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

A review on the advancement in photocatalytic degradation of poly/perfluoroalkyl substances in water: Insights into the mechanisms and structure-function relationship

Poly/perfluoroalkyl substances (PFAS) are persistent organic pollutants and ubiquitous in aquatic environment, which are hazardous to organisms and human health. Several countries and regions have taken actions to regulate or limit the production and emission of some PFAS. Even though a series of water treatment technologies have been developed for removal of PFAS to eliminate their potential adverse effects, the removal and degradation performance are usually unsatisfactory. Photocatalytic degradation of PFAS is considered as one of the most effective approaches due to the mild operation conditions and environmental friendliness. This review systematically summarized the recent advances in photocatalytic degradation of PFAS based on heterogeneous photocatalysts, including TiO2-, Ga2O3-, In2O3-, ZnO-, Bi-based, and others. Overall, two mainly degradation mechanisms were involved, including photo-oxidation (involving the holes and oxidative radicals) and photo-reduction types (by e- and reductive radicals). The band structures of the photocatalysts, degradation pathways, structure-function relationship, and impacting factors were further discussed to elucidate the essential reasons for the enhanced degradation of PFAS. Furthermore, the review identified the major knowledge gaps to solve the issues of photocatalysis in real application. This paper also propounded several strategies to promote the design and optimization of high-efficient photocatalysts, and meet the challenges to remove PFAS through photodegradation technologies.

Enhanced perfluorooctanoic acid (PFOA) degradation by electrochemical activation of peroxydisulfate (PDS) during electrooxidation for water treatment

Improved treatment of per- and polyfluoroalkyl substances (PFAS) in water is critically important in light of the proposed United States Environmental Protection Agency (USEPA) drinking water regulations at ng L-1 levels. The addition of peroxymonosulfate (PMS) during electrooxidation (EO) can remove and destroy PFAS, but ng L-1 levels have not been tested, and PMS itself can be toxic. The objective of this research was to test peroxydisulfate (PDS, an alternative to PMS) activation by boron-doped diamond (BDD) electrodes for perfluorooctanoic acid (PFOA) degradation. The influence of PDS concentration, temperature, and environmental water matrix effects, and PFOA concentration on PDS-EO performance were systematically examined. Batch reactor experiments revealed that 99 % of PFOA was degraded and 69 % defluorination was achieved, confirming PFOA mineralization. Scavenging experiments implied that sulfate radicals (SO4-) and hydroxyl radicals (HO) played a more important role for PFOA degradation than 1O2 or electrons (e-). Further identification of PFOA degradation and transformation products by liquid chromatography-mass spectrometry (LC-MS) analysis established plausible PFOA degradation pathways. The analysis corroborates that direct electron transfers at the electrode initiate PFOA oxidation and SO4- improves overall treatment by cleaving the CC bond between the C7F15 and COOH moieties in PFOA, leading to possible products such as C7F15 and F-. The perfluoroalkyl radicals can be oxidized by SO4- and HO, resulting in the formation of shorter chain perfluorocarboxylic acids (e.g., perfluorobutanoic acid [PFBA]), with eventual mineralization to CO2 and F-. At an environmentally relevant low initial concentration of 100 ng L-1 PFOA, 99 % degradation was achieved. The degradation of PFOA was slightly affected by the water matrix as less removal was observed in an environmental river water sample (91 %) compared to tests conducted in Milli-Q water (99 %). Overall, EO with PDS provided a destructive approach for the elimination of PFOA.

Plant-mediated synthesis of silver-doped ZnO nanoparticles with high sonocatalytic activity: Sonocatalytic behavior, kinetic and thermodynamic study

Together with the rapid growth of technology, the discharge of wastewater from industry into environment had become a hot topic among society nowadays. More attention had been given to the development of water treatment techniques. In this study, sonocatalysis was proposed to degrade the organic pollutants using silver-doped zinc oxide (Ag-ZnO) nanoparticles which were synthesized via green synthesis process using Clitoria ternatea Linn (Asian Pigeonwings flower). The characterization results revealed that the incorporation of Ag into the ZnO lattice decreased the crystallite size and increased the specific surface area of ZnO nanoparticles. It is noteworthy that about 98% of sonocatalytic degradation efficiency of malachite green (MG) was successfully achieved within 30 min in the presence of 5 wt.% Ag-ZnO with 1.0 g/L of catalyst loading under 500 mg/L of initial dye concentration, 80 W of ultrasonic power, 45 kHz of ultrasound frequency, and 2.0 mM of oxidant concentration. The kinetic study showed that the sonocatalytic degradation of organic dye was fitted well into second-order kinetic model with high R2 value (0.9531). In the thermodynamic study, negative value of standard Gibbs free energy and low value of activation energy (+ 24.43 kJ/mol) were obtained in the sonocatalytic degradation of MG using the green-synthesized Ag-ZnO sample. HIGHLIGHTS: • Facile synthesis of silver-doped zinc oxide nanoparticles using plant extract which act as reducing and stabilizing agents • Optical, physical, and chemical characterization of green-synthesized nanomaterials were performed • Evaluation of sonocatalytic degradation of organic dye using green-synthesized nanomaterials • Sonocatalytic behavior, kinetic and thermodynamic studies of sonocatalytic reaction.

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.

Investigation of ternary Zn-Co-Fe layered double hydroxide as a multifunctional 2D layered adsorbent for moxifloxacin and antifungal disinfection

Layered double hydroxides have recently gained wide interest as promising multifunctional nanomaterials. In this work, a multifunctional ternary Zn-Co-Fe LDH was prepared and characterized using XRD, FTIR, BET, TEM, SEM, and EDX. This LDH showed a typical XRD pattern with a crystallite size of 3.52 nm and a BET surface area of 155.9 m2/g. This LDH was investigated, for the first time, as an adsorbent for moxifloxacin, a common fluoroquinolones antibiotic, showing a maximum removal efficiency and equilibrium time of 217.81 mg/g and 60 min, respectively. Its antifungal activity, for the first time, was investigated against Penicillium notatum, Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, and Mucor fungi at various concentrations (1000-1.95 µg/mL). This LDH was found to be effective against a variety of fungal strains, particularly Penicillium and Mucor species and showed zones of inhibition of 19.3 and 21.6 mm for Penicillium and Mucor, respectively, with an inhibition of 85% for Penicillium species and 68.3% for Mucormycosis. The highest antifungal efficacy results were obtained at very low MIC concentrations (33.3 and 62 µg/ml) against Penicillium and Mucor, respectively. The results of this study suggest a promising multifunctional potential of this LDH for water and wastewater treatment and disinfection applications.

Y-mediated optimization of 3DG-PbO2 anode for electrochemical degradation of PFOS

In our previous study, the three-dimensional graphene-modified PbO2 (3DG-PbO2) anode was prepared for the effective degradation of perfluorooctanesulfonat (PFOS) by the electrochemical oxidation process. However, the mineralization efficiency of PFOS at the 3DG-PbO2 anode still needs to be further improved due to the recalcitrance of PFOS. Thus, in this study, the yttrium (Y) was doped into the 3DG-PbO2 film to further improve the electrochemical activity of the PbO2 anode. To optimize the doping amount of Y, three Y and 3DG codoped PbO2 anodes were fabricated with different Y3+ concentrations of 5, 15, and 30 mM in the electroplating solution, which were named Y/3DG-PbO2-5, Y/3DG-PbO2-15 and Y/3DG-PbO2-30, respectively. The results of morphological, structural, and electrochemical characterization revealed that doping Y into the 3DG-PbO2 anode further refined the β-PbO2 crystals, increased the oxygen evolution overpotential and active sites, and reduced the electron transfer resistance, resulting in a superior electrocatalytic activity. Among all the prepared anodes, the Y/3DG-PbO2-15 anode exhibited the best activity for electrochemical oxidation of PFOS. After 120 min of electrolysis, the TOC removal efficiency was 80.89% with Y/3DG-PbO2-15 anode, greatly higher than 69.13% with 3DG-PbO2 anode. In addition, the effect of operating parameters on PFOS removal was analyzed by response surface, and the obtained optimum values of current density, initial PFOS concentration, pH, and Na2SO4 concentration were 50 mA/cm2, 12.21 mg/L, 5.39, and 0.01 M, respectively. Under the optimal conditions, the PFOS removal efficiency reached up to 97.16% after 40 min of electrolysis. The results of the present study confirmed that the Y/3DG-PbO2 was a promising anode for electrocatalytic oxidation of persistent organic pollutants.

Advanced electrocatalytic redox processes for environmental remediation of halogenated organic water pollutants

Halogenated organic compounds are widespread, and decades of heavy use have resulted in global bioaccumulation and contamination of the environment, including water sources. Here, we introduce the most common halogenated organic water pollutants, their classification by type of halogen (fluorine, chlorine, or bromine), important policies and regulations, main applications, and environmental and human health risks. Remediation techniques are outlined with particular emphasis on carbon-halogen bond strengths. Aqueous advanced redox processes are discussed, highlighting mechanistic details, including electrochemical oxidations and reductions of the water-oxygen system, and thermodynamic potentials, protonation states, and lifetimes of radicals and reactive oxygen species in aqueous electrolytes at different pH conditions. The state of the art of aqueous advanced redox processes for brominated, chlorinated, and fluorinated organic compounds is presented, along with reported mechanisms for aqueous destruction of select PFAS (per- and polyfluoroalkyl substances). Future research directions for aqueous electrocatalytic destruction of organohalogens are identified, emphasizing the crucial need for developing a quantitative mechanistic understanding of degradation pathways, the improvement of analytical detection methods for organohalogens and transient species during advanced redox processes, and the development of new catalysts and processes that are globally scalable.