Metal ion endogenous cycles of CoFe₂O_4−x induced boosted photocatalytic/PMS degradation toward polycyclic aromatic hydrocarbons

Abstract

The rational design of photocatalytic nanomaterials with unique structures is critical for remediating environmental problems and thus reducing ecological risks. In this work, we employed H₂ temperature-programmed reduction to modify MFe₂O₄ (M = Co, Cu, and Zn) photocatalysts for obtaining additional oxygen vacancies. After activation of PMS, naphthalene and phenanthrene degradation rates in the soil phase were increased by 3.24-fold and 1.39-fold, respectively, and 1.38-fold for naphthalene in the aqueous phase by H-CoFe₂O_4−x. The extraordinary photocatalytic activity is attributed to the oxygen vacancies on the H-CoFe₂O_4−x surface, which promote electron transfer and thus enhance the redox cycle from Co(III)/Fe(III) to Co(II)/Fe(II). In addition, oxygen vacancies are used as electron traps to hinder the recombination of photogenerated carriers and accelerate the generation of hydroxyl and superoxide radicals. Quenching tests showed that the addition of p-benzoquinone resulted in the greatest decrease in the degradation rate of naphthalene (inhibition of about 85.5%), demonstrating that O₂˙⁻ radicals are the main active species in the photocatalytic degradation of naphthalene. H-CoFe₂O_4−x showed improved degradation performance in synergy with PMS (82.0%, k_app = 0.00714 min⁻¹) while maintaining excellent stability and reusability. Hence, this work provides a promising approach for the design of efficient photocatalysts to degrade persistent organic pollutants in soil and aqueous environments.

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Introduction

With the development of global industrialization, polycyclic aromatic hydrocarbons (PAHs) have become a widespread pollutant in the environment.¹ Long-term exposure to PAHs can trigger respiratory, circulatory, and neurological damage to the liver and kidneys² and the International Agency for Research on Cancer has classified some PAHs as potential human carcinogens.³ Therefore, finding an efficient and green technology to degrade PAHs has become an urgent requirement.

For efficient removal of persistent organic pollutants from water and soil, advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) have received a lot of attention.^4,5 Sulphate radicals (SO₄˙⁻) are typical radicals for AOP remediation of soils⁶ and differ from hydroxyl radicals (˙OH) in the following advantages: (i) SO₄˙⁻ can provide a greater oxidation potential (2.5–3.1 V vis. NHE) than ˙OH (1.8–2.7 V vs. NHE).⁷ (ii) Compared with ˙OH, SO₄˙⁻ shows excellent selectivity and effectiveness. (iii) SO₄˙⁻ can operate over a wide pH range. (iv) The half-life of SO₄˙⁻ is usually longer (30–40 μs) than that of ˙OH (<1 μs).⁸ Transition metal catalysts have the advantages of high efficiency and ease of operation, and are one of the most feasible methods for activating PMS to produce SO₄˙⁻.

With the further exploration of nano-photocatalytic materials,^9–12 a spinel-type metal oxide (MFe₂O₄, where M = Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, etc.) has attracted widespread attention. Among them, CoFe₂O₄, as a good semiconductor photocatalytic material, can effectively activate PMS to produce active substances.^13–16 Organic pollutants can be oxidized by oxidative species created by semiconductors, such as ˙OH, O₂˙⁻, and photogenerated holes (h⁺).^17–19 These highly oxidative species (h⁺, ˙OH, and O₂˙⁻) can degrade and even mineralize adjacent organic pollutants on the CoFe₂O₄ surface into CO₂, H₂O, and inorganic ions.^20,21 In previous studies, CoFe₂O₄ has been widely used for activating PMS to degrade 2-phenyl benzimidazole-5-sulfonic acid,²² lomefloxacin hydrochloride,²³ sulfamethoxazole,²⁴ clofibrate,²⁵ bisphenol A,²⁶etc. However, due to the wide bandgap energy, the common CoFe₂O₄ photocatalyst has an inferior ability to separate photogenerated electron–hole pairs, which limits the degradation effect.²⁷ To attenuate these negative effects, researchers have inhibited the recombination of photogenerated electrons and holes and improved the photocatalytic performance of transition metal oxides by adjusting the electronic structure of cobalt ferrites.

Defect engineering is regarded as an important method in nanomaterial modification;^28–30 the electronic structure of the photocatalyst can be altered, thus significantly improving the catalytic performance.³¹ As is well known, oxygen vacancies are the most common anionic defects in metallic compounds. When the oxygen vacancy is formed, the electrons near the oxygen vacancy occupy the O 2p orbital before delocalization, which promotes the electron transfer capability of the catalyst.^31,32 Oxygen vacancies can be adsorbed and continuously captured by PMS for inducing photocatalytic materials to generate O₂˙⁻ and ¹O₂ using dissolved oxygen. It acts as an electron transfer medium to accelerate the redox cycle from Co(III)/Fe(III) to Co(II)/Fe(II) for the synergistic degradation of organic pollutants. In addition, the localized state of the oxygen vacancy modification can extend the photoresponse and effectively trap the charge carriers.³³ Oxygen vacancy defects can be caused in a variety of ways. For example, H₂-TPR is an effective method to build surface defects, using a reducing atmosphere where a reduction reaction occurs at high temperatures to build oxygen vacancies.³⁴ In addition, thermal annealing under anoxic conditions is also a modulation method to generate oxygen vacancies.³⁵ The application in PMS activation is still limited though the oxygen-rich vacant nanoparticles have gained a lot of interest in the field of AOP. A few of the studies have used oxygen-rich cobalt ferrite activation of PMS in the aqueous and soil phases to degrade POPs.

Herein, we used the co-precipitation approach to make a series of ferrite MFe₂O₄ (M = Co, Cu, Zn) photocatalysts. Three procedures were applied to modify the samples at different temperatures to increase the oxygen vacancy content: H₂-TPR, N₂-TPD, and vacuum calcination. This work found that cobalt ferrite had the best catalytic activity for methylene blue (MB) degradation in the aqueous phase. The implications of the morphological, optical, and electrochemical properties of cobalt ferrite modified samples on photocatalytic degradation of organic contaminants were then thoroughly explored. The structure of modified cobalt ferrite and the mechanisms that increase its activity were extensively investigated by using PAHs in the aqueous and soil phases as model contaminants. This work provides new insights into natural mineral catalysts for water and soil remediation.

Experimental

Reagents and instruments

Ferric nitrate (Fe(NO₃)₃·9H₂O), cobalt nitrate (Co(NO₃)₂·6H₂O), zinc nitrate (Zn(NO₃)₂·6H₂O), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), dichloromethane (CH₂Cl₂), tert-butyl alcohol (C₄H₁₀O), edetate disodium ([–CH₂N(CH₂CO₂Na)CH₂CO₂H]₂), and p-benzoquinone (C₆H₄O₂) were provided by Aladdin Reagent Co., Ltd. Naphthalene (C₁₀H₈) was purchased from McLean Biochemical Technology Co., Ltd. Sodium hydroxide (NaOH) and ethanol (C₂H₆O) were purchased from China Phenol Reagent Co., Ltd. Methanol (CH₄O) and acetonitrile (C₂H₃N) are high-performance liquid chromatography grades, purchased from McLean Biochemical Technologies Co., Ltd.

The instruments used in the experiment are analytical balance (Gill International Trading (Shanghai) Co., Ltd); centrifuge (Shanghai Anting Scientific Instrument Factory); ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd); multiple photochemical reaction instrument (Beijing Zhongjiao Jinyuan Technology Co., Ltd); Uv-vis spectrophotometer (Platinum Elmer China); ultra-performance liquid chromatography system (Shimadzu Company, Japan); scanning electron microscope, transmission electron microscope, and solid-state ultraviolet spectrophotometer (Hitachi); Fourier transform infrared spectrometer and X-ray photoelectron spectrometer (Thermo Fisher, UK); fluorescence spectrometer (JASCO); electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd); high performance analog solar xenon lamp source (Beijing Pophilai Technology Co., Ltd); automatic multi-purpose adsorption instrument (Tianjin First Right Company); and tubular furnace (Hefei Kejing Material Technology Co., Ltd).

Preparation of ferrite catalysts

Analytical grade high purity chemicals and reagents were used without further purification. Preparation of MFe₂O₄ (M = Co, Zn, Cu) magnetic nanomaterials by co-precipitation.

CoFe₂O₄. Initially, Co(NO₃)₂·6H₂O (0.001 mol) and Fe(NO₃)₃·9H₂O (0.002 mol) were dispersed in deionized water, and kept under sonication until it was uniformly distributed. NaOH solution was added to the mixture and the pH was adjusted to 6. The solution was then stirred magnetically at 90 °C for 30 min. After the reaction, the brown-red viscous liquid was placed in a high-speed centrifuge for 4000 rpm and centrifuged for 10 min. High-speed centrifugal precipitates were washed with deionizing water. Finally, the samples were dried for 5 hours in an oven at 80 °C.

Detailed preparation processes of ZnFe₂O₄ and CuFe₂O₄ are described in the ESI (section 2.2†).

Preparation of modified ferrite catalysts

Temperature is well known to play a vital role in materials modifications. The optimum treatment temperature for the nanomaterial MFe₂O₄ (M = Co, Cu, Zn) was explored using a fully automatic multi-purpose adsorber (N₂ Temperature Programmed Desorption, TP-5080D). The tests were conducted under a dry 5% H₂/N₂ atmosphere with a 20 mL min⁻¹ flow rate. The sample was purged by opening the gas cylinder during the pre-treatment phase, and after the electrical signal of the sampled values had stabilized, the first stage of temperature increase began. The sample is first warmed up to 100 °C at a rate of 10 °C min⁻¹ and activated at a constant temperature for 30 min to remove moisture and adsorbed gases from the surface. The temperature was then increased at a rate of 10 °C min⁻¹ to 750 °C, and the electrical signals of the sampled values are displayed in Fig. 1a, b, and Fig. S2.† The temperature that corresponds to the sample's lowest value of the electrical signal is considered the optimum modification temperature, and the findings are shown in Table S1†.

Fig. 1 (a) CoFe₂O₄ TPR spectrum and (b) TPR spectrum of CoFe₂O₄ treated at 520 °C.

A series of modified ferrite samples with different oxygen vacancy contents was obtained by annealing MFe₂O₄ (M = Co, Cu, Zn) under a 5% H₂/N₂, N₂, and vacuum atmosphere, the obtained catalysts were named H-MFe₂O_4−x, N-MFe₂O_4−x, and V-MFe₂O_4−x, respectively. All samples obtained were used for further characterization and applications.

H-CoFe₂O_4−x. To eliminate moisture and surface adsorbed gases from the sample, the temperature is increased to 100 °C at a rate of 10 °C min⁻¹ and held at that temperature for 30 minutes. The temperature was then increased at a rate of 10 °C min⁻¹ for 90 minutes from 100 °C to the optimum modification temperature (Table S1†). Fig. 1b and Fig. S2† confirm that the material starts to activate at the optimum temperature for exploration, with the potential for oxygen vacancies to arise during the activation phase due to the combination of high temperature and a reducing atmosphere.

N-CoFe₂O_4−x. To demonstrate the importance of the reducing atmosphere, throughout the trials, a dry N₂ gas stream was used. The rest of the experiment followed the same procedures as the H₂-TPR modification.

V-CoFe₂O_4−x. To investigate the effect of N₂, a vacuum atmosphere was used. CoFe₂O₄ was placed in a tube furnace. The vacuum environment was pumped for 30 min before the start of the ramp-up phase, the vacuum was ramped up to 400 °C, 450 °C, and 500 °C during the ramp-up phase and maintained for 90 min and the rest of the experiment was performed in the same way as the H₂-TPR modification.

The modification processes of ZnFe₂O₄ and CuFe₂O₄ are described in detail in the ESI (section 2.3†).

Characterization

All characterization means and test conditions are shown in the ESI.†

MB degradation experiment

The optimal modification temperature of the sample was evaluated by photocatalytic degradation of MB. The photocatalyst (25 mg) was mixed with a solution of MB (1 × 10⁻⁵ mol L⁻¹, 50 mL). The reaction temperature was maintained at 5 °C using a cooling water circulation system and stirred for 30 min under light-proof conditions to establish adsorption equilibrium. A xenon lamp of 250 W was used to provide visible light at an intensity of 8 × 10⁴ Lux, and a filter (λ > 420 nm) was provided to filter out the UV fraction. Samples were taken from the solution every 20 minutes, filtered using a 0.22 μm filter membrane, and analyzed using a spectrophotometer at the maximum absorption wavelength (λ = 664 nm).

The degradation rate (%) for MB can be calculated using eqn (1):

where C₀ (mg L⁻¹) is the initial concentration after adsorption and C_t (mg L⁻¹) is the pollutant concentration at the end of the reaction.

Aqueous-phase PAH degradation experiments

To evaluate the photodegradation of PAHs in aqueous and soil phases by synthetic samples under simulated sunlight irradiation, naphthalene was used as the target pollutant, and the temperature was controlled below 5 °C by circulating cooling water, so as to effectively avoid the volatilization of pollutants. An aqueous solution of 1 ppm naphthalene was prepared. 50 ml of naphthalene solution and 25 mg of photocatalyst were added to the photocatalytic device. The mixture was stirred for 30 min under dark conditions to reach the adsorption–desorption equilibrium. Afterward, photocatalytic degradation was carried out with a 250 W xenon lamp simulating sunlight. A cut-off filter (λ > 420 nm) was installed between the reactant and the xenon lamp to filter UV light and the light intensity was set to 8 × 10⁴ lux. A 2 ml sample was taken at 30 min intervals and filtered through a 0.22 µm membrane for analysis.

Finally, free radical capture experiments were performed for photocatalytic degradation in naphthalene. Three trapping agents, tert-butyl alcohol (tBuOH), p-benzoquinone (PBQ), and disodium ethylenediaminetetraacetate (EDTA-2Na), and methanol at a concentration of 5 × 10⁻⁴ mol L⁻¹ were added to the reaction solution for the photocatalytic experiments, which were consistent with the experimental procedure for the degradation of PAHs described above.

Soil phase PAH degradation experiments

The degradation of the soil phase is different from that of the aqueous phase. Garden soils were collected from Daxing district, Beijing, and collected in polyethylene sealed bags before being taken to the laboratory. Naphthalene was dissolved in methylene chloride and configured to 1 ppm of contaminated soil. 5 g of contaminated soil was taken and the ratio of soil to water was adjusted to 1 : 10 (v/v). Follow-up experiments were consistent with aqueous phase degradation.

Analytical methods

The concentration of naphthalene in the aqueous phase for photocatalytic degradation was obtained by the following method. The extracted sample was transferred into a centrifuge tube, and then the mixture was centrifuged at 4000 rpm for 10 min to achieve solid–liquid separation. After centrifugation, an appropriate amount of supernatant was filtered through a 0.22 μm filter membrane. The qualitative analysis was performed by ultra-performance liquid chromatography (UPLC)³⁶ using a mobile phase of methanol : water (65 : 35, v/v) at a flow rate of 1.0 mL min⁻¹ and a detector wavelength of 270 nm, respectively.

Concentrations of naphthalene undergoing photocatalytic degradation in soil were obtained by the following method. The samples were transferred into centrifuge tubes, and 10 ml methanol solution was used as the extraction agent. Each sample was extracted for 30 min in an ultrasonic bath, in which the water temperature was lower than 30 °C. The mixture was then centrifuged at 4000 rpm for 10 min to achieve solid–liquid separation. The post-centrifugation operation is the same as that used for the analysis of PAHs in the aqueous phase.

The degradation efficiency of naphthalene or phenanthrene was evaluated by the ratio of the post-degradation concentration to the adsorption equilibrium concentration (C_t/C₀). The reaction rate constants were calculated using eqn (2).

−k_appt = ln(C_t/C₀) (2)

where k_app is the constant of the pseudo-first-order kinetics, t is the reaction time (min), C₀ is the concentration at which the target pollutant reaches adsorption equilibrium, and C_t is the end-of-reaction concentration.³⁷

Results and discussion

Optimal temperature exploration

To evaluate the optimal modification temperature of synthetic samples, the degradation performance of the photocatalyst in MB solution was used for screening. The results showed that the optimal modification temperatures of cobalt ferrite under nitrogen, vacuum, and hydrogen atmospheres were 400 °C, 520 °C, and 520 °C, respectively. In addition, the degradation curves in Fig. 2a and b and the degradation efficiency plots in Fig. 2c, d, and Fig. S3† indicate that CoFe₂O₄ has the best degradation effect among the three samples under the three different modification procedures. After the creation of the oxygen defect, the change in the density of bulk CoFe₂O₄ is affected by the removal of the oxygen element, which causes a potential difference between metal and oxygen atoms. Coulomb forces act as a driving force for further electron migration, resulting in the migration of electrons. The valence electrons of general transition elements include the outermost electron and the d-electrons of the lower outer shell. The outer electron groupings of Co, Cu, and Zn are 3d⁷4s², 3d¹⁰4s¹, and 3d¹⁰4s², respectively.³⁸ Cu and Zn atoms are more likely to lose the electrons first in the outermost s-orbital, while the valence electrons in the d-orbital are completely filled and the structure is very stable. In contrast, only seven electrons in the d-orbital of the Co atom are in the unfilled state, so it is easier to lose two valence electrons in the d-orbital and become a half-filled steady-state structure. From this point of view, oxygen defects cause the change of d-orbital electron density, which further interferes with electron migration and transformation of different transition state metals, and finally results in the difference of CoFe₂O_4−x performance compared with the other two ferrites.

Fig. 2 MB degradation curves for (a) MFe₂O₄ (M = Co, Cu, Zn) modified by N₂-TPD at 400 and 450 °C (b) the bulk material and H₂-TPR, vacuum calcined modified material; (c) k_app for degradation of MB by N₂-TPD modified samples, and (d) k_app for degradation of MB by the bulk material and H₂-TPR, vacuum calcined modified material.

Surface morphology analyses

SEM and TEM tests were performed to observe the morphological structure and characteristics of the samples. All samples show different nanoscale morphologies. CoFe₂O₄ consists of uniform three-dimensional nanoparticles with an average size of 2.5 μm as shown in Fig. 3a and e.³⁹ Moreover, CoFe₂O₄ particle agglomeration can be observed due to its magnetic properties.⁴⁰ The surface morphology of N-CoFe₂O_4−x is shown in Fig. 3b and f. The particles on the surface of the sample disappear and take up a lumpy shape, due to the adsorption of nitrogen that occurs on the surface of the nanoparticles. This may cover the surface active site of N-CoFe₂O_4−x and therefore will be detrimental to the contaminant degradation. Fig. 3c and g show that there are fine particles on the surface of V-CoFe₂O_4−x with an average particle size of 50 nm. The morphology of H-CoFe₂O_4−x shows a sponge-like shape with a distinct irregular porous structure and a large specific surface area. Its average particle size is 40 nm, which is the smallest among the four samples (Fig. 3d and h). It is speculated that its porous structure would be more conducive to the adsorption of pollutants and have a better catalytic effect.

Fig. 3 SEM images of (a) CoFe₂O₄, (b) N-CoFe₂O_4−x, (c) V-CoFe₂O_4−x, and (d) H-CoFe₂O_4−x; TEM images of (e) CoFe₂O₄, (f) N-CoFe₂O_4−x, (g) V-CoFe₂O_4−x, and (h) H-CoFe₂O_4−x; and O 1s chart of (i) CoFe₂O₄, (j) N-CoFe₂O_4−x, (k) V-CoFe₂O_4−x, and (l) H-CoFe₂O_4−x.

To gain further insights into the samples’ surface element environment and surface defects of the synthesized CoFe₂O₄, N-CoFe₂O_4−x, V-CoFe₂O_4−x, and H-CoFe₂O_4−x, XPS spectra were recorded. Fig. S4a† confirms that only Fe, Co, and O elements are present in the cobalt–iron ore. In the O 1s XPS spectra of the samples (Fig. 3i–l), three peaks at 528.20, 529.57, and 530.35 eV were assigned to lattice oxygen (O_L), oxygen vacancies (O_V), and adsorbed oxygen (O_A).⁴¹ The interaction between the surface oxygen defects and lattice oxygen has an important influence on the electron internal circulation of metal ions in the whole system. Specifically, when the oxygen vacancy is generated, the coulomb force becomes the driving force for further electron migration and transformation. In addition, the lack of lattice oxygen leads to a weak coordination structure and significant electrostatic repulsion between electrons, which increases the reducibility of the metal species, thus promoting the electron cycle between the metal ion and oxygen atom.⁴² The peak area ratio of oxygen vacancies/lattice oxygen (O_V/O_L) in H-CoFe₂O_4−x is 1.17, 2.02 times higher than that in CoFe₂O₄ (0.578), indicating a higher amount of oxygen vacancies in H-CoFe₂O_4−x. This is because hydrogen is a very strong reducing agent and can reduce cobalt ferrite at high temperatures.³⁴ Under thermal conditions, hydrogen atoms interact with lattice oxygen on the surface of CoFe₂O₄ to form oxygen vacancies and interstitial hydrogen. Particularly, hydrogen molecules transfer their internal electrons to surface oxygen on CoFe₂O₄, at high temperatures, causing oxygen atoms to undergo desorption and forming oxygen vacancies. The defect formation process is shown in eqn (3). In contrast, the N-CoFe₂O_4−x oxygen vacancy content is only 0.211, which indicates that the formation of oxygen defects is inhibited (Table 1).⁴³

Table 1 XPS results of variation in the ratio of oxygen and iron species as revealed

Samples	Atomic percentage (%)
	Fe(II)	Fe(III)	Fe(II)/Fe(III)	OL	OV	OA	OV/OL
CoFe2O4	22.54	77.46	0.2909	42.35	24.49	33.15	0.5782
V-CoFe2O4−x	24.49	75.51	0.3242	37.05	39.00	23.95	1.0526
N-CoFe2O4−x	27.48	72.52	0.3789	69.45	14.66	15.89	0.211
H-CoFe2O4−x	39.53	60.47	0.6537	34.11	39.79	26.10	1.1665

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Besides, Fe in CoFe₂O₄ has an oxidation state of +3; therefore, the higher Fe(II)/Fe(III) ratio could be a sign of a higher number of oxygen vacancies (Table 1). The Fe 2p XPS spectrum in Fig. S4(b–e)† shows that Fe 2p_1/2 has two peaks at 725.25 and 711.43 eV, which proves the existence of Fe(III) in this system. A peak of Fe 2p_3/2 at 713.81 eV indicates the presence of Fe(II) in the system. The satellite peaks at 716.64 eV further confirmed the existence of the Fe(III) oxidation state in CoFe₂O₄.^25,44,45 The calculated Fe(II)/Fe(III) ratio of H-CoFe₂O_4−x is 0.65, 2.16 times higher than that of CoFe₂O₄ (0.29), in good agreement with the analysis of O 1s spectra (Table 1). The peaks of Co 2p_3/2 (779.04 eV and 782.0 eV) and Co 2p_1/2 (795.0 eV) are shown in Fig. S4(f–i),† together with two satellite features of 785.49 eV and 801.02 eV, respectively. The peaks at 779.04 eV and 782.0 eV indicated the presence of Co(II) oxide species, while the peak at 795.0 eV indicated the presence of Co(III) oxide species.⁴⁵ After modified treatment, the peaks of Co 2p_3/2 and Co 2p_1/2 of three modified samples shifted slightly to the higher binding energy region, which indicated the conversion between Co(II) and Co(III) in the catalyst. Metal cycling was attributed to the action of oxygen defects. Due to the generation of oxygen defects, an electron-rich surface is formed around the oxygen atom and dissolved oxygen gets electrons to form O₂˙⁻ (eqn (4)). After the oxygen defect is produced, it promotes electron transfer due to coulomb forces and participates in the redox cycle from Co(III)/Fe(III) to Co(II)/Fe(II) to produce ¹O₂ (eqn (5) and (6)).

O₂ + e⁻ → O₂˙⁻ (4)

O₂ + Fe(III) + e⁻ → ¹O₂ + Fe(II) (5)

O₂ + Co(III) + e⁻ → ¹O₂ + Co(II) (6)

As shown in Fig. 4, the XRD patterns of CoFe₂O₄ and H-CoFe₂O_4−x nanoparticles showed that at 35.08°, 41.39°, 43.31°, 50.45°, 62.96°, 67.27° and 74.19°, they belong to the (220), (311), (222), (400), (422), (511) and (440) planes of the cubic spinel structure (JCPDS 22-1086).⁴⁶ The results confirmed the existence of spinel-structured CoFe₂O₄ nanoparticles. There is no phase transition between H-CoFe₂O_4−x and original CoFe₂O₄, but the XRD peak of H-CoFe₂O_4−x is significantly reduced. In addition, the peak (222) of H-CoFe₂O_4−x moves toward lower 2θ values compared to the cube spinel standard position (JCPDS 22-1086). The results show that H-CoFe₂O_4−x has a large d-spacing due to lattice distortion and lattice defects. The above phenomena are caused by the defects of the crystal structure and may be related to the formation of oxygen vacancies.⁴⁷

Fig. 4 XRD patterns of CoFe₂O₄ and H-CoFe₂O_4−x.

Optical and electrochemical characterization

The functional groups on the sample surface were investigated by infrared spectroscopy (FTIR), as shown in Fig. 5a. The FT-IR spectra of the samples at 3417, 1762, 1344, and 1049 cm⁻¹ show the characteristics of nanocomposite samples. Bands at 3417 and 1762 cm⁻¹ are attributed to O–H stretching vibrations and C–O stretching vibrations of the COOH CoFe₂O₄ groups, respectively, and bands at 1344 and 1049 cm⁻¹ correspond to C–OH stretching and C–O–C stretching vibrations.⁴⁸ The high-intensity band of 600–750 cm⁻¹ is caused by the inherent stretching vibrations of the tetrahedra and octahedra occupied by metal–oxygen bonds.⁴⁹ The presence of these characteristic peaks proves that the sample contains Fe(III) and Co(II). The low valent metal ions of Co(II)/Fe(II) on the catalyst surface can capture the hydroxyl groups, and in addition Co(II)/Fe(II) can serve as an active site for PMS decomposition and formation of reactive oxygen species, so the surface hydroxyl group concentration is positively correlated with the pollutant degradation rate.⁵⁰

Fig. 5 (a) FT-IR, (b) diffuse reflectance UV-vis spectrum, (c) PL spectra (λ = 420 nm), (d) photocurrents under visible light irradiation, (e) linear sweep voltammetry (LSV) and (f) Tafel images of CoFe₂O₄, N-CoFe₂O_4−x, V-CoFe₂O_4−x, and H-CoFe₂O_4−x.

To determine the optical absorption characteristics and band gap energy (E_g) of the samples, UV-vis DRS analysis was performed in the range of 300 to 800 nm, and the results are shown in Fig. 5b. It can be seen that H-CoFe₂O_4−x has a characteristic peak near 350 nm and shows light absorption ability in the UV region, indicating that H-CoFe₂O_4−x has the ability to generate electron/hole pairs under UV light. However, compared with CoFe₂O₄, H-CoFe₂O_4−x has significant absorption in both ultraviolet and visible regions, thus generating more photoinduced carriers to significantly improve its photocatalytic capacity. H-CoFe₂O_4−x has a significant absorption effect in the visible range, which may be influenced by the black character of its sample surface.⁵¹ The band gap energy values of the prepared samples were calculated using Kubelka–Munk theory, as shown in Fig. S5.† The band gap energy can be estimated from the tangent intercept of (ahv)² with the radiation energy. The obtained band gap energy for CoFe₂O₄ is 1.965 V. As for N-CoFe₂O_4−x, V-CoFe₂O_4−x, and H-CoFe₂O_4−x, the values of band gap energy are 2.097 V, 2.197 V, and 2.916 V, respectively.

Fig. 5c presents photoluminescence (PL) emission spectra of the photocatalyst monitored at an excitation wavelength of 390 nm. Of all the samples, CoFe₂O₄ showed the strongest fluorescence peak at 657 nm, and its fluorescence intensity was the highest, indicating that the resulting electron holes are prone to recombination. The emission intensity of the H-CoFe₂O_4−x heterostructure is much lower than that of CoFe₂O₄, indicating that the recombination of photogenerated electrons and holes in the H-CoFe₂O_4−x system is greatly inhibited.³⁶

In addition, photocurrent measurements can also provide a basis for the separation rate of photogenerated carriers in heterojunctions. By analysing the relationship between photocurrent, photovoltage and optical intensity, the photocatalytic performance of the sample can be confirmed. The photocurrents of the four samples under the xenon lamp (λ > 420 nm) are shown in Fig. 5d. It is concluded that H-CoFe₂O_4−x has the highest current density, photocurrent response value, and separation of light-generated electron–hole pairs. Linear scanning voltammetry (LSV) tests were performed on the samples and the highest cathodic currents in the curves further indicate that H-CoFe₂O_4−x has the best electron transfer and charge separation efficiency compared with the other three samples (Fig. 5e). The Tafel slopes (Fig. 5f) of 30.1 mV dec⁻¹ in H-CoFe₂O_4−x are much lower than those in CoFe₂O₄ (165 mV dec⁻¹), N-CoFe₂O_4−x (228 mV dec⁻¹) and V-CoFe₂O_4−x (97.8 mV dec⁻¹), indicating the fastest growth in current density and the smallest change in overpotential.⁵² This also indicates that the reduction of hydrogen enhances the photogenerated carrier transport rate and enables better photoelectron mobility of the sample. Based on PL spectroscopy, photocurrent, LSV diagram, and Tafel analyses, the cobalt-ferrite heterojunction formed by H-CoFe₂O_4−x is found to be the most beneficial for carrier separation. Thus, it is more favourable for the production of active substances and promotes pollutant degradation.

To investigate the band gap energy of cobalt ferrite and modified samples, Mott–Schottky experiments were carried out. The Mott–Schottky diagram can accurately determine the type of semiconductor. A positive slope indicates that the sample has n-type properties, while a negative slope indicates p-type properties. Fig. 6(a–d) show the Mott Schottky plots obtained for the four samples at different frequencies of 1k Hz and 2k Hz, as well as the linear region of the inverse squared capacitance versus applied potential curves for the corresponding cobalt ferrite and its defective samples. It is shown that CoFe₂O₄ and H-CoFe₂O_4−x have n-type properties, and N-CoFe₂O_4−x and V-CoFe₂O_4−x have p-type properties. The flat-band (FB) potentials of CoFe₂O₄, N-CoFe₂O_4−x, V-CoFe₂O_4−x, and H-CoFe₂O_4−x catalys are −0.558 V, −0.279 V, 0.094 V, and −0.723 V, respectively, when the extrapolated capacitance is 0. For n-type semiconductor materials, the conversion relationship between the conduction band (E_CB) and E_FB is eqn (7); for p-type semiconductors, the conversion relationship between the valence band (E_VB) and E_FB is eqn (8).

E_CB = E_FB − 0.1 V (7)

E_VB = E_FB + 0.1 V (8)

E_SCE = E_NHE − 0.241 V. (9)

Fig. 6 Mott Schottky curves for (a) CoFe₂O₄, (b) N-CoFe₂O_4−x, (c) V-CoFe₂O_4−x, and (d) H-CoFe₂O_4−x; (e) energy band diagrams of n-type semiconductor photocatalytic samples CoFe₂O₄ and H-CoFe₂O_4−x and (f) energy band diagrams of p-type semiconductor photocatalytic samples N-CoFe₂O_4−x and V-CoFe₂O_4−x.

The E_CB values were −0.658 and −0.823 V for CoFe₂O₄ and H-CoFe₂O_4−x (vs. SCE), and the E_VB values were −0.179 and 0.194 V for N-CoFe₂O_4−x and V-CoFe₂O_4−x (vs. SCE). The normal hydrogen electrode (NHE) has a certain conversion relationship with the SCE electrode, as shown by eqn (9).^53–55 The E_NHE values of CoFe₂O₄, N-CoFe₂O_4−x, V-CoFe₂O_4−x, and H-CoFe₂O_4−x were calculated to be −0.417 V, −0.062 V, −0.435 V, and −0.582 V, respectively.

Combined with the calculation results, the band gap energy of n-type CoFe₂O₄ and H-CoFe₂O_4−x and p-type N-CoFe₂O_4−x and V-CoFe₂O_4−x is 1.965, 2.916, 2.097, and 2.197 eV, respectively. H-CoFe₂O_4−x can reach higher redox potentials and produce more active substances, which was due to its wider and adjustable energy band structure. Fig. 6e and f imply that the active species are ˙OH and ˙O₂⁻, thus improving the degradation of organic pollutants.

The electron transfer resistance of the synthesized samples was investigated using the electrochemical impedance spectroscopy (EIS) technique. Fig. 7a depicts the Nyquist plots of the four photocatalytic samples under different conditions with the lights on and off. The high frequency region in the EIS plot is the charge transfer resistance, which mainly includes the resistance of the measured salt solution, the resistance of the experimentally made electrodes, and the contact resistance. Under the condition of turning on the lamp, the semicircles in the EIS diagram are arranged in the order of CoFe₂O₄ > N-CoFe₂O_4−x > V-CoFe₂O_4−x > H-CoFe₂O_4−x, which verifies that H-CoFe₂O_4−x has lower charge separation and migration resistance.^56,57

Fig. 7 (a) Electrochemical impedance spectroscopy (EIS), (b) cyclic voltammetry (CV) curves of four samples, (c) CV curves of H-CoFe₂O_4−x at different voltages (−0.1–0.1, and −0.2–0.2 V) and (d) CV curves of H-CoFe₂O_4−x at different scan rates (5, 10, 20, and 100 mV s⁻¹).

The integrated areas on the CV curves were in a descending order of H-CoFe₂O_4−x > V-CoFe₂O_4−x > N-CoFe₂O_4−x > CoFe₂O₄. H-CoFe₂O_4−x has the largest surface area, thus providing more active sites and thus allowing carriers to pass through (Fig. 7b) and it can be seen from the figure that the CV curve shape shows a good shuttle shape, and the H-CoFe₂O_4−x electrode has a higher redox potential, which is favorable for the photocatalytic reaction. Fig. 7c shows the CV curves of H-CoFe₂O_4−x at different potentials (−0.1–0.1 V, and −0.2–0.2 V). The variation of current with voltage is symmetric, indicating that H-CoFe₂O_4−x has excellent stability, regenerative capacity, and capacitive properties. Fig. 7d shows the CV curves of H-CoFe₂O_4−x at scan rates of 5, 10, 20, and 100 mV s⁻¹. The current response increases with increasing scan rate and remains well reproducible after 40 segments of the scan, indicating that H-CoFe₂O_4−x has optimal rate capability and electrochemical reversibility.⁵⁸

Assessment of photocatalysis

The microstructure, spectral properties, and photoelectrochemical properties of cobalt ferrite were analysed. The results suggest that H-CoFe₂O_4−x has an efficient photogenerated charge separation ability and is suitable for photocatalytic degradation of pollutants. The photocatalytic activity of the samples in practical applications was assessed by the photodegradation of naphthalene and phenanthrene in the soil phase under visible light.

The plots of naphthalene degradation in soil by cobalt ferrite and modified samples are shown in Fig. 8a and b. Only 2.07% of naphthalene was degraded by CoFe₂O₄ within 180 min. However, the degradation rate of H-CoFe₂O_4−x was 24.7%, which was 12 times higher compared with CoFe₂O₄. Fig. 8c shows the degradation efficiency, with the k_app in the naphthalene/H-CoFe₂O_4−x system as 0.000913 (min⁻¹), which is 9 times higher than that of the bulk material. The degradation rate improved to 46.7% in the H-CoFe₂O_4−x/PMS system, and the k_app increased by 27.7 times. For the degradation of phenanthrene in PAHs, H-CoFe₂O_4−x also showed the best degradation (Fig. 8d) with 35.1%. As shown in Fig. 8e, in the H-CoFe₂O_4−x/PMS system, the degradation rate increased to 55.2%, and k_app reached 0.00211 (min⁻¹), which increased by 10 times (Fig. 8f).

Fig. 8 Degraded naphthalene in the soil phase: (a) degradation curve ([NAP] = 1 ppm), (b) synergistic PMS degradation curve, (c) k_app value; degraded phenanthrene in the soil phase, (d) degradation curve ([PHE] = 1 ppm), (e) synergistic PMS degradation curve, and (f) k_app value.

An unexpected finding was obtained from the degradation rate graphs in Fig. 8b and e. After 30 min of light-sensitive adsorption, the degradation rate of all samples showed a significant rebound. This is because soil is a complex system containing soil humus, soil colloids, etc. During the dark adsorption process, the soil's natural active agent (HA) also has a certain adsorption effect, which temporarily reduces the concentration of organic pollution.⁵⁹ When visible light was provided, the desorption of NAP in the soil was enhanced and the concentration gradually rebounded.

The photocatalytic degradation of naphthalene at 1 ppm in the aqueous phase was carried out in the presence of 25 mg of catalyst. It can be seen from Fig. 9a that H-CoFe₂O_4−x has the best degradation effect within 180 min, and the degradation rate can reach about 60%, which is 23% higher than that of CoFe₂O₄. The k_app value was 0.00289 min⁻¹, 3.5 times that of the bulk material (Fig. 9c and Fig. S6†). After that, the photocatalyst/PMS system was constructed by adding PMS at a concentration of 1 mmol L⁻¹ to the aqueous phase, which also degraded 1 ppm aqueous naphthalene solution. The degradation curves within 180 min are shown in Fig. 9b, and the degradation effect was significantly improved. Among them, H-CoFe₂O_4−x had the best degradation effect in cooperation with PMS (82.0%, k_app = 0.00714 min⁻¹), and it can be concluded that H-CoFe₂O_4−x can effectively activate PMS to generate active species and promote the degradation of pollutants.⁶⁰ In practical applications, recyclability and stability are important indicators to evaluate the catalyst performance. For this purpose, the degree of pollutant mineralization was evaluated by four consecutive cycles of the H-CoFe₂O_4−x/PMS system for the degradation of 1 ppm naphthalene. As shown in Fig. 9d and Fig. S7,† no significant degradation was observed after four repeated runs, and the high catalytic performance (>73.6%) was still maintained. In addition, H-CoFe₂O_4−x has good magnetic properties (Fig. S8†) and can be used as a recyclable catalyst with high application potential and good economic benefits.

Fig. 9 (a) Naphthalene degradation curve ([NAP] = 1 ppm), (b) synergistic PMS degradation curve, (c) k_app value, (d) naphthalene degradation cycling experiment ([NAP] = 1 ppm), (e) CV curves of H-CoFe₂O_4−x after cyclic degradation, (f) EIS profile of H-CoFe₂O_4−x after cyclic degradation (illustration is an EIS enlarged view), (g) effect of free radical scavengers on naphthalene degradation ([NAP] = 1 ppm) and (h) degradation rate by the action of free radical scavengers.

Cyclic voltammetry was used to analyse the H-CoFe₂O_4−x after cyclic degradation (Fig. 9e). The sample was subjected to an electric potential (−0.1–0.1 V) and different sweep rates (5, 10, 20, and 100 mV s⁻¹). The results were essentially the same as for the samples before cyclic degradation. Furthermore, the stability of of H-CoFe₂O_4−x can be analyzed by the EIS spectra before and after cyclic degradation (Fig. 9f). The intersection of the EIS and the real axis indicates the solution resistance (R_s) of the electrode,⁶¹ and the values of R_s before and after cyclic degradation of H-CoFe₂O_4−x are 38.26 Ω and 41.11 Ω, respectively, with little change in resistance, indicating that the sample has high stability.

Photocatalytic mechanism

The quenching experiment was carried out to determine the type of free radical generated. tert-Butanol (TBA), disodium ethylenediaminetetraacetate (EDTA-2Na) and p-benzoquinone (BQ) were used to quench the radicals against ˙OH, h⁺ and ˙O₂⁻, respectively, and SO₄˙⁻ and ˙OH radicals were quenched by methanol (Fig. 9g). As can be seen from Fig. 9h, the degradation rate of naphthalene solution with the addition of BQ and EDTA-2Na was the lowest, indicating that a large amount of ˙O₂⁻ and h⁺ was captured. Therefore, ˙O₂⁻ and h⁺ radicals play an important role in the H-CoFe₂O_4−x/PMS system. Meanwhile, ˙OH and SO₄˙⁻ play an auxiliary effect in the degradation process of organic pollutants.

Structural changes of the samples after modification and the mechanism diagram of degradation of PAHs by activated PMS with H-CoFe₂O_4−x are shown in Fig. 10. The high temperature treatment of CoFe₂O₄ under a hydrogen atmosphere resulted in a porous structure, which promoted the adsorption of pollutants. Hydrogen reacts with oxygen in the sample to form H₂O, and oxygen vacancies appear on the surface of the material. H-CoFe₂O_4−x undergoes charge separation and generates electrons and holes under visible light irradiation (eqn (10)). Electrons can then combine with dissolved oxygen to produce O₂˙⁻ (eqn (11)). The process of oxidation of water molecules to generate ˙OH radicals is mainly through the transfer of photogenerated h⁺ to water molecules and the breaking of O–H bonds to generate ˙OH (eqn (12)).

Fig. 10 Structural changes of the samples after modification and the mechanism diagram of degradation of PAHs by activated PMS with H-CoFe₂O_4−x.

After the oxygen defect is produced, the change of oxygen atomic density is affected by the removal of the oxygen element, resulting in the potential difference with the metal. The coulomb force acts as the driving force for further electron migration, promoting electron transfer and enhancing the redox cycle (eqn (13)) from Co(III)/Fe(III) to Co(II)/Fe(II), thus promoting the formation of ¹O₂ and O₂˙⁻. Due to the existence of oxygen vacancies on the surface of H-CoFe₂O_4−x, the electronic properties of metal sites can be effectively regulated to form an electron-rich surface. In addition, more active sites can be provided as PMS activation centers to generate ˙OH and SO₄˙⁻ for efficient degradation of organic pollutants (eqn (14)). Finally, combined with quenching experiments, O₂˙⁻ and h⁺ radicals play an important role in the H-CoFe₂O_4−x/PMS system. At the same time, ˙OH and SO₄˙⁻ play auxiliary roles in the degradation of organic pollutants (eqn (15)).

H-CoFe₂O_4−x + hv → H-CoFe₂O_4−x(e) + H-CoFe₂O_4−x(h) (10)

H-CoFe₂O_4−x(e) + O₂ → H-CoFe₂O_4−x + O₂˙⁻ (11)

H-CoFe₂O_4−x(h) + OH⁻ → H-CoFe₂O_4−x + ˙OH (12)

O₂ + Fe(III)/Co(III) + 2e⁻ → O₂˙⁻ + Fe(II)/Co(II) (13)

Fe(II)/Co(II) + PMS → Fe(III)/Co(III) + SO₄˙⁻ + ˙OH (14)

O₂˙⁻ (h⁺, ˙OH, ¹O₂) + naphthalene → CO₂ + H₂O (15)

Conclusions

In summary, we have synthesized H-CoFe₂O_4−x containing suitable oxygen vacancies by using H₂-TPR to meet the needs of aqueous and soil phase pollution remediation. H-CoFe₂O_4−x coupled with a PMS activation system exhibited excellent photocatalytic activity. The degradation rates of naphthalene and phenanthrene in the soil phase increased by 3.24-fold and 1.39-fold, respectively, and 1.38-fold for naphthalene in the aqueous phase by H-CoFe₂O_4−x. Electrochemical methods were used to confirm that the oxygen vacancies on the H-CoFe₂O_4−x surface accelerate electron transfer. Meanwhile, it enhances the redox cycle from Co(III)/Fe(III) to Co(II)/Fe(II), and then promotes the formation of ˙O₂⁻. In the H-CoFe₂O_4−x/PMS system, the radical scavenging experiments confirmed that O₂˙⁻ and h⁺ were the main reactive radicals in the degradation of PAHs. In particular, the optimized H-CoFe₂O_4−x photocatalyst was magnetic and easy to recover, and had superior cycling stability. This study provides a promising remediation method for designing efficient photocatalysts to improve the photocatalytic performance of PAHs in the aqueous phase and soil.

Author contributions

Xiaojuan Bai: conceptualization, writing – review, supervision and resources. Wei Song: experiment, conceptualization, validation, data curation and writing. Xuan Ling: experiment and writing – review. Linlong Guo: conceptualization and resources. Derek Hao: supervision. Xiao Zhang: supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partly supported by the National Key R&D Program of China (2020YFC1808805), National Natural Science Foundation of China (22276011 and 21607034), Beijing Natural Science Foundation (8192011), Science and Technology General Project of Beijing Municipal Education Commission (KM202010016006), and the Pyramid Talent Training Project of Beijing University of Civil Engineering and Architecture (JDYC20200313).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr00727h

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