A flow electrode liquid, a preparation method, an FCDI device and a water treatment method

Abstract

The application discloses a kind of flowing electrode liquid, preparation method, FCDI device and water treatment method, the device includes anode chamber, cathode chamber, electrode liquid retention chamber and water inlet chamber.Anode chamber and cathode chamber are equipped with current collector and ion exchange membrane, and the cavity filled with flowing electrode liquid is formed between the two.The flowing electrode liquid is made by mixing lignin biochar, conductive carbon black, electrolyte and water according to a certain mass ratio, wherein the lignin biochar is obtained by hydrothermal, catalysis, alkalization and acid separation of lignin extracted from papermaking black liquor.When working, wastewater is pre-filtered by nanofiltration membrane, and under the driving of electric field, ions pass through ion exchange membrane and are adsorbed by flowing electrode liquid, and saturated flowing electrode liquid is circulated to electrode liquid retention chamber for regeneration, while organic pollutants can be degraded by ultraviolet irradiation.The application uses low-cost and high-performance lignin biochar electrode, combines cascade filtration and online monitoring, and realizes efficient, continuous and pollution-resistant deep desalination and resource utilization of complex wastewater.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a capacitive deionization (FCDI) technology, and particularly to a flowing electrode solution, its preparation method, an FCDI device, and its application. Background Technology

[0002] Capacitive deionization (CDI), as an emerging desalination technology, works by electrostatically adsorbing ions from the solution through the formation of an electric double layer on the electrode surface. However, traditional fixed-electrode CDI technology suffers from electrode saturation during operation, requiring frequent regeneration steps (such as voltage reversal or rinsing) to restore adsorption capacity, resulting in a discontinuous process and limited efficiency.

[0003] To overcome the aforementioned shortcomings, Flow Electrode Capacitive Deionization (FCDI) technology has emerged. This technology uses a continuously circulating conductive liquid as the flow electrode, achieving spatiotemporal separation of the adsorption and regeneration processes, enabling continuous output of fresh water, and significantly improving the system's continuous operation capability and treatment throughput. However, existing FCDI technology still faces two major technical bottlenecks in practical applications: First, membrane fouling is a significant problem. When treating industrial wastewater or slightly polluted water sources with complex compositions (such as those containing natural organic matter, colloids, and microorganisms), large molecular organic pollutants and suspended solids in the water are easily adsorbed and deposited on the ion exchange membrane surface, or clog the flow channels, leading to decreased membrane permeability, increased system energy consumption, and reduced desalination efficiency, severely restricting the long-term stable operation and application expansion of the technology. Second, the performance of the flow electrode needs optimization. Traditional flow electrodes mostly use commercial activated carbon materials, which have limited specific capacitance, poor conductive network construction, and the tendency for carbon particles to aggregate and settle in the flow electrode, affecting charge transfer efficiency and ion adsorption kinetics. Therefore, developing novel flow electrode materials that combine high adsorption capacity, excellent conductivity and stable suspension, as well as low cost and environmental friendliness, is key to promoting the development of FCDI technology.

[0004] To address the challenges of membrane fouling in treating complex water bodies using existing FCDI technology, and the low efficiency of flow electrode materials, this invention proposes an innovative solution for cascaded FCDI water treatment technology. By integrating a pre-filter nanofiltration unit with a subsequent FCDI desalination unit, and introducing a novel flow electrode based on lignin biochar, this technology aims to synergistically optimize both the process structure and core materials. This approach significantly enhances the system's anti-fouling capabilities while achieving highly efficient desalination, providing an efficient, stable, and sustainable technological path for the deep purification and resource recovery of complex water bodies. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an FCDI water treatment device and method based on a lignin biochar flow electrode that has strong anti-fouling ability, high desalination efficiency and stable operation.

[0006] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0007] In a first aspect, the present invention provides a high-performance flow electrode liquid. The flow electrode liquid comprises lignin biochar, conductive carbon black, electrolyte and water, and its key feature is that the mass ratio of the four components is (7~10):(1~2):(0.1~0.2):(170~500). This specific ratio ensures that the flow electrode has excellent conductivity, stable suspension and high ion adsorption capacity.

[0008] In some embodiments, the electrolyte is preferably sodium chloride or potassium chloride, and those skilled in the art may also choose sodium sulfate or potassium sulfate.

[0009] Secondly, the present invention provides a method for preparing the above-mentioned flowing electrode liquid. It mainly includes: S1, extracting lignin powder from papermaking black liquor, and subjecting it to hydrothermal treatment (100~300℃, 5~10 hours), catalytic treatment (using catalysts such as KCl, LiCl, MgCl2, CaCl2, etc.), alkalization (using 1~3M NaOH or KOH solution, treatment for 24~72 hours), and acid precipitation (using 3M~5M HCl solution; stopping the addition of HCl solution when a large amount of black flocculent matter is observed), to obtain porous lignin biochar with multiple oxygen-containing functional groups; S2, mixing and stirring the obtained lignin biochar with conductive carbon black, electrolyte, and water in the above proportions to obtain the final product.

[0010] Thirdly, the present invention provides an integrated FCDI device, which includes an anode chamber, a cathode chamber, an electrode solution retention chamber, and an inlet chamber. The anode and cathode chambers are respectively equipped with a current collector (such as graphite or titanium) and ion exchange membranes (cation exchange membrane and anion exchange membrane), forming cavities filled with the flowing electrode solution. The electrode solution retention chamber connects the two electrode cavities and is used for the regeneration of the flowing electrode; it can be equipped with an online dissolved organic matter monitoring device and an ultraviolet light source.

[0011] In some embodiments, the device is powered by a DC power supply (0.5~3 V) and the inlet water flow rate (5~30 mL / min) is controlled by a peristaltic pump.

[0012] Fourthly, the present invention provides a method for water treatment using the above-described apparatus. This method includes the following steps:

[0013] (a) The wastewater to be treated is filtered through a nanofiltration membrane in the inlet chamber to remove large molecular organic pollutants;

[0014] (b) Under the drive of an electric field, cations in the wastewater pass through the cation exchange membrane and enter the cathode cavity to be adsorbed by the flowing electrode liquid, while anions pass through the anion exchange membrane and enter the anode cavity to be adsorbed by the flowing electrode liquid.

[0015] (c) The adsorbed ion flowing electrode liquid is circulated to the electrode liquid retention chamber, where it is mixed to desorb the ions into the solution, thereby regenerating the flowing electrode liquid;

[0016] (d) The regenerated flow electrode solution is returned to the anode and cathode chambers for recycling.

[0017] In some embodiments, the flowing electrode solution can be irradiated with an ultraviolet light source in the electrode solution retention chamber to degrade the dissolved organic matter carried therein; and the concentration of dissolved organic matter in the electrode solution can be monitored in real time by the online monitoring device to determine the cleaning or replacement cycle of the nanofiltration membrane module.

[0018] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0019] 1. This invention uses lignin biochar derived from papermaking waste liquid as the core electrode material, which is low in cost and environmentally friendly. Its unique pore structure and surface functional groups can provide high specific capacitance and abundant adsorption sites, and synergistically construct a good conductive network with conductive carbon black, thereby improving the desalination rate and capacity.

[0020] 2. This invention integrates online monitoring of dissolved organic matter and an advanced ultraviolet oxidation unit, enabling real-time monitoring and active elimination of contamination in the flowing electrode solution, and scientifically formulating membrane module maintenance plans to optimize the economy and reliability of system operation.

[0021] 3. Converting papermaking waste (black liquor) into high-value-added water treatment materials can achieve "waste treatment with waste", which is in line with the concept of green and sustainable development.

[0022] 4. The pre-filtration nanofiltration membrane of this invention serves as a pretreatment barrier, effectively trapping large molecular organic matter, greatly reducing contamination of subsequent ion exchange membranes and channels, and ensuring long-term stable operation of the system. Attached Figure Description

[0023] Figure 1 This is a schematic diagram illustrating the working principle of an FCDI device provided in an embodiment of the present invention;

[0024] Figure 2 An exploded view illustrating the working principle of an FCDI device provided in an embodiment of the present invention;

[0025] Figure 3 Scanning electron microscope image of lignin biochar provided in an embodiment of the present invention;

[0026] In the diagram: 1. Inlet chamber; 2. Nanofiltration membrane; 3. Cation exchange membrane; 4. Anion exchange membrane; 5. Cathode current collector; 6. Anode current collector; 7. Cathode cavity; 8. Anode cavity; 9. Electrode solution retention chamber; 10. Ultraviolet light source; 11. Online monitoring device; 12. Fixing plate. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Various terms and phrases used in this invention have general meanings known to those skilled in the art. The present invention provides a general or specific description of the materials and experimental methods used in the experiments. Although many materials and operating methods used to achieve the objectives of this invention are known in the art, they are still described in as much detail as possible herein. Unless otherwise specified, the instruments, reagents, materials, etc., involved in the following embodiments are all conventional instruments, reagents, materials, etc., already existing in the prior art; and unless otherwise specified, they are all conventional experimental methods, detection methods, etc., already existing in the prior art, and can all be obtained commercially.

[0029] This invention provides a flowing electrode solution comprising lignin biochar, conductive carbon black, electrolyte, and water in a mass ratio of (7~10):(1~2):(0.1~0.2):(170~500). In some embodiments, the electrolyte comprises one or both of sodium chloride and potassium chloride. It should be noted that the electrolyte may also be other optional products, and this invention does not impose any limitations.

[0030] The present invention also provides a method for preparing a flowing electrode liquid, comprising the following steps:

[0031] S1: Lignin biochar is prepared by extracting lignin powder from black liquor of a paper mill wastewater treatment plant and then subjecting it to hydrothermal treatment, catalysis, alkalization and acid precipitation.

[0032] S2: A flowing electrode solution is prepared by mixing and stirring lignin biochar, conductive carbon black, electrolyte and water in the proportions described in claim 1.

[0033] In some embodiments, in step S1, the hydrothermal temperature is 100~300℃, the hydrothermal time is 5~10 hours, the catalyst used for catalysis is any one or more of KCl, LiCl, MgCl2, and CaCl2, the alkalization solution is any one or a mixture of NaOH and KOH, the concentration of the alkalization solution is 1M~3M, and the alkalization time is 24~72 hours. The acid precipitation solution is an HCl solution with a concentration of 3M~5M. When a large amount of black flocculent matter is observed, the addition of HCl solution is stopped, and the lignin biochar is obtained by filtration.

[0034] This invention also provides an FCDI device, comprising: an anode chamber, a cathode chamber, an electrode solution retention chamber 9, a water inlet chamber 1, and a power supply. The anode chamber contains an anode current collector 6 and an anion exchange membrane 4, forming an anode cavity 8 between the anode current collector 6 and the anion exchange membrane 4. A fixing plate 12 is provided on the outside of the anode current collector 6. The cathode chamber contains a cathode current collector 5 and a cation exchange membrane 3, forming a cathode cavity 7 between the cathode current collector 5 and the cation exchange membrane 3. A fixing plate 12 is provided on the outside of the cathode current collector 5. The electrode solution... The retention chamber 9 is connected to the anode chamber 8 and the cathode chamber 7 respectively, and is used to receive and process the flowing electrode liquid from the two chambers; nanofiltration membranes 2 are respectively provided on the cation exchange membrane 3 and the anion exchange membrane 4; the water inlet chamber 1 is connected to the nanofiltration membrane 2, and the anode chamber 8 and the cathode chamber 7 are filled with the flowing electrode liquid; the two ends of the power supply are respectively connected to the current collectors in the anode chamber and the cathode chamber, and are used to supply power to the two current collectors, so that the fluid electrode liquid can circulate in the anode chamber 8, the cathode chamber 7 and the electrode liquid retention chamber 9 respectively.

[0035] The electrode solution retention chamber 9 is also equipped with an online dissolved organic matter monitoring device 11 and an ultraviolet light source 10. In this invention, the flow rate of wastewater entering the FCDI device can be 5~30 mL / min, and the power supply voltage can be 0.5~3 V. The anode current collector 6 and / or the cathode current collector 5 can be graphite current collectors or titanium current collectors. As those skilled in the art will know, the current collector can also be other optional products, and this invention does not impose any limitations.

[0036] The present invention also provides a method for water treatment using an FCDI device, comprising the following steps:

[0037] (a) The wastewater to be treated is filtered through nanofiltration membrane 2 in inlet chamber 1 to remove macromolecular organic pollutants;

[0038] (b) Under the drive of the electric field, cations in the wastewater pass through the cation exchange membrane 3 and enter the cathode cavity 7 to be adsorbed by the flowing electrode liquid, while anions pass through the anion exchange membrane 4 and enter the anode cavity 8 to be adsorbed by the flowing electrode liquid.

[0039] (c) The adsorbed ion flowing electrode liquid is circulated to the electrode liquid retention chamber 9, where it is mixed to desorb the ions into the solution, thereby regenerating the flowing electrode liquid. Specifically, the present invention uses the ultraviolet light source 10 in the electrode liquid retention chamber 9 to irradiate the flowing electrode liquid to degrade the dissolved organic matter carried therein. The concentration of dissolved organic matter in the electrode liquid is monitored in real time by the online monitoring device 11, and the cleaning or replacement cycle of the membrane module is determined accordingly.

[0040] (d) The regenerated flowing electrode solution is returned to the anode chamber 8 and the cathode chamber 7 for recycling.

[0041] The present invention will be described in detail below through embodiments, but the scope of protection of the present invention is not limited thereto.

[0042] Example 1

[0043] This invention provides a flowing electrode solution and its preparation method. The flowing electrode solution of this invention mainly uses lignin biochar as the main material, and the preparation process of the lignin biochar is as follows:

[0044] 50g of lignin powder was weighed and placed in a hydrothermal solution containing KCl and LiCl catalysts, with 15g of KCl and 10g of LiCl. The mixture was then hydrothermally reacted at 180℃ for 10 hours. After the reaction, the product was washed and then alkalized with 2M NaOH solution for 48 hours. HCl solution was added dropwise at a concentration of 5M until a large amount of black flocculent material appeared. The addition of HCl solution was then stopped. Finally, the mixture was filtered, washed, dried, ground, and sieved to obtain lignin biochar. It should be noted that the lignin was extracted from the black liquor of an alkaline pulping process at a paper mill. Those skilled in the art can also extract lignin from the black liquor of alkaline pulping processes at other paper mills; this invention is not limited to this method.

[0045] Weigh out 8g of lignin biochar, 1.5g of conductive carbon black, 0.15g of NaCl and 300g of deionized water respectively. Use a magnetic stirrer to continuously stir the lignin biochar, conductive carbon black, electrolyte and deionized water to mix them. Stir thoroughly until a uniform slurry is formed to obtain a flowing electrode solution.

[0046] It should be noted that KCl has a pore-forming effect, while LiCl facilitates the breaking of β-O-4 ether bonds and methoxy bonds in lignin. KCl and LiCl have a synergistic catalytic effect, enabling the formation of oxygen-containing functional groups in lignin biochar. Figure 3 Here are SEM images of lignin biochar, from bottom to top. Figure 3 As can be seen, the surface of the prepared lignin biochar has a large number of microporous structures, and its unique pore structure can provide high specific capacitance and abundant adsorption sites.

[0047] The oxygen-containing functional groups of the prepared lignin biochar were determined by Boehm titration to be 8.5 mmol / g for carboxyl groups, 12.35 mmol / g for carbonyl groups, and 11.95 mmol / g for phenolic hydroxyl groups. This indicates that the oxygen-containing functional groups of the prepared lignin biochar were significantly increased and the functional groups underwent substantial changes.

[0048] Example 2

[0049] 50g of lignin powder was weighed and placed in a hydrothermal solution containing KCl and LiCl catalysts, with 10g of KCl and 15g of LiCl. The mixture was then hydrothermally reacted at 150℃ for 8 hours. After the reaction, the product was washed and then alkalized with 1M NaOH solution for 72 hours. 4M HCl solution was then added dropwise. When a large amount of black flocculent matter appeared, the addition of HCl solution was stopped. Finally, the mixture was filtered, washed, dried, ground, and sieved to obtain lignin biochar.

[0050] 10g of lignin biochar, 2g of conductive carbon black, 0.1g of NaCl and 400g of deionized water were weighed out respectively. The lignin biochar, conductive carbon black, electrolyte and deionized water were continuously stirred with a magnetic stirrer to mix them until they were mixed into a uniform slurry, and then the flowing electrode liquid was obtained.

[0051] Example 3

[0052] 50g of lignin powder was weighed and placed in a hydrothermal solution containing KCl catalyst (30g of KCl). The mixture was then hydrothermally reacted at 300℃ for 5 hours. After the reaction, the product was washed and then alkalized with 3M NaOH solution for 24 hours. 3M HCl solution was then added dropwise. When a large amount of black flocculent matter appeared, the addition of HCl solution was stopped. Finally, the mixture was filtered, washed, dried, ground, and sieved to obtain lignin biochar.

[0053] Weigh out 7g of lignin biochar, 1g of conductive carbon black, 0.2g of NaCl and 500g of deionized water respectively. Use a magnetic stirrer to continuously stir the lignin biochar, conductive carbon black, electrolyte and deionized water to mix them. Stir thoroughly until a uniform slurry is formed to obtain a flowing electrode solution.

[0054] The oxygen-containing functional groups of the prepared lignin biochar were determined by Boehm titration to be 8.1 mmol / g for carboxyl groups, 11.9 mmol / g for carbonyl groups, and 8.87 mmol / g for phenolic hydroxyl groups.

[0055] Example 4

[0056] 50g of lignin powder was weighed and placed in a hydrothermal solution containing LiCl catalyst, of which 25g of LiCl was present. The mixture was then hydrothermally reacted at 100℃ for 10 hours. After the reaction, the product was washed and then alkalized with 2.5M NaOH solution for 48 hours. 4M HCl solution was then added dropwise. When a large amount of black flocculent matter appeared, the addition of HCl solution was stopped. Finally, the mixture was filtered, washed, dried, ground, and sieved to obtain lignin biochar.

[0057] 9g of lignin biochar, 1.2g of conductive carbon black, 0.18g of NaCl and 170g of deionized water were weighed out respectively. The lignin biochar, conductive carbon black, electrolyte and deionized water were continuously stirred with a magnetic stirrer to mix them until they were in a uniform slurry, and then the flowing electrode liquid was obtained.

[0058] The oxygen-containing functional groups of the prepared lignin biochar were determined by Boehm titration to be 8.3 mmol / g for carboxyl groups, 12.1 mmol / g for carbonyl groups, and 10.03 mmol / g for phenolic hydroxyl groups.

[0059] Comparative Example 1

[0060] In comparison, Comparative Example 1 provides a method for preparing a flowing electrode solution and the flowing electrode solution itself. This comparative example aims to demonstrate that raw lignin, without the hydrothermal, catalytic, alkalization, and acid precipitation modification treatments described in this invention, exhibits poor performance as a flowing electrode material, thus highlighting the crucial role of the lignin biochar modification process in improving the electrode performance of this invention. The preparation process of the flowing electrode solution in Comparative Example 1 is as follows:

[0061] Weigh 50g of lignin powder, wash and dry it, then grind and sieve it.

[0062] Weigh out 8g of lignin, 1.5g of conductive carbon black, 0.15g of NaCl and 300g of deionized water respectively. Use a magnetic stirrer to continuously stir the lignin, conductive carbon black, electrolyte and deionized water to mix them. Stir thoroughly until a uniform slurry is formed to obtain a flowing electrode solution.

[0063] Comparative Example 2

[0064] This comparative example aims to investigate the importance of the catalytic step in the preparation of lignin biochar. By omitting the catalytic step, the key influence of the catalyst on the construction of lignin biochar rich in microporous structure and specific oxygen-containing functional groups is verified.

[0065] Weigh 50g of lignin powder and, without adding any catalyst, directly perform a hydrothermal reaction at 180℃ for 10 hours. After the reaction, the product is washed and then alkalized with 2M NaOH solution for 48 hours. 4M HCl is then added dropwise. When a large amount of black flocculent matter appears, the addition of HCl solution is stopped. Finally, the product is filtered, washed, dried, ground, and sieved to obtain lignin biochar.

[0066] 8g of lignin biochar, 1.5g of conductive carbon black, 0.15g of NaCl and 300g of deionized water were weighed out respectively. The lignin, conductive carbon black, electrolyte and deionized water were continuously stirred with a magnetic stirrer to mix them until they were in a uniform slurry, and then the flowing electrode liquid was prepared.

[0067] The oxygen-containing functional groups of the prepared lignin biochar were determined by Boehm titration to be 7.8 mmol / g for carboxyl groups, 11.65 mmol / g for carbonyl groups, and 8.5 mmol / g for phenolic hydroxyl groups.

[0068] Comparative Example 3

[0069] This comparative example aims to demonstrate the indispensable role of conductive carbon black in the formulation of flowing electrode solutions. By examining the performance of flowing electrode solutions without conductive carbon black, the contribution of conductive carbon black to constructing a continuous conductive network, improving charge transfer efficiency, and enhancing overall desalination performance is verified.

[0070] 50g of lignin powder was weighed and placed in a hydrothermal solution containing KCl and LiCl catalysts, with 15g of KCl and 10g of LiCl. The mixture was then hydrothermally reacted at 180℃ for 10 hours. After the reaction, the product was washed and then alkalized with 2M NaOH solution for 48 hours. 5M HCl was added dropwise, and the addition of HCl solution was stopped when a large amount of black flocculent matter appeared. Finally, the mixture was filtered, washed, dried, ground, and sieved to obtain lignin biochar.

[0071] 8g of lignin biochar, 0.15g of NaCl and 300g of deionized water were weighed out respectively. The lignin, conductive carbon black, electrolyte and deionized water were continuously stirred with a magnetic stirrer to mix them until a uniform slurry was formed, thereby obtaining a flowing electrode solution.

[0072] Performance testing

[0073] The flowing electrode solutions prepared in Examples 1-4 and the materials prepared in Comparative Examples 1-3 were applied in an FCDI device to conduct treatment tests on simulated slightly polluted water. The FCDI device consisted of a polyamide nanofiltration membrane, a homogeneous cation exchange membrane 3 (Shandong Tianwei Membrane Technology Co., Ltd., TWEDC1S), and a homogeneous anion exchange membrane 4 (Shandong Tianwei Membrane Technology Co., Ltd., TWEDA1R) cascaded sequentially from the inlet to the outlet. The anode chamber 8 and cathode chamber 7 were filled with the flowing electrode solution, specifically, the anode and cathode flowing electrode channels were filled with lignin-biochar flowing electrode solution, and the cathode and anode flowing electrode solutions had the same composition. For a schematic diagram of the working principle of the FCDI device, please refer to [link to schematic diagram]. Figure 1 .

[0074] Graphite is used as the anode current collector 6 and the cathode current collector 5, respectively. The prepared flowing electrode liquid is placed in the anode chamber 8 and the cathode chamber 7, respectively. The nanofiltration membrane 2 arranged on the cation exchange membrane 3 and the anion exchange membrane 4 is a polyamide membrane. The two ends of the power supply are connected to the current collectors in the anode chamber and the cathode chamber, respectively, to supply power to the two current collectors. Figure 2 An exploded view illustrating the working principle of an FCDI device provided in an embodiment of the present invention.

[0075] The object of treatment was simulated slightly polluted water containing 20 mg / L humic acid and 1000 mg / L NaCl. Before starting this embodiment, the peristaltic pump was started with an operating external voltage of 1.2 V to control the influent flow rate at 5 mL / min. This allowed the simulated slightly polluted water to first pass through the polyamide membrane in the influent chamber 1 to remove the large molecular organic pollutant humic acid, and then enter the desalination chamber.

[0076] Driven by an electric field, cations in simulated micro-polluted water pass through cation exchange membrane 3 and enter cathode chamber 7, where they are adsorbed by the flowing electrode solution. Anions pass through anion exchange membrane 4 and enter anode chamber 8, where they are adsorbed by the flowing electrode solution. The flowing electrode solution with adsorbed ions is then circulated to electrode solution retention chamber 9, where it is mixed. The flowing electrode solution is then irradiated by ultraviolet light source 10 within electrode solution retention chamber 9 to degrade the humic organic matter it carries, causing ions to desorb and enter the solution. The concentration of dissolved organic matter in the electrode solution is monitored in real time by the online monitoring device 11, which determines the cleaning or replacement cycle of the nanofiltration membrane 2 module, thus regenerating the flowing electrode solution. Table 1 shows the desalination rate of the system after 24 hours of operation for Examples 1-4 and Comparative Examples 1-3 under different applied voltages and influent flow rates.

[0077] Table 1 shows the desalination rate of the systems in Examples 1-4 and Comparative Examples 1-3 after 24 hours of operation under different applied voltages and influent flow rates.

[0078]

[0079] As shown in Table 1, the flowing electrode solutions prepared using the method of this invention (Examples 1-4) consistently maintained a desalination rate of over 85% under both operating conditions, with a maximum of 93%, significantly superior to all comparative examples. The desalination rate of Comparative Example 1 was only about 20%, a stark contrast to the examples (>90%). This clearly demonstrates that untreated lignin has an underdeveloped pore structure and insufficient surface functional groups, failing to provide effective ion adsorption sites and capacitance, directly leading to a near loss of desalination function.

[0080] Although the desalination rate of Comparative Example 2 was slightly higher than that of Comparative Example 1, it was still far lower than that of the Example. This indicates that the catalytic step is a key step in optimizing the modification process. Without a catalyst, it is difficult to achieve the directional depolymerization of lignin molecules and the reconstruction of functional groups, resulting in limited improvement in the specific capacitance and adsorption performance of the obtained biochar.

[0081] The desalination rate of Comparative Example 3 was significantly higher than that of Comparative Examples 1 and 2, but still significantly lower than that of the Examples. This result directly verifies the crucial role of conductive carbon black in the flowing electrode. Although lignin biochar has adsorption potential, its conductivity is usually insufficient. The addition of conductive carbon black can construct efficient electronic conduction pathways between particles, significantly reducing the internal resistance of the electrode, thereby improving the ion adsorption kinetics and overall desalination efficiency under electric field drive.

[0082] In this invention, lignin powder is extracted from black liquor from paper mill wastewater, and lignin biochar is obtained after hydrothermal treatment, catalysis, and alkalization. When the flowing electrode solution prepared by mixing lignin biochar, conductive carbon black, electrolyte, and water in a certain proportion is applied to the FCDI device, the pre-nanofiltration membrane 2 serves as a pretreatment barrier, which can effectively intercept large molecular organic matter, greatly reduce the pollution of subsequent ion exchange membranes and channels, and ensure the long-term stable operation of the system. Driven by an electric field, cations in slightly polluted water pass through the cation exchange membrane 3 and enter the cathode cavity 7, where they are adsorbed by the flowing electrode liquid. Anions pass through the anion exchange membrane 4 and enter the anode cavity 8, where they are adsorbed by the flowing electrode liquid. The adsorbed ions in the flowing electrode liquid are then circulated to the electrode liquid retention chamber 9, where they mix and desorb into the solution. By placing an ultraviolet light source 10 in the electrode liquid retention chamber 9 to irradiate the flowing electrode liquid, the dissolved organic matter brought to the electrode liquid retention chamber 9 is degraded. In Examples 1-4 of this invention, the desalination rate of the flowing electrode liquid is consistently maintained above 85%, indicating that the lignin biochar prepared by this invention has a large number of microporous structures and significantly increased oxygen-containing functional groups. Its unique pore structure and surface functional groups can provide high specific capacitance and abundant adsorption sites, synergistically constructing a good conductive network with conductive carbon black, which can improve the desalination rate and capacity. This invention uses lignin biochar derived from papermaking wastewater as the core electrode material, which is low in cost and environmentally friendly. Converting papermaking waste (black liquor) into high-value-added water treatment materials can achieve "waste treatment with waste," which is in line with the concept of green and sustainable development.

[0083] This invention utilizes an online monitoring device 11 within the electrode solution retention chamber 9 to monitor the concentration of dissolved organic matter in the electrode solution in real time. Based on this, the cleaning or replacement cycle of the membrane module is determined, enabling the regeneration of the flowing electrode. The regenerated flowing electrode solution is then returned to the anode chamber 8 and cathode chamber 7 for recycling. In other words, this invention integrates online monitoring of dissolved organic matter with an advanced ultraviolet oxidation unit, enabling real-time monitoring and proactive elimination of contamination in the flowing electrode solution. It also allows for the scientific formulation of membrane module maintenance plans, optimizing the economy and reliability of system operation.

[0084] This invention also investigated the desalination rates of Examples 1-4 and Comparative Examples 1-3 after 24 hours of operation at different cycle numbers under the same applied voltage and influent flow rate. Table 2 shows the desalination rates of Examples 1 and Comparative Examples 1-3 after 24 hours of operation at different cycle numbers when the applied voltage is 1.2 V and the influent flow rate is 5 mL / min. It should be noted that the cycle number refers to the number of times the same slightly polluted water was treated.

[0085] Table 2 shows the desalination rate of the systems in Examples 1-4 and Comparative Examples 1-3 after 24 hours of operation, under different cycles at an applied voltage of 1.2 V and an influent flow rate of 5 mL / min.

[0086] As shown in Table 2, after five consecutive cycles of testing, Examples 1-4 maintained a desalination rate of approximately 90% with minimal fluctuation, demonstrating excellent operational stability and anti-fouling capabilities. In contrast, the performance of the comparative examples showed instability or a declining trend. The flowing electrode solution prepared by this invention, when used with the FCDI device and method of this invention, exhibits good long-term operational stability and anti-fouling capabilities.

[0087] This invention uses lignin biochar derived from papermaking wastewater as the core electrode material, which is low-cost and environmentally friendly. Its unique pore structure and surface functional groups provide high specific capacitance and abundant adsorption sites, synergistically forming a good conductive network with conductive carbon black, thereby improving desalination rate and capacity. Furthermore, this invention integrates online monitoring of dissolved organic matter and an advanced UV oxidation unit, enabling real-time monitoring and proactive elimination of contamination in the flowing electrode solution. It also allows for the scientific formulation of membrane module maintenance plans, optimizing the system's economic efficiency and reliability.

[0088] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A flowing electrode liquid, characterized in that, The mass ratio of lignin biochar, conductive carbon black, electrolyte, and water is (7~10):(1~2):(0.1~0.2):(170~500).

2. The flowing electrode liquid according to claim 1, characterized in that, The electrolyte includes one or more of sodium chloride, potassium chloride, sodium sulfate, or potassium sulfate.

3. A method for preparing the flowing electrode liquid according to any one of claims 1 to 2, characterized in that, Includes the following steps: S1: Lignin biochar is prepared by extracting lignin powder from black liquor of a paper mill wastewater treatment plant and then subjecting it to hydrothermal treatment, catalysis, alkalization and acid precipitation. S2: A flowing electrode solution is prepared by mixing and stirring lignin biochar, conductive carbon black, electrolyte and water in the proportions described in claim 1.

4. The method for preparing the flowing electrode liquid according to claim 3, characterized in that, In step S1, the hydrothermal temperature is 100~300℃, the hydrothermal time is 5~10 hours, the catalyst used for catalysis is any one or more of KCl, LiCl, MgCl2, and CaCl2, the alkalization solution is any one or more of NaOH and KOH, the concentration of the alkalization solution is 1M~3M, the alkalization time is 24~72 hours, and the acid precipitation solution is HCl solution with a concentration of 3M~5M. When a large amount of black flocculent matter is found, the addition of HCl solution is stopped, and the lignin biochar is obtained by filtration.

5. An FCDI device, characterized in that, include: An anode chamber is provided with an anode current collector and an anion exchange membrane, and an anode cavity is formed between the anode current collector and the anion exchange membrane; A cathode chamber, wherein a cathode current collector and a cation exchange membrane are provided in the cathode chamber, and a cathode cavity is formed between the cathode current collector and the cation exchange membrane; An electrode liquid retention chamber is connected to both the anode chamber and the cathode chamber, and is used to receive and process the flowing electrode liquid from the two chambers. Nanofiltration membranes are respectively provided on the cation exchange membrane and the anion exchange membrane; The water inlet chamber is connected to the nanofiltration membrane, and the anode cavity and cathode cavity are filled with the flowing electrode liquid according to any one of claims 1 to 2 or the flowing electrode liquid prepared by the preparation method according to any one of claims 3 to 4. The power supply is connected at both ends to the current collectors in the anode chamber and the cathode chamber, respectively, to supply power to the two current collectors, so that the fluid electrode liquid can circulate in the anode chamber, the cathode chamber and the electrode liquid retention chamber, respectively.

6. The FCDI device according to claim 5, characterized in that, The electrode solution retention chamber is also equipped with an online monitoring device for dissolved organic matter and an ultraviolet light source.

7. The FCDI device according to claim 6, characterized in that, The flow rate of wastewater entering the FCDI device is 5~30 mL / min, and the power supply voltage is 0.5~3 V.

8. The FCDI device according to claim 7, characterized in that, Both current collectors are either graphite current collectors or titanium current collectors.

9. A method for water treatment using the FCDI device according to any one of claims 5 to 8, characterized in that, Includes the following steps: (a) The wastewater to be treated is filtered through a nanofiltration membrane in the inlet chamber to remove macromolecular organic pollutants; (b) Under the drive of an electric field, cations in the wastewater pass through the cation exchange membrane and enter the cathode chamber to be adsorbed by the flowing electrode liquid, and anions pass through the anion exchange membrane and enter the anode chamber to be adsorbed by the flowing electrode liquid. (c) The adsorbed ion flowing electrode liquid is circulated to the electrode liquid retention chamber, where it is mixed to desorb the ions into the solution, thereby regenerating the flowing electrode liquid; (d) The regenerated flowing electrode liquid is returned to the anode chamber and cathode chamber for recycling.

10. The method according to claim 9, characterized in that, In step (c), the flowing electrode solution is irradiated with an ultraviolet light source in the electrode solution retention chamber to degrade the dissolved organic matter carried therein; and the concentration of dissolved organic matter in the electrode solution is monitored in real time by the online monitoring device to determine the cleaning or replacement cycle of the nanofiltration membrane module.

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