Humic acid prepared from kitchen waste and its preparation method

Abstract

This invention discloses a method for preparing humic acid from kitchen waste. The method includes: (1) drying and pulverizing the kitchen waste to obtain kitchen waste powder; (2) mixing the kitchen waste powder with water, adjusting the pH to acidic, adding Fenton's reagent to carry out an oxidation reaction, and obtaining pretreated kitchen waste; (3) subjecting the pretreated kitchen waste to a primary hydrothermal reaction to obtain a primary hydrothermal product; (4) classifying and separating the primary hydrothermal product, either individually or in different combinations, and then carrying out a secondary hydrothermal reaction to obtain different humic acids. This method is not only time-efficient but also significantly improves the yield of humic acid and reduces the yield of solid byproducts. Furthermore, it can obtain humic acids with different structures, broadening the application pathways of humic acid. It has enormous application potential in the field of humic acid production from kitchen waste.

Description

Technical Field

[0001] This invention belongs to the field of waste treatment technology, and specifically relates to a method for preparing humic acid from kitchen waste and the preparation of humic acid with adjustable structure using this method. Background Technology

[0002] Food waste constitutes a significant proportion of solid waste generated by humans. Statistics show that approximately 1.3 billion tons of food waste are produced globally each year, accounting for about one-third of total food production. In my country, food waste accounts for 50-70% of household waste. Food waste has a relatively high water content, resulting in low efficiency for incineration power generation. Furthermore, the incineration process is accompanied by a series of problems, including high-temperature corrosion and dioxin pollution. Therefore, exploring the resource utilization of food waste, while minimizing food waste, is one of the measures for sustainable development.

[0003] The hydrothermal process is an important method for the resource utilization of agricultural solid waste and kitchen waste. Utilizing high-temperature and high-pressure reaction conditions, the hydrothermal process not only kills all pathogens and microorganisms but also produces numerous high-value-added products. Studies have shown that kitchen waste can produce hydrothermal humic acid through high-temperature hydrothermal reactions. Compared to traditional composting, this process can shorten the entire production cycle to a few hours, requires less space, and avoids the generation of unpleasant odors, making it suitable for the rapid resource utilization of kitchen waste in areas where composting is inconvenient. Hydrothermal humic acid produced from kitchen waste using the hydrothermal process is structurally similar to natural humic acid and can replace it in agriculture, environmental remediation, and other fields. Although the hydrothermal process can shorten the synthesis time of hydrothermal humic acid, directly using kitchen waste as a precursor still requires a relatively long reaction time, resulting in low production efficiency and a high yield of final solid waste. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies that use kitchen waste as raw material to prepare humic acid via hydrothermal methods, such as long reaction time, low production efficiency, and high yield of solid waste.

[0005] This invention provides a method for preparing humic acid from kitchen waste, the preparation method comprising:

[0006] S1: Dry the kitchen waste and crush it to obtain kitchen waste powder.

[0007] S2: Mix kitchen waste powder with water to obtain a mixed system, adjust the pH of the mixed system to acidic, add Fenton reagent to carry out an oxidation reaction, and obtain pretreated kitchen waste.

[0008] S3: Pre-treated kitchen waste undergoes a hydrothermal reaction to obtain a primary hydrothermal product.

[0009] S4: Post-process the primary hydrothermal product to obtain an extract, and extract humic acid from the extract; wherein the post-processing includes a secondary hydrothermal reaction to obtain a secondary hydrothermal product; the extraction of humic acid from the extract includes: adjusting the pH of the extract to 11-13, stirring and extracting for 4-6 hours, and then performing solid-liquid separation to obtain a humic acid solution and an extracted solid phase; adjusting the pH of the humic acid solution to 1-3 to produce a precipitate, and separating the precipitate to obtain solid humic acid.

[0010] The above scheme involves pretreating kitchen waste with Fenton oxidation before hydrothermal reaction, combining the two processes. The Fenton oxidation pretreatment shortens the hydrothermal reaction time, thus improving its efficiency. Furthermore, different reaction products obtained from the first hydrothermal reaction can be further processed, including undergoing a second hydrothermal reaction. Depending on the post-processing steps, humic acid products with varying structures can be obtained.

[0011] According to another specific embodiment of the present invention, in the method for preparing humic acid disclosed in the present invention, in step S2, the pH value of the mixed system is adjusted to 4; the amount of Fenton's reagent added is: the mass ratio of H2O2 to kitchen waste powder is 1-4:20, and Fe... 2+ The molar ratio of H2O2 to H2O2 is 1:7.5; the oxidation reaction time is 1-1.5 h.

[0012] Using the above method, the Fenton oxidation reaction is more effective and can significantly shorten the hydrothermal reaction time.

[0013] According to another specific embodiment of the present invention, the method for preparing humic acid disclosed in the present invention has the following dosage of Fenton's reagent: the mass ratio of H2O2 to kitchen waste powder is 1:10; and the oxidation reaction time is 1 hour.

[0014] Using the above scheme, the Fenton oxidation reaction is most effective and can also significantly improve the yield of the hydrothermal reaction.

[0015] According to another specific embodiment of the present invention, in the method for preparing humic acid disclosed in the present invention, in step S3, the reaction temperature of the first hydrothermal reaction is 180°C and the reaction time is 3h; in step S4, the reaction temperature of the second hydrothermal reaction is 180°C and the reaction time is 3h.

[0016] By adopting the above scheme, the efficiency of the hydrothermal reaction is improved due to the Fenton oxidation pretreatment. The temperature of the primary and secondary hydrothermal reactions is 180℃, which allows for the rapid acquisition of high-yield humic acid at a relatively low temperature, thus reducing energy consumption.

[0017] According to another specific embodiment of the present invention, in step S4 of the method for preparing humic acid disclosed in the present invention, the pH value of the extract is adjusted to 12 using KOH, and the extraction is carried out by stirring for 5 hours; the pH value of the humic acid solution is adjusted to 2.

[0018] The above method is beneficial to the stability of iron ions and allows them to enter the final humic acid, thereby ensuring that the final humic acid product contains a certain amount of iron and enhancing the efficacy of the humic acid.

[0019] According to another specific embodiment of the present invention, the method for preparing humic acid disclosed in the present invention includes step S4, which includes: S41: performing solid-liquid separation on the first hydrothermal product to obtain a first post-reaction solid phase and a first hydrothermal liquid; adding water to the first post-reaction solid phase and mixing it to obtain a first extract, extracting humic acid from the first extract to obtain a first humic acid and a first post-extraction solid phase.

[0020] S42: Dry and pulverize the solid phase after the first extraction to obtain the solid phase powder after the first extraction; add alkali to the solid phase powder after the first extraction to carry out a secondary hydrothermal reaction to obtain the secondary hydrothermal product.

[0021] Using the above scheme, after the primary hydrothermal product is extracted, the remaining solid phase is subjected to a secondary hydrothermal reaction to obtain at least two humic acid products with different structures. This process regulates the structure of the produced humic acid and significantly improves the yield of the humic acid product. Furthermore, the first humic acid has a high molecular weight and high carboxyl C content, making it suitable for soil improvement in arid regions.

[0022] According to another specific embodiment of the present invention, the method for preparing humic acid disclosed in the present invention includes step S42, which includes adding water and alkali to the solid phase powder after the first extraction to carry out a second hydrothermal reaction, and using the second hydrothermal product obtained after the reaction as a second extract to extract humic acid to obtain a second humic acid; and step S4 further includes step S43: adding alkali to the first hydrothermal solution to carry out a second hydrothermal reaction, and using the second hydrothermal product obtained after the reaction as a third extract to extract humic acid to obtain a third humic acid.

[0023] Using the above scheme, three humic acid products with different structures were obtained, which can be used for different purposes and can significantly improve the yield of humic acid; and the third humic acid has a high iron content and the highest degree of aromatization, which can be an ideal precursor for the preparation of high-performance functional carbon materials.

[0024] According to another specific embodiment of the present invention, the method for preparing humic acid disclosed in the present invention includes step S42, which involves adding a first hydrothermal solution and an alkali to the solid phase powder after the first extraction to carry out a second hydrothermal reaction, using the second hydrothermal product obtained after the reaction as a fourth extract, and extracting humic acid from the fourth extract to obtain fourth humic acid.

[0025] Using the above scheme, the solid powder after the first extraction and the first hydrothermal liquid are combined and then subjected to a second hydrothermal reaction and extraction, which saves the amount of acid and alkali used and saves energy consumption; two humic acid products with different structures are obtained, and the fourth humic acid has a lower molecular weight and a high O / C ratio, which can be used as the main raw material for the production of organic liquid fertilizer.

[0026] According to another specific embodiment of the present invention, the method for preparing humic acid disclosed in the present invention includes step S4, which includes: S41′: performing solid-liquid separation on the primary hydrothermal product to obtain a first reaction solid phase and a first hydrothermal liquid; drying and pulverizing the first reaction solid phase to obtain a first reaction solid phase powder; adding alkali to the first reaction solid phase powder to carry out a secondary hydrothermal reaction.

[0027] By employing the above method, the primary hydrothermal product is not extracted but instead undergoes a secondary hydrothermal reaction, significantly reducing the amount of acid and alkali used and energy consumption. The fifth humic acid obtained after the secondary hydrothermal reaction has the highest alkyl carbon content and high degree of esterification, which endows it with hydrophobicity, flexibility, and biodegradability. Attached Figure Description

[0028] Figure 1 This is a flowchart of the preparation of humic acid from kitchen waste in Embodiment 2 of the present invention;

[0029] Figure 2 This is a flowchart of the preparation of humic acid from kitchen waste in Embodiment 3 of the present invention;

[0030] Figure 3 This is a flowchart of the preparation of humic acid from kitchen waste in Embodiment 4 of the present invention;

[0031] Figure 4 This is a flowchart of the preparation of humic acid from kitchen waste in Embodiment 5 of the present invention;

[0032] Figure 5 This is a line graph showing the yield of humic acid prepared in Examples 1, 6, and 7 of this invention and Comparative Examples 1 and 2 at different reaction times;

[0033] Figure 6 This is a bar chart showing the yield of three types of humic acid obtained in Example 2 of the present invention;

[0034] Figure 7 This is a bar chart showing the yields of two humic acids obtained in Example 3 of the present invention;

[0035] Figure 8 This is a bar chart showing the yields of two humic acids obtained in Example 4 of the present invention.

[0036] Figure 9 This is a bar chart showing the yield of the solid waste remaining after preparing humic acid in Examples 1-5 and Comparative Examples 1 and 2 of this invention.

[0037] Figure 10 Examples 1-5 of this invention yielded different humic acids. 13 C10 NMR chemical shift spectrum;

[0038] Figure 11 The bar charts showing the Fe content of different humic acids obtained in Examples 1-5 of this invention are shown. Detailed Implementation

[0039] The hydrothermal process is an important method for the resource utilization of agricultural solid waste and kitchen waste. Kitchen waste can be processed into hydrothermal humic acid through high-temperature hydrothermal reaction. Compared with traditional composting, it can shorten the production cycle, require less space, and avoid the generation of unpleasant odors. It is suitable for the rapid resource utilization of kitchen waste in places where composting is inconvenient. Although the hydrothermal process can shorten the synthesis time of hydrothermal humic acid, the production of hydrothermal humic acid directly using kitchen waste as a precursor still requires a long reaction time, resulting in low production efficiency and high solid waste yield.

[0040] To address the aforementioned problems, this invention discloses a method for preparing humic acid from kitchen waste. The method involves first pre-treating the kitchen waste with Fenton's reagent through oxidation, followed by a hydrothermal reaction to prepare humic acid. By pre-treating the kitchen waste raw material through oxidation, the hydrothermal humification process time is shortened, costs are reduced, and the production efficiency of hydrothermal humic acid is significantly improved. Furthermore, the yield of the final solid waste is relatively low. Therefore, the humic acid preparation method of this invention effectively utilizes kitchen waste.

[0041] The present invention discloses a method for preparing humic acid from kitchen waste, such as... Figures 1-4 The process includes the following steps S1-S4 (where step S4 has different specific procedures in different figures).

[0042] S1: Dry the kitchen waste and crush it to obtain kitchen waste powder.

[0043] Specifically, the kitchen waste of this invention refers to easily perishable waste containing organic matter generated in daily life, including but not limited to discarded vegetable leaves, leftover food, fruit peels, eggshells, tea dregs, bones, etc.; preferably, discarded meat, rice, pasta, vegetables, expired food, food processing waste, etc. The specific operation method for drying and pulverizing kitchen waste is not particularly limited, as long as it can achieve the goal of obtaining dry powder; for example, kitchen waste can be dried at a temperature of 80-105℃ and then pulverized to below 30 mesh for subsequent processing.

[0044] S2: Add a certain amount of water to the kitchen waste powder, mix evenly to obtain a mixed system, adjust the pH value to acidic, preferably 2-4, add a certain amount of Fenton's reagent to carry out an oxidation reaction, and obtain pretreated kitchen waste.

[0045] The purpose of adding water is to form an aqueous solution system to facilitate the oxidation reaction. The amount of water added is only enough to form a reaction system. For example, the amount of water added can be based on a solid-liquid ratio of kitchen waste powder to water of 0.5-1.5g:9ml, preferably 1g / 9ml.

[0046] S3: Pre-treat kitchen waste through a hydrothermal reaction, with stirring during the reaction to obtain a primary hydrothermal product. Preferably, the reaction temperature of the primary hydrothermal reaction is 160-200℃, and the reaction time is 2-7 hours; more preferably, the reaction temperature of the primary hydrothermal reaction is 180℃, and the reaction time is 3 hours.

[0047] The product of the Fenton oxidation pretreatment in step S2, when used as a precursor for the hydrothermal synthesis of hydrothermal humic acid, is more prone to humification, thus improving the efficiency of humic acid production. The inventors believe that the main reason for this effect may be that H2O2 in the Fenton reagent reacts more readily with Fe... 2+Under the influence of ·OH, ·OH radicals are generated. ·OH can non-selectively attack the reactants, breaking them down into smaller molecules, and also enhances the hydrolysis potential of the pre-oxidized products. The main components of kitchen waste are protein, starch, and fat. ·OH can break the 1,4-glycosidic bonds, 1,6-glycosidic bonds, and hydrogen bonds in amylopectin, causing amylopectin to degrade into short-chain structures, disaccharides, and even glucose and fructose. Subsequently, under the conditions of ·OH or subsequent hydrothermal reactions, glucose and fructose undergo dehydration to produce furfural and 5-hydroxymethylfurfural, both of which are important intermediates in the hydrothermal humic acid synthesis process. In addition, ·OH can extract H atoms from the CH bonds of aldose rings to generate carbon-centered radicals. When these radicals are present at the aberrant carbon, they can promote the depolymerization of polysaccharide chains. Proteins are highly sensitive to ·OH-mediated oxidation reactions. ·OH oxidation can induce cross-linking, cleavage, and side-chain carbonylation of protein molecules, thereby promoting polypeptide chain hydrolysis. Polypeptide chains can directly polymerize into hydrothermal humic acid fragments, and can be hydrolyzed into amino acids and amines under ·OH or subsequent hydrothermal reactions. Amino acids and aldoses form Schiff bases via Maillard reactions, which then rearrange and cyclize to generate pyrazine derivatives. The mechanism of ·OH-mediated lipid oxidation involves the cleavage of ester bonds, generating long-chain fatty acids and glycerol. Glycerol is further oxidized to acrolein, which reacts with amino acids via Michael addition reactions to form Schiff bases. These compounds can be used to synthesize heterocyclic pyridine derivatives under high-temperature conditions. Subsequently, these intermediates condense to form aromatic polymers, further forming hydrothermal fulvic acids and even hydrothermal humic acids. In summary, oxidative pretreatment of kitchen waste can break down large molecules into smaller molecules that are dispersed in the liquid phase, accelerating subsequent hydrothermal hydrolysis and hydrothermal polymerization reactions. Furthermore, it can make some insoluble components in kitchen waste more susceptible to hydrolysis at high temperatures, thereby increasing the yield of hydrothermal humic acid synthesis.

[0048] This invention pre-treats kitchen waste with Fenton oxidation before subjecting it to hydrothermal reaction, combining the Fenton oxidation and hydrothermal reactions. The Fenton oxidation pre-treatment shortens the hydrothermal reaction time, thus improving the efficiency of the hydrothermal reaction. Furthermore, due to the increased efficiency of the hydrothermal reaction, high-yield humic acid can be obtained more quickly without requiring particularly high temperatures, reducing energy consumption.

[0049] In one specific embodiment, the pH of the mixing system is adjusted to 4, and the amount of Fenton's reagent added is such that the mass ratio of H2O2 to kitchen waste powder is 1-4:20, preferably 1:10; Fe 2+The molar ratio of H2O2 to 1:5-10, preferably 1:7.5, is used; the oxidation reaction time is 1-1.5 hours, preferably 1 hour. The above ratios were chosen because the inventors found that while adding H2O2 above or below this range shortens the hydrothermal reaction time, it does not significantly improve the yield of humic acid. This may be because a higher H2O2 content leads to over-oxidation of kitchen waste, which has little effect on promoting the efficiency of the subsequent hydrothermal reaction and increases reagent costs; while a lower H2O2 content may result in poor oxidation pretreatment, limiting the hydrothermal humification effect.

[0050] S4: Post-processing the primary hydrothermal product to obtain an extract, and extracting humic acid from the extract; wherein the post-processing includes a secondary hydrothermal reaction to obtain a secondary hydrothermal product; preferably, the reaction temperature of the secondary hydrothermal reaction is 160-200℃, and the reaction time is 2-7 hours; more preferably, the reaction temperature of the secondary hydrothermal reaction is 180℃, and the reaction time is 3 hours. Extraction of humic acid from the extract includes: adjusting the pH of the extract to 11-13, stirring and extracting for 4-6 hours, then performing solid-liquid separation to obtain a humic acid solution and a post-extraction solid phase; adjusting the pH of the humic acid solution to 1-3 to produce a precipitate, and separating the precipitate to obtain solid humic acid.

[0051] Specifically, the different reaction products obtained after the first hydrothermal reaction in step S3 can be subjected to a second hydrothermal reaction for further processing and extraction, depending on the post-processing. Different humic acid products with different structures can be obtained. For example, the humic acid product can be extracted from the product after the first hydrothermal reaction, and the remaining waste can be subjected to a second hydrothermal reaction to extract the humic acid product from the product after the second hydrothermal reaction. Alternatively, a second hydrothermal reaction can be performed directly after the first hydrothermal reaction, followed by extraction of the humic acid product from the product after the second hydrothermal reaction. The alkali used to adjust the pH can be a commonly used alkali in the field, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH); the acid used to adjust the pH can be a commonly used acid in the field, such as hydrochloric acid or sulfuric acid.

[0052] In one specific embodiment, the extraction of humic acid from the extract includes: adjusting the pH of the extract to 12 with KOH, stirring and extracting for 5 hours, and then performing solid-liquid separation to obtain a humic acid solution and an extracted solid phase; adjusting the pH of the humic acid solution to 2 to produce a humic acid precipitate, and separating the precipitate to obtain a solid humic acid.

[0053] By pretreating kitchen waste using the Fenton reaction, iron is introduced into the reaction process, ensuring the presence of iron ions in the reaction system. During extraction, KOH is used to adjust the pH of the extract to 12 and the pH of the humic acid solution to 2, which is beneficial for the stability of iron ions and their entry into the final humic acid. This results in a certain amount of iron in the final humic acid product. Iron ions can activate enzyme systems and electron transport chains, driving core processes in plants such as photosynthesis, respiration, and nitrogen metabolism. Iron deficiency significantly affects plant growth and yield. Therefore, the hydrothermal humic acid produced by this invention also functions as an iron fertilizer, enhancing the effectiveness of humic acid.

[0054] In one specific implementation, such as Figure 1 and Figure 2 As shown, the post-processing in step S4 includes steps S41 and S42. S41: After the hydrothermal reaction is completed, the hydrothermal product undergoes solid-liquid separation to obtain a first post-reaction solid phase and a first hydrothermal liquid; water is added to the first post-reaction solid phase and mixed to obtain a first extract; humic acid is extracted from the first extract to obtain first humic acid and a first post-extraction solid phase. When adjusting the pH after adding water during the extraction of the first post-reaction solid phase, an alkaline solution can be directly added to the post-reaction solid phase. The alkaline solution can be a commonly used alkaline in the art, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH); in one specific embodiment, the alkaline solution is a KOH solution with a concentration of 0.05-0.20 mol / L, preferably 0.10 mol / L.

[0055] S42: Dry and pulverize the solid phase after the first extraction to obtain the solid phase powder after the first extraction; add alkali to the solid phase powder after the first extraction to carry out a secondary hydrothermal reaction to obtain the secondary hydrothermal product. It should be noted that the hydrothermal reaction is carried out in an aqueous solution system. Therefore, in addition to adding alkali, water (which can be just water or an aqueous solution containing other components) needs to be added to the solid phase powder after the first extraction to form an aqueous solution system.

[0056] like Figure 1 As shown, in one specific embodiment, step S42 is S42': the first extracted solid phase is dried and pulverized to obtain a first extracted solid phase powder; water and alkali are added to the first extracted solid phase powder and mixed to carry out a second hydrothermal reaction; the second hydrothermal product obtained after the reaction is used as the second extract; humic acid is extracted from the second extract to obtain the second humic acid. Furthermore, step S4 also includes step S43: alkali is added to the first hydrothermal solution to carry out a second hydrothermal reaction; the second hydrothermal product obtained after the reaction is used as the third extract; humic acid is extracted from the third extract to obtain the third humic acid.

[0057] like Figure 2As shown, in one specific embodiment, step S42 is S42″: the first extracted solid phase is dried and pulverized to obtain the first extracted solid phase powder; the first hydrothermal liquid is added to the first extracted solid phase powder, and alkali is added and mixed to carry out a second hydrothermal reaction. The second hydrothermal product obtained after the reaction is used as the fourth extract. Humic acid is extracted from the fourth extract to obtain the fourth humic acid.

[0058] In one specific implementation, such as Figure 3 As shown, the post-processing includes step S41': solid-liquid separation of the primary hydrothermal product to obtain a first post-reaction solid phase and a first hydrothermal liquid; drying and pulverizing the first post-reaction solid phase to obtain a first post-reaction solid phase powder; adding alkali to the first post-reaction solid phase powder for a secondary hydrothermal reaction. Further, in one specific embodiment, as... Figure 3 As shown, after the first reaction, water and alkali are added to the solid powder to carry out a second hydrothermal reaction, and the second hydrothermal product is obtained as the fifth extract; humic acid is extracted from the fifth extract to obtain the fifth humic acid; at the same time, S4 also includes step S43.

[0059] In one specific implementation, such as Figure 4 As shown, the post-processing includes step S41″: adding alkali to the first hydrothermal product to carry out a second hydrothermal reaction, obtaining a sixth extract after the reaction is completed, extracting humic acid from the sixth extract to obtain sixth humic acid.

[0060] In summary, different reaction products are obtained by classifying and separating the primary hydrothermal products after a single hydrothermal reaction. Further processing of these products, either through secondary hydrothermal reactions or in different combinations, yields humic acid products with varying structures, depending on the post-processing steps. The above examples illustrate how one to three of six different humic acid products can be obtained as needed.

[0061] The present invention will be described in detail below through specific embodiments, wherein the kitchen waste is a mixture of 50 wt% rice, 15 wt% vegetables, and 35 wt% chicken (all wet weight). However, the present invention is not limited to these embodiments.

[0062] Example 1

[0063] The preparation methods for humic acid from kitchen waste include:

[0064] S1: Dry the kitchen waste at 105℃ to constant weight, crush it, and pass it through a 30-mesh sieve to obtain kitchen waste powder.

[0065] S2: Mix 15g of kitchen waste powder with 135ml of water to obtain a mixed system. Adjust the pH of the mixed system to 4 with HCl. Add Fenton's reagent, which includes 5g of 30% H2O2 solution and 1.635g of FeSO4·7H2O solid. Stir the reaction at room temperature for 1 hour to obtain pretreated kitchen waste.

[0066] S3: Transfer the pretreated kitchen waste to a high-pressure reactor and carry out a hydrothermal reaction at 180°C. After reacting for 2h, 3h, 4h, 5h and 6h respectively, a hydrothermal product is obtained.

[0067] S41: Solid-liquid separation is performed on the hydrothermal products to obtain the first post-reaction solid phase and the first hydrothermal liquid; 0.10 mol / L KOH solution is added to the post-reaction solid phase as the alkali-added extract, and the mixture is stirred and extracted for 5 h, followed by solid-liquid separation to obtain the first humic acid solution and the first post-extraction solid phase; the pH of the first humic acid solution is adjusted to 2 with 6 mol / L HCl solution to produce humic acid precipitate, the precipitate is separated and washed multiple times, and dried at 105℃ to obtain the first humic acid, denoted as FHA.

[0068] Example 2

[0069] Using kitchen waste to prepare humic acid, such as Figure 1 As shown, the preparation method is the same as in Example 1, except that the hydrothermal reaction time in step S3 is 3 hours, and the preparation step further includes:

[0070] S42′: The first post-extraction solid phase obtained in step S41 is dried at 105℃ and pulverized to below 100 mesh to obtain the first post-extraction solid phase powder. It is added to a high-pressure reactor, water is added at a solid-liquid mass ratio of 1:9, and then 0.32g KOH is added and mixed. A second hydrothermal reaction is carried out at 180℃ for 3h. The system after the reaction is used as the extraction solution. KOH is added to adjust the pH to 12, and the mixture is stirred and extracted for 5h. Solid-liquid separation is then performed to obtain the second humic acid solution and the second post-extraction solid phase. The pH of the second humic acid solution is adjusted to 2 with 6mol / L HCl solution to produce humic acid precipitate. After separating the precipitate, it is washed multiple times and dried at 105℃ to obtain the second humic acid, denoted as HA-HC.

[0071] S43: The first hydrothermal liquid obtained in step S41 is added to a high-pressure reactor, and 1.6g of KOH is added and mixed. A second hydrothermal reaction is carried out at 180℃ for 3h. The system after the reaction is used as the extract. KOH is added to adjust the pH to 12, and the mixture is stirred and extracted for 5h. Solid-liquid separation is then performed to obtain the third humic acid solution and the third solid phase after extraction. The pH of the third humic acid solution is adjusted to 2 with 6mol / L HCl solution to produce humic acid precipitate. After separating the precipitate, it is washed multiple times and dried at 105℃ to obtain the third humic acid, denoted as HA-L.

[0072] Example 3

[0073] Using kitchen waste to prepare humic acid, such as Figure 2 As shown, the preparation method is the same as in Example 1, except that the hydrothermal reaction time in step S3 is 3 hours, and the preparation step further includes:

[0074] S42″: The first post-extraction solid phase obtained in step S41 is dried at 105℃ and pulverized to below 100 mesh to obtain the first post-extraction solid phase powder. This powder is added to a high-pressure reactor, along with the first hydrothermal liquid obtained in step S41. 3.6g of KOH is then added and mixed. A second hydrothermal reaction is carried out at 180℃ for 3 hours. The resulting system is used as the extraction liquid. KOH is added to adjust the pH to 12, and the mixture is stirred and extracted for 5 hours. Solid-liquid separation is then performed to obtain the fourth humic acid solution and the fourth post-extraction solid phase. The pH of the fourth humic acid solution is adjusted to 2 using 6mol / L HCl solution to produce humic acid precipitate. After separating the precipitate, it is washed multiple times and dried at 105℃ to obtain the fourth humic acid, denoted as HA-(L+HC).

[0075] Example 4

[0076] Using kitchen waste to prepare humic acid, such as Figure 3 As shown, the preparation method is the same as in Example 1, except that the hydrothermal reaction time in step S3 is 3 hours, and step S41 is omitted and replaced with the following steps:

[0077] S41′: Solid-liquid separation of the primary hydrothermal product was performed to obtain the first post-reaction solid phase and the first hydrothermal liquid; the first post-reaction solid phase was dried at 105℃ and pulverized to below 100 mesh to obtain the first post-reaction solid phase powder; the first post-reaction solid phase powder was added to a high-pressure reactor, water was added at a solid-liquid mass ratio of 1 / 9, and 2.0 g KOH was added, and a second hydrothermal reaction was carried out at 180℃ for 3 h; the reaction system was used as the extraction liquid, KOH was added to adjust the pH to 12, and after stirring and extraction for 5 h, solid-liquid separation was performed to obtain the fifth humic acid solution and the fifth post-extraction solid phase; the pH of the fifth humic acid solution was adjusted to 2 with 6 mol / L HCl solution to produce humic acid precipitate, the precipitate was separated and washed multiple times, and dried at 105℃ to obtain the fifth humic acid, denoted as HA-(FHA+HC).

[0078] S43: The first hydrothermal liquid obtained in step S41′ is added to a high-pressure reactor, and 1.6g of KOH is added and mixed. A second hydrothermal reaction is carried out at 180℃ for 3h. After the reaction, KOH is added to adjust the pH to 12, and after stirring and extraction for 5h, solid-liquid separation is performed to obtain the third humic acid solution and the third extracted solid phase. The pH of the third humic acid solution is adjusted to 2 with 6mol / L HCl solution to produce humic acid precipitate. After separating the precipitate, the precipitate is washed multiple times and dried at 105℃ to obtain the third humic acid, denoted as HA-L.

[0079] Example 5

[0080] Using kitchen waste to prepare humic acid, such as Figure 4 As shown, the preparation method is the same as in Example 1, except that the hydrothermal reaction time in step S3 is 3 hours, and step S41 is omitted and replaced with the following steps:

[0081] S41″: 3.6 g of KOH was added to the primary hydrothermal product, and a secondary hydrothermal reaction was carried out at 180 °C for 3 h. The system after the reaction was used as the extract. KOH was added to adjust the pH to 12, and the mixture was stirred for 5 h before solid-liquid separation was performed to obtain the sixth humic acid solution and the sixth extracted solid phase. The pH of the sixth humic acid solution was adjusted to 2 with 6 mol / L HCl solution to produce humic acid precipitate. After separating the precipitate, it was washed multiple times and dried at 105 °C to obtain the sixth humic acid, denoted as HA-(L+FHA+HC).

[0082] Example 6

[0083] Humic acid was prepared using kitchen waste. The preparation method is the same as in Example 1, except that in step S2, when adding Fenton's reagent, 10g of 30% H2O2 solution and 3.27g of FeSO4·7H2O solid were added.

[0084] Example 7

[0085] Humic acid was prepared using kitchen waste. The preparation method is the same as in Example 1, except that in step S2, when adding Fenton's reagent, 2.5g of 30% H2O2 solution and 0.818g of FeSO4·7H2O solid were added.

[0086] Comparative Example 1

[0087] Humic acid was prepared using kitchen waste. The preparation method is the same as in Example 1. The difference is that in the preparation method of Comparative Example 1, 15g of kitchen waste powder was mixed with 135ml of water in step S2 to obtain a mixed system. There was no step of adding Fenton's reagent for oxidation reaction.

[0088] Comparative Example 2

[0089] Humic acid was prepared using kitchen waste. The preparation method was the same as that of Comparative Example 1, except that in the preparation method of Comparative Example 2, a hydrothermal reaction was carried out in step S3 at 200°C.

[0090] Effect evaluation

[0091] 1. Weigh the humic acids prepared in Examples 1-7 and Comparative Examples 1-2 and calculate the humic acid yield; and dry and weigh the remaining solid waste after preparing humic acids in Examples 1-5 and Comparative Example 1 and calculate the waste yield.

[0092] The yields of humic acid prepared by different reaction times in Comparative Examples 1, 6, 7 and Comparative Examples 1 and 2 are shown in the following results. Figure 5 Example 1 ( Figure 5 (1 / 10 H2O2 line), Example 6 ( Figure 5 The yield of humic acid in the 1 / 5H2O2 line reached its highest point when the hydrothermal reaction time was 3 hours, as shown in Example 7. Figure 5 The yield of humic acid in the 1 / 20H2O2 line reached its highest point when the hydrothermal reaction time was 5 hours; while the yield of comparative example 1 (under the same reaction conditions) was the highest. Figure 5 The yield of humic acid in the 0H2O2 line only reached its maximum when the hydrothermal reaction time was 6 hours. Comparative Example 2, with enhanced reaction conditions (temperature increased to 200℃), showed a higher yield. Figure 5 The yield of humic acid in the OH2O2 (200℃ line) reaches its highest level when the hydrothermal reaction time is 5 hours; this indicates that the Fenton oxidation pretreatment of kitchen waste in this invention can significantly shorten the hydrothermal reaction time, and that kitchen waste exhibits better hydrothermal humification ability after Fenton oxidation pretreatment.

[0093] Furthermore, the yield of Example 1 (H2O2 to kitchen waste powder mass ratio of 1:10) after 3 hours of reaction was 18.43±0.21wt%, the yield of Example 6 (H2O2 to kitchen waste powder mass ratio of 1:5) after 3 hours of reaction was 13.57±0.35wt%, the yield of Example 7 (H2O2 to kitchen waste powder mass ratio of 1:20) after 5 hours of reaction was 14.63±0.06wt%, the yield of Comparative Example 1 after 6 hours of reaction was 14.37±0.61wt%, and the yield of Comparative Example 2 after 5 hours of reaction was 15.03±0.40wt%. The humic acid yield in Example 1 was significantly higher than that in Examples 6 and 7. This indicates that excessive or insufficient H2O2 dosage may lead to poor oxidation of kitchen waste and may not achieve the best effect in accelerating the hydrothermal reaction time. Therefore, the optimal dosage of Fenton's reagent is achieved when the mass ratio of H2O2 to kitchen waste powder is 1-4:20.

[0094] Furthermore, the yield of humic acid obtained in Comparative Example 2 (hydrothermal reaction at 200℃) was higher than that in Comparative Example 1 (hydrothermal reaction at 180℃). This indicates that at relatively high temperatures, the macromolecules in the raw materials can exhibit higher hydrolytic activity. Generally, temperatures exceeding 200℃ may adversely affect the activity of humic acid. Therefore, the hydrothermal reaction for preparing humic acid is generally controlled below 200℃. However, even under these conditions, the humic acid yield of Example 1 (18.43±0.21wt%) was still significantly higher than that of Comparative Example 2 (15.03±0.40wt%), where the hydrothermal reaction conditions were altered. This shows that the yield obtained by directly hydrothermally reacting kitchen waste as a precursor cannot reach the humic acid yield achieved by pretreating kitchen waste under optimal conditions (Fenton reaction conditions of Example 1) using Fenton oxidation. Therefore, this invention performs Fenton oxidation pretreatment on kitchen waste, controlling the mass ratio of H2O2 to kitchen waste powder in the Fenton reagent to be 1:10, Fe 2+ The optimal molar ratio of humic acid to H2O2 is 1:7.5, which has the best effect on the subsequent hydrothermal reaction, significantly shortens the hydrothermal reaction time and significantly improves the yield of humic acid.

[0095] The yields of the first humic acid (FHA), the second humic acid (HA-HC), and the third humic acid (HA-L) obtained in the process of Example 2 are as follows: Figure 6 As shown, the values ​​are 18.43±0.21wt%, 13.65±0.16wt%, and 9.28±0.10wt%, respectively, meaning that 41.36±0.47wt% of hydrothermal humic acid can be obtained through Example 2.

[0096] The yields of the first humic acid (FHA) and the fourth humic acid (HA-(L+HC)) obtained in the process of Example 3 are as follows: Figure 7 As shown, the values ​​are 18.43±0.21wt% and 20.00±0.74wt%, respectively. That is, 38.43±0.95wt% hydrothermal humic acid can be obtained through Example 3.

[0097] The yields of the third humic acid (HA-L) and the fifth humic acid (HA-(FHA+HC)) obtained in the process of Example 4 are as follows: Figure 8 As shown, the values ​​were 9.28 ± 0.10 wt% and 27.58 ± 0.26 wt%, respectively. That is, 36.86 ± 0.36 wt% hydrothermal humic acid can be obtained through Example 4.

[0098] In Example 5, only one type of humic acid was obtained, namely sixth humic acid (HA-(L+FHA+HC), with a yield of 27.27±0.98wt).

[0099] The yields of the remaining solid waste after preparing humic acid in Examples 1-5 and Comparative Examples 1 and 2 are as follows: Figure 9 As shown, the values ​​are 21.53±0.60wt%, 3.30±0.32wt%, 2.76±0.56wt%, 6.32±0.41wt%, 9.65±0.45wt%, 24.73±0.35wt%, and 28.50±1.21wt%, respectively.

[0100] In summary, the yields of humic acid in Examples 2-5 were significantly higher than those in Comparative Examples 1 and 2, while the yields of residual solid waste were also significantly lower. Even Example 5, with the lowest yield, achieved a humic acid yield of 27.27 ± 0.98 wt%, far exceeding the yield of Comparative Example 1 (1.9 times), and the yield of residual solid waste was significantly lower than that of Comparative Example 1. In particular, Example 2 achieved a humic acid yield as high as 41.36 ± 0.47 wt%, 2.9 times that of Comparative Example 1, while the yield of residual solid waste was only 3.30 ± 0.32 wt%, 13.3% of that of Comparative Example 1. This indicates that the method for preparing humic acid from kitchen waste presented in this application can significantly improve the resource utilization efficiency of kitchen waste. Furthermore, the hydrothermal reaction times used in Examples 2-5 were the same as those in Comparative Example 1, both being 6 hours, demonstrating that the method provided by this invention has greater advantages in large-scale production.

[0101] Furthermore, the humic acid yields of Examples 2-4 were significantly higher than those of Example 5, while the yield of the remaining solid waste was lower than that of Example 5. This indicates that performing solid-liquid separation on the primary hydrothermal products obtained after the first hydrothermal reaction, followed by secondary hydrothermal reactions on different products obtained from the separation, or performing secondary hydrothermal reactions in different combinations and then processing and extracting them separately, not only yields a variety of humic acid products with different structures, but also significantly improves the humic acid yield and reduces the yield of the remaining solid waste, compared to directly performing secondary hydrothermal reactions on the primary hydrothermal products.

[0102] Furthermore, such as Figure 9 As shown, the residual solid phase yields of Examples 2 and 3 were only 3.30±0.32wt% and 2.76±0.56wt%, respectively. For these two implementation methods, without stringent requirements on the purity and type of humic acid obtained, the secondary hydrothermal products of steps S42 and S42′ can be directly mixed and extracted with the first hydrothermal liquid obtained in step S41 without pH adjustment and solid-liquid separation. This not only simplifies the hydrothermal humic acid extraction process but also reduces the amount of acid and alkali used.

[0103] 2. The molecular weights of the humic acids obtained in Examples 1-5 were determined by gel permeation chromatography, and the results are shown in Table 1.

[0104] Table 1 Molecular weight of different humic acids

[0105]

[0106] According to Table 1, the different types of humic acids obtained after Fenton oxidation pretreatment, first hydrothermal reaction, and second hydrothermal reaction show significant differences in relative molecular weight.

[0107] 3. The content of major elements in different humic acid products obtained in Examples 1-5 was determined by elemental analysis, and the results are shown in Table 2.

[0108] Table 2. Content of major elements in different humic acids (wt%)

[0109]

[0110] According to Table 2, different humic acids showed significant differences in carbon content. The humic acid produced by the secondary hydrothermal reaction (Examples 2-5) showed significantly higher N content and lower C content.

[0111] 4. The humic acid obtained in Examples 1-5 was obtained through... 13 The chemical structures of different humic acid products were determined by C10 NMR characterization, and the results are shown in Table 3. Figure 10 .

[0112] Table 3 Different humic acids 13 C NMR analysis results

[0113]

[0114]

[0115] Note: Aromaticity = 100 × Aromatic C (93-165ppm) / [Aromatic C (93-165ppm) + Aliphatic C (0-93ppm)]; Esterification = 100 × Aliphatic C (0-93ppm) / [Aromatic C (93-165ppm) + Aliphatic C (0-93ppm)].

[0116] According to Table 3 and Figure 10 This reflects the differences in chemical structure among different humic acid products. It can be seen that different secondary hydrothermal pathways can significantly regulate the C structure in the generated humic acid products. The different hydrothermal humic acids generated have different fatty acidity and aromaticity. For example, the third humic acid HA-L has the highest aromaticity and the lowest carboxyl carbon content, while the fifth humic acid HA-(FHA+HC) has the highest fatty acidity.

[0117] 5. The iron (Fe) content was determined by humic acid obtained in Examples 1-5, and the results are as follows: Figure 11 .

[0118] according to Figure 11 Compared to Example 1, Examples 2-5 underwent a second hydrothermal reaction, significantly increasing the Fe content in the hydrothermal humic acid, especially the third humic acid HA-L, which had an Fe content as high as 33.9 mg / g. Although Fe does not directly constitute chlorophyll or protein, it drives core processes such as photosynthesis, respiration, and nitrogen metabolism by activating enzyme systems and electron transport chains. Iron deficiency significantly affects plant growth and yield; therefore, the third humic acid produced in this application also has the function of an iron fertilizer and has promising applications in agricultural production.

[0119] In summary, the method for preparing humic acid from kitchen waste in this invention improves the efficiency of hydrothermal reaction by pretreating the kitchen waste with Fenton oxidation; and after the hydrothermal reaction, the structure of the humic acid product can be controlled by selecting different raw materials for a secondary hydrothermal reaction, thus making the product suitable for different uses.

[0120] The first humic acid FHA and the sixth humic acid HA-(L+FHA+HC) have high molecular weight and high carboxyl C content, which can enhance soil stability, help retain water and fertilizer for a long time, and are suitable for soil improvement in arid areas.

[0121] Secondary humic acid HA-HC and tertiary humic acid HA-(L+HC) have low molecular weight and high O / C ratio, making them easily absorbed by plant roots and stimulating the growth of plant root cells. They can be used as the main raw materials for the production of organic liquid fertilizers.

[0122] The third humic acid HA-L has an iron content as high as 39.9 mg / g, which can not only provide Fe element for plant growth in agriculture, but also has the highest degree of aromatization. With its polycyclic aromatic hydrocarbon skeleton and low defect characteristics, it can become an ideal precursor for the preparation of high-performance functional carbon materials, especially suitable for applications that require high conductivity, high stability and controllable pore structure.

[0123] Fifth humic acid HA-(FHA+HC) has a high alkyl carbon content and high degree of esterification, which endows it with hydrophobicity, flexibility, and biodegradability. The hydrophobic properties of high alkyl carbon content can efficiently adsorb non-polar pollutants such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons, and organochlorine pesticides, and can be used for the adsorption of such pollutants in the environment and the treatment of oily wastewater (e.g., to make hydrophobic sponges or composite membranes). The flexibility of aliphatic chains can improve the elongation at break and impact resistance of polymers (such as PLA and rubber), and replace some petroleum-based plasticizers.

[0124] It should be noted that, in addition to the specific embodiments described above, those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with the embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details are included in the above description, and the invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0125] While the invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A method for preparing humic acid from kitchen waste, characterized in that, Preparation methods include: S1: Dry and crush kitchen waste to obtain kitchen waste powder; S2: The kitchen waste powder is mixed with water to obtain a mixed system. The pH value of the mixed system is adjusted to 4, and Fenton's reagent is added to carry out an oxidation reaction to obtain pretreated kitchen waste. The amount of Fenton's reagent added is: the mass ratio of H2O2 to the mass of the kitchen waste powder is 1-4:20, Fe... 2+ The molar ratio of H2O2 to H2O2 is 1:7.5; the oxidation reaction time is 1-1.5 h. S3: The pretreated kitchen waste undergoes a hydrothermal reaction to obtain a primary hydrothermal product; S41: The hydrothermal product is subjected to solid-liquid separation to obtain a first post-reaction solid phase and a first hydrothermal liquid; water is added to the first post-reaction solid phase and mixed to obtain a first extract; humic acid is extracted from the first extract, including: adjusting the pH of the first extract to 11-13, stirring and extracting for 4-6 hours, and then performing solid-liquid separation to obtain a humic acid solution and a first post-extraction solid phase; adjusting the pH of the humic acid solution to 1-3 to produce a precipitate, and separating the precipitate to obtain the first humic acid; S42: Dry the first extracted solid phase and pulverize it to obtain the first extracted solid phase powder; add alkali to the first extracted solid phase powder to carry out a second hydrothermal reaction to obtain a second hydrothermal product, use the second hydrothermal product as the second extract, and extract humic acid from the second extract to obtain the second humic acid.

2. The method for preparing humic acid as described in claim 1, characterized in that, The dosage of Fenton's reagent is as follows: the mass ratio of H2O2 to the mass of the kitchen waste powder is 1:10; the oxidation reaction time is 1 hour.

3. The method for preparing humic acid as described in claim 1, characterized in that, In step S3, the reaction temperature of the primary hydrothermal reaction is 180°C and the reaction time is 3 hours. In step S42, the reaction temperature of the secondary hydrothermal reaction is 180°C and the reaction time is 3 hours.

4. The method for preparing humic acid as described in claim 1, characterized in that, The method for preparing humic acid also includes step S43: adding alkali to the first hydrothermal liquid to carry out a secondary hydrothermal reaction, extracting the humic acid from the secondary hydrothermal product obtained after the reaction, including adjusting the pH value of the secondary hydrothermal product to 11-13, stirring and extracting for 4-6 hours, and then performing solid-liquid separation to obtain a humic acid solution and a third solid phase after extraction. The pH of the humic acid solution was adjusted to 1-3 to produce a precipitate, which was then separated to obtain the third humic acid.

5. The method for preparing humic acid as described in claim 1, characterized in that, Step S42 also includes, The first hydrothermal liquid and alkali are added to the first extracted solid powder to carry out a second hydrothermal reaction to extract humic acid from the second hydrothermal product obtained after the reaction, including adjusting the pH value of the second hydrothermal product to 11-13, stirring and extracting for 4-6 hours, and then performing solid-liquid separation to obtain humic acid solution and fourth extracted solid phase. The pH of the humic acid solution was adjusted to 1-3 to produce a precipitate, which was then separated to obtain the fourth humic acid.

6. The method for preparing humic acid according to any one of claims 1-5, characterized in that, In the step of extracting humic acid, the pH value was adjusted to 12 using KOH, and the extraction was carried out by stirring for 5 hours; the pH value of the humic acid solution was then adjusted to 2.

7. A method for preparing humic acid from kitchen waste, characterized in that, Preparation methods include: S1: Dry and crush kitchen waste to obtain kitchen waste powder; S2: The kitchen waste powder is mixed with water to obtain a mixed system. The pH value of the mixed system is adjusted to 4, and Fenton's reagent is added to carry out an oxidation reaction to obtain pretreated kitchen waste. The amount of Fenton's reagent added is: the mass ratio of H2O2 to the mass of the kitchen waste powder is 1-4:20, Fe... 2+ The molar ratio of H2O2 to H2O2 is 1:7.5; the oxidation reaction time is 1-1.5 h. S3: The pretreated kitchen waste undergoes a hydrothermal reaction to obtain a primary hydrothermal product; S41′: The first hydrothermal product is subjected to solid-liquid separation to obtain a first post-reaction solid phase and a first hydrothermal liquid; the first post-reaction solid phase is dried and pulverized to obtain a first post-reaction solid phase powder; alkali is added to the first post-reaction solid phase powder to carry out a second hydrothermal reaction to obtain a second hydrothermal product; humic acid is extracted from the second hydrothermal product obtained after the reaction, including adjusting the pH value of the second hydrothermal product to 11-13, stirring and extracting for 4-6 hours, and then performing solid-liquid separation to obtain a humic acid solution and a fifth post-extraction solid phase; the pH value of the humic acid solution is adjusted to 1-3 to produce a precipitate, and the precipitate is separated to obtain the fifth humic acid.

8. The method for preparing humic acid as described in claim 7, characterized in that, The dosage of Fenton's reagent is as follows: the mass ratio of H2O2 to the mass of the kitchen waste powder is 1:10; the oxidation reaction time is 1 hour.

9. The method for preparing humic acid as described in claim 7, characterized in that, In step S3, the reaction temperature of the primary hydrothermal reaction is 180°C and the reaction time is 3 hours. In step S41′, the reaction temperature of the secondary hydrothermal reaction is 180°C and the reaction time is 3 hours.

10. The method for preparing humic acid according to claim 7, characterized in that, In step S41′, the pH of the secondary hydrothermal product was adjusted to 12 using KOH, and the product was extracted by stirring for 5 hours; the pH of the humic acid solution was adjusted to 2.

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