Ce-MOF-coated CMC aerogel adsorbent as well as preparation method and application thereof

Abstract

The invention discloses a Ce-MOF-coated CMC aerogel adsorbent as well as a preparation method and application thereof, and belongs to the technical field of functional materials and wastewater treatment. The preparation method comprises the following steps: firstly, carrying out coordination self-assembly reaction by taking ellagic acid and cerium chloride hexahydrate as ligands to obtain Ce-MOF; then, the Ce-MOF is loaded on carboxymethyl cellulose, and the Ce-MOF coated CMC aerogel adsorption material is prepared and obtained. The Ce-MOF-coated CMC aerogel adsorption material prepared by the invention can be used for treating high-concentration and high-alkalinity printing and dyeing wastewater, and the dosage of the adsorption material is small.

Description

Technical Field

[0001] The present invention relates to a Ce-MOF@CMC aerogel adsorbent and a preparation method and application thereof, belonging to the technical field of functional materials and wastewater treatment. Background Art

[0002] Printing and dyeing wastewater refers to the various wastewaters discharged during the pretreatment, dyeing, printing, and finishing processes of textile products. Printing and dyeing wastewater has high chroma, complex chemical composition, and high dye concentration. Conventional methods for treating printing and dyeing wastewater include biological treatment, chemical flocculation, chemical oxidation, adsorption, electrochemical methods, and membrane separation. However, these methods are costly and difficult to implement. Therefore, adsorption is currently the most common method.

[0003] It is to use some porous adsorbents to purify wastewater, adsorb some of the components on its surface, and then separate these adsorbents from the wastewater by separation methods, so as to achieve the effect of purifying wastewater. Commonly used adsorbents include activated carbon (Cao, Y., Yang, L., Liu, F., et al. (2024) Adsorption Experiments and Mechanisms of Methylene Blue on Activated Carbon from Garden Waste via Deep Eutectic Solvents Coupling KOH Activation. Biomass and Bioenergy, 182, Article 107074. https: / / doi.org / 10.1016 / j.biombioe.2024.107074), ion exchange resins (Barman, MK, Bhattarai, A. and Saha, B. (2023) Applications of Ion Exchange Resins in Environmental Remediation. Vietnam Journal of Chemistry, 61, 533-550. https: / / doi.org / 10.1002 / vjch.202300027), modified bio-based materials (CN108658160A, CN104984742B), nanomaterials (nano-Fe3O4 adsorbents (Zhang Xiaoshan, Wen Chunxiao, He Ning, et al. Research progress of adsorbent nano-Fe3O4 in dye wastewater treatment [J]. Guangdong Chemical Industry, 2020, 047(024): 80-81.), MOF materials (Zhou Y, Wang D, Feng QA facile synthesis of a Ce-based MOF at room temperature for effective adsorption of methylene blue [J]. CrystEngComm, 2024, 26(14): 2009-2017.), etc.; However, these methods have the problem that it is difficult to adsorb high-concentration and high-alkalinity printing and dyeing wastewater and a large amount of adsorbent is required.

[0004] Moreover, the properties of different dyes vary greatly, so dye adsorbents cannot be completely universal; it is also impossible to simply judge what dyes the prepared adsorbent can be used to adsorb based on the properties of the raw materials; that is, it is also difficult to select adsorbents for adsorbing dyes.

[0005] In addition, aerogels have a three-dimensional porous structure, high porosity and low density. These characteristics make aerogels have great application potential in the field of printing and dyeing wastewater treatment. At present, the document (CN 117358216 A) discloses a MOF / coconut composite aerogel and its preparation method and application. The composite aerogel uses coconut aerogel as a matrix and loads MOFs-808 (Zr 4+ ) material, which can be used to adsorb Rhodamine B, and what it can adsorb is an acidic system at pH = 3.0. Summary of the Invention

[0006] [Technical Issues]

[0007] Conventional methods for treating printing and dyeing wastewater have the problem of difficulty in adsorbing high-concentration, high-alkalinity dye wastewater and requiring a large amount of adsorbent;

[0008] The selection of adsorbents for adsorbing dyes is also difficult;

[0009] The current method of combining MOF and aerogel to treat printing and dyeing wastewater cannot adsorb dyes under alkaline conditions.

[0010] [Technical solution]

[0011] To address this issue, the present invention first uses ellagic acid and cerium chloride hexahydrate as ligands in a coordinated self-assembly reaction to produce Ce-MOF. The Ce-MOF is then loaded onto carboxymethyl cellulose to create a Ce-MOF@CMC aerogel adsorption material. The Ce-MOF@CMC aerogel adsorption material prepared in this invention can treat high-concentration, highly alkaline printing and dyeing wastewater while requiring minimal adsorption material.

[0012] The first object of the present invention is to provide a method for preparing Ce-MOF, comprising the following steps:

[0013] (1) adding ellagic acid and cerium chloride hexahydrate into a solvent and mixing them uniformly to obtain a mixture; wherein the molar ratio of ellagic acid to cerium chloride hexahydrate is 1:0.4-0.6;

[0014] (2) adding acetic acid to the mixture and carrying out coordination self-assembly reaction at 110-125° C. for 1-3 hours; after the reaction is completed, cooling, washing, and drying to obtain Ce-MOF.

[0015] In one embodiment of the present invention, the solvent in step (1) is a mixed solution of water and N,N-dimethylformamide, wherein the volume ratio of water to N,N-dimethylformamide is 1:1 to 3, more preferably 1:2.

[0016] In one embodiment of the present invention, the usage ratio of ellagic acid, cerium chloride hexahydrate, solvent and acetic acid in step (1) and step (2) is 1 mmol: 0.4-0.6 mmol: 10-20 mL: 7-8 mL.

[0017] In one embodiment of the present invention, after acetic acid is added in step (2), the mixture can be stirred and mixed uniformly before reacting.

[0018] In one embodiment of the present invention, the washing in step (2) is performed sequentially with DMF, ethanol, and deionized water.

[0019] In one embodiment of the present invention, the drying in step (2) is performed at 75-85° C. for 10-20 h.

[0020] In one embodiment of the present invention, the water in step (1) and step (2) is preferably deionized water.

[0021] The second object of the present invention is the Ce-MOF prepared by the method described in the present invention.

[0022] The third object of the present invention is to provide a method for preparing Ce-MOF@CMC aerogel adsorption material, comprising the following steps:

[0023] Ce-MOF is dispersed in a carboxymethyl cellulose solution to form a mixed solution, which is then injected into an aluminum nitrate solution for cross-linking to form microspheres. The microspheres are taken out, washed, and then freeze-dried to obtain Ce-MOF@CMC aerogel adsorption material.

[0024] In one embodiment of the present invention, the carboxymethyl cellulose solution is a carboxymethyl cellulose aqueous solution, wherein the mass ratio of carboxymethyl cellulose to water is 1:25 to 35, more preferably 1:30.

[0025] In one embodiment of the present invention, the mass ratio of Ce-MOF to carboxymethyl cellulose solution is 5-30 mg:1 g.

[0026] In one embodiment of the present invention, the aluminum nitrate solution is an aluminum nitrate aqueous solution with a concentration of 2-4%, where % is mass percentage.

[0027] In one embodiment of the present invention, the injection speed of the mixed solution is 0.3-1 mL / min.

[0028] In one embodiment of the present invention, the crosslinking is carried out at 20-30° C. for 20-30 h.

[0029] In one embodiment of the present invention, the microspheres can be removed by filtration.

[0030] In one embodiment of the present invention, washing is performed with water.

[0031] In one embodiment of the present invention, freeze drying is freeze drying at -60 to -40°C for 10 to 30 hours.

[0032] The fourth object of the present invention is the Ce-MOF@CMC aerogel adsorption material prepared by the method described in the present invention.

[0033] The fifth object of the present invention is the application of the Ce-MOF@CMC aerogel adsorption material described in the present invention in the field of wastewater treatment.

[0034] In one embodiment of the present invention, the wastewater is printing and dyeing wastewater.

[0035] The sixth object of the present invention is to provide a method for treating printing and dyeing wastewater, which adopts the Ce-MOF@CMC aerogel adsorption material described in the present invention.

[0036] In one embodiment of the present invention, the printing and dyeing wastewater contains cationic dyes, and the cationic dyes are one or more of methylene blue, crystal violet, and acid chrome blue K.

[0037] In one embodiment of the present invention, the concentration of various dyes in the printing and dyeing wastewater is in the range of 0.005-0.5 mmol / mL.

[0038] The seventh object of the present invention is to provide a methylene blue dye adsorbent, which adopts the Ce-MOF@CMC aerogel adsorption material described in the present invention.

[0039] [Beneficial Effects]

[0040] (1) The Ce-MOF and Ce-MOF@CMC aerogel adsorption materials in the present invention have a short synthesis route, a simple preparation method, and mild preparation conditions.

[0041] (2) The Ce-MOF prepared by the present invention has good acid-base stability.

[0042] (3) The present invention loads Ce-MOF on cellulose aerogel, which is conducive to secondary recycling.

[0043] (4) The raw materials used in the present invention are all green and environmentally friendly materials, which are environmentally friendly and will not cause secondary pollution.

[0044] (5) The Ce-MOF@CMC aerogel adsorption material used in the present invention can treat high-concentration, high-alkalinity dye wastewater, and the amount of adsorption material used is small. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is the SEM image of Ce-MOF prepared in Example 1.

[0046] Figure 2 This is a schematic diagram of XRD of Ce-MOF prepared in Example 1.

[0047] Figure 3 This is an analysis chart of the acid-base stability of Ce-MOF prepared in Example 1.

[0048] Figure 4 Schematic diagram of BET and pore size distribution of Ce-MOF prepared in Example 1.

[0049] Figure 5 This is the SEM image of Ce-MOF prepared in Comparative Example 1.

[0050] Figure 6 This is the SEM image of Ce-MOF prepared in Comparative Example 2.

[0051] Figure 7 This is the SEM image of Ce-MOF prepared in Comparative Example 3.

[0052] Figure 8 This is a physical picture of the Ce-MOF@CMC aerogel adsorption material prepared in Example 2.

[0053] Figure 9 This is the adsorption effect diagram of the Ce-MOF@CMC aerogel adsorption material prepared in Example 2.

[0054] Figure 10 This is a diagram showing the adsorption effect of the Ce-MOF@CMC aerogel adsorption material prepared in Example 2 in treating dye wastewater at different pH values. DETAILED DESCRIPTION

[0055] The following describes preferred embodiments of the present invention. It should be understood that the embodiments are for better explanation of the present invention and are not intended to limit the present invention.

[0056] Test method:

[0057] 1. Test method for acid and alkali stability:

[0058] The synthesized MOF material was immersed in solutions with different pH values ​​(pH=3.0, 7.0, 11.0) for 24 hours, and then taken out and tested for XRD to determine whether its structure had changed.

[0059] 2. BET and pore size distribution test:

[0060] The BET test (Brunauer-Emmett-Teller test) is an analytical method for measuring the specific surface area and pore size distribution of a substance.

[0061] It is based on the principle of physical adsorption and calculates the specific surface area of ​​the sample by measuring the relationship between the amount of gas (usually nitrogen) adsorbed on the sample surface and the relative pressure.

[0062] The raw materials used in the embodiment are:

[0063] Ellagic acid: purity 99%, CAS: 476-66-4, purchased from Shanghai Titan Technology Co., Ltd.

[0064] Cerium chloride hexahydrate: purity 99%, purchased from Shanghai Titan Technology Co., Ltd.

[0065] Acetic acid: purity is 99.5%.

[0066] Carboxymethyl cellulose: molecular weight: 10 5 -10 6， Viscosity: 600-1000mPa.s.

[0067] Water: deionized water.

[0068] Example 1

[0069] A method for preparing Ce-MOF comprises the following steps:

[0070] (1) 1 mmol of ellagic acid and 0.5 mmol of cerium chloride hexahydrate were added to 5 mL of water and 10 mL of N,N-dimethylformamide and mixed to obtain a mixture;

[0071] (2) Add 7.5 mL of acetic acid to the mixture, stir at 500 rpm for 30 min to mix evenly, then pour into a reactor and carry out coordination self-assembly reaction at 120 ° C for 2 h; after the reaction is completed, cool naturally to room temperature, wash with DMF, ethanol, and deionized water in sequence, and dry in a thermostat at 80 ° C for 12 h to obtain Ce-MOF.

[0072] The Ce-MOF was subjected to performance testing, and the test results are as follows:

[0073] Figure 1 This is the SEM image of Ce-MOF prepared in Example 1. Figure 1 It can be seen that the structure of MOF material is regular and uniform.

[0074] Figure 2 This is a schematic diagram of XRD of Ce-MOF prepared in Example 1. Figure 2 It can be seen that there are no obvious impurity peaks, indicating that the sample is relatively pure and has a high degree of crystallinity.

[0075] Figure 3 This is an analysis chart of the acid-base stability of Ce-MOF prepared in Example 1. Figure 3 It can be seen that MOF materials can still maintain their structural integrity under strong acid (pH=3) and strong base (pH=11) conditions.

[0076] Figure 4 The BET and pore size distribution diagram of Ce-MOF prepared in Example 1. Figure 4 It can be seen that Ce-MOF material has a large specific surface area, reaching 44.063m 2 / g, and the pore diameters are all mesopores and micropores.

[0077] Comparative Example 1

[0078] The coordination self-assembly reaction in step (2) of Example 1 was adjusted to be carried out at 120°C for 12 h; the other steps were kept consistent with Example 1 to obtain Ce-MOF (SEM as shown in FIG. Figure 5 ).

[0079] Comparative Example 2

[0080] The coordination self-assembly reaction in step (2) of Example 1 was adjusted to be carried out at 80°C for 12h; the other steps were kept the same as in Example 1 to obtain Ce-MOF (SEM as shown in FIG. Figure 6 ).

[0081] Comparative Example 3

[0082] The coordination self-assembly reaction in step (2) of Example 1 was adjusted to be carried out at 25°C (room temperature) for 48h; the other steps were kept consistent with Example 1 to obtain Ce-MOF (SEM as shown in FIG. Figure 7 ).

[0083] The obtained Ce-MOF was subjected to performance testing, and the test results are as follows:

[0084] Table 1

[0085]

[0086]

[0087] It can be seen from Table 1 that the surface area of ​​Ce-MOF prepared in Example 1 is the largest.

[0088] Example 2

[0089] A method for preparing Ce-MOF@CMC aerogel adsorption material comprises the following steps:

[0090] 15 mg of Ce-MOF prepared in Example 1 was dispersed in 1 g of carboxymethyl cellulose aqueous solution (the mass ratio of carboxymethyl cellulose to water was 1:30) to form a mixed solution;

[0091] The mixed solution was then injected into a 3% aluminum nitrate aqueous solution at an injection rate of 0.5 mL / min and cross-linked at 25°C (room temperature) for 24 h to form microspheres.

[0092] The microspheres were filtered out, washed with water, and then freeze-dried at -40 °C for 20 h to obtain Ce-MOF@CMC aerogel adsorption material (see the actual picture). Figure 8 ).

[0093] Comparative Example 4

[0094] The carboxymethyl cellulose in Example 2 was adjusted to polyimide;

[0095] The details are as follows:

[0096] (1) Under nitrogen protection, first add diaminodiphenyl ether (ODA) (10.2 mmol, 2.041 g), biphenyltetracarboxylic dianhydride (BPDA) (10.5 mmol, 3.089 g) and 37.620 g of N-methylpyrrolidone (NMP) into a beaker, and stir magnetically for 5 hours in an ice-water bath under nitrogen sealing conditions to form a transparent solution to prepare an anhydride-terminated PAA solution. Then add 1,3,5-tris(4-aminophenoxy)benzene (TAB) (0.2267 mmol, 0.0905 g) and stir for 15 minutes to obtain a polyamic acid (PAA) solution with a certain viscosity;

[0097] (2) Add dehydrating agent acetic anhydride (84 mmol, 7.95 mL) and catalyst pyridine (84 mmol, 6.75 mL) to the polyamic acid (PAA) solution in sequence. After stirring for 5 minutes, add 220 mg of Ce-MOF and stir evenly to obtain a bright yellow viscous PI@MOF solution.

[0098] (3) The PI@MOF solution was placed in a sealed container and aged for 24 hours. The solution was then taken out and immersed in acetone to replace the internal solvent. Each displacement lasted for 24 hours and was repeated three times. The solution was then immersed in 10 times tert-butanol to continue the solvent replacement. Each displacement lasted for 24 hours and was repeated three times to obtain a wet gel.

[0099] (4) The wet gel was placed directly in a cold trap in a freeze dryer and frozen at -70 °C for 4 h. After ensuring that the internal solvent was completely converted into a solid state, it was taken out and placed in the upper layer for vacuum freeze drying for 48 h to obtain an aerogel adsorption material.

[0100] Comparative Example 5

[0101] The carboxymethyl cellulose in Example 2 was replaced with silicon dioxide, and the other aspects remained the same as in Example 2 to obtain an aerogel adsorption material;

[0102] The details are as follows:

[0103] (1) Anhydrous ethanol (EtOH), tetraethyl orthosilicate (TEOS) and an appropriate amount of hydrochloric acid solution were mixed in a volume ratio of 30:1 (to adjust the pH value of the mixed solution to 3-4), and then stirred at room temperature for 90 minutes to obtain a precursor solution;

[0104] (2) Dispersing 1 g of Ce-MOF in 9 g of ethanol to obtain a Ce-MOF dispersion; adding 0.01 g of the Ce-MOF dispersion to 1 g of the precursor solution to obtain a Ce-MOF-doped sol;

[0105] (3) After the Ce-MOF-doped sol was magnetically stirred for 12 h and fully hydrolyzed and dispersed, 1 mol / L ammonia solution was added to 1 g of MOFs / SiO2 sol until the pH value reached 6-7. To avoid precipitation of Ce-MOF powder, the amount of ammonia was controlled to keep the gelation time within 10 min.

[0106] (4) The sol is poured into a mold and aged at room temperature. The wet gel is then placed in an ethanol bath at room temperature to remove any remaining impurities or moisture. Finally, the gel is freeze-dried to obtain a MOFs / SiO2 aerogel adsorption material.

[0107] Example 3

[0108] A method for treating printing and dyeing wastewater, which uses the aerogel adsorption materials prepared in Example 2 and Comparative Examples 4 and 5;

[0109] The details are as follows:

[0110] The aerogel adsorption material was fully dried, and then 10 mg of aerogel adsorption material and 10 mL of a 0.5 mmol methylene blue aqueous dye solution were added to a sealable 25 mL glass bottle. Adsorption was performed in an oscillating water bath at room temperature, and the remaining dye concentration was measured at intervals until equilibrium was reached.

[0111] The test results are as follows:

[0112] Figure 9 This is the adsorption effect diagram of the Ce-MOF@CMC aerogel adsorption material prepared in Example 2. Figure 9 It can be seen that Ce-MOF@CMC has a good adsorption capacity for the simulated pollutant methylene blue.

[0113] Table 2

[0114] example substrate Adsorption capacity (mg / g) Example 2 Carboxymethyl cellulose 300 Comparative Example 4 Silicon dioxide 49.2 Comparative Example 5 polyimide 244

[0115] It can be seen from Table 2 that the adsorption amount of methylene blue by the Ce-MOF@CMC aerogel adsorption material prepared in Example 2 is much greater than that of the aerogel adsorption materials in Comparative Examples 4 and 5.

[0116] Example 4

[0117] The mass of Ce-MOF in Example 2 was adjusted to 5 mg, 10 mg, 15 mg, 25 mg, and 30 mg, and the others remained consistent with Example 2 to obtain Ce-MOF@CMC aerogel adsorption materials with different MOF loading amounts.

[0118] The obtained Ce-MOF@CMC aerogel adsorption material was subjected to performance testing, and the testing method was as follows:

[0119] The aerogel adsorption material was fully dried, and then 10 mg of aerogel adsorption material and 10 mL of a 0.5 mmol methylene blue aqueous dye solution were added to a sealable 25 mL glass bottle. Adsorption was performed in an oscillating water bath at room temperature, and the remaining dye concentration was measured at intervals until equilibrium was reached.

[0120] The test results are as follows:

[0121] Table 3

[0122] Mass of Ce-MOF (mg) Adsorption capacity (mg / g) Adsorption rate (%) 5 150 47 10 200 62 15 (Example 2) 300 94 25 287 89 30 253 79

[0123] Note: Adsorption rate = (substance concentration before adsorption - substance concentration after adsorption) / substance concentration before adsorption.

[0124] It can be seen from Table 3 that the performance test of Ce-MOF@CMC aerogel adsorption material shows that the adsorption capacity of methylene blue solution with a concentration of 0.5 mmol / L can reach 300 mg / g.

[0125] Example 5

[0126] A method for treating printing and dyeing wastewater with different pH values, which uses the aerogel adsorption material prepared in Example 2;

[0127] The details are as follows:

[0128] The aerogel adsorption material was fully dried, and then 10 mg of the aerogel adsorption material and 10 mL of a 0.5 mmol methylene blue aqueous dye solution were added to a sealable 25 mL glass bottle. The pH was adjusted to 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 by adding 1 mol / L aqueous hydrochloric acid solution or 1 mol / L aqueous sodium hydroxide solution. Adsorption was carried out in an oscillating water bath at room temperature, and the residual dye concentration was measured at intervals until equilibrium was reached.

[0129] The test results are as follows:

[0130] Figure 10 This is the adsorption effect diagram of the Ce-MOF@CMC aerogel adsorption material prepared in Example 2 when treating dye wastewater at different pH values. Figure 10 It can be seen that the adsorption effect of Ce-MOF@CMC aerogel adsorption material is best under alkaline conditions, which can reach 99%.

[0131] Example 6

[0132] A method for treating printing and dyeing wastewater, which uses the aerogel adsorption material prepared in Example 2;

[0133] The details are as follows:

[0134] The aerogel adsorption material was fully dried, and then 10 mg of aerogel adsorption material and 10 mL of a 0.25 mmol dye solution were added to a sealable 25 mL glass bottle. Adsorption was performed in an oscillating water bath at room temperature. The remaining dye concentration was measured at intervals until equilibrium was reached.

[0135] Among them, the dyes are methylene blue, crystal violet, acid chrome blue K, and methyl orange.

[0136] The test results are as follows:

[0137] Table 4

[0138] Dye name Adsorption rate (%) Methylene blue ＞99 Crystal violet 40 Acid Chrome Blue K 35 Methyl orange 36

[0139] Although the present invention has been disclosed above in terms of preferred embodiments, it is not intended to limit the present invention. Anyone familiar with this technology can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the definition of the claims.

Claims

1. A method for preparing Ce-MOF, characterized in that: The steps include: (1) adding ellagic acid and cerium chloride hexahydrate into a solvent and mixing them uniformly to obtain a mixture; wherein the molar ratio of ellagic acid to cerium chloride hexahydrate is 1:0.4-0.6; (2) adding acetic acid to the mixture and carrying out coordination self-assembly reaction at 110-125° C. for 1-3 hours; after the reaction is completed, cooling, washing, and drying to obtain Ce-MOF.

2. The method according to claim 1, characterized in that The usage ratio of ellagic acid, cerium chloride hexahydrate, solvent and acetic acid in step (1) and step (2) is 1 mmol: 0.4-0.6 mmol: 10-20 mL: 7-8 mL.

3. Ce-MOF prepared by the method according to claim 1 or 2.

4. A method for preparing Ce-MOF@CMC aerogel adsorption material, characterized in that: The steps include: The Ce-MOF according to claim 3 is dispersed in a carboxymethyl cellulose solution to form a mixed solution, and then the mixed solution is injected into an aluminum nitrate solution for cross-linking to form microspheres. The microspheres are taken out, washed, and then freeze-dried to obtain a Ce-MOF@CMC aerogel adsorption material.

5. The method according to claim 4, characterized in that The mass ratio of Ce-MOF and carboxymethyl cellulose solution is 5-30 mg:1 g.

6. Ce-MOF@CMC aerogel adsorption material prepared by the method according to claim 4 or 5.

7. Use of the Ce-MOF@CMC aerogel adsorption material according to claim 6 in the field of wastewater treatment.

8. A method for treating printing and dyeing wastewater, characterized in that: The Ce-MOF@CMC aerogel adsorption material according to claim 6 is used.

9. The method according to claim 8, characterized in that The printing and dyeing wastewater contains cationic dyes, which are one or more of methylene blue, crystal violet, and acid chrome blue K; the concentration range of various dyes in the printing and dyeing wastewater is 0.005-0.5mmol / mL.

10. A methylene blue dye adsorbent, characterized in that: The Ce-MOF@CMC aerogel adsorption material according to claim 6 is used.

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