Process for photocatalytic reduction of hexavalent chromium

Abstract

The application relates to the technical field of photocatalysis and heavy metal treatment, and particularly discloses a method for photocatalytic reduction of hexavalent chromium, which comprises the following steps: mixing a photocatalyst with a hexavalent chromium solution, adjusting the pH value to 1-7, stirring, and irradiating; wherein the photocatalyst is a bismuth molybdate nanoring; the method uses the novel bismuth molybdate nanoring as the photocatalyst to efficiently reduce the hexavalent chromium for the first time, the reaction rate is fast, the photocatalyst can be recycled, the method is green and environment-friendly, and the cost is low.

Description

Technical Field

[0001] This invention relates to the fields of photocatalysis and heavy metal treatment technology, specifically to a method for photocatalytic reduction of hexavalent chromium. Background Technology

[0002] Chromium is a common and essential material in our daily lives, widely used in leather making, electroplating, textiles, and other fields. In nature, it mainly exists in two oxidation states: hexavalent chromium [Cr(VI)] and trivalent chromium [Cr(III)]. Cr(VI) is extremely toxic and carcinogenic even at very low concentrations, posing a threat to human health and causing various diseases such as liver damage, pulmonary congestion, fetal malformations, and cancer. Long-term occupational exposure to Cr(VI) can lead to bronchial and respiratory cancers. According to the World Health Organization (WHO), the maximum permissible concentration of Cr(VI) discharged into inland rivers and the maximum permissible concentration of Cr(VI) in drinking water are 0.1 mg / L and 0.05 mg / L, respectively. Cr(III), on the other hand, is an important trace element essential for human metabolism. Cr(III) can form hydroxides with hydroxyl groups, easily forming insoluble Cr(OH)3 precipitates in alkaline water bodies, which are adsorbed onto soil particles and exhibit low mobility. Therefore, reducing highly toxic Cr(VI) to Cr(III) is a promising method for treating Cr(VI)-containing wastewater.

[0003] Although traditional technologies for removing Cr(VI), such as adsorption, membrane filtration, electrochemical precipitation, and biological methods, have been extensively studied, these methods mostly suffer from drawbacks such as low efficiency, high energy consumption, secondary pollution, and high cost, severely limiting their application in the treatment of chromium-containing wastewater. Therefore, developing green and efficient materials to effectively remove Cr(VI) from the aquatic environment is particularly important. Photocatalysis, as a promising technology for wastewater treatment, air purification, and solar energy development, has attracted widespread attention due to its advantages of being non-toxic, safe, low-cost, and highly efficient. It is based on the irradiation of semiconductors by high-energy photons, generating electron-hole pairs, which induce a photoredox reaction, in which Cr(VI) can be reduced to Cr(III) under the action of electrons.

[0004] In recent years, BiVO4, Bi2WO6, and Bi2MoO6 (denoted as Bi) have become increasingly important. a AO b Bismuth oxides, as a class of compelling semiconductor materials, have become a hot topic in photocatalysis research due to their stable crystal structure, high quantum transport efficiency, high electron transport efficiency, and outstanding energy utilization capabilities. Bi₂MoO₆, a typical bismuth oxide with layered structures, consists of alternating layers of [Bi₂O₂]. 2+ The layer, through oxygen atoms and [MoO4] 2-Octahedral shared corner structures connect the layers. This interesting layered structure not only endows Bi₂MoO₆ nanostructures with unique advantages but also provides an internal electric field (IEF), promoting carrier separation and transfer. Due to its inherent layered structure, low cost, and non-toxicity, it has attracted widespread attention as a promising photocatalyst. However, due to its structural characteristics, Bi₂MoO₆ primarily exhibits a two-dimensional sheet-like morphology. Other reported morphologies of Bi₂MoO₆ include nanoparticles and nanotubes, but no reports have been found regarding Bi₂MoO₆ with nanoring structures, and even fewer technologies and patents exist for the reduction of hexavalent chromium using Bi₂MoO₆ with nanoring structures. Summary of the Invention

[0005] The purpose of this invention is to provide a method for the photocatalytic reduction of hexavalent chromium, which for the first time uses a novel bismuth molybdate nanoring as a photocatalyst for the efficient reduction of hexavalent chromium.

[0006] To achieve the above objectives, the present invention provides a method for photocatalytic reduction of hexavalent chromium, the method comprising: mixing a photocatalyst with a hexavalent chromium solution, adjusting the pH to 1-7, stirring, and irradiating with light; wherein the photocatalyst is a bismuth molybdate nanoring.

[0007] Through the above technical solution, the present invention uses bismuth molybdate nanorings as photocatalysts to catalytically reduce hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] under acidic conditions. The reaction is highly efficient and the photocatalyst can be recycled, making it green, environmentally friendly, and low in cost.

[0008] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0009] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the following detailed description to explain the invention, but do not constitute a limitation thereof. In the drawings:

[0010] Figure 1 In Figure a, bismuth molybdate nanorings prepared in Preparation Example 1 are SEM images, and in Figure b, bismuth molybdate nanosheets prepared in Preparation Example 2 are SEM images.

[0011] Figure 2 XRD patterns of bismuth molybdate nanorings prepared in Example 1 and bismuth molybdate nanosheets prepared in Example 2;

[0012] Figure 3 In the diagram, a is a TEM image of the bismuth molybdate nanorings prepared in Preparation Example 1, b is an HRTEM image of the bismuth molybdate nanorings prepared in Preparation Example 1, c is a selected area electron diffraction image of the bismuth molybdate nanorings prepared in Preparation Example 1, and d is a model diagram of the crystal plane structure of the bismuth molybdate nanorings prepared in Preparation Example 1.

[0013] Figure 4 The reduction curves of Cr(VI) reduction catalyzed by bismuth molybdate nanorings prepared in Preparation Example 1 and the reduction curves of Cr(VI) reduction catalyzed by bismuth molybdate nanosheets prepared in Preparation Example 2 in Comparative Example 1 are shown below.

[0014] Figure 5 This is a quasi-first-order kinetic model fitting diagram of the catalytic reduction of Cr(VI) (100 mg / L) using bismuth molybdate nanorings prepared in Preparation Example 1 in Example 1;

[0015] Figure 6 Examples 1-3 show the degradation curves of bismuth molybdate nanorings prepared in Preparation Example 1 catalyzing the reduction of Cr(VI) at different concentrations.

[0016] Figure 7 The degradation curves of Cr(VI) (100 mg / L) in Examples 1, 4 and Comparative Examples 2-3 are shown.

[0017] Figure 8 Examples 1-3 show the degradation curves of bismuth molybdate nanorings prepared in Example 1 with different masses catalytically reducing Cr(VI) (100 mg / L);

[0018] Figure 9 Example 7 shows the reduction efficiency of bismuth molybdate nanorings to Cr(VI) in the presence of different free radical scavengers;

[0019] Figure 10 Example 8 is a cyclic experiment of photoreduction of Cr(VI) using bismuth molybdate nanorings prepared in Preparation Example 1;

[0020] Figure 11 These are SEM images of the bismuth molybdate nanorings prepared in Example 1 before and after a cycling experiment.

[0021] Figure 12 The XRD patterns of the bismuth molybdate nanorings prepared in Example 1 before and after cycling experiments are shown.

[0022] Figure 13 The diffuse reflectance (DRS) spectra of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 are shown.

[0023] Figure 14 The bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 are (αhν) 1 / 2 and hν curve;

[0024] Figure 15 The Mott-Schottky curves are for the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2.

[0025] Figure 16 This is a schematic diagram of the band structure of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2.

[0026] Figure 17 The electron paramagnetic resonance (EPR) spectrum of the bismuth molybdate nanorings prepared in Example 1 is shown.

[0027] Figure 18 The photocurrent density of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 is shown.

[0028] Figure 19 The room-temperature fluorescence spectra (λ) of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 are shown. ex =375nm);

[0029] Figure 20 The images show the electrochemical impedance spectroscopy of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2. Detailed Implementation

[0030] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0031] This invention provides a method for photocatalytic reduction of hexavalent chromium, the method comprising: mixing a photocatalyst with a hexavalent chromium solution, adjusting the pH to 1-7, stirring, and irradiating with light;

[0032] The photocatalyst is a bismuth molybdate nanoring.

[0033] In a preferred embodiment of the present invention, the ratio of the photocatalyst to the hexavalent chromium solution is 1:100-200.

[0034] In a preferred embodiment of the present invention, the pH adjustment to 1-7 can be achieved by selecting citric acid and / or sulfuric acid.

[0035] In a preferred embodiment of the present invention, the stirring conditions include: a dark environment, a time of 40-60 minutes, and a rotation speed of 500-800 rpm.

[0036] In a preferred embodiment of the present invention, the illumination conditions include: an illumination intensity of 80-100 W·cm. 2 The time is 20-30 minutes.

[0037] In a preferred embodiment of the present invention, the light source can be provided by a xenon lamp and / or sunlight.

[0038] In a preferred embodiment of the present invention, the method for preparing the bismuth molybdate nanorings includes:

[0039] In the presence of a solvent, the precursor and an optional bismuth source are first mixed, and ammonium molybdate is added for a second mixing, followed by a reaction.

[0040] The precursor is bismuth oxalate nanosheets.

[0041] In a preferred embodiment of the present invention, the molar ratio of the precursor to the bismuth source is 1:0-1.2.

[0042] In a preferred embodiment of the present invention, the bismuth source is selected from bismuth nitrate and / or bismuth chloride.

[0043] In a preferred embodiment of the present invention, the solvent is selected from one or more of ethylene glycol, ethanol and water.

[0044] In a preferred embodiment of the present invention, the concentration of ammonium molybdate is 0.001-0.005 mol / L.

[0045] In a preferred embodiment of the present invention, the conditions for the second mixing include: time 10-30 min and rotation speed 100-500 rpm.

[0046] In a preferred embodiment of the present invention, the reaction conditions include: a temperature of 140-160°C and a time of 3-12 hours.

[0047] In a preferred embodiment of the present invention, the preparation method further includes: cooling, collecting, washing, and drying.

[0048] In a preferred embodiment of the present invention, the washing conditions include washing the product 3-6 times with water and ethanol respectively.

[0049] In a preferred embodiment of the present invention, the drying conditions include: a temperature of 60-80°C and a time of 4-6 hours.

[0050] The present invention will be described in detail below through examples. Unless otherwise specified, all pharmaceutical products used in the following examples are conventional commercially available products.

[0051] Preparation Example 1

[0052] Add 50 mg (0.143 mmol) of bismuth oxalate nanosheets and 0.1 mmol of Bi(NO3)3·5H2O to 15 mL of ethylene glycol solution and stir until homogeneous to obtain mixture A;

[0053] 0.1 mmol (NH4)6Mo7O 24· 4H2O was dissolved in 15 mL of deionized water and then added dropwise to the above mixture A. After mixing and stirring for 30 min, the mixture was transferred to a polytetrafluoroethylene reactor and the reaction temperature was controlled at 160 °C for 12 hours.

[0054] After natural cooling, the product was collected, washed 2-3 times with deionized water and ethanol, and dried in an oven at 60℃ for 5 hours to prepare bismuth molybdate nanorings, denoted as A1.

[0055] Preparation Example 2

[0056] The preparation method was followed as in Example 1, except that 0.1 mmol (NH4)6Mo7O was used. 24 Replace "·4H2O" with "0.2mmol(NH4)6Mo7O". 24 Bismuth molybdate nanosheets were prepared by adding 4H2O, denoted as A2.

[0057] Depend on Figure 1 a is an SEM image of the bismuth molybdate nanorings prepared in Preparation Example 1, and b is an SEM image of the bismuth molybdate nanosheets prepared in Preparation Example 2. It can be seen that the product prepared in Preparation Example 1 is in the shape of a nanoring, and the product prepared in Preparation Example 2 is in the shape of a nanosheet.

[0058] Depend on Figure 2 The XRD spectra of the bismuth molybdate nanorings prepared in Preparation Example 1 and the bismuth molybdate nanosheets prepared in Preparation Example 2 show that the synthesized samples are orthorhombic Bi2MoO6 (JCPDS No. 21-0102) nanorings and Bi2MoO6 nanosheets, and no other impurity peaks were detected.

[0059] from Figure 3 The TEM image of the bismuth molybdate nanorings prepared in Preparation Example 1 shows that the Bi₂MoO₆ product prepared in Preparation Example 1 has a nanoring structure; from Figure 3 b. HRTEM images of the bismuth molybdate nanorings prepared in Example 1 show that the nanorings have clear lattice fringes, and the lattice spacing of 0.275 nm belongs to the Bi₂MoO₆(200) crystal plane; Figure 3 The selected area electron diffraction results are as follows: Figure 3 As shown in c, the neat diffraction spots indicate that the synthesized nanorings are single-crystal structures. Figure 3 The crystal plane model diagram of the bismuth molybdate nanorings prepared in Example 1 shows that Bi2MoO6 exposes the (010) crystal plane.

[0060] Depend on Figure 13 The diffuse reflectance spectra (DRS) of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 show that both the bismuth molybdate nanorings and the bismuth molybdate nanosheets exhibit visible light absorption.

[0061] Depend on Figure 14 The (αhν) of bismuth molybdate nanorings prepared in Preparation Example 1 and bismuth molybdate nanosheets prepared in Preparation Example 2. 1 / 2 As can be seen from the hν curve, according to αhν=A(hν-Eg) n / 2 The equations estimate the band gaps (ΔEg) of Bi₂MoO₆ nanorings and nanosheets to be 2.28 eV and 2.51 eV, respectively.

[0062] Depend on Figure 15 From the Mott-Schottky curves of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2, it can be seen that the slope of their Mott-Schottky curves is positive, which suggests that both the Bi₂MoO₆ nanorings and nanosheets are n-type semiconductors. By simulating the intersection of the slope line and the X-axis, the flat-band potential E of the Bi₂MoO₆ nanorings and nanosheets was obtained. f (Relative to the saturated calomel electrode) are -0.38 eV and -0.50 eV respectively. The vacuum hydrogen electrode is 0.24 eV higher than the calomel electrode, so the flat-band potential E f (Relative to the standard hydrogen electrode) -0.14 and -0.26 eV, respectively.

[0063] The corresponding maximum value of the price band (E) VB ) and conduction band minimum (E) CB It can be calculated using the following formula:

[0064] Ev = E f +0.2 (for p-type semiconductors)

[0065] Ec = E f -0.2 (for n-type semiconductors)

[0066] Therefore, the E of Bi2MoO6 nanorings and nanosheets CB The values ​​are -0.34 eV and -0.46 eV, respectively.

[0067] like Figure 16 The band structure diagrams of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 are shown below. According to formula E... VB =Eg+E CB The E values ​​of nanorings and nanosheets were calculated. VB The values ​​are 2.14 and 2.05 eV, respectively.

[0068] The CB potential (-0.34 eV) of Bi₂MoO₆ nanorings relative to E 0 (O2 / ·O2 - When the photogenerated electrons are negative (-0.33 eV vs. NHE), they are excited into the conduction band and then reduce the O2 adsorbed on their surface to ·O2 through a single-electron reduction reaction. - Then O2- Cr(VI) can be reduced to Cr(III) via the following pathway: Cr(VI) + O2 - (or e) - )→Cr(III)+O2. Therefore, ·O2 - E and e are the main active species for the photocatalytic reduction of Cr(VI), which is consistent with the results of free radical capture experiments.

[0069] Bi₂MoO₆ nanorings exhibit higher photocatalytic activity than Bi₂MoO₆ nanosheets, primarily due to their unique nanoring structure. This structure significantly shortens the distance photogenerated carriers travel to the active sites on the catalyst surface, greatly reducing the recombination probability during migration and increasing the photogenerated current density per unit time, thereby effectively enhancing catalytic activity. Simultaneously, the narrower band gap allows the nanorings to absorb more visible light, exciting more photogenerated electrons and increasing the photocurrent density per unit time, thus improving photocatalytic activity.

[0070] Depend on Figure 17 The electron paramagnetic resonance (EPR) spectrum of the bismuth molybdate nanorings prepared in Example 1 showed that no obvious ·OH and ·O2 were detected in the dark. - The signal was detected, but a stronger O2 signal was observed after 10 minutes of light irradiation. - The characteristic signal and weak ·OH signal peak indicate that light is required to excite electrons from the valence band to the conduction band, where they combine with O2 adsorbed on the catalyst surface to form ·O2. - Free radicals. Simultaneously, photogenerated holes react with surface-adsorbed water to generate ·OH free radicals.

[0071] The photoelectric properties of Bi₂MoO₆ nanorings and nanosheets were analyzed, and the results are as follows: Figure 18-20 As shown.

[0072] Depend on Figure 18 The photocurrent densities of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 show that the photocurrent density of the Bi2MoO6 nanorings is significantly greater than that of the nanosheets, indicating that the Bi2MoO6 nanorings can generate more photogenerated carriers under light irradiation.

[0073] Depend on Figure 19 The room temperature fluorescence spectra (λ) of bismuth molybdate nanorings prepared in Example 1 and bismuth molybdate nanosheets prepared in Example 2 are shown. ex As can be seen from the photoluminescence emission intensity of Bi2MoO6 nanorings (375nm), the photoluminescence emission intensity of Bi2MoO6 nanorings is lower than that of Bi2MoO6 nanosheets, indicating that the recombination rate of photogenerated carriers in Bi2MoO6 nanorings is low.

[0074] Figure 20 Electrochemical impedance spectroscopy (EICs) of the bismuth molybdate nanorings prepared in Example 1 and the bismuth molybdate nanosheets prepared in Example 2 show that the Bi₂MoO₆ nanorings have very small arc radii, implying low interfacial resistance during photogenerated carrier transport, allowing for rapid separation of photogenerated electrons and holes. Therefore, the Bi₂MoO₆ nanorings exhibit excellent photocatalytic activity. These photoelectric performance results fully corroborate our hypothesis regarding the excellent photocatalytic performance of the Bi₂MoO₆ nanorings.

[0075] Example 1

[0076] At room temperature (25°C), 30 mg of bismuth molybdate nanorings prepared in Preparation Example 1 were dispersed in 30 mL of Cr(VI) solution (100 mg / L), and an appropriate amount of citric acid was added to adjust the pH of the solution to 3. The suspension was stirred for 50 min in the dark to ensure the adsorption-desorption balance between the solution and the photocatalyst.

[0077] Irradiate the light source (using a 300W Xe lamp), take out an appropriate amount of suspension every 10 minutes and centrifuge, and measure the ultraviolet-visible absorption spectrum of the supernatant.

[0078] Example 2

[0079] The procedure was carried out according to Example 1, except that “Cr(VI) solution (100 mg / L)” in Example 1 was replaced with “Cr(VI) solution (150 mg / L)”, while other conditions remained unchanged.

[0080] Example 3

[0081] The procedure was carried out according to Example 1, except that “Cr(VI) solution (100 mg / L)” in Example 1 was replaced with “Cr(VI) solution (200 mg / L)”, while other conditions remained unchanged.

[0082] Example 4

[0083] The procedure was carried out according to Example 1, except that “pH is 3” in Example 1 was replaced with “pH is 1”, while other conditions remained the same.

[0084] Example 5

[0085] The procedure was carried out according to Example 1, except that “30 mg of bismuth molybdate nanorings prepared in Example 1” was replaced with “10 mg of bismuth molybdate nanorings prepared in Example 1”, while other conditions remained unchanged.

[0086] Example 6

[0087] The procedure was carried out according to Example 1, except that “30 mg of bismuth molybdate nanorings prepared in Example 1” was replaced with “20 mg of bismuth molybdate nanorings prepared in Example 1”, while other conditions remained unchanged.

[0088] Example 7

[0089] The procedure was followed as in Example 1, except that different free radical scavengers were added to the Cr(VI) solution to conduct free radical scavenging experiments, and the results are as follows: Figure 9 As shown;

[0090] Among them, tert-butanol (TBA), ammonium oxalate (AO), potassium bromate (KBrO3), and 1,4-benzoquinone (BQ) are used as hydroxyl radicals (·OH) and holes (h + ), electron (e - ) and superoxide radicals (·O 2- ) capture agent.

[0091] Depend on Figure 9 It was found that the addition of 1,4-benzoquinone (BQ) and potassium bromate (KBrO3) significantly inhibited the photoreduction of Cr(VI), while the addition of tert-butanol (TBA) and ammonium oxalate (AO) had no effect. This demonstrates that the active species playing a role in the photocatalytic reduction of Cr(VI) is superoxide radical (·O). 2- ) and electrons (e - ).

[0092] Example 8

[0093] The method was followed as in Example 1, except that the same set of bismuth molybdate nanorings was used for 5 cycles of the experiment. The experimental results are shown in [Figure 1]. Figure 10-12 .

[0094] Depend on Figure 10 It can be seen that the bismuth molybdate nanorings prepared in Preparation Example 1 still have good photocatalytic activity after 5 cycles of use.

[0095] Depend on Figure 11-12 It can be seen that after five cycles of use, the bismuth molybdate nanorings prepared in Preparation Example 1 not only still have a nanoring morphology, but also their XRD characteristic diffraction has not changed, indicating that the bismuth molybdate nanorings have good photostability and reusability.

[0096] Comparative Example 1

[0097] The procedure was carried out according to Example 1, except that “30 mg of bismuth molybdate nanorings prepared in Example 1” was replaced with “30 mg of bismuth molybdate nanosheets prepared in Example 2”, while other conditions remained unchanged.

[0098] Comparative Example 2

[0099] The procedure was carried out according to Example 1, except that “pH 3” in Example 1 was replaced with “pH 5”, while other conditions remained the same.

[0100] Comparative Example 3

[0101] The procedure was carried out according to Example 1, except that “pH 3” in Example 1 was replaced with “pH 7”, while other conditions remained the same.

[0102] Comparative Example 4

[0103] The procedure was carried out according to Example 1, except that “30 mg of bismuth molybdate nanorings prepared in Example 1” was replaced with other bismuth molybdate-based photocatalysts, while other conditions remained unchanged. The specific replacement conditions are shown in Table 1.

[0104] Table 1

[0105] Table 1. Degradation of Cr(VI) by different Bi2MoO6-based photocatalysts

[0106]

[0107]

[0108]

[0109] As can be seen from the data in Table 1 and the experimental results of Example 1, by comparing the degradation rates of Cr(VI) by different Bi2MoO6-based photocatalysts reported previously, it is fully demonstrated that Bi2MoO6 nanorings have excellent performance in degrading Cr(VI).

[0110] Table 2. Variables and experimental results of Examples 1-6 and Comparative Examples 1-4

[0111]

[0112]

[0113] Depend on Figure 4 The reduction curves of Cr(VI) catalyzed by the bismuth molybdate nanorings prepared in Example 1 and the reduction curve of Cr(VI) catalyzed by the bismuth molybdate nanosheets prepared in Comparative Example 1 in Example 2 show that the Bi₂MoO₆ nanorings exhibit excellent photocatalytic performance, completely reducing 100 mg / L Cr(VI) within 30 minutes, while the Bi₂MoO₆ nanosheets only reduce 68% of the Cr(VI) (100 mg / L) solution. This indicates that the photocatalytic performance of the nanorings is significantly better than that of the nanosheets.

[0114] Depend on Figure 5The quasi-first-order kinetic model fitting plot of the bismuth molybdate nanorings prepared in Example 1 for the catalytic reduction of Cr(VI) (100 mg / L) shows that the apparent rate constant k is calculated from the slope of the -ln(C / C0)vs t curve, and the k value of the Bi2MoO6 nanorings is 0.0752 min. -1 It is 3.7 times larger than Bi2MoO6 nanosheets.

[0115] Depend on Figure 6 The degradation curves of bismuth molybdate nanorings prepared in Example 1-3 using bismuth molybdate nanorings catalyzed by the reduction of different concentrations of Cr(VI) show that after 50 minutes of illumination, the removal rate of 150 mg / L Cr(VI) solution reached 96%, and the removal rate of 200 mg / L Cr(VI) solution reached 87% within 60 minutes.

[0116] Depend on Figure 7 The degradation curves of Cr(VI) (100 mg / L) in Examples 1, 4, and Comparative Examples 2-3 show that Cr(VI) is difficult to reduce under neutral conditions (pH = 7), but is easily reduced under acidic conditions. The photoreduction time of Cr(VI) gradually shortens as the pH decreases. The reduction product of Cr(VI) is Cr(III). When the pH is high, Cr(III) exists in the form of Cr(OH)3, which adsorbs onto the surface of the photocatalyst, reducing its catalytic activity. When the pH decreases, the acidity of the solution increases, and Cr(III) exists in the solution in ionic form. Therefore, the photocatalytic performance of the photocatalyst is improved.

[0117] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0118] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0119] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for photocatalytic reduction of hexavalent chromium, characterized in that, The method includes: mixing the photocatalyst with a hexavalent chromium solution, adjusting the pH to 1-7, stirring, and irradiating with light; The photocatalyst is a bismuth molybdate nanoring; The mass concentration ratio of the photocatalyst to the hexavalent chromium solution is 1:100-200; Adjust the pH to 1-7 using citric acid and / or sulfuric acid; The stirring conditions include: a dark environment, a time of 40-60 minutes, and a speed of 500-800 rpm; The lighting conditions include: a light intensity of 80-100 mW / cm². 2 The time is 20-30 minutes; The light source is a xenon lamp and / or sunlight; The method for preparing the bismuth molybdate nanorings includes: In the presence of a solvent, the precursor and bismuth source are first mixed, and ammonium molybdate is added for a second mixing, followed by reaction. The precursor is bismuth oxalate nanosheets.

2. The method according to claim 1, characterized in that, The molar ratio of the precursor to the bismuth source is 1:0-1.2, wherein the bismuth source is calculated in terms of bismuth atoms; The bismuth source is bismuth nitrate and / or bismuth chloride.

3. The method according to claim 1 or 2, characterized in that, The solvent is selected from one or more of ethylene glycol, ethanol, and water.

4. The method according to claim 1, characterized in that, The concentration of the ammonium molybdate is 0.001-0.005 mol / L.

5. The method according to claim 1, characterized in that, The conditions for the second mixing include: time 10-30 min, rotation speed 100-500 rpm.

6. The method according to claim 1, characterized in that, The reaction conditions for preparing the bismuth molybdate nanorings include: a temperature of 140-160℃ and a time of 3-12h.

7. The method according to claim 1, characterized in that, The preparation method of the bismuth molybdate nanorings also includes: cooling, collecting, washing, and drying.

8. The method according to claim 7, characterized in that, The washing conditions include washing the product 3-6 times with water and ethanol respectively.

9. The method according to claim 7, characterized in that, The drying conditions include: temperature 60-80℃, time 4-6h.

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