A xanthan gum-polyacrylic acid bonded porous graphite carbon nitride composite electrode and its preparation method and application in uranium electrosorption

Abstract

The present invention discloses a xanthan gum-polyacrylic acid bonded porous graphitic carbon nitride composite electrode, its preparation method and application in electro-adsorbing uranium. After uniformly mixing a carbon and nitrogen source and a strong base by a liquid phase method, porous g-C3N4 is obtained through drying and calcination in sequence; the porous g-C3N4 is mixed with conductive carbon black, a xanthan gum-polyacrylic acid composite binder and a solvent to form a slurry, and the slurry is coated on a plate electrode, dried and cured to obtain the composite electrode. The xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode is used for electro-adsorbing U(VI) in water, and has the characteristics of fast rate, high adsorption efficiency, large adsorption capacity, easy desorption and recyclability, greatly reducing the cost of electro-adsorbing and recovering uranium.

Description

Technical Field

[0001] The invention relates to an electrode, in particular to a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, and also to a preparation method thereof and application in electrical adsorption of uranium. Background Art

[0002] As a clean and low-carbon energy source, uranium-based nuclear power plays a vital role in partially replacing fossil fuel energy. At the same time, its sustainability has also increasingly attracted public attention. One of the concerns is the global shortage of uranium supply. Currently, the use of uranium mainly comes from limited terrestrial ores, and it is estimated that these ores will only be available for human use for about 70 years. Another concern is the environmental impact. During the mining and processing of nuclear fuel, a large amount of aqueous solution containing uranium (U(VI)) will be produced. Due to the radioactivity and chemical toxicity of U(VI), if it is not properly treated, its emission will threaten the ecosystem and even harm human health. Therefore, the capture and removal of uranium (VI) is of great significance because it can simultaneously solve the shortage of conventional uranium ores and the negative impact on the ecological environment. To date, there are many technologies for capturing and removing U(VI) from aqueous solutions, including chemical precipitation, membrane separation, ion exchange, solvent extraction, and adsorption. However, these technologies have problems such as complex processes, large amounts of reagents, easy secondary pollution, high cost, and low efficiency.

[0003] Electrosorption, also known as capacitive deionization (CDI), is an emerging separation technology that was originally developed for seawater desalination based on the electrical double layer (EDL) theory. Recently, it has been explored for the separation and enrichment of U(VI) from aqueous solutions due to its low energy consumption, environmental friendliness, and long-term reusability. During the electrosorption process for extracting U(VI), by applying a low voltage (≤2.0 V) across the positive and negative electrode pairs, the uranyl cations and counter anions migrate to the oppositely charged electrode surfaces, respectively, and are separated from the aqueous solution. By reversing or removing the voltage, the uranyl cations are released back into the solution, and the electrode materials are regenerated. As a key component of CDI technology, researchers have invested a lot of effort in the design of electrode materials in the past few years. Among them, carbon-based materials have been widely used for the electrosorption of U(VI), and graphene, as a two-dimensional (2D) carbon material, is particularly attractive due to its high specific surface area, good conductivity, and chemical stability. However, their adsorption performance for U(VI) is still limited by their limited effective active sites and the irreversible agglomeration and restacking of graphene sheets. In addition, the high manufacturing cost and complicated preparation process also hinder their large-scale practical application.

[0004] In recent years, nitrogen (N)-doped graphene has become a mainstream non-metallic electrocatalyst due to its excellent activity and the ability to provide more active sites. Two-dimensional (2D) graphitic carbon nitride (g-C3N4), a nitrogen-doped graphene analogue with periodic heptazine subunits bridged by planar tertiary amino groups, has attracted extensive attention in catalysis and energy conversion due to its rich nitrogen content, good electrical and optical structure, physicochemical stability, and simple preparation process and low cost. Its unique structure meets the key requirements as an ideal electrode for U(VI) capture. Most importantly, g-C3N4 has a two-dimensional sheet structure, which provides abundant diffusion pathways for ion embedding, which is beneficial to improve the specific capacity and also facilitate the full penetration of U(VI) ions into its internal structure. At the same time, the abundant nitrogen-containing groups in its structure can serve as active sites to anchor more U(VI) ions. So far, the application of g-C3N4 electrodes in electrosorption is still in its infancy, but shows great potential. For example, the literature (Efficient treatment of brine wastewater through a flow-through technology integrating desalination and photocatalysis. G. Ye, et al., Water Res. 157 (2019) 134-144) successfully achieved efficient purification and desalination of wastewater by adding g-C3N4 to the membrane electrode system. (Enhanced capacitive deionization of a low-concentration brackish water with protonated carbon nitride-decorated graphene oxide electrode. J. Yu, et al., Chemosphere 293 (2022) 133580) reported the incorporation of g-C3N4 into the membrane electrode system, and the results showed that the lone pair of electrons in the N atom of g-C3N4 can easily attract positively charged ions. Despite these advantages, the electrosorption performance of 2D g-C3N4 is still limited by the stacking properties of its 2D nanomaterial sheets, which may hinder the diffusion of U(VI) ions into the internal structure and reduce the effective contact with active sites. At the same time, the electrosorption performance of existing g-C3N4 electrode materials is still limited by the small number of surface active sites, poor wetting properties and its inherent common ion repulsion effect. Summary of the invention

[0005] In view of the defects of the prior art, the first object of the present invention is to provide a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, which uses porous graphite carbon nitride (g-C3N4) as an active material and multifunctional cross-linked xanthan gum-polyacrylic acid (XG-PAA) as a binder. The porous g-C3N4 has abundant in-plane nanopores, and the xanthan gum-polyacrylic acid binder gives its surface more oxygen-containing and nitrogen-containing active sites, and the two-dimensional porous g-C3N4 forms a strong three-dimensional interconnected conductive structure after bonding. These characteristics give the electrode a fast electron / ion transmission channel, reduce the common ion rejection performance, and improve the electrochemical performance and hydrophilicity. These unique structural features enable U(VI) to react with negatively charged COO - The groups form a micro-electric field to reduce the repulsion of the common ions, and then migrate to the active site and coordinate with the active site, and are finally electrocatalytically reduced to U(IV) deposition, greatly improving the electrical adsorption efficiency of U(VI).

[0006] The second object of the present invention is to provide a method for preparing a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, which is simple, low-cost and conducive to large-scale production.

[0007] The third object of the present invention is to provide an application of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, which is used for the electrosorption of U(VI) in water, and has the characteristics of fast rate, high adsorption efficiency, large adsorption capacity, easy desorption, and recyclability, thereby greatly reducing the cost of uranium recovery by electrosorption.

[0008] In order to achieve the above technical objectives, the present invention provides a method for preparing a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, the method comprising: mixing a carbon nitrogen source and a strong base evenly by a liquid phase method, and then drying and calcining in sequence to obtain porous g-C3N4; mixing the porous g-C3N4 with conductive carbon black, a xanthan gum-polyacrylic acid composite binder and a solvent to form a slurry, coating the slurry on a plate, drying, and curing to obtain the electrode.

[0009] The invention provides a method for preparing a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode. On the one hand, a two-dimensional sheet-like structure of g-C3N4 is etched to form pores, and then calcined at high temperature using a strong base to form a large number of nanopores on the surface of the two-dimensional nano-sheet-like g-C3N4. These pores form fast transmission channels for electrons / ions, which are conducive to the rapid diffusion and transmission of U(VI), thereby improving its electrical adsorption performance. On the other hand, a binder is used to bond the two-dimensional sheet-like structure of g-C3N4 into a stable three-dimensional spatial structure, so that it has an interlayer effect, which greatly improves the adsorption capacity of U(VI). At the same time, the three-dimensional structure of g-C3N4 has better conductivity, which greatly improves the electrical adsorption efficiency. Thirdly, a special xanthan gum-polyacrylic acid composite binder is used. Since the g-C3N4 is affected by its poor surface wettability and inherent common ion repulsion effect during the adsorption of U(VI) , while the traditional PVDF electrode binder is hydrophobic, resulting in poor surface wettability of the g-C3N4-based electrode, which hinders the full penetration of U(VI) ions into the internal structure and destroys the long-term capture of U(VI). More importantly, an intrinsic common ion repulsion effect occurs during the electrosorption process. When the positively charged U(VI) ions are electrosorbed onto the negatively charged cathode, ions with the same charge as the cathode will be repelled by the cathode. At the same time, the electrosorbed U(VI) ions will have a great repulsive effect on the U(VI) ions in the aqueous solution due to electrostatic repulsion. Obviously, this effect will seriously waste electrical energy and greatly reduce the electrosorption efficiency. The use of multifunctional cross-linked xanthan gum-polyacrylic acid composite binder can provide abundant oxygen- and nitrogen-containing active sites, reduce common ion repulsion, and have excellent electrochemical and hydrophilic properties. These unique structural features enable U(VI) ions to bind to the negatively charged COO in advance. - The groups form a micro-electric field to attract and reduce the repulsion of the co-ions, and then migrate to the active site and coordinate with the active site, and are finally electrocatalytically reduced to U(IV) deposition.

[0010] As a preferred solution, the mass ratio of the carbon nitrogen source to the strong base is 15:(0.1-1.0). If the ratio of the strong base is too low, a good etching pore-forming effect cannot be achieved. If the relative ratio of the strong base is too high, the g-C3N4 pores will penetrate or even collapse.

[0011] As a preferred solution, the carbon and nitrogen source is at least one of urea, melamine, dicyandiamide and cyanamide. These carbon and nitrogen sources can easily form g-C3N4 through complex reactions such as condensation and oxidation during high-temperature calcination.

[0012] As a preferred embodiment, the strong base is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide and barium hydroxide.

[0013] As a preferred solution, the calcination conditions are: calcination at a temperature of 500-600°C for 1-3 hours. If the calcination temperature is too low or the calcination time is too short, a good etching pore-forming effect cannot be achieved. On the contrary, if the calcination temperature is too high or the calcination time is too long, the g-C3N4 pores will penetrate or even collapse.

[0014] As a preferred solution, the xanthan gum-polyacrylic acid composite adhesive is composed of xanthan gum and polyacrylic acid in a mass ratio of (0.5-2):1. The two components of xanthan gum (XG) and polyacrylic acid (PAA) can form covalent crosslinks through polyester reaction, and can also form non-covalent crosslinks through intramolecular / intermolecular hydrogen bonds, and the abundant hydroxyl and carboxylic acid groups on the XG-PAA adhesive can interact with the N-containing groups of the porous g-CN through hydrogen bonds. - There may be electrostatic interaction between the NHP and the porous g-C3N4, so by controlling the ratio of the two within a suitable range, better bonding performance can be obtained to ensure that the bonding of the two-dimensional porous g-C3N4 can form a solid three-dimensional interconnected conductive structure, thereby enhancing the electrochemical performance of the composite electrode and maintaining its mechanical stability and cyclability. At the same time, the amount of polyacrylic acid introduced can be controlled to regulate the content of carboxyl groups introduced into the cross-linking system, and the negatively charged COO - The group can attract UO2 in advance 2+ ions, thereby weakening the CO - The repulsive effect of ions and the introduction of an appropriate amount of carboxyl groups can improve the wettability of the g-C3N4-based electrode and the contact performance between the U(VI) ions and the electrode in the aqueous solution. Therefore, by controlling the ratio of xanthan gum and polyacrylic acid within an appropriate range, a composite electrode with better performance can be obtained.

[0015] As a preferred solution, the mass percentage of porous g-C3N4, conductive carbon black, and xanthan gum-polyacrylic acid composite binder is 70-80%: 5-15%: 10-20%, measured with the total mass as 100%. If the proportion of xanthan gum-polyacrylic acid composite binder (XG-PAA) is too low, it is difficult to form a tough cross-linked network, the stability is poor, and the modification effect on porous g-C3N4 is also poor, and it is difficult to obtain a higher U (VI) adsorption efficiency. If the proportion of xanthan gum-polyacrylic acid composite binder is too high, the porosity of porous g-C3N4 will be reduced, affecting electron / ion transmission, and the resistance of the electrode will also be increased, reducing the current efficiency.

[0016] As a preferred solution, the curing condition is: curing at a temperature of 140-180° C. for 3-5 hours. Under the preferred conditions, chemical crosslinking can be achieved between xanthan gum and polyacrylic acid. If the temperature is too low, the degree of curing will be reduced, and it is difficult to obtain a highly stable bonding system.

[0017] The present invention also provides a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, which is obtained by the preparation method.

[0018] The present invention also provides an application of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, which is applied to the electrosorption of U(VI).

[0019] As a preferred solution, a graphite electrode is used as an anode, a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode is used as a cathode, U(VI) is used as an electrolyte, the pH of the electrolyte is 3-8, and the voltage applied between the anode and the cathode is less than 1.5V.

[0020] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0021] The xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode provided by the present invention uses porous graphite carbon nitride (g-C3N4) as an active material and multifunctional cross-linked xanthan gum-polyacrylic acid (XG-PAA) as a binder. The porous g-C3N4 has abundant in-plane nanopores, and the xanthan gum-polyacrylic acid binder gives its surface more oxygen-containing and nitrogen-containing active sites. The two-dimensional porous g-C3N4 forms a solid three-dimensional interconnected conductive structure after bonding. These characteristics give the electrode a fast electron / ion transmission channel, reduce the common ion rejection performance, and improve the electrochemical performance and hydrophilic performance. These unique structural features enable U(VI) to react with negatively charged COO - The groups form a micro-electric field to attract and reduce the repulsion of the common ions, and then migrate to the active site and coordinate with the active site, and finally be electrocatalytically reduced to U(IV) deposition, which greatly improves the electrosorption efficiency of U(VI). The electrosorption U(VI) experiment shows that the removal rate of the composite electrode can reach 98.1% (vs 59.7%) within 60 minutes. At the same time, U(VI) can be efficiently reduced to U(IV) and fixed on the electrode surface during the electrosorption process. Its cumulative adsorption capacity can reach 1459.1 mg g within 6 cycles. -1 , and has great application prospects in recycling and selective adsorption of U(VI).

[0022] The preparation method of the xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode of the present invention is simple, low in cost and is conducive to large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1:(a) Schematic diagram of the preparation of XG-PAA combined with porous g-C3N4, (b) the formation mechanism of porous g-C3N4 under the action of KOH, and (c) the possible interaction between XG-PAA binder and porous g-C3N4.

[0024] Figure 2 : (a)~(b) are SEM images of g-C3N4 at different magnifications, (c)~(d) are TEM images, (e) is the elemental mapping of g-C3N4, (f) is the HRTEM image of g-C3N4, and (g) is the d-spacing corresponding to the (002) crystal plane of 0.314nm.

[0025] Figure 3 : (a)~(b) are SEM images of g-C3N4-P-75, (c)~(e) are SEM images of g-C3N4-X-75 at different magnifications, and (f) is the element mapping of g-C3N4-X-75 electrode.

[0026] Figure 4 : (a) is the XRD spectrum of all samples, (b) is the FTIR spectrum, (c) is the fluorescence spectrum of g-C3N4-X-75 and (d) is the fluorescence spectrum of g-C3N4-P-75 electrodes, (e) is the TG of all samples, and (f) is the DTA curve.

[0027] Figure 5 : Changes of contact angles of g-C3N4, g-C3N4-P-75 and g-C3N4-X-75 over time.

[0028] Figure 6 :(a) 4 samples at 5mV -1 Nyquist plot under scanning rate, (b) CV curve, (c) CV curves of g-C3N4-X-75 and (d) g-C3N4-P-75 at several different scanning rates, and (e) specific capacitance of the four samples at different scanning rates.

[0029] Figure 7 :(a) Effect of applied voltage on removal of U(VI) (pH = 5.0, T = 298K, C0 = 50mgL -1 , test time: 70min), (b) is pH value (applied voltage: 1.5V, T = 298K, C0 = 50mgL -1 , test time: 70min), (c) is the initial concentration (applied voltage: 1.5V, pH = 4.0, T = 298K, test time: 70min), (d) is the g-C3N4-X-75 and g-C3N4-P-75 electrodes at 1.5V and g-C3N4-X from 0 to 1.5V (pH 482 = 4.0, T = 298K, C0 = 50mgL-1 ), (e) is the time-dependent change of the UV-visible absorbance of g-C3N4-X-75 at 0 V, (f) g-C3N4-P-75 at 1.5 V and C3N4-X-75 at 1.5 V, (h) is the fitting result of the isothermal adsorption model (pH = 4.0, T = 298 K, C = 25-200 mgL -1 , test time: 70 min), (i) is the desorption ratio of U(VI) in the cyclic adsorption-desorption test, (j) is the relationship between the removal rate and the number of cycles, and (k) is the cumulative adsorption capacity of g-C3N4-X-75 and g-C3N4-P-75 electrodes after 6 cycles. DETAILED DESCRIPTION

[0030] The following specific embodiments are intended to further illustrate the content of the invention in detail rather than to limit the protection scope of the claims.

[0031] In the following specific examples, urea (≥99%), potassium hydroxide (KOH, 99.99%), nitric acid (HNO3, 0.1%), xanthan gum (XG, USP grade), N-methylpyrrolidone (NMP, 299%), polyacrylic acid (PAA, Mw4.5×10 5 ), polyvinylidene fluoride (PVDF) was purchased from Maclean Regent Co., Ltd., and conductive carbon black was provided by TIMCAL Spain;

[0032] In the following specific examples, uranyl nitrite hexahydrate (UO2(NO4)2 6H2O, 99%) was purchased from Sigmat Aldrich Regency. It was dissolved in a certain amount of ultrapure water to obtain U(VI) solutions of different concentrations. Hydrochloric acid (HCl, 36-38%) was purchased from Sinopharm Chemical Reagent Co., Ltd. All purchased reagents were used as received.

[0033] In the following examples, XPS was used to determine the elemental composition. Transmission electron microscopy (TEM) tests were performed on a JEM-2100F electron microscope to observe the morphology of g-C3N4 at an accelerating voltage of 100kV. The surface morphology and elements were detected on a Czech TESCAN MIRALMS field emission scanning electron microscope (FE-SEM) mapping of the electrode and operated on a Smartedx energy dispersive spectrometer (EDS). A Bruker D2 Phaser Ka diffractometer (XRD) with a scanning step of 0.02 (20) was used to study the crystallinity of the samples before and after electrosorption. FTIR tests were performed on a Nicolet-is10 spectrometer to study the electrode surface at 400 to 4000 em. -1wavelength range. The wettability of different electrode surfaces was studied using a Theta Flex dynamic contact angle instrument. The concentration of U(VI) ions in the samples was determined using a Hitachi U3900 UV-Vis spectrophotometer. The electrochemical properties of the electrodes were studied using a CHI660D electrochemical workstation. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were studied using a three-electrode system, using 0.1 M sodium sulfate solution as the electrolyte, platinum sheet as the auxiliary electrode, and Hg / Hg2Cl2 as the reference electrode to prepare the electrode material as the working electrode.

[0034] Example 1

[0035] Step 1: First, mix 15g of urea with 0.1g of potassium hydroxide and dissolve them in 30ml of deionized water. After the mixed solution is completely dissolved, evaporate and dry in an oven at 80℃ for 24h. Subsequently, the formed mixed crystals are transferred to a crucible and then calcined in a muffle furnace at 550℃ for 2h. Then, 0.1molL -1 The obtained powder was washed with nitric acid and deionized water in sequence to remove residual alkali and dried in an oven at 80 ° C. Finally, the dried product was ground into a light yellow powder.

[0036] Step 2: The preparation of g-C3N4-X electrode includes three main steps: slurry preparation, casting and drying / annealing. To prepare g-C3N4-X slurry, g-C3N4, conductive carbon black and XG-PAA (weight ratio of XG:PAA is 1:1) binder were mixed in weight ratios of 8:1:1, 7.5:1:1.5 and 7:1:2 respectively and ground into powder mixture in an agate mortar. Then 0.5 g of the mixture was suspended in 5 mL of solvent, i.e., deionized water, and magnetically stirred for 24 h to form a uniform slurry.

[0037] The preparation process of g-C3N4-P-75 slurry is similar to that of g-C3N4-X electrode. The mass ratio of g-C3N4, conductive carbon black, and PVDF binder is 7.5:1:1.5, and NMP is used as solvent.

[0038] Step 3: Pour the above slurry evenly on the graphite plate, cast it with a coater with a gap height of 150μm, and dry it at 60℃ for 12 hours to remove residual solvent. In addition, the g-C3N4-X electrode was cured at 155℃ for 4h to achieve covalent crosslinking between XG and PAA. According to the mass fraction of g-C3N4 in the mixed slurry, the g-C3N4-X electrodes were marked as g-C3N4-X-80, g-C3N4-X-75 and g-C3N4-X-70, respectively.

[0039] Experimental results of U(VI) electrosorption on g-C3N4-X-75 composite electrode (see application examples for specific adsorption process, in the case of an electrolyte containing U(VI) with an initial concentration of 100 mg / L -1 , pH = 5, the voltage applied between the anode and cathode is 1.2 V): the uranium removal rate of the composite electrode is 98.1% within 60 min, and the cumulative adsorption capacity after 6 cycles is 1459.1 mg g -1 .

[0040] Example 2

[0041] Step 1: First, 15g of melamine and 0.5g of sodium hydroxide were mixed and dissolved in 30ml of deionized water. After the mixed solution was completely dissolved, it was evaporated and dried in an oven at 80°C for 24h. Subsequently, the formed mixed crystals were transferred to a crucible and then calcined in a muffle furnace at 500°C for 3 hours. Then, 0.1molL -1 The obtained powder was washed with nitric acid and deionized water in sequence to remove residual alkali and dried in an oven at 80 ° C. Finally, the dried product was ground into a light yellow powder.

[0042] Step 2: The preparation of g-C3N4-X-75-A electrode includes three main steps: slurry preparation, casting and drying / annealing. To prepare g-C3N4-X-75-A slurry, g-C3N4, conductive carbon black and XG-PAA (weight ratio of XG:PAA is 1:2) binder were mixed in a weight ratio of 7.5:1:1.5 and ground into a powder mixture in an agate mortar. Then 0.5 g of the mixture was suspended in 5 mL of solvent, i.e., deionized water, and magnetically stirred for 24 hours to form a uniform slurry.

[0043] Step 3: Pour the above slurry evenly on the graphite plate, cast it with a coater with a gap height of 150 μm, and dry it at 60 ° C for 12 h to remove the residual solvent. In addition, the g-C3N4-X-75-A electrode was cured at 140 ° C for 5 h to achieve covalent cross-linking between XG and PAA.

[0044] Experimental results of U(VI) electrosorption on g-C3N4-X-75-A composite electrode (see application examples for specific adsorption process, in the case of an electrolyte containing U(VI) with an initial concentration of 100 mg / L -1 , pH = 5, the voltage applied between the anode and cathode is 1.2 V): the uranium removal rate of the composite electrode is 97.5% within 60 min, and the cumulative adsorption capacity after 6 cycles is 1366.8 mg g -1 .

[0045] Example 3

[0046] Step 1: First, mix 15g of cyanamide and 1.0g of calcium hydroxide and dissolve them in 30ml of deionized water. After the mixed solution is completely dissolved, evaporate and dry in an oven at 80℃ for 24h. Subsequently, the formed mixed crystals are transferred to a crucible and then calcined in a muffle furnace at 600℃ for 1 hour. Then, 0.1molL -1 The obtained powder was washed with nitric acid and deionized water in sequence to remove residual alkali and dried in an oven at 80 ° C. Finally, the dried product was ground into a light yellow powder.

[0047] Step 2: The preparation of g-C3N4-X-75-B electrode includes three main steps: slurry preparation, casting and drying / annealing. To prepare g-C3N4-X-75-B slurry, g-C3N4, conductive carbon black and XG-PAA (weight ratio of XG:PAA is 2:1) binder were mixed in a weight ratio of 7.5:1:1.5 and ground into a powder mixture in an agate mortar. Then 0.5 g of the mixture was suspended in 5 mL of solvent, i.e., deionized water, and magnetically stirred for 24 hours to form a uniform slurry.

[0048] Step 3: Pour the above slurry evenly on the graphite plate, cast it with a coater with a gap height of 150 μm, and dry it at 60 ° C for 12 h to remove the residual solvent. In addition, the g-C3N4-75-B electrode was cured at 180 ° C for 3 h to achieve covalent cross-linking between XG and PAA.

[0049] Experimental results of U(VI) electrosorption on g-C3N4-75-B composite electrode (see application example for specific adsorption process, in the case of an electrolyte containing U(VI) with an initial concentration of 100 mg L -1 , pH = 5, the voltage applied between the anode and cathode is 1.2 V): the uranium removal rate of the composite electrode is 97.9% within 60 min, and the cumulative adsorption capacity after 6 cycles is 1426.6 mg g -1 .

[0050] Comparative Example 1

[0051] The operation steps were similar to those in Example 1, except that curing was not performed, and the prepared g-C3N4-X electrode was labeled as g-C3N4-mX electrode.

[0052] Application Examples

[0053] The electrosorption device includes a CDI electrode pair, a liquid reservoir, a circulation pump, a conductivity meter and a DC power supply. The CDI electrode pair consists of a cathode and an anode, with a graphite plate as the anode and g-C3N4-X or g-C3N4-P-75 as the cathode. A non-conductive gasket is placed between the cathode and the anode to avoid short circuit. Two holes with a diameter of about 0.4 cm are drilled on the top and bottom of the cathode and the anode. 2The holes were arranged so that the electrode pair was in full contact with the U(VI) solution. The U(VI) solution was transported from the bottom hole through the CDI unit to the upper hole by a peristaltic pump. The voltage was provided and regulated by a DC power supply. In each experiment, the weight of each electrode was about 10.5 mg and the active area was ∼49 cm 2 60mLLU(VI) solution at 36.8mLmin -1 The initial U(VI) concentration is 25-200 mg L -1 The applied voltage between the cathode and the anode was controlled in the range of 0 to 1.5 V. In the electrosorption experiments, the pH value of the U(VI) solution was adjusted by adding 0.1 M nitric acid or sodium hydroxide.

[0054] The formula used is as follows:

[0055] Specific Capacitance:

[0056]

[0057] i(A) represents the average current, U(Vs -1 ) represents the voltage scanning rate, and m(g) represents the weight of the electrode material.

[0058] Adsorption capacity:

[0059]

[0060] Removal rate:

[0061]

[0062] Analysis ratio:

[0063]

[0064] Among them C o and C t represents the initial concentration and the concentration at time t (min). V (L) is the volume of U (VI) solution. Q des (mg g -1 ) represents the capacity of U(VI) ions desorbed from the electrode, Q ads (mg g -1 ) represents the capacity adsorbed on the electrode. Pseudo-first-order kinetic equation:

[0065]

[0066] Pseudo-second-order kinetic equation:

[0067]

[0068] Langmuir isotherm equation:

[0069]

[0070] Freundlich isotherm equation:

[0071]

[0072] Performance Testing and Characterization:

[0073] The preparation of XG-PAA-bound porous g-C3N4 electrode mainly includes the formation of porous g-C3N4 and the assembly of XG-PAA-bound g-C3N4 electrode, as shown in Figure 1a. In the first step, a certain amount of KOH and urea are dissolved in deionized water in advance, and white mixed crystals are formed after evaporation overnight. Then calcined at 550 ° C, the white crystals turned into light yellow powder, which means the generation of porous g-C3N4. The addition of KOH is crucial to the formation of the porous structure of g-C3N4, because the OH produced by the decomposition of KOH - It can react with the intermediate amine group during the thermal polymerization process to generate N vacancies and cyano groups (-C≡N) ( Figure 1 b).

[0074] Porous g-C3N4, PAA, XG and conductive carbon black were uniformly mixed, coated on a graphite collector plate, and then heated to 155°C to induce covalent crosslinking between XG and PAA to assemble into XG-PAA-bound g-C3N4 electrodes. A robustly interconnected 3D crosslinked conductive network was eventually generated. Among them, XG has a millipede-like structure with a main double helical backbone and a series of side chains, which can firmly attach various nano / micro materials such as g-C3N4 or carbon black. XG and PAA can form covalent crosslinks through polyester reactions or non-covalent crosslinks through intramolecular / intermolecular hydrogen bonds, while the abundant hydroxyl and carboxylic acid groups on the XG-PAA binder can interact with the n-containing groups of porous g-CN through hydrogen bonds ( Figure 1 c) In addition, XG’s COO - There may be electrostatic interactions between the NHPs and porous g-C3N4.

[0075] It is worth mentioning that the generation of defects in the g-C3N4 structure, namely N vacancies or in-plane pores, can increase the accessible specific surface area and serve as a shortcut for U(VI) ion transfer between adjacent g-C3N4 nanosheets. Compared with PVDF, XG-PAA has a triple role: first, it has excellent bonding properties to bond the porous g-C3N4 active materials together and to the graphite sheet. Unlike the relatively weak "physical adhesion" of PVDF binders, the cross-linked XG-PAA binder can form a tough 3D interconnected conductive structure, thereby enhancing the electrochemical performance of g-C3N4-based electrodes and maintaining their mechanical stability and cyclability. Secondly, as an ion exchange membrane, through the negatively charged COO - Groups attract UO2 in advance 2+ ions, thereby weakening the CO - Third, as a modifier, it provides O-containing active sites to coordinate U(VI) ions and improve the wettability of g-C3N4-based electrodes.

[0076] from Figure 2 a and Figure 2 b It can be seen that the obtained porous g-C3N4 presents a thin flower-like two-dimensional sheet structure. The lateral size ranges from tens of nanometers to hundreds of nanometers, and there are some irregular convex folds on the edge of the plane ( Figure 2 c and 2d). From Figure 2 It can be seen from the elemental mapping diagram of e that the pattern of N elements and C follows well, indicating that the rich N-containing groups are evenly distributed on the surface of the g-C3N4 layer. Further magnification of the transmission electron microscope image shows that many in-plane nanopores of several nanometers in size can be clearly seen, proving the successful etching of g-C3N4 by KOH during the thermal polymerization process. The formation of a porous structure can significantly increase the specific surface area, which is beneficial to the transport of U(VI) ions. At the same time, Figure 2 moir in f <s:1>The pattern shows that the sheets of porous g-C3N4 are superimposed at the van der Waals distance without obvious overlap. The presence of lattice fringes illustrates the crystal structure of g-C3N4, with a visible lattice fringe of 0.314 nm on the (002) crystal plane ( Figure 2 g), which indicates that the corrosion of g-C3N4 sheets by KOH does not destroy their crystal structure.

[0077] The surface morphology of g-C3N4-P-75 and g-C3N4-X-75 electrodes was characterized by SEM. Figure 3 As can be seen from a to d, both g-CsNeP-75 and g-CN-X-75 electrodes present a loose porous structure, and layered g-C3N4 and granular conductive carbon black can be clearly observed. It is worth noting that g-C3N4-X-75 exhibits a more uniformly distributed three-dimensional interconnected network, with conductive carbon black well dispersed on the surface of g-C3N4 sheets, while the filamentous XG-PAA binder connects the carbon black and g-C3N4 sheets together ( Figure 3 e). On the contrary, the carbon black particles of the g-C3N4-P-75 electrode tend to aggregate, and the mapping of the g-C3N4-X-75 electrode shows that the pattern of the O element is consistent with the pattern of the C and N elements ( Figure 3 f), indicating that the O-containing active groups are evenly covered on the electrode surface.

[0078] Figure 4 a is the XRD spectra of porous g-C3N4, g-C3N4-P-75, and g-C3N4-X-75. The characteristic diffraction peaks of all samples near 27° are attributed to the (002) crystal plane of g-C3N4, which is caused by the periodic stacking of the interlayers of its conjugated orientation structure. At the same time, compared with porous g-C3N4, the diffraction peaks of g-C3N4-P-75 and g-C3N4-X-75 decreased by 0.32° and 0.26°, respectively, indicating that the interlayer space increased. This phenomenon may be due to the embedding of the polymer binder into the g-C3N4 sheets. Compared with porous g-C3N4, the peak intensity of g-C3N4-X-75 remains basically unchanged, indicating that its high crystallinity and mechanical properties remain basically unchanged after being compounded with the XG-PAA binder.

[0079] FTIR results are as follows Figure 4 As shown in b, porous g-C3N4 is at ~3155cm -1 The broad absorption peak at 888cm comes from the stretching vibration of the remaining NH or OH bonds. -1 、1237cm -1 、1318cm -1 、1409cm -1 、1574cm -1 and 1644cm -1 Corresponding to CN and C=N stretching vibration, 807cm -1 The breathing vibration at 2800 cm corresponds to the triazine ring. The absorption peak on the FTIR spectrum is almost the same as that of g-C3N4, except for a characteristic peak overlap. The g-C3N4-X-75 electrode with XG-PAA as the binder has a peak at 2800 cm -1 ~3600cm -1 The broad peak intensity near 3161m -1 To 3157cm -1 There is a slight change between the two, indicating that COO- is formed by the esterification reaction between PAA and XG. Both covalent and non-covalent (hydrogen bonding) crosslinks between XG and PAA help to form an interconnected conductive network, improving the mechanical strength and recyclability of the electrode. The XPS spectra of g-C3N4-X-75 and g-C3N4-P-75 electrodes are also shown in Figure 2. Figure 4 c and 4d. For g-C3N4-X-75 and g-C3N4-P-75, in addition to several peaks representing graphite C, CC / C=C, CN / C=N and NC=N components, the presence of peaks representing C-OH / CO, C=O and O=CO as well as FC=F is consistent with the introduction of XG-PAA and PVDF. Obviously, the peak area of ​​hydrophilic O- and N-containing groups of g-C3N4-X-75 is significantly larger than that of g-C3N4-P-75, indicating that the U(VI) ions of g-C3N4-X-75 electrode have more accessible active sites and better surface wettability.

[0080] The TG and DTA data of the samples are as follows: Figure 4 e and 4f. From the thermogravimetric curves, all samples below 500 °C have only slight thermal degradation. Compared with porous g-C3N4, the degradation rate of other samples between 200 and 500 °C is slightly faster, which may be related to more water volatilization and decomposition of groups in the polymer binder. Between 500 and 750 °C, all samples show sharp thermal degradation, corresponding to the decomposition of g-C3N4 into carbon and nitrogen. It was also observed that porous g-C3N4 and g-C3N4-X-70 began to degrade g-C3N4-P-75, g-C3N4-X-75 and g-C3N4-X-80 at about 590 °C, indicating that the latter three samples have better thermal stability. The initial degradation temperature of g-C3N4-X-70 is close to that of porous g-C3N4, most likely because a tough cross-linked network has not yet been formed at this content of XG-PAA binder. From the DTA curve ( Figure 4 f) It can be seen that a small endothermic peak appears near 50-150°C and a large endothermic peak appears near 650°C, corresponding to the above-mentioned volatilization of water and the decomposition of g-C3N4, respectively. In addition, an exothermic peak appears at 710°C, which is due to the rapid combustion of the decomposition products and the formation of nitrogen, cyanide and carbon dioxide. At the same time, it can be clearly observed that the endothermic peak of g-C3N4-X-75 decomposing g-C3N4 is located at 676°C, which is higher than the endothermic peak of g-C3N4-P-75 (662°C), indicating that g-C3N4-X-75 has better thermal stability. This phenomenon may be due to the introduction of the cross-linked XG-PAA binder, which can form a stronger network structure than the PVDF binder due to its stronger covalent / non-covalent molecular forces. Good thermal stability and mechanical stability are conducive to the long-term application of g-C3N4-X-75 electrodes in radioactive environments.

[0081] In order to compare the effects of different binders on the wettability of the material surface, dynamic contact angle tests were carried out. Figure 5 The following is a graph showing the change of water droplets over time on porous g-C3N4, g-C3N4-X-75, and g-C3N4-P-75. The initial contact angle of g-C3N4 is 109.48°, and there is no obvious change in a short period of time, indicating its hydrophobicity. The initial contact angle of the g-C3N4-P-75 electrode is 127.08°, which is even greater than that of g-C3N4, indicating that the introduction of the PVDF binder further aggravates the hydrophobicity of the electrode. However, when the XG-PAA binder is introduced, the initial contact angle of the g-C3N4-X-75 electrode is much smaller, at 84.55°, and drops to 82.67° within 45 seconds, indicating that it is more hydrophilic than g-C3N4 and g-C3N4-P-75. Generally speaking, hydrophilicity is related to the surface polar groups and morphology (i.e., pores) of the material. The good hydrophilicity of the g-C3N4-X-75 electrode may be related to its porous surface and the large number of polar groups such as hydroxyl and carboxyl groups in the XG-PA binder. PVDF is a non-polar polymer and has no polar groups to interact with water molecules. Therefore, the hydrophilicity of the g-C3N4-P-75 electrode is poor. Overall, the good hydrophilicity of the g-C3N4-X-75 electrode is conducive to the penetration of U(VI) solution, further improving its ability to capture U(VI).

[0082] Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to study the electrochemical performance of g-C3N4-X-80, g-C3N4-X-75g-C3N4-X-70 and g-C3N4-P-75 electrodes. Figure 6 The graphs of the four samples all show oblique lines in the low-frequency region, short 45° Warburg regions in the mid-frequency region, and semi-arcs in the high-frequency region ( Figure 6 a inset). In the Nyquist plot, the intercept of the real axis represents the bulk resistance, which includes the solution resistance, the natural electrode resistance, and the contact resistance between the current and the electrodes. The diameter of the arc corresponds to the charge transfer resistance, and the oblique line corresponds to the ion diffusion. Figure 6 It can be seen from a that the bulk resistance of the g-C3N4-P-75 electrode is 23.12 ohms, which is greater than the bulk resistance of the g-C3N4-X-80, g-C3N4-X-75 and g-C3N4-X-70 electrodes (7.26, 3.54 and 7.02 ohms, respectively). In the high-frequency region, the arc diameters of the g-C3N4-X-80, g-C3N4-X-75 and g-C3N4-X-70 electrodes are also significantly smaller than those of the g-C3N4-P-75, indicating that the charge transfer resistance of the g-C3N4-X electrode is lower. In the low-frequency region, the slope of the curve of the g-C3N4-X electrode is higher than that of the g-C3N4-P-75 electrode. These results indicate that the use of XG-PAA binder is beneficial to improving the conductivity, charge transfer and ion diffusion capabilities of the material compared to the PVDF binder. At the same time, the g-C3N4-X-75 electrode has the best electrochemical performance among several electrodes.

[0083] Figure 6 b shows the voltage window from -0.8 V to 0.8 V with a scan rate of 5 mVs -1 CV curves of all electrodes at 100 ℃ and 70 ℃. It is obvious that all electrodes show similar curves, indicating that they have near-ideal capacitors and efficient ion transport performance. The closed area of ​​a general CV curve represents the value of the specific capacitance. Compared with the g-C3N4-P-75 electrode, the CV curve closed area of ​​the g-C3N4-X electrode is larger, which means a higher specific capacitance. Among all g-C3N4-X electrodes, the g-C3N4-X-75 electrode has the largest value. The superior specific capacitance of the g-C3N4 electrode of XG-PAA comes from its excellent surface hydrophilicity, the porous conductive network structure that is conducive to ion penetration, and the contribution of pseudocapacitance containing O and N groups.

[0084] Further comparison of each electrode at 5-100mV -1 The CV curve under the scanning speed is as follows: Figure 6 c~d. At all scanning speeds, the g-C3N4-X-75 electrode can be observed to have a nearly rectangular CV shape, which is consistent with its good capacitive properties. In contrast, g-C3N4-P-75 ( Figure 6 d) The CV curve area of ​​the electrode is much smaller than that of g-C3N4-X-75, and its shape is significantly different from the ideal rectangle, indicating that its specific capacitance is relatively poor. It is worth mentioning that when the scan rate is greater than 20mVs -1 When the scan rate increases, the CV curves of g-C3N4-X-70 and g-C3N4-X-80 have slight deviations from the ideal rectangular shape. This phenomenon is due to the fact that as the scan rate increases, the ions do not have enough time to penetrate into the internal porous structure to form a double electric layer, resulting in a decrease in specific capacitance. Further calculation of the specific capacitance at different scan rates, such as Figure 6 As shown in e. Obviously, for all electrodes, the specific capacitance decreases with increasing scan rate. -1 At the scan rate of , the specific capacitance of g-C3N4-X-75 electrode is the largest, which are 213.742, 161.164, 115.360, 79.761 and 59.744 F respectively. -1 From the above data, it can be seen that the g-C3N4-X-75 electrode has the best electrochemical performance and is expected to have good electrosorption performance for U(VI) ions.

[0085] The electroadsorption behaviors of g-C3N4-X and g-C3N4-P-75 electrodes were compared. Figure 7 a shows the effect of applied voltage on the removal of U(VI) by different electrodes. All four electrodes have different degrees of U(VI) adsorption at 0V, indicating the existence of physical and chemical adsorption, such as coordination adsorption. Among them, g-C3N4-P-75 has the lowest physicochemical removal rate (30%), which may be due to its hydrophobic surface and fewer active sites. As the voltage increases from 0V to 1.5V, the removal rate of each electrode increases, indicating that the applied voltage has a positive effect on the electrosorption performance. Further observation found that the removal rate of the g-C3N4-X-75 electrode was higher than that of the g-C3N4-P-75 electrode in each voltage range, indicating that the XG-PAA binder has a stronger anchoring ability for U(VI) than the PVDF binder, which may be due to its superior hydrophilicity, rich binding active sites, and reduced common ion repulsion effect. At the same time, among all g-C3N4-X electrodes, the g-C3N4-X-75 electrode has the best electrosorption performance for U(VI), which is consistent with the above electrochemical results. Therefore, g-C3N4-X-75 and g-C3N4-P-75 electrodes were mainly compared in the following electrosorption tests.

[0086] The effect of pH value on U(VI) removal efficiency was further explored. Figure 7 b It can be seen that in the entire pH range, the removal rate of the g-C3N4-X-75 electrode at 0 V is comparable to that of the g-C3N4-P-75 electrode. At the same time, the removal rate of U(VI) by the g-C3N4-X-75 electrode at 1.5 V is much higher than that of the g-C3N4-X-75 electrode at 1.5 V, reaching a maximum value at pH 4.0. As the pH value further increases, the removal rate gradually decreases. These phenomena can be explained by the combined effects of protonation of the binding site and uranyl species. At lower pH, the positively charged UO2 2+ 、UO2OH + etc. are the main substances in U(VI) solution. At the same time, most of the binding sites are occupied by H + The uranyl groups with more positive charges can bind to negatively charged groups (such as COO - Groups) form a micro electric field attraction. In addition, COO - The groups can anchor UO2 before electrosorption 2+ , thus avoiding unnecessary adsorption of anions. This can weaken the repulsion of co-ions and improve the electrosorption efficiency. Ultimately, all these factors affect the maximum removal rate of the g-C3N4-X-75 electrode at pH 4.0. When the pH is further increased to above 6.0, U(VI) anion complexes are generated, which produce electrostatic repulsion with the electrode, making adsorption difficult.

[0087] Figure 7 c shows the effect of initial concentration on the electrode electrosorption performance. At 1.5 V, the g-C3N4-X-75 electrode has the highest removal rate for U(VI), and the initial concentration of U(VI) is 25-200 mg / L -1 Especially in the range of 25-100 mg / L -1 The removal rate was over 90% within the range of U(VI). -1 The removal efficiency decreased when , which may be due to the longer time required to capture more U(VI) ions.

[0088] The adsorption kinetics of g-C3N4-X-75 at 0 V and 1.5 V and g-C3N4-P-75 at 1.5 V were further compared. Figure 7 It can be seen that g-C3N4-X-75 has the fastest kinetic rate at 1.5 V, with a removal rate of up to 98.1% within 70 min and an adsorption capacity of 280.35 mg g -1 . In contrast, the kinetic rate of g-C3N4-X-75 at 0V is much slower, and the removal rate within 70min is relatively low, at 55.0%; further extending the time to 200min, the removal rate can reach 93.4%. Therefore, it can be concluded that voltage plays an important role in the improvement of kinetic rate. At the same time, the study also found that the kinetic rate of the g-C3N4-P-75 electrode at 1.5V is also slower than that of the g-C3N4-X-75 electrode, with a removal rate of 59.7% within 70min. Figure 7 e to g show the changes of the UV-visible spectra of the above three electrodes over time. Obviously, the peak intensity at 652nm of the g-C3N4-X-75 electrode at 1.5V decays fastest over time ( Figure 7 f). Within 70 min, the peak intensity is close to zero, which means that the U(VI) in the solution is almost completely adsorbed. In contrast, the peak intensity of the g-C3N4-P-75 electrode at 1.5 V decreases the slowest, while the peak intensity of the g-C3N4-X-75 electrode at 0 V is between the two. This shows that the type of binder has a great influence on the electrosorption capacity of the electrode, and coordination is as important as the electric field in the removal of U(VI).

[0089] To further explore the long-term adsorption capacity, the cumulative adsorption capacity of g-C3N4-X-75 and g-C3N4-P-75 electrodes over six adsorption-desorption cycles was studied. The electrodes were operated at a reverse voltage of 1.5 V for 30 min and then washed with deionized water to desorb the absorbed U(VI) ions. The results showed that the desorption rate of U(VI) from both electrodes first decreased and then stabilized with increasing cycle number ( Figure 7 i). The desorption rates of g-C3N4-X-75 and g-C3N4-P-75 were maintained at about 77% and 71.5% respectively within 6 cycles, with limited decreases (~12% and ~16.0%). Figure 7 It can be seen that with the increase in the number of cycles, the removal rate of the g-C3N4-X-75 electrode decreased slightly, while the removal rate of the g-C3N4-P-75 electrode decreased significantly, with a value of ~24%. For the former, the slight decrease in removal rate may be related to the reduction of active sites caused by incomplete desorption, while for the latter, in addition to the reduction of active sites, the destruction of the electrode structure during multiple cycles (mentioned later) should be another important factor. In addition, after 6 cycles (30 min desorption, 70 min adsorption), the cumulative adsorption capacity of the g-C3N4-X-75 electrode reached 1459.1 mg g -1 , which is much higher than 1000.0 mg g of g-C3N4-P-75 electrode. -1 ( Figure 7 k).

Claims

1. An application of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode, characterized in that: Using graphite electrode as anode and xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode as cathode, it was applied to the electrosorption of U(VI); The xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode is obtained by the following preparation method: a carbon nitrogen source and a strong base are uniformly mixed by a liquid phase method, and then dried and calcined in sequence to obtain porous g-C3N4; the porous g-C3N4 is mixed with conductive carbon black, a xanthan gum-polyacrylic acid composite binder and a solvent to form a slurry, and the slurry is coated on an electrode plate, dried, and cured to obtain the electrode; The xanthan gum-polyacrylic acid composite binder is composed of xanthan gum and polyacrylic acid in a mass ratio of (0.5-2):1; the mass percentages of porous g-C3N4, conductive carbon black and xanthan gum-polyacrylic acid composite binder are 70-80%: 5-15%: 10-20%.

2. The use of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode according to claim 1, characterized in that: The mass ratio of the carbon-nitrogen source to the strong base is 15:(0.1-1.0).

3. The use of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode according to claim 1 or 2, characterized in that: The carbon and nitrogen source is at least one of urea, melamine, dicyandiamide and cyanamide; The strong base is at least one of sodium hydroxide, potassium hydroxide, calcium hydroxide and barium hydroxide.

4. The use of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode according to claim 1, characterized in that: The calcination conditions are: calcination at a temperature of 500-600° C. for 1-3 hours.

5. The use of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode according to claim 1, characterized in that: The curing conditions are: curing at a temperature of 140-180° C. for 3-5 hours.

6. The use of a xanthan gum-polyacrylic acid bonded porous g-C3N4 composite electrode according to claim 1, characterized in that: The electrolyte contains U(VI), the pH of the electrolyte is 3~8, and the voltage applied between the anode and the cathode is less than 1.5V.

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