Method for removing and recovering soluble organic matters in water

Abstract

The invention provides a method for removing and recovering soluble organic matters in water, which comprises the following steps: sequentially adding a metal salt and an organic ligand into water to be treated, wrapping DOM to quickly form a precipitate, and recovering the soluble organic matters through filtration, the metal salt comprising at least one of zinc acetate, zinc sulfate and zinc nitrate, the organic ligand comprising at least one of zinc acetate, zinc sulfate and zinc nitrate, and the organic ligand comprising at least one of zinc acetate, zinc sulfate and zinc nitrate. The organic ligand comprises a ligand containing an imidazole structure. The method provided by the invention has the advantages of high DOM removal efficiency, cheap and easily available raw materials, low production cost, simple preparation process, short preparation period, low requirements on equipment, good DOM recovery stability and strong repeatability.

Description

Technical Field

[0001] The invention belongs to the technical field of sewage treatment, and in particular relates to a method for removing and recovering soluble organic matter in water. Background Art

[0002] Dissolved organic matter (DOM) is a complex heterogeneous mixture composed of macromolecules such as phenols, quinones, olefins, amines, sulfides and heterocyclic groups. It is widely present in various water bodies, such as surface water, groundwater and sewage. The concentration in surface water and groundwater is 1-10 mgC / L, while in sewage it is 5-30 mgC / L. In the advanced oxidation process of sewage, DOM will inhibit the removal of micropollutants through processes such as light shielding, consumption of active species and reduction of pollutant free radical intermediates. In the process of drinking water disinfection, DOM will react with disinfectants such as chlorine and chloramine to produce some carcinogenic disinfection by-products, such as trihalomethanes and haloacetic acids. Therefore, the presence of DOM has a negative impact on the water treatment process.

[0003] In the process of sewage treatment, coagulation and sedimentation methods (adding coagulants, chemical reactions and flocculation to form flocs and sink) and membrane treatment methods (ultrafiltration, microfiltration, nanofiltration and reverse osmosis) are generally used to reduce DOM in water. The coagulation and sedimentation method has low efficiency in removing DOM and cannot completely remove DOM from sewage. The membrane treatment method is expensive, takes a long time to treat, occupies a large space, and is prone to membrane clogging and other problems. Common methods for recovering DOM also include ion resin method, ultrafiltration membrane method and solid phase extraction method. The ion resin method is based on the principle of ion exchange. It adsorbs and removes DOM in water by exchanging ions in water with ions on the resin, and then elutes DOM by changing the pH of the solution. The disadvantage of this method is that the cost is relatively high, frequent replacement or regeneration of ion resins may increase operating costs, and some organic matter may form strong chemical bonds on ion resins, making their regeneration more difficult. The effect of ion resins is usually affected by pH value, and pH adjustment is required before use. Ultrafiltration membrane method is a membrane separation technology commonly used to recover DOM. This method uses the micropores of the ultrafiltration membrane to effectively intercept organic molecules dissolved in water and achieve the recovery of organic matter. The disadvantage of this method is that the interception of organic matter may cause clogging of the membrane, reduce permeability, and require regular maintenance and cleaning; the preparation and maintenance costs of ultrafiltration membranes are relatively high, especially when dealing with large amounts of water flow, membrane replacement and maintenance may become an expensive problem; it is usually necessary to equip appropriate equipment, such as high-pressure pumps, to ensure that the water flow has sufficient pressure when passing through the membrane. Solid phase extraction (SPE) is to use the specific adsorption of organic matter fixed on the adsorbent to achieve the enrichment and recovery of organic matter by adjusting the conditions. SPE usually has certain restrictions on the capacity of water samples, and requires pretreatment and enrichment within a certain range, so it may not be able to process a large number of water samples; the operation is relatively cumbersome, requiring accurate sample processing and experimental condition control, and requiring high operator skills; the preparation cost of SPE columns and adsorbents is relatively high, and the solutions used for regeneration and maintenance may also increase operating costs.

[0004] In view of this, it is necessary to develop a more efficient and cost-effective method to remove and recycle DOM. Summary of the invention

[0005] The present invention aims to solve at least one of the above technical problems existing in the prior art. To this end, the present invention provides a method for removing and recovering dissolved organic matter in water, which is more efficient and less costly.

[0006] The first aspect of the present invention provides a method for removing and recovering soluble organic matter in water, the method comprising sequentially adding a metal salt and an organic ligand to the water to be treated, encapsulating the soluble organic matter to form a precipitate, and then recovering the soluble organic matter by filtration, wherein the metal salt comprises at least one of zinc acetate, zinc sulfate and zinc nitrate, and the organic ligand comprises a ligand containing an imidazole structure.

[0007] The present invention relates to a technical solution for a method for removing and recovering dissolved organic matter in water, which has at least the following beneficial effects:

[0008] In the method of the present invention, the metal ions in the metal salt are first coordinated with the soluble organic matter (DOM) and wrapped on the surface of the DOM, and then the organic ligands are coordinated with the metal ions in the metal salt wrapped on the surface of the DOM to form a precipitate, the precipitate is taken away, and acidification can destroy the chemical bond between the metal and the ligand to dissolve the precipitate, and finally the DOM is intercepted by filtering, thereby realizing the removal and recovery of DOM in water.

[0009] Compared with ultrafiltration, the volume of water sample that needs to be passed through ultrafiltration to remove DOM is much larger than that of this method, resulting in a large space occupation and a long time. However, the sample that needs to be filtered by the method of the present invention is only the acid-soluble sample after precipitation, so the sample volume is small and the time required is short.

[0010] The method of the invention has high DOM removal efficiency, cheap and readily available raw materials, low production cost, simple preparation process, short preparation cycle, low requirements on equipment, and good DOM recovery stability and strong repeatability.

[0011] The method of the present invention is to firstly remove DOM quickly by coordination polymerization encapsulation, and then recycle DOM. The common coagulation precipitation method is double-layer compression of organic matter, while the method of the present invention is encapsulation adsorption.

[0012] The method of the present invention belongs to in-situ encapsulation. If prepared or commercially available MOFs such as ZIF-8 are directly added to the water to be treated, the MOFs such as ZIF-8 will adsorb DOM instead of the in-situ encapsulation of the present invention.

[0013] In the method of the present invention, the metal ions in the metal salt adsorb DOM, and after forming coordination in water, the coordination can be easily released by acid dissolution.

[0014] According to some embodiments of the present invention, the metal salt is zinc acetate, and the organic ligand is 2-methylimidazole.

[0015] Zinc acetate and 2-methylimidazole can be replaced by other reagents, as long as they can form a metal + imidazole structure with DOM, but it is necessary to consider whether the material can be formed in water and whether it is easy to untie the coordination under acidic conditions. Such as zinc acetate, zinc sulfate, zinc nitrate, etc. Untie the coordination to separate DOM and the metal + coordination on the surface. Acid is added to untie the coordination, and then filtered to obtain pure DOM.

[0016] According to some embodiments of the present invention, the added amount of the metal salt is 5-100 mmol / L.

[0017] According to some embodiments of the present invention, the added amount of the metal salt is 5 mmol / L, 20 mmol / L, 100 mmol / L or a range formed by any two of them.

[0018] According to some embodiments of the present invention, the added amount of the organic ligand is 20-400 mmol / L.

[0019] According to some embodiments of the present invention, the added amount of the organic ligand is 20 mmol / L, 80 mmol / L, 400 mmol / L or a range formed by any two of them.

[0020] According to some embodiments of the present invention, after metal salts and organic ligands are sequentially added to the water to be treated, the solution is centrifuged.

[0021] According to some embodiments of the present invention, the rotation speed of the centrifugal treatment is 100-10000 rpm.

[0022] According to some embodiments of the present invention, the centrifugal treatment time is 0.1-2h.

[0023] According to some embodiments of the present invention, the method of filtering and recovering the soluble organic matter comprises: dissolving the precipitate in an acid solution and filtering using a dialysis membrane.

[0024] According to some embodiments of the invention, the pH of the acid solution is 3-6.

[0025] According to some embodiments of the present invention, the dialysis membrane has a molecular weight cut-off of 100-500 Da.

[0026] According to some embodiments of the present invention, the molecular weight cut-off of the dialysis membrane is 100 Da, 200 Da, 500 Da or a range formed by any two of them.

[0027] According to some embodiments of the present invention, the method comprises the following steps:

[0028] Step S1: Removal of DOM: adding metal salt to a solution containing DOM, and then adding an organic ligand, placing the solution in a centrifuge tube and centrifuging in a centrifuge, and DOM is precipitated to the bottom of the centrifuge tube by the metal organic framework material;

[0029] Step S2: Recovery of DOM: The precipitate is dissolved in an acid solution and filtered through a dialysis membrane to completely filter out the metal salts and organic ligands in the system, thereby recovering DOM.

[0030] According to some embodiments of the present invention, in step S1, the reaction temperature is 0-60°C.

[0031] According to some embodiments of the present invention, in step S2, the acid solution includes dilute sulfuric acid, dilute hydrochloric acid or other dilute acids.

[0032] Zinc acetate and 2-methylimidazole can pass through the membrane, but DOM cannot, so DOM can be retained. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the process of the present invention.

[0034] Figure 2 This is the fluorescence spectrum test result of zinc acetate complexing with DOM in Example 1.

[0035] Figure 3 This is the Fourier transform infrared spectrum test result of zinc acetate complexing DOM in Example 1.

[0036] Figure 4 This is a macroscopic morphology of the ZIF-8@DOM precipitate recovered by the method of Example 1.

[0037] Figure 5 This is a microscopic morphology of the ZIF-8@DOM precipitate recovered by the method of Example 1.

[0038] Figure 6 The figure shows the removal and recovery effect of DOM in different actual water bodies by the method of Example 1.

[0039] Figure 7 This is a diagram showing the removal and recovery effects of DOM using the method of Example 1 and different processes. DETAILED DESCRIPTION

[0040] The following are specific embodiments of the present invention, and the technical solutions of the present invention are further described in conjunction with the embodiments, but the present invention is not limited to these embodiments.

[0041] In some embodiments of the present invention, a method for removing and recovering soluble organic matter in water is provided, the method comprising sequentially adding a metal salt and an organic ligand to the water to be treated, encapsulating the soluble organic matter to form a precipitate, and then recovering the soluble organic matter by filtration, wherein the metal salt comprises at least one of zinc acetate, zinc sulfate and zinc nitrate, and the organic ligand comprises a ligand containing an imidazole structure.

[0042] It should be noted that in the method of the present invention, the metal ions in the metal salt are first coordinated with the dissolved organic matter (DOM) and wrapped on the surface of the DOM, and then the organic ligands are coordinated with the metal ions in the metal salt wrapped on the surface of the DOM to form a precipitate, and the DOM is intercepted by filtration, thereby achieving the removal and recovery of DOM in water.

[0043] It should be noted that compared with ultrafiltration, the volume of water sample that needs to be passed through ultrafiltration to remove DOM is much larger than that of this method, resulting in a large space occupation and a long time. However, the sample that needs to be filtered by the method of the present invention is only the acid-soluble sample after precipitation, so the sample volume is small and the time required is short.

[0044] It can be understood that the method of the present invention has high DOM removal efficiency, cheap and readily available raw materials, low production cost, simple preparation process, short preparation cycle, low equipment requirements, and good DOM recovery stability and strong repeatability.

[0045] It should also be noted that the method of the present invention is a method of first removing DOM quickly by coordination polymerization encapsulation and then recovering DOM. The common coagulation precipitation method is double-layer compression of organic matter, while the method of the present invention is encapsulation adsorption.

[0046] It should also be noted that the method of the present invention belongs to in-situ encapsulation. If prepared or commercially available MOFs such as ZIF-8 are directly added to the water to be treated, the MOFs such as ZIF-8 will adsorb DOM instead of the in-situ encapsulation of the present invention.

[0047] The special feature of the method of the present invention is that the metal ions in the metal salt adsorb DOM and after forming coordination in water, the coordination can be easily released by acid dissolution.

[0048] In some embodiments of the present invention, the metal salt is zinc acetate, and the organic ligand is 2-methylimidazole.

[0049] Zinc acetate and 2-methylimidazole can be replaced by other reagents, as long as they can form a metal + imidazole structure with DOM, but it is necessary to consider whether the material can be formed in water and whether it is easy to untie the coordination under acidic conditions. Such as zinc acetate, zinc sulfate, zinc nitrate, etc. Untie the coordination to separate DOM and the metal + coordination on the surface. Acid is added to untie the coordination, and then filtered to obtain pure DOM.

[0050] In some embodiments of the present invention, the amount of metal salt added is 5-100 mmol / L.

[0051] In some embodiments of the present invention, the added amount of the metal salt is 5 mmol / L, 20 mmol / L, 100 mmol / L, or a range formed by any two of them.

[0052] In some embodiments of the present invention, the amount of organic ligand added is 20-400 mmol / L.

[0053] In some embodiments of the present invention, the amount of the organic ligand added is 20 mmol / L, 80 mmol / L, 400 mmol / L, or a range formed by any two of them.

[0054] In some embodiments of the present invention, after metal salts and organic ligands are sequentially added to the water to be treated, the solution is centrifuged.

[0055] In some embodiments of the present invention, the rotation speed of the centrifugal treatment is 100-10000 rpm.

[0056] In some embodiments of the present invention, the centrifugation time is 0.1-2h.

[0057] In some embodiments of the present invention, the method of filtering and recovering the soluble organic matter comprises: dissolving the precipitate in an acid solution and filtering using a dialysis membrane.

[0058] In some embodiments of the present invention, the pH of the acid solution is 3-6.

[0059] In some embodiments of the present invention, the molecular weight cut-off of the dialysis membrane is 100-500 Da.

[0060] In some embodiments of the present invention, the molecular weight cut-off of the dialysis membrane is 100 Da, 200 Da, 500 Da, or a range formed by any two thereof.

[0061] In some embodiments of the present invention, the method of the present invention may include the following steps:

[0062] Step S1: Removal of DOM: adding metal salt to a solution containing DOM, and then adding an organic ligand, placing the solution in a centrifuge tube and centrifuging in a centrifuge, and DOM is precipitated to the bottom of the centrifuge tube by the metal organic framework material;

[0063] Step S2: Recovery of DOM: The precipitate is dissolved in an acid solution and filtered through a dialysis membrane to completely filter out the metal salts and organic ligands in the system, thereby recovering DOM.

[0064] In some embodiments of the present invention, in step S1, the reaction temperature is 0-60°C.

[0065] In some embodiments of the present invention, in step S2, the acid solution includes dilute sulfuric acid, dilute hydrochloric acid or other dilute acids.

[0066] Zinc acetate and 2-methylimidazole can pass through the membrane, but DOM cannot, so DOM can be retained.

[0067] The technical solution of the present invention will be better understood with reference to specific embodiments below.

[0068] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0069] Example 1

[0070] A method for removing and recovering dissolved organic matter in water comprises sequentially adding a metal salt and an organic ligand to the water to be treated, wrapping the dissolved organic matter to form a precipitate, removing the precipitate, adding acid to acidify the water to destroy the chemical bond between the metal and the ligand, dissolving the precipitate, and finally recovering the DOM by filtering, wherein the metal salt is zinc acetate and the organic ligand is 2-methylimidazole.

[0071] The specific steps include:

[0072] (1) Take 100 mL of water sample, add 20 mmol / L zinc acetate solution, and then add 80 mmol / L 2-methylimidazole.

[0073] (2) Place the tube in a centrifuge and centrifuge at 10,000 rpm for 5 min. ZIF-8@DOM will precipitate to the bottom of the centrifuge tube.

[0074] (3) Remove the supernatant from the solution, add dilute sulfuric acid, adjust the pH value of the solution to 3-6, and make up the volume to 100 mL.

[0075] (4) The pH-adjusted solution was filtered using a dialysis membrane with a size of 100 Da to obtain the solid DOM after the reaction, which was labeled as ZIF-8@DOM precipitate.

[0076] The initial concentration of DOM in the water sample was 10 mgC / L.

[0077] Combination Figure 1 The method of the present invention can be better understood by steps (1) and (2) as follows Figure 1 As shown in (1) to (4) in , steps (3) and (4) are as follows Figure 1 As shown in (5) to (8) in .

[0078] Example 2

[0079] The difference between this embodiment and embodiment 1 is that the addition amount of zinc acetate solution is 5 mmol / L, and the addition amount of 2-methylimidazole is 20 mmol / L.

[0080] Example 3

[0081] The difference between this embodiment and embodiment 1 is that the addition amount of zinc acetate solution is 100 mmol / L, and the addition amount of 2-methylimidazole is 400 mmol / L.

[0082] Comparative Example 1

[0083] The difference between this comparative example and Example 1 is that the addition amount of the zinc acetate solution is 1 mmol / L, and the addition amount of 2-methylimidazole is 1 mmol / L.

[0084] Experimental testing

[0085] The complexation of zinc ion and DOM in Example 1 was characterized by synchronous fluorescence spectroscopy. Figure 2 shown.

[0086] from Figure 2 It can be seen that after adding zinc acetate to the DOM solution, the characteristic fluorescence peak of DOM shifted from 344nm to 384nm, and the fluorescence intensity decreased, indicating that Zn 2+ There is a strong interaction with the DOM.

[0087] The complexation of zinc ions with DOM was characterized by simultaneous Fourier transform infrared spectroscopy. Figure 3 shown.

[0088] from Figure 3 It can be seen that in the Fourier transform infrared spectrum, Zn 2+ Link DOM(Zn 2+ -DOM), the C=O stretching vibration of the DOM carboxyl group changes from 1710 cm -1 Red shifted to 1540 cm -1 , and at the same time appeared -1 The Zn-O stretching vibration absorption is centered on the Zn-O 2+Coordination occurs between the carboxyl group of DOM, thus weakening the electronic strength of the C=O bond and causing a red shift in the Fourier transform infrared spectrum. 2+ As the amount of addition increases, the OH stretching vibration (range 3100-3600cm -1 ) gradually weakened, which indicates that there is a coordination interaction between Zn-carboxyl or Zn-hydroxyl.

[0089] The DOM powder was recovered by the method of Example 1 and the ZIF-8@DOM precipitate was as follows: Figure 4 shown.

[0090] from Figure 4 It can be seen that the ZIF-8@DOM precipitate recovered by the method of Example 1 has a regular morphology and uniform particle size in macroscopic terms.

[0091] The ZIF-8@DOM precipitate recovered by the method of Example 1 was characterized by scanning electron microscopy. Figure 5 As shown. Figure 5 It can be seen that the ZIF-8@DOM precipitate recovered by the method of Example 1 also has a regular morphology and uniform particle size at the microscopic level.

[0092] Using the same DOM solution, the removal and recovery effects of DOM in Examples 1 to 3 and Comparative Example 1 were compared. The results showed that when the addition amount of the metal salt was 5-100 mmol / L and the addition amount of the organic ligand was 20-400 mmol / L, the removal and recovery effects of DOM were better.

[0093] Furthermore, the removal and recovery of landfill leachate, sewage, surface water and DOM solution were tested by the method of Example 1. The results are shown in Tables 1 and Figure 6 shown.

[0094] The initial concentration of DOM in the landfill leachate was 1181 mgC / L. The DOC of the initial DOM was measured by the NPOC method of Shimadzu total organic carbon analyzer.

[0095] The initial concentration of DOM in the sewage was 52.3 mgC / L.

[0096] In surface water, the initial concentration of DOM was 3.3 mgC / L.

[0097] In the DOM solution, the initial concentration of DOM was 10 mgC / L.

[0098] Table 1

[0099] Landfill leachate Sewage Surface water DOM solution Removal rate 0.74 0.84 0.82 0.90 Recovery rate 0.59 0.72 0.76 0.77

[0100] The method of Example 1 is also compared with a conventional method for recycling DOM. Figure 7 shown.

[0101] Figure 7 In the process, the process parameters of ion exchange resin are as follows: XAD-8 ion resin is used to separate DOM in actual water samples.

[0102] Data comes from the literature: Fujita, Y., Ding, W.-H. & Reinhard, M. Identification of wastewater dissolved organic carbon characteristics in reclaimed wastewater and recharged groundwater. Water Environ Res 68, 867-876, (1996).

[0103] Minor, EC et al. Structural characterization of dissolved organic matter: A review of current techniques for isolation and analysis. Environ Sci-Proc Imp 16, 2064-2079, (2014).

[0104] Maurice, PA et al. A comparison of surface water natural organic matter in raw filtered water samples, XAD, and reverse osmosis isolates. WaterRes 36, 2357-2371, (2002).

[0105] Batra, S. et al. Adsorption of Bisphenol-A from aqueous solution using amberlite XAD-7 impregnated with aliquat 336: Batch, column, and designstudies. Process Saf Environ 122, 232-246, (2019).

[0106] Hughes,DD et al. Rapid, Semi-automated fractionation of freshwaterdissolved organic carbon using DAX 8(XAD 8) and XAD 4resins in tandem. NaturalScience 8, 487-498, (2016).

[0107] Green, NWet al. An intercomparison of three methods for the large-scale isolation of oceanic dissolved organic matter. Mar Chem 161,14-19, (2014).

[0108] The process parameters of ultrafiltration are: using 1k Da ultrafiltration membrane to separate DOM in actual water samples.

[0109] The data comes from the literature: Minor, ECet al. Structural characterization of dissolved organic matter: A review of current techniques for isolation and analysis. Environ Sci-Proc Imp 16, 2064-2079, (2014).

[0110] Benner, R. et al. Abundance, size distribution, and stable carbon and nitrogen isotopic compositions of marine organic matter isolated bytangential-flow ultrafiltration. Mar Chem57, 243-263, (1997).

[0111] Drouiche, M. et al. Economic study of the treatment of surface water by small ultrafiltration units. Water Sa 27, 199-204, (2001).

[0112] The process parameters of solid phase extraction are: using PPL-SPE chromatographic column to separate DOM in actual water samples.

[0113] The data comes from the literature: Minor, ECet al. Structural characterization of dissolved organic matter: A review of current techniques for isolation and analysis. Environ Sci-Proc Imp 16, 2064-2079, (2014).

[0114] Green, NWet al. An intercomparison of three methods for the large-scale isolation of oceanic dissolved organic matter. Mar Chem 161,14-19, (2014).

[0115] Dittmar, T. et al. Asimple and efficient method for the solid-phase extraction of dissolved organic matter (SPE-DOM) from seawater. Limnol Oceanogr-Meth 6, 230-235, (2008).

[0116] from Figure 7 It can be seen that the recovery efficiency of the method according to the embodiment of the present invention is the best.

[0117] The above embodiments illustrate that the method of the present invention for rapidly removing and recovering DOM by using a coordination polymerization encapsulation strategy has high DOM removal efficiency, cheap and readily available raw materials, low production cost, simple preparation process, short preparation cycle, low requirements on equipment, good DOM recovery stability, strong repeatability, and has great practical application potential.

[0118] The above embodiments are only used to help understand the method and core idea of ​​the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications are also included in the scope of protection of the claims of the present invention.

[0119] The present invention has been described in detail above in conjunction with the embodiments, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge scope of ordinary technicians in the relevant technical field without departing from the purpose of the present invention.

Claims

1. A method for removing and recovering dissolved organic matter in water, characterized in that: The method comprises sequentially adding a metal salt and an organic ligand to water to be treated, encapsulating soluble organic matter to form a precipitate, and then recovering the soluble organic matter by filtering, wherein the metal salt comprises at least one of zinc acetate, zinc sulfate and zinc nitrate, and the organic ligand comprises a ligand containing an imidazole structure.

2. The method according to claim 1, characterized in that The metal salt is zinc acetate, and the organic ligand is 2-methylimidazole.

3. The method according to claim 1 or 2, characterized in that: The added amount of the metal salt is 5-100 mmol / L.

4. The method according to claim 1 or 2, characterized in that: The added amount of the organic ligand is 20-400 mmol / L.

5. The method according to claim 1 or 2, characterized in that: After metal salt and organic ligand are sequentially added to the water to be treated, the solution is centrifuged.

6. The method according to claim 5, characterized in that The rotation speed of the centrifugal treatment is 100-10000 rpm.

7. The method according to claim 5, characterized in that The centrifugal treatment time is 0.1-2h.

8. The method according to claim 1 or 2, characterized in that: The method for recovering the soluble organic matter by filtering comprises: dissolving the precipitate in an acid solution and filtering using a dialysis membrane.

9. The method according to claim 8, characterized in that The pH of the acid solution is 3-6.

10. The method according to claim 8, characterized in that The molecular weight cut-off of the dialysis membrane is 80-500Da.

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