Effects of pH on photochemical decomposition of perfluorooctanoic acid in different atmospheres by 185nm vacuum ultraviolet

Vacuum-ultraviolet based advanced oxidation and reduction processes for water treatment

The use of vacuum-ultraviolet (VUV) photolysis in water treatment has been gaining significant interest due to its efficacy in degrading refractory organic contaminants and eliminating oxyanions. In recent years, the reactive species driving pollutant decomposition in VUV-based advanced oxidation and reduction processes (VUV-AOPs and VUV-ARPs) have been identified. This review aims to provide a concise overview of VUV photolysis and its advancements in water treatment. We begin with an introduction to VUV irradiation, followed by a summary of the primary reactive species in both VUV-AOPs and VUV-ARPs. We then explore the factors influencing VUV-photolysis in water treatment, including VUV irradiation dose, catalysts or activators, dissolved gases, water matrix components (e.g., DOM and inorganic anions), and solution pH. In VUV-AOPs, the predominant reactive species are hydroxyl radicals (˙OH), hydrogen peroxide (H2O2), and ozone (O3). Conversely, in VUV-ARPs, the main reactive species are the hydrated electron (eaq-) and hydrogen atom (˙H). It is worth noting that VUV-based advanced oxidation/reduction processes (VUV-AORPs) can transit between VUV-AOPs and VUV-ARPs based on the externally added chemicals and dissolved gases in the solution. Increase of the VUV irradiation dose and the concentration of catalysts/activators enhances the degradation of contaminants, whereas DOM and inorganic anions inhibit the reaction. The pH influences the redox potential of ˙OH, the speciation of contaminants and activators, and thus the overall performance of the VUV-AOPs. Conversely, an alkaline pH is favored in VUV-ARPs because eaq- predominates at higher pH.

Kinetics and proposed mechanisms of hexafluoropropylene oxide dimer acid (GenX) degradation via vacuum-UV (VUV) photolysis and VUV/sulfite processes

We investigated the degradation of hexafluoropropylene oxide dimer acid (GenX) in water via VUV photolysis and VUV/sulfite reactions under nitrogen-saturated conditions. Approximately 35% and 90% of GenX were degraded in 3 h in the VUV photolysis and VUV/sulfite reaction. While GenX removal rate was highest at pH 6 in VUV photolysis, it increased under alkaline pHs, especially at pH 10, in VUV/sulfite reaction. Radical scavenging experiments showed that, while both eaq- and •H contributed to VUV photolysis, eaq- played a significant role and •OH had a negative effect during VUV/sulfite reaction. Two transformation products (TPs) (TFA and PFPrA) were identified in VUV photolysis, whereas five TPs (TFA, PFPrA, TP182, TP348, and TP366) were identified in VUV/sulfite reaction by LCMS/MS and LCQTOF/MS. Defluorination of GenX was observed with the defluorination efficiency after 6 h reaching 17% and 67% in the VUV photolysis and VUV/sulfite reactions, respectively. Degradation mechanism for GenX based on the identified TPs and the theoretical calculation confirmed the susceptibility of GenX to nucleophilic attack. The initial reactions for GenX decomposition were C-C and C-O bond cleavage in both reactions, whereas sulfonation followed by decarboxylation was observed only in the VUV/sulfite reaction. ECOSAR ecotoxicity simulation showed that the toxicities of the TPs were not as harmful as those of GenX.

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.

New insights into ferric iron-facilitated UV254 photolytic defluorination of perfluorooctanoic acid (PFOA): Combined experimental and theoretical study

UV/Fe³⁺-facilitated PFOA defluorination was often reported and recognized to proceed through a "ligand-to-metal charge transfer (LMCT)" mechanism in the literatures. Sufficient Fe³⁺ supply is important for sustaining the LMCT reaction pathway. In this study, an interesting "excessive defluorination" was observed, even the continuous Fe³⁺ supply was cut off, implying a parallel mechanism strengthening PFOA defluorination. Based the results of intermediate products detection, ¹⁹F NMR analysis, and exploration of electron density alternation, transition energy evolution, and bonds characteristics, remarkable electron density perturbation in [PFOA-Fe]²⁺ was revealed. This effect was triggered by the complexation between PFOA anion and Fe³⁺, diminishing electron shielding on the perfluorinated carbon chain. Hence, the dissociation energy of C-C bonds was reduced by maximally 53% (C4-C5). Once attacked by high-flux UV₂₅₄ photons, the perfluorinated carbon chain underwent scission, and subsequent defluorination was achieved via hydrolysis reactions. This parallel mechanism cooperated with the LMCT mechanism, leading to the observed "excessive defluorination." The degree of UV/Fe³⁺-synergized PFOA defluorination depended on UV₂₅₄ photon flux and Fe³⁺ dosage. High UV₂₅₄ intensity guaranteed fast defluorination kinetics. A [Fe³⁺]/[PFOA] molar ratio near 1 showed the best UV/Fe³⁺ synergic effect on PFOA defluorination.

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.

Total organic fluorine (TOF) analysis by completely converting TOF into fluoride with vacuum ultraviolet

Quantifying total organic fluorine (TOF) in water is vital in monitoring the occurrence and persistence of all fluorine-containing organic compounds in the environment, while currently most studies focus on analyzing individual fluorine-containing organic compounds. To fill the technology gap, we herein proposed to convert TOF completely into fluoride with vacuum ultraviolet (VUV) photolysis, followed by analysis of fluoride with ion chromatography. Results showed that the tailored VUV photoreactor achieved satisfying recoveries of fluorine from ten model TOF compounds not only in ultrapure water (83.9 ± 2.0% to 109.4 ± 0.8%) but also in real water samples (92.1 ± 1.0%-106.2 ± 15.7%). Unlike other ultraviolet-based processes that favor alkaline conditions, this VUV process preferred either neutral or acidic conditions to defluorinate selected compounds. While the mechanisms remain to be explored in the future, it has been evidenced that the photo-degradation and photo-defluorination rates of these TOF compounds varied significantly among compounds and operation conditions. The method obtained a method detection limit (MDL) of 0.15 μg-F/L, which is lower than the MDLs of many other TOF analytical methods, along with excellent calibration curves for concentrations ranging from 0.01 to 10.0 mg-F/L. Notably, minimizing fluoride in sample prior to photoconversion was necessary to avoid subtraction-induced errors for TOF measurement, especially when the fluoride/TOF ratio was high. The robust VUV is also green for sample pretreatment due to its unreliance of chemicals or additives.

Per/polyfluoroalkyl substances production, applications and environmental impacts

The per/polyfluoroalkyl substances (PFAS) are growing contaminants which are extremely difficult to get degraded naturally. PFAS have been produced for nearly a century using electrochemical flourination and more relomerization processes. High chemical resistance, hydrophobicity, lipophobicity, heat resistace, extremly low friction coefficient make this class of chemicals invaluable for many applications. These same properties useful unfortunately make them 'forever chemicals' once released into the envrironment. This review focuses on the production and applications of PFAs, determining the concentration of PFAs in environmental and biological matrices and their efficient degradation. Various methods of detection of PFAS have been developed but insitu methods of detction are still in the early stages of development. Current chemical and biological remediation technologies are expensive/not effective and thus new remediation technolgies must be developed. It is imperative to focus on methods for detection of the short chain PFAS with their projected increased use.

Reductive and Oxidative UV Degradation of PFAS—Status, Needs and Future Perspectives

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) consist of a group of environmentally persistent, toxic and bio-accumulative organic compounds of industrial origin that are widely present in water and wastewater. Despite restricted use due to current regulations on their use, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) remain the most commonly detected long-chain PFAS. This article reviews UV-based oxidative and reductive studies for the degradation of PFAS. Most of the UV-based processes studied at lab-scale include low pressure mercury lamps (emitting at 254 and 185 nm) with some studies using medium pressure mercury lamps (200–400 nm). A critical evaluation of the findings is made considering the degradation of PFAS, the impact of water quality conditions (pH, background ions, organics), types of oxidizing/reducing species, and source of irradiation with emphasis given to mechanisms of degradation and reaction by-products. Research gaps related to understanding of the factors influencing oxidative and reductive defluorination, impact of co-existing ions from the perspective of complexation with PFAS, and post-treatment toxicity are highlighted. The review also provides an overview of future perspectives regarding the challenges in relation to the current knowledge gaps, and future needs.

The defluorination of perfluorooctanoic acid by different vacuum ultraviolet systems in the solution

Perfluorooctanoic acid (PFOA) is one kind of persistent organic pollutants that is often detected in water. In recent years, the effective degradation technologies of PFOA have attracted widespread attentions. Thus, in this study, the defluorination efficiency of PFOA in different systems (i.e., ultraviolet (UV), vacuum ultraviolet (VUV), vacuum ultraviolet/persulfate (VUV/PS) and vacuum ultraviolet/residual chlorine (VUV/RC)) was evaluated. Moreover, the different impact factors (i.e., the initial concentrations of persulfate and PFOA, temperature, anions, and initial pH values) on PFOA degradation by VUV/PS system were investigated. The results showed that VUV system was more effective than UV system for PFOA defluorination. VUV system combined with persulfate would further enhance the defluorination efficiency while residual chlorine would decrease it. In VUV/PS system, the defluorination efficiency of PFOA was the best as the molar ratio of PFOA and persulfate at 1:60. Moreover, higher temperature, lower initial PFOA concentration, and acid condition were favorable for the defluorination of PFOA. Under the different influence factors, the defluorination efficiency of PFOA fitted well to the first-order reaction kinetic model. When the temperature was range from 20°C to 40°C, the value of activation energy was 8.73 kJ/mol. Besides, the inhibition effect of three kinds of anions on PFOA defluorination followed the order: NO 3 -  > Cl^-  >  CO 3 2 - . PRACTITIONER POINTS: The defluorination efficiency of perfluorooctanoic acid (PFOA) in water by different VUV systems was compared. VUV system is more effective than UV system for PFOA defluorination. Persulfate will enhance the defluorination efficiency by VUV system. Hypochlorite will decrease the defluorination efficiency by VUV system.

In-situ fabrication of a spherical-shaped Zn-Al hydrotalcite with BiOCl and study on its enhanced photocatalytic mechanism for perfluorooctanoic acid removal performed with a response surface methodology

Perfluorooctanoic acid (PFOA), a widely used compound, is harmful to the environment and human health. In this study, a facile one pot solvothermal method of integrating BiOCl with Zn-Al hydrotalcite to form spherical-shaped BiOCl/Zn-Al hydrotalcite (B-BHZA) sample is reported. The characteristics and main factors affecting photocatalytic PFOA and photocatalytic mechanism of BiOCl/Zn-Al hydrotalcite (B-BHZA) are systematically investigated. It is found that spherical-shaped B-BHZA possesses abundant defects and a larger surface area of 64.4 m² g^-1. The factors affecting photocatalytic removal PFOA (e.g., time, pH, initial concentration and dosage) are investigated by modeling the 3D surface response. The removal rate of PFOA is over 90 % in 6 h under UV light at an optimal pH of 2, an initial concentration of 500 μg/L and a dose of dosage 0.5 g/L. The main mechanism occurs by photo-generated h⁺ oxidation and synergistic effects from the photocatalysis process. Though investigating the intermediates of PFOA degradation and F^-, a possibility was proposed that h⁺ initiated the rapidly decarboxylation of PFOA. The unstable perfluoroheptyl group is formatted and further conversed to short chain perfluorocarboxylic acid. This study provides a new insight for the preparation of highly efficient photocatalysts to the treatment of halogenated compounds in UV system.

Rapid photochemical decomposition of perfluorooctanoic acid mediated by a comprehensive effect of nitrogen dioxide radicals and Fe3+/Fe2+ redox cycle

Developing efficient methods to degrade perfluorochemicals (PFCs), an emerging class of highly recalcitrant contaminants, are urgently needed in recent years, due to their persistence, high toxicity, and resistance to most regular treatment procedures. Here, a UV-photolysis system is reported for efficient mineralization of perfluorooctanoic acid (PFOA) via irradiation of ferric nitrate aqueous solution, where in-situ generating •NO₂ and the effective Fe³⁺/Fe²⁺ redox cycle synergistically play great roles on rapidly mediating the mineralization of PFOA. A fast PFOA removal kinetics with first-order kinetic constants of 2.262 h^-1 is observed at initial PFOA concentration of 5 ppm (50 mL volume), reaching ∼ 92 % removal efficiency within only 0.5-h irradiation. Near-stoichiometric fluoride ions liberation and high total organic carbon (TOC) removal efficiency (∼100 %) further validated the capability for completely destructive removal of PFOA. A tentative pathway for PFOA destruction is proposed. This work, by UV photolysis of abundant existing iron/nitrate-based systems in natural environment, provides an economical, sustainable and highly efficient approach for complete mineralization of perfluorinated chemicals.

Highly effective adsorption removal of perfluorooctanoic acid (PFOA) from aqueous solution using calcined layer-like Mg-Al hydrotalcites nanosheets

To study the influence factors of calcined layer-like Mg-Al hydrotalcites nanosheets adsorbing perfluorooctanoic acid (PFOA) in aqueous solution, Mg-Al hydrotalcite (HMA) nanosheets were prepared by one-step hydrothermal synthesis. The effect of calcination temperature on adsorption properties and structure of HMA (CHMA-x, x means different calcination temperature) was investigated. The prepared samples were systematically characterized by the Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), X-ray diffraction (XRD), scanning electronic microscopy (SEM), and nitrogen adsorption-desorption isotherms. The adsorption isotherms and kinetics showed the adsorption equilibrium reached within 2 h, and the factors, such as adsorption dosage, pH, and cycles were investigated. It was found that CHMA with 600 °C displayed a uniformly morphology, higher surface area about 106.3 m²/g, and excellent adsorption properties (1969 mg/g). The equilibrium adsorption data perfectly fitted to the pseudo-second-order kinetic model (R² = 0.999) and the Freundlich model (R² = 0.994). The main mechanism of CHMA adsorbing PFOA might be the "memory effect." This study provided a new insight to prepare highly effective adsorbents in water treatment.

Hydrated electron based decomposition of perfluorooctane sulfonate (PFOS) in the VUV/sulfite system

As one of the most reactive species, hydrated electron (e_aq^-) is promising for reductive decomposition of recalcitrant organic pollutants, such as perfluorooctane sulfonate (PFOS). In this study, PFOS decomposition using a vacuum ultraviolet (VUV)/sulfite system was systematically investigated in comparison with sole VUV and ultraviolet (UV)/sulfite systems. A fast and nearly complete (97.3%) PFOS decomposition was observed within 4h from its initial concentration of 37.2μM in the VUV/sulfite system. The observed rate constant (k_obs) for PFOS decomposition in the studied system was 0.87±0.0060h^-1, which was nearly 7.5 and 2 folds faster than that in sole VUV and UV/sulfite systems, respectively. Compared to previously studied UV/sulfite system, VUV/sulfite system enhanced PFOS decomposition in both weak acidic and alkaline pH conditions. In weak acidic condition (pH6.0), PFOS predominantly decomposed via direct VUV photolysis, whereas in alkaline condition (pH>9.0), PFOS decomposition was mainly induced by e_aq^- generated from both sulfite and VUV photolytic reactions. At a fixed initial solution pH (pH10.0), PFOS decomposition kinetics showed a positive linear dependence with sulfite dosage. The co-presence of humic acid (HA) and NO₃^- obviously suppressed PFOS decomposition, whereas HCO₃^- showed marginal inhibition. A few amount of short chain perfluorocarboxylic acids (PFCAs) were detected in PFOS decomposition process, and a high defluorination efficiency (75.4%) was achieved. These results suggested most fluorine atoms in PFOS molecule ultimately mineralized into fluoride ions, and the mechanisms for PFOS decomposition in the VUV/sulfite system were proposed.

Nitrogen dioxide radicals mediated mineralization of perfluorooctanoic acid in aqueous nitrate solution with UV irradiation

Effective decomposition of perfluorooctanoic acid (PFOA) has received increasing attention in recent years because of its global occurrence and resistance to most conventional treatment processes. In this study, the complete mineralization of PFOA was achieved by the UV-photolysis of nitrate aqueous solution (UV/Nitrate), where the in-situ generated nitrogen dioxide radicals (NO₂) efficiently mediated the degradation of PFOA. In particular, when the twinborn hydroxyl radicals were scavenged, the production of more NO₂ radicals realized the complete mineralization of PFOA. DFT calculations further confirm the feasibility of PFOA removal with NO₂. Near-stoichiometric equivalents of fluoride released rather than the related intermediates were detected in solution after decomposition of PEOA, further demonstrating the complete degradation of PFOA. Possible PFOA degradation pathways were proposed on the basis of experimental results. This work offers an efficient strategy for the complete mineralization of perfluorinated chemicals, and also sheds light on the indispensable roles of nitrogen dioxide radicals for environmental pollutants removal.

Combined sonochemical and short-wavelength UV degradation of hydrophobic perfluorinated compounds

Perfluorochemicals (PFCs), which are common in the aquatic environment, are toxic substances that have high chemical and heat resistance because of their strong C-F bonds. We investigated the effect of ultrasonication and short-wavelength UV irradiation on the degradation of perfluorooctane, perfluoropropionic acid, and perfluorooctanoic acid, which are examples of hydrophobic, hydrophilic, and intermediate PFCs, respectively. The results confirmed that ultrasonication was more effective for decomposing hydrophobic PFCs and UV irradiation was more effective for decomposing hydrophilic PFCs. Therefore, defluorination of the degradation intermediates was improved by a combination of ultrasonication and UV irradiation. Our results can be applied to the decomposition treatment of PFCs that have various levels of water solubility in the aquatic environment.

Experimental and theoretical insights into the photochemical decomposition of environmentally persistent perfluorocarboxylic acids

Decomposition of perfluorocarboxylic acids (PFCAs) is of great significance due to their global distribution, persistence and toxicity to organisms. In this study, the photodegradation of a series of PFCAs (∼C2C12) in water by a medium-pressure mercury lamp was experimentally and theoretically examined. We found that photolysis of PFCAs all follow pseudo-first-order kinetics with the rate constant (k_app) increasing with carbon chain lengths, except for trifluoroacetic acid (TFA) which cannot be degraded by the polychromatic irradiation. Product analysis showed that the PFCAs were mainly decomposed into shorter carbon chain length PFCAs in a stepwise manner, with the accumulation of TFA and fluoride ions as the end products. Moreover, a small amount of perfluoroolefins (C_nF_2n) was determined as gas-phase products. Wiberg bond order calculations confirmed the cleavage of the CC bond between carboxylic carbon and the adjacent carbon as the first reaction step, and density functional theory-based calculations revealed that k_app value is correlated with some molecular structural parameters. In the case of mixture irradiation, the evolution profiles of individual PFCAs were different from that in single-component systems, due to the dynamic balance between production and degradation. This work reveals the main molecular descriptors controlling the degradation rate of different PFCAs species, and improves the general understanding on the photodegradation mechanisms, which will provide useful information for future researches.

Photochemical defluorination of aqueous perfluorooctanoic acid (PFOA) by Fe(0)/GAC micro-electrolysis and VUV-Fenton photolysis

Perfluorooctanoic acid (PFOA) is extremely persistent and bioaccumulative in the environment; thus, it is very urgent to investigate an effective and moderate technology to treat the pollution of PFOA. In this study, a process combined iron and granular activated carbon (Fe(0)/GAC) micro-electrolysis with VUV-Fenton system is employed for the remediation of PFOA. Approximately 50 % PFOA (10 mg L(-1)) could be efficiently defluorinated under the following conditions: pH 3.0, dosage of Fe 7.5 g L(-1), dosage of GAC 12.5 g L(-1), and concentration of H2O2 22.8 mmol L(-1). Meanwhile, during the process, evident defluorination was observed and the concentration of fluoride ion was eventually 3.23 mg L(-1). The intermediates including five shorter-chain perfluorinated carboxylic acids (PFCAs), i.e., C7, C6, C5, C4, and C3, were also analyzed by high-performance liquid chromatography tandem mass spectrometry (HPLC/MS/MS) and defluorination mechanisms of PFOA was proposed, which involved photochemical of OH·, direct photolysis (185-nm VUV), and photocatalytic degradation of PFOA in the presence of Fe(3+) (254-nm UV).

Decomposition of perfluorooctanoic acid by ultraviolet light irradiation with Pb-modified titanium dioxide

Perfluorooctanoic acid (PFOA, C7H15COOH) is widely used in industrial and commercial applications. It has become a global concern due to its widespread occurrence in water bodies and adverse environmental impact. PFOA could not be effectively removed by the conventional UV/TiO2 system. This study synthesized Pb-modified TiO2 catalyst and used it as a catalyst with light irradiation for PFOA decomposition. It was found that the Pb-TiO2 catalyst could produce traps to capture photo-induced electrons or holes that lead to better photocatalytic efficiencies. Rate constant values for PFOA decomposition by the UV/TiO2 and UV/Pb-TiO2 systems were determined to be 0.0158 and 0.5136 h(-1), respectively. The PFOA decomposition in the UV/Pb-TiO2 system is 32.5 times faster than that in the UV/TiO2 system. The UV/Pb-TiO2 system yielded a better performance than those of the UV/Fe-TiO2 and UV/Cu-TiO2 systems. During the reaction, PFOA decomposed stepwisely into shorter-chain perfluorocarboxylic acids and F(-).

Enhancement of photocatalytic decomposition of perfluorooctanoic acid on CeO2/In2O3

The xCeO₂/In₂O₃ catalysts were synthesized and used for photocatalytic decomposition of PFOA. The excellent activity and stability was derived from inhibition of electron-holes recombination caused by the charge transfer between CeO₂ and In₂O₃.