Skip to main content
NCBI home page
RSC Advances logoLink to RSC Advances
. 2026 Oct 5. Online ahead of print. doi: 10.1039/d6ra06402g

Sunlight-assisted activation of peroxymonosulfate using NiFe2O4 supported on a 2D g-C3N4 nanosheet heterojunction for the degradation of methylene blue in wastewater

Siphumelele Thandokwazi Mkhondwane a,✉, Yogendra Kumar b, Vinod c, Viswanadha Srirama Rajasekhar Pullabhotla a
PMCID: PMC13636767  PMID: 42836029

Abstract

This study explores the utilization of a 0D/2D NiFe2O4/g-C3N4 heterojunction for the photocatalytic activation of peroxymonosulfate (PMS) to degrade methylene blue in wastewater. The NiFe2O4/g-C3N4 composite was synthesized from 0D NiFe2O4 and 2D g-C3N4 using wet impregnation and calcination. The NiFe2O4/g-C3N4 composite was then characterised using various spectroscopic and microscopic techniques and used for the photocatalytic degradation of methylene blue in the presence of PMS (1 mL, 0.1 M). The NiFe2O4/g-C3N4 composite showed a higher degradation efficiency of 97% compared to NiFe2O4 and g-C3N4 alone, which showed 69% and 53%, respectively, after 90 min of degradation. The concentration of PMS was optimized across 0.2 M, 0.4 M and 1.6 M, yielding degradation efficiencies of 99%, 91% and 74%, respectively; this indicates that the highest degradation performance is obtained at 0.2 M. Subsequently, the pH was also optimized, with pH 9 and 11 achieving 100% degradation efficiency. The radical quenching experiments showed that sulfate radicals (SO4˙−) were the dominant reactive oxygen species (ROS) responsible for the degradation of methylene blue. The study showed that NiFe2O4/g-C3N4 has the potential to be used as a catalyst for PMS activation to degrade organic pollutants in wastewater under light irradiation.

This study explores the utilization of a 0D/2D NiFe2O4/g-C3N4 heterojunction for the photocatalytic activation of peroxymonosulfate (PMS) to degrade methylene blue in wastewater.graphic file with name d6ra06402g-ga.webp

1. Introduction

Over the past years, high concentrations of organic compounds such as textile dyes, chlorinated compounds and pharmaceuticals have been detected in wastewater. These organic compounds are highly persistent; hence, they cannot be effectively removed from wastewater using conventional methods.1 Improper discharge of wastewater containing organic pollutants leads to severe health hazards and environmental pollution.2 Therefore, developing efficient technologies to remove organic compounds from wastewater is highly needed. Advanced oxidation processes (AOPs) such as Fenton and Fenton-like processes,3 electrocatalysis,4 catalytic ozonation,5 photocatalysis6 and persulfate oxidation7 have received attention due to their effectiveness in the degradation of organic pollutants.

Persulfate oxidation is a promising technology because of its high degradation efficiency and ability to function across a wide pH range.8 PMS is often preferred over other oxidants, such as oxygen, ozone, peroxide and persulfate, because it generates both sulfate radicals (SO4˙−) and hydroxide radicals (HO˙), whereas oxygen, ozone and peroxide generate predominantly hydroxide radicals (HO˙). Sulfate radicals have a higher reduction potential (2.5–3.1 V) than hydroxide radicals (˙OH) (1.8–2.7 V). There are several processes in which PMS can be activated to generate radical species. These processes include photocatalysis,9 sonocatalysis10 and redox reactions.11 PMS activation is a sluggish process due to the high activation energy required to break the bonds; hence, a good and suitable catalyst is always required. Various catalysts ranging from carbon-based to metal and metal oxide catalysts, such as graphitic carbon nitride,12 spinel metal ferrites,13 and electrospun nanofibers,14 have shown tremendous photocatalytic and persulfate oxidation abilities.

g-C3N4 is a prominent photocatalyst with excellent photostability, good visible-light absorption and a moderate band gap (∼2.7 eV).15 g-C3N4 has unique electronic properties and can be used as an electron donor to reduce the leaching of metal ions. It has been used in photocatalysis, and the photocatalytic activation of PMS has shown promising results. However, g-C3N4 suffers from a few drawbacks such as fast electro–hole pair recombination, low quantum efficiency and low surface area.15 To combat these drawbacks, the construction of a heterojunction through the fabrication of a composite has been employed.16,17 The heterojunction promotes the separation of the electron–hole pairs and increases the photocatalytic efficiency.18 Spinel NiFe2O4 has low solubility, a stable crystalline structure, and good magnetic properties, which facilitate catalyst recovery and tremendous persulfate oxidation properties.19 Spinel NiFe2O4 particles function as 0D nanoparticles; however, when utilized in conjunction with 2D nanoparticles, the interfacial contact is enhanced, subsequently facilitating the separation of charge carriers (electron–hole pairs). This phenomenon improves photocatalytic efficiency.20

Although a composite of NiFe2O4/g-C3N4 has been reported previously by Silva et al.21 and Palanivel et al.,22 the scope of these studies differs from the present work. Specifically, Silva et al. reported the utilization of NiFe2O4/g-C3N4 for peroxymonosulfate activation to degrade tetracycline hydrochloride, whereas this study focuses on methylene blue degradation. Meanwhile, Palanivel et al. used NiFe2O4/g-C3N4 for the degradation of methylene blue and rhodamine B, with H2O2 as the oxidant or radical initiator, whereas in this study, peroxymonosulfate was used as the radical initiator, thereby generating sulfate radicals. Alternatively, graphene oxide-encapsulated g-C3N4/ZnO ternary composites were reported for degradation of methylene blue, with a degradation efficiency of 90%.23 In another study, NiFe2O4 was utilized for the photocatalytic-assisted Fenton degradation of methylene blue, with a degradation efficiency of 99% after 250 min.24 These studies have shown that both NiFe2O4 and g-C3N4 are promising photocatalysts for methylene blue degradation. Hence, based on these studies, the construction of the 0D/2D NiFe2O4/g-C3N4 heterojunction is anticipated to improve the photocatalytic activation of PMS and the degradation efficiency of methylene blue. Methylene blue was chosen in this study because it is extensively used in the textiles, leather and pharmaceutical industries, making it one of the most common wastewater pollutants.25

2. Experimental

2.1. Materials

The following chemicals were purchased and used without further purification:

Nickel acetate (Ni(CH3COO)2, Sigma-Aldrich), sodium hydroxide (NaOH, Merck, 99%), dimethylformamide (99%, Merck), methylene blue (99%, Merck), peroxymonosulphate (PMS, Aldrich, 99%), hydrochloric acid (HCl, Merck, 37%), furfuryl alcohol (FFA), iron(iii) chloride hexahydrate (FeCl3·6HO), 4-benzoquinone (BQ, Sigma), ethanol (Merck), tert-butyl alcohol (TBA, Merck), and melamine (C3H6N6, Merck). Wastewater was collected from the Mhlathuze wastewater plant, KwaZulu Natal, South Africa; coordinates: latitude −28.76; longitude 32.07.

2.2. Catalysts preparation

2.2.1. Synthesis of 0D g-C3N4

g-C3N4 was synthesized by thermal polymerization, as reported in the literature with a slight modification.17 Instead of using urea as a starting material, which was previously used,17 10 g of melamine was placed in a porcelain crucible and heated to a temperature of 500 °C at a rate of 2 °C min−1 and maintained at this temperature for 5 h. The resultant light-yellow powder (g-C3N4 NS) was cooled and collected (Scheme 1A).

Scheme 1. (A) Preparation of g-C3N4, (B) preparation of NiFe2O4 NPs and (C) preparation of the NiFe2O4/g-C3N4 catalyst.

Scheme 1

Open in a new tab

2.2.2. Synthesis of 0D NiFe2O4 nanoparticles

NiFe2O4 nanoparticles were synthesized as previously reported.11 Briefly, in a beaker, aqueous solutions of ferric nitrate (0.02 mol) and nickel acetate (0.01 mol) were mixed and consistently agitated before adding HNO3 (0.44 mol). The mixture was named solution A. A solution containing 0.09 moles of sugar was slowly mixed with 0.25 moles of monoethanolamine (ETA) and named solution B. Solution B was slowly transferred to solution A. The mixture was heated on a hot plate until it experienced a significant change and turned into a large, black material. The large, black precursor material was subjected to calcination at 600 °C in a muffle furnace under air flow for roughly 7 h until a brown powder was obtained,26 as shown in Scheme 1B. The muffle furnace was heated and stabilized at 600 °C before the precursor was subjected to heating.

2.2.3. Synthesis of 0D/2D NiFe2O4/g-C3N4

Two solutions of NiFe2O4 (1 g) and g-C3N4 (10 g) were prepared by dissolving both NiFe2O4 and g-C3N4 in 5 mL of ethanol in separate containers, followed by stirring for 1 h. The solution of NiFe2O4 was slowly (dropwise) added to a solution of g-C3N4 with continuous stirring. The mixture was further stirred for 3 h. The solvent was dried in a high vacuum fume hood, followed by calcination at 300 °C in a furnace for 3 h. The NiFe2O4/g-C3N4 composite was collected and transferred to a container and stored in the dark, as shown in Scheme 1C.

2.3. Photocatalytic degradation studies

The degradation studies were performed using natural sunlight. The temperature, precipitation, humidity and wind were reported to be 29 °C, 5%, 68% and 24 km h−1, respectively. These conditions were chosen because they are more common in South Africa in summer, which would be beneficial for practical application. 5 mL of 40 mg L−1 methylene blue was used for all the studies. PMS (1 mL, 0.1 M) and NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts (20 mg) were used. The untreated wastewater samples were collected from the Mhlathuze wastewater plant (KwaZulu Natal, South Africa, coordinates: latitude −28.76; longitude 32.07). The water in the Mhlathuze wastewater plant is a combination of sewage, industrial effluent and domestic wastewater from households in Empangeni, Richards Bay, and Felixton. The Mhlathuze wastewater plant was chosen because of its location. Its wastewater system is generated by major industries in the region, including factories and chemical processing plants, particularly in the industrial areas of Richards Bay, Empangeni and Felixton. These samples were used for the preparation of methylene blue solutions. The studies were performed in triplicate, and the arithmetic mean was calculated. Before the degradation studies were performed, the mixture of the catalysts and the solution were kept in the dark for 60 min to reach adsorption–desorption equilibrium. Eqn (1) was used to calculate the degradation efficiency, where Co is the initial concentration of methylene blue, and Ct is the concentration of methylene blue at time t during degradation.

2.3. 1

Initially, control experiments for methylene blue degradation were conducted in the presence of light alone and PMS alone. Comparative degradation studies were then performed under various combinations: PMS with light, PMS with catalysts and light with catalysts. Subsequently, methylene blue degradation studies were performed in the system combining PMS, sunlight and catalysts. The kinetic data were obtained by fitting ln(Co/Ct) against time (min). The rate constants (kobs) were obtained from the slopes. The half-lives were calculated using the equation t1/2 = 0.693/kobs. The reaction setup is shown in Fig. S1.

2.4. Catalyst characterization

A Bruker Tensor 27 FT-IR spectrometer was used to acquire the FT-IR spectra and carry out the analysis with a standard ATR cell. The analysis was performed in the mid-IR region between 450 and 4000 cm−1.

XRD spectra were acquired using a Bruker AXS D8 diffractometer instrument equipped with monochromatic Cu Kα radiation (λ = 1.5406 Å) operated at 40 mA and 40 kV. The instrument was operated over a scan range of 10°–90° with a wavelength of 1.5406 Å and a scan speed of 0.3° min−1.

Scanning electron microscopy (SEM) imaging was performed using a Carl Zeiss FE-SEM Sigma VP-03-67 instrument. The acceleration voltage was set at 20 kV. The sample holder was a carbon tape mounted on a stub. Gold was used as a coating material.

Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-resolution electron microscopy (HRTEM) analyses were performed using a JEOL 2100 HRTEM instrument. The analyses were performed at an accelerating voltage of 200 kV.

The absorption properties of g-C3N4, NiFe2O4 and the NiFe2O4/g-C3N4 composite were determined using a PerkinElmer Lambda 1050 NIR UV-visible spectrophotometer.

The Brunauer–Emmett–Teller (BET) analysis was performed using a Micrometrics ASAP 2020 surface area and porosity analyzer to obtain nitrogen adsorption/desorption isotherms. The samples were degassed with helium prior to analysis.

3. Results and discussions

3.1. Characterization of the catalysts

3.1.1. Fourier transform infrared (FT-IR) spectroscopy

The FT-IR spectra of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 are shown in Fig. 1. The spectrum of g-C3N4 displays an intense peak at 800 cm−1, corresponding to the compression modes and vibrational stretching of the triazine ring. Several peaks are observed between 1000 and 1650 cm−1, corresponding to the stretching vibrations of tri-s-triazine units. A broad peak observed at 3140 cm−1 corresponds to the stretching vibration of the O–H bond emanating from the surface-adsorbed water.27,28 The spectrum of NiFe2O4 (Fig. 1) displays a strong peak at 530 cm−1 attributed to the stretching vibrations of the metal oxide in the tetrahedral and octahedral sites.29 The spectrum of NiFe2O4/g-C3N4 (Fig. 1) displays peaks at 800 and between 1000 and 1650 cm−1, corresponding to g-C3N4. A weak peak is observed at 530 cm−1, corresponding to NiFe2O4.

Fig. 1. FT-IR spectra of NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4.

Fig. 1

Open in a new tab

3.1.2. X-ray diffraction (XRD)

The XRD patterns of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 are shown in Fig. 2. The XRD pattern of g-C3N4 displays a peak at 2θ = 27°, corresponding to the (002) Miller indices of g-C3N4.30 The XRD pattern of NiFe2O4 displays peaks at 2θ = 18.3°, 30.1°, 35.4°, 37.1°, 43.1°, 53.8°, 57.3°, and 62.6°, corresponding to (111), (220), (311), (222), (400), (422), (511), and (440) planes, respectively, of the spinel NiFe2O4.31,32 The XRD pattern of NiFe2O4/g-C3N4 displays an intense peak at 2θ = 27.2°, corresponding to the (002) Miller index, and low intensity peaks at 2θ = 18.3°, 30.1°, 35.4°, 37.1°, 43.1°, 53.8°, 57.3°, and 62.6°, corresponding to (111), (220), (311), (222), (400), (422), (511), and (440) planes, respectively, of the spinel NiFe2O4 are observed. This confirms the existence of both g-C3N4 and NiFe2O4. The results showed that there was no observable shift in the (002) peak corresponding to g-C3N4 upon the incorporation of NiFe2O4 NPs. The crystallite size of g-C3N4 and NiFe2O4 NPs and g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 were investigated using the Scherrer equation, eqn (2).

3.1.2. 2

where D is the crystaliine size, K is the shape factor (K = 0.90), λ is the wavelength (λ = 0.15406 nm), β is line broadening measured as the full width at half maximum of the diffraction peak, and θ is Bragg's angle (corresponding to half of the experimental 2θ peak position). Both 2θ and β are measured in radians. The mean size of pristine g-C3N4 was calculated to be 12 nm, while the mean size of NiFe2O4 was calculated to be 29 nm. The mean size of NiFe2O4/g-C3N4 was calculated to be 17 nm.

Fig. 2. XRD patterns of NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4.

Fig. 2

Open in a new tab

However, the Scherrer method only provides information regarding crystal size and ignores internal lattice strain. Hence, the William–Hall method was utilized to investigate the crystal size and microstrain. The William–Hall equation is shown in eqn (3).

3.1.2. 3

where ε is the effective microstrain. The equation represents a straight line in which ε is the slope. The plot of NiFe2O4/g-C3N4 is shown in Fig. S5 as an example. The mean crystalline sizes of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 were 16 nm, 31 nm and 23 nm, respectively. The mean crystalline sizes obtained from the Williamson–Hall method were slightly higher than those obtained from the Scherrer method. This is attributed to the fact that the Scherrer method assumes that peak broadening arises solely from finite crystal size, whereas the William–Hall method decouples the additional peak broadening contributions from crystal defects and lattice strain.

The lattice strains calculated for g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 were 0.000642, 0.000873 and 0.00112, respectively. The lattice strain of the NiFe2O4/g-C3N4 composite was higher than those obtained from pristine g-C3N4 and NiFe2O4, suggesting that the composite has a high density of defect sites.

3.1.3. Raman spectroscopy

Raman spectroscopy was performed to further investigate the interaction between g-C3N4 and NiFe2O4. The Raman spectra of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 are displayed in Fig. 3. g-C3N4 displays the anticipated peaks at 1280 cm−1 and 1600 cm−1 attributed to the D-band (sp3) and G-band (sp2), respectively.33 The Raman spectrum of NiFe2O4 displays peaks corresponding to A1g, Eg and T2g mode. The peak for the A1g mode is due to the stretching of oxygen atoms in the tetrahedral sites of the Fe–O bonds. The peak for the Eg mode was attributed to the bending of oxygen with respect to the metal ion, while the peak for the T2g mode was assigned to the asymmetric oxygen bond bending. The spectrum of NiFe2O4/g-C3N4 displays peaks associated with g-C3N4 and NiFe2O4. However, the formation of the composite induced a shift in the D and G bands of g-C3N4 to higher frequencies. This is believed to have emanated from the strong interactions between g-C3N4 and NiFe2O4. The ID/IG ratios calculated for g-C3N4 and NiFe2O4/g-C3N4 were 0.27 and 0.96, respectively. An apparent increase in the ID/IG ratio in NiFe2O4/g-C3N4 was caused by defect sites in the NiFe2O4/g-C3N4composite compared to g-C3N4 due to the strong interaction between g-C3N4 and NiFe2O4.22

Fig. 3. Raman spectra of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4.

Fig. 3

Open in a new tab

3.1.4. Brunauer–Emmett–Teller surface analysis

The BET surface analysis was carried out to determine the surface areas, pore volumes and pore diameters of the g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts. The BET isotherms are shown in Fig. 4. All the catalysts displayed properties of a type IV material with a hysteresis loop. This suggests that the materials have mesopores.34 The surface areas of the g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts were 30.4 m2 g−1, 26.0 m2 g−1 and 37.4 m2 g−1, respectively. The results suggests that the NiFe2O4/g-C3N4 composite had the highest surface area, whereas the NiFe2O4 nanoparticles had the lowest surface area. The pore volumes for g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts were 0.347 cm3 g−1, 0.264 cm3 g−1 and 0.608 cm3 g−1, respectively. The pore volume of the NiFe2O4/g-C3N4 composite was higher than those of g-C3N4 and NiFe2O4. The BET surface areas and pore volumes imply that the porous structure of the catalysts is characterized by micropores. The pore widths were 29 nm, 21 nm and 27 nm for g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts, respectively. The pore width of the g NiFe2O4/g-C3N4 catalysts is lower than those of g-C3N4 and NiFe2O4. The NiFe2O4 are hosted within the g-C3N4 pores.

Fig. 4. BET isotherms of g-C3N4, NiFe2O4 and the NiFe2O4/g-C3N4 composite.

Fig. 4

Open in a new tab

3.1.5. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) elemental mapping

SEM was performed to study the morphology and topography of the catalysts, while EDX elemental mapping was performed to confirm the elemental composition and distribution of the catalysts. The SEM images of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 (Fig. 5A) show characteristics of a rough material with a poorly defined structure. The SEM image of g-C3N4 revealed a highly porous structure, which is ideal for the subsequent immobilization of active nanoparticles. The SEM image of NiFe2O4 shows a particular morphology characterized by agglomerated nanoparticles sounds perfect. The SEM image of NiFe2O4/g-C3N4 revealed that NiFe2O4 occupies the pores in g-C3N4, resulting in a less porous surface compared to that of g-C3N4. The SEM results further revealed that NiFe2O4 is homogeneously dispersed on g-C3N4 without significantly altering the g-C3N4 structure.

Fig. 5. SEM images (A) EDX electron microscope images (B) and the corresponding EDX elemental mappings (C) of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4.

Fig. 5

Open in a new tab

The EDX electron images of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 are shown in Fig. 5B, and their corresponding elemental mapping images are shown in Fig. 5C. The EDS elemental mapping of g-C3N4 shows the presence of carbon (C) and nitrogen (N) as anticipated. The EDS elemental mapping of NiFe2O4 reveals the presence of nickel (Ni), iron (Fe) and oxygen (O) elements, which are homogeneously distributed. As expected, the EDS elemental mapping of NiFe2O4/g-C3N4 shows the presence of C, N, Ni, Fe and O. Hence, the presence of all the elements was confirmed using EDS elemental mapping analysis.

3.1.6. Atomic force microscopy (AFM)

AFM was utilised to study the surface morphology and roughness of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4. The topographical data provide information on texture, roughness and structural changes, which are very important in photocatalysis. The two-dimensional and three-dimensional AFM topographic images of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 are shown in Fig. 6. The 2D topographic image of g-C3N4 shows a smooth surface without visible particles, as shown in Fig. 6A. The 3D topographic image also shows a smooth surface with low peak-to-valley contrast. The root mean square (RMS) value was found to be 2.75 nm, as shown in Fig. 6A. This is evidence of a higher-order conjugated microporous polymer material. The AFM 2D topographic image of NiFe2O4 shows a relatively rough surface with visible particles, as shown in Fig. 6B. The 3D topographic image shows a rough surface with high peak-to-valley contrast, as shown in Fig. 6B. The obtained RMS was 12.3 nm, which is higher than that obtained in g-C3N4. The 2D topographic image of NiFe2O4/g-C3N4 displayed a rough surface with visible particles (Fig. 6C). The 3D structure also displayed a rough surface with high peak-to-valley contrast. The obtained RMS was 7.8 nm, which is higher than that obtained for pristine g-C3N4, as shown in Fig. 6C. This suggests that the fabrication of the composite resulted in an increase in surface roughness. An enhanced surface roughness of the nanomaterials in photocatalysis significantly increases the available active surface area, improves the light-harvesting ability through multiple scattering, and accelerates the turbulent mass transfer of pollutants.35

Fig. 6. Two-dimensional and three-dimensional topographic images of the g-C3N4 (A), NiFe2O4 (B) and NiFe2O4/g-C3N4 (C) catalysts.

Fig. 6

Open in a new tab

3.1.7. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns

TEM was used to study the morphologies of the g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts. The TEM images of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts are shown in Fig. 7A. The TEM images of NiFe2O4 and g-C3N4 display two-dimensional structures. The TEM image of g-C3N4 displays the properties of highly porous, elongated 2D nanosheets. The TEM image of the NiFe2O4/g-C3N4 composite shows that 2D NiFe2O4 nanoparticles are homogeneously distributed on the surface of 2D g-C3N4 to form NiFe2O4/g-C3N4.

Fig. 7. TEM images (A), HRTEM images (B) and SAED patterns (C) of g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4.

Fig. 7

Open in a new tab

HRTEM was used to study the atomic arrangements of the g-C3N4, NiFe2O4 and NiFe2O4/g-C3N4 catalysts. The HRTEM images of NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts are shown in Fig. 7B. The HRTEM image of NiFe2O4 shows lattice fringes with d-spacings of 0.246 nm and 0.209 nm, which correspond to the (311) and (400) Miller indices, respectively.36 The HRTEM image of g-C3N4 did not show any d-spacing, which suggests that the material is amorphous. This is in correlation with the XRD results, which showed a broad diffraction peak corresponding to the (002) Miller index. The HRTEM image of NiFe2O4/g-C3N4 shows lattice fringes with a d-spacing of 0.209 nm for the (400) Miller index of NiFe2O4. Another lattice fringe with d-spacing of 0.294 nm was observed, which corresponds to the (220) lattice plane of NiFe2O4 .37 From the HRTEM image of NiFe2O4/g-C3N4, the interface between NiFe2O4 and g-C3N4 was perceived.

The selected area electron diffraction (SAED) patterns for NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts are shown in Fig. 7C. The pattern of NiFe2O4 shows diffraction rings corresponding to (111), (220), (311), (400), (422) and (440) planes.32 The pattern of g-C3N4 shows only one ring corresponding to the (002) Miller index. The SAED pattern of NiFe2O4/g-C3N4 shows diffraction rings corresponding to (111), (220), (311), (400), (422) and (440) planes of NiFe2O4 and an additional diffraction ring corresponding to the (002) Miller index of g-C3N4. This confirms that both NiFe2O4 and g-C3N4 are present in the NiFe2O4/g-C3N4 composite.

4. Degradation studies

4.1. Spectral changes

Prior to irradiation, the mixture was kept in the dark for 30 min while monitoring adsorption using UV-vis spectroscopy at 15 min intervals. The UV-vis spectra are shown in Fig. S3. The results suggest that the adsorption–desorption equilibrium was achieved after 45 min. The degradation of methylene blue was evaluated by assessing the UV-vis spectral changes at 665 nm.38 The aliquots were collected and analysed at 15 min intervals for 90 min using UV-vis spectroscopy. When methylene blue degradation was performed in the presence of PMS alone, light alone and PMS combined with light, no spectral changes were observed. These results suggest that no methylene blue degradation occurred under these conditions. When degradation studies were performed in the presence of light combined with catalysts (photocatalytic process), PMS combined with catalysts (catalytic PMS activation), and PMS combined with light and catalysts (photocatalytic PMS activation), methylene blue spectral changes were recorded, which suggested that degradation occurred. The colour fading of the methylene blue solution can be observed. The UV-vis spectral changes of methylene blue recorded at 15 min intervals during degradation using NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 in the presence of PMS and sunlight are shown in Fig. 8a–c. The images of the methylene blue solutions (aliquots) collected from the reaction mixture prior to degradation and after 90 min of degradation in the presence of sunlight and PMS using NiFe2O4/g-C3N4 catalyst are displayed in Fig. 8d. The FT-IR results obtained before and after methylene blue degradation are shown in Fig. S4. The spectra of the reaction mixture after 60 min of degradation and after 90 min of degradation reveal the presence of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O peaks, which originate form intermediate degrdation product. Ultimately,CO2 is the final product of methylene blue degradation.

Fig. 8. Methylene blue spectral changes observed during methylene blue degradation using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts in the presence of PMS (a)–(c), and images of the methylene blue solutions (aliquots) collected from the reaction mixture prior to degradation and at various time intervals (d).

Fig. 8

Open in a new tab

4.2. Catalytic performance

The results for methylene blue degradation using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts in the presence of PMS (PMS + catalyst) are shown in Fig. 9a and Table 1. When methylene blue degradation studies were performed in the presence of PMS using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts, degradation efficiencies of 44%, 19% and 74%, respectively, were obtained.

Fig. 9. Methylene blue degradation efficiencies (a) and kinetics (b) in the presence of PMS + catalyst (PMS activation), light + catalyst (photocatalysis) and PMS + light + catalyst (photocatalytic PMS activation) using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts.

Fig. 9

Open in a new tab

Table 1. Catalytic degradation efficiencies and reaction kinetics for methylene blue degradation using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalystsa.

Catalyst Degradation efficiency (%) k obs (min−1) τ 1/2 (min)
PMS + cat Light + cat PMS + light + cat PMS + cat Light + cat PMS + light + cat PMS + cat Light + cat PMS + light + cat
NiFe2O4 44 32 69 0.016 0.012 0.025 43 57 28
g-C3N4 19 28 53 0.005 0.008 0.019 138 87 37
NiFe2O4/g-C3N4 74 63 97 0.028 0.021 0.034 25 33 20
Open in a new tab
a

Cat = catalyst.

NiFe2O4 is more catalytically active than g-C3N4. This is because NiFe2O4 has transition metals that easily undergo redox reactions due to their multiple oxidation states. In contrast, g-C3N4 requires light to activate PMS.21 The NiFe2O4/g-C3N4 composite showed higher catalytic activity than both NiFe2O4 and g-C3N4. This is due to the synergic effect between NiFe2O4 and g-C3N4, which promotes the activation of PMS.

When degradation of methylene blue was performed using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts in the presence of sunlight (light + catalyst), degradation efficiencies of 32%, 28% and 63%, respectively, were obtained, as shown in Fig. 9a and Table 1. This suggests that NiFe2O4 is more active than g-C3N4 in the photocatalytic degradation of methylene blue. Generally, g-C3N4 exhibits lower photocatalytic activity than NiFe2O4 due to the rapid recombination of the photogenerated electron–hole pairs. In contrast, NiFe2O4 has a more favourable band structure and superior charge separation compared to g-C3N4, resulting in a higher degradation efficiency.22 Notably, the NiFe2O4/g-C3N4 composite outperformed both NiFe2O4 and g-C3N4. This was attributed to the fact that the NiFe2O4/g-C3N4 composite exhibits improved light absorption and enhanced electron–hole separation due to the formation of a heterojunction at the interface between NiFe2O4 and g-C3N4.

The methylene blue degradation efficiencies obtained from the PMS activation process using the NiFe2O4 and NiFe2O4/g-C3N4 catalysts were higher than those obtained in the photocatalytic process using similar catalysts. This is because of the presence of NiFe2O4, which can act as a redox catalyst during PMS activation. However, when g-C3N4 was used, the photocatalytic process was more effective for the degradation of methylene blue than the PMS activation process. This is because g-C3N4 is a poor redox catalyst, although it can absorb sunlight to activate PMS.39,40 When degradation of methylene blue was performed using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts in the presence of sunlight and PMS (PMS + light + catalyst), degradation efficiencies of 69%, 53% and 97%, respectively, were obtained, as shown in Fig. 9a and Table 1. Similarly, NiFe2O4 was more catalytically active than g-C3N4. This is because NiFe2O4 can activate PMS through the redox pathway and light absorption pathway, in which NiFe2O4 is first excited by sunlight, followed by the activation of PMS by the photo-excited electrons. In contrast, g-C3N4 does not easily undergo a redox reaction to activate PMS; instead, it is first excited by sunlight, followed by the activation of PMS by photo-excited electrons.41 The kinetics results for the PMS activation process (Fig. 9b and Table 1) using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts show that NiFe2O4/g-C3N4 had a higher kobs value than those of the NiFe2O4 and g-C3N4 catalysts. This was expected since NiFe2O4/g-C3N4 was more catalytically active than NiFe2O4 and g-C3N4 catalysts. The half-life (t1/2) was shorter for NiFe2O4/g-C3N4 than those observed for the NiFe2O4 and g-C3N4 catalysts during methylene blue degradation. The kinetics results for the photocatalytic process (Fig. 9b and Table 1) using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts show that NiFe2O4/g-C3N4 had a higher kobs value than those of NiFe2O4 and g-C3N4 catalysts, and the half-life was shorter for the NiFe2O4/g-C3N4 catalyst. The kobs values obtained from the PMS activation process using NiFe2O4 and NiFe2O4/g-C3N4 were higher than those obtained from the photocatalytic process using similar catalysts. This is because NiFe2O4 and NiFe2O4/g-C3N4 exhibited higher methylene blue degradation efficiencies in the PMS activation process than in the photocatalytic process. However, the kobs value obtained using the g-C3N4 catalyst was higher and the half-life was lower in the photocatalytic process than in the PMS activation process. This was anticipated since the photocatalytic process was more active than the PMS activation process in the presence of g-C3N4. Moreover, the use of a 0D/2D hybrid compared to a 0D/0D hybrid creates a large contact area, speeds up electron movement, stops charge loss, and captures more light, thereby increasing efficiency.42

The kinetics results for the photocatalytic PMS activation process (Fig. 9b and Table 1) using the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts revealed that NiFe2O4/g-C3N4 had a higher kobs value than those of the NiFe2O4 and g-C3N4 catalysts, and the half-life was shorter for the NiFe2O4/g-C3N4 catalyst. The kobs obtained from the photocatalytic PMS activation process were higher than those obtained from the photocatalytic and PMS activation processes, and t1/2 was shorter for all the catalysts. This was expected since the photocatalytic PMS activation process outperformed the photocatalytic and PMS activation processes.

5. Effect of various parameters on the degradation of methylene blue

5.1. Effect of the initial concentration of methylene blue

The degradation efficiency of organic pollutants is also affected by the initial concentration of the organic pollutants.43 The effect of the methylene blue concentration was studied using 80 mg L−1, 120 mg L−1 and 160 mg L−1 in the presence of NiFe2O4/g-C3N4 and PMS under sunlight irradiation. The results were then compared with those obtained with 40 mg L−1 methylene blue concentration. The methylene blue degradation results are displayed in Fig. 10a. The methylene blue degradation efficiencies achieved using 40 mg L−1, 80 mg L−1, 120 mg L−1 and 160 mg L−1 were 97%, 86%, 81% and 78%, respectively. The results revealed that increasing the methylene blue concentration resulted in a decrease in degradation efficiency. However, the efficiency was still higher even with 160 mg L−1 concentration of methylene blue.

Fig. 10. Degradation of methylene blue at different initial concentrations (a), with various PMS concentrations (b) and at various pH values (c).

Fig. 10

Open in a new tab

5.2. Effect of the PMS concentration

The effect of the PMS concentration on the degradation efficiency of methylene blue (40 mg L−1) was studied using NiFe2O4/g-C3N4 under sunlight irradiation. The PMS (1 mL) concentrations of 0.2 M, 0.4 M and 1.6 M were used, and the results were compared with those obtained from 0.1 M under similar conditions. The methylene blue degradation results are shown in Fig. 10b. The methylene blue degradation efficiencies obtained using 0.1 M, 0.2 M, 0.4 M and 1.6 M were 97%, 99%, 91% and 74%, respectively. The results suggest that an increase in the concentration of PMS from 0.1 to 0.2 M demonstrated an increase in methylene blue degradation efficiency from 97% to 99%, respectively. However, a further increase in the PMS concentration to 0.4 M and 1.6 M resulted in a decrease in degradation efficiency. This is caused by the excess PMS quenching the free radicals to form the weaker oxidizing SO5− free radicals, as shown in eqn (4) and (5).44

HSO5− + SO4˙− → SO5− + SO42− + H+ 4
HSO5− + ˙OH → SO5− + H2O 5

5.3. Effect of pH

The effect of the pH of the solution on the degradation efficiency of methylene blue (40 mg L−1) was studied using NiFe2O4/g-C3N4 and 0.2 M of PMS under sunlight irradiation. The studies were performed at pH 3, 5, 7, and 11. The plots are shown in Fig. 10c. The degradation efficiency obtained at pH 3, 5, 7, 9 and 11 were 59%, 64%, 83%, 100% and 100%, respectively. The complete degradation efficiency was obtained at pH 9 and 11. Under alkaline solutions, an excess of hydroxide ions (OH−) promotes the production of radical species. In addition, the hydroxide anions increase the negetive surface charge on the catalyst, thereby facilitating the adsorption of the positively charged methylene blue dye.45

6. Reactive oxygen species (ROS) scavenging studies

In order to identify the radicals responsible for the degradation of methylene blue, radical quenching experiments were performed using NiFe2O4/g-C3N4 in the presence of PMS and sunlight. Methanol (MeOH) was used to quench ˙OH and SO4˙− since MeOH has an affinity for both ˙OH and SO4˙−. Tert-butyl alcohol (TBA) was utilized to quench ˙OH. 4-Benzoquinone (BQ) was utilized for O2˙− quenching. The methylene blue degradation results in the absence and presence of radical scavengers are compared in Fig. S7 and Table S3. MeOH can quench both ˙OH and SO4˙−, while TBA has a high affinity for ˙OH and less affinity for SO4˙−.46 When MeOH, TBA and BQ were used as quenchers, methylene blue degradation efficiencies were 64%, 73% and 88%, respectively. MeOH is a highly hydrophilic scavenger; hence, it does not effectively adsorb onto the surface of the catalyst. Hence, potassium iodide (KI) was utilized to quench SO4˙−. KI is hydrophobic; thus, it can easily adsorb on the catalyst's surface and capture SO4˙−. When KI was utilized as a radical scavenger, a methylene blue degradation efficiency of 46% was obtained. From the obtained results, it can be postulated that SO4˙− was the dominant degradation species responsible for the degradation of methylene blue.

7. Catalyst stability test

The catalyst stability was examined by evaluating the degradation efficiency after 5 cycles. The catalyst was separated from the reaction mixture using a magnet, dried in an oven at 80 °C and re-used in the photocatalytic degradation of methylene blue (40 mg L−1) at pH 9 using the NiFe2O4/g-C3N4 composite. The plot showing the degradation efficiencies obtained after five cycles is shown in Fig. S4. The degradation efficiency obtained after five cycles was 79%. This suggests that the catalyst exhibited high stability even after five cycles, indicating its suitability for industrial applications.

8. Reaction mechanism

The proposed mechanism was predicted following a series of analysis. Firstly, the UV-vis absorption behaviour of NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts were compared, as shown in Fig. S2A. The catalysts displayed absorption below 500 nm. The NiFe2O4/g-C3N4 composite displayed enhanced absorption compared to the NiFe2O4 and g-C3N4 analogues, with an apparent red shift. This suggests that more visible light can be harvested by the NiFe2O4/g-C3N4 composite. The band gap energies (Eg) of the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts were determined using Tauc plots, as shown in Fig. S2B. The band gap energies of the NiFe2O4, g-C3N4 and NiFe2O4/g-C3N4 catalysts were 1.77 eV, 2.70 eV, and 1.45 eV, respectively. The NiFe2O4/g-C3N4 composite has a lower band gap energy than those of NiFe2O4 and g-C3N4. This accounts for its enhanced light-harvesting character compared to pristine NiFe2O4 and g-C3N4, as observed in the UV-vis spectrum.

Valence band-X-ray photoelectron spectroscopy (VB-XPS) was utilized to determine the valence bands (VBs) of NiFe2O4 and g-C3N4. The VB edges of NiFe2O4 and g-C3N4 were obtained to be 2.16 eV and 1.63 eV, respectively. The conduction band edges (ECB) of NiFe2O4 and g-C3N4 were obtained using eqn (6).

ECB = EVB − Eg, 6

where EVB is the valence band potential, ECB is the conduction band potential and Eg is the band gap energy. The ECB values of NiFe2O4 and g-C3N4 were 0.39 V and −1.07 eV, respectively. Based on the positions of the VBs and CBs of NiFe2O4 and g-C3N4, two possible types of heterojunctions can be proposed for the separation of the electron–hole pairs. These heterojunctions operate via (i) a type-II charge transfer mechanism and (ii) a Z-scheme heterojunction mechanism.

Under light illumination, as shown in Fig. 11 and eqn (7), the electrons in the VBs of NiFe2O4 and g-C3N4 become excited to the CBs of NiFe2O4 and g-C3N4, respectively. In the type-II heterojunction, the electrons in the CB of g-C3N4 are transferred to the CB of NiFe2O4 since the ECB of NiFe2O4 is more positive than that of g-C3N4. The holes in the VB of NiFe2O4 are transferred to the VB of g-C3N4 since the EVB of NiFe2O4 is more positive than that of g-C3N4. The reduction reaction occurs in the CB of NiFe2O4 (to produce superoxide radicals, E° O2/˙O2− = −0.33 eV vs. NHE), while the oxidation reaction occurs in the VB of g-C3N4. However, the E° for the production of ˙OH is +1.99 eV against NHE; hence, ˙OH is not anticipated to participate in the degradation of methylene blue in the type-II heterojunction.47,48

Fig. 11. Reaction mechanism for photocatlytic activation of PMS (A), and redox assisted activation of PMS during degradation of methylene blue using the NiFe2O4/g-C3N4 heterojunction.

Fig. 11

Open in a new tab

However, the ROS quenching studies revealed that ˙OH participated in methylene blue degradation. For this reason, the type-II heterojunction is disregarded, and the Z-scheme heterojunction is proposed, as shown in Fig. 11. The Z-scheme heterojunction suggests that upon irradiation with light and excitation, the electrons in the CB of NiFe2O4 combine with holes in the VB of g-C3N4. The electrons accumulated in the CB of g-C3N4 initiate PMS activation and O2 reduction to generate SO4˙− and O2˙−, respectively, as shown in Fig. 11, eqn (8) and (9). The holes in the VB of NiFe2O4 initiate the oxidation reaction to generate ˙OH, as shown in Fig. 11 and eqn (10). The separation of the photogenerated electron–hole pairs results in enhanced degradation of methylene blue.

NiFe2O4/g-C3N4 + visible light → NiFe2O4 (e−/h+) 7
HSO5− + e− → SO4˙− + −OH 8
O2 + e− → O2˙− 9
H2O + h+ → ˙OH + H+ 10

Based on ROS scavenging studies, optical analyses and literature reports, the mechanism for PMS activation in the degradation of methylene blue using NiFe2O4/g-C3N4 was also proposed.9,49–51 Firstly, SO4˙− is produced through redox activation of PMS by Ni2+ and Fe2+, as shown in eqn (11) and(12), respectively. Ni2+ and Fe2+ are reduced by PMS in the reactions that produce SO5˙−, as shown in eqn (13) and (14), respectively. At the surface of the NiFe2O4/g-C3N4 catalyst, PMS decomposes into SO52− and H+, as shown in eqn (15). In the presence of H2O, SO52− produces O2˙−, SO42− and H+, as shown in eqn (16). In another process, PMS is activated by light to generate ˙OH and SO4˙− species, as shown in eqn (17). Finally, the ROS (˙OH, O2˙− and SO4˙−) participate in the degradation of methylene blue, eqn (18).

Fe(ii) + HSO5− → Fe(iii) + SO4˙− + −OH, 11
Ni(ii) + HSO5− → Ni(iii) + SO4˙− + −OH, 12
Fe(iii) + HSO5− → Fe(ii) + SO5˙− + H+, 13
Ni(iii) + HSO5− → Ni(ii) + SO5˙− + H+, 14
HSO5− → H+ + SO52−, 15
SO52− + H2O → O2˙− + SO42− + H+, 16
HSO5− + light → ˙OH + SO4˙−, 17
Organic pollutant + O2˙− + ˙OH + SO4˙− → degradation products. 18

9. Conclusion

A NiFe2O4/g-C3N4 Z-scheme heterojunction was successfully synthesized and used for the first time for the activation of PMS to degrade methylene blue in wastewater. The composite showed high photocatalytic activity with a degradation efficiency of 97% after 90 min. SO4˙− was the dominant radical species responsible for the degradation of methylene blue. The results showed that NiFe2O4/g-C3N4 is a promising catalyst for the removal of organic pollutants from wastewater. We strongly believe that the obtained results may lead to new avenues for designing highly efficient PMS-based photocatalytic technologies for the removal of organic pollutants in wastewater.

Author contributions

Siphumelele Thandokwazi Mkhondwane: writing – original draft, investigation, conceptualization, methodology, data curation. Yogendra Kumar: methodology, validation. Vinod: methodology, visualisation. Viswanadha Srirama Rajasekhar Pullabhotla: supervision, resources, funding acquisition.

Conflicts of interest

The authors declare no conflicts of interest.

Supplementary Material

RA-OLF-D6RA06402G-s001

Acknowledgments

The authors acknowledge Dr Siphamandla Masikane for high-resolution electron microscopy analysis and the HRTEM unit at the University of Zululand. Siphumelele Thandokwazi Mkhondwane acknowledges the NRF for financial support in the form of the Research Infrastructure Professionals Programme (RIPP). Rajasekhar Pullabhotla would like to acknowledge the National Research Foundation (NRF, South Africa) for the financial support in the form of the Incentive Fund Grant (grant no. 132468) and the Research Developmental Grant for Rated Researchers (grant no. 112145).

Data availability

Data will be available on request.

Supplementary information (SI): plots for Williamson–Hall, UV-vis from adsorption studies, band gaps and valence band edge. See DOI: https://doi.org/10.1039/d6ra06402g.

References

  1. Li J. Li Y. Xiong Z. Yao G. Lai B. The electrochemical advanced oxidation processes coupling of oxidants for organic pollutants degradation: A mini-review. Chin. Chem. Lett. 2019;30:2139–2146. doi: 10.1016/j.cclet.2019.04.057. [DOI] [Google Scholar]
  2. Wang J. Wang S. Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants. Chem. Eng. J. 2018;334:1502–1517. doi: 10.1016/j.cej.2017.11.059. [DOI] [Google Scholar]
  3. Qin L. Chen W. J. Fu Y. K. Tang J. Yi H. Li L. Xu F. H. Zhang M. M. Cao W. C. Huang D. L. Lai C. Hemin derived iron and nitrogen-doped carbon as a novel heterogeneous electro-Fenton catalyst to efficiently degrade ciprofloxacin. Chem. Eng. J. 2022;449:137840. doi: 10.1016/j.cej.2022.137840. [DOI] [Google Scholar]
  4. Shen P. Yang T. S. Li Q. C. Chen Z. Wang Y. L. Fu Y. L. Wan J. Wu Z. X. Wan L. Hollow-structured amorphous Cu(OH)(x) nanowires doped with Ru for wide pH electrocatalytic hydrogen production. J. Colloid Interface Sci. 2022;628:1061–1069. doi: 10.1016/j.jcis.2022.08.128. [DOI] [PubMed] [Google Scholar]
  5. Manivel A. Lee G. J. Chen C. Y. Chen J. H. Ma S. H. Horng T. L. Wu J. J. Synthesis of MoO3 nanoparticles for azo dye degradation by catalytic ozonation. Mater. Res. Bull. 2015;62:184–191. doi: 10.1016/j.materresbull.2014.11.016. [DOI] [Google Scholar]
  6. Mkhondwane S. T. Matshitse R. Nyokong T. porphyrin-graphitic carbon nitride quantum dots decorated on titanium dioxide eletrospun nanofibers for photocatlytic degradation of organic pollutants. J. Coord. Chem. 2022;75:2150–2169. doi: 10.1080/00958972.2022.2132153. [DOI] [Google Scholar]
  7. Mkhondwane S. T. Sebiawu G. Mgidlana S. Openda Y. Nwahara N. Mack J. Nyokong T. Photosono activation of peroxymonosulfate using A3B phthalocyanines supported on titanium dioxide nanofibers for degradation of Rhodamine 6G. Synth. Met. 2024;307:117699. doi: 10.1016/j.synthmet.2024.117699. [DOI] [Google Scholar]
  8. Wang S. Xu L. Wang J. Iron-based dual active site mediated peroxymonosulfate for degradation of emerging organic pollutants. Environ. Sci. Technol. 2021;55:15412–15422. doi: 10.1021/acs.est.1c06205. [DOI] [PubMed] [Google Scholar]
  9. Lin Z. F. Tsai C. K. Lai Y. J. S. Nguyen T. B. Venkatesan P. Doong R. A. S-scheme heterojunction of N-carbon dots/g-C3N4/MIL-101(Fe) photocatalytic membrane for peroxymonosulfate-activated photodegradation of enrofloxacin. J. Chem. Eng. 2025;522:167477. doi: 10.1016/j.cej.2025.167477. [DOI] [Google Scholar]
  10. Cheng X. Li P. Liu W. Luo C. Wu D. Zhou W. Zheng L. Zhu X. Liang H. Activation of peroxymonosulfate by metal oxide nanoparticles for mitigating organic membrane fouling in surface water treatment. Sep. Purif. Technol. 2020;246:116935. doi: 10.1016/j.seppur.2020.116935. [DOI] [Google Scholar]
  11. Mkhondwane S. T. Kumar Y. Vinod Pullabhotla V. S. R. R. Photocatalytic activation of peroxymonosulphate using spinel MnFe2O4, NiFe2O4 and CoFe2O4 catalysts for degradation of organic pollutants in wastewater. Catal. Today. 2026;467:115695. doi: 10.1016/j.cattod.2026.115695. [DOI] [Google Scholar]
  12. Dikdim J. M. D. Gong Y. Noumi G. B. Sieliechi J. M. Zhao X. Ma N. Yang M. Tchatchueng J. B. Peroxymonosulfate improved photocatalytic degradation of atrazine by activated carbon/graphitic carbon nitride composite under visible light irradiation. Chemosphere. 2019;217:833–842. doi: 10.1016/j.chemosphere.2018.10.177. [DOI] [PubMed] [Google Scholar]
  13. Zhang B. T. Zhang Y. Teng Y. Electrospun magnetic cobalt–carbon nanofiber composites with axis-sheath structure for efficient peroxymonosulfate activation. Appl. Surf. Sci. 2018;452:443–450. doi: 10.1016/j.apsusc.2018.05.065. [DOI] [Google Scholar]
  14. Humayun M. Ullah H. Cao J. Pi W. Yuan Y. Ali S. Tahir A. A. Yue P. Khan A. Zheng Z. Fu Q. Luo W. Experimental and DFT Studies of Au Deposition Over WO3/g-C3N4 Z-Scheme Heterojunction. Nano-Micro Lett. 2019;12:7. doi: 10.1007/s40820-019-0345-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Ahmed M. A. Mahmoud S. A. Mohamed A. A. Unveiling the photocatalytic potential of graphitic carbon nitride (g-C3N4): a state-of-the-art review. RSC Adv. 2024;14:25629–25662. doi: 10.1039/D4RA04234D. [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Li T. Ge L. Peng X. Wang W. Zhang W. Enhanced degradation of sulfamethoxazole by a novel Fenton-like system with significantly reduced consumption of H2O2 activated by g-C3N4/MgO composite. Water Res. 2021;190:116777. doi: 10.1016/j.watres.2020.116777. [DOI] [PubMed] [Google Scholar]
  17. Mkhondwane S. T. Mgidlana S. Nyokong T. Asymmetric phthalocyanine-graphitic carbon nitride nanosheets conjugate on zinc oxide fibers for combined ultrasound and visible light driven degradation of Rhodamine 6G. J. Photochem. Photobiol., A. 2024;447:115245. doi: 10.1016/j.jphotochem.2023.115245. [DOI] [Google Scholar]
  18. Chen Y. Liu X. Hou L. Guo X. Fu R. Sun J. Construction of covalent bonding oxygen-doped carbon nitride/graphitic carbon nitride Z-scheme heterojunction for enhanced visible-light-driven H2 evolution. J. Chem. Eng. 2020;383:123132. doi: 10.1016/j.cej.2019.123132. [DOI] [Google Scholar]
  19. Du Y. Ma W. Liu P. Zou B. Ma J. Magnetic CoFe2O4 nanoparticles supported on titanate nanotubes (CoFe2O4/TNTs) as a novel heterogeneous catalyst for peroxymonosulfate activation and degradation of organic pollutants. J. Hazard. Mater. 2016;308:58–66. doi: 10.1016/j.jhazmat.2016.01.035. [DOI] [PubMed] [Google Scholar]
  20. Han Y. Y. Lu X. L. Tang S. H. Yin X. P. Wei Z. W. Lu T. B. Metal-Free 2D/2D Heterojunction of Graphitic Carbon Nitride/Graphdiyne for Improving the Hole Mobility of Graphitic Carbon Nitride. Adv. Energy Mater. 2018;8:1702992. doi: 10.1002/aenm.201702992. [DOI] [Google Scholar]
  21. Silva R. R. M. Ruotolo L. A. M. Nogueira F. G. E. Peroxymonosulfate activation by magnetic NiFe2O4/g-C3N4 for tetracycline hydrochloride degradation under visible light. J. Chem. Eng. 2023;476:146621. doi: 10.1016/j.cej.2023.146621. [DOI] [Google Scholar]
  22. Palanivel B. Ayappan C. Jayaraman V. Chidambaram S. Maheswaran R. Mani A. Inverse spinel NiFe2O4 deposited g-C3N4 nanosheet for enhanced visible light photocatalytic activity. Mater. Sci. Semicond. Process. 2019;100:87–97. doi: 10.1016/j.mssp.2019.04.040. [DOI] [Google Scholar]
  23. Shakoor M. H. Shakoor M. B. Jilani A. Ahmed T. Rizwan M. Dustgeer M. R. Iqbal J. Zahid M. Yong J. W. H. Enhancing the Photocatalytic Degradation of Methylene Blue with Graphene Oxide-Encapsulated g-C3N4/ZnO Ternary Composites. ACS Omega. 2024;9:16187–16195. doi: 10.1021/acsomega.3c10172. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Biswas B. Ahmed M. F. Rahman M. L. Khanam J. Bhuiyan M. H. R. Sharmin N. Investigation of structural, optical, and magnetic properties of NiFe2O4 for efficient photocatalytic degradation of organic pollutants through photo fenton reaction. Heliyon. 2024;10:37199. doi: 10.1016/j.heliyon.2024.e37199. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Mgidlana S. Nyokong T. Asymmetrical zinc(II) phthalocyanines cobalt tungstate nanomaterial conjugates for photodegradation of methylene blue. J. Photochem. Photobiol., A. 2021;418:113421. doi: 10.1016/j.jphotochem.2021.113421. [DOI] [Google Scholar]
  26. Kumar Y. Sharma S. Mishra K. B. Singh M. V. Pullabhotla V. S. R. R. Pramanik P. Nickel ferrite-based graphite-paste electrochemical sensor for detection of paracetamol and dopamine molecules. J. Indian Chem. Soc. 2025;102:101633. doi: 10.1016/j.jics.2025.101633. [DOI] [Google Scholar]
  27. Lee D. E. Khan A. Husain A. Anwar K. Lee H. C. Shin S. H. Lee J. Y. Danish M. Jo W. K. Synergistic performance of Z-scheme based g-CN/Zn0.5Cd0.5S/GO nanocomposite towards sustainable model organic pollutants detoxification: Mechanistic insights and practical implications. J. Alloys Compd. 2025;1022:179820. doi: 10.1016/j.jallcom.2025.179820. [DOI] [Google Scholar]
  28. Gupta S. Kumar R. Enhanced photocatalytic performance of the N-rGO/g-C3N4 nanocomposite for efficient solar-driven water remediation. Nanoscale. 2024;16:6109–6131. doi: 10.1039/D3NR06203A. [DOI] [PubMed] [Google Scholar]
  29. Masry M. M. E. Ramadan R. The effect of CoFe2O4, CuFe2O4 and Cu/CoFe2O4 nanoparticles on the optical properties and piezoelectric response of the PVDF polymer. Appl. Phys. A. 2022;110:128. [Google Scholar]
  30. Mkhondwane S. T. Nyokong T. The use of phthalocyanines@g-C3N4 nanosheets conjugates supported on TiO2 fibers for photosono-catalytic degradation of rhodamine 6G. Diamond Relat. Mater. 2025;153:112067. doi: 10.1016/j.diamond.2025.112067. [DOI] [Google Scholar]
  31. Tian N. Madani Z. Giannakis S. Isari A. A. Arjmand M. Hasanvandian F. Noorisepehr M. Kakavandi B. Peroxymonosulfate assisted pesticide breakdown: Unveiling the potential of a novel S-scheme ZnO@CoFe2O4 photo-catalyst, anchored on activated carbon. Environ. Pollut. 2023;334:122059. doi: 10.1016/j.envpol.2023.122059. [DOI] [PubMed] [Google Scholar]
  32. Han Z. Wang N. Fan H. Ai S. Ag nanoparticles loaded on porous graphitic carbon nitride with enhanced photocatalytic activity for degradation of phenol. Solid State Sci. 2017;65:110–111. doi: 10.1016/j.solidstatesciences.2017.01.010. [DOI] [Google Scholar]
  33. Giegold V. Lange L. Ciesielski R. Hartschuh A. Non-linear Raman scattering intensities in graphene. Nanoscale. 2020;12:5612–5617. doi: 10.1039/C9NR10654E. [DOI] [PubMed] [Google Scholar]
  34. Zhang Z. B. Wang C. C. Zakaria R. Ying J. Y. Role of particle size in nano-crystalline TiO2-based photo-catalysts. J. Phys. Chem. B. 1998;102:10871. doi: 10.1021/jp982948+. [DOI] [Google Scholar]
  35. Hosseini Z. S. Haghparast F. Masoudi A. A. Mortezaali A. Enhanced visible photocatalytic performance of un-doped TiO2 nanoparticles thin films through modifying the substrate surface roughness. Mater. Chem. Phys. 2022;279:125530. doi: 10.1016/j.matchemphys.2021.125530. [DOI] [Google Scholar]
  36. Kumar N. Ansari M. R. Khaladkar S. Maurya O. Peta K. R. Kalekar A. Singha M. K. Dash J. K. NiFe2O4 nanoparticles as highly efficient catalyst for oxygen reduction reaction and energy storage in supercapacitor. Mater. Chem. Phys. 2024;316:129072. doi: 10.1016/j.matchemphys.2024.129072. [DOI] [Google Scholar]
  37. Lai X. Qin J. Wang C. CoFe2O4/carbon nitride Z-scheme heterojunction photocatalytic PMS activation for efficient tetracycline degradation: Accelerated electron transfer. Process Saf. Environ. Prot. 2024;191:2522–2532. doi: 10.1016/j.psep.2024.09.120. [DOI] [Google Scholar]
  38. Sindelo A. Nyokong T. Photoinactivation of microorganisms and photodegradation of pollutants using phthalocyanines supported on nanofibers and glass wool. J. Photochem. Photobiol., A. 2024;447:115236. doi: 10.1016/j.jphotochem.2023.115236. [DOI] [Google Scholar]
  39. Wang S. Liu Y. Wang J. Peroxymonosulfate Activation by Fe–Co–O-Codoped Graphite Carbon Nitride for Degradation of Sulfamethoxazole. Environ. Sci. Technol. 2020;54:10361–10369. doi: 10.1021/acs.est.0c03256. [DOI] [PubMed] [Google Scholar]
  40. Xiang S. Lin Y. Chang T. Mei B. Liang Y. Wang Z. Sun W. Cai C. Oxygen doped graphite carbon nitride as efficient metal-free catalyst for peroxymonosulfate activation: Performance, mechanism and theoretical calculation. Chemosphere. 2023;338:139539. doi: 10.1016/j.chemosphere.2023.139539. [DOI] [PubMed] [Google Scholar]
  41. Wang Y. Yan X. Sun Z. Liu J. Wang Y. Hu C. Deng Y. Xie M. Xie J. Zhang W. Xu Y. Synergistic catalysis of oxygen vacancy and S-scheme heterojunction in NiFe2O4–x/NiS regulates peroxymonosulfate activation forenhanced photo-Fenton-like reaction. Chin. J. Catal. 2025;79:186–204. doi: 10.1016/S1872-2067(25)64841-8. [DOI] [Google Scholar]
  42. Kavitha R. Manjunatha C. Yu J. Girish Kumar S. Rational design and interfacial engineering of hierarchical S-scheme heterojunction and their photocatalytic applications. EnergyChem. 2025;7:100159. doi: 10.1016/j.enchem.2025.100159. [DOI] [Google Scholar]
  43. Mkhondwane S. T. Mgidlana S. Openda Y. I. Nene L. Nyokong T. Photosono catalytic behaviour of phthalocyanine when supported on electrospun nanofibers: The effect of radical initiators. Synth. Met. 2023;299:117484. doi: 10.1016/j.synthmet.2023.117484. [DOI] [Google Scholar]
  44. Li M. Ding D. Lin K. A. Kwon E. Cobalt-based coordination polymers as heterogeneous catalysts for activating Oxone to degrade organic contaminants in water: a comparative study. Sep. Purif. Technol. 2020;236:116245. doi: 10.1016/j.seppur.2019.116245. [DOI] [Google Scholar]
  45. Saint U. K. Baral S. C. Sasmal D. Maneesha P. Datta S. Naushin F. Sen S. Effect of pH on photocatalytic degradation of methylene blue in water by facile hydrothermally grown TiO2 nanoparticles under natural sunlight. JCIS Open. 2025;19:100150. doi: 10.1016/j.jciso.2025.100150. [DOI] [Google Scholar]
  46. Zhang R. Wan Y. Peng J. Yao G. Zhang Y. Lai B. Efficient degradation of atrazine by LaCoO3/Al2O3 catalysed peroxymonosulfate: Performance, degradation intermediates and mechanism. Chem. Eng. J. 2019;372:796–808. doi: 10.1016/j.cej.2019.04.188. [DOI] [Google Scholar]
  47. Mishra P. Behera A. Kandi D. Parida K. Facile construction of a novel NiFe2O4@P-doped g-C3N4 nanocomposite with enhanced visible-light-driven photocatalytic activity. Nanoscale Adv. 2019;1:1864–1867. doi: 10.1039/C9NA00018F. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Muhammad W. Ali W. Khan M. A. Ali F. Zada A. Ansar M. Z. Yap P. S. Construction of visible-light-driven 2D/2D NiFe2O4/g-C3N4 Z-scheme heterojunction photocatalyst for effective degradation of organic pollutants and CO2 reduction. J. Environ. Chem. Eng. 2024;12:113409. doi: 10.1016/j.jece.2024.113409. [DOI] [Google Scholar]
  49. Kohantorabi M. Moussavi G. Giannakis S. A review of the innovations in metal- and carbon-based catalysts explored for heterogeneous peroxymonosulfate (PMS) activation, with focus on radical vs. non-radical degradation pathways of organic contaminants. J. Chem. Eng. 2021;411:127957. doi: 10.1016/j.cej.2020.127957. [DOI] [Google Scholar]
  50. Zhao W. Shen Q. Nan T. Zhou M. Xia Y. Hu G. Zheng Q. Wu Y. Bian T. Wei T. Zhang C. Cobalt-based catalysts for heterogeneous peroxymonosulfate (PMS) activation in degradation of organic contaminants: Recent advances and perspectives. J. Alloys Compd. 2023;958:170370. doi: 10.1016/j.jallcom.2023.170370. [DOI] [Google Scholar]
  51. Wang Z. Du Y. Liu Y. Zou B. Xiao J. Ma J. Degradation of organic pollutants by NiFe2O4/peroxymonosulfate: efficiency, influential factors and catalytic mechanism. RSC Adv. 2016;6:11040–11048. doi: 10.1039/C5RA21117D. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA06402G-s001
RA-OLF-D6RA06402G-s001.pdf (233KB, pdf)

Data Availability Statement

Data will be available on request.

Supplementary information (SI): plots for Williamson–Hall, UV-vis from adsorption studies, band gaps and valence band edge. See DOI: https://doi.org/10.1039/d6ra06402g.

Articles from RSC Advances are provided here courtesy of Royal Society of Chemistry

RESOURCES