Plastic materials are widely used in everyday life and their presence is found in fabrics, electronics, hygiene products and packaging. Synthetic plastics are made up of artificial or semi-artificial organic compounds. The properties of mechanical strength, flexibility and durability have made plastics an indispensable material for society. However, they pose several environmental problems:Their transfer to the landfill determines the exhaustion of the same with a consequent increase in disposal costs;Their incineration produces toxic substances and greenhouse gases that contribute to pollution and climate change;The plastic accumulates in the oceans, and the products they generate by fragmentation (microplastics) have been found in many parts of the planet where they have contaminated the food web;Plastics also take hundreds of years to biodegrade, remaining very long in the environment.
The growing demand for plastics together with the long time needed for their biodegradation, underlines the need to reduce the use of plastics and their replacement with degradable bioplastics and produced through sustainable methods. Generally bioplastics can be produced from biomasses such as corn, sugars and potatoes.Bioplastics have several advantages such as a lower carbon footprint, the possibility of greater independence from fossil sources and greater energy efficiency and eco-compatibility. However, at present, they have higher production costs, recycling problems (it is in fact necessary to separate them from traditional plastics so as not to compromise the process) and low mechanical resistance. Last but not least, the use of food resources for the production of bioplastics, poses an important ethical question that needs appropriate assessments.Bioplastics, as part of a circular bioeconomy, can be designed alternatively to totally degrade to CO2 in a few months or years, or to contribute as a CCS technology (Carbon Capture and Storage) by integrating them into non-biodegradable infrastructures such as pipe-based of plastics for municipal waters and sewers, building materials and road surfaces.
Non-biodegradable plastics as CO2 tanks
Non-degradable bioplastics (such as bio PE) could play an important role in the future for the development of sustainable infrastructures that act as CO2 reservoirs. Any legislative recognition could also make these infrastructures suitable for issuing CO2 credits.
Degradable bioplastics can be used to produce objects that totally degrade to minimize their environmental impact. The timing with which the plastics degrade can be designed based on the use of the object. It is also essential that the plastics can totally degrade to CO2 and water in industrial compostors, on the ground and in water without releasing toxic by-products.
Bioplastics production process from biomass
The photosynthesis carried out by plants, cyanobacteria and microalgae determines the reduction of CO2 thanks to solar energy and the formation of a complex of biomolecules that make up biomass. A simple method to integrate this biomass in the petrochemical industry for the production of plastics is to convert it to methane by fermentation. Methane can be used to produce polyhydroxyalkanoates (PHA), lactic acid, ethanol (precursor of bio polyethylene and bio-polyvinyl chloride). This approach has the advantage of minimizing initial and operational costs as the processes involved are relatively simple and inexpensive. The disadvantage is that all the solar and chemical energy to generate the biomolecules is lost in their conversion to methane from which the bioplastic precursor molecules must be synthesized again. Although the procedure is therefore not very efficient in this sense, it represents a simple method to obtain bioplastics.
Bioplastics through refining
A possible alternative and more targeted strategy is to break down the biomass into its biomolecular components through a biorefining approach. Processes based on mechanical cell rupture and hydrothermal liquefaction are under development to allow the release of proteins, lipids, carbohydrates, nucleic acids and cellulose. These materials can then be used to produce the different classes of bioplastics. The success of this approach is conditioned by the achievement of a good cost-benefit ratio, despite the high initial cost required to fractionate the biomass into its components. Cyanobacteria can be used as a resource for the coproduction of polyhydroxyalkanoates (PHA), pigments, methane and fertilizers. It is also possible to enhance food waste by fungal and microalgal hydrolysis with consequent production of plasticizing components and lactic acid. The production of multiple bioplastics starting from the same biomass is fundamental and contributes to leveling the high cost required for fractionation and purification.
Generic engineering for the production of bioplastics
Genetically modified cyanobacteria can already use sunlight to enhance the production of polyhydroxyalkanoates and it is believed that with the advancement of CRISPR (Clustered Regularly Interspaced Palyndromic Repeats) technology it will be possible to optimize the light capture efficiency. Furthermore, the engineering of specific biochemical pathways will allow the production of new precursor molecules that will confer a wide range of physical and chemical properties to future generations of bioplastics.
Mode of biodegradation of biodegradable bioplastics
The introduction into the market of biodegradable biopolymers cannot be separated from accurate analyzes of biodegradability in different environmental conditions, aimed at avoiding further pollution by plastic materials. The biodegradation of biodegradable polymers follows three different steps: biodeterioration, bioframmentation and assimilation and can take place thanks to bacteria, algae and fungi. Their degradation depends on environmental conditions such as temperature, water, oxygen and the chemical conditions of the polymer itself. The basic biodegradation mechanisms are oxidation or hydrolysis by enzymes that improve the hydrophilicity of the plastic material, from which a polymer with a lower molecular weight is obtained, suitable for assimilation by microorganisms.After degradation, the fragmentation causes the breakage of the plastic polymer chains that make their assimilation possible.Different polymers have different biodegradation properties. The bioplastics in polylactate (PL), for example, show a rather slow degradation (up to a year) while those in cellulose acetate take only a few months. Furthermore, the biodegradation of these materials varies according to the environmental sector: they often have high degradability in the soil sector and in composting systems, but low in the case of aquatic compartments. The acidic nature of the environment can also influence biodegradation as the pH changes the rate of hydrolysis and growth of microorganisms. A further factor that influences the biodegradation rate is the flexibility of the polymer chains: the higher it is, the greater the biodegradation as the hydrolysis reactions will proceed at greater speed. Anaerobic conditions, on the other hand, are suitable for the degradation of polylactate and polyhydroxyalkanoates. The energy released by the mineralization process is used by microorganisms. In this case they use an electron acceptor alternative to oxygen, such as the sulphate ion or the nitrate ion. Generally the rates of degradation under anaerobic conditions are slower than aerobic due to the lack of oxygen. However, degradation rates for polylactate and polyhydroxybutyrate (PHB) are higher in anaerobic conditions.
Plastic materials are a peculiar feature of modern societies thanks to their mechanical properties and their use in many sectors of the economy. However, these advantages are largely offset by the negative effects on the environment. In this perspective the need to replace these materials with bioplastics able, depending on the objective, to easily biodegrade or to act as a CO2 reservoir in the infrastructure is inserted. There are different ways of synthesizing precursor molecules from which bioplastics can be obtained, ranging from the simple fermentation of biomass to the use of engineered bacteria. As regards biodegradation capacity, an essential property to avoid further pollution, bioplastics have different biodegradation rates conditioned by environmental factors and the physico-chemical properties of the polymer. In any case, for a totally sustainable industrial development, it will be necessary that the bioplastics designed to biodegrade, totally degrade to CO2 and water without the release of dangerous chemical residues.In light of the above, it is still necessary to combine technological innovations in this field with a legislative apparatus that allows the transition to a circular renewable bioeconomy based on the use of bioplastics.
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