Enhanced performance of photocatalytic treatment of Congo red wastewater by CNTs-Ag-modified TiO2 under visible light

In order to improve the treatment efficiency of printing and dyeing wastewater, the carbon nanotubes-silver-modified-titanium dioxide (CNTs-Ag-TiO2, CAT) ternary composite was prepared by a mechanical mixing method. It was found that the morphology of the prepared CAT sample was uniformly coated with strips of CNTs, speckled Ag, and lumpy TiO2. The (002) crystal plane of CNTs, the (101) crystal plane of TiO2, and the (111) crystal plane of Ag were observed, which possessed functional groups such as Ti-OH and Ti-O-C, indicating that the prepared CAT sample had photocatalytic reaction sites. The visible light utilization of titanium dioxide can be improved. The treatment effect of different proportions of CNTs-Ag-TiO2 on Congo red wastewater was tested, and the results showed that the optimum degradation effect of Congo red wastewater was CNTs: Ag = 10:1, and the doped amount of CNTs/Ag was 15%, and the removal rate of Congo red wastewater could reach 100% within 140 min. The excellent removal effect of CAT ternary composite on Congo red wastewater provided a new idea and way for the modification of TiO2 and its composites for the potential of organic dyes degradation.


Introduction
Printing and dyeing wastewater was one of the main sources of industrial wastewater, which discharges a large number of colored wastewater with various components. Organic dyes, which produced printing and dyeing wastewater, had a wide range of uses in many industries, including textiles, leather, paper, printing, cosmetics, and plastics ). Most of these dyes had the characteristics of stable structure, difficult degradation, and high toxicity, which had brought serious problems to the environment (Eltaweil et al. 2021). Congo red (CR) was a typical azo dye (benzidine direct azo anion), and its metabolite benzidine was a known carcinogen, which had caused serious harm to human health (Yang et al. 2017).
At present, the methods of removing organic dyes from wastewater mainly include physical adsorption (Hou et al. 2021), chemical oxidation (Xi et al. 2021), acoustic catalysis (Chai et al. 2020), and photocatalytic degradation (Rizal et al. 2021). Physical adsorption-its principle was that adsorbent and pollutants would reach adsorption equilibrium, and the process was reversible, which could not completely remove pollutants and might cause secondary pollution. Chemical method had the advantages of rapid reaction and thorough degradation, but its cost was often higher, and other toxic by-products might be produced in the chemical reaction, causing harm to the environment. These shortcomings limit their application in the treatment of printing and dyeing wastewater. Therefore, we urgently need an environmentally friendly, economical, and efficient purification method.
The appearance of photocatalytic technology had brought new idea to industrial wastewater treatment. This technology aimed to make photocatalyst produce active particles with redox ability by sunlight to oxidize and decompose pollutants, and the whole process was environmentally friendly and nontoxic (Mohammed et al. 2021). TiO 2 semiconductor material was the photocatalyst with the most natural affinity. Due to its high catalytic activity, low cost, and stable chemical properties, it had received extensive attention in the field of environmental governance (Sheng et al. 2020). TiO 2 catalyst had strong oxidation capacity, which could oxidize some toxic and not easily degradable pollutants, and eventually transform them into non-toxic and harmless small molecule. It could effectively protect the environment and personal safety, and was a very ideal photocatalyst (Arfanis et al. 2017;Athanasekou et al. 2017). Brooke et al. found that compared with ultraviolet photolysis alone, TiO 2 photocatalysis under ultraviolet (UV) irradiation significantly improved the removal rate of aromatic macromolecular organic compounds (Mayer et al. 2019). Using UV-TiO 2 to treat the mixed wastewater of urban and textile dyes was very effective for removing color and other organic compounds. The metal-metal codoping changed the band gap energy and the structure of codoped nanoparticles, which showed higher photocatalytic activity. The silicon-tungsten co-doped titanium dioxide had a positive effect on the photocatalytic degradation of methyl orange dye under 40-W low-pressure mercury lamp (Xu et al. 2018;El Mragui et al. 2019).
The surface modification of TiO 2 could extend the light absorption capacity to visible wavelengths, increase the utilization of visible light, improve the separation of photogenerated electron hole pairs, enhance the overall efficiency of CO 2 photoconversion process, and reduce the processing cost (Athanasekou et al. 2018). It was found that plasma metal Ag and Au loaded on TiO 2 could form resonance, cause intense oscillation of surface electrons, and transfer hot electrons generated on the metal surface to TiO 2 and induce photocatalytic reaction (Low et al. 2017). As the plasma absorption of Au and Ag located in the visible light range, these visible light active plasma metals could be used as visible photosensitizers for TiO 2 . At the same time, the Schottky potential barrier formed when the noble metal particles were in contact with the semiconductor could effectively inhibit the recombination of hole-electron pairs and increase the photocatalytic activity (Chen et al. 2020). Therefore, precious metal deposition could not only improve the separation of photo-generated carriers, but also boost the utilization rate of visible light due to local ionic resonance.
It was necessary to find a carrier for photocatalyst because nanoparticles were easy to agglomerate and hinder the photocatalytic performance. Carbon nanotubes, with large specific surface area and layered hollow structure, were a new adsorbent with unique mechanical, electrical, and thermal properties. Several studies had shown that the photocatalytic degradation rate was closely related to the concentration of pollutants adsorbed on the catalyst surface, so carbon nanotubes (CNTs) could be used as carriers to adsorb pollutants and effectively separate photogenerated hole-electron pairs (Djellabi et al. 2019;Tibodee et al. 2019;Chávez et al. 2020). By combining TiO 2 , precious metal, and CNTs, not only improved the light absorption efficiency of TiO2, but also promoted the degradation of pollutants (Peng et al. 2016). However, there were currently few studies on the modification of TiO2 with precious metals and nanocarbon. Most of the research on modified TiO2 ternary composite materials focused on semiconductor materials and nanomaterials with good conductivity. Fu et al. synthesized a threedimensional(3D) photocatalyst by self-assembly of graphite carbon nitride (g-C3N4), titanium dioxide (TiO 2 ), graphene (Gr), and carbon nanotubes (CNTs). Highly interconnected carbon nanotubes could not only effectively promote the transfer of photo-generated charge carriers, but also provide a larger specific surface area for photoreaction, which had a remarkable effect in phenol degradation experiments (Fu et al. 2020). Huang et al. synthesized a novel carbon nanotube/ titanium dioxide/zinc oxide composite material, which showed significant degradation effect of rhodamine B under visible light (Huang et al. 2018).
In this study, CNTs-Ag-TiO 2 ternary composite was prepared by a mechanical mixing method, and the treatment effects of CNTs-Ag-TiO 2 with different proportions on CR were studied, and the best ratio of the ternary composites was found out. The structure of the composites was characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), and the morphology of the composites was analyzed by field emission scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The degradation mechanism was analyzed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) spectra before and after the reaction. This paper not only studied the degradation efficiency of CR by this composite material, but also provided experimental basis and theoretical support for the modification of TiO 2 and its composites for the potential of organic dyes degradation.

Pretreatment of carbon nanotubes (CNTs)
The modified CNTs were prepared by measuring 0.5g CNTs (MWCNTs, purity > 95%, Beijing, China) into the mixed solution of 10mL concentrated nitric acid (HNO 3 , purity > 65%, Yantai, China) and 30mL concentrated sulfuric acid (H 2 SO 4 , purity > 95%, Yantai, China), and the suspension was stirred at 300 r/min for 30 min and heated to 80°C. The oxidized CNTs were washed to neutrality from the mixture solution with deionized water, and the solid were taken out and dried at 70°C, and then, the solid was stored in containers for subsequent use.
Preparation of carbon nanotube-silver(CNTs-Ag, CA) binary composites The modified CNTs and nanosilver powder (AgNPs, purity > 99%, Shanghai, China) were added into 30 mL absolute ethyl alcohol (purity > 95%, Shandong, China) at the same time according to a certain proportion, and the CNTs-Ag composite material was obtained by standing after ultrasonic treatment for 30 min, taking out the solid after centrifugal treatment for 5 min, and grinding. To explore the influence of different Ag loadings on CNTs on the treatment effect of ternary material CNTs-Ag-TiO 2 , CA materials with CNTs-Ag ratio of 20:1, 10:1, and 5:1 were prepared respectively, and their degradation performance on CR was tested under the same conditions.
Synthesis of CNTs-Ag-TiO 2 (CAT) ternary composites 0.1g TiO 2 (TiO 2 , purity > 98%, Sinopharm, China) powder was put into 30 mL of absolute ethyl alcohol for ultrasound of 30 min marked as solution A; a certain amount of prepared CA material was added into 1.2 mL of sodium dodecyl benzene sulfonate aqueous solution with mass fraction of 2%, then 20 mL of absolute ethyl alcohol was added into it, and ultrasound of 30 min marked as solution B. The A solution is slowly added to the B solution, magnetic stirring, and ultrasonic for 30 min, and the suspension was centrifuged for 5 min, followed by drying in an oven at 70°C for 48 h. In order to explore the best compounding ratio, CAT samples with CA content of 5%, 10%, 15%, and 20% were prepared. At the same time, CNTs-TiO 2 (CT) samples with the same mass percentage were prepared for comparative study. The flow chart of material preparation is shown in Fig. 1.

Characterization
The FESEM (Hitachi S-4800) and EDS (Oxford 6498) were used to characterize the micromorphology and element distribution of the samples. A XRD (MiniFlex600) was used to test the crystal structure of the prepared materials, and FTIR (NEXUS-470) was used to detect the surface functional groups of the materials in the wave number range of 4000-400 cm −1 .

Photocatalytic degradation experiment
In this experiment, CR wastewater was used to simulate printing and dyeing wastewater. One hundred milliliters of CR solution with a concentration of 100mg/L was taken, and 50 mg CAT sample was added to it. The sample stirred in the dark for further 60 min in order to attain the adsorptiondesorption equilibrium and to ensure that the removal of dye was completed by the photocatalytic process. The degradation performance of CAT on CR wastewater under 1000-W xenon lamp was tested in photochemical reaction device (HF-GHX-II). The concentration of CR wastewater solution was measured every 10 min with a dual-beam(UV-8000s) spectrophotometer. The adsorption-photocatalytic degradation effect was determined by the ratio of the CR wastewater concentration C t to the initial concentration C 0 at the corresponding time. In other words, the degradation efficiency was denoted by C t /C 0 .

Results and discussion
Morphology and structure of CAT Figure 2 showed the SEM images of the synthesized CAT composites with different proportions. When the mass ratio was 20:1, the SEM image of CA sample was shown in Fig 2a, and the amount of AgNPs coated on CNTs surface was less. This was mainly because the content of treated nanosilver was too low to effectively adhere to the surface of CNTs. When the mass ratio increased to 10:1, as shown in Fig. 2b, the number of silver particles on the surface of CNTs increased significantly. However, when the proportion of CNTs/Ag was increased to 5:1, as shown in Fig. 2c, the coating effect on CNTs does not further improve. This was mainly because too much nanosilver was easy to cause agglomeration, resulting in poor loading effect. Comparing the morphologies, it was considered that the CNTs/Ag was 10:1; the CA composite with uniform surface coating of AgNPs could be obtained. When the content of CA was 10%, only a small amount of CA material could be seen from Fig. 2d, which indicated that TiO 2 was not uniformly dispersed on the surface of CA, because it might be due to the low content of CA, and it could not provide sufficient active sites, which makes TiO 2 particles agglomerate together. When the content of CA was 15% and 20%, as shown in Fig. 2e and f, the outline of carbon tubes was clearer, and TiO 2 loaded on the surface was not agglomerated on a large scale, indicating that TiO 2 particles were well attached to the surface of samples. In order to further determine the dispersion uniformity of the composite material, the energy spectrum analysis of CAT samples with 10% CA content was tested. The EDX spectrum of nanocomposite was depicted in Fig. 3, confirming the presence of Ti and AgNPs. The sample showed that TiO 2 was evenly dispersed, and AgNPs coated on CNTs surface well; the three components were evenly distributed.
The peak position and strength of the sample FTIR could be used to reflect the changes of functional groups on the surface of the prepared nanocomposites. As shown in Fig.  4a, CNTs showed a characteristic strong vibration band at 3430 cm − 1, which was attributed to the stretching of -COOH group and OH adsorbed water molecules. The bands at 1420 cm −1 were attributed to the OH deformation vibration  The crystal structure of the composite samples was characterized by XRD. In order to determine whether the mixing process has an effect on the crystal structure of the sample.   . 21-1272), respectively. In the XRD pattern of CA samples, when 2θ was 25.8°, the characteristic diffraction peak of CNTs was observed, which corresponds to the (002) crystal plane of typical graphite sheet (Zhou et al. 2020). However, there was no diffraction peak corresponding to CNTs in the XRD pattern of CAT samples, which might be the proximity between the main characteristic peak of CNTs at 25.8°and the main peak of anatase TiO 2 at 25.3°, resulting in the overlapping of diffraction peaks and the increase of peak width (Ahmad et al. 2017;Zhou et al. 2020). The diffraction peak of Ag was obvious in the XRD pattern of CAT samples (Rtimi et al. 2013;Hajjaji et al. 2018), and we could see the diffraction peaks of Ag and TiO 2 were very close ((Ag (111) and TiO 2 (004)) (Chaudhary et al. 2017;Tan et al. 2017;Zhang et al. 2019). The results clearly indicated that the prepared CAT sample had photocatalytic reaction sites.

Degradation performance of CAT on CR wastewater
In order to explore the photocatalytic performance of CAT sample under visible light, the treatment effect of CNTs/Ag with different proportions of CAT and the ability of CAT with different CA content to degrade CR wastewater were tested under visible light, and the results were shown in Fig. 5. The uniformity of CA composites was affected by different CNTs/Ag ratios. As shown in Fig. 5a, CR wastewater was completely degraded within 150 min when CNTs/Ag ratio was 10:1. This was because the CA composite with AgNPs coated on the surface was uniform. Therefore, in the prepared CAT sample with CA-doped content of 15%, AgNPs were better dispersed on the surface of the sample, so that the role of Ag as an induced electron could be better exerted, the electronspace separation efficiency of TiO 2 could be improved, and the degradation effect of TiO 2 on CR wastewater could be promoted (Espino-Estévez et al. 2016). Different treated amounts of CA had a direct impact on CAT degradation of CR wastewater, as shown in Fig. 5b. Due to the separation of photogenerated electrons by Ag as visible photosensitizer and CNTs, CAT sample degradation rate of CR wastewater could reach 90% when CAtreated amount was 10%. With the increase of CA-treated amount, the treatment effect first increased and then decreased. The maximum removal effect was obtained when the content of CA was 15%, and 100mL of CR wastewater with a concentration of 100mg/L could be degraded in 140min. When the mass fraction of CA was 20%, the treatment effect was not as good as 15% CA. The possible reason was that the excessive CA content reduced the uniformity of the TiO 2 coating, affected the absorption of photons by TiO 2 , produced a shielding effect, and affected the degradation efficiency. The removal rate of CR wastewater in CT samples with mass fraction of 15% prepared by CNTs and TiO 2 was 82%. The reason was that TiO 2 nanoparticles were attached to the sidewall of CNTs, which made TiO 2 and CNTs in a good bonding state at the interface, and the formed Ti-O-C bond separates the photogenerated hole-electron pair and reduces the band gap width. Therefore, the catalytic activity of CT samples was improved, the absorption of visible light was significantly enhanced, and the photocatalytic performance of composite materials was improved (Zhao et al. 2020). In addition, on the one hand, CNTs could be used as a carrier of photocatalytic reaction to block photogenerated hole-electron recombination, and on the other hand, Ti-O-C could be formed to expand the interface area and improve the degradation effect of CR wastewater  (Nguyen et al. 2016).

Number of reuses for CAT degradation on CR wastewater
To study the stability of CAT samples with CNTs/Ag ratio of 10:1 and CA-doped ratio of 15%, repeated cyclic degradation experiments were carried out, and the results were shown in Fig. 6. After the catalytic degradation was finished, the catalyst was repeatedly washed with deionized water, and the catalytic degradation experiment was carried out again in the same environment, repeated five times. It could be found that the degradation efficiency of 15% CAT remained stable in the first four experiments, and decreased slightly in the fifth experiment, which indicated that 15% CAT had high catalytic performance and excellent stability.

Photocatalytic degradation mechanism of CAT
In order to further determine the degradation mechanism of CAT composites, FTIR and XRD of CAT samples with 15% CA treated after reaction were measured. As shown in Fig. 7a, CAT showed a characteristic strong vibration band at 3424 and 1567 cm −1 , and the tensile strength of Ti-OH group and OH adsorbed by water molecules decreased, which might be the reaction between activated electron holes and adsorbed water or OH − to form highly active superoxide radical ions, thus achieving the effect of catalytic degradation (Jung et al. 2015). After the end of the reaction, the absorption bands of CAT at 3424 and 1567 cm −1 might be related to the formation of OH formed by hydrogen bonds between the benzene ring on the surface of CNTs and organics containing oxygen functional groups (Zhao et al. 2018). The bending vibration strength of the Ti-O-Ti and Ti-O-C bonds in the wide absorption band between 1000 and 500 cm −1 was weakened, and the tensile vibration strength of the -C-O bond in the absorption band between 1112 and 1038 cm −1 was weakened. This was attributed to the adsorption of CR wastewater onto CNTs by complexation with oxygen-containing functional groups. Therefore, the adsorption mechanism of CNTs for CR wastewater might include the electrostatic adsorption on the sample surface, π-π interaction, and the complexation of oxygen-containing functional groups of the adsorbents (Wang et al. 2015). The bending vibration intensity of T-O-C bond was weakened, which indicated that Ag was mainly used as photosensitizer in the photocatalysis of samples, which played an important role in the efficient photocatalytic activity of TiO 2 under visible light. It was found from Fig. 7b that the structure of CAT composite had not changed obviously during the photocatalytic reaction, which indicated that TiO 2 , Ag, and CNTs were only carriers of photo-generated electron transfer in the photocatalytic reaction, and their own structure had not changed. The oxidation-reduction reaction with organic pollutants was mainly caused by superoxide radicals with high activity (El Mragui et al. 2021). CNTs mainly played a role in the separation of photogenerated electrons. Due to the local surface plasmonic resonance effect of Ag, it could not only improve the separation of photogenerated charge carriers, but also generate hot electron transfer to TiO 2 and induce photocatalytic reaction, which improved the utilization rate of TiO 2 for visible light (Chen et al. 2021). Therefore, the photocatalytic reaction of the prepared CAT ternary composites under visible light might be as follows: visible light resonates with Ag, and electrons were injected into the TiO 2 conduction band. At the same time, the Ti-O-C bond formed by TiO 2 and CNTs reduces the band gap width, so that the electrons in the TiO 2 conduction band were excited, and the excited electrons were further transferred by a well-conducted Ag and CNTs to promote the separation of electron holes. When electrons and holes flow, a Schottky potential barrier was formed at the interface between Fig. 6 Cycling tests of 15% CAT Ag and TiO 2 , which inhibits the recombination of electron holes in the degradation process, and the resulting electrons and holes reacted with oxygen and water molecules adsorbed on the surface of CAT material to produce strongly oxidizing hydroxyl radicals and superoxide radicals that degraded Congo red by oxidizing the azo structure of the dye, thus further improves the catalytic activity of CAT materials in visible light.

Conclusion
In this paper, CNTs-Ag-TiO 2 ternary composites were prepared by a mechanical mixing method. By using the good adsorption ability, electron transfer ability and visible light photosensitization activity of CA material, the degradation ability of the composites to organic pollutants under visible light was improved. The best treated ratio of this ternary material was explored. The CA material with good morphology and uniform package was prepared by mixing CNTs and Ag at a ratio of 10:1, and the CAT ternary composite nano material prepared by adding 15% CA sample to TiO 2 could degrade CR wastewater completely in 140 min under visible light. By characterizing the structure of the samples before and after the reaction, the possible mechanism of photocatalytic treatment of CR wastewater by CNTs-Ag-modified TiO 2 under visible light was expounded. The adsorption mechanism of CNTs on CR wastewater included electrostatic adsorption, π-π interaction and complexation. The role of CNTs in photocatalysis mainly included providing highly active superoxide radicals and separating photogenerated electrons. Ag could not only improve the separation of photogenerated charge carriers, but also increase the utilization rate of visible light of TiO 2 as a visible photosensitizer. In addition, the prepared CAT ternary composite material had stable structure in photocatalysis experiment, did not produce intermediate products, and could be reused. The action mechanism of CAT ternary composite material was determined in the photocatalytic degradation of organic matter, which would provide a new idea and way for the modification of TiO 2 and its composites for the potential of organic dyes degradation.
Materials availability We declare that the materials supporting the findings of this study are available within the article.