Method for determining molecular weight distribution of humic acid in fertilizer

Abstract

The invention discloses a method for determining molecular weight distribution of humic acid in a fertilizer, and belongs to the technical field of humic acid. The humic acid adsorbent is obtained by mixing the graphene oxide and the biochar nanoparticles. Grading power is provided by utilizing the adsorption effect of the humic acid adsorbent, and the humic acid is graded according to the molecular weight under the condition that the regenerated cellulose membranes with different pore diameters form physical barrier. Molecular weight grading is carried out through the adsorption effect on humic acid, compared with a traditional method, the effect is close, the process is faster, each step can be completed within 15 min, and the grading efficiency is greatly improved. The method provided by the invention can be used for measuring humic acids from different sources, such as mineral source humic acid fertilizers or biochemical humic acid fertilizers, and has important economic value and social benefit.

Description

Technical Field

[0001] The invention relates to the technical field of humic acid, and in particular to a method for measuring the molecular weight distribution of humic acid in fertilizers. Background Art

[0002] Humic acid is the most widely distributed organic matter in nature, widely present in soil, sediments, rivers, lakes and oceans. It is a product formed by chemical and biological processes of animal and plant residues.

[0003] As an important indicator of chemical substances, molecular weight has an important influence on the function of humic acid. In media such as soil, sediment and water, there are differences in the contact area and probability between humic acids of different molecular weights and minerals and microorganisms, which affects their function. Since humic acid is a mixture of macromolecular organic matter with multiple functional groups, there is no definite molecular weight for this mixed macromolecular substance composed of natural complex organic matter, only a molecular weight distribution range. Therefore, it is not easy to accurately determine the molecular weight of humic acid. The patent "CN 105548081 A A method for determining the molecular weight distribution of humic acid in water" uses an asymmetric field flow analyzer connected in series with a multi-angle laser light scattering instrument and an ultraviolet-visible light detector to detect the molecular weight distribution of humic acid, but this method has high requirements for instruments and equipment and is not suitable for large-scale promotion. Patent "CN107436332 A A method for separating and determining different polarity and molecular weight components of humic acid" determines the molecular weight of humic acid by building a special high-performance liquid chromatography module, but it is still relatively complicated because it needs to rebuild the modules including mobile phase, binary pump, six-way valve, injector, column oven, diode array detector (DAD) and fluorescence detector (FLD) based on HPLC. Therefore, it is promising to find a simpler and faster method for determining the molecular weight of humic acid. Summary of the invention

[0004] In view of the above prior art, the object of the present invention is to provide a method for determining the molecular weight distribution of humic acid in fertilizers.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A first aspect of the present invention provides a method for determining the molecular weight distribution of humic acid in a fertilizer, comprising the following steps:

[0007] (1) The humic acid adsorbent is placed in dialysis bags of different molecular weights, and the two ends are tightened to prepare adsorption membrane packages of different molecular weight grades;

[0008] (2) dissolving humic acid in an alkaline solution to obtain a humic acid solution; placing the adsorption membrane packages obtained in step (1) into the humic acid solution in order of molecular weight of the dialysis bags from small to large, stirring, and performing an adsorption reaction;

[0009] (3) After the adsorption reaction is completed, the adsorption membrane package is taken out and the concentration of the humic acid solution is measured; the humic acid in the membrane package is a narrow molecular weight humic acid, and the molecular weight distribution of the humic acid is finally obtained by using the subtraction method;

[0010] The humic acid adsorbent comprises graphene oxide and biochar nanoparticles.

[0011] Furthermore, the graphene oxide has a thickness of 0.8-2 nm, a sheet diameter of 1-20 μm, and a monolayer rate of 99%; and the particle size of the biochar nanoparticles is 1 μm.

[0012] Furthermore, the mass ratio of graphene oxide and biochar nanoparticles is (1-2): (1-2).

[0013] Furthermore, in step (1), the molecular weight of the dialysis bag is 1k-100kDa.

[0014] Furthermore, in step (2), the pH of the humic acid solution is adjusted to 8.5-12.

[0015] Furthermore, in step (2), the adsorption reaction temperature is 40-60° C., the stirring speed is 60-100 rpm, and the adsorption time is 5-30 min.

[0016] Furthermore, in step (3), the absorbance of humic acid is measured at 254 nm, and the concentration of humic acid in the membrane package is determined by subtraction method.

[0017] Beneficial effects of the present invention:

[0018] The present invention obtains a humic acid adsorbent by mixing graphene oxide and biochar nanoparticles. The adsorption effect of the humic acid adsorbent is used to provide classification power, and the humic acid is classified according to the molecular weight under the condition that the regenerated cellulose membrane with different pore sizes forms a physical barrier. The humic acid adsorbent used in the present invention has a large specific surface area and a mesoporous structure, and has a strong adsorption capacity for humic acid. The present invention utilizes the adsorption effect of humic acid to perform molecular weight classification, which is similar to the traditional method in effect and faster in the process. Each step can be completed within 15 minutes, and the classification efficiency is greatly improved. The present invention can be used to measure humic acid from different sources, such as mineral humic acid fertilizers or biochemical humic acid fertilizers, and has important economic value and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1The invention aims to improve the fixing methods of two dialysis bags for making adsorption membrane packages during dialysis. DETAILED DESCRIPTION

[0020] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0021] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below in conjunction with specific embodiments.

[0022] The test materials used in the embodiments of the present invention that are not specifically described are all conventional test materials in the art and can be purchased through commercial channels. The dialysis bag used in the present invention is tubular and is made of seamless regenerated cellulose membrane through a viscose process. The molecular weight indicated in the molecular weight grade is the molecular weight cutoff, which can be purchased conventionally. The CAS number of the graphene oxide used in the present invention is 2640657-49-2, with a thickness of 1.2nm, a sheet diameter of 10μm, and a monolayer rate of 99%; the CAS number of the biochar nanoparticles is 7440-44-0, and the particle size is 1μm.

[0023] Example 1

[0024] 1.1 Pretreatment of materials

[0025] (1) Pretreatment of humic acid

[0026] Add 200 mg of mineral humic acid into a 500 ml volumetric flask, add 1 M NaOH solution dropwise until the humic acid is completely dissolved, and dilute to 500 ml with ultrapure water to obtain a humic acid solution. The pH of the humic acid solution is about 12.

[0027] (2) Pretreatment of dialysis bags

[0028] Cut the regenerated cellulose dialysis bag into 20cm pieces, wet it in ultrapure water, take it out after three minutes, clamp one end with a clip, and open the other end for later use. The molecular weight grades of the five dialysis bags for pretreatment are 1kDa, 5kDa, 10kDa, 50kDa and 100kDa respectively.

[0029] (3) Preparation of humic acid adsorbent

[0030] Graphene oxide and biochar nanoparticles were mixed in a mass ratio of 1:1 to obtain a humic acid adsorbent.

[0031] (4) Preparation of adsorption membrane package

[0032] Take 25 g of the humic acid adsorbent prepared in step (3) and put it into the dialysis bags of five molecular weight grades prepared in step (2), with 5 g in each dialysis bag, to obtain five adsorption membrane packages of different molecular weight grades.

[0033] 1.2 Adsorption classification of humic acid

[0034] Take the humic acid solution prepared in step (1), put it into a beaker, add the adsorption membrane package with the smallest molecular weight grade prepared in step (4), put the center part of the adsorption membrane package into the humic acid solution, and let the two ends extend beyond the outer wall of the beaker and fold back. Use rubber bands to tightly wrap the two ends of the adsorption membrane package with the beaker wall. At this time, the dialysis membrane clamp can be replaced to prevent leakage. Stir at 100 rpm for 15 minutes at 50°C, and the adsorption reaction of the adsorption membrane package is completed. Take out the adsorption membrane package and measure the concentration of the humic acid solution outside the membrane package. According to the molecular weight grade of the adsorption membrane package from small to large, put in the adsorption membrane package of the next molecular weight grade. After the reaction of 5 adsorption membrane packages is completed, the classification is completed.

[0035] 1.3 Molecular weight and distribution determination of humic acid

[0036] After the adsorption reaction of each adsorption membrane package is completed, the humic acid solution outside the membrane package is taken and the concentration of humic acid is measured by ultraviolet spectrophotometry at 254nm, and the concentration of humic acid of different molecular weights in the membrane package is determined by subtraction. Humic acid is graded into 6 components according to the molecular weight of the dialysis bag, and the content percentage distribution of different components is shown in Table 1.

[0037] Table 1 Content distribution of each fraction (wt%)

[0038]

[0039] Example 2

[0040] 2.1 Pretreatment of materials

[0041] (1) Pretreatment of humic acid

[0042] Add 200 mg of mineral humic acid into a 500 ml volumetric flask, add 1 M NaOH solution dropwise until the humic acid is completely dissolved, and dilute to 500 ml with ultrapure water to obtain a humic acid solution. The pH of the humic acid solution is about 12.

[0043] (2) Pretreatment of dialysis bags

[0044] Cut the regenerated cellulose dialysis bag into 20cm pieces, wet it in ultrapure water, take it out after three minutes, clamp one end with a clip, and open the other end for later use. The molecular weight grades of the five dialysis bags for pretreatment are 1kDa, 5kDa, 10kDa, 50kDa and 100kDa respectively.

[0045] (3) Preparation of humic acid adsorbent

[0046] Graphene oxide and biochar nanoparticles were mixed in a mass ratio of 1:2 to obtain a humic acid adsorbent.

[0047] (4) Preparation of adsorption membrane package

[0048] Take 25 g of the humic acid adsorbent prepared in step (3) and put it into the dialysis bags of five molecular weight grades prepared in step (2), with 5 g in each dialysis bag, to obtain five adsorption membrane packages of different molecular weight grades.

[0049] 2.2 Adsorption classification of humic acid

[0050] Take the humic acid solution prepared in step (1), put it into a beaker, add the adsorption membrane package with the smallest molecular weight grade prepared in step (4), put the center part of the adsorption membrane package into the humic acid solution, and let the two ends extend beyond the outer wall of the beaker and fold back. Use rubber bands to tightly wrap the two ends of the adsorption membrane package with the beaker wall. At this time, the dialysis membrane clamp can be replaced to prevent leakage. Stir at 100 rpm for 15 minutes at 50°C, and the adsorption reaction of the adsorption membrane package is completed. Take out the adsorption membrane package and measure the concentration of the humic acid solution outside the membrane package. According to the molecular weight grade of the adsorption membrane package from small to large, put in the adsorption membrane package of the next molecular weight grade. After the reaction of 5 adsorption membrane packages is completed, the classification is completed.

[0051] 2.3 Molecular weight and distribution determination of humic acid

[0052] After the adsorption reaction of each adsorption membrane package is completed, the humic acid solution outside the membrane package is taken and the concentration of humic acid is measured by ultraviolet spectrophotometry at 254nm, and the concentration of humic acid of different molecular weights in the membrane package is determined by subtraction. Humic acid is graded into 6 components according to the molecular weight of the dialysis bag, and the content percentage distribution of different components is shown in Table 2.

[0053] Table 2 Content distribution of each fraction (wt%)

[0054]

[0055] Example 3

[0056] 3.1 Material pretreatment

[0057] (1) Pretreatment of humic acid

[0058] Add 200 mg of mineral humic acid into a 500 ml volumetric flask, add 1 M NaOH solution dropwise until the humic acid is completely dissolved, and dilute to 500 ml with ultrapure water to obtain a humic acid solution. The pH of the humic acid solution is about 12.

[0059] (2) Pretreatment of dialysis bags

[0060] Cut the regenerated cellulose dialysis bag into 20cm pieces, wet it in ultrapure water, take it out after three minutes, clamp one end with a clip, and open the other end for later use. The molecular weight grades of the five dialysis bags for pretreatment are 1kDa, 5kDa, 10kDa, 50kDa and 100kDa respectively.

[0061] (3) Preparation of humic acid adsorbent

[0062] Graphene oxide and biochar nanoparticles were mixed in a mass ratio of 2:1 to obtain a humic acid adsorbent.

[0063] (4) Preparation of adsorption membrane package

[0064] Take 25 g of the humic acid adsorbent prepared in step (3) and put it into the dialysis bags of five molecular weight grades prepared in step (2), with 5 g in each dialysis bag, to obtain five adsorption membrane packages of different molecular weight grades.

[0065] 3.2 Adsorption classification of humic acid

[0066] Take the humic acid solution prepared in step (1), put it into a beaker, add the adsorption membrane package with the smallest molecular weight grade prepared in step (4), put the center part of the adsorption membrane package into the humic acid solution, and let the two ends extend beyond the outer wall of the beaker and fold back. Use rubber bands to tightly wrap the two ends of the adsorption membrane package with the beaker wall. At this time, the dialysis membrane clamp can be replaced to prevent leakage. Stir at 100 rpm for 15 minutes at 50°C, and the adsorption reaction of the adsorption membrane package is completed. Take out the adsorption membrane package and measure the concentration of the humic acid solution outside the membrane package. According to the molecular weight grade of the adsorption membrane package from small to large, put in the adsorption membrane package of the next molecular weight grade. After the reaction of 5 adsorption membrane packages is completed, the classification is completed.

[0067] 3.3 Molecular weight and distribution determination of humic acid

[0068] After the adsorption reaction of each adsorption membrane package is completed, the humic acid solution outside the membrane package is taken and the concentration of humic acid is measured by ultraviolet spectrophotometry at 254nm, and the concentration of humic acid of different molecular weights in the membrane package is determined by subtraction. Humic acid is graded into 6 components according to the molecular weight of the dialysis bag, and the content percentage distribution of different components is shown in Table 3.

[0069] Table 3 Content distribution of each fraction (wt%)

[0070]

[0071] Comparative Example 1

[0072] The humic acid adsorbent used in this comparative example 1 is graphene oxide.

[0073] Comparative Example 2

[0074] The humic acid adsorbent used in this comparative example 2 is biochar nanoparticles.

[0075] Comparative Example 3

[0076] The humic acid adsorbent used in this comparative example 3 is nano-ferroferric oxide, which has a particle size of 10-20 nm, a molecular weight of 100 kDa-200 kDa, and an isoelectric point pH of 6.2.

[0077] Test example

[0078] Take the humic acid adsorbents in Example 1 and Comparative Examples 1 to 3 and operate according to the steps in Example 1.

[0079] When preparing the adsorption membrane package, 5 g of each of the humic acid adsorbents in Example 1 and Comparative Examples 1 to 3 were taken and placed in dialysis bags with a molecular weight grade of 1 kDa, respectively, to obtain adsorption membrane packages for standby use.

[0080] Four groups of humic acid solutions prepared in step (1) of Example 1 were prepared, three in each group, and the above-mentioned adsorption membrane packages were added thereto respectively. The mixture was stirred at 100 rpm at a temperature of 50°C. The concentration of the humic acid solution outside the membrane package was measured every minute. When the concentration was constant, the experiment was terminated and the required time was counted.

[0081] According to statistics, the time required for constant concentration in Example 1 and Comparative Examples 1-3 is 12±0.3min, 19±0.5min, 22±0.4min and 28±0.9min respectively. It can be seen that the method of the present invention can achieve humic acid adsorption faster, and improve the efficiency of humic acid classification and molecular weight measurement. The data of Examples 1-3 are consistent with previous studies in the field, such as "Ultrafiltration classification humic acid structural characteristics and its regulation of phosphorus effectiveness", which shows that this method improves efficiency while ensuring the accuracy of measurement results.

[0082] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining the molecular weight distribution of humic acid in fertilizers, characterized in that: The following steps are involved: (1) The humic acid adsorbent is placed in dialysis bags of different molecular weights, and the two ends are tightened to prepare adsorption membrane packages of different molecular weight grades; (2) dissolving humic acid in an alkaline solution to obtain a humic acid solution; placing the adsorption membrane packages obtained in step (1) into the humic acid solution in order of molecular weight of the dialysis bags from small to large, stirring, and performing an adsorption reaction; (3) After the adsorption reaction is completed, the adsorption membrane package is taken out and the concentration of the humic acid solution is measured; the humic acid in the membrane package is a narrow molecular weight humic acid, and the molecular weight distribution of the humic acid is finally obtained by using the subtraction method; The humic acid adsorbent comprises graphene oxide and biochar nanoparticles.

2. The method for determining the molecular weight distribution of humic acid in fertilizer according to claim 1, characterized in that: The thickness of graphene oxide is 0.8-2nm, the sheet diameter is 1-20μm, and the monolayer rate is 99%; the particle size of biochar nanoparticles is 1μm.

3. The method for determining the molecular weight distribution of humic acid in fertilizer according to claim 1, characterized in that: The mass ratio of graphene oxide and biochar nanoparticles is (1-2):(1-2).

4. The method for determining the molecular weight distribution of humic acid in fertilizer according to claim 1, characterized in that: In step (1), the molecular weight of the dialysis bag is 1k-100kDa.

5. The method for determining the molecular weight distribution of humic acid in fertilizer according to claim 1, characterized in that: In step (2), the pH of the humic acid solution is 8.5-12.

6. The method for determining the molecular weight distribution of humic acid in fertilizer according to claim 1, characterized in that: In step (2), the adsorption reaction temperature is 40-60° C., the stirring speed is 60-100 rpm, and the adsorption time is 5-30 min.

7. The method for determining the molecular weight distribution of humic acid in fertilizer according to claim 1, characterized in that: In step (3), the absorbance of humic acid is measured at 254 nm, and the concentration of humic acid in the membrane package is determined by subtraction method.

Images