PLA, or polylactic acid, is a biodegradable plastic that can leave behind microplastics if it is not properly composted. Microplastics are small pieces of plastic that are less than 5 mm in size and can pose environmental and health risks. PLA is one of the most widely used bioplastics, accounting for 33% of all bioplastics produced in 2021. It is made from renewable resources such as corn, starch, and sugar cane, and it has applications in agriculture, medicine, packaging, and textile. However, PLA is not completely degradable under natural environmental conditions, especially in aquatic environments, where it can disintegrate into microplastics faster than petroleum-based plastics. In this article, we will explore the synthesis, biodegradability, conversion to microplastics, and toxicity of PLA, and discuss some possible solutions to reduce its environmental impact.
PLA is a polyester that is synthesized from lactic acid, which can be produced by fermenting renewable resources such as bagasse, corn, starch, and food waste. Lactic acid can be polymerized by two main methods: direct condensation and ring-opening polymerization. Direct condensation involves the removal of water molecules from lactic acid molecules to form PLA chains, but this method has low efficiency and produces low molecular weight PLA. Ring-opening polymerization involves the use of a catalyst to open the cyclic ester (lactide) derived from lactic acid and link the monomers into PLA chains. This method has higher efficiency and produces higher molecular weight PLA with better properties.
Biodegradability of PLA
PLA is considered a biodegradable plastic because it can be degraded by microorganisms such as bacteria and fungi into carbon dioxide and water under certain conditions. The biodegradability of PLA depends on several factors, such as the molecular weight, crystallinity, additives, temperature, humidity, pH, and oxygen availability. PLA can be biodegraded in industrial composting facilities, where the temperature is maintained at 60°C and the humidity is above 50%. PLA can also be biodegraded in soil and landfill environments, where the temperature is lower and the humidity is variable, but the degradation rate is slower and may take several months or years. PLA is not biodegradable in marine environments, where the temperature is below 30°C and the salinity is high, and it can persist for decades.
PLA can be converted to microplastics by physical, chemical, and biological processes in the environment. Physical processes include mechanical abrasion, UV radiation, and hydrolysis, which can break down PLA into smaller fragments. Chemical processes include oxidation, acidification, and alkalization, which can alter the molecular structure and properties of PLA. Biological processes include enzymatic hydrolysis and microbial degradation, which can cleave the ester bonds and reduce the molecular weight of PLA. These processes can result in the formation of microplastics, which are defined as plastic particles that are less than 5 mm in size and can be further classified into primary and secondary microplastics. Primary microplastics are intentionally produced or used as microbeads, pellets, or fibers, while secondary microplastics are unintentionally generated from the fragmentation of larger plastic items.
Toxicity of microplastics
Microplastics can pose environmental and health risks due to their persistence, accumulation, and potential to carry pollutants and pathogens. Microplastics can be ingested by aquatic organisms, such as zooplankton, fish, and shellfish, and cause physical damage, inflammation, reduced feeding, and altered behavior. Microplastics can also be transferred along the food chain and reach humans, who may consume seafood contaminated with microplastics. The effects of microplastics on human health are not well understood, but some studies have suggested that microplastics can cause oxidative stress, inflammation, genotoxicity, and endocrine disruption. Moreover, microplastics can act as vectors for persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), which can adsorb onto the surface of microplastics and be released into the organisms that ingest them. Microplastics can also harbor microorganisms, such as bacteria and fungi, that can cause infections or diseases.
There are several possible solutions to reduce the generation and impact of microplastics from PLA and other plastics. These include:
Improving the design and production of PLA to enhance its biodegradability and reduce its fragmentation. For example, adding additives, such as plasticizers, fillers, or nucleating agents, can modify the crystallinity, molecular weight, and mechanical properties of PLA and influence its degradation rate and mode. Another example is embedding enzymes in PLA that can degrade it faster and more completely under specific conditions, such as heat and water.
Increasing the collection and recycling of PLA and other plastics to prevent them from entering the environment. For example, implementing separate waste streams for bioplastics and conventional plastics, and developing efficient sorting and identification methods, can facilitate the recycling process and avoid contamination. Another example is using chemical or enzymatic recycling methods that can depolymerize PLA and other plastics into their monomers or oligomers, which can be reused to produce new plastics.
Promoting the use of alternative materials that are more sustainable and environmentally friendly than PLA and other plastics. For example, using natural materials, such as cellulose, starch, or chitin, that are biodegradable, biocompatible, and abundant, and can be processed into films, fibers, or foams for various applications. Another example is using bio-based materials, such as polyhydroxyalkanoates (PHAs), that are produced by microorganisms from renewable resources, such as sugar, and can be biodegraded in various environments, including marine.
Conclusion
PLA is a biodegradable plastic that can leave behind microplastics if it is not properly composted. Microplastics are small pieces of plastic that are less than 5 mm in size and can pose environmental and health risks. PLA is one of the most widely used bioplastics, accounting for 33% of all bioplastics produced in 2021. It is made from renewable resources such as corn, starch, and sugar cane, and it has applications in agriculture, medicine, packaging, and textile. However, PLA is not completely degradable under natural environmental conditions, especially in aquatic environments, where it can disintegrate into microplastics faster than petroleum-based plastics. To reduce the generation and impact of microplastics from PLA and other plastics, some possible solutions include improving the design and production of PLA, increasing the collection and recycling of PLA and other plastics, and promoting the use of alternative materials that are more sustainable and environmentally friendly than PLA and other plastics.
