Pig manure aerobic composting method for improving heavy metal passivation and compost humification
By adding iron oxide nanoparticles to pig manure compost, the problems of low degree of compost humification and high activity of heavy metals are solved, and efficient heavy metal passivation and compost humification are achieved, improving the quality of compost products and soil improvement effect.
Patent Information
- Application Number
- CN202510557621.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-29
- Publication Date
- 2025-08-15
AI Technical Summary
In the traditional aerobic compost method, the degree of humification of compost is low, the heavy metal activity is high, and the greenhouse gas emissions are severe during the compost process, which affects the quality of compost products and the soil improvement effect.
Iron oxide nanoparticles (Fe2O3NPs and Fe3O4NPs) with particle sizes of 19-22 nm were added to pig manure and wheat straw compost raw materials, and the initial moisture and C/N ratio were adjusted to 55-65%. Composting was carried out in an aerobic compost reactor to control temperature and humidity, and promoting microbial activity and heavy metal passivation.
Significantly improve the degree of humification of compost, reduce the bioavailability of heavy metals, reduce environmental pollution, improve the stability of compost products and the resource utilization of agricultural waste, and improve soil quality and crop growth.
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Figure CN120483784A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of environmentally friendly agricultural waste treatment, and in particular relates to a pig manure aerobic composting method for improving heavy metal passivation and compost humification. Background Art
[0002] Traditional aerobic composting of pig manure is a common method for treating livestock and poultry manure and is widely used for the resource recovery of agricultural waste. However, many issues remain in this process, including low compost humification, high heavy metal activity, and greenhouse gas emissions during the composting process. These issues seriously affect the quality of the compost product and its soil improvement effects. In particular, pig manure often contains high concentrations of heavy metals such as copper (Cu) and zinc (Zn). These heavy metals are highly bioavailable during the composting process, posing a potential threat to the environment and plant growth.
[0003] With the development of nanotechnology, iron oxide nanomaterials (such as Fe2O3 NPs and Fe3O4 NPs) have been increasingly used in environmental remediation and agriculture due to their large surface area, excellent chemical stability, and strong adsorption capacity. Iron oxide nanoparticles not only promote the degradation of organic matter in compost but also improve the humification of compost by enhancing microbial activity and regulating its physicochemical properties. Iron oxide nanoparticles interact with humic substances (such as humic acid (HA)) in compost, forming a stable structure and enhancing the stability of the compost product. Furthermore, by interacting with heavy metals, they effectively reduce the bioavailability of heavy metals, thereby passivating them in the compost and reducing their toxicity to soil and crops. Despite the broad potential for application of iron oxide nanomaterials in composting, the comprehensive effects of different iron oxide nanoparticle concentrations on compost quality, humification, and heavy metal passivation have not been fully investigated. Most studies focus on a single composting method, lacking in-depth exploration of the optimal iron oxide nanoparticle concentration and addition mechanism. Furthermore, the admixtures commonly used in traditional composting (such as chemical fertilizers and organic fertilizers) have limited effectiveness in improving compost quality and deactivating heavy metals, and long-term use can lead to soil degradation and environmental pollution. Therefore, a new composting method is urgently needed. Summary of the Invention
[0004] The present invention overcomes the deficiencies of the prior art and proposes a method for aerobic composting of pig manure that improves heavy metal passivation and compost humification; it solves the problems of poor quality of aerobic composting of pig manure, high risk of heavy metal pollution, and slow composting process in the prior art.
[0005] The present invention is achieved through the following technical solutions: A method for aerobic composting of pig manure to improve heavy metal passivation and compost humification comprises the following steps: S1. Preparation of compost raw materials: The compost raw materials include pig manure and wheat straw. The initial moisture of the compost raw materials is adjusted to 55-65%, and the C / N ratio is 24-26; S2. Adding iron oxide nanoparticles to the compost raw material, wherein the amount of the iron oxide nanoparticles added is 0.025%-0.1%; the particle size of the iron oxide nanoparticles is 19-22 nm; S3. aerobically composting the compost material to which the iron oxide nanoparticles are added.
[0006] Preferably, the length of the wheat straw is 1.5-2.5 cm.
[0007] Preferably, the iron oxide nanoparticles are Fe2O3NPs or Fe3O4NPs.
[0008] Preferably, the added amount of the iron oxide nanoparticles is 0.025%.
[0009] Preferably, a composting reactor is used for aerobic composting, and the temperature, humidity and oxygen supply in the composting reactor are adjusted according to the composting process.
[0010] More preferably, the composting reactor is a glass bottle reaction device, and the effective capacity of the composting reactor is 3 kg.
[0011] More preferably, three holes are provided on the composting reactor, one of which is connected to an air pump, and the other two holes are used for temperature measurement and air outflow respectively.
[0012] Preferably, the iron oxide nanoparticles have a particle size of 20 nm and a purity greater than 99.90%.
[0013] The beneficial effects of the present invention compared to the prior art are: 1. Improve compost quality: The iron oxide nanoparticles (Fe2O3NPs and Fe3O4NPs) have a particle size of 20 nm and a purity of 99.90%. Different particle sizes exhibit different properties. Therefore, the iron oxide nanomaterial used in this invention can significantly increase the degradation rate of organic matter during composting, prolong the high-temperature phase of the composting process, promote microbial activity, and thus accelerate the humification process of the compost. The addition of iron oxide nanoparticles significantly increases the humic acid (HA) content in the compost product, significantly reduces the fulvic acid (FA) content, significantly improves the humification degree of the compost, and enhances the compost stability.
[0014] 2. Significant heavy metal passivation effect: The addition of iron oxide nanoparticles effectively passivates heavy metals (such as copper (Cu) and zinc (Zn)) in compost. By promoting the conversion of heavy metals from exchangeable forms to more stable forms (such as residual and oxidizable forms), this significantly reduces the bioavailability of heavy metals, minimizing their potential harm to the environment and plants. This approach has important implications for the remediation of heavy metal-contaminated soils.
[0015] 3. Promote microbial activity during composting: Iron oxide nanoparticles can provide a favorable growth environment for microorganisms during the composting process, enhancing their metabolic activity, raising the compost temperature and the rate of organic matter degradation, thereby improving the overall quality of the compost. The high surface area and good bioactivity of iron oxide nanoparticles provide an ideal habitat for microorganisms, promoting faster degradation of organic matter.
[0016] 4. Strong environmental protection and reducing environmental pollution: The iron oxide nanoparticles used in this invention have high chemical stability and are environmentally friendly. They do not produce harmful side effects during the composting process. Compared to traditional composting methods, this invention significantly reduces environmental pollution, particularly in the passivation of heavy metals. Furthermore, the composting process does not produce harmful gases such as greenhouse gases, thereby enhancing the environmental friendliness of the compost product.
[0017] 5. Promote the resource utilization of agricultural waste: This invention uses iron oxide nanoparticles to optimize the pig manure composting process, improving not only the quality of the compost but also promoting the resource utilization of agricultural waste, such as pig manure. The compost product has a higher degree of humification and lower heavy metal content, making it safe for use in soil improvement, fertilizer supply, and crop cultivation, reducing environmental pollution from waste.
[0018] 6. Improve soil quality and crop growth: By improving the quality of compost, this invention provides a new solution for agricultural soil improvement. The high humification level and low heavy metal content of the compost product improve soil structure, enhance soil fertility, and promote crop growth. In particular, the composting method of this invention helps improve soil productivity and ecological benefits in the treatment of organic waste, such as pig manure.
[0019] Through the above technical solution, the present invention not only solves the problems of poor compost quality and high heavy metal activity in traditional composting methods, but also innovatively combines iron oxide nanoparticles with pig manure compost raw materials, significantly improving composting efficiency and the quality of compost products. In particular, the present invention has broad application prospects in the treatment of agricultural waste and soil improvement. By providing an efficient and environmentally friendly composting solution, the present invention can promote the resource utilization of agricultural waste and enhance the sustainability of agricultural production, with significant technical and economic advantages. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Figure 1 The graph shows the effect of iron oxide nanomaterials on aerobic composting parameters, where (a) is temperature, (b) is pH, (c) is conductivity, and (d) is the effect of C / N. Figure 2 The graph shows the effect of iron oxide nanomaterials on the changes of organic matter and Kjeldahl nitrogen in compost; (a) is organic matter, (b) is Kjeldahl nitrogen; Figure 3 The effects of iron oxide nanomaterials on the changes and polymerization degree of humus in compost; Figure 4 The effect of iron oxide nanomaterials on the three-dimensional fluorescence spectrum of composting on the third day; Figure 5 The effect of iron oxide nanomaterials on the three-dimensional fluorescence spectrum of composting on the 36th day; Figure 6 is the change in the fluorescence response percentage Pi,n value; Figure 7 The effects of iron oxide nanomaterials on the proportion of different Zn forms and bioavailability in compost; Figure 8 The effects of iron oxide nanomaterials on the proportion of different Cu forms and bioavailability in compost; Figure 9 This is the correlation analysis between the various forms of heavy metals Cu and Zn and the physical and chemical properties of compost. DETAILED DESCRIPTION
[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail with reference to the embodiments and the accompanying drawings. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. The technical solutions of the present invention will be described in detail below with reference to the embodiments and the accompanying drawings, but the scope of protection is not limited thereto.
[0022] Example 1 This embodiment provides a method for aerobic composting of pig manure to improve heavy metal passivation and compost humification, which specifically includes the following steps: S1. Preparation of compost raw materials: Pig manure and wheat straw were selected as composting materials. The wheat straw was chopped to approximately 2 cm, and the initial moisture content of the compost material was ensured to be 60%, with a C / N ratio of 25. The wet basis ratio of pig manure to wheat straw was 85% to 15%, and the dry basis ratio was approximately 62% to 38%. The initial physical and chemical properties of compost materials are shown in the table below: Iron oxide nanoparticles were added to compost raw materials at different mass percentages of 0.025%, 0.05%, and 0.1%. The iron oxide nanoparticles had a particle size of 20 nm and a purity greater than 99.90%.
[0023] In this embodiment, two types of iron oxide nanoparticles are selected, namely Fe2O3NPs and Fe3O4NPs: A total of seven treatment samples were formed: CK (blank), F2-1 (250 mg / kg Fe2O3NPs), F2-2 (500 mg / kgFe2O3NPs), F2-3 (1000 mg / kg Fe2O3NPs), F3-1 (250 mg / kg Fe3O4NPs), F3-2 (500 mg / kgFe3O4NPs), and F3-3 (1000 mg / kg Fe3O4NPs).
[0024] S2. Composting process: All ingredients were mixed thoroughly in the above proportions to ensure uniform dispersion of the nanoparticles, and then prepared for placement in the compost reactor. The compost reactors were glass bottles (27.5 cm high, 17 cm in diameter) with a working capacity of 3 kg. Each reactor had three ports: one for connecting an air pump to the glass reactor, and the remaining two ports in the bottle cap were for temperature measurement and air flow. During the composting process, maintain a temperature of 50-60°C, humidity at approximately 60%, and adequate oxygen supply. Regularly turn the compost to ensure uniform composting. Record the compost and ambient temperatures at noon each day. To ensure uniform oxygen distribution, manually turn and stir the compost pile weekly. Samples were collected on days 0, 7, 12, 21, and 36 of composting. One sample was air-dried and ground through a 0.1 mm sieve for determination of heavy metal speciation and other indicators, while the other was stored in a refrigerator at 4°C for fresh sample analysis.
[0025] S3. Sample analysis after composting: Measure the temperature, pH value, conductivity and other physical and chemical properties of the compost every day and record the changes during the composting process. (See Figure 1 ).
[0026] After the composting was completed (about 36 days), samples were taken to determine the content of humus (humic acid HA and fulvic acid FA) in the compost.
[0027] The speciation of heavy metals (e.g. copper (Cu) and zinc (Zn)) in compost was analyzed, especially their transformation from exchangeable forms (EXC) to more stable forms (e.g. residual form (RES) and oxidizable form (OXI)).
[0028] Measurement indicators and methods: The pH value was determined by referring to the method of He Shilei et al. (2021). Distilled water and fresh samples were mixed evenly in a ratio of 1:10, shaken in an oscillator for 30 min to ensure sufficient mixing, and measured using a pH meter.
[0029] The electrical conductivity (EC) was measured using a conductivity meter.
[0030] Organic matter (OM) was determined by the muffle furnace ignition method.
[0031] Total Kjeldahl nitrogen (TKN) was determined by the Kjeldahl method according to NY5252012.
[0032] The extraction process of water-soluble organic matter (DOM) was referred to Ren Xiu-na (2022), and the method was as follows: ultrapure water was used as the extraction agent, and the compost sample was mixed with ultrapure water in a ratio of 1:10 (mass / volume) and stirred for 2 h, followed by centrifugal filtration to extract DOM.
[0033] Fluorescence spectrum of DOM: Scanned by F-4600 spectrometer, with excitation Ex (220-450 nm) and emission Em (220-550 nm) for fluorescence spectrum scanning.
[0034] Humic substances (HSs) were extracted as follows: a dry compost sample (60-mesh sieve) was mixed with a solution (1:1 0.1 mol / L sodium hydroxide (pH 13) and 0.1 mol / L sodium pyrophosphate) at 20°C for 2 hours, centrifuged, and the supernatant (HSs) was separated from the residue. The supernatant was adjusted to pH 1-2, incubated in a 70°C water bath for 1.5 hours, and allowed to stand overnight. The following day, the supernatant was filtered and centrifuged to obtain the FA. The precipitate was washed and then dissolved in sodium hydroxide to obtain the HA solution, which was then measured using a total organic carbon analyzer.
[0035] Compost BCR extraction methods were described in Wang et al. (2019) and are as follows: Weak acid extraction (EXC): air-dried, sieved compost samples were mixed with glacial acetic acid solution, shaken for 16 h, and centrifuged. The supernatant was collected for analysis. The remaining solids were washed with ultrapure water, centrifuged, and the supernatant was discarded, retaining the solid residue.
[0036] Reducible state (RED): Mix the solid residue from the previous step with hydroxylamine hydrochloride solution, shake well for 16 hours, centrifuge and filter, and use the supernatant for analysis. Wash the remaining solid as in the previous step and retain the solid residue.
[0037] Oxidizable state (OXI): Add hydrogen peroxide solution to the solid residue from the previous step, cover, and digest at room temperature for 1 hour. Remove from a hot water bath, cool, and then add ammonium acetate. Shake well for 16 hours. Centrifuge and filter, and the supernatant is analyzed. Wash the remaining solid as in the previous step, and retain the solid residue.
[0038] Residue state (RES): Dry the solid residue from step 3 at 60°C to constant weight, grind it, weigh 0.2 g, and add hydrochloric acid, nitric acid, perchloric acid, and hydrofluoric acid to digest it.
[0039] Bioavailability Factor BF (Bioavailability Factor) = (EXC+RED) / (EXC+RED+OXI+RES).
[0040] S4. Conclusion 4.1 Effects of different iron oxide nanomaterials on the maturity of aerobic composting The temperature of the compost pile reflects the maturity of the compost. All treatments showed similar trends, corresponding to the four stages of composting (thermal, thermophilic, cooling, and mature) (Bernal et al. 2009). As shown in Figure 1(a), all treatments experienced a five-day temperature increase, reaching 50°C on the sixth day and entering the thermophilic stage. CK, F2-1, F2-2, F2-3, F3-1, F3-2, and F3-3 maintained temperatures above 50°C for 7, 9, 9, 8, 8, 9, and 8 days, respectively, with peak temperatures reaching 57.2°C, 58.9°C, 59.2°C, 58.7°C, 57.5°C, 57°C, and 57.1°C, respectively. The high temperature duration for each treatment exceeded 5 days, which is beneficial for eliminating pathogens, insect eggs, and weed seeds, a key aspect of ensuring the sanitation and quality of the composting process. Microorganisms use easily degradable organic matter to metabolize, and the heat generated during the process is released into the pile, causing the compost to enter the thermophilic stage.
[0041] The above results show that compared with the control treatment, the addition of iron oxide nanomaterials will prolong the high temperature period of composting to varying degrees. This may be because iron oxide nanomaterials have a high specific surface area, providing a suitable habitat for microbial activities and enhancing microbial metabolic capacity.
[0042] pH is an important factor affecting the degradation of organic matter and the passivation of heavy metals in the composting process. Figure 1 As shown in Figure b, the pH change trends of all composting treatments are similar. In the early stage of composting, as the temperature rises, the compost pH value rises rapidly and reaches a peak level on the 7th day, which is 9.20 (CK), 9.29 (F2-1), 9.21 (F2-2), 9.16 (F2-3), 9.28 (F3-1), 9.23 (F3-2), and 9.2 (F3-3). This may be due to the rapid degradation of nitrogen-containing organic matter by microorganisms under high temperature, resulting in the production and accumulation of a large amount of ammonia. After the peak, the pH shows a downward trend, which is due to the formation of small molecular organic acids and the nitrification of H + After composting, the pH of all treatments was less than 8.65 (8.41-8.63). The increase in pH of the compost products may be due to the large specific surface area of the iron oxide nanomaterials, which may affect the cation exchange process in the compost material and allow protons to be freely exchanged.
[0043] Electrical conductivity (EC) is an important indicator of compost salt content and is crucial for assessing its potential inhibitory effects on plant growth and potential threats to soil health. As shown in Figure 1(c), the EC of all treatments decreased during the composting process. However, the EC of the treatments that added iron oxide nanomaterials after composting was lower than that of the CK treatment. This is likely due to the addition of iron oxide nanomaterials, which stimulated redox reactions, generating more OH-ions and leading to the precipitation of more mineral salts. In this experiment, the final EC of all composting treatments was below 4000 μS / cm, meeting the requirements for compostable products.
[0044] As shown in Figure 1(d), the C / N ratio showed an upward trend during the warming period. This was primarily due to environmental conditions favoring the growth and reproduction of ammonia-oxidizing bacteria, which resulted in the volatilization of large amounts of ammonia, leading to an increase in the C / N ratio. When the compost entered the high-temperature period, thermophilic microorganisms rapidly decomposed carbohydrates, polysaccharides, and other substances, causing a continuous decrease in the compost's organic matter content. Ultimately, the C / N ratio for all compost products remained below 20 (ranging from 14.18 to 15.68), indicating that all compost products had reached maturity.
[0045] Changes in organic matter (OM) content in compost can reflect the quality of organic matter mineralization and nutrient conversion during the composting process. As shown in Figure 2a, OM content in each treatment exhibited the same trend: a rapid decrease during the thermophilic period, followed by a decrease in compost temperature, followed by a slow decrease until reaching stability at the end of composting. OM loss is primarily due to microbial decomposition of organic matter as an energy source and the release of CO2. The OM decline was most pronounced between days 7 and 12, when the compost temperature was highest, indicating that thermophilic microorganisms were rapidly decomposing organic matter and generating heat. The decline then leveled off, likely due to a decrease in microbial activity. At the end of composting, OM content in each treatment decreased by 28.06%, 33.69%, 31.64%, 31.00%, 34.33%, 33.54%, and 30.34%, respectively. The degradation efficiency of each treatment was higher than that of the control (CK), indicating that the addition of different concentrations of iron oxide nanomaterials promoted the degradation of organic matter in the compost, with the most significant effect observed at a concentration of 0.025%.
[0046] As shown in Figure 2(b), total Kjeldahl nitrogen (TKN) showed a downward trend in the early stages of composting, likely due to the release of NH3 during this period, while the higher temperature and pH conditions were not conducive to nitrification. However, as composting progressed, TKN values continued to increase, likely due to a combination of microbial degradation of nitrogen-containing organic matter, reduced ammonia volatilization, and the corresponding concentration effect. At the end of the composting process, the TKN contents of the various treatments were 21.43 g / kg, 22.09 g / kg, 21.68 g / kg, 20.75 g / kg, 22.02 g / kg, 21.82 g / kg, and 21.51 g / kg, respectively, showing no significant differences. These results indicate that the addition of iron oxide nanomaterials has no significant effect on nitrogen retention in aerobic composting.
[0047] 4.2 Effects of different iron oxide nanomaterials on the humification degree of aerobic composting Humic substances in compost mainly include humic acid (HA) and fulvic acid (FA). HA is a structurally stable macromolecular compound, while FA is a small molecule compound with high activity. The content of FA and HA during composting is also used as one of the indicators for evaluating the quality of compost products. The increase of HA not only improves the stability of compost products, but also promotes crop growth by improving the available nutrients for plants in the soil. The changes of HA and FA in this embodiment are as follows: Figure 3As shown, the FA content showed an overall downward trend. This phenomenon is attributed to the fact that during the composting process, microorganisms utilize easily degradable small-molecule FA as a carbon source, generating the energy needed to sustain their life activities. After composting, the FA content of each treatment ranged from 15.86 g / kg to 19.04 g / kg. The FA degradation rates for CK, F2-1, F2-2, F2-3, F3-1, F3-2, and F3-3 were 33.52%, 46.18%, 37.04%, 42.79%, 42.99%, 50.60%, and 43.30%, respectively. These results indicate that the addition of iron oxide nanomaterials optimizes aerobic composting conditions (temperature, microbial activity, etc.), thereby promoting the decomposition of soluble organic matter.
[0048] In contrast to the FA content, HA content increased continuously as the composting process progressed, with the growth rate after 12 days accounting for 66.30%-80.50% of the total change. This indicates that the increase in HA content primarily occurred during the high-temperature and cooling phases, during which high microbial activity continuously decomposed FA and other non-humic substances and aggregated them into HA. At the end of composting, the HA content of each treatment was 32.18 g / kg, 38.34 g / kg, 30.74 g / kg, 30 g / kg, 38.49 g / kg, 35.16 g / kg, and 34.40 g / kg, respectively. With the exception of treatments F2-2 and F2-3, HA content increased by 19.14% (F2-1), 19.61% (F3-1), 9.26% (F3-2), and 6.90% (F3-3) compared to the control, respectively. This is primarily because HSs contain abundant oxygen-containing functional groups, such as carboxyl and quinone groups. These groups ionize into negative ions and readily bind to positively charged metal ions through ion adsorption or chemical interactions, forming a metal layer on their surface that protects them from microbial degradation. Results indicate that the addition of low concentrations (0.025%) of Fe2O3 and Fe3O4 NPs significantly promotes the polymerization and retention of HA in aerobic composting.
[0049] When HA / FA is greater than 1.60, the compost product has reached the maturity standard. After the end of this experiment, the HA / FA of all treatments from small to large were 1.69 (CK), 1.71 (F2-2), 1.89 (F2-3), 1.94 (F3-3), 2.11 (F3-2), 2.14 (F2-1), and 2.42 (F3-1), all greater than 1.60. The treatments with added iron oxide nanomaterials were all higher than the control treatment, and the treatment with low concentration of iron oxide nanomaterials had the highest HA / FA. The addition of iron oxide nanomaterials improved the humification degree of the compost product.
[0050] EEM is commonly used as one of the methods to evaluate the maturity of compost products, where the excitation / emission wavelengths (Ex / Em) of 200-250 / 280-330, 200-250 / 330-380, 200-250 / 380-550, 250-340 / 280-380 and 250-400 / 380-550 nm correspond to region I (aromatic protein I), region II (aromatic protein II), region III (fulvic acid (FA) compounds), region IV (soluble microbial product compounds) and region V (related to humic acid (HA) compounds), respectively. Figure 4 shows the fluorescence intensity of the area corresponding to the aromatic protein on the third day of composting. As the composting process progresses, the fluorescence intensity of areas I and II corresponding to the aromatic protein gradually disappears, indicating that the aromatic protein is decomposed during the composting process. Except for the F2-3 treatment, peaks of different fluorescence intensities appeared in area IV, indicating the presence of soluble microbial products in the compost. The fluorescence intensity of area III in the F2-1 and F3-2 treatments is significantly stronger than that in other treatments, corresponding to the FA content.
[0051] After the composting was completed, the EEM of each treatment was shown in Figure 5. According to the corresponding areas of excitation wavelength / emission wavelength (Ex / Em), it can be seen that as the composting progressed, the fluorescence intensity of the corresponding areas of fulvic acid compounds and humic acid compounds increased. Compared with the CK treatment, obvious fluorescence intensity peaks appeared in area V of treatments F2-1, F3-1, F3-2, and F3-3, indicating that the addition of Fe2O3NPs and Fe3O4NPs promoted the synthesis of HA in the compost, corresponding to the HA content of each treatment.
[0052] like Figure 6As shown in the figure, the cumulative fluorescence response percentage (Pi,n) corresponding to each area was obtained by referring to the fluorescence regional integral (FRI) method. By comparing the cumulative fluorescence response percentages of each treatment before and after composting, it was found that the treatment areas I, II and IV showed different degrees of reduction (7.62%-18.86%) after the end of composting. Among them, the degradation rates of areas I, II and IV treated with low concentrations of Fe2O3NPs and Fe3O4NPs were 16.28% and 18.86%, respectively, which were higher than those of other addition amounts, indicating that the addition of low doses of Fe2O3NPs and Fe3O4NPs played the greatest role in promoting the degradation of simple aromatic proteins and soluble microbial products during composting. After the composting was completed, the cumulative fluorescence response percentages of each treatment V area were 9.92%, 20.64%, 16.10%, 14.01%, 20.83%, 19.65% and 17.82%, respectively, which indicated that the addition of Fe2O3NPs and Fe3O4NPs could enhance the transformation of compost humus from unstable organic matter structure to stable structure (such as HA).
[0053] 4.3 Effects of different iron oxide nanomaterials on heavy metal passivation in aerobic composting Heavy metals in pig manure are highly reactive, posing significant environmental risks to their utilization. In this experiment, the bioavailability of Zn in the initial pig manure was high, reaching 80.77%. The active chemical form is crucial in determining the actual hazard of heavy metals. As shown in Figure 7a, the bioavailability of Zn decreased in all treatments after aerobic composting. The proportion of EXC-Zn (exchangeable form) decreased significantly after composting, decreasing by 17.84% (CK), 23.86% (F2-1), 18.41% (F2-2), 18.86% (F2-3), 18.16% (F3-1), 19.27% (F3-2), and 20.48% (F3-3), respectively, with the most significant decrease in F2-1. The residual Zn content remained relatively stable during the composting process, with no significant fluctuations. The proportion of less active OXI-Zn (oxidizable form) increased after aerobic composting, increasing by 4.52%, 12.72%, 14.39%, 9.87%, 11.82%, 15.72%, and 7.21%, respectively. The bioavailability of Zn in each treatment, as shown in Figure 7b, showed an overall downward trend as the composting process progressed. The initial decrease and subsequent increase in Zn bioavailability during composting may be due to the chemical additive's ability to adsorb heavy metals through strong electrostatic forces and ion exchange, but the binding is not strong, making it difficult to maintain long-term heavy metal passivation stability. After the experiment, the bioavailability of Zn in each treatment decreased by 7.02%, 15.98%, 18.65%, 13.73%, 16.12%, 20.91%, and 11.38%, respectively. These results indicate that the addition of iron oxide nanomaterials promotes the conversion of exchangeable to oxidizable Zn, enhancing Zn passivation.
[0054] In this experiment, the initial bioavailability of Cu in pig manure was 47.36%, which was less than that of Zn, and decreased after the aerobic composting process. The changes in the proportion of each form during the composting process are shown in Figure 8a. Although the proportion of EXC-Cu (exchangeable state) increased slightly after the end of composting, the proportion of RED-Cu (reducible state) decreased by 14.50% (CK), 17.47% (F2-1), 18.03% (F2-2), 18.43% (F2-3), 12.26% (F3-1), 20.18% (F3-2), and 18.21% (F3-3) after the end of composting compared with the beginning of composting. As the composting process progressed, the proportion of RES-Cu continued to increase, and the RES-Cu of each treatment accounted for 13.47%, 12.79%, 12.80%, 12.27%, 15.72%, 17.09% and 13.58%, respectively. It can be seen that compared with Fe2O3NPs, the addition of Fe3O4NPs further promoted the conversion of Cu in pig manure compost to the residual state. After the composting was completed, the bioavailability of Cu decreased by 6.7%, 13.43%, 14.36%, 15.16%, 8.42%, 15.27% and 12.7%, respectively, indicating that the addition of different iron oxide nanomaterials can significantly promote the passivation effect of Cu in the compost products. The action modes may be different. Fe2O3NPs promotes the passivation effect of heavy metal Cu by promoting the degree of humification, allowing humus to complex Cu and thus promote the passivation effect of heavy metal Cu. Fe3O4NPs not only promotes the passivation effect of Cu by promoting the degree of compost humification, but also further promotes the passivation effect of Cu by causing Cu precipitation.
[0055] As shown in Figure 9, correlation analysis results show that the content of compost humus has a significant impact on the speciation of heavy metals Cu and Zn. It can be seen that the HA content is positively correlated with the residual and oxidizable forms of heavy metal Cu, while the FA content is negatively correlated with the residual form of heavy metal Cu. This indicates that a higher degree of humification during the composting process, that is, a higher stable HA content, leads to better passivation of heavy metal Cu. Humic content also has a similar effect on the speciation of heavy metal Zn: HA content is positively correlated with the residual and oxidizable forms of heavy metal Zn, while FA content is negatively correlated with the residual and oxidizable forms of Zn and positively correlated with the weakly acid extractable form. These results indicate that a higher degree of humification promotes the passivation of heavy metals Cu and Zn, and the passivation effect on Cu is superior to that on Zn.
[0056] In summary, (1) compared with the control, the addition of different concentrations of Fe2O3NPs and Fe3O4NPs can prolong the thermophilic stage of aerobic composting and promote the degradation of organic matter during the composting process. The low concentration (0.025%) has the best effect, while all treatments have no significant effect on the retention of nitrogen in aerobic composting. After the end of composting, all treatments reached the maturity requirements. (2) The addition of low doses (0.025%) of Fe2O3NPs and Fe3O4NPs during the composting process has a positive effect on the degradation of simple aromatic proteins and soluble microbial products, promotes the conversion of simple organic matter to stable HA, increases the HA content of compost, and improves the humification degree of pig manure compost. (3) After the aerobic composting process, the proportion of the more stable oxidative and residual states of Cu and Zn increases, reducing the bioavailability. The addition of Fe2O3NPs and Fe3O4NPs has different degrees of promotion on the conversion of Cu and Zn from active metals to more stable forms in aerobic composting.
[0057] The addition of iron oxide nanoparticles not only accelerates the degradation of organic matter during composting and improves the humification of the compost, but also significantly reduces the bioavailability of heavy metals in the compost, minimizing their potential environmental hazards. This composting method effectively enhances the resource utilization of agricultural waste, such as pig manure.
Claims
1. A method for aerobic composting of pig manure to improve heavy metal passivation and compost humification, characterized in that: The following steps are involved: S1. Preparation of compost raw materials: The compost raw materials include pig manure and wheat straw. The initial moisture of the compost raw materials is adjusted to 55-65%, and the C / N ratio is 24-26; S2. Adding iron oxide nanoparticles to the compost raw material, wherein the amount of the iron oxide nanoparticles added is 0.025%-0.1%; the particle size of the iron oxide nanoparticles is 19-22 nm; S3. aerobically composting the compost material to which the iron oxide nanoparticles are added.
2. The method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 1, characterized in that: The length of wheat straw is 1.5-2.5 cm.
3. The method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 1, characterized in that: The iron oxide nanoparticles are Fe2O3 NPs or Fe3O4 NPs.
4. The method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 1, characterized in that: The added amount of the iron oxide nanoparticles is 0.025%.
5. The method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 1, characterized in that: Aerobic composting is carried out using a composting reactor, in which the temperature, humidity and oxygen supply are adjusted according to the composting process.
6. The method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 5, characterized in that: The composting reactor is a glass bottle reaction device, and the effective capacity of the composting reactor is 3 kg.
7. The method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 6, characterized in that: Three holes are set on the composting reactor, one of which is connected to the air pump, and the other two holes are used for temperature measurement and air outflow respectively.
8. A method for aerobic composting of pig manure for improving heavy metal passivation and compost humification according to claim 1, 3 or 4, characterized in that: The iron oxide nanoparticles have a particle size of 20 nm and a purity greater than 99.90%.