Abstract
Abstract
COVID-19, a disease caused by severe acute respiratory syndrome-corona virus-2 (SARS-CoV-2) has taken the lives of millions of people across the globe. Researchers are working on the development of different aids to cure and prevent the disease. Personal protection equipment (PPE) has become an essential prerequisite for healthcare workers. PPE kits are reported to provide sufficient protection against pathogens but their disposal can be devastating to environment. The proper disposal of PPE is being taken care of by the national authorities according to the guidelines provided by WHO.The plethora of disposed PPE kits will further increase the burden of polymer on our earth.This review presents a strategy to dispose the PPE kits by their conversion to alternative fuel.
Introduction
In the last week of December 2019, the China health authority alarmed the World Health Organization (WHO) regarding numerous cases of pneumonia of unfamiliar etiology, in Wuhan City in Hubei Province of central China [1]. In the first week of January 2020, a new virus was isolated from the throat swab sample of a patient, which was renamed by International Committee on Taxonomy of Viruses as severe acute respiratory syndrome corona virus (SARS-CoV-2) [2]. The disease popularly known as COVID-19 was declared as pandemic by WHO on march 11, 2020. On April 30, 2020, the confirmed cases crossed 3.0 million with a death toll of more than 0.21 million in 213 different countries [3].
Large droplets generated during coughing and sneezing are the prime cause of transmission of infection [4]. These infected droplets can spread to couple of meter distances and get deposited on surfaces. The virus can remain viable on surfaces for days under optimum conditions [5].
In case of Covid-19, therapies are based on supportive care and treatment of symptoms to prevent the additional transmission to other people (family, other contacts, patients, healthcare workers, cleaning staff etc).
World is still under progression to find an approved treatment to combat the disease, however, prevention can help us to slow the rate of spread of virus. Several properties of this virus make prevention difficult which include non-specific features of the disease, the infectivity even before onset of symptoms in the incubation period, transmission from asymptomatic people, long incubation period, tropism for mucosal surfaces such as the conjunctiva, prolonged duration of the illness and transmission even after clinical recovery [6, 7]. The community of frontline workers is at maximum risk of getting infected with SARS-CoV-2 infection [8].
Personal protection equipment (PPE) becomes a life-saving item for all the healthcare team directly or indirectly involved in the hospital set-up. It includes gloves, medical/surgical face masks (including face-piece respirators such as N95), goggles, face shield, and gowns, as well as items for specific procedures filtering (viz., FFP2 and FFP3 facepiece respirator).
Although the world is concerned about the shortage of PPE and is trying to manufacture more such items, there is a need to understand their disposal and fate in the environment. Guidelines for their disposal are given by world health organization (WHO) and National Center for Disease Control (CDC). Indian CDC has issued guidelines regarding the management and disposal of coronavirus related biomedical waste [9, 10].
Components & chemical constituents of PPE
The raw material used for manufacturing of components of PPE can be summarized as Table 1 [11]:
Table 1. Raw materials used for manufacturing of components of PPE.
Table 2. Reported physical properties of Polypropylene [12].
The polymer which is largely used in making of PPE is Polyproylene (PP), a non woven material that can be used once. It is the lightest known downstream petrochemical product obtained by polymerization of monomer propylene. The basic structure of PP is a saturated carbon chain having methyl group attached to alternate carbon atom (Figure 1). The presence of methyl group makes PP different from Polyethylene and imparts hardness to it. The hardness, lightweight (density of 0.90 g/cm3) i.e. high strength to weight ratio makes it suitable for various industrial applications [12].
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Figure 1. Structure of polypropylene.
Polypropylene can be synthesized in three different stereo specific configurations [13]:
Isotactic: The configuration of all the chiral carbons, bearing methyl group, is same. All the methyl groups are present above the plane or below the plane. [Figure 2 (i)]
Syndiotactic: The configuration ofchrial carbons is alternatively similar to each other, the methyl groups are present in an alternate manner, one above the plane and next below the plane. [Figure 2 (ii)]
Atactic: The configuration of chiral carbons have no regualrity, the methyl groups are irregularly present on both the sides. [Figure 2 (i)]
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Figure 2. i) Isotactic, ii) Syndiotactic and iii) Atactic polypropylene.
Out of these three, the isotactic PP is more crystalline due to regular arrangement of chains and the melting point of perfectly isotactic PP is 171 °C [14]. The thermal stability of Isotactic PP upto 171 °C allows steam sterilization of the molded articles, which supports the use of the PP in medical accessories.
In addition to high thermal stability, bacterial resistance, high dimensional stability, especially through repeated sterilisation cycles, resistance to various chemicals, acid alcohols, bases, aldehydes, esters, aliphatic hydrocarbons, ketones and flexibility for molding [15] also favors the application of PP for surgical instruments and PPE.
During Covid-19, the PPE is being designed for single use followed by their disposal. Once these plastic materials are discharged into environment they end up in the land fills or oceans as their natural degradation is difficult at ambient temperature. They need decades to get decomposed by the microbial organism. The other way is to recyle these polymers include physical methods and chemical methods. Reduction, reuse and recycling are the three pillars of sustainable development that can help to prevent the disposal of plastic to the environment [16].
Chemical processing of polypropylene is one of the most promising method. It involves thermal cracking of large hydrocarbon chains, known as pyrolysis. It is an efficient and economical method of recycling polypropylene.
Value addition by conversion of PPE into biofuels
Plastics can be converted to their constitute chemicals via catalytic/non catalytic chemical treatment or thermal treatment to manage the accumulated plastics in a better and more efficient manner. Chemical treatment results inchange of the chemical structure of the plastic material [17]. It comprises of various processes that result in conversion of plastic waste into value added products. Some methods are - glycolysis [18, 19], hydrogenation [20], aminolysis [21], hydrolysis [22] pyrolysis [23–25] and gasification [26–28].
Of the various processes mentioned above, used to convert solid waste into fuels, the pyrolysis process is the most commonly used technique [29, 30]. Pyrolysis is a thermo-chemical plastic waste treatment technique that can be an alternate to plastic dumping & pollution problems. It involves thermal degradation of long-chain polymer molecules into simple, smaller molecules, in the absence of oxygen at high pressure and temperature for a small duration. Pyrolysis does not require prior separation of different types of waste plastics, thus a mixture of plastcis can also be converted into liquid fuel, which can be used for the generation of energy for any industrial applications [31, 32].
This chemical recycling method which unlike mechanical recycling utilizes mixtures of waste plastics is an environmentally friendly alternative to incineration and inefficient landfilling. . The process involve treating PP at high temperature (573-773 K, (300 °C to 500 °C)) in absence of oxygen to facilitate thermal cracking of macromolecules in the form of – liquid, char and gas.
Many researchers utilize pyrolysis for treating plastic waste due to its ability to produces a large quantity of liquid oil, up to 80 wt%, at temperatures around 773 K(500 °C) [33]. This liquid oil has various applications in gas turbines, boiler systems, generators and sterling engines.
An hour demands to protect the world from COVID-19 but there is a necessity of timely realization of the ill effects of the procedures associated with the disposal of PPE. Polypropylene, a single use plastic, being the major component of most PPE, can cause a significant threat to the environment in coming months. Pyrolysis based conversion of PP polymer used in PPE into biofuel can help to overcome this challenge substantially.
Many researchers have studied the process of pyrolysis of PP by altering the parameters to optimize the liquid oil yield, and also the research is underway worldwide to get liquid products from various plastics [34, 35]. In a study conducted by Martynis et al. (2019), a 125 dm3 pyrolysis reactor was designed and used for pyrolysis of 1 kg polypropylene at a temperature of 523 K, 573 K, 623 K, 673 K (250 °C, 300 °C, 350 °C and 400 °C respectively), for 30 min and 60 min. The results showed that at a temperature of 673 K (400 °C), for 60 min, is favorable to yield 88.86% w/w liquid fuel. The obtained pyrolysis liquid fuel is comparable with the commercial fuel set standards. Ahmad et al. (2014) conducted pyrolysis of polypropylene over a temperature range of 523 K-673K (250-400 °C) [36]. Product comprising of 98.7%w/w liquid; 69.8%w/w, gas; 28.8%w/w, andresidue;1.34%w/wwasachieved at 673 K (400 °C). Reserachers have studied the thermal decomposition of PE and PP in an autoclave [37]. They found that the optimum conditions were 723 K (450 °C) temperature, 0.14 MPa pressure, and 30 min of reaction time. The liquid product formed was rich in alkanes with carbon atoms between 5 and 11. Branched and cycled alkanes were present in very low concentrations. Sakata et al. (1999) studied the pyrolysis of PP at a temperature of 653 K(380 °C) and obtained a liquid yield of 80.1 w/w, gas yield of 6.6 wt% and 13.3 wt% solid residue [38]. Miskolczi et al.studied pyrolysis of mixed polyolefins (PE, PP) for production of liquid fuel like and concluded that the final product yield and the composition mainly dependsupon the residence time and the type of waste polymer [39]. Fakhrhoseini and Dastanian studied PP pyrolysis of PP at 773 K(500 °C) and reported a liquid yield of 82.12 wt% [40]. The liquid yield was reduced when the temperature is more than 500 °C . Demirbas team also conducted pyrolysis of PP at high temperature 1013 K (740 °C), the product obtained were 48.8 wt% liquid, 49.6 wt% gas, and 1.6 wt%. The study inferred a reduction in liquid fuel amount with rise intemperature .
In recent studies, pyrolysis based biorefineries have also been documented to have potential to convert plastic waste to energy [41]. Budsaereechai et al. have studied the catalytic pyrolysis of waste plastics (PP etc) with the use of economical binder-free pelletized bentonite clay. The process yielded pyrolysis oils as drop-in replacements for commercial liquid fuels such as diesel and gasohol 91 [42].
Chemistry of pyrolysis of PP
The thermal pyrolysis of PP is favorable as it is an easily degradable polyolefin due to the presence of the tertiary carbon atom or its branching structure [43, 44] that makes the chain scission easy. The overall reaction involves three steps: initiation, propagation, and termination. Initiation is generation of free radicals due to homogenous cleavage of PP chain at high temperature. The free radicals and the molecular species can be further cracked into smaller radicals and molecules during the propagation reactions. The free radicals are unstable and undergo coupling and disproportionation to form stable molecules in the termination step.
The pictorial representation of thermal pyrolysis of PP [Figure 3 (i-iii)]:
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Figure 3. (i) Initiation (ii) Propagation (iii) Termination.
Need of alternate fuels
There is accelerated growth in primary energy consumption and carbon emission due to rapid growth of the world’s population. According to BP statistical review 2018 [45], Primary energy consumption grew at a rate of 2.9% last year, almost double its 10-year average of 1.5% per year, and the fastest since 2010.
There is always a need for alternative fuels or energy resources to meet our energy demands. The pyrolysis of plastics is one of the methods to mitigate our energy crisis. It is well documented, that the liquid fuel obtained from plastic needs no further upgrading like biofuels. The absence of oxygen and larger content of carbon and hydrogen in the fuel obtained by pyrolysis decreases the need for further upgrading. They also have high calorific value owing to the absence of water. Further, the absence of oxygen also makes the fuel non-acidic and non-corrosive, unlike biofuel [46–49].
Conclusion
The proposed strategy is a suggestive measure addressing the anticipated problem of disposal of PPE. Presently, the world focusing to combat COVID-19, however, we can foresee the issues of economic crisis and ecological imbalance also. We have to prepare ourselves to meet the challenges which are forcefully imposed by COVID-19 pandemic, so as to maintain the sustainability. There is a high production and utilization of PPE to protect the community of health workers and the other frontline-warriors of COVID-19. The disposal of PPE is a concern owing to its material i.e. non woven polypropylene. Authors proposing an effective means of recycling PPE kits (used and defective) using pyrolysis. The pyrolysis of the PPE kit can be done in a closed thermal reactor between 300-400 °C for 60 min, which will convert the polypropylene into liquid fuels. This conversion will not just prevent the severe after-effects to humankind and the environment but also produce a source of energy. Thus, the challenges of PPE waste management and increasing energy demand could be addressed simultaneously by the production of liquid fuel from PPE kits. The liquid fuel produced from plastics is clean and have fuel properties similar to fossil fuels.
| Component of PPE | Raw material used |
|---|---|
| N95 respirators | Polypropylene |
| Powered Air Purifying Respirators | Rubber or silicone |
| Face shields | Polycarbonate, Propionate, Acetate, Polyvinyl chloride, and Polyethylene terephthalate glycol |
| Normal surgical masks | Polypropylene |
| Goggles | High quality polycarbonates |
| Single use protetive gowns | Normally polypropylene |
| Coveralls | High density polyethylene |
| Physical Property | Value |
|---|---|
| Glass Transition Temperature | −10 °C |
| Density | 0.09 − 1.06 g/cm3 |
| Elastic modulus | 1.5 − 3 GPa |
| Impact strength, Charpy notched | 2 − 6 kJ/m2 at 20 °C |
| Coefficient of thermal expansion | 6*10-5 − 1*10-4 1/K at 20 °C |
| Max. service temperature, short | 140 °C |
| Melting point | 160 − 168 °C |
| Specific heat capacity | 1520 J/(kg.K) at 20 °C |
| Thermal conductivity | 0.41 W/(m.K) at 20 °C |
| Flammability | UL 94 HB |
| Dielectric constant | 2.8 at 20 °C |
| Electrical resistivity | 1*1013 − 1*1014 Ω.m at 20 °C |
Disclosure statement
No potential conflict of interest was reported by the authors.
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