the lab scale, while others have already been tested in the field.

Emerging technologies for PFOS/PFOA degradation and removal: A review

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are highly recalcitrant anthropogenic chemicals that are ubiquitously present in the environment and are harmful to humans. Typical water and wastewater treatment processes (coagulation, flocculation, sedimentation, and filtration) are proven to be largely ineffective, while adsorption with granular activated carbon (GAC) has been the chief option to capture them from aqueous sources followed by incineration. However, this process is time-consuming, and produces additional solid waste and air pollution. Treatment methods for PFOS and PFOA generally follow two routes: (1) removal from source and reduce the risk; (2) degradation. Emerging technologies focusing on degradation are critically reviewed in this contribution. Various processes such as bioremediation, electrocoagulation, foam fractionation, sonolysis, photocatalysis, mechanochemical, electrochemical degradation, beams of electron and plasma have been developed and studied in the past decade to address PFAS crisis. The underlying mechanisms of these PFAS degradation methods have been categorized. Two main challenges have been identified, namely complexity in large scale operation and the release of toxic byproducts. Based on the literature survey, we have provided a strength-weakness-opportunity-threat (SWOT) analysis and quantitative rating on their efficiency, environmental impact and technology readiness.

Catalytic degradation of dinotefuran by dielectric barrier discharge plasma combined with La-doping TiO2

Degradation of neonicotinoid insecticide dinotefuran (DIN) in dielectric barrier discharge (DBD) non-thermal plasma combined with lanthanum-doped titanium dioxide (La-TiO₂) system was investigated. A La-TiO₂ catalyst was prepared by the sol-gel method and characterized by SEM, XRD, and DRS. The effects of various factors (initial concentration, initial pH, input power, and addition of metal ions) on the removal rate of DIN were evaluated. The results indicated that when the initial concentration, input power, initial pH, and Fe²⁺ catalyst ions were 100 mg/L, 150 W, 10.5 and 50 mg/L, respectively, the DIN degradation efficiency was improved to 99.0% by coupling 10 wt% La-TiO₂ at 180 min. La-TiO₂ showed excellent catalytic performance on DIN degradation in a DBD system. The removal rate decreased with the presence of H₂O₂ and a scavenger, manifesting that HO∙ plays an imperative role in the degradation process. Furthermore, intermediate products were analyzed by MS and the possible degradation pathway of DIN was proposed.

Photocatalytic degradation of perfluoroalkyl substances in water by using a duo-functional tri-metallic-oxide hybrid catalyst

The recalcitrant nature of perfluoroalkyl substances (PFASs) urges scientists to discover solutions to permanently remove PFAS contaminations from water with less energy in contrast to incineration. Herein, a duo-functional tri-metallic-oxide (f-TMO) hybrid photocatalyst was developed via a facile process, 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 such as O₂^•- and SO₄^•-. Particularly the Langmuir adsorption capacities up to 827.84 and 714.46 mg g^-1 along with the adsorption efficiency of 99.8% and 99.4% for PFOS and PFOA were respectively achieved. A defluorination ratio of as high as 74.8% with PFOA and a ratio up to 67.6% with PFOS were respectively received. Over 98% PFOA molecules were degraded within as fast as 15 min under initial concentrations ranging from 1 ppb to 1000 ppb, which demonstrates an excellent degradation kinetics. As for the sulfonic acid of PFOS, an as high as 95.5% degradation efficiency was obtained within 300 min. The degradation rates were 4.5 mg L^-1 h^-1 for PFOA and 0.54 mg L^-1 h^-1 for PFOS, respectively. In parallel, the f-TMO photocatalyst still exhibited a >96.2% degradation efficiency after eight regeneration cycles. The high physical adsorption capacity and high defluorination rate make this f-TMO catalyst promising applications in removing various PFASs from a broad range of residential and industrial water systems.

Catalytic degradation of clothianidin with graphene/TiO2 using a dielectric barrier discharge (DBD) plasma system

Clothianidin served as the model pollutant to investigate the performance and mechanism of pollutant removal by dielectric barrier discharge plasma (DBD) combined with the titanium dioxide-reduced graphene oxide (rGO-TiO₂) composite catalyst. In this study, different ratios of titanium dioxide-graphene catalysts were loaded onto honeycomb ceramic plates via the sol-gel method, and the modified catalytic ceramic plates were characterized by XRD, SEM, FTIR, DRS, and energy dispersive X-ray. The results suggested that the rGO-TiO₂ was well loaded on the surface of the honeycomb ceramic plates. According to the results of the characterization experiments and the degradation of the clothianidin solution with different proportions of the catalyst, 8 wt% rGO-TiO₂ was selected as the optimum ratio for degradation. Clothianidin degradation efficiency was significantly influenced by input power, clothianidin concentration, pH value, liquid conductivity, free radical quencher. Finally, six degradation products of clothianidin were identified by HPLC-MS, and the possible transformation pathways of clothianidin degradation were identified. Graphical abstract.

Internal Energy Deposition in Dielectric Barrier Discharge Ionization is Significantly Lower than in Direct Analysis in Real-Time Mass Spectrometry

The extent of internal energy deposition using three different plasma-based ionization mass spectrometry (MS) methods, atmospheric pressure chemical ionization (APCI), direct analysis in real time (DART), and active capillary dielectric barrier discharge ionization (DBDI), was investigated using benzylammonium ‘thermometer’ ions. Ions formed by DBDI were activated significantly less than those that were formed by DART and APCI under these conditions. Thermal ion activation by DART can be reduced slightly by positioning the DART source further from the capillary entrance to the MS and reducing the heat that is applied to metastable atoms exiting the DART source. For example, the average ion internal energy distribution decreased by less than 10 % (166.9 ± 0.3 to 152.2 ± 1.0 kJ mol−1) when the distance between the DART source and the MS was increased by 250 % (10 to 25 mm). By lowering the DART temperature from 350 to 150°C, the internal energy distributions of the thermometer ions decreased by ~15 % (169.93 ± 0.83 to 150.21 ± 0.52 kJ mol−1). Positioning the DART source nozzle more than 25 mm from the entrance to the MS and decreasing the DART temperature further resulted in a significant decrease in ion signal. Thus, varying the major DART ion source parameters had minimal impact on the ‘softness’ of the DART ion source under these conditions. Overall, these data indicate that DBDI can be a significantly ‘softer’ ion source than two of the most widely used plasma-based ion sources that are commercially available.