Highly photocatalytic activity of natural halloysite - Based material for the treatment of dyes in wastewater

Nội dung text: Highly photocatalytic activity of natural halloysite - Based material for the treatment of dyes in wastewater

1. Journal of Mining and Earth Sciences Vol. 62, Issue 3 (2021) 19 - 28 19 Highly photocatalytic activity of natural halloysite - based material for the treatment of dyes in wastewater Son Ha Ngo*, Nui Xuan Pham, Tuan Ngoc Tran Faculty of Oil and Gas, Hanoi University of Mining and Geology, Vietnam ARTICLE INFO ABSTRACT Article history: th In this study, the halloysite nanotube material will be fabricated from a Received 19 Feb. 2021 natural halloysite mineral and used as a support for the photocatalytic Accepted 17th Apr. 2021 activity phase based on TiO2. The material is characterized by modern Available online 30th June 2021 physicochemical methods such as XRD, SEM, BET, UV - vis spectrum, and Keywords: EDX. Accordingly, the refined halloysite has a nanoscale with a length of about 1.3 μm and a capillary size of about 5 nm. After the deposition of Ag Ag - TiO2, - TiO on the halloysite, the specific surface of the material measured by Halloysite, 2 the BET method was about 60 m2/g, and the structure of the halloysite Photocatalyst, was intact. The band - gap energy of as - prepared materials is also RR - 195, significantly improved in comparison to pure TiO2, makes the material TiO2. capable of absorbing longer wavelengths of light in the photocatalytic process. The Photocatalyst based on Halloysite and TiO2 showed very high efficiency, up to more than 95% in the decomposition of typical organic pollutant RR - 195. This result shows great potential in this novel material in environmental treatment applications. Copyright â 2021 Hanoi University of Mining and Geology. All rights reserved. about 50% of the total amount of dyes used on the 1. Introduction market. However, the use of dye also leads to Reactive dyes are one of the most important environmental problems such as residual dyeing technological advances of the 20th century in the compounds and chemical additives used in the field of textile. They are used more and more dyeing process in wastewater (Bagane et al., 2000). because they have bright colours, rich varieties, These wastes are dangerous to the environment, and high colour fastness. Currently, the number of especially aquatic habitats, if not being treated reactive dyes in the textile industry accounts for thoroughly (Imamura et al., 2002) due to its solubility in water and stability in this environment ___ even with low concentrations. Therefore, the *Corresponding author removal of dyes has received the huge attention of E - mail: ngohason@humg.edu.vn researchers. DOI: 10.46326/JMES.2021.62(3).03
2. 20 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 Adsorption using activated carbon has long other noble metals, acts as a trap of been used to remove various pollutants in water. photogenerated electrons and as a result, improves However, adsorption is only the first step to collect the charge carrier separation (Ohtani et al., 1997). pollutants and cannot treat them completely. Moreover, silver nanoparticles can absorb visible Therefore, many scientists have studied the light due to localized surface plasmon resonance feasibility of using inexpensive, commercially (Zhou et al., 2012), leading to new applications available materials as dye removal agents by highly such as antibacterial textiles, medical devices, food efficient processes without secondary pollutant preparation surfaces as well as air conditioning generation. filters and coated sanitary wares (Zielińska et al., Recently, natural halloysite has been attracted 2010). far - reaching interest for effective water treatment. Currently, the combination of TiO2 This is a low - priced and abundant material with nanoparticles with halloysite nanotubes has been numerous advantages such as good dispersibility, done by scientists with varied methods, and the rich in active groups, low toxicity, good biological results are promising. A typical example of this compatibility and availability in nature (Zhang et combination is Halloysite - TiO2 composite al., 2016). photocatalytic material, which has a much higher In the field of photocatalysis for decomposition rate for azo Basic Blue 41 dye in environmental treatment, the most popular water than commercial TiO2 under UV light nanomaterial is TiO2 nanoparticles. TiO2 is a non - (Szczepanik et al., 2017). toxic material which has been applied in the field of For that reason, the aim of this study is to solar energy and especially in environmental fabricate a photocatalytic material based on HNT treatment because of their strong photocatalytic carrier with Ag - doped TiO2 and to evaluate the properties and chemical stability (Amin et al., 2009, activity of this catalyst in the photocatalytic Guo et al., 2009). However, TiO2 has a large band - degradation of RR - 195, which is a persistent gap energy that requires ultraviolet irradiation to pollutant present in textile wastewater. stimulate the catalytic ability. Furthermore, the rapid recombination of electron - hole pairs can 2. Experimental drastically reduce quantum efficiency. So, narrowing the band - gap energy of TiO2 to increase 2.1. Chemicals the visible light absorption ability is a commonly Raw halloysite was obtained from kaolinite used method to improve photocatalytic efficiency mines. Sulfuric acid (H2SO4, 98%); Ethanol (Amin et al., 2009). Such methods could be the (C2H5OH); Acetic acid (CH3COOH, 99%); Titanium addition of metals, metal oxides, various elements iso - propoxide (TTIP) (Ti(OC3H7)4); Silver nitrate into TiO2 lattice such as Zn, Fe, Cr, Eu, Y, Ag, Ni; or (AgNO3); Glucose (C6H12O6); NH3 solution; the insertion of nonmetals such as N, C, S, F, Cl; or Hydroperoxide (H2O2); Reactive Red - 195 simultaneously insertion of different elements into (RR195) were purchased from China; Deionized the TiO2 crystal lattice, etc. Most of the modified water and ice were produced directly at the lab and products have higher catalytic activity than the used throughout. All the chemicals were used initial TiO2 in the visible light (Cotolan et al., 2016). without any further purification (except raw Among them, silver is one of the most highly halloysite). efficient ingredients (Guo et al., 2009; H. Dong et al., 2015). Over the years, the use of silver deposited 2.2. Halloysite (HNT) purification on semiconductors in the photocatalytic process has much interest (for example, in the degradation Raw halloysite was crushed and sieved to of organic pollutants, hydrogen production, remove particles and dried at 1000C, then 40 g of disinfection) because it can enhance the the material was dissolved with 55 ml of distilled photocatalytic activity and extend the light water. After that, a solution of 2 ml of 98% sulfuric absorption to the visible light region (Oros - Ruiz et acid was slowly added and stirred slowly at 900C al., 2013; Grabowska et al., 2013; Shan et al., 2008; for 90 minutes to obtain solution 1. This solution Ansari et al., 2013; Khan et al., 2013). Silver, like was filtered and washed several times with distilled water to remove excess H2SO4 and was
3. Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 21 dried then. Next, 1.5 litters of distilled water were 2θ = 800C. Nitrogen adsorption analyses were added to the mixture that has just been dried in carried out on Micromeritics ASAP 2000 at - 1960C. previous step. The mixture was stirred for 24 hours All the samples were outgassed at 1500C under more to acquire solution 2. Solution 2 was settled vacuum for 1 hour prior to the measurements. down for 96 hours and followed by a filtration. At Pore size distributions were determined using the this step, the upper part of solution 2 was decanted BJH method from desorption branches of the and, the residues at the bottom of the beaker were isotherms. EDX spectra was recorded by EDS JED - discarded. These separations were repeated 3 2300 Analysis Station. UV - vis spectra was times. Finally, pure halloysite was obtained via the obtained using Shimadzu UV - 3101 PC in the centrifugation, filtration and drying. ultraviolet region in the wavelength range of 200ữ800 nm. Transmission electron microscopy 2.3. Synthesis of Ag - TiO2, TiO2 - HNT, Ag - (TEM) was observed by a JOEL JEM - 1010 Electron TiO2/HNT Microsope while scanning electron microscope was taken by Philips XL30 microscope. First, 3.3 ml of CH3COOH was cooled down in ice until completely frozen. Then, 6.6 ml of C H OH 2 5 2.5. Measurement of photocatalytic activity was added and stirred for about 30 minutes to obtain a completely transparent solution. TTIP (3.3 The photocatalytic activity was evaluated ml) was added to the solution and under magnetic through the photocatalytic degradation of RR - 195. stirring for 30 minutes. Then slowly added 280 ml The efficiency of this reaction is calculated by of distilled H2O to the solution, stirring for about 30 determining the concentration of the dye before minutes, to obtain TiO2 sol. Next, 3 g of purified and after the reaction by UV - vis ultraviolet halloysite was put to TiO2 sol, and stirred at 650C spectroscopy. Specifically, the concentration of RR for 24 hours, then kept it in an autoclave for 5 hours - 195 was determined through the optical density at 1800C. The final solution was filtered, washed, obtained from ultraviolet spectroscopy at 541 nm. and dried at 700C to TiO2 - HNT. The efficiency of the degradation process is Repeat the procedure until the step of TTIP calculated by the following formula: addition. After that, 0.1 g of glucose was added H (%) = (C - C )/C (1) under mild stirring, and the solution was heated up o t o to 800C for 45 minutes. In which: Co - initial concentration of RR - 195 In the following step, a solution of AgNO3 (ppm); Ct - concentration of RR - 195 (ppm) after t 0.002M in excess NH3 was added into the previous (mins). solution under hard stirring for 15 minutes to All the concentrations were identified based obtain sol Ag - TiO2. After overnight aging, the gel on calibration curves obtained by UV - vis. was washed with acetonitrile repeatedly, vacuum - dried at 800C, and calcined in air at 5500C for 2 h to 3. Results and discussions produce Ag/TiO2 powders. To synthesize Ag - TiO2/HNT, the sol was 3.1. Characterization of purified HNT material gradually cooled down to 650C with the addition of 3g of purified halloysite under normal stirring for 3.1.1. X - ray diffraction results 24 hours. The sol was kept at 1800C in an autoclave To determine the crystallographic structure of for 5 hours. Finally, the solution was washed purified halloysite material, the X - ray diffraction several times with filtration, then dried at 700C to spectrum was performed in the range 2θ from 50C obtain Ag - TiO2/HNT. to 800C. The XRD results of halloysite material are shown in Figure 1. 2.4. Characterization Wide - angle X - ray spectra of the purified The phase structure of fabricated products halloysite sample show the appearance of was characterized by X - ray diffraction (Bruker, D8 characteristic peaks of halloysite nanotubes, in advance) using CuKλ (λ= 0.15406 nm) radiation. which the typical reflectance angles are at 2θ at The data were collected in the range of 2θ = 50C to 12.20 (001); 19.90 (100); 24.840 (002); 35.020 (110); 37.980 (003) and 54.340 (211) are
4. 22 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 consistent with standard X - ray diffraction data of method was conducted, and the scanning electron halloysite (HNT) nanotubes (Kamble et al., 2012). microscopy (SEM) image was obtained as shown in Figure 2. 3.1.2. SEM images of purified HNT The SEM image reveals that most of the To determine the specific surface morphology purified halloysite has the form of tubes in nano for the purified halloysite material, the SEM size with porous structure and high uniformity. Figure 1. X - ray patterns (XRD) of purified halloysite. Figure 2. SEM images of purified Halloysite.
5. Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 23 Specifically, the length of the tubes is about 1.3 μm, and diameter is about 130 nm, which is in accordance with results published in the literature (Rooj et al., 2010). The above results prove that halloysite (HNT) nanotubes have been successfully achieved from the raw halloysite source in Vietnam. 3.1.3. Nitrogen adsorption - desorption results To determine the specific surface area and pore volume as well as pore diameter of HNT, the BET nitrogen adsorption - desorption isotherm at 77.3 K according was carried out for halloysite nanotube material samples. Figure 4. X - ray (XRD) spectrum of different The curves are exhibited in Figure 3, showing catalysts. that the adsorption - desorption isothermal curve of the HNT is of type IV and has a hysteresis curve H3 corresponding to the mesoporous material according to IUPAC classification. The pore size distribution curve illustrates that the mean pore diameter of HNT was 4.8 nm. Typical parameters of HNT obtained from BET measurement results: - Specific surface area (SBET): 28.0186 m2/g - Total pore volume (Vpore): 0.137840 cm3/g - Pore diameter (DBJH): 21.8489 nm. Figure 5. Graph of [F(R)hν]2 as a function of hν for Ebg calculation. Figure 3. Nitrogen adsorption - desorption isotherm at 77.3 K of HNT. 3.1.4. Characterization of Ag - TiO2/HNT The results of XRD, UV - vis solid, EDX and SEM of Ag - TiO2/HNT materials are exhibited in Figures 4ữ7. X - ray spectrum of Ag - TiO2/HNT material (Figure 4) still confirms the existence of the characteristic peaks of TiO2, which proves that Figure 6. EDX spectrum of Ag - TiO2/HNT. there is no change in the structure of TiO2 material.
6. 24 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 Figure 7. SEM (a and b), TEM (c and d) images of Ag - TiO2/HNT. There are additional peaks which appear at 2θ = TiO2/HNT catalyst and no different elements 38.1 (111) and 64.4 (220) and weaker intensity appear. Additionally, the mass percentage of the peak at 2θ = 44.3 (200) characterizing the presence elements are quite compatible with the theoretical of metallic silver (JCPDS 65 - 2871). The obtained calculation used in the first step to synthesize Ag - results demonstrate that the insertion of TiO2 and TiO2/HNT. Ag - TiO2 on the HNT support causes no effect on SEM, TEM images (Figure 7) presents that the the halloysite nanotube structure. implantation of Ag - TiO2 onto halloysite nanotubes The band - gap energy Eg of the photocatalytic did not break the halloysite's tube structure. The materials TiO2, Ag - TiO2, TiO2 - HNT, Ag - TiO2/HNT images show the uniform dispersion of the Ag - were 3.2 eV, 2.8 eV, 3.0 eV, and 2.6 eV, respectively TiO2 nanoparticles on halloysite (HNT) nanotubes that were indicated in Figure 5. Those values and the Ag nanoparticles that are tightly attached appear to verify that the combination of HNT and on the surface of the TiO2 nanorods to form Ag - TiO2 has adjusted the band - gap of TiO2 3.2ữ3 eV. TiO2 nanorods with an average size is about 10ữ20 Furthermore, the addition of Ag onto the surface of nm. TiO2 has drastically reduced the band - gap of TiO2 As can be described in Figure 8 and Table 1, Ag to 2.8 eV that could consequently lead to a - TiO2/HNT has twice as much specific surface area significant influence on the photocatalytic and significantly increased pore volume as the HNT properties of TiO2. Thereby, the excited zone of Ag support. It could be explained that Ag - TiO2 also - TiO2/HNT shifted considerably from the has its own specific surface area and has its porous ultraviolet (UV) to the visible (Vis) region, and the system. As a result, the presence of Ag - TiO2 on the material has a narrower band - gap (2.6 eV) inner and outer surface of halloysite nanotubes compared to pure TiO2 (3.2 eV) could contribute to the number of pores Also, the X - ray energy dispersive spectrum of representing for the higher surface area as well as Ag - TiO2/HNT in the binding energy range of 0ữ10 the total pore volume. The higher pore volume of keV in Figure 6 points out the corresponding Ag - TiO2/HNT could guarantee the better signals of the elements that make up the Ag - performance of this catalyst in terms of pollutants
7. Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 25 adsorption. Based on the evidence given by various photocatalytic activity. This can be elucidated by characterization techniques, it could be concluded the existence of Ag - TiO2 on the surface of that Ag - TiO2 was successfully grafted on the halloysite reduced the band - gap energy of TiO2 surface of HNT. Moreover, Ag - TiO2 particles in (2.6 eV). In addition, Ago can be a center of charge nano size, which were distributed evenly on the transition and has high ability to trap electrons to surface, could bring efficient catalytic activity to the prevent recombination of photo - generated as - prepared nanocomposite. electrons and holes on TiO2 surface. At the same time, silver also has a plasmon resonance effect resulting in the more OH radical generation and increased photocatalytic efficiency. Figure 8. Nitrogen adsorption - desorption isotherm at 77.3K of Ag - TiO2/HNT catalyst. Figure 9. Conversion of RR - 195 in photocatalytic Table 1. Parameters of Ag - TiO - HNT obtained 2 degradation process using different catalysts. from BET method. Parameter Value 3.2.2. Impact of H O concentration on the Specific surface area (S - m2/ g) 57.3668 2 2 BET conversion of RR - 195 Pore diameter (DBJH - nm) 10.6906 Pore volume (Vpore - cm3/ g) 0.163701 H2O2 has an important impact on the Micropore volume (cm3/ g) 0.000193 photocatalytic degradation of pollutants. The effect of H2O2 in RR - 195 was investigated, and the results are presented in Figure 10. 3.2. Photocatalytic activity of Ag - TiO2/HNT As can be observed in Figure 10, when using 2 ml of H2O2 with the presence of Ag - TiO2/HNT 3.2.1. Comparison of photocatalytic activity of catalyst under UV irradiation, the conversion of RR synthesized materials - 195 was 79% in a period of 360 minutes. When increasing the amount of H O to 4ml, the The photocatalytic decomposition of RR - 195 2 2 mineralization of RR - 195 after 360 minutes did was performed at room temperature with the not occur faster but tended to decrease (76%). In following conditions: the initial concentration of addition, if the amount of H O is reduced to 1ml RR - 195 in water was 50 ppm, then 1.5 ml of H O 2 2 2 2 and 1.5 ml, the rate as well as the decomposition was added with 0.05 g solid catalyst. Next, the efficiency RR - 195 decreased also standing at 66% mixture was stirred for 3 hours to reach and 76%, respectively. equilibrium adsorption. After that, the mixture was This is because more •OH radicals generated irradiated with UV light for the next 3 hours. The from H O promote the reaction leading to results of the catalytic activity evaluation are 2 2 increased decomposition rate and efficiency. presented in Figure 9. However, when the amount of H O in the solution The results pointed out that the Ag - TiO /HNT 2 2 2 is too high or too low will reduce the free radicals synthesized by the direct method has the highest •OH occurs according to the equation (Szczepanik
8. 26 Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28 et al., 2017; Abdel Fattah et al., 2016; Natarajan et results with a relatively new photocatalytic al., 2015): material system based on the halloysite natural mineral. Specifically, purified halloysite (HNT) H2O2 + •OH •HO2 + H2O (2) nanotubes have been processed from raw •HO2 + •OH O2 + H2O halloysite sources in kaolin mines in a specific surface area of 28.0186 m2/g with an average In addition, a high amount of H2O2 also results in the saturation of the active sites of the catalyst, length of 1.3 μm and average diameter of 130 nm. thereby diminishing the reaction rate. For that Ag - TiO2/HNT photocatalytic material was then synthesized by the direct method. The data showed reason, 1.5 ml of H2O2 was chosen to be applied to all remaining research processes to best assess the that the doping of Ag significantly narrowed the catalytic activity of the catalyst materials. band - gap energy of pure TiO2. Thus, the material could be easily excited in the visible region, which 3.2.3. Influence of catalyst weight makes it more efficient in the photocatalytic process. In addition, the specific area and pore The influence of the catalyst content on the volume of Ag - TiO2/HNT were significantly conversion of dye RR - 195 on Ag - TiO2/HNT improved in comparison with HNT and TiO2. The catalyst was investigated. The results are shown in photocatalytic activity evaluation indicated that Figure 11. the Ag - TiO2/HNT had superior efficiency The results showed that after 360 minutes of compared to the other catalysts namely TiO2; Ag - UV irradiation using 0.15 g of Ag - TiO2/HNT, the TiO2; TiO2/HNT thanks to the presence of Ag and efficiency in RR - 195 degradation was 96%. This the better distribution of active phase on HNT value is considerably higher than the ones obtained support. As a result, Ag - TiO2/HNT could almost when 0.05g and 0.1g catalyst was applied, which completely decomposed RR - 195 at room only reached the conversion of 66% and 90%, temperature and neutral pH environment. This respectively. This can be explained as follows: suggests that Ag - TiO2/HNT material could offer when the amount of catalyst increases, the amount extreme potential in the treatment of polluted of catalytic activity sites increases, causing the wastewater. diffusion rate of the anions RR - 195 to the active sites on the surface, leading to an increase the Author contributions number of 96% shows the almost complete decomposition of contaminants under normal Son Ha Ngo and Nui Xuan Pham conceived and conditions. planned the experiments; Tuan Ngoc Tran carried out the experiments; Son Ha Ngo and Tuan Ngoc 4. Conclusions Tran contributed to sample preparation. Son Ha Ngo and Nui Xuan Pham contributed to the This study has obtained some remarkable interpretation of the results; Son Ha Ngo and Tuan Figure 10. The conversion of RR - 195 when using Figure 11. Influence of catalyst weight in the Ag - TiO2/HNT at different H2O2 concentrations. degradation process of RR - 195.
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