Synergistic Catalysis: Sodium Perborate-Mediated Advanced Oxidation Processes Coupled with MIL-88B(Fe) for Photocatalytic Degradation of Methylene Blue

DongjieHe1Email1902099799@qq.com

XinLi1Email2191820040@qq.com

NuoXu1Email2405079443@qq.com

YimengGu1Email3409124377@qq.com

JiaboWang1Emailwangjb794@nenu.edu.cn

PengHe1Emailhepeng@jlict.edu.cn

QunLiu1✉Emailqunliu@jlict.edu.cn

YuZhang8,9✉Emailzhang99yu@jlict.edu.cn

1Dongjie HeJilin University of Chemical TechnologyJilin CityChina

2Xin LiJilin University of Chemical TechnologyJilin CityChina

3Nuo XuJilin University of Chemical TechnologyJilin CityChina

4Yimeng GuJilin University of Chemical TechnologyJilin CityChina

5Jiabo WangJilin University of Chemical TechnologyJilin CityChina

6Peng HeJilin University of Chemical TechnologyJilin CityChina

7Qun LiuJilin University of Chemical TechnologyJilin CityChina

8Jilin University of Chemical TechnologyJilin CityChina

9School of Petrochemical TechnologyJilin University of Chemical Technology132022JilinChina

Dongjie He, Xin Li, Nuo Xu, Yimeng Gu, Jiabo Wang, Peng He, Qun Liu*, Yu Zhang*

Author information:

1. Dongjie He, Jilin University of Chemical Technology, Jilin City, China, 1902099799@qq.com

2. Xin Li, Jilin University of Chemical Technology, Jilin City, China, 2191820040@qq.com

3. Nuo Xu, Jilin University of Chemical Technology, Jilin City, China, 2405079443@qq.com

4. Yimeng Gu, Jilin University of Chemical Technology, Jilin City, China, 3409124377@qq.com

5. Jiabo Wang, Jilin University of Chemical Technology, Jilin City, China, wangjb794@nenu.edu.cn

6. Peng He, Jilin University of Chemical Technology, Jilin City, China, hepeng@jlict.edu.cn

7. Qun Liu, Jilin University of Chemical Technology, Jilin City, China, qunliu@jlict.edu.cn

8. Yu Zhang, Jilin University of Chemical Technology, Jilin City, China, zhang99yu@jlict.edu.cn

Corresponding author:

1. Qun Liu, qunliu@jlict.edu.cn

2. Yu Zhang, zhang99yu@jlict.edu.cn

School of Petrochemical Technology, Jilin University of Chemical Technology, Jilin 132022, China;

Corresponding author E-mail addresses: qunliu@jlict.edu.cn (Q. Liu), zhang99yu@jlict.edu.cn (Y. Zhang)

Abstract

Addressing the growing concerns of water pollution, requires the development of efficient advanced oxidation processes (AOPs). This study investigates a novel AOPs system that integrates the metal-organic framework MIL-88B(Fe) with sodium perborate (SPB) under visible light for the degradation of methylene blue (MB). The choice of SPB over traditional oxidants is motivated by its solid stability, ease of transport, and environmental friendliness. The coupling of MIL-88B(Fe) with SPB creates a remarkable synergy: MIL-88B(Fe) acts as an activator for SPB, while the presence of SPB inhibiting the recombination of electrons and holes and facilitates the crucial photo-reduction of Fe(III) to Fe(II) in MIL-88B(Fe), establishing a sustainable catalytic cycle. This synergistic effect resulted in a dramatic enhancement in MB degradation efficiency, achieving 95% within 30 minutes, higher than that of the MIL-88B(Fe)/vis system, exhibited enhancement of degradation efficiency (SI = 6.07). Radical quenching experiments and EPR measurements identified ·OH and ·O₂⁻ as the primary reactive oxygen species. This MIL-88B(Fe)/SPB/vis system showcases a promising, sustainable strategy for wastewater treatment by effectively combining a stable heterogeneous catalyst with a green oxidant.

Keywords:

Fe-MOFs

Visible light irradiation

Perborate

Synergistic effect

Oxidative species role

1. Introduction

Due to the industry's recent rapid development, persistent organic pollutants have become widely present in wastewater from chemical, dyeing, pharmaceutical, and paper industries^{1, 2}. The discharge of heavily contaminated effluents into aquatic environments has emerged as a critical environmental issue, adversely impacting aquatic ecosystems and posing risks to human and animal health^{3, 4}. Various wastewater treatment approaches have been investigated, including physical⁵, physicochemical⁶, chemical⁷, and biological treatments⁸; however, limitations such as poor adsorbent regeneration, elevated ion-exchange costs, secondary pollutant formation, and low ecological efficiency constrain their widespread implementation⁹. Fujishima and Honda made a groundbreaking discovery in 1972¹⁰, photocatalysis has garnered considerable academic interest owing to its energy efficiency and environmental compatibility. However, conventional semiconductor photocatalysts such as TiO₂¹¹, ZnO¹², and CdS¹³, suffer from aqueous corrosion and potential dissolution^{14, 15}. Limiting their practical applications. Consequently, creating stable and effective photocatalytic materials remains a critical research priority.

Metal-organic frameworks (MOFs) are crystalline porous materials composed of metalions or clusters linked by organic linker¹⁶. Compared to traditional organic and inorganic porous materials, MOFs exhibit a high surface areas, tunable pore structures, uniformly dispersed active sites, and multifunctionality¹⁷, rendering them promising candidated for gas adsorption^{18, 19}, molecular separation²⁰, and photocatalysis^{21, 22}. Iron-based metal-organic framework materials (Fe-MOFs) have recently garnered significant owing to their environmental sustainability, cost-effectiveness, facile magnetic separation, and the high absorption of visible light by iron oxide clusters²³. Still, their photocatalytic properties are hindered through swift recombination of photo-excited electron-hole pairs²⁴. Various strategies, including elemental doping^{25, 26} and surface modification²⁷ have been employed to suppress charge recombination and enhance photocatalytic performance²⁸. The combining of photocatalysis with advanced oxidation processes represents a promising approach to improves pollutant degradation efficiency, while maintaining cost-effectiveness, and minimizing secondary contamination^{23, 29–31}.

Advanced oxidation processes (HR-AOPs) were first introduced for drinking water treatment in the 1980s. Characterized by hydroxyl radicals (·OH) generation, have proven successful in water and wastewater remediation^{32, 33}. These hydroxyl radicals exhibit exceptional reactivity, enabling non-selective oxidation of organic contaminants into smaller inorganic compounds or biodegradable intermediates, thereby achieving pollutant removal^{32, 34–36}. Although, combining AOPs with electricity or thermal inputs enhances performance, such approaches often incur substantial energy costs and potential secondary contamination. Light-driven AOPs present an attractive alternative, harnessing renewable energy with minimal environmental impact^{23, 37}. Introducing oxidants like hydrogen peroxide (H₂O₂) as electron acceptors suppresses charge carrier recombination in photocatalytic systems while generating abundant reactive radical species for contaminant degradation^{29, 31, 38, 39}. In 2013, Ai and colleagues demonstrated that MIL-53(Fe) could activate 20 mM H₂O₂ under illumination to completely degrade 10 mg·L⁻¹ Rhodamine B (RhB), achieving 100% degradation efficiency within 50 minutes³⁹. Subsequently, in 2020, Sablas et al. employed an Fe²⁺-MOF to activate 60 mM sodium percarbonate (SPC), achieving complete mineralization of 1 mM imidacloprid after 120 minutes of irradiation⁴⁰. Despite these advances, conventional oxidants such as H₂O₂ face challenges including elevated storage and transportation costs, as well as susceptibility to catalytic decomposition by metal-based catalysts, compromising stability in photocatalytic applications⁴¹. Additionally, high concentrations of sulfate ions (SO₄²⁻) can contribute to wastewater acidification and may be converted into harmful byproducts in SPC-based advanced oxidation processes (AOPs), potentially causing secondary pollution⁴². Therefore, to further optimize and enhance the overall efficiency of photocatalytic-AOP systems in pollutant degradation, identifying more suitable oxidants remains a critical research focus.

This article is the first to use the MIL-88B (Fe) coupled with sodium perborate (SPB, NaH₄B₂O₈) system for photo assisted degradation of methylene blue (MB) and reveals its unique synergistic catalytic mechanism. SPB offers distinct advantages including environmental compatibility, high stability, shock resistance, facile storage and transportation, and cost-effectiveness, making it widely utilized in disinfection, cleaning, and bleaching applications^{31, 43}. Upon dissolution, SPB functions as a stable solid H₂O₂ source⁴⁴, enabling sustained release of reactive oxygen species (ROS)⁴⁵. This investigation characterizes the physicochemical properties of MIL-88B(Fe), elucidates the degradation mechanism within the MIL-88B(Fe)/SPB/vis light system, analyzes the influence of key operational parameters on MB degradation efficiency, and evaluates catalyst stability and reusability. Additionally, electron paramagnetic resonance (EPR) spectroscopy was employed to elucidate the reactive species generated during the catalytic reaction, and revealing a synergistic catalytic mechanism between MIL-88B(Fe) and SPB under light irradiation.

2. Experimental

2.1 Material

Supporting Information Text S1 has comprehensive details about chemicals and reagents. Every reagent is analytically pure and utilized straight away, requiring no additional purification. Deionized water is used to prepare all of the solutions.

2.2 Preparation of MIL-88B(Fe)

We prepared MIL-88B(Fe) via solvothermal synthesis, as depicted in Scheme 1. The Supporting Material (Text S2) contained information.

Scheme 1

Diagrammatic representation of MIL-88B(Fe) synthesis.

2.3 Analytical techniques

Patterns exhibiting X-ray diffraction were acquired using the Empyrean Razor diffractometer in the Netherlands, covering the 5° to 55° 2θ range. Infrared spectra were recorded with the Alpha Centaurt FT-IR spectrophotometer, employing a wavenumber range from 4000 to 500 cm⁻¹ for spectroscopic analysis. The BELSORP-mini II automatic volumetric analyzer was used to perform nitrogen adsorption-desorption tests. We vacuum-treated all samples for four hours at 393K before testing to degas them properly. We examined the surface morphology using a JSM-7610F Plus scanning electron microscope (Japan). Surface electronic properties were examined through X-ray photoelectron spectroscopy (XPS) using Al Kα radiation from Thermo Fisher Scientific in the United States. To assess the material's optical absorption characteristics, BaSO₄ was used to perform UV-visible diffuse reflectance spectroscopy (UV-vis DRS), serving as the reflectance benchmark. Photoluminescence (PL) spectra were captured with a fluorescence spectrophotometer (F-7000), utilizing the wavelength of excitation of 345 nm. Electron paramagnetic resonance (EPR) measurements utilized an X-band frequency-operating Bruker ELEXSYS E500 instrument under normal atmospheric conditions. Electrochemical evaluations—including electrochemical impedance spectroscopy (EIS), photocurrent measurements (I-t), Mott-Schottky analysis, and linear sweep voltammetry (LSV)—were performed within a typical three-electrode configuration connected to a CHI electrochemical workstation. To prepare the photoanode, the photocatalyst was mixed into a solution of Nafion at a level of 10 mg·mL^− 1 and subjected to ultrasonication for an hour to achieve a uniform dispersion. Next, 0.2 mL of this colloidal mixture was carefully drop-cast onto a glass substrate and left to dry organically at room temperature. The photoelectrochemical tests were conducted in an aqueous solution of sodium sulfate. In order to suppress ultraviolet radiation, a 300 W Xenon lamp with a 420 nm UV filter created the illusion of sunlight.

2.4 Photocatalytic procedure

Assessment breakdown of MB took place in a multireactor visible light setup (CEL-LAB200E7) with a rotating lamp column for consistent illumination. To guarantee the robustness of the outcomes, the tests were conducted three times under the same conditions. The figures display the average measurements, with error bars illustrating the standard deviation (SD) to reflect variability around the mean. In each experiment, the photocatalyst and sodium perborate oxidizer spread throughout 50 milliliters of MB aqueous solution (10 mg·L⁻¹) under constant magnetic stirring. At 30-minute intervals, 3 mL aliquots were withdrawn and immediately quenched with 3 mL methanol to terminate the reaction. We passed the samples through 0.2 µm syringe filters to prepare them for further analysis. The level of MB was determined by recording its distinctive absorbance at 664 nm with a UV-Vis spectrophotometer (UV-7000). To assess how effectively the organic compound broke down, the degradation efficiency was calculated using a specific formula⁴⁶:

η=(

$\:1-\frac{{C}_{t}}{{C}_{0}}$

) × 100%

the starting concentration of MB is denoted by C₀, the degradation efficiency by η, the residual concentration at reaction time t by C_t. In order to attain equilibrium between adsorption and desorption, the system was agitated for 90 minutes in the dark before each photocatalytic experiment. To shed light on the entire process of MB breakdown by catalysts, the experimental data was described using the pseudo-first-order kinetic model⁴⁷:

$\:\text{l}\text{n}({C}_{t}/{C}_{0})=-{k}_{obs}t$

the pseudo-first-order rate constant (min^− 1) is represented by kobs; C_t (mg·L^− 1), C₀ (mg·L^− 1), and t (min) are consistent with the previous text.

3. Results and discussion

3.1 Characteristic

MIL-88B (Fe) was characterized using XRD, SEM, FT-IR, XPS, BET, and UV-Vis DRS; carrier migration and free radical production in the solution were determined using PL, electrochemical tests (Eis, I-t, LSV), and EPR (Text S3).

Figure 1. Structural characterization of MIL-88B(Fe) XRD(a) and FT-IR(b).

Figure 2 shows the SEM pictures and energy-dispersive X-ray spectrum (EDS) elemental mapping of MIL-8B(Fe). The material exhibits well-crystallized spindle-shaped morphology containing particles that range in size from approximately 600 to 800 nm. These physical characteristics align with the MIL-88B(Fe) structure that has been characterized⁴⁸. EDS elemental analysis (Fig.S1a) reveals the presence of C, O, and Fe elements in within the framework structure. The matching elemental mapping shows a consistent distribution of metal centers and H₂BDC ligands throughout the crystalline matrix, indicating successful framework assembly.

Fig. 2

(a-c) SEM for MIL-88B(Fe) and mapping elemental analysis images.

It is confirmed by the spectrum from the XPS survey (Fig. 3a) that MIL-88B(Fe) is composed only of the components C, O, and Fe. XPS C 1s high-resolution spectrum (Fig. 3b), reveal three deconvoluted at 284.8, 286.4, and 287.7 eV binding energies, corresponding to O–C = O, C–C, and C–O–C bonds, in that order. The O 1s spectrum with great resolution (Fig. 3c) exhibits two distinct peaks at 531.4 and 533.0 eV, attributed to oxygen species within the organic linker and Fe–O coordination bonds, respectively. High-resolution Fe 2p spectra (Fig. 3d) display characteristic Fe 2p₃/₂ and Fe 2p₁/₂ peaks at 711.3 and 725.3 eV, respectively, consistent with Fe(III) oxidation states. A minor Fe 2p₃/₂ component at 715.0 eV indicates the presence of Fe(II) species. The predominance of Fe(III) is further corroborated by satellite features at 717.0 and 732.0 eV, characteristic of high-spin Fe(III) configurations.

Fig. 3

XPS spectra of MIL-88B(Fe): (a) Survey scan; (b) High-resolution C 1s spectrum; (c) High-resolution O 1s spectrum; (d) High-resolution Fe 2p spectrum.

The matching pore size distribution and nitrogen adsorption-desorption isotherms of MIL-88B(Fe) were analyzed to evaluate its porous structure and specific surface area (Fig. 4a and b). The isotherms display a characteristic Type IV pattern with an H3-type hysteresis loop, indicating the mesoporous nature of the material⁴⁹. Key physicochemical specifications of MIL-88B(Fe) are summarized in Table 1. Brunauer-Emmett-Teller (BET) analysis revealed a specific surface area of 15.5 m²/g with an average pore diameter of 13.34 nm. High specific surface area is a critical characteristic that enables photocatalysts to exhibit superior adsorption capacity. The comprehensive characterization data collectively confirm the successful synthesis of the MIL-88B(Fe) photocatalyst.

Fig. 4

(a) N₂ adsorption-desorption curves and (b) mesoporous size distribution of MIL-88B(Fe).

Table 1
The N₂ adsorption-desorption data.
Materials	Specific surface aera (m²·g^− 1, BET)	Pore diameter(nm)	Pore volume (cm³·g^− 1, STP)
MIL-88B(Fe)	15.50	13.34	0.052

The features of MIL-88B(Fe) optical absorption using ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS). H₂BDC and MIL-88B(Fe) exhibited absorption edges at 354 nm and 470 nm respectively (Fig. 5a). These results indicate that while H₂BDC visible light response, while MIL-88B(Fe) exhibits excellent visible-light absorption. The band gap energy, a crucial optical parameter for evaluating photoinduced electron-hole pair generation and charge transfer in semiconductor photocatalysts⁵⁰, was determined for MIL-88B(Fe). The optical band gap was calculated using the Tauc plot method^{51, 52}. The band gap energy of MIL-88B(Fe) was estimated to be 2.76 eV. This suitable band gap position suggests that MIL-88B(Fe) possesses excellent photocatalytic potential under visible light irradiation. The valence band (VB) of MIL-88B(Fe) is primarily composed of C 2p and O 2p orbitals, while Fe 3d orbitals dominate the conduction band (CB)⁴⁸. When exposed to visible light, the iron centers in MIL-88B(Fe) generate electron-hole (e⁻-h⁺) pairs, which function as active sites for MB degradation (Eq. (1)).

$\:\text{M}\text{I}\text{L}-88\text{B}\left(\text{F}\text{e}\right)\:\:+\:\:\text{h}\text{v}\:\:\to\:\:{\text{e}}^{-}\:\:+\:{\text{h}}^{+}$

Fig. 5

(a) UV-Vis diffuse reflectance spectra (DRS) and (b) the corresponding Tauc plot for band gap (Eg) determination of MIL-88B(Fe).

3.2 Comparison of various treatment processes

After preliminary experiments, the degradation of methylene blue (MB) was compared across six water treatment processes under identical operational parameters. These processes included: no catalyst (None), adsorption (MIL-88B(Fe)), oxidation with sodium perborate (SPB/vis), photoexcited MIL-88B(Fe) (MIL-88B(Fe)/vis), catalysis with MIL-88B(Fe) in the presence of SPB (MIL-88B(Fe)/SPB), and photoexcited MIL-88B(Fe) activated SPB (MIL-88B(Fe)/SPB/vis). Figure 6a illustrates the temporal evolution of normalized MB concentration (C_t/C₀) under various experimental conditions. MB exhibited remarkable stability under exposure to visible light for 120 min in the absence of both photocatalyst and oxidant, with negligible decolorization observed. The SPB/vis system achieved only 18% MB removal, indicating that SPB neither directly decolorizes MB nor undergoes activated by visible light alone. MIL-88B(Fe) alone adsorbed 37.27% of MB in 120 min, primarily through adsorption⁴⁹. Under visible light exposure without SPB, MIL-88B(Fe) demonstrated moderate photocatalytic activity with 50% MB degradation. This activity likely originates from reactive charge carriers generated upon visible light excitation, although the relatively modest efficiency suggests rapid photogenerated electron-hole pair recombination. To investigate potential SPB activation by iron-based catalysts, we performed additional experiments with both SPB and MIL-88B(Fe) in dark conditions, which resulted in 64.32% MB removal. Notably, the MIL-88B(Fe)/SPB/vis system achieved 99.8% decolorization of MB within 120 min, demonstrating exceptional photocatalytic performance through synergistic SPB activation. A pseudo-first-order model suited the MB photodegradation kinetic curves quite well (Fig. 6b). The catalytic degradation of MB (Fig. 6c) exhibited a more pronounced kinetic enhancement for MIL-88B(Fe) when both SPB and visible light are present (i.e., the MIL-88B(Fe)/SPB/vis system), MIL-88B(Fe)/SPB/vis is k_obs = 5.18×10^− 2 min^− 1, MIL-88B(Fe)/SPB gets 0.455×10^− 2 min^− 1, and MIL-88B(Fe)/vis has 0.398×10^− 2 min^− 1. Furthermore, the MIL-88B(Fe)/SPB/vis system had a higher MB degradation rate (5.18×10⁻² min⁻¹) than the sum of MIL-88B(Fe)/vis and MIL-88B(Fe)/SPB systems (0.853×10⁻² min⁻¹). This confirms their synergy among the MIL-88B(Fe) catalyst and SPB when exposure to visible light significantly enhanced the overall catalytic activity for MB degradation^{39, 53}. To quantitatively evaluate the synergistic effect, we introduced a synergy index (SI) was introduced⁴⁷:

$\:\text{S}\text{I}=\frac{{k}_{\text{M}\text{H}\text{V}}}{{k}_{\text{H}V}+{k}_{\text{M}V}}$

the apparent rate constants k_MVH, k_MV, and k_MH correspond to the MIL-88B(Fe)/SPB/vis, MIL-88B(Fe)/vis, and MIL-88B(Fe)/SPB systems in Fig. 6c, respectively, with the MB degradation rates for each system also shown in the figure. The calculated SI value of 6.07 for the MIL-88B(Fe)/SPB/vis system relative to the MIL-88B(Fe)/vis and MIL-88B(Fe)/SPB systems demonstrates a substantial synergistic effect, enhancing the catalytic activity by 507% compared to the sum of the individual MIL-88B(Fe)/vis and MIL-88B(Fe)/SPB systems³⁹.

Fig. 6

(a) MB concentration (C_t/C₀) changes with reaction time under various catalytic methods. (b) MB deterioration pseudo-first-order kinetic curves under various circumstances. (c) Evaluation of apparent reaction rate constants and MB degradation rates under various circumstances. Conditions of reaction: MB (50 mL, 10 mg·L^− 1); MIL-88B(Fe) (30 mg); SPB (1.95 mM); initial pH 8.1.

3.3 Kinetic and the synergistic effect study

To elucidate the crucial role of SPB oxidant in enhancing the photodegradation efficiency of MIL-88B(Fe), we employed photoluminescence (PL) spectroscopy to analyze charge carrier entrapment, migration, and transfer dynamics. Figure 7a displays the PL spectra of photocatalyst suspensions in ethanol with and without SPB oxidant. Under 345 nm excitation, MIL-88B(Fe) exhibited a broad emission band (400–600 nm) with maximum intensity at 470 nm, corresponding to an energy of 2.8 eV - consistent with the optical band gap (2.76 eV) determined from UV-Vis DRS measurements (Fig. 5). The strong PL emission originates from electron-hole recombination, where higher intensity indicates faster recombination rates. Notably, the PL intensity decreased substantially upon SPB addition, demonstrating effective suppression of charge recombination through SPB's electron-accepting capability. Electrochemical impedance spectroscopy (EIS) measurements (Fig. 7b) revealed reduced charge transfer resistance for MIL-88B(Fe) with SPB, as evidenced by the smaller arc radius in Nyquist plots²³. This finding confirms improved charge carrier mobility and more efficient electron-hole separation at the catalyst/electrolyte interface in the present of SPB. Transient photocurrent response (I-t) tests (Fig. 7c) showed that SPB incorporation substantially decreased the photocurrent density of MIL-88B(Fe) under visible light, consistent with PL results and confirming SPB's role as an effective electron scavenger. Linear sweep voltammetry (LSV) measurements were conducted to investigate the charge transfer characteristics (Fig. 7d). MIL-88B(Fe) exhibited marginally higher current density under visible light illumination compared to dark conditions, indicating that photoexcitation facilitates electron transfer. More remarkably, the presence of SPB led to substantially enhanced current densities regardless of illumination conditions. This observation demonstrates that: (i) SPB incorporation significantly promotes charge transfer processes. (ii) Visible light excitation synergistically accelerates charge transfer over the interface between MIL-88B(Fe) and SPB, and (iii) The combined system exhibits cooperative enhancement effects.

Fig. 7

(a) PL spectra, (b) EIS Nyquist plots, (c) I-t curves of MIL-88B(Fe) systems with/without SPB and (d) LSV profiles of MIL-88B(Fe)/SPB with/without light.

3.4 Mechanistic Investigation

To elucidate the reaction mechanism of MIL-88B(Fe) in activating SPB under exposure to visible light for enhanced MB degradation, we systematically examined the band MIL-88B(Fe) structure and redox characteristics. The charge carrier behavior was evaluated through Mott-Schottky analysis. The Mott-Schottky plot's positive slope is displayed in Fig. 9a which measured at 800, 1000, and 1200 Hz in dark conditions confirms the n-type semiconductor characteristics of MIL-88B(Fe). It was found that the flat band potential (E_FB) was − 0.35 V vs NHE, with the conduction band minimum (E_CB, equivalent to LUMO level) positioned at -0.45 V vs NHE (E_CB = E_FB − 0.1 eV). Notably, this LUMO potential is less than the redox potential of O₂/O₂⁻ (-0.33V vs. NHE), thermodynamically enabling generation of superoxide radicals (·O₂⁻) through electron transfer (Eq. (2)).

$\:{\text{O}}_{2}+\:\text{M}\text{I}\text{L}-88\text{B}\left(\text{F}\text{e}\right)\left({\text{e}}^{-}\right)\:\to\:\:·{\text{O}}_{2}^{-}\:+\:\text{M}\text{I}\text{L}-88\text{B}\left(\text{F}\text{e}\right)\:\:\:$

Complementary UV-Vis DRS analysis (Fig. 5b) revealed that the valence band maximum (E_VB, equivalent to HOMO level) of MIL-88B(Fe) lies at + 2.31 V vs NHE, calculated using the equation Eg = E_VB - E_CB⁵⁴. This potential is less positive than the redox potential of OH^-/·OH (+ 2.38V vs. NHE), indicates that MIL-88B(Fe) cannot directly oxidize OH^- to ·OH radicals. The derived band structure is schematically illustrated in Fig. 9b. Remarkably, in the MIL-88B(Fe)/vis light system for MB degradation, the introduced SPB undergoes hydrolysis in aqueous solution, releasing H₂O₂ (Eq. (3)). H₂O₂ captures electrons produced by photolysis from MIL-88B(Fe), rapidly producing ·OH radicals ((Eq. (4))³⁹. Furthermore, the valence band potential of MIL-88B(Fe) exceeds the oxidation potential of MB (+ 0.532V vs. NHE)⁴⁹, confirming the feasibility of direct hole-mediated MB degradation. These conclusions are well supported by the EPR-·OH spectra presented in (Fig. 9c), photoexcited MIL-88B(Fe) produces minimal ·OH without SPB, but effectively activates SPB under illumination to generate abundant ·OH, demonstrating markedly enhanced activity compared to dark conditions, demonstrating that visible-light-excited MIL-88B(Fe) effectively activates SPB and accelerates MB degradation through radical-mediated pathways.

$\:\text{N}\text{a}{\text{H}}_{4}{\text{B}}_{2}{\text{O}}_{8}+\:4{\text{H}}_{2}\text{O}\:\to\:\:2\text{N}\text{a}\text{B}{\text{O}}_{2}+\:2{\text{H}}_{2}{\text{O}}_{2}$

$\:{\text{H}}_{2}{\text{O}}_{2}+\text{M}\text{I}\text{L}-88\text{B}\left(\text{F}\text{e}\right)\left(\:{\text{e}}^{-}\:\right)\to\:\:\bullet\:\text{O}\text{H}+\text{O}{\text{H}}^{-}$

Fig. 8

(a) Mott-Schottky curve of MIL-88B(Fe), (b) estimated energy level diagram of MIL-88B(Fe), and (c) EPR spectra of DMPO-·OH in different systems.

Therefore, to further distinguish for the contribution of each radical, trapping experiments radical and Electron spin resonance (EPR) analysis were carried out (Fig. 9). According to previous reports⁴⁸, ·OH can be effectively scavenged by tert-butanol (TBA), while benzoquinone (BQ) predominantly quenches ·O₂^⁻ with minor reactivity toward ·OH. Additionally, capture ¹O₂ using (sodium azide) NaN₃, disodium ethylenediaminetetraacetate (EDTA-2Na) was used to trap holes (h⁺). As illustrated in Fig. 9a, the inhibition effects of all scavengers were relatively weak during the dark reaction (1.5 h). However, upon light irradiation, the suppression became progressively more pronounced.Notably, the addition of TBA reduced the degradation efficiency from 99.8% to 53.7%, indicating that hydroxyl radicals (·OH) play a critical role in MB degradation. The reaction was also partially inhibited by BQ, with the degradation rate decreasing to 67.8%, suggesting a minor contribution of superoxide radicals (·O₂⁻). In contrast, the addition of NaN₃ and EDTA-2Na resulted in only mild inhibition, with degradation rates of 91.8% and 88.9%, respectively, implying that singlet oxygen (¹O₂) and holes (h⁺) have negligible effects on MB degradation. These findings were further corroborated by electron spin resonance (ESR) spectroscopy, which detected characteristic signals of DMPO-·O₂⁻ and TEMPO-¹O₂ adducts (Fig. 9b). Combined with the data in Fig. 9c, these results confirm that the MIL-88B(Fe)/SPB/vis system can generate four reactive species: ¹O₂, h⁺, ·O₂⁻, and ·OH.

Fig. 9

(a) The effect of various scavengers on theMB degradation by MIL-88B(Fe )/SPB/vis; molar ratio of scavenger vs.MB = 100:1. (b) EPR signals of the reaction solution with MIL-88B(Fe) and SPB in darkness and under light illumination in the presence of DMPO (·O₂⁻) and TEMPO (¹O₂) as the spin-trapping reagent. (c) Contribution of ·OH, ·O₂⁻ and ¹O₂ in the degradation of MB after 120 min of the treatment.

Scheme 2 illustrates the proposed catalytic reaction mechanism through which MB degrades in the MIL-88B(Fe)/SPB/vis system. The visible-light response and related photocatalytic activity of MIL-88B(Fe) are caused by the Fe–oxo clusters' ability to absorb incident photons when exposed to visible light; These reactive oxygen species (·OH and ·O₂⁻) are generated during photocatalysis and are essential to the process of degradation:

(i)

Under exposure to visible light, MIL-88B(Fe) generates electrons (e⁻) and holes (h⁺) (Eq. (1)). SPB produces H₂O₂ in solution (Eq. (3)). Photogenerated electrons then react with H₂O₂ to form ·OH (Eq. (4)). Simultaneously, the photoexcited electrons can react with O₂ to generate ·O₂⁻ (Eq. (2)).

(ii)

Visible light excites the iron-oxo clusters in MIL-88B(Fe), forming an excited state (MIL-88B*(Fe))³⁹. Photoinduced electron transfer from the organic linkers to the iron-oxo clusters (analogous to VB-to-CB excitation) leads to the formation of Fe(II)⁵⁵. These electrons in the conduction band (CB) can further H₂O₂, producing ·OH. Generally, in photocatalytic processes, two main mechanisms exist for metal-organic frameworks (MOFs): (i) charge transfer from ligand to metal (LMCT)^56–58 and (ii) excitation of metal-oxo clusters^{59, 60}. In Fig. 5(a), terephthalic acid shows no visible light absorption, ruling out the LMCT mechanism. Thus, the photocatalytic activity of MIL-88B(Fe) is probably driven by direct excitation of Fe-O clusters, as observed in Fe(III)-based MOFs by Roeffares et al⁵⁹. H₂O₂ and surface Fe³⁺ combine to generate Fe²⁺ (Eqs. (5)-(6)), enhancing ·OH generation⁴⁸. XPS analysis (Fig. S3b) shows Fe 2p binding energies react with at 715.0 and 728.9 eV. This confirms Fe²⁺ formation increases after the reaction. ·O₂⁻ further converts to ¹O₂ via the Haber-Weiss reaction (Eq. (7))⁶¹, with h⁺, ·OH, and ·O₂⁻ degrade MB (Eq. (8)).

$\:{\text{F}\text{e}}^{3+}\:+\:{\text{H}}_{2}{\text{O}}_{2}\to\:\:{\text{F}\text{e}}^{2+}\:\:+\:{\text{H}}^{+}+\:·{\text{O}\text{H}}_{2}$

$\:{\text{F}\text{e}}^{2+}\:+\:{\text{H}}_{2}{\text{O}}_{2}\to\:\:{\text{F}\text{e}}^{3+}\:\:+\:0{\text{H}}^{-}\:+\:·\text{O}\text{H}$

$\:·{\text{O}}_{2}^{-}\:+\:\:\:·\text{O}\text{H}\to\:\:+\:\text{O}{\text{H}}^{-}\:+\:{}_{\:}{}^{1}\text{O}{\:}_{2}$

$\:\text{M}\text{B}\:+\:·\text{O}\text{H}/·{\text{O}}_{2}^{-}\:/{}_{\:}{}^{1}\text{O}{\:}_{2}/{\text{h}}^{+}\:\to\:\:\text{P}\text{r}\text{o}\text{d}\text{u}\text{c}\text{t}\text{s}$

Scheme 2

The reaction mechanism proposed in the MIL-88B(Fe)/SPB/vis reaction process.

3.5 Reusability and Stability of MIL-88B(Fe)

The reusability and structural stability of photocatalysts critically determine their practical application potential and operational lifetime. To systematically evaluate these characteristics, MIL-88B(Fe) was subjected to five consecutive MB degradation cycles under identical experimental conditions (Fig. 10a). The photocatalytic performance showed no significant deterioration throughout the testing period, demonstrating exceptional recyclability. Remarkably, the photocatalytic activity remained well-maintained within five cycles confirming outstanding structural integrity under prolonged photocatalytic conditions. To assess the practical application performance of the MIL-88B(Fe)/SPB/vis system in degradation by photocatalysis, numerous experiments on the degradation of pollutants were carried out (Fig. 10b). The results demonstrated that after 120 minutes of treatment, the degradation rates for methylene blue (MB), rhodamine B (RhB), tetracycline hydrochloride (TC), malachite green (MG), and Congo red (CR) reached 99.8%, 99.3%, 99.5%, 98.8%, and 95.4% respectively. These findings further confirm the broad-spectrum capability of the MIL-88B(Fe)/SPB/vis system for treating organic contaminants in wastewater. Meanwhile, the MIL-88B(Fe) synthesized exhibits comparable or even superior performance in degrading organic pollutants across different systems (Table S1). XRD, FT-IR and SEM analyses verified the structural and morphological integrity of MIL-88B(Fe), with no detectable phase transitions or functional group alterations observed after light irradiation (Fig. S2). The XPS survey spectra of MIL-88B(Fe) before and after photocatalysis (Fig. S3a) are identical, confirming that only C, O, and Fe are present with no detectable impurities. This demonstrates the structural and compositional stability of MIL-88B(Fe) during the process of photocatalysis, underscoring its robust stability.

Fig. 10

(a) Reusability of MIL-88B (Fe) activated SPB destruction of MB by photocatalysis in the presence of visible light LED light. (b) Experimental conditions: [MB] = [TC] = [RhB] = [MG] = [CR] = 10 mg·L⁻¹, MIL-88B(Fe) 30 mg, SPB 1.95mM, solution volume = 50 mL)

3.6 Parameter Influence

To comprehensively evaluate the practical application performance of MIL-88B(Fe) as a photocatalyst for activating SPB to degrade MB under visible light, this study conducted in-depth research and analysis. The effects of various factors, including the dosage of MIL-88B(Fe), the concentration of SPB oxidant, initial pH, and the existence of different inorganic ions, regarding the photocatalytic performance were systematically investigated. MB degradation improved from 70% to 99.8% as MIL-88B(Fe) dosage rose from 0.1 to 0.6 g·L⁻¹. Beyond 0.6 g·L⁻¹, efficiency declined (Fig. 11a). This phenomenon is explained by the fact that a greater amount of MIL-88B(Fe) generates more photogenerated holes and electrons when exposed to visible light. These photogenerated electrons are effectively captured by the oxidant, while the oxidative holes react with generated OH- to produce a large number of hydroxyl radicals (·OH) (Eqs. (1) and (4)). However, when the dosage of MIL-88B(Fe) was further increased to 0.8 g·L⁻¹, the degradation efficiency of MB exhibited a slight decline. This reduction may be due to particle aggregation in the suspension, which could scatter incident radiation rather than allowing it to be absorbed²³. As a result, the scattered radiation energy is no longer available to drive the photocatalytic process, thereby reducing the degradation efficiency. Meanwhile, as the oxidant in this system, SPB exhibits a constant electron-hole pair generation rate under photoexcitation for a given catalyst amount. Therefore, increasing the SPB concentration promotes the reaction between electron-hole pairs and SPB to generate ·OH until the oxidation process reaches a stable plateau. This indicates a direct correlation between the SPB concentration and the amount of ·OH radicals produced. As shown in Fig. 11b, when the SPB dosage was raised from 0.1 g·L⁻¹ to 0.3 g·L⁻¹, the degradation efficiency of MB improved because of the enhanced production of ·OH. However, when the SPB dosage exceeded 0.3 g·L⁻¹, no further increase in MB degradation was observed. This may be attributed to the alkaline nature of SPB in aqueous solutions (pH 10–11), where it is highly unstable and prone to releasing reactive oxygen species⁶². pH affects SPB stability. Higher alkalinity accelerates its decomposition. When the SPB concentration reached 0.5 g·L⁻¹ or higher, the solution pH rose beyond a certain threshold, destabilizing ·OH radicals and reducing the oxidative capacity of SPB. This triggered a scavenging effect on ·OH radicals (Eqs. (9) and (10))³⁹. Additionally, excessive H₂O₂ generated by SPB produced hydroperoxyl radicals (HOO·), which exhibit lower oxidative potential compared to ·OH:

$\:{\text{H}}_{2}{\text{O}}_{2}\:+\:·\text{O}\text{H}\:\to\:\:\:\text{H}\text{O}\text{O}·+\:{\text{H}}_{2}\text{O}\:$

$\:\text{H}\text{O}\text{O}·+·\text{O}\text{H}\to\:{\text{O}}_{2}\:+\:{\text{H}}_{2}\text{O}$

Hence, 0.3 g·L⁻¹ SPB is identified as the optimal concentration for effective oxidation in this process.

pH is another critical parameter in photocatalytic reactions, as it directly influences the surface charge and active sites of the photocatalyst, as well as the generation and reactivity of reactive oxygen species. Figure 11c illustrates the effect of initial pH on the degradation of MB by the MIL-88B(Fe)/SPB system under visible light irradiation. The results demonstrate that the system remains effective over a broad pH range (6–11), with pH 8.1 showing the quickest rate of degradation. MB, a strongly cationic dye, exhibits a pKa value of 1.3⁶³. When the pH exceeds 1.3, MB predominantly exists in its molecular form. Under acidic conditions, the adsorption of MB is relatively low due to the protonation of its negatively charged moieties by H⁺ ions, which neutralizes its positive charge and reduces its affinity for the catalyst surface⁶⁴. Consequently, the molecular form of MB is less susceptible to degradation. In contrast, alkaline conditions enhance the adsorption and subsequent oxidation of MB⁶⁵. As the pH increases, the adsorption capacity improves, leading to higher degradation efficiency. However, excessively alkaline conditions (pH > 11) can adversely affect the oxidative capacity of SPB, suppressing the generation of ·OH radicals and ultimately diminishing the degradation performance⁴⁴. This phenomenon may be attributed to the instability of SPB under strong alkaline conditions, where rapid decomposition reduces the availability of reactive oxygen species for the degradation of MB. These results emphasize how crucial it is to optimize pH to balance MB adsorption and oxidant activity, with pH 8.1 identified as the optimal condition for maximizing degradation efficiency in this system.

Extensive research has demonstrated that coexisting anions that are inorganic can significantly alter the degradation kinetics of organic pollutants in ·OH-mediated advanced oxidation processes, with reported effects showing considerable variability across different systems⁴⁸. In natural aquatic environments, NO₃⁻ represents one of the most prevalent anions, while elevated Cl⁻ concentrations are characteristic of industrial effluents from textile manufacturing, leather tanning operations, and waste leachates. To assess the robustness of the MIL-88B(Fe)/SPB/vis system under realistic conditions, we evaluated the impact of four representative inorganic anions (H₂PO₄⁻, SO₄²⁻, NO₃⁻, and Cl⁻) on the photocatalytic degradation of MB. The experimental results revealed distinct anion-dependent effects on degradation efficiency (Fig. 11d). The system tolerated Cl⁻, SO₄²⁻, and NO₃⁻ well. MB removal rates remained high: 88.9% (Cl⁻), 94.9% (SO₄²⁻), and 93% (NO₃⁻). This resilience suggests that these anions neither significantly interfere with the ·OH generation pathway nor strongly compete with MB for the catalyst surface's active spots. However, the degradation efficiency dropped significantly to 57.3% when H₂PO₄⁻ was present, indicating substantial inhibition of the photocatalytic process. This detrimental effect can be mechanistically explained by: (i) the formation of stable iron(III)-phosphate complexes that sequester active iron species from the catalytic cycle, and (ii) the strong surface affinity of phosphate anions, which leads to catalyst surface passivation and concomitant radical scavenging through reactions described in Eqs. (11) and (12)⁶⁶. These findings are particularly relevant for applications involving phosphate-rich water matrices, where pretreatment may be necessary to ensure optimal system performance.

$\:{\text{H}}_{2}\text{P}{\text{O}}_{4}^{-}\:+\:·\text{O}\text{H}\:\to\:\:{\text{H}}_{2}\text{P}{\text{O}}_{4}·+\:\text{O}{\text{H}}^{-}\:$

$\:{\text{H}}_{2}\text{P}{\text{O}}_{4}^{-}\:+\:{\text{h}}^{+}\:\to\:{\text{H}}_{2}\text{P}{\text{O}}_{4}$

Chloride ions (Cl⁻) compete with MB for ·OH radicals, generating a series of reactions that result in fewer reactive chlorine species (Cl∙, ClOH∙⁻) (Eqs. (13)-(17))^{67, 68}. These chlorine radicals exhibit lower redox potentials (2.0–2.4 V vs. NHE) compared to ·OH (2.8 V vs. NHE), thereby significantly suppressing MB degradation efficiency.

$\:\text{C}\text{l}·\:+\:·\text{O}{\text{H}}^{-}\:\to\:\:\text{C}\text{l}\text{O}\text{H}{·}^{-}$

$\:\text{C}\text{l}·\:+\:{\text{H}}_{2}0\:\to\:\text{C}\text{l}\text{O}\text{H}{·}^{-}\:+\:{\text{H}}^{+}$

$\:\text{C}\text{l}\text{O}\text{H}·-\to\:\text{C}{\text{l}}^{-}+·\text{O}\text{H}$

$\:\text{C}\text{l}·+\text{C}{\text{l}}^{-}\to\:\text{C}{\text{l}}_{2}{·}^{-}$

$\:\text{C}\text{l}\text{O}\text{H}{·}^{-}+\text{C}{\text{l}}^{-}\to\:\text{O}{\text{H}}^{-}+\text{C}{\text{l}}_{2}{·}^{-}$

NO₃⁻ competes for radicals, lowering MB degradation efficiency. Nitrate ions react with ·OH to form nitrate radicals (NO₃∙, Eq. (18))⁶⁹. However, unlike chloride-derived radicals, NO₃∙ maintains sufficient oxidative capacity to participate in MB degradation, resulting in a less pronounced inhibitory effect on overall removal efficiency.

$\:\text{N}{\text{O}}_{3}^{-}+·\text{O}\text{H}\to\:\text{N}{\text{O}}_{3}·+\text{O}{\text{H}}^{-}$

Notably, sulfate ions (SO₄²⁻) exhibit a unique dual-role behavior in the MB degradation system. Under dark conditions, SO₄²⁻ can surprisingly promote MB degradation by serving as an additional electron donor. SO₄²⁻ can react with ·OH to generate SO₄⁻∙, as shown in Eq. (19). The SO₄⁻∙ exhibits a greater redox potential (2.5–3.1 V vs. NHE) than hydroxyl radicals (·OH, 2.8 V vs. NHE)⁷⁰. More significantly, SO₄⁻∙ demonstrates superior kinetic advantages in MB degradation. However, under photocatalytic conditions, the presence of SO₄²⁻ demonstrates an inhibitory effect. This paradoxical behavior can be explained by three primary methods: (i) the potential formation of surface precipitates containing SO₄⁻∙ species that may block active sites and hinder light absorption; (ii) competitive consumption of photogenerated electrons/holes by hydroxide ions (OH⁻) reacting with SO₄⁻∙; and (iii) interference with the electron-hole separation process crucial for SPB activation^{71, 72}.

$\:·\text{O}\text{H}+\text{S}{\text{O}}_{4}^{2-}\to\:\text{S}{\text{O}}_{4}^{-}·+\text{O}{\text{H}}^{-}$

Without inorganic anions, the system degraded 99.8% MB. However, all tested anions reduced efficiency to varying degrees. This observation underscores the complex interplay between inorganic anions and photocatalytic processes, where the same anion can exhibit either promoting or inhibiting effects depending on the reaction conditions.

Fig. 11

Parameter optimization for MB degradation in the MIL-88B(Fe)/SPB/vis system:(a) Catalyst loading effect of MIL-88B(Fe) (5mg-40mg); (b) Oxidant concentration effect of SPB (0.1–0.9 g·L⁻¹); (c) pH-dependent performance (pH 6–11); (d) Anion interference studies (H₂PO₄⁻, Cl ⁻, NO₃⁻, and SO₄²⁻).

4. Conclusion

This study successfully synthesized MIL-88B (Fe) material with a spindle shaped porous structure, and comprehensively characterized it through various characterization methods such as XRD, FT-IR, SEM, BET, and UV-vis DRs. And MIL-88B (Fe) was used for efficient synergistic degradation of MB by light assisted activation of SPB. The MIL-88B (Fe)/SPB/vis combined system followed pseudo first order reaction kinetics, and the synergistic effect of MIL-88B (Fe) and SPB under light assistance was confirmed based on the apparent rate constant. The mechanism of the combined process was elucidated based on electrochemical analysis and direct electron transfer. Visible light excites MIL-88B (Fe) with semiconductor properties, generating electron hole pairs (e ^-/h⁺), which in turn activate SPB to generate active species mainly composed of strongly oxidizing hydroxyl radicals (·OH). The study also examined the effects of pH, types of pollutants, and coexisting anions in water. The key role of these active species in the efficient degradation of MB was confirmed through free radical capture experiments. Based on the quenching effect of the capture agent, the contribution of each active species to MB degradation was ranked as follows: ·OH > ·O₂^- >h⁺ >¹O₂.

Declaration of competing interest

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

Acknowledgments

This work was financially supported by the Development Plan Project of Jilin Province Science and Technology Department (YDZJ202201ZYTS623), and the National Natural Science Foundation of China (22305094). The authors acknowledge the assistance of JLICT Center of Characterization and Analysis.

Electronic Supplementary Material

Below is the link to the electronic supplementary material

Supplementary Material 1

Author Contribution

H：Conceptualization; Data curation; Investigation; Methodology; Roles/Writing - original draft.L：Validation.X：Formal analysis.G：Validation.W：Investigation.Peng He：Writing - review &editing; Funding acquisition.Qun Liu：Funding acquisition; Supervision; Writing - review Yu Zhang：Funding acquisition; Supervision.All authors reviewed the manuscript

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