Table of Contents:
Current status of plastic pollution
One of the most serious environmental pollution that we face today is plastic pollution which affects our environment and health badly. Plastic is being utilized on a large scale in transport, cables, plastic packaging, telecommunications, clothing, etc. The extensive use of this polymer has led to the accumulation of plastic in our environment. Now this plastic which has been discarded is accumulated and damaging our terrestrial and aquatic biosphere (Richard et al. 2009). Plastics are produced from petroleum in the same way as refined gasoline, with the entire process releasing harmful gases like carbon monoxide, ozone, benzene, hydrogen sulfide and methane into our environment (Goel and Tripathi 2019). Similarly, binning of plastics and plastic products also releases carbon dioxide leading to global wanning. Plastic on degradation releases toxic chemicals like polystyrene and bisphenol A (BPA) which gets mixed with soil and water (Knight 2012) causing pollution and impacting human health. Moreover, additives present in plastics are chemicals and have also been reported to disturb endocrine glands’ functions (Alabi et al. 2019). Ingestion, contact and inhalation of the plastic additives cause serious health issues in human and animals. Besides this, consumption of micro-plastics by marine animals is also detrimental. Marine wildlife is influenced by plastic pollution through entanglement, ingestion and bioaccumulation and changes the integrity and functioning of wildlife habitats.
The plastic debris are classified as mega debris (> 100 mm), macro-debris (> 20 mm diameter), meso-debris (5-20 mm) and micro-debris (< 5 mm) and these debris are estimated to have longevity of thousands years (Barnes et al. 2009). Micro-plastics are the major pollutants deteriorating the ecosystem; they are either produced as design or as result of degradation of macro-plastic. Rising human population and its rising demand for plastics and plastic products has led to its greater accumulation in the environment. Single-use plastic accounts for almost 40% of the overall plastic usage (Worm et al. 2017). According to an estimate, 13 million tons of plastic bags reach ocean, killing 100,000 marine lives (Alabi et al. 2019) and by 2050, oceans will have more plastics than fish in terms of weight. Leaching of plastic additives like coloring materials, additives, and heavy metals through degradation of plastics leads to soil and water contamination and pollution. Therefore, this is high time to reduce plastic usage by avoiding single-use plastics or by raising awareness among the
Figure 1. Global plastic production (source: Statista 2019).
people on hazardous effects of plastic pollution. Moreover, reuse and recycling of plastic material is also needed along with searching alternative biodegradable options for the scientific management of plastic.
Classification of plastics based on biodegradability
Based on biodegradability, the fossil-based and bio-based plastics can be classified into two groups such as non-biodegradable and biodegradable plastic (Table 1). Non-biodegradable plastics are high molecular weight derivative of hydrocarbon and petroleum compound with high stability and do not readily enter into the degradation cycles of the biosphere (Vijaya and Reddy 2008, Ghosh et al.
2013), for example, polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyethylene (PE) and polyurethane (PUR) (Ahmed et al. 2018). Biodegradable plastics depend upon the degree of biodegradability and microbial assimilation (Wackett and Hershberger 2001). This process involves the degradation of complex polymer compound into smaller compounds in the presence of enzymes which are secreted by microorganisms (Artham and Doble 2008), for example, starch (Chattopadhyay et al. 2011), Polyhydroxyalkanoates (Shimao 2001), Polylactic acid (Ikada and Tsuji 2000), Polyethylene succinate (Hoang et al. 2007) and Polycaprolactone (Wu 2005). Biodegradable plastic is further classified into two categories, which is bio-based biodegradable plastic and fossil-based biodegradable plastic.
Bio-based biodegradable plastic
Bio-based biodegradable plastics are derived from renewable resources and are considered as eco-friendly plastics which can be completely degraded biologically after their application (Kale et al. 2007), such as cellulose, starch and starch-based polymers. Starch is the most commonly used bio-based polymer for the production of biodegradable plastics. The starch contains some important properties such as high richness, ready availability, inexpensiveness and biodegradability under certain environmental condition and is utilized frequently to synthesize bio-based biodegradable plastics (Chattopadhyay et al. 2011, Kyrikou and Briassoulis 2007, Nanda et al. 2010). The main constituent of starch is amylopectin and amylase polymers, which makes it a viable substitute. Various microorganisms have been reported to degrade bio-based polymers under both anaerobic and aerobic conditions (Shah et al. 2008), for example, Variovoraxparadoxus, Comamonas sp., Aspergillus fiumigatus, Acidovoraxfaecilis and P lemoignei. There are two types of bio-based biodegradable plastics, viz. Polyhydroxyalkanoates (PHA) and Polylactic acid (PLA) (Elbanna et al. 2004).
Polyhydroxyalkanoates are bio-based biodegradable plastics obtained by bacterial fermentation of sugar and lipids (Shimao 2001). Due to their biodegradability, it can be used in packaging, medical, pharmaceutical industries (Philip et al. 2007), fast food service materials, disposable medical tools, etc. (Flieger et al. 2003). The biodegradation of PHA by microorganisms, i.e., Bacillus, Burkholderia, Nocardiopsis, Cupriavidus, Mycobacterium and Micromycetes, etc. involves both aerobic and anaerobic degradation mechanisms (Boyandin et al. 2013).
3.1.2 Polylactic acid
Polylactic acid is produced from com starch, tapioca roots, or sugarcane. It has been used extensively in medicine because of the ability of the polymer to be incorporated into human and animal bodies (Ikada and Tsuji 2000). It is the most important among the bio-based biodegradable plastics because of its availability, biodegradability, and good mechanical attributes.
Fossil-based biodegradable plastics
Fossil-based biodegradable plastics are mainly used in the packaging industry, and these are non- biodegradable plastics which cause a serious problem to the environment (Hoshino et al. 2003, Vert et al. 2002). Its degradation takes more time, which involves various microbes and enzymes under different environmental conditions (Chen and Patel 2011, Shah et al. 2008, Chen 2010, Mir et al. 2017), for example, enzyme glycosidase (A. flavus) is involved in polycaprolactone (PCL) degradation (Tokiwa et al. 2009).
3.2.1 Polyethylene succinate
It is thermoplastic polyesters which are developed by the process of copolymerization of ethylene oxide and succinic anhydride or via ethylene glycol and succinic acid poly-condensation (Hoang et al. 2007). It is used in plastic films production for agriculture sector, paper coating agent, and for the manufacturing of shopping bags. The degradation of this polymer is reported by Pseudomonas sp. AKS2 (Tribedi and Sil 2014), Bacillus and Paenibacillus genera (Tezuka et al. 2004, Tokiwa et al. 2009).
It is a fossil-based biodegradable polymer that can easily be degraded by aerobic and anaerobic microorganisms. It is highly flexible and biodegradable, which is made up with partially crystalline polyester mingled with other copolymers (Wu 2005). The polycaprolactone (PCL) is commonly used in packaging material, biomedical, catheters and blood bags (Wu 2005). It is degraded by the microbial lipases and esterases (Karakus 2016), viz. Rhizopusdelemar C. botulinum (Tokiwa et al. 2009). The Aspergillus sp. ST-01 has also been found efficient in PCL degradation into butyric, succinic, caproic, and valeric acids (Sanchez et al. 2000).