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. Author manuscript; available in PMC: 2023 Jul 18.
Published in final edited form as: Chemosphere. 2021 Apr 27;280:130660. doi: 10.1016/j.chemosphere.2021.130660

Photooxidative decomposition and defluorination of perfluorooctanoic acid (PFOA) using an innovative technology of UV–vis/ZnxCu1-xFe2O4/oxalic acid

Sanny Verma a,b,1, Bineyam Mezgebe a,1, Endalkachew Sahle-Demessie c,*, Mallikarjuna N Nadagouda d,**
PMCID: PMC10353478  NIHMSID: NIHMS1768694  PMID: 33962294

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a large group of perfluorinated organic molecules that have been in use since the 1940s for industrial, commercial, and consumer applications. PFAS are a growing concern because some of them have shown persistent, bioaccumulative and toxic effects. Herein, we demonstrate an innovative technology of UVevis/ZnxCu1-xFe2O4/oxalic acid for the degradation of perfluorooctanoic acid (PFOA) in water. The magnetically retrievable nanocrystalline heterogeneous ferrite catalysts, ZnxCu1-xFe2O4 were synthesized using a sol-gel auto-combustion process followed by calcination at 400 °C. The combination of ZnxCu1-xFe2O4 and oxalic acid generate reactive species under UV light irradiation. These reactive species are then shown to be capable of the photodegradation of PFOA. The degree of degradation is tracked by identifying transformation products using liquid chromatography coupled with quadrupole time-of-flight mass spectroscopy (LC-QTOF-MS).

Keywords: Magnetically retrievable catalyst, Heterogeneous photocatalytic system, UV–Vis, Oxalic acid, PFAS, PFOA

Graphical Abstract

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1. Introduction

Per- and polyfluoroalkyl substances (PFAS) are a group of fluorine-containing organic molecules that have been utilized in a large number of products, including aqueous film-forming foams (AFFF) to fight fires, as a repellent to grease and water, and as a material resistant to heat, friction, and degradation by other chemicals or bacteria (Ghisi et al., 2019; Post et al., 2012). Over the past two decades, scientists have detected PFAS, particularly perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), in the environment across the globe. The strong C–F bond and other properties that make PFAS useful also make them recalcitrant to biological and photodegradation. Thus, PFAS are ubiquitous in the environment (Nakayama et al., 2017), and they are commonly detected in human biomonitoring studies (Corsini et al., 2014), with studies showing potential health concerns (Yu et al., 2009).

As the risks of releasing PFAS throughout their lifecycle, during manufacturing, use, and end-of-life continue to be studied, there are increasing efforts to regulate PFAS by local, state, and federal authorities. Some health-based regulations for PFAS in drinking water have been developed (La Farre et al., 2008). A lifetime drinking water health advisory of 70 ng/L (70 ppt) for the combined concentration of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) has been established by USEPA (US EPA, 2016; Cordner et al., 2019; Ahrens et al., 2014; Sunderland et al., 2019).

Currently, activated carbon adsorption, high-pressure membrane filtration, and anion exchange (AIX) resins are typically employed to treat water to remove PFAS (Crone et al., 2019). Although it depends on many factors, nanofiltration (NF) and reverse osmosis (RO) membranes are perhaps the most successful in removing a wide range of PFAS. However, NF and RO are very expensive, and the concentrated waste stream of PFAS requires further treatment (Appleman et al., 2013; Cummings et al., 2015; Franke et al., 2019). Activated carbon adsorption, specifically granular activated carbon (GAC), is the most studied and utilized. However, many PFAS, specifically the smaller chain PFAS, do not have high adsorption potentials. Also, other factors such as competition from other contaminants and natural organic matter can lower GAC performance. Finally, it is unknown as to the efficacy of carbon reactivation (Kucharzyk et al., 2017). AIX processes can also eliminate PFAS effectively. However, although capacities are generally higher than GAC, the cost of the media is also higher, resulting in an unclear economic advantage. The regeneration of used AIX is very difficult and generates an additional residual stream to treat (Franke et al., 2019). High-capacity resin is often recommended in a once-used situation with the media destroyed after use (Emery et al., 2019).

Other approaches have been tried for removing PFAS from high-strength solutions, such as foam fractionation techniques (Dai et al., 2019). Thus, this process’s operating cost is low; it generates a small volume of highly accumulated PFAS containing water. However, this protocol is not effective toward short-chain PFAS, and the concentrated waste stream still needs to be treated (Dai et al., 2019). Numerous promising dissociative methods such as biological, electrochemical, sonolysis, thermolysis, chemical reduction and oxidation, radiochemical treatment, subcritical treatment process, and photochemical treatment have been considered and evaluated for the removal of PFAS (Ross et al., 2018). Previous studies have concluded that the photochemical process establishes itself as a feasible technique for the degradation of PFOA at ambient reaction parameters in water treatment (George et al., 2015). Photocatalytic degradation comprises many steps such as adsorption-desorption, electron-hole pair generation, electron-hole recombination, and chemical reaction. These phenomena generate reactive species such as superoxide (O2−) and hydroxyl radicals (•OH) (Zhang et al., 2014; Giri et al., 2011). These reactive species have a significant role in the degradation of organic contaminants (Yamamoto et al., 2007; Jing et al., 2007).

Due to its strength, the carbon-fluorine bond in PFOA requires a forceful process to ease C–F cleavage (Bentel et al., 2019). Nanomaterials such as titanium dioxide, titanium nanotubes, indium oxide, graphene oxide, carbon nanotubes, etc., have been utilized in photocatalytic degradation PFOA (Saleh et al., 2019). However, all these reported methods require prolonged treatment duration, and after the reaction process, separation of catalysts requires extra attention (Saleh et al., 2019).

Consequently, additional studies on the use of nanocomposites have been promised (Trojanowicz et al., 2018; Enami et al., 2014). Fenton’s reagent has been known since 1894. However, it is a homogeneous process in its simplest form, and the ferrous catalyst cannot be recovered. Iron ions have been immobilized into activated zeolite, mesoporous silica-alumina, clay, and other porous materials to make heterogeneous Fenton-like catalyst systems (Zhang et al., 2019) to resolve this. However, these heterogeneous catalytic systems showed a slow degradation rate due to the high mass transfer limitation between organic pollutants and the catalytic surface. The use of nano-ferrites as a heterogeneous magnetic-catalyst overcomes many of the above limitations because it has fast mass transfer, high activity, large surface area, and potential recoverability using an external magnetic field (Rossi et al., 2014; Ortiz-Quiñ;onez et al., 2018). Valdés-Solís et al. developed manganese ferrite, MnFe2O4 nanoparticles, as a magnetically separable heterogeneous catalyst for the photo-Fenton dissociation of H2O2 (Valdés-Solís et al., 2007 ). Liu et al. studied the magnetic nickel ferrite, NiFe2O4, as a heterogeneous photo-Fenton catalyst in the presence of oxalic acid under UV light (Liu et al., 2012). Hydroxyl radicals are generated through this heterogeneous catalytic system utilized to degrade rhodamine B (Liu et al., 2012). Ni and his research group used the combination of additives (such as hydrogen peroxide, oxalic acid, and KOH) and ZnFe2O4 nanocrystal as a photocatalyst for the effective dissociation of methylene blue organic dye (Liu et al., 2014). Recovery of ZnFe2O4 catalyst at the end of the reaction using an extrinsic magnet is very tough due to its weak magnetic nature. Hence, the ZnFe2O4 catalyst is not compatible with environmental sustainability principles (Huang et al., 2018). Nevertheless, this problem can be overcome by introducing another transition metal such as copper into ZnFe2O4 (Singh et al., 2014).

Engaged in the design and development of treatment technologies for perfluorinated compounds (Dionysiou et al., 2020; Parenky et al., 2020), herein, we explore an innovative UVevis/ZnxCu1-xFe2O4/oxalic acid process for the effective degradation and treatment of PFOA. To the extent of our knowledge, this heterogeneous nano-ferrites catalyst, in combination with oxalic acid, has not yet been reported for PFAS degradation. This method presents an example of the heterogeneous photocatalyst, leading to the in-situ generation of reactive species under UV irradiation. Based on extensive experimental results, this heterogeneous magnetic catalyst demonstrates a high photoproduction of reactive species due to the synergic effect of Zn and Cu. This method holds promise as a sustainable, low-cost approach for the degradation of PFAS, with the minimum generation of byproducts at ambient temperatures.

2. Materials and methods

2.1. Materials

All the reagents have been utilized as received without additional purification. Perfluorooctanoic acid, Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), copper nitrate hemi(pentahydrate (Cu(NO3)2·2.5H2O), ferric nitrate nonahydrate (Fe(NO3)3·9H2O) and citric acid (H2C2O4.H2O) have been purchased from Sigma-Aldrich (USA). All solutions have been made with 17.6 MΩ cm deionized Milli-Q water.

2.2. Synthesis of nanocrystalline heterogeneous nano-ferrites, ZnxCu1-xFe2O4 catalyst via the sol-gel auto-combustion process

Nanosized metal oxides can be made in the sol-gel using an auto-combustion method where an exothermic reaction occurs between oxidants, such as metal nitrates, and fuel sources such as citric acid, urea, and organic amines (Niu et al., 2017). Citric acid also acts as a complexing agent by generating [citrate]3- ions species and allows smaller size and crystallinity of the nanoparticles (Niu et al., 2017). In this regard, herein, we synthesized the heterogeneous nano-ferrites, ZnxCu1-xFe2O4 (x = 0.2, 0.4, 0.5, 0.6 and 0.8) catalysts by a sol-gel auto-combustion process. Zn(NO3)2·6H2O, Cu(NO3)2·2.5H2O, and Fe(NO3)3·9H2O at the designated stoichiometric ratio were dissolved in Milli-Q water. Then citric acid was mixed into the solution until the molar ratio of iron (III) to citric acid came to 2:3. After homogenizing, the reaction mixture was heated at 90 °C for 2 h to form a thick gel. The resultant gel was activated by combustion and altered into powder by an auto-combustion method at 400 °C for 3h. Finally, the calcined powder was grounded using mortar and pestle, washed with Milli-Q water, and dried in a vacuum oven at 100 °C for 12 h (Fig. 1).

Fig. 1.

Fig. 1.

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Pictorial representation of ZnxCu1-xFe2O4 catalyst synthesis.

2.3. Characterization of ZnxCu1-xFe2O4

The crystal phase and structure of the synthesized catalysts were determined using PANalytical X’Pert Pro, 2-theta diffractometer (PANalytical, Almelo, The Netherlands) at 2-theta range 2–90° under CuKα radiation and a wavelength of 1.54 μm to detect crystalline phases of the synthesized samples. The crystallinity and surface morphology of synthesized heterogeneous catalysts were explored using A JEOL (JEM-2010F) high resolution-transmittance electron microscope (HR-TEM) with field transmission of 200 kV. The scanning electron microscopy (SEM) images were taken at 10 KV using a FEI SCIOS SEM instrument. Gold sputter coating is accomplished on solid catalysts at a pressure ranging from 0.1 to 1 Pa. Energy-dispersive X-ray spectroscopy (EDX) was conducted with an EDAX Octane Elite System at 10 kV 2 nA. The variations of chemical oxidation states of the elements on the surface of nanocrystalline heterogeneous ferrites catalyst were studied using high-resolution X-ray photoelectron spectroscopy equipped with a PHI 5000 VersaProbe II XPS system (Physical Electronics) using a monochromatic Al-Kα source (15 kV, 50 W) with a photon energy of 1486.7 eV and dual-beam charge compensation. All spectra were recorded in a vacuum of 1.4 × 10−7 Pa and at 22 °C. For the high-resolution spectra, pass energy of 23.500 eV and step size of 0.200 eV were applied. The XPS patterns were assessed with the Multi-Pak (Ulvac-PHI, Inc.) software. Additionally, values of binding energy values were referenced to the C1s peak at 286.2 eV.

2.4. Photo-degradation of PFOA

Batch photocatalytic experiments were performed using a 450W (Medium-pressure mercury lamp, quartz, Hanovia Lamp Model Number PC451.050) and power supply (Catalog Number 7830–56, ACE Glass Incorporated) for Ultraviolet (UV) light irradiation (Fig. 2). Initially, the stability of the system, including the light intensity on the samples and water temperature, were established during the test. The light intensity and the temperature were found uniform among the tubes, and stable with an irradiation value of 20 mW/cm2. Glass tubes (8 ml) were filled with known PFOA concentrations such as 100 ppm, 10 p.m., and 100 ppb. ZnxCu1-xFe2O4 nanoparticles (50 mg) were dispersed in a mixture of 6 mL PFOA and 1 mL oxalic acid (0.1 M). The mixtures were then shaken for 10 min to equilibrate the PFOA and ZnxCu1-xFe2O4 catalyst before placing these vials in the reactor. After completing the reaction, the heterogeneous catalyst was recovered using an external magnetic field, and then the reaction mixture was analyzed by LC-Q-TOF. All experiments were conducted in triplicate at 27 °C (Room temperature).

Fig. 2.

Fig. 2.

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Pictorial representation of the photocatalytic experimental setup.

2.5. Analysis methods

The concentration of reduced PFOA were determined using an Agilent 6540 Ultra High Definition (UHD) Accurate Mass Q-TOF with a jet stream ESI interface (Agilent Technologies, Palo Alto, CA, USA). This instrument was furnished with a vacuum degasser, a thermostated autosampler, a binary pump, and a column compartment. Mixtures were isolated using a column, ZORBAX Eclipse XDB-C18, Narrow Bore 3.5um, 2.1 × 100mm; Agilent Technologies, Palo Alto, CA, USA). Buffered water (5 mM ammonium acetate, v/v) and acetonitrile (2% water, 5 mM ammonium acetate, v/v) were applied as mobile phases A and B, respectively. The elution gradient were performed at 0.5 mL/min (Constant flow rate) as follows: 0 min, 5% B; 9 min, 80% B; 10 min, 95% B; 11 min, 5% B, and 12 min, 5% initial conditions until 12 min (Re-equilibration step). 5 μL sample volume was injected, with the column and autosampler being maintained at ambient temperatures.

3. Results

3.1. Characterization of ZnxCu1-xFe2O4 catalysts

The crystallinity of heterogeneous magnetic ferrites, ZnxCu1-xFe2O4 catalysts (x = 0.2, 0.4, 0.5, 0.6 and 0.8) were characterized and confirmed by XRD (Fig. 3). The diffraction peaks at 18.34°, 30.20°, 35.53°, 36.93°, 43.05°, 53.53°, 56.90°, 62.23°, and 73.89° are, respectively, representing the (111), (220), (222), (311), (400), (422), (511), (440), and (533) Bragg reflection which can be further referenced to the spinel copper zinc ferrite (Cu0.5Zn0.5Fe2O4, JCPDS#01–077-0012) (Huang et al., 2018; Singh et al., 2014). Furthermore, the morphology and nanostructure of Zn0.8Cu0.2Fe2O4 and Zn0.5Cu0.5Fe2O4 were established by SEM, EDX, and HR-TEM. The SEM images (Fig. 4) demonstrate that the sol-gel auto-combustion preparation of heterogeneous magnetic ferrites leads to the agglomeration of nanoparticles. Also, EDX analysis confirms the presence of Zn, Cu, and Fe. For complete EDX analysis, see supporting information. HR-TEM analysis (Fig. 5 a & Fig. 5 c) exhibited a primary ~20 nm particle size. A selected area diffraction (SAED) analysis (Fig. 5 b and 5 d) also confirmed the crystallinity of the materials. The surface valence states and functional sites of elements were investigated by XPS analysis (Fig. 6). XPS analysis of Zn0.5Cu0.5Fe2O4 depicted in Fig. 6 and in supporting information. All XPS analyses agreed with previous studies (Huang et al., 2018; Ding et al., 2013; Zhang et al., 2016). As compared with the NIST XPS database and previous studies, peaks at 533.1 eV, 531.5 eV, and 530.1 eV in O1s spectra corresponded to adsorbed oxygen, surface hydroxyl groups, and lattice oxygen, respectively (Zhang et al., 2016).

Fig. 3.

Fig. 3.

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X-ray diffraction analysis of prepared of ZnxCu1-xFe2O4 (x = 0.2, 0.4, 0.5, 0.6 and 0.8).

Fig. 4.

Fig. 4.

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(a, c) SEM and EDX analysis of Zn0.5Cu0.5Fe2O4; (b, d) SEM and EDX analysis of Zn0.8Cu0.2Fe2O4.

Fig. 5.

Fig. 5.

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Transmission electron microscopy (TEM) images of catalysts (a) Zn0.5Cu0.5Fe2O4 (b) Zn0.8Cu0.2Fe2O4; and crystallographic analysis using selected area electron diffraction (SAED) analysis of (c) Zn0.5Cu0.5Fe2O4; (d) Zn0.8Cu0.2Fe2O4.

Fig. 6.

Fig. 6.

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X-ray photoelectron spectroscopy (XPS) analysis of Zn0.5Cu0.5Fe2O4.

The peak at 1021.4 eV demonstrated the occurrence of ≡Zn(II) and was designated to Zn 2p3/2. In Cu 2p spectrum, two peaks at 934 eV and 931.3 eV correspond to ≡Cu(II) and ≡Cu(I), respectively. The peak at 934.0eV with the satellite peaks at 940.1 eV and 942.9 eV were assigned to Cu 2p3/2. The binding energy 710.3 eV and 711.8 eV corresponds to ≡Fe(II) and ≡Fe(III) in Fe 2p3/2. The satellite peak at 719 eV is a representative peak of Fe3+ in γ-Fe2O3 (Fig. 6) (Verma et al., 2015).

3.2. Leaching studies of ZnxCu1-xFe2O4 catalysts

To ascertain that no toxic elements were released from the catalyst, leaching tests were performed in the presence of oxalic acid (10 mg/L). The mixture of ZnXCu1-XFe2O4 (100 mg) and 10 ml oxalic acid (10 mg/L) solution was stirred for 1 h and 18 h. The supernatant was analyzed with ICP-MS. All catalysts showed very low Cu and Zn detection, which were below the EPA drinking water advisory limits 1.3 ppm for Cu and 5 ppm for Zn (Table S1; Supplementary Information) (US EPA, 2018). Zn0.8Cu0.2Fe2O4 and Zn0.5Cu0.5Fe2O4 demonstrated higher stability among the various compositions ZnXCu1-XFe2O4 (Table S1; Supplementary Information).

3.3. Photocatalytic degradation of PFOA

Multiple tests were conducted to determine factors affecting PFOA degradation using UV–vis/ZnxCu1-xFe2O4/oxalic acid. Due to the high stability of Zn0.8Cu0.2Fe2O4 and Zn0.5Cu0.5Fe2O4 during leaching studies, these two compositions of ZnXCu1-XFe2O4 catalyst were tested for the photocatalytic degradation of PFOA. Closed glass tubes with different PFOA concentrations were mixed with catalyst Zn0.5Cu0.5Fe2O4 or Zn0.8Cu0.2Fe2O4 (50 mg) and oxalic acid (0.1 M) placed in a photoreactor. All the experiments were performed in triplicate.

The effects of reductants additives such as oxalic acid, 28–30% ammonium hydroxide, hydrazine hydrate, sodium borohydride, and formic acid on PFOA degradation (C0 = 100 ppb) were tested. Control experiments were performed for a reaction time of 2 h in the absence of a catalyst under dark, visible light, and UV light, respectively.

Two stock solutions of these additives were prepared at 0.1 M and 0.5 M, their pH values were recorded, and PFOA concentrations were adjusted to 100 ppb (Table S2; Supplementary Information). All results obtained from Q-TOF analysis have been summarized in Fig. 7. Both 0.1 M and 0.5 M concentrations of these reductant additives gave slight but encouraging results. However, oxalic acid yielded high degradation of PFOA at pH = 1.28–1.74 (Table S2; Supplementary Information). The results suggest that a renewable source, oxalic acid, can be utilized as a reductive additive for PFOA degradation. This study confirmed that noncatalytic photo-degradation of PFOA is also possible in the presence of a reductive additive such as oxalic acid. However, this approach does not give a high degree of dissociation. Therefore, there is a necessity for a catalyst to stimulate the degradation process at optimal parameters. These observations also supported the previous report, which states that the addition of oxalic acid enhances the photo-degradation of PFOA over P25 TiO2 with the irradiation of 254 nm UV light under a nitrogen atmosphere (Wang and Zhang, 2011). Their research group investigated only 10.5% and 12.4% of PFOA (C0 = 24 μM ≈ 10 ppb) degradation in oxygen and nitrogen atmosphere, respectively, in the presence of TiO2 after 3 h of UV light irradiation. Following nitrogen purging of PFOA solution to remove dissolved oxygen, treatment with 3 mM oxalic acid (pH 2.47) gave 86.7%, whereas the oxygenated solution of PFOA showed only 6.6% PFOA photocatalytic decomposition under the same reaction condition. However, P25 TiO2 photocatalyzed treatment tested only on very low concentration, i.e.,10 ppb of PFOA solution. Similarly, other previous titanium-based photocatalysts have been studied for PFOA photodegradation (Chen et al., 2011, 2015a, 2016; Gatto et al., 2015; Huang et al., 2016; Li et al., 2016; Sansotera et al., 2014; Song et al., 2012; Tian et al., 2017; Wang and Zhang, 2011) (Table S3; Supplementary information). It seems that several photocatalysts require to prolong treatment time, unique arrangements, and primarily applicable for low concentrations of PFOA (Table S3; Supplementary Information).

Fig. 7.

Fig. 7.

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Effects of A) lower (0.1 M) and B) higher (0.5 M) concentrations of various additives/hole-scavengers on the degradation of PFOA (C0 = 100 ppb).

In this study, preliminary tests were conducted for the degradation of PFOA at four conditions, namely, the use of 0.1 M oxalic acid as a reductive additive for the degradation of very high concentration PFOA solution (6 ml, 100 ppm), under visible light, dark condition, and in combination with fresh Zn0.5Cu0.5Fe2O4 (50 mg, 8.3 g/L) as shown in Fig. 8 A. All experiments were conducted in triplicate, and LC-QTOF results are summarized in tabular form in supporting information (Table S2; Supplementary Information). PFOA (100 ppm) alone under room light and UV light showed minimal removal 6.10% and 10.20%, respectively, after 2 h of reaction time. When the mixture of PFOA (100 ppm, 6 ml), fresh 8.3 gL−1 Zn0.5Cu0.5Fe2O4, and 0.1 M Oxalic acid (1.0 ml) at pH 1.73 was tested in the dark, it showed only 26.90% degradation of PFOA. Under UV light, the degradation increased to 32.10% after 2 h of reaction time (Fig. 8 A). Hence, the process of UV/Zn0.5Cu0.5Fe2O4/oxalic acid showed positive results for the photodegradation of very high concentration PFOA.

Fig. 8.

Fig. 8.

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A) PFOA (C0 = 100 ppm, 6 ml, pH = 2.03) under a) room light only, b) UV only, c) Zn0.5Cu0.5Fe2O4 (8.3 g L−1) + Oxalic acid (0.1 M, 1 ml) under dark, d) Zn0.5Cu0.5Fe2O4 (8.3 g L−1) + Oxalic acid (0.1 M, 1 ml) under UV; B) PFOA(C0= 10 ppm, 6 ml, pH = 2.03) under a) room light only; b) UV only; c) Zn0.5Cu0.5Fe2O4 (8.3 g L−1) + Oxalic acid (0.1 M,1 ml) under UV; C) PFOA (C0 = 100 ppb, 6 ml, pH = 2.03) under a) room light only, b) UV only, c) Zn0.5Cu 0.5Fe2O4 (8.3 g L−1) + Oxalic acid (0.1 M,1 ml)+ UV; D) PFOA (C0 = 100 ppb, 6 ml, pH = 2.03) under UV light a) Control, b) Zn0.5Cu0.5Fe2O4 (8.3 g L−1) + Oxalic acid (0.1 M, 1 ml), c) Zn0.8Cu0.2Fe2O4 (8.3 g L−1) + Oxalic acid (0.5 M, 1 ml), d) Zn0.8Cu 0.2Fe2O4 (8.3 g L−1) + Oxalic acid (0.1 M, 1 ml), e) Zn0.5Cu0.5Fe2O4 (8.3 g L−1) + Oxalic acid (0.5 M, 1 ml).

Next, the combination of UV/Zn0.5Cu0.5Fe2O4/oxalic acid was tested for the degradation of another one with high concentration PFOA solution (10 ppm). As depicted in Fig. 8 B and Table S2 (Supplementary Information), the degradation of PFOA (10 ppm) under room light and UV light were low. However, when the mixture of PFOA (10 ppm, 6 ml), fresh Zn0.5Cu0.5Fe2O4 (8.3 g L−1), and oxalic acid (0.1 M, 1 ml) with pH 1.73 was exposed to UV light for 2 h, it yielded 20 ± 1.0% degradation (Fig. 8 B). When exposed to UV light alone for 2 h, 78.9 ± 5.5% of PFOA (100 ppb) was unreacted (Fig. 8C). Again, when the mixture of PFOA (100 ppb, 6 ml), fresh Zn0.5Cu0.5Fe2O4 (8.3 g L−1), and Oxalic acid (0.1 M, 1 ml) with pH 1.73 was treated under UV light for 2 h, it gave 63 ± 2.70% degradation. In the absence of catalyst and oxalic acid, PFOA (100 ppb) in room light, PFOA was stable (97.20 ± 2.70% remaining). Hence, the degradation using the innovative UV/Zn0.5Cu0.5Fe2O4/Oxalic acid process showed positive results over broad concentration ranges representative of contaminated sites.

Other additional experiments were also conducted using the combination of fresh Zn0.5Cu0.5Fe2O4 and a higher concentration of oxalic acid (0.5 M) for the photodegradation of PFOA (C0 = 100 ppb). However, 0.5 M oxalic acid was not found helpful in the photodegradation of PFOA (Fig. S2; Supplementary Information). The performance of another catalyst formulation, Zn0.8Cu0.2Fe2O4, that was tested with different concentrations of oxalic acid are summarized in the Supporting Information (Figure S2; Supplementary Information) and Fig. 8 D. All studies conclude that the combination of catalyst, Zn0.5Cu0.5Fe2O4, and 0.1 M oxalic acid is the most effective for the photodegradation of PFOA. Although it is recognized that 2 h of reaction time at low pHs are not optimal for widespread application of this process, it does demonstrate that this approach is possible. Additional optimization such as adsorption and photocatalytic kinetics for PFAS degradation under dark and light conditions of this method may expand its applicability. It may also find usefulness in treating smaller volume wastes that are already at low pHs or can be easily brought to lower pHs. Similarly, smaller volumes of wastes, such as residual streams of other technologies, may allow sufficient time for treatment offline of the main process.

3.4. A tentative pathway for the oxidative decomposition and defluorination of PFOA

In the presence of oxalic acid, the interaction of nano-ferrites ZnxCu1-xFe2O4 catalyst with PFOA under UV light irradiation was investigated to understand the photodegradation mechanisms. Based on previous reports (Chen et al., 2015b; Han et al., 2018; Wang et al., 2017), herein, a tentative reaction mechanism for the photoproduction of reactive species has been proposed (Fig. 9). The formation of reactive species with exalted photochemical oxidation potential in the degradation of organic contaminants. Initially, this tentative reaction mechanism involves the adsorption of oxalic acid on the nano-ferrites surface in the ZnxCu1-xFe2O4 catalyst (Fig. 9). Then, it forms an active photochemical [≡FeIII(C2O4)] complex on the surface of nano-ferrites as well as in the reaction mixture (Chen et al., 2015b; Han et al., 2018; Wang et al., 2017). These photoactive species generate oxalate radicals [C2O4]·− which are further transformed into carbon-centered radicals CO2•− and carbon dioxide. After this, the excited electrons transfer from CO2•− to the absorbed O2 resulting in the formation of superoxide ions (O2·−). Further, in the acidic solution, O2·− reacts with iron (II) and transforms into hydrogen peroxide (H2O2) and Fe3+. Subsequently, strong oxidant potential such as hydroxyl radicals (•OH) are produced by the reaction of Fe2+ with •OH, and simultaneously, Fe3+ was generated in the reaction mixture (Chen et al., 2015b; Han et al., 2018; Wang et al., 2017).

Fig. 9.

Fig. 9.

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A tentative pathway for the oxidative decomposition and defluorination of PFOA.

Finally, the reaction mechanism of PFOA degradation may be predominantly initiated by hydroxyl radicals (•OH). The magnetic catalyst likely enhanced the photoproduction of hydroxyl radicals (•OH) due to the synergic effect of Zn and Cu. Current research demonstrates that, along with adsorption and photocatalytic kinetics, applying the technology to other PFAS, such as PFOS and GenX.

Herein, a tentative pathway for the oxidative decomposition and defluorination of PFOA has been intended (Fig. 9). This pathway is consistent with previous work (Singh et al., 2019; Bo et al., 2014; Wang et al., 2017). First, PFOA was adsorbed on the photocatalyst support. Next, intermediate A (m/z = 369.9839) was formed by the attack of aqueous electrons on the carboxylic group of PFOA via a photoinduced electron transfer-promoted decarboxylative radical reaction pathway (photoinduced Kolbe decarboxylation reaction). Subsequently, perfluoroalcohol B (m/z = 384.9788) has been produced by the attack of OH on intermediate A. Elimination of HF from the thermally less stable intermediate B enol (C–OH) group could yield intermediate C, which has a more stable keto (C=O) group. Then, hydrolysis of C generates intermediate D, C6F13COOH (m/z = 362.9769), and one more molecule of HF. C6F13COOH further yields short-chain perfluorinated carboxylic acids (e.g., perfluorobutanoic acid (J) trifluoroacetic acid (N), and short-chain organic acid (e.g., formic acid (P)) via chain propagation reactions of redox species followed by photoinduced Kolbe decarboxylation hydrolysis. All these short-chain acids are then further mineralized to H2O, CO2, and F−.

The chemical transformations of PFOA using the ZnxCu1-xFe2O4/oxalic acid combination can yield several stable and transient components. Further, several byproducts have been recognized based on exact mass measurements using Q-TOF analysis. When reaction mixture obtained after completion of reaction using PFOA (C0 = 100 ppm and 10 ppm), four byproducts such as A, I, J, and K were detected by Q-TOF (Fig. S4 and Fig. S5; Supplementary Information). In another case, PFOA with an initial concentration of 100 ppb shows five byproducts (A, I, O, K, and L) when a reaction mixture infused into LC-QTOF for analysis (Fig. S6; Supplementary Information).

4. Conclusions

In summary, UVevis/ZnxCu1-xFe2O4/oxalic acid as a heterogeneous photocatalytic system was demonstrated for PFOA degradation. The magnetic nanocrystalline ferrites, ZnxCu1-xFe2O4 catalyst in the presence of biomass-derived oxalic acid under UV light irradiation, lead to a process that generated in-situ reactive species in a potentially more sustainable manner than previously shown. These photogenerated reactive species predominantly initiate the reaction mechanism of PFOA degradation. Further, the optimal degradation conditions have been found to be at an oxalic acid initial concentration of 0.1 M, a pH of 1.73, and 8.3 mg/ml of Zn0.5Cu0.5Fe2O4 under 450W (Medium-pressure mercury lamp) of UV light irradiation. Up to six organic transformation products were detected and identified by Q-TOF analysis. These transformation products demonstrated oxidative dissociation and defluorination of PFOA.

Supplementary Material

Supplementary Data

HIGH LIGHTS.

  • Synthesis, characterization, and UV light performance of ZnxCu1-xFe2O4 catalysts.

  • UV light mediated in-situ generation of. •OH radicals.

  • Heterogeneous photo-Fenton like system without the addition of H2O2.

  • Photo-mineralization of PFOA has been observed.

Disclaimer

The U.S. Environmental Protection Agency funded and collaborated in the work described here. It has been subjected to the Agency’s review and has been approved for publication. Note that approval does not signify that the contents necessarily reflect the views of the Agency. Mention of trade names, products, or services does not convey official EPA approval, endorsement, or recommendation. S. Verma and B. Mezgebe were supported in part by appointment to the Postdoctoral Research Program at the National Risk Management Research Laboratory administered by the Oak Ridge Institute for Science and Education through Interagency Agreement No. DW-8992433001 between the U.S. Department of Energy and the U.S. Environmental Protection Agency.

Footnotes

Credit author statement

Sanny Verma: Investigation, Methodology, Formal analysis, Visualization, Data curation, Writing – original draft. Bineyam Mezgebe: Investigation, Methodology, Data curation, Formal analysis, Visualization, Writing – original draft. Endalkachew Sahle-Demessie: Conceptualization, Validation, Supervision, Funding acquisition, Writing – review & editing. Mallikarjuna N. Nadagouda: Conceptualization, Supervision, Validation, Funding acquisition, Writing – review & editing.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2021.130660.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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