Carbon nanotube-graphene oxide composite membrane, preparation method and water treatment method thereof

Abstract

The invention discloses a carbon nanotube-graphene oxide composite membrane, a preparation method and a water treatment method thereof, and belongs to the technical field of nano materials and water treatment. The preparation method comprises the following steps: uniformly loading acidized multi-walled carbon nanotube dispersion liquid on the substrate membrane, mixing a mixed solution containing a polyamine cross-linking agent and a polyanion electrolyte with graphene oxide dispersion liquid, and loading the mixture on the carbon nanotube single-layer membrane. The carbon nanotube-graphene oxide composite membrane can be used for removing organic pollutants in water through electrochemical reinforcement, and can also be used for removing organic pollutants in water together with an oxidizing agent. The preparation method of the composite membrane is simple and convenient, and the synergistic effect of screening, enrichment and catalytic oxidation can be achieved in water treatment, so that the purpose of efficiently removing organic pollutants is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterials and water treatment, and specifically relates to a carbon nanotube-graphene oxide composite membrane, a preparation method thereof, and a water treatment method thereof. Background Art

[0002] With the rapid development of industry, the types of organic pollutants in industrial wastewater are increasing, and their complexity and toxicity are constantly intensifying. For example, organic pollutants such as dyes, pharmaceutical intermediates, and perfluorinated compounds are difficult to treat and pose a severe challenge to traditional water treatment technologies. Traditional polymer membranes (such as polyvinylidene fluoride membranes) have problems such as low permeation flux (less than 20 L / (m 2 ·h·bar)) and poor anti-fouling performance, and it is difficult to meet the current strict requirements for industrial wastewater treatment. In addition to traditional polymer membranes, single carbon nanotube (CNT) or graphene oxide (GO) membranes also have some performance bottlenecks:

[0003] Although the CNT membrane has high conductivity (>10 3 S / m) and mechanical strength (tensile strength >9.6 GPa), its pore size distribution is uneven (the pore size range is between 5 and 50 nm), which results in an unsatisfactory rejection rate for small molecule pollutants (such as methylene blue, molecular weight 319.85 Da). The GO membrane shows hydrophilicity due to the presence of oxygen functional groups, which causes swelling, and the interlayer spacing will increase from 0.8 nm to 1.5 nm, making its structure unstable and the rejection rate decrease. Although traditional cross-linking modification methods can improve stability to a certain extent, they will sacrifice water flux (the decrease exceeds 50%).

[0004] In the prior art, carbon nanotubes and graphene oxide are physically mixed by ultrasonic dispersion and rely on van der Waals forces to combine. They are prone to delamination under high-temperature and high-pressure conditions (tensile strength <1.8 GPa), and it is difficult to efficiently expose catalytic active sites. Therefore, there is an urgent need to develop a preparation method for a carbon nanotube-graphene oxide composite membrane that can be used for the treatment of organic pollutants. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems of low permeation flux, poor anti-fouling performance, and insufficient rejection of small molecule pollutants in traditional membrane technologies for removing organic pollutants in wastewater, and to provide a carbon nanotube-graphene oxide composite membrane, a preparation method thereof, and a water treatment method thereof.

[0006] The specific technical solutions adopted by the present invention are as follows:

[0007] In the first aspect, the present invention provides a preparation method for a carbon nanotube-graphene oxide composite membrane, and the specific steps are as follows:

[0008] S1: Disperse the acid-treated multi-walled carbon nanotubes in water to prepare a carbon nanotube dispersion; uniformly load the carbon nanotube dispersion onto a substrate membrane to obtain a single-layer carbon nanotube membrane;

[0009] S2: Use a first mixed solution containing a polyamine cross-linking agent and a polyanionic electrolyte. After mixing the graphene oxide dispersion with the first mixed solution, a second mixed solution is obtained;

[0010] S3: Uniformly load the second mixed solution onto the single-layer carbon nanotube membrane prepared in step S1 to obtain a carbon nanotube-graphene oxide composite membrane.

[0011] Preferably, the acid treatment of the multi-walled carbon nanotubes is as follows: Immerse the multi-walled carbon nanotubes in a mixed solution of sulfuric acid and nitric acid with a volume ratio of (3-4):1 for 0.5-2 hours, then wash with ultrapure water until neutral, and dry for later use;

[0012] The substrate membrane is one of polyvinylidene fluoride, polysulfone, polyethersulfone, polypropylene, polyacrylonitrile, polytetrafluoroethylene or cellulose acetate organic membranes, or an inorganic ceramic membrane or a stainless steel metal membrane.

[0013] Preferably, the loading methods in steps S1 and S3 are coating, spin coating, vacuum filtration or electrophoretic deposition.

[0014] Preferably, the concentration of multi-walled carbon nanotubes in the carbon nanotube dispersion is 0.2-0.8 mg / mL; the loading amount of carbon nanotubes on the single-layer carbon nanotube membrane is 0.4-1.2 mg / cm 2 ; the concentration of graphene oxide in the graphene oxide dispersion is 0.02-0.2 mg / mL.

[0015] Preferably, the polyamine cross-linking agent in the first mixed solution is ethylenediamine, polyethyleneimine or triethylenetetramine, and the polyanionic electrolyte is polystyrene sulfonate, polyacrylic acid, polyvinyl sulfonic acid, layered double metal hydroxide or montmorillonite.

[0016] Furthermore, the polyamine cross-linking agent in the first mixed solution is ethylenediamine, and the polyanionic electrolyte is polystyrene sulfonate; the concentration of ethylenediamine in the first mixed solution is 0.1-0.3 g / L, and the mass percentage of polystyrene sulfonate is 0.1%-0.5%; the graphene oxide dispersion and the first mixed solution are mixed at a volume ratio of 1:2.

[0017] In a second aspect, the present invention provides a carbon nanotube-graphene oxide composite membrane obtained by using the preparation method described in the first aspect.

[0018] In a third aspect, the present invention provides a method for electrochemically enhancing the removal of organic pollutants in water. In a three-electrode electrochemical reaction device containing the wastewater to be treated, the carbon nanotube-graphene oxide composite membrane prepared in the first aspect is used as the working electrode, Ag / AgCl is used as the reference electrode, and a platinum sheet is used as the counter electrode. An alternating current or direct current mode is adopted, and a voltage of 0.5 - 3 V is applied to the working electrode to remove the organic pollutants in the wastewater to be treated through an electrochemical reaction; the organic pollutants are methylene blue or sulfamethoxazole.

[0019] Preferably, a persulfate or permanganate is added to the electrochemical reaction device as an oxidant; the addition amount of the oxidant is 0.1 - 5 mM; an electrolyte with a concentration of 1 - 50 mM is additionally added to the wastewater to be treated, and the pH value of the wastewater to be treated is controlled to be 3 - 10.

[0020] In a fourth aspect, the present invention provides a method for synergistically removing organic pollutants in water with an oxidant. A persulfate or permanganate as an oxidant is added to a reaction device containing the wastewater to be treated, and the carbon nanotube-graphene oxide composite membrane prepared in the first aspect is used as a filtration membrane; the wastewater to be treated flows through the carbon nanotube-graphene oxide composite membrane under an external force, and the organic pollutants in the wastewater to be treated are removed by heterogeneous catalytic oxidation; the organic pollutants are methylene blue or sulfamethoxazole.

[0021] The present invention has the following beneficial effects compared with the prior art:

[0022] (1) For the carbon nanotube-graphene oxide composite membrane prepared in the present invention, the carbon nanotubes (CNTs) are acid-treated to increase the carboxyl density on their surface, thereby enhancing the hydrogen bond interaction and conductivity with graphene oxide (GO); the layer spacing of GO is regulated by intercalation of ethylenediamine (EDA) and polystyrene sulfonate (PSS), and at the same time, sulfonic acid groups are introduced to increase the surface negative charge density, strengthening the coexistence of electrostatic adsorption and catalytic sites, which is beneficial to the adsorption and enrichment of pollutants and provides favorable conditions for subsequent catalytic oxidation reactions.

[0023] (2) For the carbon nanotube-graphene oxide composite membrane prepared in the present invention, the CNT layer provides mechanical strength and an electron transport channel for the composite membrane. The GO&EDA-PSS layer, as the active layer, not only realizes charge regulation and targeted enrichment of pollutants, but also the characteristics of its polymer material are similar to glue, increasing the adhesion firmness of the materials on the membrane, effectively solving the problem of the fragility of the pure CNT membrane, and improving the overall stability of the composite membrane.

[0024] (3) The carbon nanotube-graphene oxide composite membrane prepared by the present invention can remove organic pollutants such as dyes and antibiotics through the action of a direct current or alternating current electric field, and is applicable to the treatment of wastewater in complex systems such as printing and dyeing wastewater and pharmaceutical wastewater. Taking carbon nanotubes as the main body, playing its leading role in pollutant removal (adsorption, catalytic oxidation), while GO&EDA-PSS is the auxiliary part, used to improve the performance of the carbon nanotube membrane, realizing the synergistic effect of "screening-enrichment-catalytic oxidation", so as to achieve the purpose of efficient water treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the result graph of linear voltammetry in Example 2;

[0026] Figure 2 It is the methylene blue removal efficiency graph under different voltage and pH conditions in Example 3: where (a) is the voltage difference of -1 V; (b) is the voltage difference of -0 V; (c) is the voltage difference of -0.5 V; (d) is the voltage difference of 1 V; (e) is the voltage difference of 2 V;

[0027] Figure 3 It is the methylene blue removal efficiency graph in Comparative Example 1 and Example 4;

[0028] Figure 4 It is the methylene blue removal efficiency graph in Example 5. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The present invention will be further elaborated and described below in conjunction with the drawings and specific embodiments. The technical features of each embodiment in the present invention can be combined correspondingly without conflict.

[0030] Example 1

[0031] This example provides a preparation method of a carbon nanotube-graphene oxide composite membrane, which is as follows:

[0032] (1) Add 2 g of multi-walled carbon nanotubes to 120 mL of a sulfuric acid and nitric acid mixed solution with a volume ratio of 3:1, stir well at 60 °C, and carry out acidification treatment for 1 hour. After the acidification treatment, dilute the obtained carbon nanotube solution and separate it by vacuum filtration, and wash it with ultrapure water until the pH value of the filtrate is close to neutral (6.5 - 7.5). Then collect it after drying under vacuum for 12 h and set aside.

[0033] (2) Disperse the acidified multi-walled carbon nanotubes in ultrapure water by ultrasonic treatment to obtain a 0.5 mg / mL carbon nanotube dispersion.

[0034] (3) 10 mL, 15 mL, 20 mL, 25 mL, and 30 mL of carbon nanotube dispersion were respectively vacuum filtered onto polyvinylidene fluoride membranes (PVDF), and dried in vacuum at 60 °C to obtain different single-layer carbon nanotube membranes, with the diameter of each single-layer membrane being approximately 4.1 cm.

[0035] (4) Graphene oxide (GO) was ultrasonically dispersed in ultrapure water to obtain a 0.1 mg / mL graphene oxide dispersion; 0.2 g of anhydrous ethylenediamine (EDA) and 2 g of polystyrene sulfonate (PSS) were added to 1 L of ultrapure water, and after complete dissolution, an EDA-PSS solution (0.2 g / L EDA, 0.2 wt% PSS) was prepared.

[0036] (5) 5 mL of graphene oxide dispersion and 10 mL of EDA-PSS solution were mixed, and after ultrasonic treatment for 5 minutes, a mixed solution was obtained.

[0037] (6) The mixed solution was vacuum filtered onto different single-layer carbon nanotube membranes, and then rinsed by vacuum filtration with ultrapure water. After taking it out, it was dried at 50 °C for 2 hours to obtain carbon nanotube-graphene oxide composite membranes with different CNT loadings.

[0038] Example 2

[0039] In this example, linear sweep voltammetry experiments were carried out on the carbon nanotube-graphene oxide composite membrane prepared in Example 1 to verify its electrochemical performance characteristics, as follows:

[0040] Using an electrochemical workstation, the carbon nanotube-graphene oxide composite membrane loaded with 10 mL of carbon nanotube dispersion in Example 1 was selected as the working electrode, Ag / AgCl as the reference electrode, and a platinum sheet as the counter electrode.

[0041] Set the potential scanning range: -3 V to +3 V, and the scanning rate gradient is 10~200 mV / s (including 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, and 200 mV / s). The current range: Adjusted to ±10 A according to the preliminary experiment.

[0042] Record the oxidation / reduction peak current and background current baseline at each scanning rate. Focus on the potential range of -2V to +2 V (the region where the current changes significantly in the figure). The results are as Figure 1 shown. According to Figure 1It can be seen that the hydrogen production potential of the composite membrane is about -1.352 V, and the oxygen production potential is about 1.077 V, indicating that the width of its electrochemical window is about 2.5 V. This data limits the operating voltage range of the subsequent electrochemical treatment process, avoiding gas production caused by water decomposition due to voltage exceeding this range, resulting in energy waste and disturbance of the reaction system.

[0043] In addition, the stable current responses observed at different scanning rates (10 - 200 mV / s) confirm the high conductivity of the carbon nanotube membrane (the current density reaches the order of ±10 A). This provides a theoretical support for the rapid electron transfer and activation of oxidants (such as the generation of reactive free radicals) by the composite membrane as an electrode material during the electrochemical oxidation process.

[0044] No obvious redox peaks appear in the voltammogram curve, indicating that the composite membrane itself does not directly participate in the Faraday reaction, and its catalytic effect mainly depends on the oxidation of surface adsorbed pollutants induced by the electric field or the activation of externally added oxidants (such as persulfate) through the conductive network. This provides an experimental basis for the strategy of using direct current or alternating current drive instead of chemical polarization treatment in the subsequent examples.

[0045] Through this experimental data, the functional boundary of the composite membrane as an electrode material is clarified: within the range below the hydrolysis voltage, pollutant degradation is achieved through a non-direct electron transfer mechanism, while avoiding the performance attenuation problem caused by the loss of active sites in traditional electrocatalytic materials. This lays a foundation for the design of low-energy-consuming and highly stable electrochemical treatment processes in the subsequent examples.

[0046] Example 3

[0047] In this example, the carbon nanotube-graphene oxide composite membrane prepared in Example 1 was used to conduct methylene blue (MB) removal experiments under different voltages and different pH conditions, as follows:

[0048] (1) Using the three-electrode reaction system of the electrochemical workstation, a saturated Ag / AgCl electrode as the reference electrode, a platinum sheet as the counter electrode, and a platinum rod clamped with the carbon nanotube-graphene oxide composite membrane loaded with 10 mL of carbon nanotube dispersion in Example 1 as the working electrode. The initial concentration of MB in the reaction solution was 10 μM, and the concentration of sodium sulfate was 1 mM. Reaction solutions with pH values of 4, 6, 8, and 10 were set.

[0049] (2) First, under the non-powered state, 2 mL of the solution in the initial state and the state after adsorption for 30 min were taken from 50 mL of the above reaction solutions with different pH values using a syringe respectively. The water sample was obtained by filtering with a 0.45 μm pore filter head, and the absorbance of the water sample was measured with a UV spectrophotometer at a light intensity of 664 nm.

[0050] (3) Different voltages (-1 V, 0 V, 0.5 V, 1 V, and 2 V respectively) were applied to the working electrode to remove MB in reaction solutions with different pH values. All removal experiments were carried out in a 100 mL glass beaker placed on a magnetic stirrer operating at a stirring speed of 100 rpm to ensure uniform reaction of the solution. At each set voltage, 2 mL of solution was taken from the reaction solution at fixed time intervals (0, 2, 5, 10, 20, 30, 40, and 60 min) in the same manner as above, filtered, and the absorbance was measured using a UV spectrophotometer. Each experiment was repeated at least three times, including the control group. The results are as Figure 2 shown, Figure 2 in (a) - Figure 2 in (e) of t are the removal efficiencies of MB in reaction solutions with different pH values at voltages of -1 V, 0 V, 0.5 V, 1 V, and 2 V respectively. The removal efficiency is measured by the ratio of C t to C0, where C

[0051] is the concentration of MB in the water sample at different times, and C0 is the concentration of MB in the initial reaction solution. The abscissa in the figure is the chemical reaction time, and the negative value refers to the physical adsorption process in the first 30 minutes before the electrochemical reaction occurs. Figure 2 As can be seen from

[0052] (1) In terms of the influence of voltage:

[0053] When the voltage is 0 V, the removal efficiency depends only on physical adsorption and electrostatic interaction. The removal efficiency is the highest at pH = 4 in the reaction solution (C t / C0 = 0.3 after 60 minutes), and the lowest at pH = 10 (C t / C0 = 0.8 after 60 minutes). This indicates that in an acidic environment, it is beneficial for the positively charged MB to electrostatically bind to the negatively charged carbon nanotube membrane. At low voltages of 0.5 V or 1 V, electric field-assisted adsorption comes into play, and the voltage-driven electroosmotic flow (EOF) can accelerate mass transfer: at pH = 8 and voltage 1 V, C t / C0 drops to 0.2 after 60 minutes, and the efficiency is 60% higher than that at the same pH and voltage of 0 V. When the voltage rises to 2 V, catalytic oxidation becomes the dominant mechanism, and the high voltage activates the conductive network of carbon nanotubes to generate ·OH and SO4 - · free radicals.

[0054] (2) In terms of the regulation of pH:

[0055] Under acidic conditions (pH = 4 - 6), the electrostatic adsorption effect is enhanced. Due to the increased negative charge density of the carbon nanotube membrane, MB becomes positively charged after protonation. However, too high H + concentration will quench ·OH, limiting the free radical pathway, and the removal efficiency mainly depends on adsorption. When in a neutral to weakly alkaline (pH = 8) environment, the synergistic effect of electrostatic adsorption and free radical oxidation reaches equilibrium. Under the conditions of pH = 8 and voltage of 2 V, C t / C0 drops to 0.1, which is an optimal operating condition. Under strong alkaline (pH = 10) conditions, the negative charge density of the carbon nanotube membrane decreases (Zeta potential ≈ -10 mV), and MB deprotonates to become a neutral molecule, weakening the electrostatic adsorption effect. However, OH — can promote the free radical chain reaction (SO4 - · + OH — → SO4 2- + ·OH), but a high voltage of ≥2 V is required for activation.

[0056] (3) Comprehensive analysis of key data and mechanisms:

[0057] Under the conditions of pH = 4 and voltage of 0 V, C t / C0 is 0.3, and the dominant mechanism is electrostatic adsorption. However, due to the lack of oxidation ability, it is prone to saturation. Under the conditions of pH = 8 and voltage of 1 V, C t / C0 is 0.2, and electroosmotic flow mass transfer and weak oxidation act together. However, the generation of free radicals is insufficient. Under the conditions of pH = 10 and voltage of 2 V, C t / C0 rebounds to 0.8, which is limited by oxidation side reactions (such as the generation of O3 - ), free radical quenching, and the accumulation of by-products.

[0058] In summary, the best operating window is recommended to be the treatment under the conditions of pH = 8 and voltage of 2 V. Under these conditions, adsorption and oxidation are balanced, C t / C0 = 0.1 (removal rate 90%), achieving an equilibrium in energy consumption and stability. At the same time, situations where pH > 10 (sharp drop in efficiency) and pH < 4 (increase in side reactions) should be avoided. In terms of process adaptability, for low-concentration MB (<10 mg / L), an energy-saving mode with pH = 8 and voltage of 1 V can be adopted; while for high-concentration or refractory MB, a strengthened oxidation mode with pH = 4 and voltage of 4 V can be used.

[0059] Comparative Example 1

[0060] In this comparative example, a carbon nanotube monolayer membrane was used for the methylene blue (MB) removal experiment, as follows:

[0061] (1) 2 g of multi-walled carbon nanotubes were added to 120 mL of a sulfuric acid and nitric acid mixed solution with a volume ratio of 3:1, and stirred thoroughly at 60 °C for 1 hour for acidification treatment. After the acidification treatment, the obtained carbon nanotube solution was diluted and separated by vacuum filtration, and washed with ultrapure water until the pH value of the filtrate was close to neutral (6.5 - 7.5). Then it was dried under vacuum for 12 h and collected for standby.

[0062] (2) The acidified multi-walled carbon nanotubes were dispersed in ultrapure water by ultrasonic treatment to obtain a 0.5 mg / mL carbon nanotube dispersion.

[0063] (3) 10 mL and 30 mL of the carbon nanotube dispersion were respectively vacuum filtered onto a polyvinylidene fluoride membrane (PVDF), and dried under vacuum at 60 °C to obtain two carbon nanotube single-layer membranes with different CNT loadings. The diameter of the single-layer membrane was about 4.1 cm.

[0064] (4) Using the three-electrode reaction system of an electrochemical workstation, a saturated Ag / AgCl electrode was used as the reference electrode, a platinum sheet was used as the counter electrode, and platinum rods clamped with carbon nanotube single-layer membranes with different CNT loadings were used as the working electrodes respectively. A voltage of 1.6 V was applied to the working electrode to remove MB in the reaction solution. The initial concentration of MB in the reaction solution was 10 μM, the concentration of sodium sulfate was 1 mM, and the pH value was 8.

[0065] Example 4

[0066] In this example, the carbon nanotube-graphene oxide composite membranes with different CNT loadings prepared in Example 1 were used for the methylene blue (MB) removal experiment, as follows:

[0067] Using the three-electrode reaction system of an electrochemical workstation, a saturated Ag / AgCl electrode was used as the reference electrode, a platinum sheet was used as the counter electrode, and platinum rods clamped with the carbon nanotube-graphene oxide composite membranes with different CNT loadings prepared in Example 1 were used as the working electrodes respectively. A voltage of 1.6 V was applied to the working electrode to remove MB in the reaction solution. The initial concentration of MB in the reaction solution was 10 μM, the concentration of sodium sulfate was 1 mM, and the pH value was 8.

[0068] The MB removal efficiency results in Comparative Example 1 and Example 4 are as Figure 3 shown. The removal efficiency is measured by the ratio of C t and C0, where C t is the concentration of MB in the water sample at different times, and C0 is the concentration of MB in the initial reaction solution.

[0069] The results show that the efficiency of the single-walled carbon nanotube (SWCNT) film in removing methylene blue (MB) is better than that of the SWCNT-reduced graphene oxide (rGO) composite film. There is no obvious proportional relationship between the removal efficiencies of composite films with different CNT loadings. This indicates that the CNT layer plays a major role in the MB removal process. However, the SWCNT film has problems such as being fragile and having poor stability. By adding rGO, ethylenediamine (EDA), and polystyrene sulfonate (PSS), the adhesion strength of the materials on the film can be effectively increased. The reason is that rGO, EDA, and PSS are polymer materials that are loaded on the film like glue, thus playing a strengthening role. In addition, it is difficult to avoid electrolyte interference during the electrochemical process in a pure CNT layer. The asymmetric composite film formed by intercalating EDA-PSS into rGO / CNT realizes the simultaneous improvement of water flux and Na + rejection rate by means of the electrokinetic effect.

[0070] Generally speaking, as the main material of the membrane, CNT plays a major role and is mainly responsible for pollutant removal (including adsorption and catalytic oxidation, etc.). While rGO, EDA, and PSS, as the auxiliary parts of the membrane, are mainly used to improve the performance of the CNT membrane. On the one hand, it helps to remove salts, and on the other hand, it can enhance the firmness of the membrane.

[0071] Example 5

[0072] In this example, the SWCNT-rGO composite membrane loaded with 10 mL of CNT dispersion prepared in Example 1 was used to conduct MB removal experiments under different current modes, as follows:

[0073] Using the three-electrode reaction system of an electrochemical workstation, a saturated Ag / AgCl electrode was used as the reference electrode, a platinum sheet was used as the counter electrode, and a platinum rod clamped with the SWCNT-rGO composite membrane was used as the working electrode. The electrochemical workstation was respectively set to direct current (DC) mode, 1 Hz alternating current (AC) mode, and 400 Hz alternating current (AC) mode. A voltage of 1.6 V was applied to the working electrode to remove MB in the reaction solution. The initial concentration of MB in the reaction solution was 10 μM, the concentration of sodium sulfate was 1 mM, and the pH value was 8.

[0074] The results are as Figure 4 shown. In the direct current (DC) mode, the MB removal rate was 70% in 60 minutes (the concentration decreased from 10 mg / L to 3 mg / L), and its kinetic characteristics showed a rapid decline, with the removal rate exceeding 50% in the first 30 minutes. The key mechanism is the continuous electric field-driven electrochemical oxidation process, mainly ·OH and SO4 -Free radicals play a dominant role, and there is no polarity switching on the electrode surface, enabling the oxidation reaction to proceed continuously. In the alternating current (AC) 1 Hz mode, the removal rate of MB is 50% in 60 minutes (the concentration drops from 10 mg / L to 5 mg / L), and its kinetic characteristics show a slow linear decline. This is mainly because the low-frequency polarity switching (60 times per minute) allows intermittent oxidation reactions to occur, and at the same time, the generation of some free radicals is superimposed with the mass transfer effect, thus affecting the removal efficiency of MB. In the alternating current (AC) 400 Hz mode, the MB concentration drops from 10 mg / L to 9 mg / L, and the removal rate is less than 10%. The key reason is that the high-frequency polarity switching (24,000 times per minute) causes the oxidation and reduction reactions to cancel each other out periodically, and the lifespan of free radicals is extremely short, unable to effectively remove MB, resulting in poor removal effect.

[0075] In summary, the direct current mode has the highest removal efficiency, but its energy consumption is also relatively large (0.5 kWh / m 3 ). In the AC 1 Hz mode, the energy consumption can be reduced by 40% (0.3 kWh / m 3 ), which is suitable for treating low-concentration MB wastewater with a concentration less than 5 mg / L. It is also possible to adopt a combination of direct current and alternating current. In the early stage (0 - 30 minutes), the direct current mode is used to quickly remove MB, and in the later stage (30 - 60 minutes), the 1 Hz alternating current mode is used to maintain stability, which can achieve a total MB removal rate > 80% and an energy consumption reduction of 30%.

[0076] Example 6

[0077] In this example, the carbon nanotube-graphene oxide composite membrane loaded with 10 mL of carbon nanotube dispersion prepared in Example 1 is used to remove trace organic pollutants in surface water, specifically as follows:

[0078] Using the three-electrode reaction system of an electrochemical workstation, a saturated Ag / AgCl electrode is used as the reference electrode, a platinum sheet is used as the counter electrode, and a platinum rod clamped with a carbon nanotube-graphene oxide composite membrane is used as the working electrode respectively. The electrochemical workstation is operated in the direct current (DC) mode. A voltage of 4V is applied to the working electrode to remove sulfamethoxazole in the reaction solution. Other electrochemistry reaction settings are the same as above.

[0079] The reaction solution is a sulfamethoxazole (SMX) treatment solution prepared from natural water. The initial concentration of SMX is 5 mg / L. This natural water is from a lake in Zhejiang Province, which contains a variety of coexisting interfering substances. The basic parameters are as follows: the concentration of humic acid (HA) is 2.13 mg / L, the conductivity is 431 µS / cm, the total nitrogen content is 10.1 mg / L, Fe 3+ concentration is 0.012 mg / L, Cl -Concentration is 0.950 mg / L and total carbon content is 31.73 mg / L.

[0080] Through experiments, at a voltage of 4 V, the membrane with a diameter of 4 cm removed 0.0025 mg of sulfamethoxazole in 60 minutes. That is, the removal rate of sulfamethoxazole (SMX) per unit area of the membrane is 0.033 mg·m -2 ·min -1 . It shows that the carbon nanotube-graphene oxide composite membrane provided by the present invention helps to solve the problems of poor anti-fouling performance and insufficient retention of small molecule pollutants in traditional membrane technology when removing organic pollutants in wastewater.

[0081] It should be noted that in addition to removing organic pollutants in water through electrochemical enhancement, the carbon nanotube-graphene oxide composite membrane prepared by the present invention can also adopt oxidant synergistic treatment or electrochemical-oxidant coupling to remove organic pollutants in water, which is specifically described as follows:

[0082] I. Oxidant synergy (heterogeneous catalytic oxidation mechanism)

[0083] (1) Oxidant activation and free radical chain reaction: After adding persulfate (PS, S2O8 2- ) or permanganate (MnO4 - ), the surface characteristics of the composite membrane drive heterogeneous activation. Among them, the defect sites of carbon nanotubes: sp 2 hybrid carbon structure acts as an electron transfer medium to promote the decomposition of PS; or the carbonyl group (C=O) in the oxygen-containing groups of graphene oxide acts as a Lewis acid site to adsorb MnO4 - , triggering a reduction reaction, and the generated SO4 - · (oxidation potential 2.5 - 3.1 V) and MnO2 nanoparticles further catalyze the oxidation of pollutants.

[0084] (2) External field energy synergy. For example, light irradiation (ultraviolet / visible light) can excite the π-π* transition of graphene oxide to generate electron-hole pairs (e - -h + ), accelerating the activation of oxidants (such as the photolysis quantum efficiency of PS is increased by 2.3 times). Another example is heating (30 - 60 °C), through the Arrhenius effect, raising the temperature to 40 °C can increase the reaction rate constant k by 1.8 times. Or, for example, ultrasound (20 - 40 kHz), the cavitation effect generates local high temperature and high pressure (~5000 K, 1000 atm), promoting the surface renewal of the catalyst and mass transfer (the diffusion coefficient of pollutants is increased by 65%).

[0085] II. Electrochemical-oxidant coupling enhancement mechanism

[0086] Electric field-directed activation of oxidants: In a 0.5 - 3 V DC electric field, PS or MnO4 near the anode- It preferentially obtains electron activation, and pollutants are enriched in the cathode region to form a concentration gradient, achieving precise spatial matching of the oxidant and pollutants.

[0087] The embodiments described above are only a preferred solution of the present invention, but they are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can also make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by means of equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for preparing a carbon nanotube-graphene oxide composite membrane, characterized in that, The specific steps are as follows: S1: Disperse the acid-treated multi-walled carbon nanotubes in water to prepare a carbon nanotube dispersion; uniformly load the carbon nanotube dispersion onto a substrate membrane to obtain a carbon nanotube monolayer membrane; S2: Use a first mixed solution containing a polyamine crosslinking agent and a polyanionic electrolyte. After mixing the graphene oxide dispersion with the first mixed solution, a second mixed solution is obtained; S3: Uniformly load the second mixed solution onto the carbon nanotube monolayer membrane prepared in step S1 to obtain a carbon nanotube-graphene oxide composite membrane.

2. The preparation method of the carbon nanotube-graphene oxide composite film according to claim 1, wherein, The acid treatment of the multi-walled carbon nanotubes is specifically as follows: Immerse the multi-walled carbon nanotubes in a mixed solution of sulfuric acid and nitric acid with a volume ratio of (3-4):1 for 0.5-2 hours, and then wash with ultrapure water until neutral and dry for later use; The substrate membrane is one of polyvinylidene fluoride, polysulfone, polyethersulfone, polypropylene, polyacrylonitrile, polytetrafluoroethylene or cellulose acetate organic membranes, or an inorganic ceramic membrane or a stainless steel metal membrane.

3. The preparation method of the carbon nanotube-graphene oxide composite membrane according to claim 1, wherein, In steps S1 and S3, the loading method is coating, spin coating, vacuum filtration or electrophoretic deposition.

4. The preparation method of the carbon nanotube-graphene oxide composite film according to claim 1, characterized in that The concentration of multi-walled carbon nanotubes in the carbon nanotube dispersion is 0.2 to 0.8 mg / mL; the loading amount of carbon nanotubes on the carbon nanotube single-layer film is 0.4 to 1.2 mg / cm 2 ; the concentration of graphene oxide in the graphene oxide dispersion is 0.02 to 0.2 mg / mL.

5. The preparation method of the carbon nanotube-graphene oxide composite film according to claim 1, characterized in that, The polyamine crosslinking agent in the first mixed solution is ethylenediamine, polyethyleneimine or triethylenetetramine, and the polyanionic electrolyte is polystyrene sulfonate, polyacrylic acid, polyvinyl sulfonic acid, layered double metal hydroxide or montmorillonite.

6. The preparation method of the carbon nanotube-graphene oxide composite membrane according to claim 5, characterized in that, The polyamine crosslinking agent in the first mixed solution is ethylenediamine, and the polyanionic electrolyte is polystyrene sulfonate; the concentration of ethylenediamine in the first mixed solution is 0.1-0.3 g / L, and the mass percentage of polystyrene sulfonate is 0.1%-0.5%; the graphene oxide dispersion and the first mixed solution are mixed at a volume ratio of 1:

2.

7. A carbon nanotube-graphene oxide composite membrane obtained by using the preparation method according to any one of claims 1-6.

8. A method for electrochemically enhancing the removal of organic pollutants in water, characterized in that, In a three-electrode electrochemical reaction device containing wastewater to be treated, use the carbon nanotube-graphene oxide composite membrane obtained by the preparation method according to any one of claims 1-6 as the working electrode, Ag / AgCl as the reference electrode, and a platinum sheet as the counter electrode. Apply a voltage of 0.5-3 V to the working electrode in an alternating current or direct current mode to remove organic pollutants in the wastewater to be treated through an electrochemical reaction; the organic pollutants are methylene blue or sulfamethoxazole.

9. The method for electrochemically enhancing the removal of organic pollutants in water according to claim 8, characterized in that, Add persulfate or permanganate as an oxidant to the electrochemical reaction device; the addition amount of the oxidant is 0.1-5 mM; an electrolyte with a concentration of 1-50 mM is additionally added to the wastewater to be treated, and the pH value of the wastewater to be treated is controlled to be 3-10.

10. A method for synergistically removing organic pollutants in water by an oxidant, characterized in that, Add an oxidant persulfate or permanganate to the reaction device containing wastewater to be treated, and use the carbon nanotube-graphene oxide composite membrane obtained by the preparation method according to any one of claims 1-6 as a filtration membrane; The wastewater to be treated flows through the carbon nanotube-graphene oxide composite membrane under an external force, and the organic pollutants in the wastewater to be treated are removed by heterogeneous catalytic oxidation; the organic pollutants are methylene blue or sulfamethoxazole.

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