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

The photocatalytic property of available semiconductor catalysts still suffers from some urgent problems, such as the high excitation energy, easy agglomeration of powders, or weak recycling property. Therefore, developing novel visible light-supported catalysts and catalyst loading have aroused great attention recently. In this work, a novel Ag3PO4/BiVO4/MWCNTs@Cotton functional fabric was prepared by introducing Ag3PO4 as a plasma resonance photocatalyst and MWCNTs with cotton as composite substrates. Not only did the introduction of Ag3PO4 and MWCNTs effectively strengthen the application ability of BiVO4, but also inhibited the recombination of carriers, and promoted the transport of carriers according to spectroscopic and electrochemical tests. Degradation tests remained that Ag3PO4/BiVO4/MWCNTs @cotton retained the high photocatalytic efficiency of the powder catalyst, along with the degradation degree of active blue KN-R (50mg/L) as well as Cr (VI) (20mg/L) could reach more than 90% within 120 min. What’s more, the functional fabric has gained excellent performance in degrading pollutants for 5 cycles. Meanwhile, the prepared BiVO4 is consistent with the band structure and electron density calculated theoretically by the GGA-PBE function. Free radical trapping and scavenging experiments exhibited that functional fabrics could produce active substances such as h+,·O2-, and·OH, among which the first two are the main active substances in the reaction. To sum up, this study is an effective attempt based on the existing problems of photocatalysts together with providing some study directions for the development of photocatalytic technology in the future.


Introduction
With the continuous progress of industrial production, a series of pollution problems have emerged 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 full of complex pollutants, including metal ions, dyes, various nitrogen compounds, and other pollution waste . The long-term circulation of these pollutants in a water body will not only cause irreversible loss of water resources but also gained great harm to the sustainable development of the 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 (Boyjoo et al. 2017;Hoffmann et al. 1995;Loeb et al. 2019). However, most of the available semiconductor photocatalyst is generally in powder state which owns the problems of easy agglomeration and difficulty in recovery (Takata & Domen 2019). In addition, the efficiency and cost problems have also become important factors that limit its development Wang et al. 2018). Therefore, it is of great significance to develop low-cost, highefficiency 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 photocatalysts at present, TiO 2 is the most commonly used (Schneider et al. 2014). However, due to the relatively high band gap of TiO 2 , 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 on a large scale (Park et al. 2013;Wang et al. 2018). Therefore, we have to find out other visible light-responsive catalysts (Tong et al. 2012). As a new photocatalyst, BiVO 4 has 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 excite, so it is widely used (Qu et al. 2020;Ran et al. 2019a;Yan et al. 2020). BiVO 4 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 . CO 2 reduction is the starting point for many current research projects. By growing Cu 0.4 V 2 O 5 nanosheets on the surface of BiVO 4 microplates using a simple coprecipitate method, a new type of heterostructure was formed and showed a high CO precipitation rate of 9.49μmol g −1 h −1 under visible light irradiation, which is about 4.4 times that of the original Cu 0.4 V 22 O 5 . The chain hollow microstructure of BiVO 4 (Ag-Bi/BiVO 4 ) co-modified by metal Ag and Bi nanoparticles can greatly improve the visible light response and enrich the oxygen vacancy on its surface, so it has an amazing photocatalytic performance of converting CO 2 into CO under visible light irradiation (Duan et al. 2020). In addition, the Z scheme Cu 2 O/Bi/BiVO 4 nanocomposite photocatalyst along with (Wang et al. 2021a) Cu−Bi/BiVO 4 (Huang et al. 2021) both have high visible light absorption capacity and have made great progress in CO 2 reduction, with the reduction rate higher than that of monomer BiVO 4 . In addition, BiVO 4 has a wide range of natural sources, relatively low toxicity to the environment, and relatively good stability in water. However, the carriers of BiVO 4 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 BiVO 4 mainly focuses on inhibiting its recombination efficiency and improving its optical response range (Malathi et al. 2018). The introduction of some metal or nonmetal particles is one of the important ways to prepare modified BiVO 4 (Zhou et al. 2014). By introducing a small amount of Pt element, Wang et al. effectively prevented the charge recombination caused by excessive BiVO 4 defects, and the sample could maintain stability for 50 h at 0.8VRHE potential (Gao et al. 2021). As a non-noble metal, Bi has direct plasma photocatalytic activity. Fang (Li et al. 2019b) loaded fine Bi nanoparticles on BiVO 4 by electrodeposition process and established the Bi-BiVO 4 heterojunction 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 BiVO 4 modification. Liang et al. (2020) successfully prepared Bi 2 S 3 -BiVO 4 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 five times and the removal rates are maintained at more than 85 %. Academician Li Can group (Qi et al. 2022) has recently studied the dual-assisted method to further improve the performance of BiVO 4 . They modified BiVO 4 by selective photodeposition of dual-cocatalysts (metal Ir and FeOOH and CoCOOH composites (FeCoOx) to improve the water oxidation ability of BiVO 4 . Finally, relevant studies (Li et al. 2019a) show that the relative exposure of different crystal planes of BiVO 4 also has a certain impact on the surface charge distribution and photocatalytic activity. Ag 3 PO 4 (with a forbidden bandwidth of 2.4 eV) is a new type of photocatalyst whose catalytic properties were first discovered in 2010. Ag 3 PO 4 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 Ag 3 PO 4 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 Ag 3 PO 4 itself and the easy formation of Ag elemental deposition on the surface after a period of time, which affects the catalytic activity, Ag 3 PO 4 is generally not used alone but is used as a plasmon resonance catalyst mixed with other semiconductors (Yi et al. 2010). Aiming at the photo corrosion of Ag 3 PO 4 , Yang (Zhao et al. 2020a) et al. proposed to effectively composite Ag 3 PO 4 with a new two-dimensional layered nanomaterial titanium carbide (MXene) with high electrical conductivity to controllably construct a zero-dimensional silver phosphate/twodimensional 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 et al. (2018) constructed a BiVO 4 /Ag 3 PO 4 heterojunction on the electrondominated and hole-dominated crystal planes of BiVO 4 by different deposition methods, respectively. The results show that the performance of the monomer BiVO 4 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 onedimensional structure, large specific surface area, superior mechanical properties, and high chemical and thermal stability (Yang et al. 2006). 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 narrowseries semiconductor and can be used as an effective photosensitizer to improve the light absorption range of materials (Gangu et al. 2019). 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 (Gangu et al. 2019, Sengupta & Gupta 2017. By far, organic dyes, antibiotics, and heavy metal ions are important components of pollutants in wastewater today. Many photocatalysts have been developed to remove various pollutant molecules from wastewater. CoO@meso-CNmodified MoS 2 was once prepared to successfully degrade methylene red, methylene orange, and Congo red by using under visible light and achieve an average removal rate of about 94% within 60 min. It can also be used for environmental management and reusable area (Anonymous, Chen et al. 2020). The novel heterojunction structure formed by bridging MoS 2 /ZnO using CNTs can remove tetracycline (TC) by 94.5% after 60 min of irradiation, the treatment rate of which is about 10-15 times faster than that of bulk MoS 2 and ZnO. In addition, the material can be used for Cr(VI) removal (Chen et al. 2022c). Mosse bears a larger specific surface area compared to MOS 2 and a relatively high efficiency of photogenerated carriers, which ensures a good degradation effect on tetracycline, oxytetracycline, and aureomycin under visible light irradiation (Chen et al. 2022b) (Anonymous).
In this study, BiVO 4 was selected as the basic visible light catalyst, and Ag 3 PO 4 was used as the semiconductor plasmon resonance catalyst, which was attached to the surface of BiVO 4 by deposition method (Qi et al. 2016). In addition, MWCNTs were selected and combined with BiVO 4 by hydrothermal method. Taking advantage of the structural advantages of Ag 3 PO 4 and MWCNTs, Ag 3 PO 4 / BiVO 4 /MWCNTs were constructed to promote the transport of carriers, recombination of carriers as well as the visible light catalytic performance of BiVO 4 . What's more, cotton fiber with good physical adsorption capacity was employed to solve defects of the powder catalyst such as easy agglomeration in water and difficulty to recycle Qu et al. 2020;Ran et al. 2019b;Yu et al. 2021;Zhang et al. 2019). The Ag 3 PO 4 /BiVO 4 /MWC-NTs@Cotton functional cotton fabric retains the catalytic performance of powder catalysts and relieves its application pressure by degrading reactive blue KN-R and Cr (VI) at a fast speed (Loeb et al. 2019;Wang et al. 2021b). In addition, simulation experiments have been conducted in the reuse of reclaimed water in dyeing and finishing systems to achieve the purpose of carbon emission reduction to a certain extent. This study is an improvement to the existing problems of poor visible light catalytic efficiency and reuse of catalysts. It can also provide new ideas and methods for water pollution treatment methods at this stage (Wang et al. 2021b).

Preparation of BiVO 4 / MWCNTs
In total, 0.970g Bi (NO 3 ) 3 · 5H 2 O was dissolved into 5ml of HNO 3 with a 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 NH 4 VO 3 was dispersed in 20 mL of NaOH with a 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 to 1, 3, 5, 7, and 9, continuing to magnetically stir for 30 min (named C). The solution in C was transferred to the autoclave, the oven temperature was adjusted to 160 °C 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 °C for 4h to obtain yellow-green powder BiVO 4 / MWCNTs. In addition, BiVO 4 can be prepared by repeating the above operation without adding MWCNTs.

Preparation of Ag 3 PO 4 /BiVO 4 /MWCNTs
A total of 0.4g BiVO 4 /MWCNTs powder prepared in the second step was weighed and dispersed with 30ml absolute ethanol ( named A after magnetic stirring for 30min); 0.268g AgNO 3 was dispersed in 4ml ammonia solution (named B); The solution in A was continuously stirred and added into B (named C); 4 mL of Na 2 HPO 4 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 °C for 4h to obtain yellowish brown powder Ag 3 PO 4 /BiVO 4 /MWCNTs.

Preparation of Ag 3 PO 4 /BiVO 4 /MWCNTs @ Cotton
1. Pure cotton woven fabric was cut into a 6 cm × 6 cm experimental cloth sample (about 0.5g) and put into the prepared acetone ethanol solution (10ml: 10ml) with a 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 °C. 2. BiVO 4 and BiVO 4 /MWCNTs prepared at pH = 3 with the mass ratio of Ag 3 PO 4 to BiVO 4 /MWCNTs was controlled to be 1:2 was selected. 0.150g of dried catalyst was weighed into a beaker and 100 mL 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 °C and taken out after a constant temperature reaction for 160 min. The removed cloth was washed with clean water and dried in an oven at 60 °C for 20min as well as transferred to an oven at 165 °C for baking for 15 min to obtain Ag 3 PO 4 /BiVO 4 / MWCNTs @Cotton composite photocatalytic fabric, as shown in Fig 1.

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. TEM was employed to determine the morphology of different catalysts and the composition of heterojunctions. An X-ray photoelectron spectrometer (XPS, Escalab 250Xi) was employed to figure out the elemental profiles of different catalysts. PL, EIS, and photocurrent tests were employed to evaluate the separation efficiency of photogenerated carriers and photocatalytic performance.

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 were selected and put into a clean degradation test tube. Fifty-milliliter reactive blue KN-R dye solution with a concentration of 50 mg/L (50mL of 20mg/L Cr (VI)) was put into the reaction tank for photochemical reaction and reacted for 20min without turning on the light. After the reaction, a syringe was used to absorb the appropriate dye solution and a filter was 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 absorbance of Cr (VI) solution needs to be measured after the color is developed). The xenon lamp, refrigeration, circulation, and electric fan were turned on to repeat the above-mentioned absorbance measurement operation every 10 mins and end the reaction after 60 min of photoreaction. Decolorization rate (%) = ((A 0 -A)/A 0 ) ×100% was used to summarize the recorded absorbance data (where A 0 is the initial absorbance of the dye solution, and A is the absorbance of the dye solution measured at different 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.

Basic characterization of BiVO 4 /MWCNTs
In this work, the preparation of BiVO 4 /MWCNTs is given priority, and then Ag 3 PO 4 is deposited on the body and loaded onto the cotton fiber. Therefore, the preparation conditions of a double composite catalyst are particularly important. The N 2 adsorption capacity, UV-visible diffuse reflectance spectrum, and crystal structure of BiVO 4 / MWCNTs were characterized in Fig. 2. As shown in Fig. 2A, the specific surface was 1.7361 m 2 /g, 7.8932 m 2 /g, 11.3093 m 2 /g, 5.7543m 2 /g, and 3.1932m 2 /g when the pH value was 1, 3, 5,7, and 9, respectively. 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 freedom 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, when the 2θ angle was 18.5°, 35°, and 46°, all the diffraction peaks of BiVO 4 /MWCNTs were split peaks, while when the 2θ angle was 28.6°, 30.5°, and 55.6°, these peaks were characteristic peaks of monoclinic BiVO 4 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. In addition, the absorbance of the double composite catalyst is better in the ultraviolet region and lower in the visible region. With the change in the doping degree of MWCNTs themselves, the response degree to light varies in the measured wavelength range, as shown in Fig. 2C. Finally, hv-(Ahv) 2 spectra of catalysts prepared at different pH values were drawn, and the tangents of the spectra were shown in Fig. 2D. The introduction of MWCNTs did reduce the band gap width of BiVO 4 and obtained good light absorption performance under the preparation of pH=3. This demonstrates that the composite catalyst is more easily excited by visible light and gains a good modification effect. Figure 3A exhibits the surface morphology of BiVO 4. The monomers prepared in this work are blocky in shape and form partial stacking agglomerates. Figure 3B and C show the surface morphology of BiVO 4 /MWCNTs at different magnifications, from which it can be seen that the elongated MWCNTs accumulate on the surface of BiVO 4 and promote the crystal agglomeration of it to some extent. Figures 3D-F shows the powder dispersion of Ag 3 PO 4 /BiVO 4 /MWCNTs. The agglomerated surface of the three composite catalysts deposited many round crystal particles, which were not shaped the same as those of BiVO 4 , suggesting the surface accumulation of Ag 3 PO 4 , thus meaning the successful preparation of the composite catalysts. The composite catalyst can be adsorbed on the surface of cotton fabric under hydrothermal co-bath conditions, as shown in Fig. 3G as well as 3H. At higher magnification, the shape of the catalyst can be seen to be consistent with the characterization results in Fig  3I, proving that hydrothermal heat did not change the underlying structure of the catalyst. Figure 3J shows Fig. 5C is the characterization of the Bi 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 potential of Ag 3 PO 4 /BiVO 4 /MWCNTs was shifted to the left by 0.35ev compared with that of BiVO 4 /MWCNT, indicating that there was a charge interaction between Ag 3 PO 4 and the dual composite catalysts (Sun et al. 2022b). In addition, Figs. 4D and E 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 VO 4 3- (Zhao et al. 2020b). The binding energy peak of the C element (284.8eV) in Fig. 4H. This is due to the presence of C-C bonds in the sample (Mei et al. 2022;Wu et al. 2022). Therefore, Figs. 4C, D, E, and H fully demonstrate the existence of BiVO 4 and MWCNTs in the samples (Khazaee et al. 2021;Sun et al. 2022a). The deposition of Ag 3 PO 4 can be seen in Figs. 4F and G. Figure 4F is the characterization of the Ag element in the sample, and its peak binding energies are at 368.4eV and 374.2eV, respectively, which proves that the valence of the Ag element in the composite catalyst is +1, Fig. 4G is the characterization of P 2P and the peak energy at 133.4eV verifies the existence of P-O (Chen et al. 2022a;Ma &Cheng 2022). 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 Ag 3 PO 4 /BiVO 4 /MWCNTs (Chen et al. 2022a). Figure 5A casts the N 2 adsorption-specific surface area test for different composite catalyst components. The specific surface area of the composite catalysts gradually increases with the increase of the catalyst composite components, proving that their adsorption in the dark reaction stage will be strengthened. Then, electrochemical basis tests were performed for different catalysts. Figure 5B is the results of the EIS impedance test. The radius of curvature of the impedance curve for the monomeric bismuth vanadate is the largest, while the radius of curvature for the triple composite catalyst turns out to be the smallest, indicating that the increase in composite composition bears a negative effect on the interfacial resistance of the catalyst. Then the photocurrent of the catalyst was measured for every 20-s intervals of switching light time, as shown in Fig. 5C. The maximum photocurrent of Ag 3 PO 4 /BiVO 4 /MWCNTs in the figure ensures the superior photogenerated carrier separation efficiency of the catalyst, which is consistent with the results of the PL photoluminescence spectroscopy test in Fig. 5D. Under the excitation wavelength of 315nm, the composites obtained similar stronger diffraction peaks at wavelength differences of no more than 5 nm. The more components, the lower the diffraction peaks, suggesting that the fluorescence effect of the material gradually decreases, which is negatively correlated with the separation efficiency of photogenerated carriers. Figures 5E and F exhibit the UVvis diffuse reflectance spectra and hv-(Ahv) 2 spectra for the three catalysts, respectively. The increase of the composite composition of the catalysts can significantly change the UV absorption and redshift of the semiconductor together with reducing its band gap width, which sets up its utilization of the solar spectrum and good photocatalytic performance.

Degradation analysis of dyes and heavy metal ions
The degradation curves of BiVO 4 , BiVO 4 / MWCNTs, Ag 3 PO 4 /BiVO 4 /MWCNTs, and Ag 3 PO 4 /BiVO 4 /MWCNTs @ Cotton for reactive blue KN-R under different preparation conditions were shown, respectively ( Fig. 6A-C), 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 Ag 3 PO 4 : BiVO 4 /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 Ag 3 PO 4 / BiVO 4 /MWCNTs. Figure 6D is the 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 from 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 Ag 3 PO 4 /BiVO 4 / MWCNTs@Cotton functional cotton fabric retains three of the decolorization effect of the composite component catalyst is that the decolorization rate of the dye solution is 92% within 100 min; Fig. 6F shows the degradation of Cr (VI). With the increase of the composite component, the final content of Cr (VI) gradually decreases. The removal rate of Cr (VI) by Ag 3 PO 4 /BiVO 4 /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, as well as realize catalyst loading and retain the catalytic performance of powder catalysts. Finally, Figs. 6G and H are the before and after comparisons of reactive blue KN-R and Cr conversion.
As shown in Fig. 7, Fig. 7A-D simulates 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. 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 negatively correlated. 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 along with acid and alkali resistance. These test results can provide theoretical references for the practical application of composite catalysts.
The cycle performance of the composite catalyst is also the focus of this study. The functional cotton fabric can effectively degrade reactive blue KN-R within 150 min, and the decolorization rate is maintained at about 90% in Fig. 8A. The degradation can be cycled 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 the Cr (VI) removal experiment. The results show that the target fabric can achieve 5 cycles of Cr (VI) degradation, and the degradation effect can reach 85% within 100 min above. This shows that functional cotton fabrics can alleviate the problems of powder catalyst recollection and difficult reuse, and have good versatility. Figure 8C casts the XPS survey spectrum of the functional fabric before and after cycling, which indicates that no loss of elements occurred before and after cycling. In addition, the leaching of the composite catalyst on the fabric can be obtained by measuring the weight loss rate of the functional fabric, as shown in Fig. 8D. The weight loss rate of the functional fabric after 5 cycles was 9.6%, suggesting that most of the catalyst was retained, which is consistent with the results of cyclic degradation. Figure 8E is the surface morphology of the fabric after 5 cycles of degradation, which proved that there is still a lot of catalyst loading. Figure 8F is the macroscopic comparison before and after the degradation. Some dyes settle 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.

Simulation experiment
Combined with the dyeing principle of the overflow dyeing machine, a simulation test was carried out on the sewage treatment performance of functional fabrics to verify the applicability of functional fabrics, as shown in Fig. 9. 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. Then, the wastewater is subjected to the first catalytic treatment under the action of ultraviolet rays followed by the initially treated wastewater being 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.

Possible photodegradation mechanism
The feasibility of the experimental materials can be verified by theoretical calculation. In this work, GGA-PBE of 10A calculates its bandwidth, and the calculated result is 2.532eV, which is 0.8% different from the above experimental result of 2.51eV. It is within the allowable error range and proves that BiVO 4 is successfully synthesized. The results of state density calculation showed that the Fermi energy level of BiVO 4 is in the interval of DOS value 0, and it has good covalent bond performance, which verifies the semiconductor performance of BiVO 4 . Finally, Fig. 10D shows the catalyst heterojunction structure optimized by GGA-PBE, and the combination of BiVO 4 and Ag 3 PO 4 can be seen from the side and front respectively.
To explore the active substances in the process of dye degradation, free radical capture experiments on dyes were conducted using DMPO. The detection results are shown in Figs. 11A and B. Figure 11A shows the capture of OH· free radicals. EPR is shown as a wavy line in the figure under dark conditions, indicating that OH· does not exist. After the light was turned on, the peak spectrum of OH· free radical increased significantly, indicating that a certain amount of OH· free radical was produced under the light condition. Figure 11B shows the EPR spectrum of DMPO-·O 2-. The change of its peak value is similar to that of OH· free radical, proving that the catalyst can produce a good catalytic effect under light conditions. As shown in Fig. 11C, different scavengers were selected (including hole scavenger sodium oxalate (Na-OA), free radical ·O 2 scavenger benzoquinone (BQ), and free radical ·OH scavenger tert-butanol (t-BuOH) in the degraded solution along with the dye without scavenger is added as a blank sample. 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 ·O 2 scavenger benzoquinone (BQ) can also reduce the decolorization effect to 1/3 of the original, meaning that ·O 2 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, which stated that ·OH was not the main active substance of the system. As shown in Fig. 11D, the addition of MWCNTs can increase the wave-absorbing properties of the monomer BiVO 4 , whose absorption edges are at 505nm and 520 nm, respectively. The forbidden bandwidth of the catalyst can be calculated according to the absorption edge, and the specific calculation method is as follows: 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 BiVO 4 and BiVO 4 /MWCNTs can be obtained to be 2.51 eV and 2.38 eV, respectively.
As shown in Fig. 12, taking BiVO 4 /MWCNTs(BM)as a whole and depositing Ag 3 PO 4 on this basis is the main goal of this experiment. According to previous reports, the CB of Ag 3 PO 4 is 0.29 eV, VB is 2.6 eV (Gao et al. 2018), and the modified BM is still an N-type semiconductor. Considering its (1) E g = 1239.6∕λ g Fig. 11 A DMPO-OH· and B DMPO-·O 2of the catalyst; C Effects of different free radical scavengers on degradation efficiency; D Uv-visible diffuse reflectance spectra of samples valence band and conduction band potential, it can form a type II heterojunction with Ag 3 PO 4 . Under the irradiation of visible light, both BM and Ag 3 PO 4 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 Ag 3 PO 4 , and the photogenerated electrons generated by Ag 3 PO 4 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 CO 2 and H 2 O and other harmless substances, which is the main process of the composite catalyst to degrade the dye. In addition, 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 ·O 2 -, ·O 2 further react with pollutants in wastewater to generate CO 2 and H 2 O. The possible reaction formulas in the photocatalysis process are as follows: (2) BM + hv → h1 + + e1 − (3) OH − + h1 + → ⋅OH (4) ⋅OH + dye → CO 2 + H 2 O (5) Ag 3 PO 4 + hv → h2 + + e2 −

Conclusions
In summary, Ag 3 PO 4 /BiVO 4 /MWCNTs@Cotton is an important change to the inhibition of visible light catalysis of BiVO 4 and the difficulty about the reuse of catalyst powder. 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 beyond 90%) but also can be recycled 5 times. Moreover, the catalyst loading rate remains at a high level after the cycle according to the weight loss test. 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. The GGA-PBE of CASTEP was first used to optimize the structure of the experimental subject BiVO 4 to make sure its synthesis.
(6) Cr 6+ + e2 − → Cr 3+ (7) h1 + → h2 + + dye → CO 2 + H 2 O (8) e2 − → e1 − + O 2 → ⋅O − 2 + dye → CO 2 + H 2 O Fig. 12 Schematic diagram of the degradation mechanism of the composite catalyst Mechanism studies show that the main active groups of composite catalysts and functional fabric catalysis under the irradiation of visible light are ·O 2and h + , while the effect of ·OH is relatively small. This research is an effective attempt to apply photocatalysis to environmental treatment, along with new improvements for the study of solar light conversion, which gains some new insights and ideas into the preparation of novel materials for visible light catalysis treatment of wastewater.