Modified enzymatic hydrolysis lignin-based adsorbent as well as preparation method and application thereof

Abstract

The invention relates to a modified enzymatic hydrolysis lignin-based adsorbent and a preparation method and application thereof.The preparation method comprises the following steps that enzymatic hydrolysis lignin residues, NaHCO3 and a citric acid solution are evenly mixed and react, and citric acid modified enzymatic hydrolysis lignin residues are obtained; the citric acid modified enzymatic hydrolysis lignin residues are immersed in a mixed solution of FeSO4 and FeCl3 for a reaction, then the pH is adjusted to be alkaline, and the magnetic citric acid modified enzymatic hydrolysis lignin-based adsorbent is obtained. According to the invention, waste enzymatic hydrolysis lignin residues are used as raw materials to prepare the efficient and recyclable magnetic citric acid modified enzymatic hydrolysis lignin-based adsorbent; the adsorbent can be used for efficiently adsorbing heavy metal pollutants such as Cu < 2 + > and reducing the toxicity.

Description

Technical Field

[0001] The present invention relates to the technical field of adsorbents, and in particular to a modified enzymatically hydrolyzed lignin-based adsorbent, a preparation method thereof, and an application thereof. Background Art

[0002] Heavy metals enter the human body mainly through breathing, skin contact and ingestion. Since heavy metals have significant biological toxicity and are not easily degraded, when they accumulate to a certain level in the human body, they will cause changes in various physiological functions of the human body, and eventually manifest as carcinogenic, teratogenic and mutagenic properties, posing a major risk to human health. 2+ It will inhibit the activity of many enzymes in the human body and seriously damage the cell membrane. In order to reduce the content of heavy metals in the urban environment and effectively curb its harm to the human body and the ecological environment, heavy metal pollution control and remediation methods are gradually receiving widespread attention from researchers at home and abroad. Currently, the commonly used heavy metal removal methods include chemical precipitation, ion exchange, chemical oxidation, reverse osmosis, ultrafiltration, electrodialysis, electrocoagulation and adsorption. Among them, the adsorption method overcomes the problems of low efficiency, high cost, complex treatment equipment, large amount of sludge production and secondary pollution in the original heavy metal removal methods. It is one of the most effective heavy metal removal methods. It uses cheap and readily available adsorption materials to quickly and effectively remove heavy metals through specific functional groups and a high specific surface area, and has been widely favored by people.

[0003] How to develop green, low-carbon, and highly regenerative heavy metal adsorbents from the perspective of effective resource utilization and ecological environmental protection has become a bottleneck problem that needs to be solved urgently in the field of heavy metal adsorption and detoxification. Enzymatic lignin is a type of renewable resource with a wide range of sources and low prices. It is mainly produced as a residual component after biomass is used to prepare cellulosic ethanol, functional polysaccharides and other high-value-added chemicals. In recent years, due to the increasing problems of environmental pollution and energy crisis, the use of bioconversion technology to process biomass into liquid fuels and a variety of high-value-added products has become a new development trend, resulting in a large amount of enzymatic lignin residues every year. However, most of the enzymatic lignin residues are often used for combustion to generate electricity or are directly discarded, and are not effectively utilized, which not only wastes resources but also pollutes the environment.

[0004] While enzymatic lignin has many advantages, the adsorption capacity of pure, untreated enzymatic lignin is limited. Furthermore, the regeneration performance of the adsorbent is crucial for industrial applications. Since adsorbents may pollute the environment after adsorption, to avoid solid waste contamination, they need to be regenerated using a desorbent to allow them to enter the next adsorption cycle. However, the adsorption capacity of the adsorbent may decrease after desorption, which is attributed to the loss of active sites and incomplete desorption of the adsorbate during the regeneration process.

[0005] Patent CN110813250A discloses a method for preparing a modified straw fiber adsorption material. After being modified with citric acid, the surface of the straw fiber has some clustered structures and flaky structures. After modification, the carboxyl group of citric acid is successfully introduced. Citric acid is a natural green tribasic organic acid that can be cross-linked to the surface of the straw fiber through esterification. The straw fiber after the introduction of carboxyl groups has a strong effect on the absorption of heavy metal Cu in wastewater. 2+ and Cr 6+ The adsorption capacity can be greatly improved. However, the above patent has the following shortcomings: first, the adsorbent synthesized in the patent cannot be recycled; second, the adsorption time is 20 hours, which is too long; third, the coupling agent A-172 used in it may be released into the environment during use, which still poses a potential risk. Summary of the Invention

[0006] The purpose of the present invention is to provide a modified enzymatically hydrolyzed lignin-based adsorbent, its preparation method and application in order to overcome at least one of the defects of the above-mentioned prior art. The enzymatically hydrolyzed lignin contains a certain amount of phenolic hydroxyl groups, ether bonds and aromatic groups, etc. These groups can be used as reactive active sites for binding to the target during the adsorption process and can be used for the adsorption of heavy metals in urban environments, which can reduce the cost of repair and reduce the risk of secondary pollution of detoxification agents.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] One of the purposes of the present invention is to provide a method for preparing a modified enzymatically hydrolyzed lignin-based adsorbent, comprising the following steps:

[0009] The enzymatically degraded lignin residue, NaHCO3 and citric acid solution are mixed and reacted to obtain citric acid-modified enzymatically degraded lignin residue;

[0010] The citric acid-modified enzymatic lignin residue is immersed in a mixed solution of FeSO4 and FeCl3 for reaction, and then the pH is adjusted to alkaline to obtain a magnetic citric acid-modified enzymatic lignin-based adsorbent.

[0011] Furthermore, the preparation method of the enzymatically hydrolyzed lignin residue is as follows: placing the alkali pretreated straw residue into a container containing cellulase and sodium citrate slow-release solution, placing the container into a constant temperature oscillator for reaction, centrifugation, neutral washing, and drying to obtain the enzymatically hydrolyzed lignin residue.

[0012] Furthermore, the concentration of the sodium citrate sustained-release solution is (45-50) mM / L, and the pH is (4-5). Preferably, the concentration of the sodium citrate sustained-release solution is 50 mM / L, and the pH is 4.5.

[0013] Furthermore, the ratio of the alkali pretreated straw residue to the sodium citrate slow-release solution is 1 g: (40-50) mL, preferably, the ratio of the alkali pretreated straw residue to the sodium citrate slow-release solution is 1 g: 50 mL;

[0014] Furthermore, the reaction time is (48-72) h, preferably, the reaction time is 72 h;

[0015] Furthermore, the mass ratio of the cellulase to the alkali pretreated straw residue is (0.05-0.1):1, preferably, the mass ratio of the cellulase to the alkali pretreated straw residue is 0.05:1;

[0016] Furthermore, the rotation speed of the constant temperature oscillator is (150-200) rpm, preferably, the rotation speed of the constant temperature oscillator is 180 rpm;

[0017] Furthermore, the drying temperature is (95-110)°C, preferably, the drying temperature is 105°C.

[0018] Furthermore, the alkali pretreated straw residue preparation method comprises: uniformly mixing rice straw powder and sodium hydroxide solution and heating in a water bath, taking out and cooling, filtering, neutralizing and drying to obtain the alkali pretreated straw residue.

[0019] Furthermore, the mass concentration of the sodium hydroxide solution is (1-2)%, and the solid-liquid ratio of the rice straw powder and the sodium hydroxide solution is 1g:(20-30)mL; preferably, the mass concentration of the sodium hydroxide solution is 2%, and the solid-liquid ratio of the rice straw powder and the sodium hydroxide solution is 1g:20mL.

[0020] Furthermore, the concentration of the citric acid solution is (0.1-0.15) M / L, preferably, the concentration of the citric acid solution is 0.1 M / L;

[0021] Furthermore, the ratio of the enzymatically hydrolyzed lignin residue, the NaHCO3 and the citric acid is 1g:0.4g:(10-30)mL. Preferably, the ratio of the enzymatically hydrolyzed lignin residue, the NaHCO3 and the citric acid is 1g:0.4g:(10-30)mL.

[0022] Furthermore, in the mixed solution, [Fe(II)] / [Fe(III)] is 0.1, 0.3 or 0.5.

[0023] Furthermore, the pH is 11 or 13.

[0024] The second object of the present invention is to provide a modified enzymatically hydrolyzed lignin-based adsorbent, which is prepared by the preparation method described above.

[0025] The third object of the present invention is to use the modified enzymatically hydrolyzed lignin-based adsorbent as described above in the adsorption of heavy metal pollutants.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) The present invention proposes to use discarded enzymatic lignin residue as raw material to prepare a highly efficient and recyclable magnetic citric acid-modified enzymatic lignin-based adsorbent.

[0028] (2) The adsorbent provided by the present invention can be used to efficiently adsorb heavy metal pollutants, such as Cu 2+ , reducing its toxicity.

[0029] (2) The present invention can also measure Cu by ICP-MS 2+ The remaining amount was calculated and the maximum removal rate was obtained to screen out the optimal enzymatic lignin modification method, and the desorption method of the modified enzymatic lignin-based adsorbent was explored to improve its recyclability.

[0030] (4) The present invention can provide a new approach to alleviate the waste of enzymatic lignin resources and efficiently detoxify heavy metals, and provide an important theoretical basis for overcoming the key technical difficulties in heavy metal pollution control. It is an innovative attempt at heavy metal pollution control. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Comparison of the adsorption effects of 18 enzymatic lignin-based adsorbents in Example 1;

[0032] Figure 2 These are SEM characterization images of the enzymatic lignin-based adsorbent S10 before and after modification and before and after adsorption in Example 1: a before modification; b after modification; c before adsorption; d after adsorption;

[0033] Figure 3 The effects of different factors on the adsorption effect of the enzymatic lignin-based adsorbent obtained in Example 1 are as follows: a. adsorbent dosage; b. initial pH; c. reaction temperature; d. adsorption time;

[0034] Figure 4 This is the adsorption and desorption cycle of the modified enzymatic lignin-based adsorbent obtained in Example 1. DETAILED DESCRIPTION

[0035] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0036] Example 1

[0037] This embodiment provides a method for preparing a modified enzymatically hydrolyzed lignin-based adsorbent, comprising the following steps:

[0038] (1) Cut rice straw into small segments (2-3 cm), wash them three times with deionized water, and dry them in an oven (set to 105°C). Place the dried straw segments in a grinder and grind them into filaments for later use. Take a certain amount of the crushed rice straw, put it into a 2% NaOH solution (solid-to-liquid ratio of 1:20), and then put it into a water bath (set to 85°C) for alkali pretreatment for 1 hour. After cooling, filter the mixture with a filter bag when the temperature drops to room temperature, wash it with deionized water, and finally dry the resulting alkali-pretreated rice straw residue.

[0039] (2) 4 g of clean alkali-pretreated rice straw residue was placed in 50 mM / L (pH 4.5) sodium citrate buffer (maintaining a solid-liquid ratio of 1:50) and enzymatically saccharified with cellulase for 72 h (maintaining a temperature of 45°C, adding 0.05 g of cellulase / g of rice straw residue, and a constant temperature oscillator at 180 rpm). The residue after enzymatic hydrolysis and saccharification was centrifuged and washed with deionized water until neutral, and then dried in an oven at 105°C overnight to obtain a clean and dry enzymatically hydrolyzed lignin residue.

[0040] (3) Enzymatic lignin residue (w), NaHCO3 (w), and citric acid (0.1 M / L) (v) were mixed at solid-liquid ratios of (i) 1:0.4:10, (ii) 1:0.4:20, and (iii) 1:0.4:30. The mixture was shaken in a magnetic stirrer (400 rpm) at 25°C for 1 h, then evenly spread on a tray, dried at 60°C for 24 h, and heated at 100°C for 4 h. Finally, the cooled sample (25°C) was washed with 1.0 M / L NaOH solution and deionized water until neutral, and dried at 60°C to constant weight to obtain citric acid-modified enzymatic lignin.

[0041] (4) The enzymatically hydrolyzed lignin modified with citric acid was immersed in a mixture of equal concentrations of Fe(II) (FeSO4) and Fe(III) (FeCl3) ([Fe(II)] / [Fe(III)] = 0.1, 0.3 and 0.5 M) at a solid-liquid ratio of 1:20 and stirred in a magnetic stirrer (400 rpm) at 70°C for 1 h. After cooling to room temperature, the pH of the solution was adjusted to 11 or 13 by adding 4 M / L NaOH solution. The black mixture was stirred again in a magnetic stirrer (400 rpm) at 70°C for 1 h, and finally centrifuged at high speed in a centrifuge and freeze-dried to obtain a sample, which was then rinsed with deionized water to pH = 7 to obtain 18 enzymatically hydrolyzed lignin-based adsorbents modified under different citric acid and [Fe(II)] / [Fe(III)] mixing ratios. Among them, the order between step (3) and step (4) cannot be reversed. This is because the stronger the magnetism of the adsorbent, the more antagonistic effect there will be, that is, the increased divalent iron and trivalent iron occupy the adsorption sites increased by citric acid modification, resulting in a decrease in adsorption performance.

[0042] The 18 enzymatic lignin-based adsorbents are as follows: S1 (1:0.4:10, 0.1, 11), S2 (1:0.4:10, 0.3, 11), S3 (1:0.4:10, 0.5, 11), S4 (1:0.4:20, 0.1, 11), S5 (1:0.4:20, 0.3, 11), S6 (1:0.4:20, 0.5, 11), S7 (1:0.4:30, 0.1, 11), S8 (1:0.4:30, 0.3, 11), S9 (1:0.4:30, 0.5, 11), S10 (1:0.4:10, 0.1, 13), S11 (1:0.4:10, 0.3, 13), S12 (1:0.4:10, 0.5, 13), S13 (1:0.4:20, 0.1, 13), S14 (1:0. 4:20, 0.3, 13), S15 (1:0.4:20, 0.5, 13), S16 (1:0.4:30, 0.1, 13), S17 (1:0.4:30, 0.3, 13), S18 (1:0.4:30, 0.5, 13). Among them, 1:0.4:10, 1:0.4:20, and 1:0.4:30 are the solid-liquid ratios of enzymatic lignin residue (w), NaHCO3 (w), and citric acid (0.1M / L); 0.1, 0.3, and 0.5 are the [Fe(II)] / [Fe(III)] ratios of Fe(II) (FeSO4) and Fe(III) (FeCl3); 11 and 13 are pH values.

[0043] 18 adsorbents (0.001 g / mL) were added to 10 mL of Cu 2+The (50 mg / L) solution was reacted for 24 h, filtered, and Cu was determined by inductively coupled plasma mass spectrometry (ICP-MS). 2+ The remaining amount is calculated and the adsorption amount and removal rate are calculated. Figure 1 It can be seen that when the amount of citric acid grafted on the surface of the raw material is too much, the adsorption amount decreases. This may be because the excess citric acid will block the pores on the surface of the material, resulting in a decrease in the specific surface area of ​​the material, thereby inhibiting the migration and adsorption of target substances (such as copper ions, etc.) on the surface and in the voids of the material, thereby reducing the adsorption amount. Secondly, the excess citric acid itself will dehydrate and produce a cross-linking effect, resulting in a decrease in the amount of carboxyl groups attached to the surface of the material, which will also reduce the adsorption amount. In the process of loading ferroferric oxide, when the [Fe(II)] / [Fe(III)] ratio increases, the adsorption amount decreases. The reason for this may be that when its content is too high, the adsorbent particles will agglomerate, thereby reducing its actual available specific surface area; at the same time, ferroferric oxide will occupy a large number of adsorption sites, resulting in other more effective adsorption components being unable to fully play their role. By comparison, a highly efficient modified enzymatic lignin adsorbent with strong adsorption effect was screened out (S10: adsorption amount of 36.64 (mg / g); removal rate of 73.28%), and its removal rate is higher than that of other adsorbents (such as Figure 1 ), therefore, S10 was selected as the optimal adsorbent for further study.

[0044] Example 2

[0045] The research object in this embodiment is S10 prepared in Example 1, and the S10 was observed by scanning electron microscopy (SEM) ( Figure 2 It can be clearly observed from the image of the adsorbent before modification that the surface of the adsorbent exhibits a smooth columnar structure ( Figure 2 a). However, after modification, the surface of the adsorbent becomes quite rough ( Figure 2 b) The increase in surface roughness is mainly due to the presence of a large number of functional groups, including hydroxyl and carboxyl groups, on the surface of the raw materials, as well as the introduction of ferrosoferric oxide. Figure 2 The images in further show that the adsorbent is Figure 2 c) After Figure 2 d) Surface changes. As can be seen from the figure, after the adsorption process, the surface of the adsorbent is smoother than before adsorption, but still retains a large number of rough structures. These rough structures are mainly adsorption sites, which play a key role in the adsorption process, enabling the adsorbent to effectively capture and fix the target substance. The presence of this surface roughness not only demonstrates the high efficiency of the adsorbent in the adsorption process, but also indicates that this adsorbent has great potential and broad application prospects in future applications.

[0046] All experiments in this example were repeated three times.

[0047] Example 3

[0048] The S10 adsorbent in Example 1 was used for the next experiment. By comparing different adsorbent dosages (specifically including: 0.005g, 0.01g, 0.02g, 0.04g, 0.06g, 0.08g), as shown in FIG. Figure 3 As shown in a, if the amount of adsorbent is too much, competition may occur between adsorbents, resulting in some adsorption sites being relatively vacant, thereby reducing the unit adsorption capacity. At the same time, when the adsorbent concentration is high, adsorbents may aggregate with each other, resulting in a reduction in the adsorbent surface area. Therefore, it is concluded that when the amount of adsorbent is 0.02g, the adsorption effect is the best. Figure 3 As shown in b, when the temperature exceeds 35℃, the adsorption amount will decrease. The possible reason is that the temperature increase will increase the thermal motion of the molecules, making it easier for the adsorbate molecules to desorb from the adsorbent surface, resulting in a decrease in the adsorption amount. By comparing the adsorption effects of adsorbents under different temperature conditions, it is concluded that the adsorption effect is best when the adsorption temperature is 35℃. Figure 3 As shown in c, at low pH values ​​(acidic conditions), heavy metals mainly exist in the form of cations. At this time, H + Ions will compete with heavy metal ions for adsorption sites, thereby reducing the adsorption effect. Under neutral or alkaline conditions, heavy metal ions may undergo electrostatic interactions or complex reactions with functional groups on the surface of the adsorbent (such as carboxyl, hydroxyl, etc.), so the adsorption capacity is higher. When the pH value is too high, heavy metals may form hydroxide precipitates, which are not easily adsorbed by the adsorbent, thus causing the adsorption effect to decrease. Therefore, by comparing the adsorption effects of adsorbents under different pH conditions, it was concluded that when pH = 9, the adsorption effect is best, and for Cu 2+ The removal rate reaches more than 90%. Figure 3 As shown in Figure d, when the adsorption time is less than 0.5 h, the reaction enters the adsorption equilibrium stage. The main reason is that the adsorbent has advantages such as higher specific surface area and more active sites, as well as moderate pore size, which is more conducive to the rapid diffusion and adsorption of adsorbate molecules, thereby accelerating the arrival of adsorption equilibrium.

[0049] Application Example 4

[0050] The S10 adsorbent in Example 1 was used to conduct the next experiment. 2+The saturated modified enzymatic lignin-based adsorbent was placed in a conical flask and 50 mL (0.1 M / L HCl) of eluent was added. Oscillate in a constant temperature oscillator at 25 ° C and 240 rpm for 90 minutes until the pollutants are completely desorbed. The desorbed adsorbent was then separated with a magnet and washed several times with deionized water until the pH = 7.0, dried at 70 ° C to constant weight, and entered the next cycle. The removal rate and recovery rate of the initial (unused) adsorbent and the adsorbent after the first to fifth reuse are shown in Figure 2. Figure 4 After five reuses, the adsorbent still has a high adsorption capacity for Cu 2+ The adsorption removal rate can be maintained at more than 95%. The main reasons are as follows. First, it is well known that magnetic composite materials or nanocomposites such as Fe3O4 have large specific surface area, excellent complexing ability and specific chemical properties (such as superparamagnetism). These properties enable them to have higher adsorption capacity and removal rate for specific adsorbents (such as heavy metal ions). In addition, the pore structure and surface chemical properties of the adsorbent will also affect its adsorption capacity. For example, the microporous structure can provide more adsorption surface area, while the surface oxide may become a selective adsorption center. Second, the surface of the adsorbent after adsorption is smoother than the surface before adsorption, but it still has a large number of rough structures, namely adsorption sites, which participate in re-adsorption. Therefore, after five cycles, it still has a high adsorption capacity.

[0051] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the present invention into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention remain within the scope of protection of the present invention.

Claims

1. A method for preparing a modified enzymatically hydrolyzed lignin-based adsorbent, characterized in that: The steps include: The enzymatically degraded lignin residue, NaHCO3 and citric acid solution are mixed and reacted to obtain citric acid-modified enzymatically degraded lignin residue; The citric acid-modified enzymatic lignin residue is immersed in a mixed solution of FeSO4 and FeCl3 for reaction, and then the pH is adjusted to alkaline to obtain a magnetic citric acid-modified enzymatic lignin-based adsorbent.

2. The method for preparing a modified enzymatically hydrolyzed lignin-based adsorbent according to claim 1, wherein: The preparation method of the enzymatic lignin residue comprises the following steps: placing alkali pretreated straw residue into a container containing cellulase and sodium citrate slow-release solution, placing the container into a constant temperature oscillator for reaction, centrifugally filtering, neutralizing and drying to obtain the enzymatic lignin residue.

3. The method for preparing a modified enzymatically hydrolyzed lignin-based adsorbent according to claim 2, wherein: The concentration of the sodium citrate sustained-release solution is (45-50) mM / L, and the pH is (4-5); The ratio of the alkali pretreated straw residue to the sodium citrate slow-release solution is 1 g: (40-50) mL; The reaction time is (48-72) h; The mass ratio of the cellulase to the alkali pretreated straw residue is (0.05-0.1):1; The rotation speed of the constant temperature oscillator is (150-200) rpm; The drying temperature is (95-110)°C.

4. The method for preparing a modified enzymatically hydrolyzed lignin-based adsorbent according to claim 2, wherein: The preparation method of the alkali pretreated straw residue comprises the following steps: uniformly mixing rice straw powder and sodium hydroxide solution, heating the mixture in a water bath, taking the mixture out, cooling the mixture, filtering the mixture, neutralizing the mixture, and drying the mixture to obtain the alkali pretreated straw residue.

5. The method for preparing a modified enzymatically degraded lignin-based adsorbent according to claim 4, wherein: The mass concentration of the sodium hydroxide solution is (1-2)%, and the solid-liquid ratio of the rice straw powder to the sodium hydroxide solution is 1g:(20-30)mL.

6. The method for preparing a modified enzymatically degraded lignin-based adsorbent according to claim 1, wherein: The concentration of the citric acid solution is (0.1-0.15) M / L; The ratio of the enzymatically hydrolyzed lignin residue, the NaHCO 3 and the citric acid is 1 g:0.4 g:(10-30) mL.

7. The method for preparing a modified enzymatically degraded lignin-based adsorbent according to claim 1, characterized in that: The [Fe(II)] / [Fe(III)] ratio in the mixed solution is 0.1, 0.3 or 0.

5.

8. The method for preparing a modified enzymatically degraded lignin-based adsorbent according to claim 1, wherein: The pH is 11 or 13.

9. A modified enzymatic lignin-based adsorbent, characterized in that: The invention discloses a novel cellulose acetate copolymer prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the modified enzymatically hydrolyzed lignin-based adsorbent according to claim 9 in adsorbing heavy metal pollutants.

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