KR102343270B1 - Method for producing hydrogen using makgeolli wastewater and microbial electrolysis cell - Google Patents

Abstract

The present invention relates to a method for producing hydrogen from makgeolli wastewater using a microbial electrolysis cell, and by using the method, high efficiency and high purity hydrogen gas can be produced from discarded makgeolli wastewater.

Description

METHOD FOR PRODUCING HYDROGEN USING MAKGEOLLI WASTEWATER AND MICROBIAL ELECTROLYSIS CELL

The present invention relates to a method for producing hydrogen from makgeolli wastewater using a microbial electrolysis cell.

Recently, due to the decrease in the reserves of non-renewable fossil fuels, the reuse of renewable resources is receiving a lot of attention, and in particular, clean and high-efficiency energy sources are being developed worldwide to solve the environmental pollution problem. Among them, hydrogen energy is considered as a clean green energy source to replace fossil fuels. Hydrogen is an environmentally clean energy fuel and has great potential as a future energy source because it can reduce the generation of greenhouse gases that adversely affect climate change.

Accordingly, microbial electrolysis cell (MEC) technology is attracting attention for high efficiency of hydrogen production. Microbial electrolysis cell technology is a technology that converts biodegradable organic matter into hydrogen gas using electrochemically active microorganisms (electron-emitting bacteria, exoelectrogens). When electrochemically active microorganisms anaerobically oxidize organic matter, electrons are generated and moved to the anode. When an electric potential is applied to the circuit, electrons move to the cathode and reduce protons to generate hydrogen gas. do.

However, the currently developed microbial electrolysis cell generates methane and carbon dioxide gas when operated for a long time, and thus the performance of the electrolysis cell is remarkably deteriorated.

1. Republic of Korea Patent No. 10-1232454

An object of the present invention is to provide a method for producing hydrogen gas with high efficiency and high purity using makgeolli wastewater.

In order to achieve the above object, one aspect of the present invention provides a method for producing hydrogen comprising the steps of:

(a) supplying a mixture of a substrate containing active sludge and acetate to a microbial electrolysis cell equipped with an anode and a hydrogen cathode on which a biofilm is formed;

(b) operating a microbial electrolysis cell while applying an external applied voltage; and

(c) supplying makgeolli wastewater as a substrate.

The present inventors have studied a method for utilizing makgeolli wastewater discarded during the production of makgeolli, and devised a method for using it for hydrogen production as described above. Makgeolli wastewater can be used as a substrate for microbial fuel cells because the content of organic matter is high and the use of chemicals is small in the manufacturing process, so there are few harmful substances.

As used herein, the term "microbial electrolysis cell (MEC)" refers to a technology for converting biodegradable organic matter into hydrogen gas using electrochemically active microorganisms. The organic matter contained in the substrate is anaerobically oxidized by microorganisms to generate electrons, and when an electric potential is applied to the circuit after the electrons move to the anode, the electrons move to the reduction electrode and reduce protons to hydrogen gas will occur

According to one embodiment of the present invention, the hydrogen production method may further include the step of forming a biofilm on the anode before step (a). Specifically, it is possible to form a biofilm on the anode by operating in a microbial fuel cell (MFC) mode until a constant voltage is generated in a state where an external resistance of 1 kΩ is applied to the MEC using the oxygen cathode. Substrates and inoculums that can be used are described in the Examples below.

According to one embodiment of the present invention, when a biofilm is formed on the anode, the step (a) can be performed by replacing the oxygen cathode with a hydrogen cathode, and thereafter, an external applied voltage is applied to the microbial electrolysis for hydrogen production. The battery can be operated (step b). When the current is stabilized by operating the microbial electrolysis cell, step (c) can be performed.

According to one embodiment of the present invention, in order to use makgeolli wastewater as a substrate for a microbial electrolysis cell, it is necessary to increase the electrical conductivity. Therefore, in the present invention, makgeolli wastewater is diluted with phosphate buffer and used, and makgeolli wastewater is diluted with phosphate buffer by 0.5 to 5 times, preferably 0.8 to 3 times, and most preferably 0.9 to 1.5 times. have.

According to one embodiment of the present invention, when operating a microbial electrolysis cell using makgeolli wastewater as a substrate, an externally applied voltage may be applied to 0.3 to 1.5 V, preferably 0.5 V to 1.0 V, more preferably 0.6 V or 0.8 V can be applied. In the external applied voltage condition, the microbial electrolysis cell may have a hydrogen generation rate of 0.7 to 3.0 m 3 /m 3 /d.

By using the hydrogen production method of the present invention, it is possible to produce high-efficiency, high-purity hydrogen gas from the waste water of makgeolli.

1 is a photograph showing a state in which gas generated in one-room-type microbial electrolysis according to an example of the present invention is captured by a water-phase substitution method.
2 is a graph showing the results of current measurement over time in a microbial electrolysis cell using sodium acetate solution and makgeolli wastewater as substrates, and applying an external voltage of 0.6 V (A) or 0.8 V (B).
3 is a graph showing the gas generation amount and gas generation efficiency according to the substrate in the microbial electrolysis cell using sodium acetate solution and makgeolli wastewater as substrates, and applying an external voltage of 0.6 V (A) or 0.8 V (B).
4 is a graph showing the percentage of gas generated in the one-room type microbial electrolysis cell using makgeolli wastewater as a substrate.
5 is a graph showing _{the hydrogen recovery rate (r H2} ) and the hydrogen production rate (Y _H2 ) according to the batch in a one-room microbial electrolysis cell using makgeolli wastewater as a substrate.

Hereinafter, one or more specific examples will be described in more detail through examples. However, these examples are for illustrative purposes of one or more embodiments, and the scope of the present invention is not limited to these examples.

experimental method

1. Manufacture of one-room type microbial electrolysis cell

The one-room type microbial electrolysis cell (MEC) reactor was used with some modifications of the method disclosed in the prior literature (PNAS 2007, 104, 18871-18873). The positive and negative electrodes were placed at both ends of a Plexiglas cylinder (3 cm in diameter and 2 cm in length) with an internal volume of 15 ml, and a 4.5 ml Teflon cylinder tube was attached to the top of the cylinder to collect the gas generated from the cell. . The Teflon cylinder tube is connected to a gas collection cylinder filled with water through a silicone tube. Carbon cloth with a surface area of 7 cm2 is used for the anode (anode), and synthesized Cu ₂ O nanoparticles and MoS ₂ nanoparticles (Alfa Aesar, USA) as the cathode (cathode). ) with carbon paper (7 cm 2 ) was used. The Cu ₂ O nanoparticles (0.5 mg/cm 2 ) and MoS ₂ nanoparticles (5 mg/cm 2 ) were loaded on one side of carbon paper using Nafion solution.

An oxygen cathode (air cathode) for microbial fuel cells (MFC) mode was prepared using Pt/C powder as a catalyst (0.5 mg/cm2) in the prior literature ( Environ. Sci. Technol. 2005, 39, 4317). ) was prepared by the method disclosed in

Microbial fuel cells (microbial fuel cells, MFC) mode. As substrate and inoculum, sodium acetate (1 g/L), NH ₄ Cl (0.31 g/L), KCl (0.13 g/L), mineral solution (12.5 mL/L) in 50 mM phosphate buffered saline (PBS) L) and a vitamin solution (12.5 ml/L) mixed with a solution (hereinafter referred to as 'acetic acid substrate solution') and 30% activated sludge was supplied in a batch manner. After that, when a constant voltage is generated, the oxygen reduction electrode is replaced with a reduction electrode (Cu ₂ O/MoS ₂ reduction electrode) having a hydrogen ion reduction catalyst, and an acetic acid substrate solution is used in a state where an external applied voltage (0.6 V) is applied. It was operated in MEC mode until stable current and hydrogen gas were generated. The current was measured with a multi-channel potentiometer (WMPG 1000, WonATech, Korea).

2. Hydrogen production using makgeolli wastewater

Makgeolli wastewater has a high content of organic matter and uses less chemicals in the manufacturing process, so there are few harmful substances but low electrical conductivity. Therefore, if it is used directly in a microbial electrolysis battery, the internal resistance of the battery increases, so it is not suitable and requires pretreatment. After collecting makgeolli wastewater from the Pocheon moving makgeolli wastewater treatment facility, it was filtered through a stainless-steel mesh to remove solid particles. The filtered makgeolli wastewater showed characteristics of chemical oxygen demand (COD) of about 1800 mg/L, conductivity 280 μS/cm, and pH 4.9. For use in MEC, PBS was added 1:1 (v/v) to filtered makgeolli wastewater to adjust COD 840 mg/L, conductivity 6.5 mS/cm, and pH 7.1.

In the MEC mode, when electric current and hydrogen gas were stably generated, the diluted makgeolli wastewater was supplied as a substrate, and when the electric current fell below 0.1 mA, an acetic acid substrate solution was supplied again. After that, when the current and hydrogen gas generation were stabilized, the diluted makgeolli wastewater was supplied as a substrate, and the above process was repeated to replace the acetic acid substrate solution and the diluted makgeolli wastewater, and the microbial electrolysis cell was operated for 40 days. When replacing the substrate, the biofilm formed on the inner surface of the reactor was removed.

3. Gas Collection and Analysis

The total volume of gas produced in the MEC was measured as the volume of water displaced by the gas in the glass cylinder where the gas mixture was collected. 1 shows a gas collection device according to an example of the present invention.

Individual gases and their exact ratios were analyzed by gas chromatography equipped with a thermal conductivity detector (DS 6200, DS Science, Korea). Argon (Ar) was used as a carrier gas, and a carbon molecular sieve column (Carbospher ^TM , Alltech, USA) was used for gas separation. The percentage of individual valences was calculated from the chromatogram area of the individual gas after correction for the detector response.

4. Parameter Calculation

To evaluate the performance of MEC, coulombic efficiency (CE), cathodic hydrogen recovery ( r _cat ), overall hydrogen recovery ( r _H2 ), hydrogen yield ( Y _H2 ) and parameters such as the hydrogen production rate ( Q _{H2 ).}

Coulombic efficiency (CE) is the ratio of the experimentally confirmed total charge (Q _exp ) to the theoretical charge ( Q _th ), and is calculated according to Equation 1 below.

[Equation 1]

The cathode hydrogen recovery rate ( r _cat ) calculated according to Equation 2 below means the ratio of the number of moles of hydrogen calculated based on the measured current and the hydrogen volume, and the total hydrogen recovery rate ( r _H2 ) is the Coulombic efficiency (CE) ) and the cathode hydrogen recovery rate ( r _cat ) is calculated from (Equation 3).

[Equation 2]

[Equation 3]

Since the exact components of makgeolli wastewater cannot be confirmed, the hydrogen yield ( Y _H2 ) was calculated based on the COD removed as shown in Equation 4 below. In Equation 4, nH2 is the number of moles of hydrogen recovered in the experiment, M _H2 is the molecular weight of hydrogen, and VL is the volume of the reactor. Hydrogen production rate ( Q _H2 ) is generally expressed in volume of hydrogen (m3) collected per day and reactor volume (m3).

[Equation 4]

Experiment result

1. Check the current change in MEC

Before using makgeolli wastewater as a substrate, when an acetic acid substrate solution is first supplied to the MEC (externally applied voltage -0.6 V), the current immediately increases to 1.4 mA (95.3 A/m3), and a current of 1 mA or more is maintained for one day. could confirm that However, upon depletion of the substrate, the current rapidly decreased, and the current was barely measured (~0.03 mA) on day 4 of the MEC run. Thereafter, diluted makgeolli wastewater was continuously added (M1-M4). After addition, the current rapidly increased to the same level as that of the acetic acid batch, and then the current of 1 mA or more continued for 16 hours (FIG. 2A). In the graph of FIG. 1 , the letter A denotes a batch using an acetic acid substrate solution (hereinafter, referred to as “acetic acid batch”), and the letter M denotes a batch using diluted makgeolli wastewater as a substrate (hereinafter, referred to as “makgeolli wastewater batch”). , and the area marked with a gray box also represents the batch of makgeolli wastewater.

As a result of calculating the coulombic efficiency (CE) of the makgeolli wastewater batch, it was found to be 57.8%, and it was found that a repetitive pattern appeared until the makgeolli wastewater batch 5 (M5). However, after batch 5 (M5) of makgeolli wastewater, the performance of MEC was significantly reduced, and although current was measured, almost no gas was generated (dark gray area in A of FIG. 1). It is presumed that this is because the microbial community of the anode was changed by the external air that penetrated through the cracks in the gas collection cylinder. The penetrating oxygen acquires electrons from the electroactive microorganisms formed on the anode, and promotes the decomposition of starch, thereby generating a substrate readily available to microorganisms.

After that, it was found that the performance was restored when the gas collection cylinder was changed and the MEC was re-run by supplying a fresh acetic acid substrate solution (light gray area in A of FIG. 1). Due to the oxidation of hydrogen at the anode, the CE was quite high, but the Q _H2 was almost the same as the previous batch, and the average Q _H2 was 0.952 _{m 3} H 2 /m 3/d.

The same experiment was performed at an external applied voltage -0.8 V. In acetate batch 1 (A1'), the current increased to 2.7 mA (185 A/m3), and in makgeolli wastewater batch 1 (M1'), the maximum current was 2.4 mA (162 A/m3). Specifically, in the makgeolli wastewater batch 1 (M1'), the current rapidly increased to about 2.0 mA, maintained for a while, and then increased again (FIG. 2B). However, a different pattern was observed in batch 2 (M2') of makgeolli wastewater. The rapid increase and decrease of current in makgeolli wastewater batch 2 (M2') is presumed to be because polysaccharides such as starch in makgeolli wastewater are not easily decomposed into useful monosaccharides or organic acids by microorganisms, which also changes the bacterial community in the MEC chamber. it means did In addition, in the makgeolli wastewater batch 2 (M2'), all MEC parameters showed low performance, but Q _H2 was maintained at a constant value of 1.43. MEC was operated by supplying an acetate substrate three times (A2'-A4') to recover the microbial community forming a biofilm.

After that, makgeolli wastewater was supplied again, and it was found that the current profile of the makgeolli wastewater batch 3 (M3') was very similar to that of the makgeolli wastewater batch 2 (M2'), and the performance of the MEC parameters was improved. The average Q _H2 was found to be 1.55 H ₂ /m 3 /d, which was significantly higher than when tested with an external applied voltage of -0.6 V. CE exceeding 100% is not uncommon in MFC, and can be confirmed in Makgeolli wastewater batch 6 (M6) tested with an external applied voltage of -0.6 V. From this, it can be inferred that hydrogen is being recycled. Hydrogen molecules generated at the cathode can be oxidized on the anode surface and used by homoacetogen to generate acetate, which is reoxidized in the cathode biofilm.

Table 1 below lists the MEC parameters measured for each batch.

arrangement applied voltage (V) Coulomb Efficiency
(CE) (%) Cathode hydrogen recovery rate ( r _cat ) (%) Total hydrogen recovery
(rH2) (%) Hydrogen Yield ( Y _H2 ) Hydrogen production rate ( Q _H2 )
(m3/m3/d) M1 -0.6 57.8 27.8 16.1 0.755 0.90 M2 -0.6 56.4 31.1 17.5 0.823 0.98 M3 -0.6 66.7 32.6 21.7 1.02 0.99 M4 -0.6 62.1 70.6 43.8 2.06 0.95 M6 -0.6 111.6 27.7 30.9 1.45 0.93 average -0.6 70.9 37.9 26.0 1.22 0.95 M1' -0.8 104.1 88.2 43.0 1.72 1.59 M2' -0.8 46.0 82.1 15.0 0.601 1.43 M3' -0.8 45.7 99.3 18.7 0.749 1.64 average -0.8 65.2 89.9 25.6 1.02 1.55

2. Check the gas produced by the MEC

The total amount of gas and the amount of individual gas of each batch generated under the conditions of applied voltages -0.6 and -0.8V are shown in FIG. 3 .

When sodium acetate was supplied to the MEC at an applied voltage of -0.6 V, the total amount of gas produced was 6 ml, _{and the percentages of collected hydrogen (H 2} ), methane (CH ₄ ) and carbon dioxide (CO ₂ ) were each about 83.7, 15.4 and 0.9% (FIG. 3A). When makgeolli wastewater (MW) was supplied to MEC under the same applied voltage condition, the average gas production from batches M1 to M6 of makgeolli wastewater was 4.9±2.3 ㎖. Except for batch M5 of makgeolli wastewater, which showed the aforementioned head space problem, the average gas production increased to 5.4±2.0 ㎖, and the purity of hydrogen gas was 91.5±3.2%. The remaining gas was methane, which was substantially free of carbon dioxide.

After replacing the headspace part of the gas collection unit, the first batch, the A2 batch, produced the highest current at -0.6 V, but the percentages of _{hydrogen (H 2} ), methane (CH ₄ ) and carbon dioxide (CO _{2 ) collected were} As 52.8, 46.2, and 1.0%, respectively, a total of 3.2 ml of gas was generated (FIG. 3A). The proportion of methane was significantly increased compared to before replacement of parts, and considering the transformation of cells after batch 5 of makgeolli wastewater and the continuous supply of acetate solution to the battery 4 times, acetoclastic methanogen that can generate methane from acetate (acetoclastic methanogen) is presumed to be due to growth. However, when the makgeolli wastewater was supplied again (makgeolli wastewater batch 6), a total gas of 6.6 ml with a hydrogen purity of about 90% was produced. It is expected that this is because the microbial community on the anode surface has begun to adapt to the new environment.

Under the condition of applied voltage -0.8 V, the total amount of generated gas increased to 9 ml and 8.0 ml in acetate batch 10 (A10) and makgeolli wastewater batch 10 (M10), respectively. On average, the acetate batch produced a total of 7.1 ± 0.7 ml of gas with a hydrogen purity of 95.1 ± 4.3%, and the makgeolli wastewater batch produced an average of 4.7 ml of gas. However, in the batch of makgeolli wastewater, there was a difference in the amount of gas produced depending on the batch, and the average purity of hydrogen gas was about 90% (FIG. 2 b). At the applied voltage -0.8 V, the total volume of generated gas was slightly lower than -0.6V, but the hydrogen production rate (Q _H2 ) increased from 0.95 to 1.59 m3H ₂ /m3/d. This is presumably because the substrate is consumed faster at the higher voltage. The amount of methane produced in the makgeolli batch was averaged at 0.43 and 0.50 ml at -0.6 and -0.8 V, respectively, and was similar regardless of the applied voltage. This means that the methane occurred in the solution phase and not the electrode. In particular, as the batch was repeated under the applied voltage -0.8V condition, the percentage of methane gradually decreased, and in the case of makgeolli wastewater 3 (M3), the percentage of methane was 0.68%, and only a very small amount was produced (FIG. 4).

Interestingly, little or no _{CO 2} was detected irrespective of the substrate and and applied voltage. _{In theory, one CO 2} molecule is produced for each carbon atom when the substrate is fully oxidized. For example, as shown in the following reaction scheme, 6 moles of carbon dioxide can be produced from 1 mole (mol) of glucose. In the present invention, _{almost no CO 2} is generated because most of the CO ₂ is assumed to be dissolved in water.

[reaction formula]

C ₆ H ₁₂ O ₆ + 6H ₂ O → 6CO ₂ + 24H ⁺ + 24 ^e-

Although the acetate batches performed better than the makgeolli wastewater batches (data not shown), the MEC parameters of Makgeolli wastewater batches 1 to 4 were improved as the batches were repeated at the applied voltage -0.6 V condition. Specifically, as the batch was repeated, the overall hydrogen recovery (r _H2 ) and hydrogen production rate (Y _H2 ) increased from 16.1% and 0.755 to 43.8% and 2.06, respectively. However, this trend could not be confirmed under the applied voltage -0.8 V condition.

_{5 shows the hydrogen recovery rate (r H2} ) and the hydrogen production rate (Y _H2 ) for different batches of makgeolli wastewater under the applied voltage conditions of -0.6V and -0.8V. Because a higher voltage was applied to the cathode, Q _H2 exhibited a higher value of _{1.55 m3H 2} /m3/d at -0.8 V applied voltage than -0.6 V applied voltage (0.95 m3H _{2 /m3/d).} .

MEC using makgeolli wastewater according to the present invention shows superior performance than previously reported literature. _{For example, 0.28 and 0.17 m3H 2} /m3/d have been reported at -0.9 V applied voltage when domestic and winery wastewater is used (RD Cusick, PD Kiely, BE Logan, Int. J. Hydrog. Energy 2010, 35 , 8855.), for food processing wastewater, Q _{H2 of 0.35 m 3} H 2 /m 3/d at -0.7 V was obtained (A. Tenca, RD Cusick, A. Schievano, R. Oberti, BE Logan, Int. J. Hydrog. Energy 2013, 38, 1859.).

Through the experimental results so far, it was confirmed that hydrogen can be produced by operating MEC using makgeolli wastewater whose pH has been adjusted with PBS as a substrate. Specifically, it was possible to obtain hydrogen with a purity of 90% or more under external voltage conditions of -0.6 V and -0.8 V, which means that makgeolli wastewater, which requires a lot of energy for decomposition, can be usefully used to produce hydrogen, a clean energy source.

Claims (7)

(a) supplying a mixture of a substrate containing active sludge and acetate to a microbial electrolysis cell equipped with an anode and a hydrogen cathode on which a biofilm is formed;
(b) operating a microbial electrolysis cell while applying an external applied voltage; and
(c) a hydrogen production method using makgeolli wastewater comprising the step of supplying the makgeolli wastewater as a substrate.
The method of claim 1, wherein step (c) is made after the current of the microbial electrolysis cell is stabilized in step (b).
The method of claim 1, wherein the substrate containing acetate of (a) is a mixture of PBS (phosphate buffered saline), sodium acetate, ammonium chloride, potassium chloride, a mineral solution and a vitamin solution.
The method of claim 1, wherein the makgeolli wastewater is diluted with PBS.
The method of claim 4, wherein the makgeolli wastewater is diluted 0.5 to 5 times with PBS.
The method of claim 1, wherein the externally applied voltage is 0.3 to 1.5 V.
The method of claim 1, wherein the microbial electrolysis cell has a hydrogen generation rate of 0.7 to 3.0 m3/m3/d under an externally applied voltage condition of 0.3 to 1.5 V.

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