Method for rapidly promoting degradation of polylactic acid household compost

Abstract

The invention belongs to the field of polylactic acid household compost, and particularly relates to a method for rapidly promoting degradation of polylactic acid household compost. The method comprises the following steps: S1, putting polylactic acid waste into a composting reactor, controlling the temperature and the relative humidity, introducing oxygen, and then adding a biological stimulant into a compost solid; s2, deionized water is added, the relative humidity is kept to be 50%, and then CO2 and polylactic acid in the composting reactor are taken for testing; s3, comparing the CO2 mineralization rate, molecular weight and crystallinity in the composting reactor before and after composting. PLA is industrially compostable, but its rapid biodegradation depends on whether a thermophilic range temperature (45-60 DEG C) is reached. At the temperature, the material can be hydrolyzed into oligomer as a microbial food source in a short time. The PLA is difficult to degrade at a home compost (20-45 DEG C). The biological stimulant can accelerate compost degradation of polylactic acid under a medium-temperature condition, and breaks through temperature limitation.

Description

Technical Field

[0001] The present invention relates to the field of polylactic acid home composting, and specifically relates to a method for rapidly promoting the degradation of polylactic acid home composting. Background Art

[0002] Polylactic acid (PLA) has attracted interest due to its low environmental footprint, ability to replace traditional polymers, and ability to be processed in industrial composting environments. PLA must be completely exposed to thermophilic conditions for a long time to be biodegradable, which limits its popularization and application, hinders its acceptance in industrial composting facilities, and negates its potential in home composting. Therefore, improving the biodegradation of polylactic acid is the key to expanding its acceptance. It is difficult to degrade PLA at lower temperatures (e.g., backyard or home composting environments - mainly mesophilic environments) because it relies on higher temperatures (about 60 °C, above the glass transition temperature) for chemical hydrolysis. In addition, compared with other more aggressive thermophilic degradation environments such as industrial composting, the degradation rate of PLA in natural environments (usually associated with lower temperatures) is slower because PLA-degrading microorganisms are sparsely distributed.

[0003] This study aims to evaluate the addition of different biostimulant compounds. Biostimulation is a method of addressing limiting factors such as nutrients (in the form of enzyme inducers), electron donor or acceptor compounds, nitrogen donor compounds, and compounds that activate biochemical processes by providing the necessary resources or chemicals to stimulate the environment. These compounds can be added to the composting medium (solid) to accelerate the biodegradation of polylactic acid under mesophilic conditions. The compounds selected as biostimulants are screened through multiple factors and are expected to meet various criteria: non-toxic to the existing microorganisms, non-antibacterial, able to decompose, consumable by microorganisms, biodegradable within an appropriate time, cost-effective, and easily available or accessible. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the object of the present invention can be achieved by the following technical solutions:

[0005] A method for rapidly promoting the degradation of polylactic acid home composting, the method comprising the following steps:

[0006] S1. Place polylactic acid waste in a composting reactor, control the temperature and relative humidity, and introduce oxygen, and then add a biostimulant to the composting solid;

[0007] S2. Add deionized water to maintain the relative humidity at 50%, and then take CO2 and polylactic acid in the composting reactor for testing;

[0008] S3. Compare the CO2 mineralization rate, molecular weight, and crystallinity in the composting reactor before and after composting.

[0009] Furthermore, the biological stimulant is one or more of Fe3O4 nano powder, skim milk, gelatin, ethyl lactate, cellulose, etc.

[0010] Furthermore, the polylactic acid waste needs to be screened using a 10-mm sieve to remove debris or lumps.

[0011] Furthermore, the reaction temperature in the compost reactor is 37 ± 2°C.

[0012] Furthermore, the addition ratio of the polylactic acid waste to the biological stimulant is (1 - 5):1.

[0013] Furthermore, the CO2 is measured and analyzed using a non-dispersive infrared gas analyzer.

[0014] Furthermore, the mineralization is defined as the total amount of carbon converted into CO2 molecules, and its calculation formula is:

[0015]

[0016] The numerator represents the CO2 precipitated in the sample, and its calculation method is to use the average cumulative mass of CO2 precipitated in the bioreactor containing the sample (CO2) t minus the average cumulative mass of CO2 precipitated in the blank (CO2) b . The denominator represents the theoretical amount of carbon dioxide that the sample can produce and is calculated as the product of the total mass of the sample (Mt) and the total carbon content of the sample (Ct) in the bioreactor. 44 is the molecular weight of carbon dioxide, and 12 is the atomic weight of carbon.

[0017] Furthermore, the crystallinity is calculated by Equation (2).

[0018]

[0019] In the formula, ΔHm is the heat of fusion, ΔHc is the cold crystallization enthalpy, and ΔH0 m is the heat of fusion of 100% crystalline pure PLA (93 J / g).

[0020] Advantages of the present invention: PLA is industrially compostable, but its rapid biodegradation depends on reaching the temperature in the thermophilic range (45 - 60°C). At this temperature, it can be hydrolyzed into oligomers in a short time as a food source for microorganisms. It is difficult to degrade PLA under home composting (20 - 45°C). The biological stimulant can accelerate the composting degradation of polylactic acid under mesophilic conditions and break through the temperature limit. Description of the Drawings

[0021] To more clearly illustrate the technical solutions in the embodiments of the present disclosure or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 Shows the reaction situation in the compost reactor of the present invention;

[0023] Figure 2 Shows the change in the molecular weight distribution of pure PLA in the present invention;

[0024] Figure 3 Shows the change in the molecular weight distribution of PLA in the presence of skim milk in the present invention;

[0025] Figure 4 Shows the degradation rate of PLA and PLA treated with skim milk in the present invention. Detailed implementation manners

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0027] Example 1:

[0028] Comparison between blank compost and compost containing PLA

[0029] 400 g of compost was filled in two compost reactors respectively, with a temperature of 37 °C and a relative humidity of 50%. The 1st compost reactor was used as a blank, and 8 g of polylactic acid was added to the 2nd compost reactor. Samples were taken every other month for six consecutive months. The results showed that about 22.5 g of carbon dioxide was produced by the polylactic acid in the compost, while about 26.1 g of carbon dioxide was produced by the particles in the blank compost. This means that the carbon in the polylactic acid was not degraded. The negative mineralization rate indicates that the blank compost reactor produced more CO2 than the compost reactor containing the PLA sample. In the blank bioreactor, microorganisms can easily absorb the organic matter in the compost, while the presence of PLA in the bioreactor reduces the working efficiency, manifested as a decrease in CO2 production. PLA provides an initial physical hydrophobic barrier to water, making it difficult for microorganisms to use it as a carbon source and establish macroscopic colonies to decompose PLA. Due to the low temperature of 37 °C, we did not observe any mineralization rate in the PLA.

[0030] Example 2:

[0031] Effect of Skim Milk on the Degradation of Polylactic Acid

[0032] 400 g of compost was filled in the compost reactor at a temperature of 37 °C and a relative humidity of 50%. 8 g of polylactic acid and 8 g of skim milk were added to the compost reactor. Samples were taken every month for six consecutive months. Skim milk consists of lactose, casein, and whey protein, making it a good precursor for the enzymatic activity of microbial proteases in the compost. The amount of CO2 evolved from the bioreactor containing PLA and skim milk was 42.1 g at the end of the test, and the maximum mineralization rate was 60.1%. This result indicates that positive mineralization was observed compared to PLA alone, suggesting that the enzymatic degradation of PLA was due to the presence of skim milk. The molecular weight and molecular weight distribution of PLA were evaluated during the process until the end of the test, as Figure 2 shows the change in the molecular weight distribution of PLA alone, Figure 3 shows the change in the molecular weight distribution of PLA and skim milk. Figure 2 The peak amplitude of PLA in [reference] remained approximately constant throughout the test. There was no broadening of the peak and the displacement was negligible, indicating that the chemical degradation process was slow at the mesophilic temperature of 37 °C. For PLA biostimulated with skim milk, significant changes and broadening were observed in Figure 3 , depicting a decrease in Mn and rearrangement of the PLA molecular chains. There was a significant difference in the degradation rates of PLA treated alone and with skim milk, as shown by the k values (PLA k = 0.0045, PLA + skim milk k = 0.0053) in Figure 4 . This increase in k can be explained by the acceleration of the biodegradation of polylactic acid by microbial assimilation in the presence of skim milk, ultimately reducing the time to reach Mn ≤ 10 kDa by approximately 75 days. The Xc of PLA increased from 28.3% to 31.4%, and the Xc of the PLA sample stimulated with skim milk increased from 28.3% to 39.5%. The significant difference between the two indicates that the degradation rate of the PLA sample was faster in the presence of skim milk.

[0033] In the description of this specification, the description referring to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials, or characteristics described in connection with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0034] The foregoing has shown and described the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and what is described in the above embodiments and the specification is only to illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements fall within the scope of the present invention claimed.

Claims

1. A method for rapidly promoting the degradation of polylactic acid by household composting, characterized in that: The method comprises the following steps: S1, placing polylactic acid waste in a composting reactor, controlling the temperature and relative humidity, introducing oxygen, and then adding a biostimulant to the compost solids; S2, adding deionized water to maintain the relative humidity at 50%, and then taking the CO2 and polylactic acid in the composting reactor for testing; S3. Compare the CO2 mineralization rate, molecular weight and crystallinity in the composting reactor before and after composting.

2. A method for rapidly promoting the degradation of polylactic acid by home composting according to claim 1, characterized in that: The biostimulant is one or more of Fe3O4 nano powder, skim milk, gelatin, ethyl lactate, cellulose and the like.

3. A method for rapidly promoting the degradation of polylactic acid by home composting according to claim 2, characterized in that: The polylactic acid waste needs to be screened using a 10 mm sieve to remove fragments or lumps.

4. The method for rapidly promoting the degradation of polylactic acid by home composting according to claim 1, characterized in that: The reaction temperature in the composting reactor is 37±2°C.

5. The method for rapidly promoting the degradation of polylactic acid by home composting according to claim 1, characterized in that: The mass ratio of the added amount of the polylactic acid waste and the biostimulant is (1-5):

1.

6. The method for rapidly promoting the degradation of polylactic acid by home composting according to claim 1, characterized in that: The CO2 is measured and analyzed using a non-dispersive infrared gas analyzer.

7. The method for rapidly promoting the degradation of polylactic acid by home composting according to claim 1, characterized in that: The mineralization is defined as the total amount of carbon converted into CO2 molecules, which is calculated as: The numerator represents the CO2 released from the sample and is calculated as the average cumulative mass of CO2 released from the bioreactor containing the sample (CO2) t Subtract the average cumulative mass of CO2 released in the blank (CO2) b The denominator represents the theoretical amount of carbon dioxide that the sample can produce and is calculated as the product of the total mass of the sample in the bioreactor (Mt) and the total carbon content of the sample (Ct); 44 is the molecular weight of carbon dioxide and 12 is the atomic weight of carbon.

8. The method for rapidly promoting the degradation of polylactic acid by household composting according to claim 1, characterized in that: The crystallinity was calculated by formula (2). Where ΔHm is the heat of fusion, ΔHc is the cold crystallization enthalpy, and ΔH0m is the heat of fusion of 100% crystalline pure PLA (93 J / g).

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