English

• 0.   Introduction

  The accelerated process of industrialization has resulted in increasingly severe pollution from organic dye wastewater. Rhodamine B (RhB), a representative cationic azo dye, poses a significant threat to both the ecosystem and human health due to its high color intensity, potent biotoxicity, and resistance to photodegradation^[1-2]. In recent years, research on dye adsorption has garnered widespread attention, with a focus on developing efficient and environmentally friendly adsorbent materials. For instance, natural polymer materials (such as chitosan^[3]) and synthetic materials [such as metal-organic frameworks (MOFs)^[4]] have shown significant effects in dye adsorption. Usman et al.^[5] successfully synthesized a novel adsorbent, NTA-β-CD-CS, by integrating the encapsulation ability of β-CD with the functional group adsorption capacity of chitosan. This resulted in efficient simultaneous removal of heavy metals and organic dyes, good reusability, and provided a feasible solution for complex industrial wastewater treatment. Vargas et al.^[6] used activated carbon prepared from flame tree pods to study its adsorption behavior towards three food dyes (acid yellow 6, acid yellow 23, and acid red 18), and detailed the adsorption performance through various isothermal and kinetic models. Yang et al.^[7] fabricated a composite by in-situ growth of Zr-based MOFs onto modified cellulose sponges. The resulting composite demonstrated excellent performance in removing organic dyes from complex wastewater. Moreover, many studies have explored the kinetics and thermodynamics of the dye adsorption process^[8].

  It is worth noting that bismuth vanadate (BiVO₄), as a photocatalytic material, is widely used for dye degradation due to its excellent photocatalytic performance. Under visible light irradiation, it can excite electrons, generating highly oxidative free radicals that effectively degrade organic dyes in water^[9]. For instance, Rajkumar et al.^[10] hydrothermally synthesized BiVO₄-MoS₂ nanocomposites for RhB dye degradation and achieved 96.5% RhB degradation under sunlight in 45 min. Narzary et al.^[11] developed an oxygen vacancy mediated Zn₃V₂O₈/BiVO₄ heterojunction at a 2∶1 molar ratio using the hydrothermal method and achieved 83.7% and 95.03% degradation of Congo red (CR) dye within 120 min under visible light and direct sunlight. Although these studies have demonstrated significant advancements in photocatalytic degradation, the adsorption behavior of BiVO₄, as a prerequisite and important step for photocatalytic reactions, has not been sufficiently studied, so its comprehensive application in dye wastewater treatment remains to be further explored.

  It is found that oxygen vacancies are crucial for the adsorption behavior of pollutants on the surface of BiVO₄. Ma et al.^[12] revealed that the creation of surface oxygen vacancies greatly shifted the d-band center of BiVO₄ and increased the adsorption energy of methylene blue (MB) on the BiVO₄ (010) surface. This phenomenon indicates that the surface oxygen-deficient BiVO₄ is more favorable for the adsorption of dye molecules. The dye adsorption performance of BiVO₄ can be enhanced by regulating the concentration of oxygen vacancies. In terms of improving adsorption efficiency and multifunctionality, the composite of BiVO₄ with other materials (such as MOFs, graphene, tungsten oxide) has shown significant potential^[13-14]. Current research has made important progress by using a heterogeneous structure strategy to composite BiVO₄ with Bi₂O₃. Jiang et al.^[15] synthesized BiVO₄/Bi₂O₃ composites using a homogeneous precipitation method and systematically studied the role of surface dispersants in regulating the morphology of BiVO₄ and its visible-light photocatalytic performance, confirming its high-efficiency degradation of RhB under visible light. Lopes et al.^[16] proposed a new strategy based on solubility differences, where BiVO₄ nanoparticles were grown on the surface of Bi₂O₃ to construct a heterogeneous structure, and systematically explored the significant improvement of this structure on visible-light photocatalytic performance. Obviously, existing research mainly focuses on optimizing the photocatalytic degradation performance of Bi₂O₃/BiVO₄ composites by regulating composite ratios and preparation processes. However, the adsorption behavior of Bi₂O₃/BiVO₄ has not attracted the attention of researchers. To expand the photocatalytic performance of Bi₂O₃/BiVO₄ in removing environmental pollutants, it is imperative to develop efficient ways to engineer its adsorption performance and understand the underlying mechanism.

  We employed a solvothermal method to synthesize Bi₂O₃@BiVO₄ composites with abundant surface oxygen vacancies by regulating the reaction temperature. The in situ growth of Bi₂O₃ induced the formation of oxygen vacancies in BiVO₄, significantly enhancing the adsorption performance for RhB. The mechanism of the enhancement is the specific interaction between RhB molecules and the surface oxygen vacancies of the material, along with electrostatic interactions at the interface. Further studies indicated that parameters such as solution pH, pollutant mass concentration, and temperature have a significant impact on the adsorption process, with the pseudo-second-order kinetic model effectively describing the adsorption behavior. This study not only provides new insights for optimizing the design of traditional photocatalysts but also offers valuable new materials for the removal of cationic dyes.

  1.   Experimental

  1.1   Materials

  Ethylene glycol was purchased from Aladdin Bio-Chem Technology Co., Ltd. Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) was purchased from Xilong Scientific Co., Ltd. RhB, MB, crystal violet (CV), CR, methyl orange (MO), titanium dioxide (TiO₂), tungsten trioxide (WO₃), and ammonium metavanadate (NH₄VO₃) were all purchased from Shanghai Macklin Biochemical Technology Co., Ltd. All chemicals used in the experiments are of analytical grade.

  1.2   Material synthesis

  Bi₂O₃@BiVO₄ composites were synthesized using a solvothermal method. As shown in Scheme 1, 8 mmol of Bi(NO₃)₃·5H₂O and 8 mmol of NH₄VO₃ were dissolved in ethylene glycol and stirred continuously until fully dissolved, yielding a yellow precursor solution. Then, 60 mL of the precursor solution was transferred to a Teflon-lined high-pressure reactor and reacted for 8 h at 120, 150, 180, and 220 ℃, respectively. After the obtained product was centrifuged, the precipitate was washed three times with anhydrous ethanol and ultrapure water, respectively. The washed product was then dried at 60 ℃ to obtain the precursor powder. This powder was heated to 450 ℃ at a heating rate of 5 ℃·min^-1, held for 1 h, and then cooled to room temperature, yielding composites synthesized at different reaction temperatures. The resulting samples were labeled as 120-BO@BVO, 150-BO@BVO, 180-BO@ BVO, and 220-BO@BVO.

  Scheme 1

  

  Scheme 1.  Schematic diagram of the synthesis of 180-BO@BVO

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  1.3   Characterizations

  The crystallographic structure of the samples was characterized using an X-ray diffractometer (XRD, Rigaku SmartLab SE) with the following experimental conditions: Cu Kα radiation λ=0.154 06 nm at an operating voltage of 40 kV and a current of 40 mA, with a 2θ scanning range of 5°-90° and a scanning rate of 2 (°)·min^-1. The chemical state and valence state of the surface elements were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha), with a monochromatic Al Kα radiation source (hν= 1 486.6 eV). The binding energy was calibrated using the carbon C1s peak (284.8 eV) as a reference. The morphology of the materials was observed using a scanning electron microscope (SEM, TESCAN MIRA LMS) and a transmission electron microscope (TEM, JEOL JEM-F200, acceleration voltage 200 kV). Molecular functional group information was collected using a Fourier-transform infrared spectrometer (FTIR, Thermo Scientific Nicolet iN10) with a spectral resolution of 4 cm^-1 and a scan range of 400-4 000 cm^-1. The specific surface area and pore structure parameters were measured using a fully automatic surface area and porosity analyzer (Brunauer-Emmett-Teller, BET, Micromeritics ASAP 2460). The surface potential (ζ potential) was determined using a laser particle size-ζ potential system (Malvern Zetasizer Pro). Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMXplus-6/1) was employed to detect and analyze the oxygen vacancies in the samples.

  1.4   Adsorption experiments

  The adsorption experiment procedure was as follows: 100 mg of Bi₂O₃@BiVO₄ adsorbent was dispersed into 100 mL of RhB solution (10 mg·L^-1) in a conical flask, and dark adsorption was conducted in a constant-temperature shaker at 25, 30, 35, 40, and 45 ℃. Solution samples were collected at 5, 10, 15, 20, 25, 30, 60, 90, and 120 min for subsequent analysis. After rapid solid-liquid phase separation was performed using a 0.45 μm microporous filter membrane, the residual RhB mass concentration was quantitatively analyzed using an ultraviolet-visible spectroscopy spectrophotometer (UV-Vis, Shimadzu UV-3600 Plus, λ=554 nm). In the adsorption cycle experiment, the used 180-BO@BVO powder samples were collected and subjected to ultrasonic treatment for 30 min in ethanol solution. After thorough washing and drying in a 70 ℃ oven for 10 h, the samples were reused in subsequent adsorption cycles to evaluate their recyclability^[17]. The dye adsorption amount and dye removal rate (R, %) were calculated using formulas 1-3^[18-19].

  $ q_t=\frac{\left(\rho_0-\rho_t\right) V}{m} $

  (1)

  $ q_{\mathrm{e}}=\frac{\left(\rho_0-\rho_{\mathrm{e}}\right) V}{m} $

  (2)

  $ R=\frac{\rho_0-\rho_{\mathrm{e}}}{\rho_0} \times 100 \% $

  (3)

  where q_t and ρ_t represent the adsorption capacity (mg·g^-1) and RhB mass concentration (mg·L^-1) at time t, respectively; ρ₀ is the initial mass concentration of the RhB solution (mg·L^-1); q_e and ρ_e are the equilibrium adsorption capacity (mg·g^-1) of the dye and the equilibrium mass concentration of the RhB solution (mg·L^-1), respectively; m (mg) and V (mL) are the mass of the adsorbent and the volume of the RhB solution, respectively.

  The adsorption kinetics of RhB onto the Bi₂O₃@ BiVO₄ composite were analyzed using the pseudo-first-order (PFO) kinetic model (Eq.4), and the pseudo-second-order (PSO) kinetic model (Eq.5). The Weber-Morris intraparticle diffusion (IPD) model (Eq.6) was applied to elucidate the mass transfer mechanism of RhB in the Bi₂O₃@BiVO₄ composite^[20-21].

  $ \ln \left(q_{\mathrm{e}}-q_t\right)=\ln q_{\mathrm{e}}-k_1 t $

  (4)

  $ q_t=\frac{t k_2 q_{\mathrm{e}}^2}{1+k_2 q_{\mathrm{e}} t} $

  (5)

  $ q_t=k_{\mathrm{di}} t^{0.5}+C $

  (6)

  where k₁ (min^-1) and k₂ (g·mg^-1·min^-1) are the rate constants; k_di (mg·g^-1·min^-1/2) and C (mg·g^-1) are the IPD constant and the intercept at different stages, respectively.

  The adsorption equilibrium isotherms and their fitting parameters were used to elucidate the adsorption mechanism. The adsorption equilibrium data were fitted using the Langmuir isotherm model (Eq.7) and the Freundlich isotherm model (Eq.8)^[22].

  $ q_{\mathrm{e}}=\frac{q_{\mathrm{m}} K_{\mathrm{L}} \rho_{\mathrm{e}}}{1+K_{\mathrm{L}} \rho_{\mathrm{e}}} $

  (7)

  $ q_{\mathrm{e}}=K_{\mathrm{F}} \rho_{\mathrm{e}}^{1 / n} $

  (8)

  where q_m (mg·g^-1) is the maximum adsorption capacity of the adsorbent; K_L (L·mg^-1) is the Langmuir constant; K_F ($ \text{mg}^{1-\frac{1}{n}} $·$ {L}^{\frac{1}{n}} $·g^-1) and n are the Freundlich adsorption constants.

  The effect of temperature on the adsorption of RhB by Bi₂O₃@BiVO₄ was explored by studying the standard free energy (ΔG^⊖) (Eq.9), standard enthalpy (ΔH^⊖), and standard entropy (ΔS^⊖) (Eq.10 and 11). The thermodynamic parameters of adsorption were calculated using the following formulas 9-12^[23-24].

  $ \Delta G=-R T \ln K_{\mathrm{a}} $

  (9)

  $ \Delta G=\Delta H^{\ominus}-T \Delta S^{\ominus} $

  (10)

  $ \ln K_{\mathrm{a}}=-\frac{\Delta H^{\ominus}}{R T}+\frac{\Delta S^{\ominus}}{R} $

  (11)

  $ K_{\mathrm{a}}=M \times 55.5 \times 1000 \times K_{\mathrm{L}} $

  (12)

  where T is the absolute temperature, R is the gas constant (8.314 J·K^-1·mol^-1), M (g·mol^-1) is the molar mass of RhB, K_a is the dimensionless adsorption equilibrium constant converted from K_L (Eq.12), and K_L can be calculated using Eq.7.

  2.   Results and discussion

  2.1   Characterization

  In this study, a temperature gradient control strategy of the solvothermal process was employed to successfully construct Bi₂O₃@BiVO₄ composites with a high concentration of oxygen vacancies. SEM analysis revealed the temperature-dependent morphological evolution pattern: in the low-temperature of 120 and 150 ℃, the material formed a nanoparticle assembly with a multi-level pore structure, showing good geometric continuity (Fig.1a, 1b)^[25]; when the temperature rose to 180 ℃, the high surface energy drove small particles to migrate and fuse, evolving into a spherical structure with closely packed submicron cubes (Fig.1c)^[26-27]. Further increasing the temperature to 220 ℃, the Ostwald ripening effect dominated crystal boundary relaxation (Fig.1d)^[28]. These observations indicate that the solvothermal temperature has a significant impact on the morphology of Bi₂O₃@BiVO₄. Fig.1e shows the XRD patterns of Bi₂O₃@BiVO₄ composites prepared at different solvothermal temperatures. The 120-BO@BVO sample exhibited characteristic diffraction peaks of the monoclinic scheelite BiVO₄ structure, with main peaks at 18.8° (011), 28.9° (121), and 30.5° (040) crystal planes (PDF No.14-0688)^[29]. As the solvothermal temperature increased, the XRD of 150-BO@BVO detected the characteristic peak of α-Bi₂O₃ (012) at 28.0°. When the temperature further rose to 180 and 220 ℃, the characteristic diffraction peaks of α-Bi₂O₃ were clearly observed at 26.9° (111) and 28.0° (012) crystal planes (PDF No.41-1449)^[30]. These results indicate that by regulating the solvothermal temperature, the Bi₂O₃@ BiVO₄ composite can be controllably synthesized.

  Figure 1

  

  Figure 1.  SEM images of (a) 120-BO@BVO, (b) 150-BO@BVO, (c) 180-BO@BVO, and (d) 220-BO@BVO; (e) XRD patterns of different Bi₂O₃@BiVO₄ samples; (f) TEM image, (g) HRTEM images, (h) SAED pattern, and (i) elemental mapping images of 180-BO@BVO

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  Fig. 1f shows that 180-BO@BVO were composed of a large number of nanoparticle assemblies. The size distribution analysis of the TEM image (Fig.S1, Supporting information) indicates that the average nanoparticle size was 7 nm. The high-resolution TEM (HRTEM) image of 180-BO@BVO (Fig. 1g) reveals multi-directional lattice fringes, with an inner 0.172 6 nm interplanar spacing corresponding to the (222) crystal plane of the monoclinic BiVO₄ phase, and an outer 0.195 9 nm spacing attributed to the (041) crystal plane of α-Bi₂O₃^[31-32]. The selected area electron diffraction (SAED) pattern of 180-BO@BVO (Fig. 1h) showed a typical polycrystalline diffraction pattern, with discrete diffraction spots arranged in concentric rings. The calibration confirmed that the significant diffraction spots originate from the (123) crystal plane of BiVO₄ (PDF No.14-0688) and the (310), (102), (404), and (205) crystal planes of α-Bi₂O₃ (PDF No.41-1499), further confirming the coexistence of BiVO₄ and Bi₂O₃. The energy dispersive X-ray spectroscopy (EDS) elemental mapping results in Fig. 1i showed that the distributions of Bi, O, and V elements were highly uniform, with no segregation phenomenon^[33]. It is inferred that the synthesis process of the Bi₂O₃@BiVO₄ composite primarily involves: under solvothermal conditions, Bi³⁺ in the solution reacted with vanadate ions (VO₃^-) to form BiVO₄ (Eq.13)^[34]. At the same time, ethylene glycol was oxidized during the reaction to generate oxalate ions (C₂O₄^2-), which subsequently combined with Bi³⁺ and OH^- in the solution to form a Bi(OH)C₂O₄ intermediate through co-precipitation (Eq.14). Bi(OH)C₂O₄ then underwent thermal decomposition to convert into Bi₂O₃ (Eq.15)^[35]. Through this series of continuous reactions, the Bi₂O₃@BiVO₄ composite was successfully synthesized.

  $ \mathrm{Bi}^{3+}+\mathrm{VO}_3^{-}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{BiVO}_4+2 \mathrm{H}^{+} $

  (13)

  $ \mathrm{Bi}^{3+}+\mathrm{C}_2 \mathrm{O}_4^{2-}+\mathrm{OH}^{-} \rightarrow \mathrm{Bi}(\mathrm{OH}) \mathrm{C}_2 \mathrm{O}_4 \downarrow $

  (14)

  $ 4 \mathrm{Bi}(\mathrm{OH}) \mathrm{C}_2 \mathrm{O}_4+4 \mathrm{O}_2 \xrightarrow{\triangle} 2 \mathrm{Bi}_2 \mathrm{O}_3+8 \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}$

  (15)

  Fig.S2a shows the XPS survey spectra, confirming that all samples exhibited characteristic peaks of Bi, V, O, and C on their surfaces, indicating that the main elements are Bi, V, and O. Additionally, the C1s peak shows no shift (Fig.S2b), ruling out instrument error. Notably, as the solvothermal temperature increased, the Bi4f and V2p spectral characteristic peaks shifted towards higher binding energies, which is attributed to the formation of Bi₂O₃@BiVO₄ composites. At the interface between Bi₂O₃ and BiVO₄, electrons transfer from BiVO₄ to Bi₂O₃, leading to a decrease in the electronic density of Bi/V atoms in the BiVO₄ lattice, causing the binding energy to shift towards higher energy^[36]. The Bi4f spectra (Fig.2a) exhibited a double peak at 158.97 eV (Bi4f_7/2) and 164.27 eV (Bi4f_5/2) with a ΔE (binding energy splitting) of 5.3 eV, indicating that all samples have Bi in the Bi³⁺ oxidation state^[37]. The main peak of the V2p spectra (Fig.2b) appeared at 516.5 eV (V2p_3/2) and 523.8 eV (V2p_1/2), which is attributed to V^5+[38]. It was observed that for 150-BO@BVO, 180-BO@BVO, and 220-BO@BVO, a weak peak appeared at 515.2 eV in the V2p spectrum, indicating that increasing the temperature can induce the formation of V⁴⁺ species. This phenomenon is closely related to the charge compensation mechanism induced by oxygen vacancies^[39].

  Figure 2

  

  Figure 2.  (a) Bi4f, (b) V2p, and (c) O1s high-resolution XPS spectra of different Bi₂O₃@BiVO₄ samples

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  Fig.2c shows the O1s spectra of all the samples. Through peak fitting, it can be observed that the O1s characteristic peak could be divided into three components: the main peak at 529.6 eV corresponds to lattice oxygen, the sub-peak at 530.6 eV corresponds to surface oxygen vacancies^[40], and the characteristic peak at 532.1 eV is attributed to surface chemisorbed oxygen^[41-42]. Notably, the oxygen vacancy characteristic peak and the presence of V⁴⁺ confirm the existence of oxygen vacancy structures in the material system. Comparing the peak area ratio of the oxygen vacancy characteristic peaks for different samples, the trend of oxygen vacancy content was found to be 180-BO@ BVO (12.9%)>220-BO@BVO (12.1%)>150-BO@BVO (10.4%)>120-BO@BVO (9.8%). It can be seen that a temperature of 180 ℃ induces the highest concentration of oxygen defects. These findings suggest that oxygen vacancies are formed via the reduction of V⁵⁺ to V⁴⁺ during the hydrothermal process. The reducing agent C₂O₄^2-, which formed ethylene glycol, also facilitated the reduction of V⁵⁺ to V⁴⁺. This redox reaction is thermodynamically favorable under hydrothermal conditions, promoting the simultaneous generation of oxygen vacancies.

  Fig.3 shows the N₂ adsorption-desorption isotherms and pore size distributions of Bi₂O₃@BiVO₄. The Bi₂O₃@BiVO₄ samples follow type Ⅳ isotherms with H3-type hysteresis loops, indicating the presence of mesoporous structures^[43-44]. Pore size distribution analysis (Fig.3b) showed that the pore sizes of all samples were concentrated below 10 nm, in line with the classification standard for mesoporous materials (2-50 nm). The mesoporous structure mainly results from the gaps formed by the stacking of nanoparticles during the crystallization process^[45]. The pore volume, specific surface area (S_BET), and average pore size of each sample are shown in Table 1. The specific surface areas of 120-BO@BVO, 150-BO@BVO, 180-BO@BVO, and 220-BO@BVO were 2, 7, 1, and 1 m²·g^-1, respectively. The specific surface area of 150-BO@BVO was significantly higher than that of the other samples. At low temperatures (120 ℃), the precursor crystallization driving force was insufficient, leading to particle agglomeration or a more compact structure, resulting in a small specific surface area. When the temperature was increased to 150 ℃, the solvation ability of ethylene glycol and the reaction activity were moderate, which also inhibited excessive particle growth, forming a loose and porous structure that increased the specific surface area. However, at higher temperatures (180 and 220 ℃), rapid grain growth, increased agglomeration, or collapse of the pore structure occurred, leading to an increase in material densification and a decrease in the specific surface area^[46].

  Figure 3

  

  Figure 3.  (a) N₂ adsorption-desorption isotherms and (b) the corresponding pore size distribution curves of different Bi₂O₃@BiVO₄ samples

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  Table 1

  

  Table 1.  Textural parameters of different Bi₂O₃@BiVO₄ samples determined by N₂ adsorption-desorption measurements

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  Sample	SBET / (m2·g-1)	Pore volume / (cm3·g-1)	Average pore size / nm
  120-BO@BVO	2	0.009 04	15.52
  150-BO@BVO	7	0.055 36	33.21
  180-BO@BVO	1	0.007 32	24.14
  220-BO@BVO	1	0.008 61	24.85

  2.2   Adsorption capacity

  The adsorption performance of Bi₂O₃@BiVO₄ prepared at different solvothermal temperatures for RhB was studied. As shown in Fig.4a, 120-BO@BVO exhibited a low removal efficiency of only 18.2%, demonstrating weak adsorption ability. When the solvothermal temperature was increased to 150 ℃, the adsorption performance significantly improved, and the removal efficiency reached 65.4%. Notably, 180-BO@BVO demonstrated the best adsorption performance, with a removal efficiency of 83.0%, which was higher than that of 220-BO@BVO (69.3%). Therefore, 180-BO@BVO was selected as the optimal adsorbent for subsequent adsorption experiments. Note that, although 150-BO@ BVO had the largest specific surface area (Table 1), its adsorption effect on RhB was inferior to that of 180-BO@BVO, suggesting that the increase in specific surface area is not the dominant factor for the enhanced adsorption performance of 180-BO@BVO. Based on the XPS analysis results in Fig.2, it can be speculated that the enhanced adsorption performance of 180-BO@BVO is due to the increased surface oxygen vacancies, which strengthen its chemical adsorption of RhB^[47]. Furthermore, as shown in Fig.4b, several common photocatalytic materials were compared with 180-BO@ BVO for RhB adsorption. The results indicate that the adsorption capacity of 180-BO@BVO for RhB was more than four times higher than that of BiVO₄, TiO₂, and WO₃. These demonstrate that constructing the Bi₂O₃@BiVO₄ heterostructure significantly enhances the adsorption performance. To further investigate the broad-spectrum adsorption performance of 180-BO@ BVO, four types of representative pollutants were selected for the same adsorption experiments, including diclofenac (DCF), tetracycline hydrochloride (TC), ammonia nitrogen (NH₃-N), and norfloxacin (NOR). As shown in Fig.S3, the adsorption performance of 180-BO@BVO for the four types of pollutants varied significantly. It exhibited good adsorption performance for NH₃-N and DCF, with removal efficiencies of 65.3% and 60.0%, respectively. However, the adsorption removal efficiencies for TC and NOR were relatively poor, at only 41.2% and 15.3%, respectively. Clearly, 180-BO@BVO is more suitable for removing positively charged or small neutral molecules, while its adsorption performance for large, negatively charged antibiotics is poor, which can be attributed to its lower specific surface area.

  Figure 4

  

  Figure 4.  (a) Removal efficiency of RhB on different Bi₂O₃@BiVO₄ samples; (b) Removal efficiency of RhB on different adsorbents; (c) ζ potential-pH curves of 180-BO@BVO; Effect of (d) pH, (e) dosage, (f) initial mass concentration of RhB, and temperature on RhB removal efficiency of 180-BO@BVO

  a-d: 100 mg of adsorbent was added to 100 mL of dye solution with an initial concentration of 10 mg·L^-1, and the adsorption was conducted at room temperature (25 ℃) for 120 min; e, f: the other experimental conditions were identical to those in a-c, except for changing the adsorbent dosage or initial RhB mass concentration.

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  The point of zero charge (pH_pzc) of an adsorbent is a key parameter for analyzing the charge behavior of the solid-liquid interface, which directly affects the environmental response characteristics of the adsorption process. The pH_pzc value of 180-BO@BVO was measured to be 3.5 (Fig.4c). When the solution pH was below 3.5, the surface of 180-BO@BVO carried a positive charge. Conversely, at pH values above 3.5, it carried a negative charge. Utilizing this charge characteristic, this study investigated the effect of pH on the adsorption of RhB (Fig.4d). The experimental results showed that under conditions where pH≥4, the surface of 180-BO@BVO consistently carried a negative charge, allowing efficient adsorption of the cationic RhB dye through electrostatic attraction. The removal efficiency could reach 82.0% when the pH was unadjusted (initial pH=6.83)^[48]. Notably, when the solution pH was further increased (pH=8 and 10), the RhB molecules underwent protonation equilibrium shift, forming zwitterionic states, which led to molecular aggregation and ultimately resulted in a decrease in adsorption performance^[49]. To further verify the electrostatic adsorption mechanism of 180-BO@BVO for dyes, adsorption experiments were conducted with various ionic dyes, including cationic dyes such as MB and CV, as well as anionic dyes like MO and CR. The results are shown in Fig.S4. It can be seen that the adsorption removal efficiencies of MB and CV by 180-BO@BVO were 69.2% and 92.2%, respectively, which were higher than those of MO and CR (12.1% and 10.3%). This difference indicated that 180-BO@BVO exhibited stronger selective adsorption for cationic dyes. Its negatively charged surface can significantly promote the adsorption process by forming electrostatic interactions with cationic dyes, which serve as a key driving force for adsorption.

  To investigate the effect of the 180-BO@BVO dosage on RhB adsorption performance, the removal efficiency was examined under various dosages ranging from 50 to 125 mg (Fig.4e). The results showed that at a dosage of 50 mg, the RhB removal efficiency after 120 min was 72.3%, and the adsorption process reached equilibrium after 60 min. When the dosage was increased to 75 mg, the removal efficiency increased to 77.8%. Further increasing the dosage to 100 mg significantly improved the removal efficiency to 83.0%, indicating that the dosage range of 50-100 mg has a significant effect on adsorption efficiency. Within this range, increasing the dosage helps to improve the adsorption efficiency. Moreover, for the 100 mg dosage group, a rapid adsorption kinetic characteristic was observed between 0-40 min, which can be attributed to the ample active sites provided by the higher dosage, facilitating the efficient adsorption of RhB molecules^[50-51]. It is worth noting that further increasing the dosage to 125 mg did not significantly improve the removal efficiency, possibly due to the poor dispersion of 180-BO@BVO in the solution at higher dosages, leading to particle agglomeration and reducing the exposure of effective active sites.

  Fig. 4f investigates the effect of solution temperature (25-45 ℃) and initial RhB mass concentration (10-25 mg·L^-1) on the adsorption performance of 180-BO@BVO. At the same initial RhB mass concentration, as the temperature increased from 25 to 45 ℃, the RhB removal efficiency increased from 82.0% to 89.5%. This indicates that increasing the temperature has a significant impact on the adsorption performance of 180-BO@BVO. According to thermodynamic calculations discussed in the later section, it can be that ΔH^⊖>0, suggesting that this adsorption process is endothermic, and the adsorption efficiency of the endothermic reaction increases with temperature^[52]. In the initial RhB mass concentration gradient experiment (Fig. 4f), when at the same temperature, as the initial RhB mass concentration increased, the removal efficiency gradually decreased. However, when the initial RhB mass concentrations were 10, 15, 20, and 25 mg·L^-1 at 25 ℃, the equilibrium adsorption capacities of 180-BO@BVO were 7.2, 7.4, 7.8, and 8.0 mg·g^-1, respectively. The equilibrium adsorption capacity slightly increased with the initial mass concentration. Clearly, as the RhB mass concentration increases, the driving force increases, causing more RhB molecules to transfer from the liquid phase to the adsorbent surface and occupy remaining adsorption sites, thereby increasing the equilibrium adsorption capacity. The decrease in removal efficiency is primarily due to the insufficient adsorption sites on 180-BO@BVO to adsorb a large number of RhB molecules. Even when the adsorbent becomes saturated, many RhB molecules remain in the solution, leading to a reduction in the removal efficiency^[53].

  2.3   Adsorption kinetics fitting

  The control mechanism of RhB adsorption by 180-BO@BVO was revealed through kinetic modeling. Based on the adsorption data from Fig.4e, PFO, PSO, and IPD models were established to analyze the interfacial process (Fig.5a, 5b), with the relevant fitting parameters shown in Table 2. Among them, the PSO model exhibited the best fitting quality (R²=0.999 9), and the theoretical adsorption capacity (q_{e, cal}=7.095 7 mg·g^-1) showed a relative deviation of only 0.8% from the experimental equilibrium adsorption capacity (q_{e, exp}=7.066 2 mg·g^-1), indicating that chemical adsorption dominates the process^[54]. The IPD model, based on the Weber-Morris theory, decoupled the multiscale mass transfer process (Fig.5c). The entire adsorption process could be divided into three stages: in the initial stage, RhB molecules rapidly diffuse from the bulk solution to the liquid film layer on the surface of the adsorbent, with a rate constant of k_d1=0.10, exhibiting efficient membrane diffusion characteristics; in the intermediate stage, the diffusion process shifts to the slow migration of RhB molecules within the adsorbent particles, with the rate constant decreasing to k_d2=0.04, where the diffusion resistance significantly increases, becoming the limiting factor of the entire adsorption rate; in the final stage, the system reaches dynamic adsorption equilibrium, with a rate constant k_d3=0, and RhB molecules have largely occupied the active sites on the 180-BO@BVO adsorbent, leading to equilibrium between adsorption and desorption^[55]. It is worth noting that the fitting curve did not pass through the origin, which indicates that, in addition to IPD, interfacial chemical reactions, surface adsorption, and other mechanisms also influence the adsorption process, suggesting that RhB adsorption in this system is typically regulated by multiple factors working synergistically^[56].

  Figure 5

  

  Figure 5.  Fitting curves of (a) PFO, (b) PSO, (c) IPD model, (d) Langmuir isotherm model, (e) Freundlich isotherm model, and (f) thermodynamic analysis via van′t Hoff plot for the adsorption of RhB on 180-BO@BVO

  下载: 全尺寸图片 幻灯片

  Table 2

  

  Table 2.  Parameters of linear kinetic models for the removal of RhB on 180-BO@BVO

  下载: 导出CSV

  Dosage / (mg·L-1)	PFO					PSO
  	qe, exp / (mg·g-1)	qe, cal / (mg·g-1)	k1 / min-1	R2		qe, cal / (mg·g-1)	k2 / (g·mg-1·min-1)	R2
  10	7.066 2	1.156 6	0.063 9	0.672 2		7.095 7	0.279 3	0.999 5
  15	7.415 7	3.580 9	0.053 0	0.899 6		7.586 7	0.048 9	0.999 2
  20	7.702 9	2.932 4	0.048 8	0.802 3		7.951 7	0.063 6	0.999 5
  25	8.027 5	3.668 0	0.046 8	0.852 5		8.209 5	0.042 9	0.998 8

  2.4   Adsorption isotherm fitting

  RhB adsorption experiments were conducted on 180-BO@BVO at different temperatures, and the results were fitted using the Langmuir and Freundlich adsorption isotherm models (Fig.5d, 5e), with the relevant parameters listed in Table 3. It could be seen that the Langmuir model showed a higher linear correlation, significantly outperforming the Freundlich model. This characteristic indicates that the adsorption of RhB on the surface of 180-BO@BVO follows a monolayer adsorption mechanism, where the adsorption sites are uniformly distributed on the material surface, and there are no lateral interactions between molecules^[57]. Furthermore, the calculated maximum adsorption capacity (q_m) was in good agreement with the experimental data (q_e) (Table S1). For the thermodynamic properties of RhB adsorption on 180-BO@BVO, the key thermodynamic parameters of the adsorption process were determined through the derivation of Eq.9, 10, and 11, providing a quantitative basis for revealing the adsorption mechanism. As shown in Fig.5f, the linear fit between ln K_a and 1 000/T was obtained. Based on the slope and intercept of the fitted line, the values of ΔH^⊖, ΔS^⊖, and ΔG^⊖ were calculated, and the results are shown in Table 4. Among them, the ΔG^⊖ values were negative, and their absolute values increased with temperature, indicating that the adsorption of RhB on 180-BO@BVO is a spontaneous and thermodynamically favorable process^[58], further confirming that the adsorption reaction of RhB on 180-BO@BVO is an endothermic reaction, which was consistent with the experimental results.

  Table 3

  

  Table 3.  Parameters of linear kinetic models for the removal of RhB on 180-BO@BVO

  下载: 导出CSV

  T / ℃	Langmuir model				Freundlich model
  	qm / (mg·g-1)	KL / (L·mg-1)	R2		KF / $ (\mathrm{mg}^{1-\frac{1}{n}} \cdot \mathrm{~L}^{\frac{1}{n}} \cdot \mathrm{~g}^{-1}) $	1/n	R2
  30	13.388 7	0.702 6	0.999 4		6.252 4	0.295 6	0.942 2
  35	14.596 4	0.795 6	0.999 4		7.004 4	0.301 3	0.923 5
  40	14.916 5	0.895 8	0.995 4		7.315 1	0.308 9	0.887 1
  45	16.345 2	0.996 6	0.999 6		8.462 0	0.293 0	0.941 0

  Table 4

  

  Table 4.  Adsorption thermodynamic parameters of 180‑BO@BVO towards RhB

  下载: 导出CSV

  T / ℃	ΔG⊖ / (kJ·mol-1)	ΔH⊖ / (kJ·mol-1)	ΔS⊖ / (kJ·mol-1·K-1)
  30	-42.18	18.71	0.20
  35	-43.19
  40	-44.20
  45	-45.18

  2.5   Adsorption mechanism analysis

  To further confirm the selective adsorption of cationic dyes by 180-BO@BVO, adsorption experiments were conducted on a mixed solution of cationic and anionic dyes. The dye combinations of RhB+MO and MB+CR were selected for the selective adsorption experiments. A 100 mL mixed solution was prepared by mixing the cationic dye (RhB, 10 mg·L^-1) and anionic dye (MO, 10 mg·L^-1) at a volume ratio of 1∶1, then treated with 100 mg of 180-BO@BVO for 120 min. The UV-Vis spectra of the resulting solutions were recorded and are presented in Fig.6a and 6b. As shown in Fig.6a, the RhB+MO mixed solution appeared orange-red (with RhB being purplish-red and MO being orange). After adsorption by 180-BO@BVO, the solution turned orange. This phenomenon indicates that RhB was almost completely adsorbed by the adsorbent, with only MO remaining in the solution. The UV-Vis spectra also confirmed this. After adsorption, the absorption peak at 554 nm corresponding to RhB disappeared, while the absorption peak at 464 nm attributed to MO remained unchanged (Fig.6a). To further verify this, the same treatment was applied to other mixed systems containing one cationic dye and one anionic dye, and similar results were observed. For instance, the MB+CR mixed solution changed from purple to deep red after adsorption (Fig.6b), and its UV-Vis spectral results were consistent with the color change: the characteristic absorption peak for MB at 668 nm disappeared, while the characteristic absorption peak for CR at 490 nm remained unchanged. The above results show that 180-BO@BVO has excellent selective adsorption performance for cationic dyes.

  Figure 6

  

  Figure 6.  Photos and UV-Vis absorption spectra of (a) RhB+MO and (b) MB+CR binary dye mixtures before and after the adsorption treatment with 180-BO@BVO

  下载: 全尺寸图片 幻灯片

  As shown in Fig.7a, after five cycles, the RhB removal efficiency by 180-BO@BVO remained above 80%, indicating good stability and repeatability. The XPS spectra of 180-BO@BVO before and after adsorption in Fig.S5 revealed that the valence states of Bi, V, and O did not undergo significant shifts or changes. This strongly indicates that 180-BO@BVO maintains excellent chemical stability during the adsorption process. Fig.7b shows the FTIR spectra of 180-BO@BVO before and after RhB adsorption. The peak at 3 437 cm^-1 corresponds to the assigned to OH^- stretching vibrations of the hydroxyl groups, signifying O—H stretching modes of surface adsorbed water molecules or hydroxyl groups, and indicates the presence of adsorbed water molecules on the 180-BO@BVO^[59]. After adsorption, the O—H peak became significantly weaker, suggesting that the surface O—H groups of 180-BO@BVO were reduced by the adsorbed RhB during the process^[60]. These results suggest that RhB regulates the surface chemical environment of oxygen vacancies of 180-BO@BVO through competitive adsorption and coordination interactions. To further verify the existence of oxygen vacancies in 180-BO@ BVO, EPR spectroscopy was employed to detect unpaired electrons associated with oxygen vacancies. As shown in Fig.7c, a distinct oxygen vacancy signal appears at g=2.003 for 180-BO@BVO^[61]. Together with the data in Fig.2, the presence of oxygen vacancies in 180-BO@BVO was confirmed.

  Figure 7

  

  Figure 7.  (a) Cyclic adsorption performance, (b) FTIR spectra before and after RhB adsorption, (c) EPR spectrum, and (d) proposed adsorption mechanism of 180-BO@BVO

  下载: 全尺寸图片 幻灯片

  In summary, the adsorption mechanism of 180-BO@BVO is derived as shown in Fig.7d. Firstly, during the solvothermal process, Bi₂O₃ was in situ grown on the surface of BiVO₄, and a large number of oxygen vacancies were formed at the interface due to the reduction of V⁵⁺ to V^4+[62]. The oxygen vacancies on the 180-BO@BVO surface play a key role in the adsorption of RhB molecules. These vacancies can adsorb adventitious oxygen species (including both free and combined oxygen) at the vacancy sites. Due to the strong electronegativity of the adsorbed oxygen species, they tend to attract electrons, thereby acquiring a negative charge (Fig.4c)^[12]. As a result, under neutral conditions, the cationic dye RhB molecules were readily adsorbed onto the 180-BO@BVO surface via electrostatic attraction. This leads to a weakening of the OH^- peak in the FTIR spectra, indicating that the —OH groups are competitively adsorbed by RhB molecules. The reduction in the hydroxyl signal further confirms that cationic dye molecules preferentially occupy the negatively charged sites around oxygen vacancies, competing with the —OH groups for adsorption.

  3.   Conclusions

  Bi₂O₃@BiVO₄ composites with abundant oxygen vacancies were successfully prepared via a solvothermal method, and their adsorption performance and mechanism for RhB were investigated. The results show that the 180-BO@BVO composite prepared at 180 ℃ exhibited the best adsorption performance for RhB, with a removal efficiency of up to 83.0%. 180-BO@BVO achieved efficient adsorption of RhB through electrostatic interactions with the oxygen vacancies on the material surface. The adsorption process follows the Langmuir isotherm model and the PSO kinetic model, confirming that the adsorption is primarily characterized by monolayer coverage and follows a chemical adsorption mechanism. Thermodynamic analysis shows that the ΔG^⊖ of the adsorption process was negative, indicating that the adsorption is thermodynamically spontaneous. After five cycles of adsorption, the RhB removal efficiency of 180-BO@BVO remained above 80%, demonstrating excellent recyclability. In summary, the Bi₂O₃@BiVO₄ composite prepared in this study exhibits efficient and specific adsorption performance for cationic dyes, highlighting its significant practical application potential for selectively removing cationic dyes from polluted water on an industrial scale.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: This work has been financially supported by Guangxi KeTechnologies R&D Program (Grants No.AB25069334, AB24010314), Scientific Research Foundation for introduced Talents of Guangxi Minzu University (Grant No.2018KJQD07), the Young Scholar Innovation Team of Guangxi Minzu University (2022). Author Contributions: The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.
  Notes: The authors declare no competing financial interest.
  
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  Scheme 1  Schematic diagram of the synthesis of 180-BO@BVO

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  Figure 1  SEM images of (a) 120-BO@BVO, (b) 150-BO@BVO, (c) 180-BO@BVO, and (d) 220-BO@BVO; (e) XRD patterns of different Bi₂O₃@BiVO₄ samples; (f) TEM image, (g) HRTEM images, (h) SAED pattern, and (i) elemental mapping images of 180-BO@BVO

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  Figure 2  (a) Bi4f, (b) V2p, and (c) O1s high-resolution XPS spectra of different Bi₂O₃@BiVO₄ samples

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  Figure 3  (a) N₂ adsorption-desorption isotherms and (b) the corresponding pore size distribution curves of different Bi₂O₃@BiVO₄ samples

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  Figure 4  (a) Removal efficiency of RhB on different Bi₂O₃@BiVO₄ samples; (b) Removal efficiency of RhB on different adsorbents; (c) ζ potential-pH curves of 180-BO@BVO; Effect of (d) pH, (e) dosage, (f) initial mass concentration of RhB, and temperature on RhB removal efficiency of 180-BO@BVO

  a-d: 100 mg of adsorbent was added to 100 mL of dye solution with an initial concentration of 10 mg·L^-1, and the adsorption was conducted at room temperature (25 ℃) for 120 min; e, f: the other experimental conditions were identical to those in a-c, except for changing the adsorbent dosage or initial RhB mass concentration.

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  Figure 5  Fitting curves of (a) PFO, (b) PSO, (c) IPD model, (d) Langmuir isotherm model, (e) Freundlich isotherm model, and (f) thermodynamic analysis via van′t Hoff plot for the adsorption of RhB on 180-BO@BVO

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  Figure 6  Photos and UV-Vis absorption spectra of (a) RhB+MO and (b) MB+CR binary dye mixtures before and after the adsorption treatment with 180-BO@BVO

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  Figure 7  (a) Cyclic adsorption performance, (b) FTIR spectra before and after RhB adsorption, (c) EPR spectrum, and (d) proposed adsorption mechanism of 180-BO@BVO

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  Table 1.  Textural parameters of different Bi₂O₃@BiVO₄ samples determined by N₂ adsorption-desorption measurements

  Sample	SBET / (m2·g-1)	Pore volume / (cm3·g-1)	Average pore size / nm
  120-BO@BVO	2	0.009 04	15.52
  150-BO@BVO	7	0.055 36	33.21
  180-BO@BVO	1	0.007 32	24.14
  220-BO@BVO	1	0.008 61	24.85

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  Table 2.  Parameters of linear kinetic models for the removal of RhB on 180-BO@BVO

  Dosage / (mg·L-1)	PFO					PSO
  	qe, exp / (mg·g-1)	qe, cal / (mg·g-1)	k1 / min-1	R2		qe, cal / (mg·g-1)	k2 / (g·mg-1·min-1)	R2
  10	7.066 2	1.156 6	0.063 9	0.672 2		7.095 7	0.279 3	0.999 5
  15	7.415 7	3.580 9	0.053 0	0.899 6		7.586 7	0.048 9	0.999 2
  20	7.702 9	2.932 4	0.048 8	0.802 3		7.951 7	0.063 6	0.999 5
  25	8.027 5	3.668 0	0.046 8	0.852 5		8.209 5	0.042 9	0.998 8

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  Table 3.  Parameters of linear kinetic models for the removal of RhB on 180-BO@BVO

  T / ℃	Langmuir model				Freundlich model
  	qm / (mg·g-1)	KL / (L·mg-1)	R2		KF / $ (\mathrm{mg}^{1-\frac{1}{n}} \cdot \mathrm{~L}^{\frac{1}{n}} \cdot \mathrm{~g}^{-1}) $	1/n	R2
  30	13.388 7	0.702 6	0.999 4		6.252 4	0.295 6	0.942 2
  35	14.596 4	0.795 6	0.999 4		7.004 4	0.301 3	0.923 5
  40	14.916 5	0.895 8	0.995 4		7.315 1	0.308 9	0.887 1
  45	16.345 2	0.996 6	0.999 6		8.462 0	0.293 0	0.941 0

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  Table 4.  Adsorption thermodynamic parameters of 180‑BO@BVO towards RhB

  T / ℃	ΔG⊖ / (kJ·mol-1)	ΔH⊖ / (kJ·mol-1)	ΔS⊖ / (kJ·mol-1·K-1)
  30	-42.18	18.71	0.20
  35	-43.19
  40	-44.20
  45	-45.18

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