Advancements in Biodegradation of Polylactic Acid Plastics

Plastics have become an integral part of our daily lives, providing versatility and convenience. However, their improper disposal and slow degradation pose significant environmental challenges. Polylactic acid (PLA) plastics are a promising alternative due to their biodegradability. In recent years, extensive research efforts have been devoted to understanding and enhancing the biodegradability of PLA plastics.

Overview of PLA Plastics

PLA is a new type of polyester material polymerized with the organic acid lactic acid, which is linked by ester bonds. Lactic acid (2-hydroxypropionic acid, lactic acid) is widely found in nature and has two stereoisomers. PLA can be categorized into three types according to the cyclicity of the lactide intermediate: levulinic poly(lactic acid) (poly L-lactide, PLLA), dextrinic poly(lactic acid) (poly D-lactide, PDLA), and racemic poly(lactic acid) (poly D, L-lactide, PDLLA).

Fig.1 Stereoisomers of lactic acid monomer, lactide and polylactic acid.^[1]

PLA plastics can be processed like all other thermoplastic polymers into various products by extrusion, injection molding, blow molding, or fiber spinning, and have become the most commonly used biopolymers in many industries, including agriculture, automotive, and packaging.

Below is our product list.

Granules

Catalog	Product Name
PL-PLA-A001	PLA Granule (2-4 mm granule size)
PL-PLA-A002	PLA Granule (3-5 mm granule size)

Powder

Catalog	Product Name
PL-PLA-A003	PLA Powder (10 mesh)
PL-PLA-A004	PLA Powder (16 mesh)
PL-PLA-A005	PLA Powder (30 mesh)
PL-PLA-A006	PLA Powder (50 mesh)
PL-PLA-A007	PLA Powder (60 mesh)
PL-PLA-A008	PLA Powder (100 mesh)
PL-PLA-A009	PLA Powder (200 mesh)
PL-PLA-A010	PLA Powder (250 mesh)
PL-PLA-A011	PLA Powder (300 mesh)
PL-PLA-A012	PLA Powder (350 mesh)
PL-PLA-A013	PLA Powder (500 mesh)

Filaments

Catalog	Product Name
PL-PLA-A014	PLA Filament (1.75 mm)
PL-PLA-A015	PLA Filament (2.85 mm)
PL-PLA-A016	PLA Filament (3 mm)

Understanding the Mechanisms of PLA Plastic Biodegradation

PLA degradation usually involves the breaking of ester bonds. Depending on the type of induced degradation factors, PLA degradation can be categorized into abiotic (i.e., hydrolysis, thermal degradation, oxidation, and photolysis) and biotic (microbial degradation, and enzymatic degradation) processes. In this paper, Alfa Chemistry will focus on reviewing the research progress on microbial and enzymatic degradation of PLA.

Microbial communities and biodegradation

Microorganisms that have been reported to degrade PLA plastics include bacteria, actinomycetes, and fungi, and exist in a variety of natural environments such as compost, freshwater, marine, anaerobic, and soil, and secrete enzymes with different optimal conditions of action. Environmental factors such as differences in humidity, temperature, pH, salinity, oxygen, and nutrient availability have important effects on microbial PLA degradation activity.

• Actinomycetes: Amycolatopsis, Pseudonocardiaceae, Micromonosporaceae and Streptomycetaceae), etc.
• Bacteria: Bacillus, Pseudomonas, Stenotrophomonas and Laceyellai, etc.
• Fungi: Fusarium, Penicillium, Tritirachium, Aspergillus and Thermomyces, etc.

Enzymatic degradation pathway

PLA can undergo ester bond breaking by microorganisms producing specific hydrolytic enzymes, such as esterases, keratases, lipases, and proteases. In recent years, researchers have worked to isolate and purify PLA-degrading enzymes from soil, sludge, and other environments.

Fig.2 3D structure display of various PLA-degrading enzymes.^[2]

The following table shows some typical PLA degrading enzymes.

Enzyme	Name	Representative Species
Protease	Proteinase K	Tritirachium album
	PLAase 3	Amycolatopsis orientalis
	Prot T16-1	Actinomadura keratinilytica T16-1
	LP175	Laceyella sacchari
Esterase	RPA1511	Rhodopseudomonas palustris
	ABO2449	Alcanivorax borkumensis
	PudA	Comamonas acidovorans TB-35
	PlaM4	Bacillus clausii ATCC10987
	ABO 1197	Alcanivorax borkumensis strain ATCC700651
Lipase	PLaA	Paenibacillus amylolyticus
	Lipase PL	Alcaligenes sp.
Cutinase	CLE	Cryptococcus sp. S-2
	Thc Cut1	Thermobifida cellulosylitica
	Thc Cut2	Thermobifida cellulosylitica
	Cut190	Saccharomonospora viridis AHK190

Natural Inspiration for PLA Biodegradation

Microbial in-situ treatment of PLA waste

PLA has a high risk of leaving plastic in the environment at the end of its use in several application scenarios, the most typical of which are waste PLA mulch and fishing gear. In agriculture, PLA can replace traditional, non-biodegradable plastic materials such as PLA mulch and twine.

Bonifer et al^[3] screened a strain from Bacillus pumilus B12 for discarded PLA mulch film, which could degrade high molecular weight PLA film detected L-lactic acid monomers within 48 h and fracture of the surface of the PLA film was observed by scanning electron microscopy. Bacillus pumilus has been shown to have significant adaptability to a variety of environmental stresses, such as intense UV illumination, high concentrations of hydrogen peroxide, and other harsh environments, and could be one of the candidate strains for the in situ treatment of PLA waste films.

Enzymatic depolymerization of PLA waste for recycling

Plastic recycling is a method of collecting plastic waste in a centralized location and then chemically or biologically depolymerizing the plastic macromolecules and recovering the plastic monomers, oligomers, or hydrocarbons that are ultimately used to reproduce polymers. Significant research progress has been made in the chemical depolymerization and recycling of PLA plastics.

The French company Carbios is committed to research on enzymatic recycling of waste plastics. The company is now experimenting with the enzymatic recycling technology C-ZYME^® for PET waste. In addition to research on enzymatic recycling of PET waste plastics, Carbios has also screened PLA plastic degrading microorganisms and announced in 2014 that its proprietary enzyme can degrade 90% of PLA materials in 48 h. Degradation experiments have been conducted on cups, trays, and plastic films^[4], and in 2015, it announced the production of PLA-degrading enzymes in a 300 L reactor.

Fig.3 Biological treatment of PLA waste.^[2]

Advances in research on biodegradation of polylactic acid (PLA) plastics demonstrate the potential for sustainable plastic waste management. Through a combination of understanding the underlying mechanisms, enhancing biodegradation strategies, and drawing inspiration from nature, scientists are revealing new ways to effectively break down PLA plastics. However, challenges associated with scaling up processes and assessing environmental impacts must be addressed. Continued interdisciplinary research and innovation will drive the development of effective and environmentally friendly solutions that contribute to a more sustainable future.

References

1. Murariu M, et al. (2016). "PLA Composites: from Production to Properties." Advanced Drug Delivery Reviews, 107, 17-46.
2. Xie B, et al. (2023). "Synthesis, Biodegradation and Waste Disposal of Polylactic Acid Plastics: A Review." Chinese Journal of Biotechnology, 39(5), 1912-1929.
3. Bonifer KS, et al. (2019). "Bacillus pumilus B12 degrades polylactic acid and degradation is affected by changing nutrient conditions." Frontiers in Microbiology, 10, 2548.
4. Qian BZ, et al. (2019). "Carbios can complete the depolymerization process of PLA within 48 h." Polyester Industry, 28(1), 35.