Unlocking the effect of Zn²⁺ on crystal structure, optical properties, and photocatalytic degradation of perfluoroalkyl substances (PFAS) of Bi₂WO₆

Abstract

Bismuth tungstate (Bi₂WO₆) with a layered structure and visible light response exhibits excellent photocatalytic activity. To enhance its photocatalytic activity for the degradation of perfluoroalkyl substances (PFAS), Zn²⁺ is partially substituted for Bi³⁺ in the Bi₂WO₆ lattice in this study. Particularly, the effect of Zn²⁺ content (0–22.5 at%) on the crystal structure, optical property, and photocatalytic activity for the photodegradation of PFAS of Bi₂WO₆ is investigated. According to the Le Bail fits, the unit-cell volume is slightly reduced from 487.7 ų to 480.8 ų by the partial substitution of smaller Zn²⁺ (0.74 Å for CN = 6) for larger Bi³⁺ (1.03 Å for CN = 6) in the Bi₂WO₆ crystal lattice, and the solubility of Zn²⁺ in the Bi₂WO₆ lattice is found to be below 17.5 at%. The partial substitution of Zn²⁺ influences the self-aggregation of nanoparticles, Ostwald ripening, and self-organization of nanoplates, resulting in different morphologies. Although the optical bandgap energy of Bi₂WO₆ is not significantly altered upon the partial substitution of Zn²⁺, the conduction and valence bands simultaneously shift upward. Among the Bi_2−xZn_xWO_6+δ photocatalysts, 2.5 at% Zn²⁺-substituted Bi₂WO₆ exhibits larger water oxidation photocurrent density (0.316 mA cm⁻² at 1.23 V_RHE) and the highest photocatalytic activity for the photodegradation of PFH_xA (k₁ = 0.012 min⁻¹). The trapping experiments confirm that the photo-excited holes (h⁺) and superoxide radicals (O₂˙⁻) are the major reactive species involved in the photodegradation of PFH_xA. Liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) reveals that decarboxylation and defluorination are the main possible routes for the photodegradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts. Our findings suggest that the partial Zn²⁺-to-Bi²⁺ substitution can enhance the photocatalytic activity of Bi₂WO₆ for the degradation of PFAS.

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Water impact

The contamination of water with per- and poly-fluoroalkyl substances (PFAS) leads to adverse health effects because of their toxicity, extreme persistency, high mobility, and accumulative nature. The conventional water treatment process is ineffective in removing PFAS from contaminated water. The partial substitution of Zn²⁺ for Bi³⁺ in the Bi₂WO₆ crystal lattice can enhance the photocatalytic removal of PFAS from the model and real wastewater.

1. Introduction

Per- and poly-fluoroalkyl substances (PFAS) are a class of persistent, water-soluble synthetic organic compounds, including perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA).^1–3 Particularly, perfluorooctane sulfonate (PFOS) has been designated as one of the target chemicals by the Stockholm Convention on Persistent Organic Pollutants (POPs). PFAS have also adverse health effects due to their toxicity, persistency, mobility, and accumulative nature.⁴ The United States Environmental Protection Agency (EPA) proposed maximum contaminant level goals (MCLG) of 4 ng L⁻¹ and set a health recommendation of 70 ng L⁻¹ for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA).⁵ PFAS are specifically characterized by having a fully or partially fluorinated carbon chain. Their high electronegativity and small ionic radius give them an extremely strong C–F bond (up to 544 kJ mol⁻¹) and high chemical and thermal stability, respectively.⁶ Therefore, the degradation of PFAS remains challenging because of this strong C–F bond, risks associated with the persistency and toxicity of intermediates and final products, long treatment time, high energy requirement, and high capital cost.⁷ For the elimination of PFAS, the conventional water treatment process has been found to be ineffective.⁸

Perfluorohexanoic acid (PFH_xA) is a short-chain, six-carbon perfluoroalkyl acid, and it is a principal contaminant, degradant, and metabolite linked to short-chain fluorotelomer-based compounds.^9,10 PFH_xA was detected in various water samples at concentrations reaching up to 1400 and 4000 ppb for drinking water and groundwater.¹⁰ Therefore, the European Chemical Agency (ECHA) suggested listing PFH_xA as a “substance of very high concern” because of its extreme persistence, multiple sources, and high mobility in the aquatic environment.¹¹ Various techniques, including adsorption,¹² membrane filtration,¹³ anion exchange resin,¹⁴ foam fractionation,¹⁵ advanced oxidation processes,¹⁶etc., were applied for the removal of PFAS.

One of the methods applied for the removal of PFAS is physical adsorption.¹⁷ In addition to its slow sorption rate because of the sluggish diffusion of PFAS molecules, physical adsorption simply converts PFAS from the liquid phase to the solid phase without completely mineralizing them. Therefore, to avoid secondary pollution, various redox treatment processes, including electrochemical,¹⁸ photocatalytic,¹⁹ photolytic,²⁰ photochemical,²¹ sonochemical,²² radiochemical,²³ thermochemical,²⁴ subcritical,²⁵ and plasma,²⁶ have been applied to completely mineralize PFAS compounds. Among the redox treatment processes, photocatalytic degradation is promising because of its operation under mind conditions and higher efficiency.²⁷ The homogeneous photocatalytic degradation of PFAS is based on the photo-Fenton process. Tang et al.²⁸ efficiently degraded PFAS by applying the photo-Fenton process and achieved more than 90% degradation and 53.2% defluorination after 5 h. Unlike P25-TiO₂ with low efficiency for PFAS degradation due to the recombination of photoexcited charge carriers,²⁹ In₂O₃ nanoporous nanospheres exhibited high and fast degradation efficiency towards PFAS (100% within 30 min) because of the presence of abundant oxygen vacancy defects ().³⁰ Having an open layer structure, BiOCl with oxygen vacancies also exhibited a high defluorination efficiency (59.3%) after 3 h.³¹ The sheaf-like β-Ga₂O₃ with nanoplates elongating in the [1 0 0] direction and a large number of nanopores exhibited a complete degradation of PFAS after 3 h of reaction.³² The photodegradation of PFAS tightly bound on the surfaces of BiOCl and β-Ga₂O₃ followed the hole-oxidation path. The limiting factor of the oxidation process was found to be the electronic structures and reactive site distribution on specific surfaces, and facet engineering was suggested to improve the removal efficiency of PFAS.³³

As one of the simplest members of the Aurivillius family (Bi₂A_n−1B_nO_3n+3), Bi₂WO₆ has an orthorhombic layer structure constructed from alternating [Bi₂O₂]_n²ⁿ⁺ and perovskite-like [WO₄]_n²ⁿ⁻ layers.³⁴ In general, this layer structure not only favors the separation and transfer of photoexcited charge carriers due to the built-in electric field vertical to the layer direction but also reduces the surface trapping of photoexcited charge carriers.³⁵ The valence band edge is more positively positioned,³⁶ providing the sufficient potential to thermodynamically drive a hole-mediated oxidation reaction. By changing the synthesis parameters and doping, the morphology and exposed facets of Bi₂WO₆ can be easily tuned, resulting in a large number of oxygen vacancies that act as electron scavengers and binding sites for adsorbates.³⁷

Due to its capability to couple the highest occupied dopant states into the valence band, reduce the band gap, hamper the formation of midgap states in the forbidden band, and distort the crystal structure, Koteski et al.³⁸ and Ren et al.³⁹ computationally investigated the effect of Zn substituted partially for Bi and W lattice sites on the optical and electronic properties of Bi₂WO₆ using density functional theory (DFT), respectively. The partial substitution of Zn for W could narrow the optical bandgap energy without the direct presence of the localized defect states and affect the mobility of photoexcited charge carriers.³⁸ The partial substitution of Zn for Bi also led to band gap reduction due to the increase in the density of electrons and an upward shift of the conduction and valence band positions, revealing the possibility of the reaction of electrons with molecular oxygen to form active radicals.³⁹ Inspired by these two theoretical studies,^38,39 we aim to experimentally validate the effect of partial substitution of Zn for Bi on the crystal structure, optical properties, and photocatalytic activity for the degradation of perfluoroalkyl substances (PFAS) of Bi₂WO₆ in this study. The kinetics and mechanism of photodegradation of PFAS on Zn-substituted Bi₂WO₆ are also discussed.

2. Experimental

2.1. Synthesis

Pristine and Zn²⁺-substituted Bi₂WO₆ photocatalysts were synthesized by a hydrothermal method.⁴⁰ For the synthesis of pristine Bi₂WO₆, Bi(NO₃)₃·5H₂O (98%, Merck) was dissolved in 5 mL of ethylene glycol (>99%, Merck), while Na₂WO₄·2H₂O (>99%, chemPUR) was dissolved in 5 mL of deionized water. For the synthesis of Zn²⁺-substituted Bi₂WO₆, both Bi(NO₃)₃·5H₂O (98%, Merck) and Zn(CH₃COO)₂·2H₂O (>98%, Merck) were simultaneously dissolved in 5 mL of ethylene glycol (>99%, Merck), while Na₂WO₄·2H₂O (>99%, chemPUR) was dissolved in 5 mL of deionized water. Both solutions were then mixed under vigorous magnetic stirring, and the pH of the well-homogenized solution was adjusted to 7–9 by adding dropwise an aqueous solution of NaOH (98%, Alfa Aesar) and the mixture was transferred into a Teflon-lined stainless-steel autoclave (Parr Instrument GmbH). The hydrothermal reaction was carried out at 200 °C for 24 h. After the completion of the hydrothermal reaction, the resulting precipitate was washed and collected using a Universal 320 centrifuge (Andreas Hettich GmbH & Co. KG, 9000 rpm for 5 min), and dried at 80 °C for 12 h in a drying oven. The amount of Zn²⁺ substituted partially for Bi³⁺ in Bi₂WO₆ was controlled at 0, 1, 2.5, 7.5, 12.5, 17.5, and 22.5 at%. The synthesized powder samples were denoted as Zn0, Zn1, Zn2.5, Zn7.5, Zn12.5, Zn17.5, and Zn22.5 according to the content of substituted Zn²⁺.

2.2. Characterization

The X-ray diffraction (XRD) patterns were acquired with a Panalytical X'Pert PRO diffractometer with Cu K_α radiation. The diffraction data were collected in a Bragg–Brentano geometry with a θ/θ-arrangement over an angular range of 2θ = 10–120° with a 0.026° step. Initial qualitative analysis of the XRD data was performed using the HighScore plus program (version 4.7) and compared to entries from the ICDD-PDF-2 powder pattern database. Le Bail fits were performed using the method implemented in the FULLPROF Suite⁴¹ in order to determine the lattice parameters and proper fit of the crystalline phases. The microstructures of the synthesized samples were examined using a Carl Zeiss GeminiSEM 500 NanoVP scanning electronic microscope (SEM). The bright-field and lattice images and selected-area electron diffraction (SAED) patterns were obtained using an EM-002B high-resolution transmission electron microscope (TOPCON) at an accelerating voltage of 200 kV. The UV-vis diffuse reflectance spectra were recorded on an Evolution 220 UV/vis spectrometer (Thermo Fisher Scientific).

Photoelectrochemical tests were performed with a DropSens μSTAT200 potentiostat in 0.1 M Na₂SO₄ deoxygenated water solution (50 μL). Irradiation was provided with a solar light LED. The Bi_2−xZn_xWO_6+δ photocatalysts were dip-coated on the carbon surface (0.13 cm²) of the working electrode of the commercial screen-printed electrode (DS110), following the procedure previously reported elsewhere.^42,43 Linear scanning voltammetry (LSV) at 5 mV s⁻¹ from 0.2 to 1.4 V (V vs. Ag–AgCl) and chronoamperometry (CA) at 1.4 V (vs. Ag–AgCl) for 1 h were conducted.

2.3. Photocatalytic activity tests

The photocatalytic activity of pristine and Zn²⁺-substituted Bi₂WO₆ samples was evaluated for the degradation of polyfluoroalkyl and perfluoroalkyl substances (PFAS). First, 0.4 mg L⁻¹ of the synthesized sample was dispersed in an aqueous solution of PFH_xA (5 mg L⁻¹) in the photochemical reactor (0.7 L, Heraeus) in the dark to achieve adsorption–desorption equilibrium for 30 min. Then, a 150 W mercury lamp, which was vertically placed in the center of the photoreactor, with emitting radiation centered at 500–550 nm, an intensity of 7.31–7.53 mW cm⁻², and a photon flux of 20.83 × 10¹⁹ m² s⁻¹ was turned on. During the photocatalytic reaction, an aliquot was collected at different times (−30, 0, 5, 15, 30, 45, 60, and 120 min), filtered, and analyzed by High-Performance Liquid Chromatography (1200 Series Gradient HPLC System, Agilent Technologies). The extracted ion chromatograms of PFH_xA and ¹³C₆-PFH_xA are shown in Fig. S1.† The details of the instrumentation and analysis conditions of HPLC are given in the ESI.† The blank test conducted without any photocatalyst sample revealed that PFH_xA was not decomposed under visible light irradiation as a stable long-chain PFAS decomposition product.⁴⁴

The ratio of the PFH_xA concentrations before and after the photocatalytic reactions C/C₀ was used to indicate the amount of PFH_xA photocatalytically removed from the aqueous solution. The effects of various parameters, such as the initial pH of PFH_xA-containing aqueous solution (controlled using 0.1 mol L⁻¹ NaOH or 0.1 mol L⁻¹ HCl), competing ions (Cl⁻, NO₃⁻, and H₂PO₄⁻ from respective sodium salts with a concentration of 5 × 10⁻³ mol L⁻¹), and water matrix (distilled water – DW, tap water – TW, and treated wastewater – TWW) on the efficiency of the photocatalytic degradation of PFH_xA over pristine and Zn²⁺-substituted Bi₂WO₆ photocatalysts were studied. TWW was collected from the municipal wastewater treatment plant “Hajdów” in Lublin, which is a mechanical-biological wastewater treatment facility with increased removal of biogenic compounds (nitrogen and phosphorus) from wastewater. TWW was characterized by significantly reduced biological (5.7 mg L⁻¹) and chemical (35.1 mg L⁻¹) oxygen demands, total suspended solids (up to 6.4 mg L⁻¹), and total nitrogen (10.72 mg L⁻¹) and phosphorous (0.27 mg L⁻¹) contents. TW was characterized by total organic carbon (<3 mg L⁻¹), chloride (35 mg L⁻¹), nitrate (<2 μg L⁻¹), and sulfate (41 mg L⁻¹) contents, and pH of 7.2.

Toxicity towards marine bacteria (Aliivibrio fischeri) was also evaluated based on the inhibition of bioluminescence using the Microtox® test.⁴⁵ The luminescence inhibition was determined after 5 min and 15 min of exposure of Aliivibrio fischeri to the water samples before and after the photocatalytic reaction according to the standard protocol (Microtox®, 1995) in a Microtox M500 analyzer with the Omni software. All obtained data were expressed with the standard deviation.

3. Results and discussion

3.1. Material characterization

The XRD patterns of Bi_2−xZn_xWO_6+δ photocatalysts with varying contents of Zn²⁺ substituent are shown in Fig. 1. The main reflections in the XRD patterns of Bi_2−xZn_xWO_6+δ photocatalysts can be indexed to the orthorhombic Bi₂WO₆ phase with a space group of Pca2₁ and lattice parameters of a = 5.4370 Å, b = 16.4300 Å, and c = 5.4580 Å (ICSD# 98-006-7647). The Bi_2−xZn_xWO_6+δ photocatalysts maintained the phase purity up to 12.5 at% Zn²⁺ (sample Zn12.5), and a further increase in the Zn²⁺ content affected the phase purity of the Bi_2−xZn_xWO_6+δ photocatalysts. That is, additional reflections corresponding to the monoclinic ZnWO₄ phase with a space group of P2/c and lattice parameters of a = 4.5160 Å, b = 5.5210 Å, and c = 4.7990 Å (ICSD# 98-016-2236) and the hexagonal ZnO phase with a space group of P6₃mc and lattice parameters of a = 3.2420 Å, b = 3.2420 Å, and c = 5.1880 Å (ICSD# 98-006-5119) appeared at 17.5 and 22.5 at% Zn²⁺ (samples Zn17.5 and Zn22.5), respectively. It can be stated that the solubility of Zn in the Bi₂WO₆ matrix is below 17.5 at%. Table 1 summarizes the results from the Le Bail fits and the agreement factors of the calculations. Profile matching of the Bi_2−xZn_xWO_6+δ photocatalysts with 17.5 and 22.5 at% Zn²⁺ (samples Zn17.5 and Zn22.5) was performed using two different phases (Fig. 2a and b). The goodness of the agreement factors confirms a satisfactory fit of the ZnWO₄ and ZnO phases. The calculated unit-cell volume decreased with increasing the Zn²⁺ content up to 12.5 at% (Fig. S2†). This is due to the fact that the ionic radius of Zn²⁺ (0.74 Å) is much smaller than that of Bi³⁺ (1.03 Å) in a six-fold coordination.⁴⁶ Thus, the partial substitution of Zn²⁺ for Bi³⁺ in the crystal lattice of Bi₂WO₆ contracts the unit-cell volume. Interestingly, the unit-cell volume was slightly increased in the Bi_2−xZn_xWO_6+δ photocatalysts containing 22.5 at% Zn²⁺. In the case of the Bi_2−xZn_xWO_6+δ photocatalysts containing 17.5 and 22.5 at% Zn²⁺ (samples Zn17.5 and Zn22.5), the unit-cell volumes could not be properly estimated due to the simultaneous presence of Zn²⁺ in Bi₂WO₆ and minor impurity phases.

Fig. 1 XRD patterns of Bi_2−xZn_xWO_6+δ powders with varying contents of Zn²⁺ substituent: (a) Zn0, (b) Zn1, (c) Zn2.5, (d) Zn7.5, (e) Zn12.5, (f) Zn17.5, and (g) Zn22.5. The orange and magenta arrows indicate the reflections belonging to ZnWO₄ and ZnO, respectively.

Table 1 Lattice parameters and agreement factors of the Le Bail fit of the Bi_2−xZn_xWO_6+δ photocatalysts

Sample	Zn0	Zn1	Zn2.5	Zn7.5	Zn12.5	Zn17.5		Zn22.5
Crystalline phase	Bi2WO6	Bi2WO6	Bi2WO6	Bi2WO6	Bi2WO6	Bi2WO6	ZnWO4	Bi2WO6	ZnO
Space group	Pca21	Pca21	Pca21	Pca21	Pca21	Pca21	P2/c	Pca21	P63mc
a (Å)	5.4482 (7)	5.4318 (2)	5.4384 (2)	5.4355 (2)	5.4280 (4)	5.4239 (3)	4.684 (1)	5.4354 (2)	3.2255 (7)
b (Å)	16.423 (2)	16.4185 (5)	16.4278 (6)	16.4299 (5)	16.426 (1)	16.4090 (9)	5.724 (1)	16.4351 (6)
c (Å)	5.4507 (6)	5.4473 (2)	5.4551 (2)	5.4511 (2)	5.3924 (3)	5.3829 (3)	4.9318 (8)	5.4549 (2)	5.288 (3)
β (°)	—	—	—	—	—	—	90.58 (2)	—	—
Volume (Å^3)	487.7 (1)	485.81 (3)	487.36 (3)	486.81 (3)	480.80 (5)	479.08 (4)	132.22 (5)	485.56 (3)	47.65 (3)
R p	2.19	2.34	2.55	2.55	2.62	2.51	7.50	2.47	5.83
R wp	3.15	3.23	3.90	3.66	3.53	3.45	7.30	3.60	6.41
R exp	1.53	1.68	1.66	1.57	1.67	1.68	3.55	1.59	2.83
χ ^2	4.26	3.71	5.54	5.39	4.47	4.23	4.23	5.14	5.14

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Fig. 2 Le Bail fit of XRD patterns of (a) Zn17.5 and (b) Zn22.5 samples. The blue line indicates the difference between observed (dots) and calculated (black line) intensities and the vertical green bars are the Bragg positions of Pca2₁ and P2/c for Zn17.5 and Pca2₁ and P6₃mc for Zn22.5.

Fig. 3 shows the SEM images of Bi_2−xZn_xWO_6+δ photocatalysts with varying contents of Zn²⁺ substituent. Unlike the Bi₂WO₆ particles with three-dimensional morphologies,^47–49 pristine Bi₂WO₆ (sample Zn0) synthesized by a hydrothermal method in this study possesses a platelet morphology with an average diameter of <600 nm (Fig. 3a), which was possibly formed by the self-assembly of ultrathin nanosheets. At 1 at% Zn²⁺ substituent, the platelet morphology was altered, and large anisotropic nanoplates with irregular shapes were formed due to the dominance of the dissolution–recrystallization (Oswald ripening) process over self-assembly (Fig. 3b). According to a recent study by Iversen and co-workers,⁵⁰ the preferential growth of Bi₂WO₆ nanoplates under hydrothermal conditions is governed by the initial presence of Bi₂O₂²⁺ molecular complexes that interact with WO₄²⁻ tetrahedra, forming disordered Bi_0.933W_0.067O_1.6, and when there are sufficient WO₄²⁻units intertwined the Bi₂WO₆ nanoplates are eventually formed by the sideways addition of units in the ac plane, which has a three times faster growth rate than the b direction. Strikingly, flower-like microstructures were formed when the content of Zn²⁺ substituent was set to 2.5 at% (Fig. 3c). Clearly, 2.5 at% Zn²⁺ substituent balanced the self-aggregation of nanoparticles, Ostwald ripening, and self-organization of nanoplates with high anisotropic surface energy,⁵¹ resulting in flower-like microstructures. By adjusting the amount of Zn²⁺ substituent to 7.5, 12.5, and 17.5 at%, the governing roles of Oswald ripening and self-organization were subdued, forming some nanoplates along with nanoparticles (Fig. 3d–f). A further increase in the concentration of Zn²⁺ to 22.5 at% gave rise to the Ostwald ripening process but not to the self-organization process (Fig. 3g). It is evident that the partial substitution of Zn²⁺ for Bi³⁺ in the Bi₂WO₆ crystal lattice changed the content of Bi₂O₂²⁺ molecular complexes interacting with WO₄²⁻ tetrahedra, surface atomic structure, and surface free energy, which ultimately affected the particle morphology of Bi_2−xZn_xWO_6+δ photocatalysts.

Fig. 3 SEM images of Bi_2−xZn_xWO_6+δ powders with varying contents of Zn²⁺ substituent: (a) Zn0, (b) Zn1, (c) Zn2.5, (d) Zn7.5, (e) Zn12.5, (f) Zn17.5, and (g) Zn22.5.

Further, the nanostructures were characterized by transmission electron microscopy. Fig. 4 shows the bright-field TEM and HRTEM images and SAED patterns of pristine (sample Zn0) and 17.5 at% Zn²⁺-substituted (sample Zn17.5) Bi₂WO₆ photocatalysts. In Fig. 4a, the low-magnification TEM image of sample Zn0 shows that pristine Bi₂WO₆ is in the form of platelets with a diameter of 580 nm and a thickness of about 75 nm. The high-magnification TEM image of sample Zn0 in Fig. S3a† confirms that the platelets were formed by the self-assembly of ultrathin nanosheets. The observed lattice fringes with a d-spacing value of 0.390 nm in Fig. S3b† manifest that the ultrathin nanosheets have exposed (1 3 0) facets along the [3̄ 1 6] direction. No obvious defects are noticed in the examined area, suggesting the high crystallinity of ultrathin nanosheets. In the corresponding selected area electron diffraction pattern (Fig. 4b), a ring diffraction pattern can be seen due to the random orientation of crystallites. The observed Debye–Scherrer ring patterns are indexed to the different Miller indices of orthorhombic Bi₂WO₆. In Fig. 4c, the bright-field TEM image of sample Zn17.5 indicates the change in the nanostructure upon 17.5 at% Zn²⁺ substitution and the formation of large irregular particles, which are enclosed by a dotted oval, along with ultrathin nanosheets. In the corresponding selected area electron diffraction pattern in Fig. 4d, the Debye–Scherrer ring patterns are indexed to the different Miller indices of orthorhombic Bi₂WO₆ and monoclinic ZnWO₄, which is consistent with the XRD result.

Fig. 4 TEM images (left) and SAED patterns (right) of Zn0 (a and b) and Zn17.5 (c and d).

The optical properties of the synthesized samples were analyzed by UV-vis spectroscopy. Fig. 5a shows the UV-vis diffuse reflectance spectra of Bi_2−xZn_xWO_6+δ photocatalysts with varying contents of Zn²⁺ substituent. A typical absorption edge at approximately 445 nm was noted for pristine Bi₂WO₆ (sample Zn0), which corresponds to an optical bandgap energy of 2.78 eV according to the Kubelka–Munk function vs. the energy of absorbed light. This is due to the transition from filled anti-bonding states, formed by the hybridization of Bi 6s and O 2p orbitals at the top of the valence band to the empty W 5d orbitals in the conduction band.⁵² With increasing the Zn²⁺ content to 1, 2.5, 7.5, and 12.5 at%, the absorption edges of Bi_2−xZn_xWO_6+δ photocatalysts were slightly redshifted toward 453, 475, 451, and 448 nm, respectively. A slight reduction in the optical bandgap energy upon partial substitution of Zn²⁺ for Bi³⁺ stemmed from the merging and coupling of the highest occupied substituent states (the Zn 3d–levels) into the valence band³⁹ and the split-off of states near the top of the valence band as a result of a substituent–host interaction (Zn–3d–states and O–2p–state).³⁸ The partial substitution led to the upward shift of the conduction band minimum and valence band maximum.³⁹ A further increase in the Zn²⁺ content to 17.5 and 22.5 at% resulted in the blueshift in the light absorption due possibly to the existence of ZnWO₄ and ZnO with absorption edges at about 355 nm (ref. 53) and 385 nm,⁵⁴ respectively. Among the Bi_2−xZn_xWO_6+δ samples, 2.5 at% Zn²⁺-substituted Bi₂WO₆ is more effective in absorbing visible light, which is important for enhancing the visible-light-driven photocatalytic activity. Unlike in previous studies,^38,55,56 no background absorption beyond the absorption edges was observed upon partial substitution of Zn²⁺, suggesting the absence of deep impurity states within the band gap.

Fig. 5 (a) UV-vis diffuse reflectance spectra of Bi_2−xZn_xWO_6+δ powders with varying contents of Zn²⁺ substituent. Photoelectrochemical results of Bi_2−xZn_xWO_6+δ as photoanodes in 0.1 M Na₂SO₄ deoxygenated water solution: LSV (b) and CA (c) at 1.4 V (V vs. Ag–AgCl) for Zn0 and Zn2.5.

The photoelectrochemical studies allow the elucidation of the characteristics of a photocatalyst for the conversion of light energy in chemical reactions. Fig. 5b presents the LSV results. The LSV response of Bi_2−xZn_xWO_6+δ photocatalysts is due to the photoinduced oxidation of water and is qualitatively consistent with previous reports.^57,58 Under light illumination, all the photocatalysts define a photocurrent with the following order: Zn2.5 > Zn1.0 > Zn0 > Zn7.5 > Zn12.5 = Zn17.5 = Zn22.5. Therefore, Bi₂WO₆ partially substituted with 2.5 at% Zn²⁺ exhibits the highest water oxidation photocurrent density (0.316 mA cm⁻² at 1.23 V_RHE), projecting itself as a photocatalyst capable of effectively promoting different photo-redox reactions. In general, the potential at which the photocurrent begins to be representative is relatively positive for all the samples, suggesting that charge carrier recombination dominates at low overpotentials. However, as the potential increases, the photocurrent is observed to increase to a pseudo-steady value at very high overpotentials. The latter is evidence that electron transfer phenomena become important at higher overpotentials, since polarization allows photo-excited electrons to be collected, effectively slowing down the recombination process. It is worth mentioning that Zn0 and Zn2.5 define the CA response with a constant photocurrent for 1 h (Fig. 5c), indicating the stability of the photocatalysts under light irradiation and high polarization conditions. Therefore, the Bi_2−xZn_xWO_6+δ photocatalysts studied have a consistent photoelectrochemical response that results from the interplay of light absorption and the balance between recombination and charge transfer processes.⁵⁹

3.2. Photocatalytic degradation of PFH_xA

The photocatalytic activity of Bi_2−xZn_xWO_6+δ photocatalysts was evaluated for the photodegradation of PFH_xA in an aqueous solution for 120 min. Fig. 6 shows the reaction time course of the photodegradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts. As shown in Fig. 6a, the content of the Zn²⁺ substituent influenced the photodegradation kinetics of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts. Although the photodegradation of PFH_xA was slow, the pronounced change in the concentration of PFH_xA was noted in the first 45 min under light irradiation. The highest photodegradation rate of PFH_xA was observed for sample Zn2.5 (up to 57%), followed by Zn1.0 (40%), Zn0 (37%), and Zn7.5 (32%). Samples Zn12.5, Zn17.5, and Zn22.5 are characterized to have a lower photodegradation rate of PFH_xA (<30%). Clearly, 2.5 at% Zn²⁺ substitution was favorable to enhance the adsorption and photodegradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts due to the optimum concentration of oxygen vacancies.

Fig. 6 Kinetics of photocatalytic degradation of PFH_xA over Bi_2−xZn_xWO_6+δ powders (a). Effects of inorganic ions in the presence of 3 mmol L⁻¹ sodium salts (b), water matrix (c), and radical scavengers (d) on photocatalytic degradation of PFH_xA over sample Zn2.5. (e) Toxicity test of PFH_xA-containing water samples before (1) and after photocatalytic treatment with (2) Zn0, (3) Zn1, (4) Zn2.5, (5) Zn7.5, (6) Zn12.5, (7) Zn17.5, and (8) Zn22.5 and after photocatalytic treatment in TW (9) and TWW (10) water samples (f).

Recently, Lovisa et al.⁵⁵ experimentally studied the impact of partial substitution of Zn for Bi on the photocatalytic and photoluminescence properties of Bi₂WO₆ and found that Zn could not effectively favor the photocatalytic property but the photoluminescence property due to the increase in the recombination rate of photoexcited charge carriers in the oxygen vacancy-related defects. On the contrary, In₂O₃ nanostructures with pores showed high photocatalytic activity toward PFAS degradation because of the presence of abundant oxygen vacancy defects on their surface,³⁰ which received an O atom from the –COOH group of PFOA molecules, forming a close contact with In₂O₃ and improving the charge transfer and photocatalytic activity under UV irradiation. The oxygen vacancy in the BiOI microspheres was tuned by Zn²⁺ substitution and an increase in the concentration of oxygen vacancies enhanced the photocatalytic activity for NO removal.⁶⁰ Also, the flower-like microstructures of sample Zn2.5 permit multiple light scattering and offer a longer optical path, which improves the efficiency of light harvesting and increases the number of photo-excited charge carriers available for the photodegradation of PFH_xA. Similarly, flower-like microspheres of Bi₂WO₆ exhibited superior photocatalytic activity for the photodegradation of RhB in comparison to the plate-like and clew-like structures due to their efficient separation and transfer of photo-excited charge carriers and enhanced light-harvesting efficiency.⁶¹ Previously, P25-TiO₂ exhibited 31.1% degradation efficiency and 3.3% de-fluorination efficiency for PFOA,⁶² while In₂O₃ with graphene showed 100% degradation efficiency and 60.9% de-fluorination efficiency for PFOA although it requires a thermal treatment at 400 °C.⁶³ As shown in Table S1 in the ESI,† although some photocatalysts and processes showed high efficiency in the degradation of PFAS, they were conducted under UV light irradiation. In contrast, 2.5 at% Zn²⁺-substituted Bi₂WO₆ exhibited a 57% removal efficiency of PFH_xA in 45 min under visible light irradiation. Apparently, the photocatalytic removal of PFH_xA by Bi_2−xZn_xWO_6+δ photocatalysts followed a pseudo-first-order kinetics, implying that the adsorption process was a rate-limiting step of the reaction, and the highest pseudo-first-order kinetic constant (k₁) was observed for sample Zn2.5 (0.012 min⁻¹ in Table 2). With increasing the concentration of Zn²⁺ substituent, the k₁ value gradually decreased. Table 2 shows the kinetic parameters, such as k₁, half-life time (t_1/2), and pseudo-second-order kinetic (k₂), for the photocatalytic degradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts.

Table 2 Kinetic parameters of photocatalytic degradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts

Photocatalyst	k 1 (×10^3)	t 1/2	R ^2	k 2 (×10^3)	R ^2
	min^−1	(min)		(mg g^−1 min^−1)
Zn0	5.32	130	0.9690	9.10	0.9686
Zn1.0	5.23	133	0.9456	12.13	0.9289
Zn2.5	12.24	57	0.9923	17.64	0.9980
Zn7.5	3.49	198	0.9032	8.67	0.8855
Zn12.5	3.19	217	0.9844	6.86	0.9870
Zn17.5	3.05	227	0.9734	6.86	0.9684
Zn22.5	2.39	290	0.9809	5.66	0.9756

---

The natural components of the water matrix, including inorganic ions, are recognized as potential competitors of active sites on the surface of a photocatalyst. Thus, additional photodegradation tests were conducted using sample Zn2.5 in the presence of Cl⁻, NO₃⁻, or PO₄³⁻. As shown in Fig. 6b, the photocatalytic removal efficiency of PFH_xA was lowered in the presence of these inorganic anions. Particularly, the PO₄³⁻ ions were more active in competition with PFH_xA molecules on the surface of Bi_2−xZn_xWO_6+δ photocatalysts, significantly reducing the photocatalytic removal efficiency of PFH_xA. Depending on the type of water matrix, the photocatalytic removal efficiency of PFH_xA can be varied due to the simultaneous existence of inorganic ions and organic molecules (dissolved organic matter). Fig. 6c presents the effect of the water matrix (distilled water – DW, tap water – TW, and treated wastewater – TWW) on the photocatalytic removal of PFH_xA over sample Zn2.5. It can be seen that the photocatalytic removal efficiency of PFH_xA was much higher in tap water in comparison to distilled water and treated wastewater possibly due to the simultaneous presence of Fe (49 mg L⁻¹) or Mg (22.9 mg L⁻¹) ions. Interestingly, the presence of Cl⁻ (35 mg L⁻¹) in treated wastewater did not reduce the photocatalytic removal efficiency of sample Zn2.5.

3.3. Photodegradation mechanism

To gain insights into the possible mechanism for the degradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts, trapping experiments were conducted using sample Zn2.5 in the presence of isopropyl alcohol – IPA (1000 mg L⁻¹), p-benzoquinone – PBQ (1 mg L⁻¹), and ethylenediaminetetraacetic acid disodium salt dihydrate – Na₂EDTA (100 mg L⁻¹) as scavengers of ˙OH, O₂˙⁻, and h⁺, respectively. Fig. 6d shows that the photodegradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts was mainly driven by the involvement of photo-excited holes and superoxide radicals. This is because the conduction band minimum and valence band maximum of Bi₂WO₆ shifted upward, preferentially generating O₂˙⁻ radicals since the reduction potential of O₂/O₂˙⁻ is −0.33 eV. Similarly, photo-excited holes and O₂˙⁻ were found to be the major contributors to the photodegradation of fluoroquinolones over Mg-substituted Bi₂WO₆.⁶⁴ Interestingly, hydroxyl radicals were not involved in the photodegradation of PFH_xA according to the results of trapping experiments. It is then inferred that the direct electron transfer (DET) from PFH_xA is an important step. In fact, this evidence is in good agreement with the proposed mechanistic study.⁶⁵ It has been reported that the formation of the radical is a determining step in the reaction.^65,66 Then, to complete the oxidation process, different chemical and redox steps took place, such as the reaction with holes, electrons, and O₂˙⁻ as possible steps to react with formed and continue the photocatalytic reaction (Fig. 8).

To understand the reaction pathway of the photodegradation of PFH_xA over Bi_2−xZn_xWO_6+δ photocatalysts, LC-QTOF/MS operating in a scan mode was used. By applying collision energy equal to 0 eV, we attempt to minimize a possible fragmentation of PFH_xA in an MS source and to identify specific products of its photodegradation. As shown in Fig. 7a, the deprotonated molecular ion of m/z = 312.97 and an ion of m/z = 268.98, which are associated with the loss of CO₂ from the carboxyl group, and an ion of m/z = 68.99 delivered from fragment CF₃ were observed in each MS spectra. This suggests the partial decomposition of PFH_xA in an MS source instead of its degradation by photocatalysis. Generally, dealkylation is the main process that takes place during the photocatalytic removal of PFAS,⁴⁴ and longer PFAS are degraded into shorter ones.⁶⁷ Interestingly, fragments indicating the photodegradation of PFH_xA into short-chain PFAS, such as perfluoropentanoic acid (M_w = 264.05 g mol⁻¹) or perfluorobutanoic acid (M_w = 214.4 g mol⁻¹) were not found. However, a thorough inspection of MS spectra acquired for the photocatalytically treated samples confirms the presence of smaller fragments of m/z = 248.96, m/z = 226.98, and m/z = 180.97, which are associated with the loss of HF, CO, CO₂, and C_nF_m (Fig. 7b) and the formation of [M–HF–CO₂]-, [M + H–CF₄]- and [M–CO + H₂–CF₅]-fragments.⁶⁸ Decarboxylation and defluorination are the main possible routes for the transformation of PFAS during the photocatalytic processes.⁶⁹ The possible mechanism for the photodegradation of PFH_xA over Zn²⁺substituted Bi₂WO₆ is shown in Fig. 8. The reusability of the synthesized photocatalyst (sample Zn2.5) for the degradation of PFH_xA in an aqueous solution was tested in four cycles. The obtained results in Fig. 6e revealed that sample Zn2.5 has good reusability since only a 27% reduction in photocatalytic activity after four runs was observed due to the loss of photocatalyst powders during collection and drying after each cycle.

Fig. 7 Mass spectra of PFH_xA-containing water samples before (a) and after (b) photocatalytic treatment in the presence of sample Zn2.5.

Fig. 8 Possible mechanism for the photodegradation of PFH_xA in an aqueous solution over the Zn²⁺-substituted Bi₂WO₆ photocatalyst.

3.4. Toxicity

The toxicity of photocatalytically treated water samples containing PFH_xA to Aliivibrio fischeri was estimated to be low (EC₅₀ = 1.27 g).⁷⁰ The presence of PFH_xA in distilled water before and after photocatalytic treatment (Fig. 6f) did not affect the bioluminescence of Aliivibrio fischeri. However, slight toxicity (<20%) was observed in the water samples containing PFH_xA after photocatalytic treatment using sample Zn0. With the Zn²⁺ substituent, the toxicity was reduced, which needs further investigation to understand this phenomenon. The toxicity of the photocatalytically treated PFH_xA-containing TWW sample was significant (nearly 34%), inhibiting the bioluminescence of Aliivibrio fischeri.

4. Conclusions

Zn²⁺-substituted Bi₂WO₆ has the potential to transform solar energy into redox reactions effectively because the partial substitution of Zn²⁺ for Bi³⁺ in the Bi₂WO₆ crystal lattice contracts the unit-cell volume, improves the absorption of visible light, and decreases the recombination of photo-excited charge carriers, favoring the photo-redox processes. The Bi₂WO₆ substituted with 2.5 at% Zn²⁺ differed from the unmodified Bi₂WO₆ by the possibility of generating O₂˙⁻ due to the displacement of the conduction band to more negative potentials. The Zn²⁺-substituted Bi₂WO₆ photocatalysts exhibited the highest pseudo-first-order kinetic constant for PFH_xA photodegradation (k₁ = 0.012 min⁻¹). Trapping experiments confirmed that photo-excited holes and superoxide radicals were the main reactive species involved in the photodegradation of PFH_xA. The direct electron transfer of PFH_xA was the important step and the reaction of holes, electrons, and O₂˙⁻ with the formed supported the photocatalytic oxidation reaction. Considering the complexity of wastewater, which simultaneously contains organic (dissolved organic matter) and inorganic substances (anions, such as nitrates, carbonates), the low photocatalytic efficiency in TTW still indicates the potential of the synthesized photocatalysts for practical application in the removal of PFAS under visible light irradiation. Furthermore, there is also a possibility to regenerate the used photocatalysts by heat treatment at temperatures above 157 °C, which is the boiling point of PFH_xA. Although the number of studies on the photocatalytic removal of PFAS has recently increased, the efficiency, stability, and scalability of such systems need to be further explored along with understanding their detailed mechanisms.

Author contributions

Mirabbos Hojamberdiev: conceptualization, investigation, methodology, validation, visualization, writing – original draft, writing – review & editing; Ana Laura Larralde: investigation, software, validation, writing – original draft; Ronald Vargas: formal analysis, investigation, software, validation, writing – original draft, writing – review & editing; Lorean Madriz: formal analysis, investigation, software, validation, writing – original draft, writing – review & editing; Kunio Yubuta: formal analysis, investigation, software, validation, writing – original draft; Lokesh Koodlur Sannegowda: investigation, writing – review & editing; Ilona Sadok: formal analysis, investigation, software, validation, writing – original draft; Agnieszka Krzyszczak-Turczyn: formal analysis, investigation, software, validation, writing – original draft; Patryk Oleszczuk: formal analysis, investigation, software, validation, writing – original draft; Bożena Czech: conceptualization, investigation, methodology, supervision, validation, visualization, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank MPWIK Lublin and The John Paul II Catholic University of Lublin, Poland, for providing wastewater samples and giving access to services and research facilities, which were co-funded by the European Regional Development Fund (POPW.01.03.00-06-003/09-00), respectively. India-Uzbekistan Collaborative Grants (No. INT/Uzbek/P-21 and UZB-Ind-2021-91) are also acknowledged. The authors would like to thank Dipl. Phys. Christoph Fahrenson from ZELMI, TU Berlin, for his kind technical support in SEM analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ew00430a

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