Method for producing caproic acid through iron anode in-situ hydrogen production enhanced microbial electrosynthesis

Abstract

The invention discloses a method for producing hexanoic acid through iron anode in-situ hydrogen production enhanced microbial electrosynthesis. The method comprises the following steps: (1) directional domestication of functional microorganisms for producing hexanoic acid; (2) pre-culturing the cathode of the electrosynthesis system; (3) construction of an electrosynthesis system and colonization of functional microorganisms: replacing the carbon fiber brush in the step (2) with a high-purity iron sheet, and mixing the caproic acid synthesis functional bacteria subjected to directional domestication in the step (1) with an electrolyte to start the electrosynthesis process; and (4) producing hexanoic acid by an iron anode in-situ hydrogen production mediated microbial electrosynthesis system. According to the method, caproic acid synthesis is carried out by adopting iron anode in-situ hydrogen production enhanced microbial electrosynthesis, electron transfer in an iron anode group is more efficient, the percentage of electrons transferred to caproic acid is higher, 54.5% of electrons are transferred to caproic acid, hydrogen production through extracellular electron transfer is enhanced by introduction of the iron anode, hydrogen is used as an additional electron donor, and the yield of caproic acid is increased. The generation of hexanoic acid is also promoted.

Description

Technical Field

[0001] The invention relates to the technical field of microbial electrosynthesis, and in particular to a method for producing caproic acid by enhancing microbial electrosynthesis through in-situ hydrogen production at an iron anode. Background Art

[0002] Microbial electrosynthesis (MES) is a technology that uses microorganisms to transfer electrons from the cathode to metabolic pathways. This process can be used to produce a variety of valuable chemicals, such as caproic acid, lactic acid, acetone, etc. Caproic acid is an important medium-chain fatty acid with a wide range of industrial applications, including as a flavor ingredient, solvent, plasticizer, etc. However, despite the great potential of MES technology, it faces some significant technical barriers in practical application. Specifically: (1) Transportation of substrates and products In the MES system, the supply of substrates and the discharge of products are essential to maintain metabolic activity in the reactor. However, due to the permeability limitation of the cell membrane, the speed at which substrates (such as acetate or other organic compounds) enter the cell is slow, and the output rate of products (such as hexanoic acid) is also hindered. This low transport rate directly affects the cumulative concentration of hexanoic acid, which in turn affects the final yield.

[0003] (2) Electron transfer efficiency Electron transfer efficiency is one of the key factors affecting the performance of MES. In some cases, microorganisms need to accept electrons from the cathode through the extracellular electron conduction mechanism (EET). However, the rate of EET is usually low, which not only limits the amount of H2 generated, but also limits the efficiency of hexanoic acid synthesis. In addition, H2 is an effective electron carrier, which can act as an electron acceptor in some microorganisms to promote the synthesis of hexanoic acid. However, due to the limitation of EET, the production of H2 is often insufficient, which limits the production of hexanoic acid.

[0004] (3) Cost and efficiency issues of carbon brush anodes Traditionally, MES systems use carbon brushes as anode materials. Although carbon brushes have good conductivity and mechanical stability, they also have significant disadvantages, such as high manufacturing costs and relatively low electron transfer efficiency. In addition, carbon brushes may degrade after long-term operation, resulting in reduced performance. Therefore, finding an alternative material that is more cost-effective and can provide a stable supply of electrons has become a research focus. Summary of the invention

[0005] The purpose of the present invention is to provide a method for in-situ hydrogen production at an iron anode to enhance microbial electrosynthesis to produce caproic acid, so as to solve the problems of slow substrate and product transport efficiency, low electron transfer efficiency and low stability of the carbon brush anode in the above-mentioned microbial electrosynthesis technology.

[0006] To achieve the above object, the present invention provides a method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode, comprising the following steps: (1) Directed domestication of functional microorganisms producing caproic acid Sodium acetate and ethanol are used as carbon sources, and wastewater plant excess sludge is used as inoculation source. The culture medium is inoculated, the pH is adjusted to 6.0-6.5, and then an anaerobic microbial culture process is carried out to obtain caproic acid-synthesizing functional bacteria that have been oriented and domesticated. (2) Cathode pre-culture of electrosynthesis system The cathode pre-culture is carried out in a single-chamber membraneless electrosynthesis system, the anode is a carbon fiber brush, the cathode is a carbon cloth, the entire pre-culture process is carried out under anaerobic conditions, sodium acetate is used as a carbon source, fresh excess sludge from a sewage treatment plant is inoculated, and a voltage is applied to carry out the pre-culture process until the electrosynthesis system reaches a state of chemical stability, completing the pre-culture process; (3) Construction of electrosynthetic system and colonization of functional microorganisms The carbon fiber brush in step (2) is replaced with a high-purity iron sheet, and the caproic acid-synthesizing functional bacteria that have been oriented and domesticated in step (1) are mixed and inoculated with the electrolyte to start the electrosynthesis process; (4) Hexanoic acid production mediated by in situ hydrogen production at the iron anode and microbial electrosynthesis system The concentrations of sodium acetate and ethanol in the electrolyte are adjusted, and voltage is applied to complete the process of electrosynthesizing caproic acid.

[0007] Preferably, in step (1), the culture medium contains 4.1 g sodium acetate, 8.7 mL ethanol, 3.6 g ammonium dihydrogen phosphate, 5.4 g sodium bicarbonate, 2 g sodium 2-bromoethane sulfonate and 5 mL trace elements per liter.

[0008] Preferably, in step (1), the anaerobic microorganism cultivation process is: After adjusting the pH value, high-purity nitrogen was introduced for deoxygenation for 10-15 minutes, and then the inoculated culture medium was placed in an air bath shaker, the speed was adjusted to 100-150 rpm, and the temperature was adjusted to 30-40°C.

[0009] Preferably, in step (1), the volume ratio of sludge to culture medium is 1:(8-10).

[0010] Preferably, in step (1), the microorganism is cultured for 2 to 3 months at a rate of 7 to 10 days per cycle until the hexanoic acid production is stable and the directional domestication is completed.

[0011] Preferably, in step (2), the concentration of sodium acetate is 1.0-1.5 g / L, the applied voltage is 0.6-0.9 V, the electrolyte in the electrosynthesis system is 100 mM phosphate buffer solution, and the pH value is adjusted to 7.0.

[0012] Preferably, in step (2), the sludge is inoculated for 3 cycles at a rate of 5 to 7 days per cycle, and then started up for 15 to 20 days at a rate of 1 to 2 days per cycle.

[0013] Preferably, in step (3), a high-purity iron sheet is used as the anode, and the volume of the iron anode is 30×40×0.2 mm 3 .

[0014] Preferably, in step (3), the electrolyte contains 2 g of sodium acetate, 4.23 mL of ethanol, 3.6 g of ammonium dihydrogen phosphate, 0.33 g of magnesium chloride hexahydrate, 0.2 g of magnesium sulfate heptahydrate, 0.5 g of calcium chloride dihydrate, 0.15 g of potassium chloride, 4 g of anhydrous potassium carbonate, 2 g of sodium 2-bromoethane sulfonate and 5 mL of trace elements per liter.

[0015] Preferably, in step (3), the volume ratio of the caproic acid-synthesizing functional bacteria that have been oriented and domesticated to the electrolyte is 1:(8-10).

[0016] Preferably, in step (3), the process of inoculating the caproic acid-synthesizing functional bacteria that have been oriented and domesticated is: three cycles of inoculation at a rate of 5 to 7 days per cycle.

[0017] Preferably, in step (4), the concentrations of sodium acetate and ethanol in the electrolyte are adjusted to 1.0-1.2 g / L and 2-2.2 mL / L, respectively, and the pH value is adjusted to 6.0.

[0018] Therefore, the present invention adopts a method for in-situ hydrogen production of an iron anode with the above structure to enhance microbial electrosynthesis to produce caproic acid, which has the following beneficial effects: (1) The introduction of the iron anode in the present invention significantly improves the electron transfer efficiency, allowing more electrons to be transferred to the hexanoic acid synthesis process. The experimental results show that the percentage of electrons transferred to hexanoic acid in the iron anode group is as high as 54.5%, while the comparison experiment is only 19.7%, which indicates that the electrons released by the corrosion of the iron anode can serve as an intermediate electron transfer medium, significantly enhancing the electron transfer performance.

[0019] (2) The addition of the iron anode in the present invention also stimulates the growth and enrichment of caproic acid-synthesizing functional bacteria and electrode-active bacteria. Clostridium_sensu_stricto_12 and Desulfovibrio The bacterial communities reached a richness of 6.8% and 30.8% respectively. The enrichment of these microorganisms further promoted the production of caproic acid and improved the overall efficiency of the system.

[0020] (3) The present invention ensures the stability and reliability of the system through detailed pre-culture and acclimatization steps. The pre-culture process gradually adapts the microbial community to the electrosynthesis system through multiple periodic inoculations and cultivations, and finally reaches a state of stable chemical properties, which lays the foundation for subsequent efficient operation.

[0021] (4) The raw materials used in the present invention, such as sodium acetate, ethanol and wastewater plant excess sludge, are all common and low-cost substances, which makes the technology highly economically feasible in practical applications and easy to promote and implement.

[0022] (5) The present invention utilizes microbial electrosynthesis technology to achieve the transformation of organic matter through anaerobic acclimation and electrochemical reaction, thereby reducing the use of chemical reagents and the generation of waste. At the same time, the reuse of excess sludge from sewage treatment plants is also in line with the concept of circular economy and contributes to environmental protection.

[0023] (6) The present invention can produce high value-added chemicals through efficient hexanoic acid synthesis while reducing environmental pollution. This technology has great application potential in the recovery and conversion of resources and energy. In addition, this technology can also be applied to wastewater treatment, bioenergy production and other fields, and has broad application prospects.

[0024] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a graph showing the change of volatile acid concentration over time during the operation of Example 1; Figure 2 This is a graph showing the change of volatile acid concentration over time during operation of Comparative Example 1; Figure 3 The electron distribution diagrams of Example 1 and Comparative Example 1 before and after operation; Figure 4 This is a diagram of the main microbial composition of the cathode after the operation of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0026] The present invention will be further described below. It should be noted that this embodiment is based on the technical solution and provides a detailed implementation method and a specific operation process, but the present invention is not limited to this embodiment.

[0027] Example 1 A method for in-situ hydrogen production at an iron anode to enhance microbial electrosynthesis of caproic acid comprises the following steps: (1) Directed domestication of functional microorganisms producing caproic acid Sodium acetate and ethanol are used as carbon sources, and the residual sludge from the sewage treatment plant is used as the inoculation source, and is inoculated with the culture medium at a volume ratio of 1:9. The pH is adjusted to 6.5 with dilute hydrochloric acid or sodium hydroxide, and high-purity nitrogen is introduced for deoxygenation for 12 minutes. The inoculated culture medium is then placed in an air bath shaker, the speed is adjusted to 120 rpm, and the temperature is adjusted to 35° C. The culture medium is carried out at a cycle of 10 days for 3 months. After each cycle, the culture medium is removed, and a new culture medium is added at a volume ratio of 1:9 to continue the microbial culture process of the next cycle until the production of caproic acid in the liquid is stable. The mixture formed by the culture medium and the sludge is centrifuged, and the remaining sludge is collected to obtain the caproic acid synthesis functional bacteria that have been oriented domesticated. The caproic acid synthesis functional bacteria obtained in this embodiment are a mixture of multiple strains. The culture medium contained 4.1 g sodium acetate, 8.7 mL ethanol, 3.6 g ammonium dihydrogen phosphate, 5.4 g sodium bicarbonate, 2 g sodium 2-bromoethane sulfonate, and 5 mL trace elements per liter; Trace elements are present in the culture medium as mineral solutions, specifically: manganese chloride 0.1 g / L, boric acid 0.01 g / L, zinc chloride 0.01 g / L, copper chloride 0.006 g / L, sodium molybdate 0.002 g / L, cobalt chloride 0.2 g / L, nickel chloride 0.02 g / L, sodium selenate 0.01 g / L; (2) Cathode pre-culture of electrosynthesis system Cathode pre-culture is carried out in a single-chamber membraneless electrosynthesis system, the anode is a carbon fiber brush, the cathode is a carbon cloth, the entire pre-culture process is carried out under anaerobic conditions, sodium acetate is used as a carbon source, the concentration of sodium acetate is 1.2 g / L, fresh residual sludge from a sewage plant is inoculated in a 100 mM phosphate buffer solution at a volume ratio of 1:9, and then a voltage of 0.8 V is applied for the pre-culture process, and a culture process of 3 cycles is carried out at a rate of 6 d / cycle. After each cycle, the sludge and the aqueous solution are all pumped away and removed, and sodium acetate, fresh sludge and phosphate buffer solution are added again according to the above ratio, and the next cycle of culture is continued until the end of three cycles, and the sludge and the aqueous solution are all pumped away and removed; Then, a start-up process of 18 days was carried out at a rate of 2 days / cycle. During this process, sodium acetate (1.2 g / L) and phosphate buffer solution (100 mM) were added to the electrosynthesis system, and a voltage of 0.8 V was applied. After each cycle, all the solutions were removed, and new sodium acetate and phosphate buffer solution were added to carry out the next cycle of cultivation until the electrosynthesis system reached a state of chemical stability (stable hydrogen production), completing the pre-cultivation process. (3) Construction of electrosynthetic system and colonization of functional microorganisms The carbon fiber brush in step (2) was replaced with a high-purity iron sheet (purity > 99%). The volume of the iron anode was 30 × 40 × 0.2 mm.3 , the caproic acid synthesis functional bacteria that have been oriented and domesticated in step (1) are mixed and inoculated with the electrolyte in a volume ratio of 1:9, and a voltage of 0.8 V is applied to carry out the electrosynthesis process. The caproic acid synthesis functional bacteria that have been oriented and domesticated are inoculated for 3 cycles at a rate of 6 days per cycle. After each cycle, the mixture of the caproic acid synthesis functional bacteria that have been oriented and domesticated and the electrolyte is extracted and removed, and new caproic acid synthesis functional bacteria that have been oriented and domesticated and the electrolyte are added in a volume ratio of 1:9, and the inoculation and cultivation of the next cycle are continued until the three cycles are completed; The electrolyte contains 2 g of sodium acetate, 4.23 mL of ethanol, 3.6 g of ammonium dihydrogen phosphate, 0.33 g of magnesium chloride hexahydrate, 0.2 g of magnesium sulfate heptahydrate, 0.5 g of calcium chloride dihydrate, 0.15 g of potassium chloride, 4 g of anhydrous potassium carbonate, 2 g of sodium 2-bromoethane sulfonate, and 5 mL of trace elements per liter; Trace elements are present in the electrolyte as mineral solutions, specifically: manganese chloride 0.1 g / L, boric acid 0.01 g / L, zinc chloride 0.01 g / L, copper chloride 0.006 g / L, sodium molybdate 0.002 g / L, cobalt chloride 0.2 g / L, nickel chloride 0.02 g / L, sodium selenate 0.01 g / L; (4) Hexanoic acid production mediated by in situ hydrogen production at the iron anode and microbial electrosynthesis system The concentrations of sodium acetate and ethanol in the electrolyte were adjusted to 1.0 g / L and 2.13 mL / L, respectively, and then the electrolyte at this concentration was added and a voltage of 0.8 V was applied to complete the process of electrosynthesizing hexanoic acid.

[0028] Comparative Example 1 The difference from Example 1 is that a carbon fiber brush is used in step (3).

[0029] Test example Figure 1 and Figure 2 The graph shows the change of volatile acid concentration over time during the operation of Example 1 and Comparative Example 1. As can be seen from the figure, the hexanoic acid content of the iron anode in-situ hydrogen production enhanced microbial electrosynthesis system increased significantly, and finally reached 2333.94±102.07 mg COD / L, which is 2.75 times that of the carbon anode group. The ethanol consumption in the iron anode group (Example 1) and the carbon anode group (Comparative Example 1) is basically the same, but the hexanoic acid production in the iron anode group is higher, the butyric acid production is lower, and acetic acid is efficiently utilized, which shows that iron as an anode can promote the process of carbon chain extension of acetic acid and butyric acid.

[0030] Figure 3: This is the electron distribution diagram of Example 1 and Comparative Example 1 before and after operation. As can be seen from the figure, 54.5% of the electrons in the iron anode group are transferred to caproic acid, while the carbon anode group is only 19.7%. At the same time, the electron recovery rate of butyric acid is reduced, and the iron anode group is reduced by 5.4% compared with the carbon anode group. After the introduction of the iron anode, the total efficiency of electron recovery increased by 9.5%, and the electron flow was more inclined to high molecular fatty acids, i.e., caproic acid rather than butyric acid. This shows that the iron anode supports caproic acid to accept more electrons from the electron donor.

[0031] Figure 4 The main microbial composition of the cathode after the operation of Example 1 and Comparative Example 1. It can be seen from the figure that the addition of the iron anode stimulates the growth and enrichment of caproic acid-synthesizing functional bacteria and electrode active bacteria. Clostridium_ sensu_stricto_12 and Desulfovibrio The abundance of the bacterial community reached 6.8% and 30.8%, respectively, indicating that the introduction of the iron anode enhanced the extracellular electron transfer to produce hydrogen, and hydrogen, as an additional electron donor, promoted the production of caproic acid. Desulfovibrio , Acinetobacte r and Bacteroides Composition, of which Desulfovibrio The bacterial abundance was as high as 30.8%, which was 19.3% higher than that of the carbon anode group, indicating that the iron anode helps to receive electrons from the cathode to accelerate the process of current generation and H2 reduction. Clostridium_sensu_stricto_12 As a typical functional bacterium for caproic acid synthesis, it is not only essential for the synthesis of caproic acid, but also plays a vital role in the fermentation of organic matter and the production of H2. It is enriched by 6.9% in the iron anode group and only 4.6% in the carbon anode group. Oscillibacter It was demonstrated that hexanoic acid could be produced using ethanol as an electron donor, reaching 3.7% in the iron anode group.

[0032] In summary, the introduction of iron as an anode into the microbial electrosynthesis system has a good effect on the production of caproic acid. The generation of hydrogen promoted by the electrochemical corrosion of iron in the early stage of chain extension provides more electron donors and appropriate hydrogen partial pressure for the subsequent production of caproic acid, resulting in a surge in the production of caproic acid in the later stage.

[0033] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for in-situ hydrogen production at an iron anode to enhance microbial electrosynthesis of caproic acid, characterized in that: The following steps are involved: (1) Directed domestication of functional microorganisms producing caproic acid Sodium acetate and ethanol are used as carbon sources, and wastewater plant excess sludge is used as inoculation source. The culture medium is inoculated, the pH is adjusted to 6.0-6.5, and then an anaerobic microbial culture process is carried out to obtain caproic acid-synthesizing functional bacteria that have been oriented and domesticated. (2) Cathode pre-culture of electrosynthesis system The cathode pre-culture is carried out in a single-chamber membraneless electrosynthesis system, the anode is a carbon fiber brush, the cathode is a carbon cloth, the entire pre-culture process is carried out under anaerobic conditions, sodium acetate is used as a carbon source, fresh excess sludge from a sewage treatment plant is inoculated, and a voltage is applied to carry out the pre-culture process until the electrosynthesis system reaches a state of chemical stability, completing the pre-culture process; (3) Construction of electrosynthetic system and colonization of functional microorganisms The carbon fiber brush in step (2) is replaced with a high-purity iron sheet, and the caproic acid-synthesizing functional bacteria that have been oriented and domesticated in step (1) are mixed and inoculated with the electrolyte to start the electrosynthesis process; (4) Hexanoic acid production mediated by in situ hydrogen production at the iron anode and microbial electrosynthesis system The concentrations of sodium acetate and ethanol in the electrolyte are adjusted, and voltage is applied to complete the process of electrosynthesizing caproic acid.

2. The method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode according to claim 1, characterized in that: In step (1), the culture medium contains 4.1 g of sodium acetate, 8.7 mL of ethanol, 3.6 g of ammonium dihydrogen phosphate, 5.4 g of sodium bicarbonate, 2 g of sodium 2-bromoethane sulfonate and 5 mL of trace elements per liter.

3. The method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode according to claim 1, characterized in that: In step (1), the anaerobic microbial culture process is: After adjusting the pH value, high-purity nitrogen was introduced for deoxygenation for 10-15 minutes, and then the inoculated culture medium was placed in an air bath shaker, the speed was adjusted to 100-150 rpm, and the temperature was adjusted to 30-40°C.

4. The method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode according to claim 1, characterized in that: In step (1), the volume ratio of sludge to culture medium is 1:(8~10).

5. The method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode according to claim 1, characterized in that: In step (2), the concentration of sodium acetate is 1.0-1.5 g / L, the applied voltage is 0.6-0.9 V, the electrolyte in the electrosynthesis system is 100 mM phosphate buffer solution, and the pH value is adjusted to 7.

0.

6. The method of claim 1, wherein: In step (2), the sludge is inoculated for 3 cycles at a rate of 5 to 7 days per cycle, and then started up for 15 to 20 days at a rate of 1 to 2 days per cycle.

7. The method of claim 1, wherein: In step (3), the electrolyte contains 2 g of sodium acetate, 4.23 mL of ethanol, 3.6 g of ammonium dihydrogen phosphate, 0.33 g of magnesium chloride hexahydrate, 0.2 g of magnesium sulfate heptahydrate, 0.5 g of calcium chloride dihydrate, 0.15 g of potassium chloride, 4 g of anhydrous potassium carbonate, 2 g of sodium 2-bromoethane sulfonate and 5 mL of trace elements per liter.

8. The method of claim 1, wherein: In step (3), the volume ratio of the caproic acid-synthesizing functional bacteria that have been oriented and domesticated to the electrolyte is 1:(8-10).

9. The method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode according to claim 1, characterized in that: In step (3), the process of inoculating the caproic acid-synthesizing functional bacteria that have been oriented and domesticated is as follows: three cycles of inoculation are performed at a rate of 5 to 7 days per cycle.

10. The method for enhancing microbial electrosynthesis of caproic acid by in-situ hydrogen production at an iron anode according to claim 1, characterized in that: In step (4), the concentrations of sodium acetate and ethanol in the electrolyte are adjusted to 1.0-1.2 g / L and 2-2.2 mL / L, respectively, and the pH value is adjusted to 6.0.

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