Supported photocatalyst for Cr (VI) conversion and removal of organic pollutants

doi:10.21203/rs.3.rs-2088191/v1

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Supported photocatalyst for Cr (VI) conversion and removal of organic pollutants

https://doi.org/10.21203/rs.3.rs-2088191/v1

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The photocatalytic property of available semiconductor catalysts still suffer from some urgent problems, such as the high excitation energy, easy agglomeration of powders, and or the weak recycling property. Therefore, developing the novel visible light supported catalysts and catalyst loading have aroused great attention recently. BiVO₄, as a hot-spot semiconductor photocatalytic material, bears the advantages of strong visible light absorption capacity, narrow band gap and fast carrier generation rate. However, the carrier recombination rate is fast and the efficiency is low, so it needs to be modified before its use. In this study, a novel BiVO₄/Ag₃PO₄/MWCNTs @Cotton functional fabric was prepared by introducing Ag₃PO₄ as plasma resonance photocatalyst and MWCNTs and cotton as composite substrates. The results showed that BiVO₄/Ag₃PO₄/MWCNTs @cotton retained the high photocatalytic efficiency of the powder catalyst, along with the degradation degree of active blue KN-R as well as Cr (VI) could reach 92% within 120 min, and it could be reused for 5 times. Free radical scavenging experiments showed that functional fabrics could produce active substances such as h⁺,·O₂^- and·OH. The introduction of Ag₃PO₄ and MWCNTs effectively improved the application ability of BiVO₄, inhibited the recombination of carriers and promoted the transport of carriers. The research can provide some study directions for the development of photocatalytic technology in the future.

BiVO4/Ag3PO4/MWCNTs

Composite catalytic cotton fabric

dye degradation

impregnation

cycle performance

With the continuous progress of industrial production, a series of pollution problems have emerged need to be handled. Water pollution is one of the most serious factors restricting the sustainable and healthy development of industrial production. Industrial wastewater discharge is large and contains complex pollutants, including metal ions, dyes, various nitrogen compounds and other pollution waste^[1]. The long-term circulation of these pollutants in water body will not only cause irreversible loss of water resources, but also cause great harm to the sustainable development of ecological environment and people’ s health (recalcitrant and refractory). Among many treatment methods, semiconductor photocatalytic technology has become a hot choice for wastewater treatment due to its simple process, green and efficient process, and low secondary pollution^[2-4]. However, most of the available semiconductor photocatalyst is generally powder state which owns the problems of easy agglomeration and difficulty in recovery^[5]. In addition, the efficiency and cost problems have also become important factors that limit its development^{[6, 7]}.Therefore, it is of great significance to develop low-cost, high-efficiency supported photocatalysts that can be recycled.

In order to obtain a supported type with excellent performance, it is first necessary to design and prepare an efficient photocatalyst, followed by the combination of the catalyst and the support. Among all the photocatalyst at present, TiO₂ is the most commonly used^[8]. However, due to the relatively high band gap of TiO₂, the excitation energy required is relatively high, which is usually only excited by ultraviolet light in the sunlight. This reaction condition is relatively special and cannot be applied in large scale^{[6, 9]}. Therefore, we have to study other visible light responsive catalysts^[10]. As a new photocatalyst, BiVO₄has the following crystal types: tetragonal scheelite, tetragonal zircon (3.1eV) and monoclinic scheelite (2.4eV). Among all crystal forms, monoclinic scheelite has a narrow band gap and is easy to be excited, so it is widely used^[11-13]. BiVO₄ has a high degree of response under visible light catalysis, and the carrier separation efficiency is relatively high under the response conditions, so it has gradually become a research hotspot in many research projects^[14]. In addition, BiVO₄has a wide range of natural sources, relatively low toxicity to the environment and relatively good stability in water. However, the carriers of BiVO₄ are extremely easy to combine after they are generated, so the quantum efficiency is relatively low and has a certain degree of photo corrosion. Therefore, in recent years, the research on BiVO₄ mainly focuses on inhibiting its recombination efficiency and improving its optical response range^[15]. The introduction of some metal or nonmetal particles is one of the important ways to prepare modified BiVO₄^[16]. By introducing a small amount of Pt element, Wang et al. effectively prevented the charge recombination caused by excessive BiVO₄ defects, and the sample could maintain stability for 50 h at 0.8VRHE potential^[17]. As a non-noble metal, Bi has direct plasma photocatalytic activity. Fang^[18] loaded fine Bi nanoparticles on BiVO₄ by electrodeposition process, and established the Bi-BiVO₄heterojunction structure, which delayed the recombination of photogenerated charge pairs. The results showed that the degradation effect of the sample on phenol was obvious, and the hydrogen production rate was effectively improved. In addition, the combination of two semiconductor catalysts to form a composite catalytic heterojunction is another method for BiVO₄ modification. Liang et al. ^[19]successfully prepared Bi₂S₃ - BiVO₄ Z-type heterojunction aerogels under light and applied them to the degradation of Cr ( VI ) and bisphenol A in water. The process can be cycled for five times and the removal rates are maintained at more than 85 %. Academician Li Can group ^[20]has recently studied the dual-assisted method to further improve the performance of BiVO₄. They modified BiVO4 by selective photodeposition of dual-cocatalysts (metal Ir and FeOOH and CoCOOH composites (FeCoOx) to improve the water oxidation ability of BiVO₄. Finally, relevant studies^[21] show that the relative exposure of different crystal planes of BiVO₄ also has a certain impact on the surface charge distribution and photocatalytic activity. Ag₃PO₄ (with a forbidden band width of 2.4 eV) is a new type of photocatalyst whose catalytic properties were first discovered in 2010. Ag₃PO₄ has a super high quantum yield, and its actual catalytic activity even far exceeds that of other narrow-series semiconductors (CdS, CdSe). In addition, the study found that the charge distribution of Ag₃PO₄ is relatively dispersed around its conduction band, which makes the actual mass of the electrons relatively small, and the difficulty of transition is greatly reduced. However, due to the poor stability of Ag₃PO₄ itself and the easy formation of Ag elemental deposition on the surface after a period of time, which affects the catalytic activity, Ag₃PO₄ is generally not used alone, but is used as a plasmon resonance catalyst mixed with other semiconductors^[22]. Aiming at the photocorrosion of Ag₃PO₄, Yang ^[23]et al. proposed to effectively composite Ag₃PO₄ with a new two-dimensional layered nanomaterial titanium carbide (MXene) with high electrical conductivity to controllably construct a zero-dimensional silver phosphate/two-dimensional MXene nanocomposite photocatalytic material system. The efficiency of photocatalytic water splitting to produce oxygen under the excitation of different LED light sources was systematically investigated, and the stability of the monomer catalyst was finally improved. Gao^[24] constructed a BiVO₄/Ag₃PO₄ heterojunction on the electron-dominated and hole-dominated crystal planes of BiVO₄ by different deposition methods, respectively. The results show that the performance of the monomer BiVO₄ is significantly improved, because the hole-dominated plane is more than the electron-dominated plane. The surface can further promote the separation of photogenerated holes and electrons. MWCNTs have been widely used due to their unique one-dimensional structure, large specific surface area, superior mechanical properties, and high chemical and thermal stability^[25]. Studies have shown that carbon nanotubes can form heterojunctions with semiconductor photocatalysts, and the recombined photoelectrons can rapidly propagate along one-dimensional directions, reducing the inhibitory effect of photogenerated potential on carrier separation. In addition, MWCNTs can be regarded as a narrow-series semiconductor and can be used as an effective photosensitizer to improve the light absorption range of materials^[26]. In terms of pollutant degradation, due to the advantage of large specific surface area, MWCNTs can enrich pollutants, so that the degradants can fully contact the photocatalyst material, thereby increasing the degradation rate and reducing secondary pollution^{[26, 27]}.

In this study, BiVO₄was selected as the basic visible light catalyst, and Ag₃PO₄ was used as the semiconductor plasmon resonance catalyst, which was attached to the surface of BiVO₄ by deposition method^[28]. In addition, MWCNTs were selected and combined with BiVO₄ by hydrothermal method. Taking advantage of the structural advantages of Ag₃PO₄ and a novel heterojunction BiVO₄/Ag₃PO₄/MWCNTs was constructed to promote the transport of carriers, improve the recombination of carriers, and improve the visible light catalytic performance of BiVO₄. Since the powder catalyst has defects such as easy agglomeration in water and difficult to recycle, consider selecting cotton fiber with good physical adsorption capacity with BiVO₄ as the load^{[12, 29-32]}, plus MWCNTs and the hydroxyl group of cotton fiber can flexibly act, and finally Synthesis of BiVO₄/Ag₃PO₄/MWCNTs @Cotton functional cotton fabric retains the catalytic performance of powder catalysts and relieves its application pressure^{[4, 33]}. The research results can also provide new ideas and new methods for water pollution treatment methods at this stage^[33].

2.1 Materials

Bismuth nitrate pentahydrate(Bi(NO₃)₃·5H₂O), ammonium metavanadate༈NH₄VO₃༉, silver nitrate༈AgNO₃༉, disodium hydrogen phosphate༈Na₂HPO₄༉, multi walled carbon nanotubes ༈MWCNTs༉, anhydrous ethanol (C₂H₅OH), sodium hydroxide ༈NaOH༉, nitric acid ༈HNO₃༉, sodium dodecyl sulfate༈C₁₂H₂₅SO₄Na༉, reactive brilliant blue KN-R༈C₂₂H₁₆N₂NaO₁₁S₃༉hydrochloric acid ༈HCl༉hydrogen peroxide solution ༈H₂O₂༉and sodium chloride༈NaCl༉were generally purchased from Sinopharm group. Pure cotton woven fabric pure cotton plain woven fabric (Specification: transverse density: 81 longitudinal rows /5cm longitudinal density: 57 transverse rows / 5 cm) were purchased from Shanghai sanqiang Group Co, Ltd.

2.2.1 Preparation of BiVO₄/ MWCNTs

0.970g Bi (NO₃) ₃ · 5H₂O was dissolved into 5ml of HNO₃ with the concentration of 4mol/L. Then, 0.1g (0.2% MWCNTs, 0.1% PVP) solution was added and mixed evenly (named A after magnetic stirring for 30min); Then, 0.234g NH₄VO₃ was dispersed in 20 mL of NaOH with the concentration of 1mol/L and stirred magnetically for 30min (named B); The solution in B was slowly added to A, the pH value of the solution was adjusted according to different requirements, continuing to magnetically stir for 30 min (named C). The solution in C was transferred to the autoclave, and the oven temperature was adjusted at 160 ℃ as well as the reaction time was 6h (named D). Finally, the solution in D was filtered by suction after cooling to room temperature, and the filter residue was dried at 60 ℃ for 4h to obtain yellow green powder BiVO₄/MWCNTs. In addition, BiVO₄ can be prepared by repeating the above operation without adding MWCNTs.

2.2.2 Preparation of BiVO₄/Ag₃PO₄/MWCNTs

0.4g BiVO₄/MWCNTs powder prepared in the second step was weighed and dispersed with 30ml absolute ethanol ( named A after magnetic stirring for 30min); 0.268g AgNO₃ was dispersed in 4ml ammonia solution (named B); The solution in A was continuously stirred and added into B (named C); 4ml of Na₂HPO₄ with the concentration of 0.12mol/L was added dropwise to C and stirred magnetically for 1h(named D); The obtained D solution was allowed to stand for 2h, centrifuged, washed and dried at 60 ℃ for 4H to obtain yellowish brown powder BiVO₄/Ag₃PO₄/MWCNTs.

2.2.3 Preparation of BiVO₄ / Ag₃PO₄ / MWCNTs @ Cotton

(1) Pure cotton woven fabric was cut into 6 cm × 6 cm experimental cloth sample (about 0.5g) and put into the prepared acetone ethanol solution (10ml: 10ml) with volume ratio of 1:1 in the beaker. Then, it was transferred to the ultrasonic instrument for cleaning for 30min and deionized water was used for ultrasonic cleaning for 20min. Finally, samples were dried at the temperature of 60 ℃.

(2) As shown in Fig. 1, BiVO₄ and BiVO₄ / MWCNTs prepared at pH = 3 with the mass ratio of Ag₃PO₄ to BiVO₄/MWCNTs was controlled to be 1:2 was selected. 0.150g of dried catalyst was weighed into a beaker and 100ml deionized water mixed with 0.015g PVP was added as well. Then, it was dispersed by ultrasonic for 15min and transferred to a magnetic stirrer. The cotton cloth treated in (1) was put into the beaker and heated to 60 ℃ and taken out after constant temperature reaction for 160 min. the removed cloth was washed with clean water and dried it in an oven at 60 ℃ for 20min as well as transferred to an oven at 165 ℃ for baking for 15 min to obtain BiVO₄/Ag₃PO₄/MWCNTs @Cotton composite photocatalytic fabric.

2.2.4 Analysis of materials

X-ray diffractometer (D / max-2550pc, RIGAKU company of Japan) was used to analyze the crystal structure of the composite catalyst with the speed of 0.02°/0.06s; Nitrogen adsorption analysis (BET) was used to determine the specific surface area of the sample and its adsorption effect; Ultraviolet visible infrared spectrometer (UV3600, America PerkinElmer) was performed to measure the absorbance spectrum of the composite catalyst at the wavelength of 200-800nm to determine the response degree and band gap width. Scanning electron microscopy (jsm-5600lv, Japan Electronics Co., Ltd.) was used to characterize the surface morphology of composite catalyst and functional cotton fabric.

2.2.5 Photodegradation experiment

The catalytic performance of the catalyst was characterized by the degradation of reactive blue KN-R dye solution and the removal of Cr (VI). The specific steps are as follows: 0.05g of different samples was selected and put into a clean degradation test tube. 50mL reactive blue KN-R dye solution with the concentration of 50 mg/L (50mL of 20mg/L Cr (VI)) was put into the reaction tank for photochemical reaction and react for 20min without turning on the light. After the reaction, a syringe was used to absorb the appropriate dye solution and a filter to transfer 3-4mL of the dye solution to the cuvette for measuring absorbance and pour the dye solution back after recording the data. The xenon lamp, refrigeration, circulation and electric fan were turned on to repeat the above-mentioned absorbance measurement operation every 10 min and end the reaction after 60 min of photoreaction. Decolorization rate (%) = (A₀-A)/A₀) × 100% was used to summarize the recorded absorbance data (where A₀ is the initial absorbance of the dye solution, and A is the absorbance of the dye solution measured at different time periods). Besides, the actual catalytic effect measurement method for functional cotton fabrics is similar.

After determining the optimal preparation conditions, the dark reaction time of the catalyst was set to 20 min, and the removal effects of catalysts with different composite degrees on dye liquor and Cr (VI) were measured every 20 min until the cut-off time of 100 min as well as the catalytic performance. Different catalysts are compared graphically.

3.1 Basic characterization of BiVO₄/MWCNTs

In this study, the preparation of BiVO₄/MWCNTs is given priority, and then Ag₃PO₄ is deposited on the body and loaded onto cotton fiber. Therefore, the preparation conditions of double composite catalyst are particularly important. The nitrogen adsorption capacity, UV-visible diffuse reflectance spectrum and crystal structure of BiVO₄ / MWCNTs were characterized in Fig. 2. As shown in Fig. 2A, the specific surface area is 1.7361 m²/g when pH = 1. When pH = 3, the specific surface area is 7.8932 m²/g. The specific surface area is 11.3093 m²/g when = 5. When pH = 7, the specific surface area is 5.7543m²/g. When pH = 9, the value of specific surface area is 3.1932m²/g. In the dark reaction stage, the adsorption effect of samples at pH = 5, pH = 7 and pH = 9 is relatively prominent due to the existence of MWCNTs in the form of free in the preparation process (black spots in the prepared powder part), which improves the adsorption capacity of the sample. However, in the photoreaction stage, free MWCNTs could not promote catalysis, resulting in the degradation effect of relatively good combined samples (prepared at pH = 3) under the same conditions, and the absorbance of the dye solution decreased more obviously.

As shown in Fig. 2B, the absorbance of the double composite catalyst is better in the ultraviolet region and lower in the visible region. With the change of doping degree of MWCNTs themselves, the response degree to light varies in the measured wavelength range. At pH = 3, the doping degree of MWCNTs and BiVO₄ is relatively high, so the absorbance of catalyst is relatively good in both ultraviolet and visible regions, which is consistent with the results of dye degradation test. As shown in Fig. 2C, When the 2θ Angle was 18.5°, 35°, 46°, all the diffraction peaks of BiVO₄/MWCNTs were split peaks, while when the 2θ Angle was 28.6°, 30.5°, 55.6°, these peaks were characteristic peaks of monoclinic BiVO₄ system compared with JCPDS. When pH = 3 and pH = 5, the single peak diffraction peak occurs at the 2θ Angle of 26.4°. Compared with the standard diffraction card, it is found that the peak belongs to the (002) crystal plane of carbon, indicating the successful composite of MWCNTs.

3.2 Surface characteristics of materials

As shown in Fig. 3, the surface morphologies of the powdered catalysts as well as the functional fabrics were specifically characterized. From Fig. 3A and Fig. 3B, it can be seen that BiVO₄/MWCNTs forms a natural loading platform, and silver phosphate molecules grow freely on the surface of this platform in a dendron, which proves the successful preparation of the powder catalyst. The powder catalyst was successfully supported on the surface of waste cotton fabric by the water bath dipping method. Figures 3CD, EF, and GH show the loading of the powder at different magnifications of X 500, X 1200, and X 3000, respectively. It can be seen that the powder catalyst can achieve different degrees of distribution on the surface of the cotton fabric.

XPS can be used to characterize the elemental composition and element valence state of composite catalysts. Figure 4 is the full spectrum of the XPS survey of the samples. The figure illustrates that the prepared composite catalyst contains basic elements such as Bi, V, O, Ag, P, and C, which preliminarily verifies the successful composite BiVO₄/Ag₃PO₄/MWCNTs. In addition, the high-resolution XPS spectrum are shown in Fig. 5. Fig A is the characterization of the Bi element in the composite catalyst, from which it can be seen that its binding energies are at 158.4 eV and 164.5 eV, respectively, which are due to Bi 4f_7/2 and formed by the presence of Bi 4f_5/2. The results of this spectrum show that the valence state of Bi element in the composite catalyst is + 3^[34]. In addition, Figs. 5B and 5C further verify the existence of V-O bonds in the composite catalysts. According to their binding energy assignments, it can be shown that V and O in the composite catalysts exist in the form of VO₄^3–[35]. The binding energy peak of C element in Fig. 5F. This is due to the presence of C-C bonds in the sample^{[36, 37]}. Therefore, Figs. 5A, 5B, 5C and 5F fully demonstrate the existence of BiVO₄ and MWCNTs in the samples, and the bi-composite catalyst BiVO₄/MWCNTs was successfully prepared^{[38, 39]}. The deposition of Ag₃PO₄ can be seen in Figs. 5D and 5E. Figure 5D is the characterization of Ag element in the sample, and its peak binding energies are at 368.4eV and 374.2eV, respectively, which proves that the valence of Ag element in the composite catalyst is + 1, Fig. 5E is the characterization of P 2P, the peak energy is 133.4eV verification the existence of P-O^{[40, 41]}. In conclusion, XPS characterization shows the existence of each element in the composite catalyst and its valence form, which can further confirm the successful preparation of BiVO₄/Ag₃PO₄/MWCNTs^[41].

3.3 Degradation analysis of dyes and heavy metal ions

The degradation curves of BiVO₄, BiVO₄ / MWCNTs, BiVO₄ / Ag₃PO₄ / MWCNTs, and BiVO₄ / Ag₃PO₄ / MWCNTs @ Cotton for reactive blue KN-R under different preparation conditions were shown, respectively(Fig. 6A-6C), to determine the optimal preparation conditions of composite catalytic cotton fabrics. The experimental results showed that the composite catalyst could be effectively loaded on the cotton fiber when the pH value was 3, the reaction time was 6 h, the system temperature was 160°C, and the mass ratio of Ag₃PO₄: BiVO₄/MWCNTs was 1: 2. What is more, the decolorization rate of the dye reached more than 90% within 60 min, indicating the high efficiency of BiVO₄/Ag₃PO /MWCNTs. Figure 6D is a further verification of the photodegradation performance of the final functional fabric. By comparing the degradation curve of the reaction without light for a long time and under the condition of light, it is determined that the catalytic degradation performance of functional cotton fabrics is mainly due to photocatalysis, rather than the pseudo-degradation caused by physical adsorption. Under the optimal preparation conditions of catalysts, this study characterizes the catalytic performance of different catalysts under the same degradation environment. Figure 6E shows the degradation of reactive blue KN-R. The results show that with the increase of composite components, the photodegradation efficiency of the catalyst for dye liquor gradually increases, and the BiVO₄/Ag₃PO₄/MWCNTs @Cotton functional cotton fabric retains three the decolorization effect of the composite component catalyst is that the decolorization rate of the dye solution is 92% within 100 minutes; Fig. 6F shows the degradation of Cr (VI). With the increase of the composite component, the final content of Cr (VI) gradually decreases, the degradation rate is increased and the results show that the removal rate of Cr (VI) by BiVO₄/Ag₃PO₄/MWCNTs @COTTON functional cotton fabric can reach 90%. This shows that the functional cotton fabric can solve the problem of easy agglomeration of powder catalysts, realize catalyst loading and retain the catalytic performance of powder catalysts. Finally, Fig. 6G and 6H are the before and after comparison of reactive blue KN-R and Cr conversion.

As shown in Fig. 7, Fig. 7A – Fig. 7D simulate the degradation ability of composite catalytic cotton fabric in different environments by changing the inorganic salt concentration, dye concentration, dye pH value and surfactant concentration of the degraded dye solution, and verifies its possibility in wastewater treatment. The results show that the inorganic salts and the concentration of surfactants have the effect of promoting first and then inhibiting the catalytic effect of functional cotton fabrics, which is considered to be related to the dyeing promotion of dyes and the critical micelle concentration of surfactants. The effect of dye liquor concentration and pH value on the decolorization rate is basically negatively correlated. That is, the higher the concentration, the greater the pH value, and the lower the catalytic effect, which may be related to the concentration of the catalyst itself and acid and alkali resistance. These test results can provide theoretical reference for the practical application of composite catalysts.

The cycle performance of the composite catalyst is also the focus of this study. As shown in Fig. 8, Fig. 8A shows that the functional cotton fabric can effectively degrade reactive blue KN-R within 150 minutes, and the decolorization rate is maintained at about 90%. The degradation can be cycled for 5 times, and the degradation effect of the sixth time is also maintained at 80% %; Fig. 8B is the repeatability test of the composite catalyst for Cr (VI) removal experiment. The results show that the target fabric can achieve 5–6 cycles of Cr (VI) degradation, and the degradation effect can reach 85% within 100min above. This shows that functional cotton fabrics can alleviate the problems of powder catalyst recollection and difficult reuse, and have good versatility. As shown in Fig. 9, the experiment further characterized the morphology of the fabrics before and after cyclic degradation. From Fig. 9A and Fig. 9B, we can see that the catalyst on the surface of the fabric has a high loading rate before being degraded without cycling. Figure 9C is the surface morphology of the fabric after 5 cycles of degradation, and there is still a lot of catalyst loading. Further It proves the recycling performance of functional fabrics. Figure 9D is the macroscopic comparison before and after the degradation. Some dyes are dyed on the surface of the fabric, which is considered to be caused by the adsorption performance of the functional fabric for multiple cycles of degradation.

3.4 Simulation experiment

In order to verify the applicability of functional fabrics, combined with the dyeing principle of overflow dyeing machine, a simulation test was carried out on the sewage treatment performance of functional fabrics, as shown in the Fig. 10. Wastewater is transferred to the overflow drum under the action of the water pump, and the functional fabric is acted on by the spray device, and the wastewater is subjected to the first catalytic treatment under the action of ultraviolet rays; the initially treated wastewater is transferred to the overflow machine through the action of the transport pipe. In the process, the functional fabric rotates in a conveyor belt-like manner under the irradiation of ultraviolet light, and the wastewater is treated twice by catalysis. Finally, the wastewater after the secondary treatment is collected for subsequent dyeing and finishing.

3.5 Possible photodegradation mechanism

Photoexcited holes h⁺, radicals ·O₂⁻ and ·OH are the main active species in the degradation process. As shown in Fig. 11A, the above experimental results show that the three composite catalysts and functional fabrics prepared in this study have a good degradation effect on the removal of reactive blue KN-R and heavy metal ions. In order to further identify the active species during the degradation process, the study chose to conduct free radical scavenging experiments on the composite catalysts. Select different scavengers (including hole scavenger sodium oxalate (Na-OA), free radical ·O₂⁻ scavenger benzoquinone (BQ) and free radical ·OH scavenger tert-butanol (t-BuOH) In the degraded solution, the dye without scavenger is added as a blank sample, and different dyes are degraded respectively to determine the active substance of the reaction.

As shown in Fig. 11, the composite catalyst can achieve a decolorization rate of more than 95% for reactive blue KN-R under visible light irradiation without adding any scavenger. After adding the hole scavenger sodium oxalate (Na-OA), the decolorization rates of the functional fabric and the composite catalyst to the dye were reduced to 28% and 18%, respectively, indicating that photogenerated holes are the main active free radicals in this reaction. In addition, the addition of free radical ·O₂⁻ scavenger benzoquinone (BQ) can also reduce the decolorization effect to 1/3 of the original, indicating that ·O₂⁻ is also the main active substance in this reaction. Finally, the addition of t-BuOH did not have a great influence on the decolorization effect of the experimental samples, indicating that ·OH was not the main active substance of the system.

As shown in Fig. 11 (B), the addition of MWCNTs can increase the wave-absorbing properties of the monomer BiVO₄, whose absorption edges are at 505nm and 520 nm, respectively. The forbidden band width of the catalyst can be calculated according to the absorption edge, and the specific calculation method is as follows:

E _g= 1239.6 / λ _g (1)

E _g is the forbidden band width (band gap energy) of the sample, λ _g is the wavelength value at the intersection of the extension line of the diffuse reflection side and the horizontal axis. According to (1), the band gap energies of BiVO₄ and BiVO₄/MWCNTs can be obtained to be 2.45 eV and 2.38 eV, respectively.

As shown in Fig. 12, taking BiVO₄/MWCNTs(BM)as a whole and depositing Ag₃PO₄ on this basis is the main goal of this experiment. According to previous reports, the CB of silver phosphate is 0.29 eV, VB is 2.6 eV, and the modified BM is still an N-type semiconductor. Considering its valence band and conduction band potential, it can form a type II heterojunction with silver phosphate. Under the irradiation of visible light, both BM and Ag₃PO₄ can generate photogenerated carriers. Due to the formation of a type II heterojunction between the two, the holes in the valence band of BM can be transferred to the valence band of Ag₃PO₄, and the photogenerated electrons generated by Ag₃PO₄ are rapidly transferred to the conduction band of the BM conductor under the action of the heterojunction. The pollutant molecules can directly react with the photogenerated holes to generate CO₂ and H₂O and other harmless substances, which is the main process of the composite catalyst to degrade the dye. In addition, regarding the degradation of heavy metal ions, the transferred photogenerated electrons can be captured by heavy metal ions, and Cr (VI) is thus reduced to Cr (III). The remaining photogenerated electrons can react with reactive oxygen species in water to form ·O₂⁻, ·O₂⁻ further react with pollutants in wastewater to generate CO₂ and H₂O. The possible reaction formulas in the photocatalysis process are as follows:

$$\text{B}\text{M}+\text{h}\text{v}\to {\text{h}1}^{+}+{\text{e}1}^{-}$$

$${\text{O}\text{H}}^{-}+{\text{h}1}^{+}\to ·\text{O}\text{H}$$

$$·\text{O}\text{H}+\text{d}\text{y}\text{e}\to {\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O}$$

$${ \text{A}\text{g}}_{3}{\text{P}\text{O}}_{4}+\text{h}\text{v}\to {\text{h}2}^{+}+{\text{e}2}^{-} （5）$$

$${\text{C}\text{r}}^{6+}+{\text{e}2}^{-}\to {\text{C}\text{r}}^{3+}$$

$${ \text{h}1}^{+}\to {\text{h}2}^{+}+\text{d}\text{y}\text{e}\to {\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O} （7）$$

$${\text{e}2}^{-}\to {\text{e}1}^{-}+{\text{O}}_{2}\to {·\text{O}}_{2}^{-}+\text{d}\text{y}\text{e}\to {\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O}$$

In summary, all the research results show that the composite catalyst as well as the functional fabric have been successfully prepared. The experimental samples not only have excellent degradation performance (dye decolorization rate and heavy metal ions are all > 90%), but also can be recycled 5–6 times for repeated use, and the catalyst loading rate remains at a high level after the cycle. Mechanism studies show that the main active groups of composite catalysts and functional fabric catalysis under the irradiation of visible light are ·O²⁻ and h⁺, while the effect of ·OH is relatively small. This is because the dual composite catalyst BM and silver phosphate can form a type II heterojunction, which can effectively transport photogenerated carriers and prevent the recombination of electron holes. In addition, simulation experiments show that the functional fabric is promising to be used in the reuse of reclaimed water in dyeing and finishing systems, and to achieve the purpose of carbon emission reduction to a certain extent. This research is an effective attempt to apply photocatalysis to environmental treatment, and new improvements have been made to the study of solar light conversion, which can provide some new insights and new ideas for the preparation of new materials for visible light catalysis treatment of sewage and wastewater.

Ethics approval and consent to participate

This article does not contain any studies with human participants or animals performed by any of the authors. Informed consent was obtained from all individual participants included in the study.

Consent for publication
The authors agree that the article will be published by the journal after acceptance.

Availability of data and materials
The authors promise that the data are valid and that the material is worth applying.

Funding

Fundamental Research Funds for the Central Universities (Grant No. 2232021G-04 and Grant No. 2232020D-20), Graduate Student Innovation Fund of Donghua University (Grant No. GSIF-DH-M-2021003) for funding this study.

Authors' contributions

Nan Xu, Chunyan Hu, Baojiang Liu designed the study and wrote the manuscript. Nan Xu is the main author of the article and responsible for the completion of the thesis experiments and the writing of the. Jingshan Chen is responsible for the supplementation of some data in the research. Wei Wang and Zhijia Zhu participated in the analysis and arrangement of the literature. Chunyan Hu, Baojiang Liu was the creator and person in charge of the project and directed the writing of the paper. Nan Xu, Wei Wang, revised the manuscript. All the authors read and agreed on the final version.

Declaration of Competing Interest

The authors declare that there are no known competing financial interests or personal relationships that could have influenced the work reported herein.

Acknowledgements

The authors gratefully acknowledge the Fundamental Research Funds for the Central Universities (Grant No. 2232021G-04 and Grant No. 2232020D-20), Graduate Student Innovation Fund of Donghua University (Grant No. GSIF-DH-M-2021003) for funding this study.

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No competing interests reported.

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Supported photocatalyst for Cr (VI) conversion and removal of organic pollutants

Archived Versions:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Materials

2.2.1 Preparation of BiVO₄/ MWCNTs

2.2.2 Preparation of BiVO₄/Ag₃PO₄/MWCNTs

2.2.3 Preparation of BiVO₄ / Ag₃PO₄ / MWCNTs @ Cotton

2.2.4 Analysis of materials

2.2.5 Photodegradation experiment

3. Results And Discussion

3.1 Basic characterization of BiVO₄/MWCNTs

3.2 Surface characteristics of materials

3.3 Degradation analysis of dyes and heavy metal ions

3.4 Simulation experiment

3.5 Possible photodegradation mechanism

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Archived Versions:

Version 1

Supported photocatalyst for Cr (VI) conversion and removal of organic pollutants

Archived Versions:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1 Materials

2.2.1 Preparation of BiVO4/ MWCNTs

2.2.2 Preparation of BiVO4/Ag3PO4/MWCNTs

2.2.3 Preparation of BiVO4 / Ag3PO4 / MWCNTs @ Cotton

2.2.4 Analysis of materials

2.2.5 Photodegradation experiment

3. Results And Discussion

3.1 Basic characterization of BiVO4/MWCNTs

3.2 Surface characteristics of materials

3.3 Degradation analysis of dyes and heavy metal ions

3.4 Simulation experiment

3.5 Possible photodegradation mechanism

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Archived Versions:

Version 1

2.2.1 Preparation of BiVO₄/ MWCNTs

2.2.2 Preparation of BiVO₄/Ag₃PO₄/MWCNTs

2.2.3 Preparation of BiVO₄ / Ag₃PO₄ / MWCNTs @ Cotton

3.1 Basic characterization of BiVO₄/MWCNTs