Biodegradation of polyethylene: A Review

Excerpt: The use of polyethylene has increased significantly over the years due to its low cost, light weight, good mechanical and permeability resistance properties.

Abstract

The use of polyethylene has increased significantly over the years due to its low cost, light weight, good mechanical and permeability resistance properties. But due to these unique properties, the production as well as usage of polyethylene which created many problems associated with disposal and their impact on the environment. Polyethylene does not easily degrade in the natural environment due to their chemical and biological inertness, thus the need for degradability of polyethylene has become a key topic of research now a days. Degradation of polyethylene is mainly divided into two categories. These include abiotic degradation caused by environmental factors such as temperature, UV irradiation and biotic degradation defined as biodegradation caused by the action of microorganisms that modify and consume the polymer leading to changes in its properties. Various types of microorganism (bacteria, fungus) with their capability to grow on the surface of polyethylene have been discussed. Also modification in polymer by natural sources as well as different additives were studied for the degradation of polyethylene. Challenges and benefits for biodegradation have been studied.

Key words: Biodegradation, Oxo-degradation, Polyethylene, Microorganisms, Bacteria, Fungus, High density polyethylene (HDPE), Low density polyethylene (LDPE), linear low density polyethylene LLDPE)

Introduction

THE chemical process used for synthesizing synthetic polymers (plastics) from crude oil was a revolution in chemistry and material science which creates the way to the production of one of the most resourceful group of materials ever produced.

A polymer consists of long chain carbon along with other additives like plasticizers, stabilizers, lubricant, UV absorbing material, flame retardants to improve performance. There are various types of polymer which are classified as plastics, fibers, resin and elastomers. Plastics are usually polymers of high molecular weight and mainly classified into thermoplastics and thermosets. Thermoplastics can be recycled and reused with no such changes in its properties whereas thermoset can't be recycled. Thermoplastics are divided into commodity, engineering and specialty plastics in which commodity plastics are used in huge tonnage than engineering and specialty plastics1.

Plastics play a major role in day today's life, but due to its single-use in most of the application such as polyethylene bags and packaging film, it has become one of the most defiled material that can pollute the environment. Nearly 64% of the synthetic plastics are wasted after use within a short period, that causes an huge and rapid accretion to the environment2. Due to single use of plastics in many application, the global plastic production has increased from 0.5 million tons in 1950 to 257 million tons in 2007 which further increased to 322 million tons in 2015 and 348 million tons in 20173,4. Fig 01 and Fig 02 shows the consumption of plastics worldwide and in India are increasing in trend5,6.

The estimated geographical breakdown for production capacity of PE in 2010 was Western Europe 19%, North America 22%, Middle East 17%, Asia 25% and rest of the world 17%1. The global market for agricultural plastic films has increased more than U.S. $5.8 billion (2012), which is projected to grow 7.6% per year through 20197. Plastics waste generated as municipal solid waste (MSW) by United States was 20% whereas Western Europe and Australia were 7.5%, 25% respectively. In Turkey, 11 million tons of plastic were disposed per year8. India generates 5.6 million tons per annum of plastic waste considering 70% of total plastics consumption is discarded as waste9.

Polyethylene has a number of advantages over other materials being multipurpose, low cost, light weight (as compared to metals), having strong, good barrier properties, non-rusting material, potentially transparent, high thermal and electrical insulation characteristics10. Polyethylene is broadly used for numerous one trip applications such as food packaging, retail industry uses and agricultural uses11,12. Major applications of polyethylene include carrier bags, packaging film, agriculture mulching film13, fabrics, toys and water container etc.14,15. High density polyethylene (HDPE) used to make milk jugs, juice bottles, bleach, detergents and household cleaner bottles, cooking oil bottles, butter and yogurt containers, Low density polyethylene (LDPE) is used for making plastic bags, Outdoor furniture, siding, floor tiles, textile products, mulching film for agriculture purpose, shower curtains, clamshell packaging netting16,17, drinking straws, and wire cables, linear low density polyethylene (LLDPE) is used for mulching film3.

Plastic materials have several disadvantages, the most important one is its long-term persistence in the environment and their resistance to degradation 18. Because of hydrophobic property, high molecular weight and three-dimensional structure, polyethylene recalcitrant in nature and thus they are not easily available to microorganisms. As the use of plastics increased continuously that creates problems for plastic waste disposal, the prerequisite for biodegradable plastics and biodegradation of plastic wastes has extended considerable importance in the last few decades. The rate of degradation of polyethylene is quite slow if it is subjected to natural environments and this has triggered an immense threat to environment 19. It is reported that less than 0.5% carbon might be mineralized in 100 years when polyethylene is thrown away into the environment and after natural ageing of polyethylene under sunlight for 2 years, only 1% of carbon could be mineralized in the same period of time 20.

In Western Europe alone it is estimated that 7.4% of municipal solid wastes are plastic, which are classified as 65% polyethylene/polypropylene, 15% polystyrene, 10% PVC, 5% polyethylene terephthalate and remaining others19. A study on plastic waste production in 60 major Indian cities revealed that per day nearly 15,340 tons of plastic waste is generated in the country21.

Mainly plastics are found in marine and dumping sites. The occurrence of plastic debris in the marine environment first reported in 1970s. The threat that widespread plastic waste poses to marine life, with conservative estimates of the overall financial damage of plastics to marine ecosystems standing at $13 billion each year 22. Based on size, waste plastics can broadly be divided into mega-plastics (100 mm ), macro-plastics (20 mm diameter), meso-plastics (5 mm – 20 cm) and micro-plastics (300 μm–5 mm) while while those <1 μm are defined as nano-plastics23–26. Plastic debris creates threat by choking and starving wildlife, distributing non-native and potentially harmful organisms, absorbing toxic chemicals and degrading to micro-plastics that may subsequently be ingested27. Such debris is mostly apparent in marine environments where items of plastic have been reported from the poles to the equator, with 60–80 percent of marine litter being plastic. The significant quantity of marine plastic debris and its durability creates physical hazards for wildlife which may ingest or become entangled in this debris28. In addition, there is evidence that ingestion of plastic debris may also present a threat as chemicals including phthalates, PCB's and organo-chlorine pesticides, either added during manufacture or absorbed from seawater have been reported on plastic fragments29–31 and may present a toxicological hazard32. Conventional plastics show high resistance to aging and minimal biological degradation33 and this increase in usage, especially disposable items of packaging, which make up 37% of all the plastic produced. LDPE accounts for 60 % of the total plastics production of plastic bags and most commonly found solid waste 34. In addition, 10–20 million tons per year of plastic materials collect in the oceans and the form of plastics varies from micro to nano-sized particles that bio- accumulate in filter-feeding animals, posing a significant threat to ocean ecosystems35. The quantity of plastic waste in Municipal Solid Waste (MSW) is increasing due to increase in population, development activities and changes in the life style. The health and environmental implications associated with Solid waste management are increasing specially in the context of developing countries and regulatory requirements for environmental clearance36. Disposal of plastic wastes in landfills and open dumps results in the generation of toxic leachate due to the interaction of plastics with groundwater and moisture-rich substances present in the dump, which is hazardous in nature30. Most of the times, the Municipal Solid Waste containing about 12% of plastics is burnt, releasing toxic gases like Dioxins, Furans, Mercury and Polychlorinated Biphenyls into the atmosphere37. Substantial amounts (22-43%) of these non-biodegradable plastic materials are disposed of in municipal land-fill sites38. The present review will cover degradation of polyethylene by Oxo-degradation, biodegradation, benefits and challenges associates with biodegradation, different types of polyethylene degrading microorganism involves in biodegradation.

Degradation of polyethylene

The most common polyethylene types are: Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE), Linear Low Density Polyethylene (LLDPE) and Cross Linked Polyethylene (XLPE). The density of HDPE is greater or equal to 0.941 g cm-3 whereas LLDPE is in the range of 0.915–0.925 g cm-3), and LDPE is 0.910–0.94 g cm-3. The different types of branching in HDPE, LDPE and LLDPE are viewed as below36,39–41. From Fig 03 it can be seen that HDPE has no branched whereas LLDPE has short branching and LDPE has long branching. Thus HDPE has more crystallinity as compare to LLDPE and LDPE and due to amorphous nature LDPE and LLDPE are more accessible by microorganism than HDPE.

Polyethylene is highly resistant to acids, alcohols, bases and esters. It is also biologically inactive as it is nonpolar in nature and considered as an obstinate material42. Its inertness is due to the high molecular weight, hydrophobicity and lack of functional groups recognized by microbial enzymatic systems2. But it was studied that the tertiary carbon atoms that are present at the branch sites are more susceptible to attack by free radicals, because they form more stable radicals when they lose a hydrogen atom. Some structural defects such as unsaturation and carbonyl and hydro-peroxide groups may also be present in all polymers, formed during polymerization and subsequent processing, but are present at very low levels43

The effectiveness of microbial attack on PE by microorganism resulting in bio-deterioration is dependent on the molecular weight, crystallinity and physical forms of the polymer. Degradation is confirmed by decrease of functional groups such as -OH and -CO in the polymer and further cleaving of the polymer chain by extracellular enzymes into low molecular weight fragments that are further used as carbon and energy sources for growth44. As it is already discussed earlier that polyethylene waste are creating environmental pollution, it was estimated that the yield of plastics worldwide is growing at a rate of about 5% per year45.

There are various methods explained in Fig 4 to resolve the environment threat caused by plastics. Thermal and UV pretreatment method had been studied by many researchers and used as primary treatment before exposed to microorganism46. Various types of degradation have been discussed in this paper including oxo-dgradation, biodegradation along with modification in polyethylene. Fig 5 explains about end products that produced after the degradation of polymers19.

Oxo-degradation

It has been reported that Oxo-degradable LDPE, HDPE and LLDPE provide a potential solution to littering issues. These plastics contain special additives that cause them to degrade40. There is a basic difference between antioxidant and pro-oxidant, anti-oxidant. Anti-oxidant are used to slow down the abiotic degradation whereas pro-oxidants are used to fasten43. There is a confusion between antioxidant and pro-degradant additives occurs because the chemical compound involved in the respective reaction mechanisms are the same, while oxo-degradants helps for oxidation and antioxidant prevents polymer from degradation. It has been reported that LDPE and LLDPE are more susceptible towards thermooxidative degradation than HDPE47. The steps of Oxo-biodegradation includes

1) Oxidative degradation (radical chain scission and oxygenation [adding hydroxyl and carboxyl groups])

2) Biodegradation by microorganisms (fungi, bacteria, etc.)

For the initial oxidative degradation of the polymer backbone oxo-biodegradable additives require the presence of oxygen and some form of activation energy like light (UV) or heat48,49. Many researchers concluded that the combination of photo-oxidation induced by UV-radiation exposure and biodegradation with new bacteria may improve plastic degradation without any impacts to the ecosystem. In sunny location, UV-radiation alone can act as a photo-oxidative inducer of abiotic polyethylene degradation but the oxidative power of UV-radiation in sunlight will vary according to geographical location and radiation angle50. The radiation by UV or sunlight reduces the polymeric chain size of PE and form oxidizing groups such as hydroperoxides, peroxides, alcohols, ketones, and perhaps some aldehyde resulted from the partial oxidation of PE are present in small amounts, but they continue to undergo oxidation. The amounts of the intermediate products depend on whether the oxidation has been started by UV light. Ketone groups attached on PE molecules are decomposed by UV light and hydroperoxide groups are decomposed both by UV light and heat51. Microbial cell contains macronutrients made up of carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium and iron that require higher amounts by microorganism. 52. Polymer containing pro-oxidant (transition metal Co, Mn, Fe, Ni, Cu) substance are known as oxo-biodegradable polymers. Two types of transition metals are used for oxo-degradation. Heat sensitive transition metals like chromium (VI) and cadmium which are more heat sensitive but less stable are not used as 'environmental solutions' on the other hand, light-sensitive transition metals, like iron, have poor thermal sensitivity and require light (UV) to be activated (to provide degradation of conventional polymer plastics). This offers good storage stability and an adequately long shelf- and use-life, and also improves recycling, but may have impact on the further degradation if the product at 'end of use' is put in a dark place (e.g. covered in a landfill) without being exposed to sunlight before this48. The addition of metal salts (Fe, Mn and Co) may accelerate the thermo and photooxidation processes of the different polyethylene (PE) types. Due to chemical and morphological characteristics, it was studied that LDPE and LLDPE are more susceptible to thermooxidative degradation than HDPE 47,53,54. Two preliminary treatments (heat and ultra violet light) are essential to modify its chemical structure. Molecular enlargement and broadening of molecular weight distribution occurred after preheating in air55. Mechanism of degradation of polymer started from oxidise (introduce oxygen in the form of hydroxyls, carbonyls, peroxides) followed biodegradation (reduce the molecular weight or increase it by crosslinking reactions) and completes at de-structure (modify the crystalline structure) the PE. Chemical groups are detected, such as OH, C=O, COOH, double bonds such as -C=C-, H2C=C-, ketones, and acids by FTIR56. Biodegradation of UV photo-oxidized polyethylene increased with increasing irradiation time49. Thermo-oxidative degradation of polyethylene films containing pro-oxidant has been studied and it was witnessed that while temperature is the most important factor influencing the rate of thermo-oxidative degradation of the materials, oxygen concentration is of negligible importance. The investigation has also shown that when the material is degraded into low molecular mass products, it is bio-assimilated57. In another experiment, biodegrability of high density polyethylene film (HDPE) and low density polyethylene film (LDPE) both containing a balance of antioxidants and pro-oxidants was studied with Rhodococcus rhodochrous and Nocardia asteroides in mineral medium. Corresponding analysis revealed that during the experiment time biodegradation processes probably affected surface layer of materials only 58. Thermally oxidized LDPE-film samples and corresponding acetone extractable fractions were submitted to the effect of microorganism flora present in river water. The effective biodegradation was assessed by monitoring the amount of CO2 developed over time in a Respirometer apparatus. Levels of biodegradation up to 12 and 48% for the degraded fragments and corresponding fractions extracted with boiling acetone were detected on a 100-day time frame 59. Also degradation in the polymer chain was monitored by changes in the mechanical, morphological structural and thermal properties60. The light-induced oxidative degradation caused by iron-based oxo-biodegradable additives in conventional plastic (PE and PP) will continue in dark thermal oxygenated conditions and is increased when more light exposure has taken place. This supports the view that e.g. iron-based oxo-biodegradable additives can be used to retain long shelf-life/use-life and low degradation during recycling of plastic and to thereafter rapidly degrade conventional plastic products that will either be exposed to sunlight as part of use or in the initial phase of disposal, even if the final location is dark5548. When HDPE, LDPE and LLDPE was exposed to pro-oxidant and antioxidant, it was observed that the main factor controlling the biodegradability of the polyethylene films is the nature of the pro-oxidant additive and to a lesser extent that of the matrix. Except for the samples containing very high content of cobalt additive, the various polymer films were used as substrates by the bacteria61. From Tab 01, it can be conclude that presence of different pro-oxidant and pretreatment helps the microorganism to degrade polyethylene to a greater extent. The oxidation level of commercial polyethylene is higher by anionic surfactant at 60°C for 1 month than the thermally oxidized polyethylene at the same temperature. 66. When UV untreated and untreated LDPE samples were compared, it was found that UV-treated LDPE showed better degradation than the non-treated LDPE in soil. This is due to higher microbial association and their better metabolic potential towards UV treated LDPE that lead to enhanced degradation of the LDPE21. Fourier transform infrared spectra of UV photo-oxidized polyethylene incubated with Rhodococcus ruber (C208) indicated that biodegradation was initiated by utilization of the carbonyl residues formed in the photo-oxidized polyethylene 67. Photo-catalytic degradation of low density polyethylene (LDPE) films using Titania, the photo catalyst generally used Titania in the nanoparticle form was studied. The degradation of pure and composite PE films made of Titania was measured in terms of photo-induced weight loss and was confirmed by FTIR, SEM, surface roughness and tensile strength testing. Thus, polyethylene films with 10% dye sensitized Titania nanotubes showed a degradation of around 50% under visible light over a short period of 45 days68. Bacillus cereus and Bacillus gottheilii degraded UV treated polyethylene 1.6% and 6.2% after 40 day69.

Biodegradation of polyethylene

Biodegradation is defined as decomposition or destruction of contaminant molecules by the action of the enzymatic machinery of biological system. Biodegradation is the process by which organic substances are broken down by living organisms70. Many attempts have been made to curb the problem at large by resorting to both chemical and biological methods. But chemical methods creates further pollution by releasing toxic gases into the atmosphere; whereas; biological methods have been found to be eco-friendly however they are not cost effective. The primary mechanism for biodegradation of polyethylene includes oxidation or hydrolysis by enzymes to create functional groups which improve its hydrophilicity. The main chains of polymer are degraded resulting in polymer of low molecular weight and mechanical properties are rather weak, thus, making it more accessible for further microbial assimilation51.

Polymers with the molecular weights lower than 620 may undergo biodegradation while those having higher molecular weight are difficult to biodegrade. In case of polyethylene, HDPE has molecular weight of 200,000 g/mol where as LDPE and LLDPE has molecular weight less than 50,000 g/mol71. Thus due to high molecular weight, these polymers doesn't degrade easily.

PE has market more than a third of the global market share of commodity thermoplastics, with a production in 2012 over 70 million tonnes, which consisted of 45% HDPE, 30% LLDPE, and 25% LDPE1. As it was already discussed that PE is used as polyethylene bags and packaging material, the onetime use creates pollution to environment as it takes decades to degrade. Researchers have made many experiments such as modification in polyethylene (by adding natural additives and metals) and biodegradation of polyethylene by bacteria and fungus, still there is no such report for making polyethylene 100 % biodegradable. Polyethylene considered to be inert can be biodegraded by microorganism if the right microbial strain is isolated72.

Biodegradation of modified polyethylene

Degradation can be defined as a change in the chemical structure of a plastic involving a deleterious change in properties. The material is degraded under environmental conditions (e.g. Microorganisms, temperature, light, water) and in a reasonable period of time in one or more steps73. LDPE along with different percentage of starch was studied. These usually contain between 6 and 15% starch, but up to 40% starch may be added in granular form. The relatively low amount of starch means that part of the granules remain isolated in the polyethylene matrix and are not susceptible to microorganisms74. Streptomyces strains demonstrated reductions in the percent elongation values (aver- age 28.5% for Streptomyces cultures) when degradable plastic used in this study was disposable polyethylene bags containing 6% starch along with pretreated for ten-day heat-treated at 70 °C 75. It was observed that the abiotic degradation breaks bonds and releases degradation products, leaving the remaining polymer rearranged with higher degree of order76. The water absorption capacity of the film improved with increase in the starch content making the film more hygroscopic and biodegradable with reduced mechanical properties77

Pro-oxidant based HDPE shows higher percentage of degradation than starch based and in case of starch based LDPE it was seen that higher percentage of starch addition to polymer gives higher percentage of degradation65,78. Another approach by Kim, in which thermal degradability and biodegradability of films prepared from blends of PE with hydroxypropylated potato starch (HPS) were evaluated by measuring the changes of the carbonyl index and mechanical properties of films. It was found that the carbonyl index of HPS/PE films increased but mechanical strength decreased in line with the increase in the degree of substitution in the starch during heat treatment. This implies that the higher the hydroxylpropylation of starch, the faster chemical degradation of the HPS/PE when film inoculated with P. aeruginosa 79.

LDPE can be modified by the grafting of 2-hydroxyethyl methacrylate (HEMA) which maintain good structural order as well as improved hydrophilicity which helps the polymer for biodegradation although there is a decrease in crystallinity80. In another study, low-density polyethylene (LDPE) films modified with polyester (Bionolle) was studied with different environment like waste coal soil, forest soil and soil from an extinct volcano crater. It was found that there is a significant reduction in the materials' molecular weight was perfectly visible through the reduction of their mechanical properties by nearly 98%81. PE films (18µ) inoculated with Bacillus subtilis with the addition of its bio-surfactant showed a weight loss percentage of 9.26% in 30 days64.

Biodegradation of polyethylene by Bacteria

Many types of bacteria have been reported which degrades polyethylene and this type of degradation is known as biotic degradation. Polyethylene degrading bacteria mainly found in dumping sites, marine environment and mangroves forest. Results from many work prove that there are many microorganisms in natural habitats that are able to degrade polyethylene. The most active bacterial isolate, IRN19, has the ability to degrade polyethylene film by 26.8 ± 3.04% over 4 weeks. Also IRN11 shows the highest cell mass production (6.29 ± 0.06 log cfu/cm2) after growth on LDPE films, showed 98.74% similarity to Sphingobacterium moltivourum50. Rhodococcus ruber (C208) utilized LDPE film by forming biofilm on the surface that degrade it up to 8% within 30 days67. In another study Pseudomonas sp. AKS2 degrade LDPE by 5±1 % in 45 days without prior oxidation. AKS2 can also be attributed to its ability to form a biofilm wherein the hydrophobicity of the cell surface may play an important role82. The biodegraded samples of LDPE and HDPE exhibited weight loss (7.02% and 7.08%, respectively) was done by Bacillus weihenstephanensis10. The highest degradation activity for bacteria was a mean of 35.72± 4.01% and 20.28± 2.30% attributed to Bacillus cereus strain A5,a (MG645264) and Brevibacillus borstelensis strain B2,2 (MG645267) respectively83. Microbacterium paraoxydans degraded 61.0% of LDPE while Pseudomonas aeruginosa degraded 50.5% in 2 months84. Extruded low-density polyethylene (LDPE) films commonly available in the market as 20-micron thick carrier bags were used for above the experiments. LDPE supplemented liquid mineral culture medium showed the presence of extracellular enzymes with approximate molecular weight of 55 kDa and 35 kDa 85. It was reported that suggests the suitability of three marine bacteria, K. palustris M16, B. pumilus M27 and B. subtilis H1584 after 30 days incubation were 1%, 1.5% and 1.75% after 30 days of incubation with the M16, M27 and H1584 isolates, respectively. The maximum (32%) cell surface hydrophobicity was observed in M16, followed by the H1584 and M27 isolates. Based on the growth results on the polyethylene surface, hydrophobicity, metabolic activity and FT-IR data, we were able to determine that B. subtilis H1584 is more efficient than the other bacteria2. In this investigation, the degradation of LDP were analyzed after 7 months of incubation period and the LDP samples were treated cattle dung and kitchen vegetable wastes and compared with control. Further this study confirmed that the microorganisms present in the cow and sheep dung have more degrading ability of low density polyethylene material than the microbes in the kitchen vegetable wastes under controlled environmental condition86. The ability of Bacillus mycoides and Bacillus subtilis (Bacillus species indigenous to the Niger Delta mangrove soil) to biodegrade polyethylene (LDPE and HDPE) was studied. The result showed that certain Bacillus sp. Indigenous to the Niger Delta mangrove soil are capable of growing on PE films and biodegrade them, after an initial abiotic degradation. Biodegradation in Erlenmeyer flasks by the bacteria after 60 days of incubation ranged between 8.41%-23.15%.87. The bacteria cultured from mangroves soils, Pseudomonas species degraded 20.54% of polythene and 8.16% of plastics in one-month period88. Subjected to in vitro biodegradation assay with the pretreated LDPE as its sole carbon source for another 2 months, .M. paraoxydans degraded 61.0% of LDPE while P. aeruginosa degraded 50.5% in 2 months84. After incubation period of 60 days, the degrading capability of the strains Bacillus amyloliquefaciens (BSM-1) and Bacillus amyloliquefaciens (BSM-2) was analyzed and it was observed thatweight loss of BSM-1 was 11% whereas BSM-2 was 16%89. It was confirmed that tailor-made indigenous marine communities comprising of polymer and hydrocarbon degrader species have the potential to degrade naturally weathered PE (LLDPE) films in the marine environment before they are turned into micro-plastics90. Chryseobacterium gleum EY1 able to deragde photo-degraded linear low density PE (LLDPE) film. Considering the experimental results of thermo- and photo-degradation of LLDPE incorporated with Co-stearate, Mn-stearate, and Fe-stearate, the last was selected as the most desirable pro-oxidant for the photo-sensitive LLDPE films. Increasing the intensity of the UV irradiation reduced the tensile properties and molecular weight of PE–Fe (LLDPE with Fe-stearate) slightly but increased biodegradability considerably91. High-density polyethylene (HDPE) is widely used in packaging industry, thus the need for biodegradation of HDPE have attracted many researchers. Biodegradation of by K. pneumoniae CH001 isolated from a landfill site observed that this strain was capable of adhering strongly on HDPE surface and form a thick biofilm, mechanical testing analysis showed a significant decrease in weight (18.4%) and reduction in tensile strength (60%) of HDPE film. Furthermore, SEM analysis showed the cracks on the HDPE surface, whereas AFM results showed an increase in surface roughness after bacterial incubation. Overall, these results indicate that K. pneumoniae CH001 can be used as potential candidate for HDPE degradation in eco-friendly and sustainable manner in the environment92. In another assessment, high-density polyethylene (HDPE)-degrading bacteria isolated from plastic waste dumpsites of Gulf of Mannar had been studiedBacteria identified from land fill were Arthrobacter sp. and Pseudomonas sp., respectivelywhich degrade HDPE nearly 12% and 15% within 30 days93. Low molecular weight polyethylene (LMWPE) powder was prepared by thermal degradation of highdensity- polyethylene (HDPE) in a strict nitrogen atmosphere and used as a sole source of carbon for Soil sample was collected 10 cm beneath the seashore soil 4 genera of Arthrobacter, Curtobacterium, Gordonia and Rhodococcus comprised the majority bodegradability of LMWPE under the compost condition at 30°C for 60 days using soil samples before and after enrichment culture with LMWPE. LMWPE in the control soil sample before enrichment cultivation was degraded by 10.1% after 60 days of biodegradation. Meanwhile, the degradability of LMWPE was 22.2%, 32.7% and 37.8%, when LMPWE was degraded in the compost inoculated with the soil after enrichment for 30, 60 and 90 days, respectively94.

Biodegradation of polyethylene by Fungus

The advantages of Fungi are they can survive in environments with low nutrient availability, low pH and low moisture as well40. Many efforts have been made to identify and isolate microorganisms capable of utilizing synthetic polymers. Current results point towards the viability of a solution for this problem based on the biodegradation of plastics resorting to selected microbial strains. It has been reported that under the tested conditions, Z.maritimumis capable of utilizing PE, resulting in the decrease, in both mass and size, of the pellets. These results indicate that this naturally occurring fungus may actively contribute to the biodegradation of microplastics, requiring minimum nutrients25. The ability of fungi to attack degradable plastics was investigated. The plastic contains disposable polyethylene bags with 6% starch and heat-treated (70°C) for ten days. It was observed that A. flavus degrade PE pretreated film 46.5% when compared with uninoculated controls75. In another experiment low-density polyethylene bags containing starch (12%) was degraded by Phanerochaete chrysosporium (ATCC 34541) to enhance polyethylene film biodegradation in soil microcosms for 6 months. LDPE/starch blend film showed a 56% reduction (range, 20±56%) in percentage elongation in inoculated soil compared to a 12% reduction in uninoculated soil, suggesting that LDPE/starch blend film degraded faster in the inoculated soil than in the uninoculated soil95. A blends made from low-density polyethylene (LDPE) and sugar cane bagasse (SCB), before and after exposure to the fungus Phanerochaete chrysosporium for 32 days was studied. The initial blends contained equal weights of each component. The composition of the blend LDPE/SCB at 32 days changed to the value (66 ± 3)/ (34 ± 3) 96. Preheated hot air oven at 70 °C for 10 days was used as a sole source of carbon for the growth of Rhizopus oryzae NS5. Fungal growth was perceived on the surface of the polyethylene when cultured in potato dextrose broth at 30° C and 120 rpm, for 1 month. About 8.4 ± 3% decrease (gravimetrically) in weight and 60% reduction in tensile strength of polyethylene was observed isolate. A thick network of fungal hyphae forming a biofilm was also observed on the surface of the polyethylene pieces. Present study shows the potential of Rhizopus oryzae NS5 in polyethylene degradation in eco-friendly and sustainable manner97. Soil sample was collected from the municipal solid waste landfill and isolated. It was found that fungus Mucor circinelloides and Aspergillus flavus degrade LDPE powder made from LDPE film within 28 days with 0.34% and 0.029%98. Thermo-oxidized LDPE powder (80°C, 15 days) sterilized with UV radiation was incubated with Aspergillus niger and Penicillium pinophilum fungi, with and without ethanol as cosubstrate for 31 months. An oxidation decrease (almost twice) on samples without ethanol with respect to the control was observed, while in those with ethanol it was increased (up to 2.5 times). Double bond index increased more than twice from 21 to 31 months99. LDPE was achieved by employing fungus and action-bacteria isolated from waste dumping site. Based on 18S rRNA and 16S rRNA analyses the isolated strains were identified as Aspergillus nomius and Streptomyces sp., respectively. The biodegradation of LDPE was determined by evaluating weight loss and morphological changes of the LDPE samples. The isolated strains; Aspergillus nomius had the capacity to degrade 4.9% and Streptomyces sp. showed 5.2% of weight loss of LDPE films respectively41. HDPE was treated with nitric acid to make it liable to microorganisms. Chemical treatment with nitric acid introduces carbonyl and nitro functional groups in HDPE as confirmed by FTIR analysis. Gravimetric analysis showed a decrease in weight of the polymer by 7.18 ± 0.15% after 20 days of incubation period. Reduction in the weight of polymer confirmed the ability of Cephalosporium species to utilize HDPE for their growth. The pH of liquid culture media was found to decrease, whereas total dissolved solids and conductivity increase with the incubation period101. High density polyethylene (HDPE) was degraded by fungus Aspergillus tubingensis VRKPT1 and Aspergillus flavus VRKPT2. After 30 days of incubation, the weight loss observed by the fungal isolates VRKPT1 and VRKPT2 was 6.02 ± 0.2% and 8.51 ± 0.1%102. Among the fungal species, Aspergillus glaucus degraded 28.80% of polythene and 7.26% of plastics in one-month period. This work reveals that the mangrove soil is a good source of microbes capable of degrading polythene and plastics88. P. simplicissimum was able to degrade treated polyethylene (38 %) more efficiently than autoclaved (16 %) and surface-sterilized polyethylene (7.7 %)103. A new fungal strain Fusarium sp. AF4 was isolated from sewage sludge, capable of not only adhering to the surface of PE film but also utilizing it as the source of carbon and energy, as evident by the increase in growth. About 1.85 g/l of CO2 was produced in case of test, whereas, 0.45 g/l in case of control104. In a study where fungal isolates are cultured from soil sample that was collected from a plastic dumping Ground, have been used to degrade polyethylene sheets (HDPE, LDPE). Two potential fungal strains, namely, Penicillium oxalicum NS4 (KU559906) and Penicillium chrysogenum NS10 (KU559907) had been isolated and identified to have plastic degrading abilities. The degradation rate of HDPE was determined as 55.598% by NS10 and 55.34% NS4 post 90 days of incubation. The degradation rate of LDPE was determined as 34.35% by NS10 and 36.60% NS4 post 90 days of incubation105.

Mechanism of biodegradation

Degradation reflects changes in material properties such as mechanical, optical or electrical characteristics in crazing, cracking, erosion, discoloration and phase separation106. Recent works revealed that microorganisms have the ability to use polyethylene as a carbon source. Microorganisms which colonize the surfaces of polyethylene have various effects on its properties. Different characteristics are have been studied to monitor the extent of biodegradation of the polymer: functional groups on the surface, hydrophobicity/hydrophilicity, crystallinity, mechanical properties, molecular weight distribution, consumption of polymer39.

Functional groups on the surface

Fourier Transform Infrared Spectroscopy analysis was used for detecting changes in the amount of existing functional groups, formation of new functional groups, carbonyl index to determine the extent of microbial degradation, the effect of natural and artificial UV-light exposure on the structural integrity 43,50,83,86. Thus degradation products, chemical moieties incorporated into the polymers molecules such as co-monomers, branches, unsaturations and presence of additives such as antioxidants can be determined by this technique. Mainly polymer control sample shows a number of peaks that reflects the complex structure of polyethylene89. The monitoring of the oxidation extent was carried out by transmission FTIR spectrophotometry with the non-fragmented films and by micro-FTIR spectrophotometry (FTIR spectrometer equipped with IR microscope) with the oxidized particles obtained after fragmentation61. Due to photo-oxidation(natural weathering), the generation of several oxygen containing functionalities(carboxylic groups, keto carbonyl, ester carbonys, vinly) observed on the surface of polymeric film which leads to an increase in intensity as well as band broadening. This indicates the presence of multiple oxidation products resulting from the abiotic exposure. Also the increase in carbonyl index verifies the oxidation of polyethylene60,65. The decrease in weight of polyethylene after exposed with bacteria was confirmed by FTIR that leads to an absence of functionalities. FTIR results revealed that the bacteria consume the oxygenated products leading to a decrease in the Carbonyl Index (CI)60. After 25 days of UV irradiance exposure of LDPE, some new peaks are observed in between 1710 cm- 1 and 1750 cm-1 which is due to the formation of carbonyl group(confirmed by FTIR analysis)107. Also it was reported that the percentage of transmittance at 2920 cm-1was directly proportional to the concentration of LDPE108. Different types of peak shows during FTIR analysis are mentioned along with their functional groups in Tab 0364. Absorption band between 1340 cm-1 and 1354 cm-1 was because of the weak hydrogen bond between starch and glycerol. Absorption band between 1340 cm-1 and 1354 cm-1was derived from C-O-H stretching bond. Alcohol absorption band was 987–1039 cm-1 and this indicated a fast degradation rate of carbon chain109. When LDPE was pretreated with UV radiation and nitric acid followed by fungal Fusarium sp. AF4, it was observed that in case of UV and nitric acid treated LDPE, peaks appeared at 1710 cm-1 and 831 cm-1, which were then reduced to 1708 cm-1and 830 cm-1 after microbial treatment indicating breakdown of polymer chain. This is due to a synergistic effect of UV-nitric acid and microbial treatment induced oxidation reaction that enhanced and accelerated the biodegradability rate of LDPE pieces51. ATR-FTIR has more accurate indication of percentage transmittance of the native bonds present in LDPE. The biodegradation of LDPE was calculated due to the formation of new peak at 1714 cm-1 using carbonyl index84. The absorbance peak for the ketone carbonyl is observed at 1715 cm-1 and the ester carbonyl at 1735 cm-1. The carbonyl index is calculated by taking the ratio of the carbonyl absorbance to the absorbance of the C–H stretching at 1465 cm-1, which remains essentially unchanged during oxidation and helps in the oxidative degradation over time110.

Mechanical properties

The mechanical property tests performed in a universal tester, in accordance with ASTM D882, ISO 527-3. It should be mentioned that strain at break is used in polymer degradation because of the great sensitivity of this property to any structural change. At very high molar masses such as that of HDPE, tensile strength and elastic modulus are significantly affected only when there are very significant variations in molar mass47,63. Natural polymers are typically highly hydrophilic, and packaging films produced with them can absorb water, leading to dimensional changes and reductions in mechanical properties111. The exposed PE samples by UV Weatherometer used to calculate the tensile strength and modulus of polymer before and after exposure48. It was observed that processing in the presence of cobalt stearate does not lead to any substantial degradation in LDPE in terms of loss in the mechanical properties by bacteria. However, after UV exposure, the tensile strength (TS) and elongation at break (EAB) decrease for PLD. This decrease in the mechanical properties of films can be attributed to chain scission of the polymer due to the pro-oxidative nature of cobalt stearate60.

Crystallinity

Polymer is not 100% crystalline and known as semi-crystalline material which consist of crystalline and amorphous region. HDPE has a high degree of crystallinity (typically 60-80%) and a high melting temperature of ~135 °C and specific gravity of about 0.96, LDPE is a partially crystalline solid with a degree of crystallinity in the 50 to 70% range, melting temperature of 100 to 120°C, and specific gravity of about 0.91 to 0.94. LLDPE are highly crystalline, with a melting point over 127°C usually about 135°C and specific gravity in the 0.94 to 0.97 range1,43,112. It was reported that when a polymer is exposed to microorganism, first the amorphous region was accessed and then the crystalline and there is an increase in crystallinity in the initial stage. Thus when the amorphous region consumed, the microorganism we progress to consume small crystals present in the polymer39. When a PE thermo-oxidative ageing, the material becomes denser due to an increase in crystallinity and the incorporation of oxygen which makes the polymer heavier. The crystallinity of the material was measured with DSC. The degree of crystallinity was obtained by dividing the melting enthalpy by the enthalpy corresponding to 100% crystallinity63,67. Blown extruded LDPE exposed to direct sunlight reveals that crystallinity increases with exposure time. The reason of these morphological changes is due to Photo-oxidative reactions leading to crosslinking in the beginning and to chain scissions for an advanced stage of ageing. The short chain segments resulting from scission reactions increase the crystallinity of the film via a chemocristallisation process113. In biopolymer, degradation first takes place with the amorphous regions of the films being degraded before the crystalline ones. Thus, the degree of crystallinity may vary throughout the degradation process114.

Hydrophobicity/hydrophilicity

The hydrophobicity/hydrophilicity of a surface depends on the nature, concentration and exposition of the functional groups present in the material. Two types of phenomena happens in oxidation and consumption of oxidized group by microorganism. If the extent of oxidation due to the action of UV light or activity of enzymes is higher than the extent of consumption of functional groups, then hydrophilicity increases while the rate of consumption of functional groups is higher than the rate of oxidation then an increase in the hydrophobicity will be observed. Hydrophobicity will determine the extent of colonization on the polymer substrate and it is accepted that more hydrophilic surfaces are more easily colonized by microorganisms39. Two methods are mainly used to determine bacterial cell-surface hydrophobicity: the bacterial adhesion to hydrocarbon (BATH) test and the salt aggregation test (SAT). The BATH is based on the affinity of bacterial cells for an organic hydrocarbon such as hexadecane. The more hydrophobic the bacterial cells, the greater their affinity for the hydrocarbon. The SAT method based on the bacterial cells in a salt solution. Higher the cell hydrophobicity, the lower the salt concentration required to reduce cell aggregation and precipitation67. Hydrophilicity and wettability of polymer is measured by contact angle. It was reported that polymer exposed to direct sun light shows higher reduction in contact angle when compared to those exposed to ocean and buried in soil65,115. The contact angle of films was measured at room temperature using contact angle measuring unit. The wetting liquid used for this purpose was Millipore grade distilled water60. Many techniques have been examined to modify the hydrophilic property of polymers. These include polymer-blend method, corona-discharge treatment, an ozone treatment, a plasma treatment, a UV irradiation, a graft polymerization, a treatment with sulfuric acid and hydrogen fluoride and the other chemical treatments116. The surface of low-density polyethylene (LDPE) was functionalized with maleic anhydride (MA) using solution grafting method in the presence of benzoyl peroxide (BPO), an initiator. The contact angle value was found to be 44±3°, which confirms the development of hydrophilicity in LDPE117. The advantage of the plasma treatment is to modify the most external layers of the material without changing its bulk characteristics118.Plasma discharge increased hydrophilicity, decreasing contact angle by 76.57% and increasing surface roughness by 99.81%. P. ostreatus colonization was 88.72% in 150 days in comparison with untreated LDPE (45.55%)119. When a very thin persulfate salt aqueous solution layer (mm) was sandwiched between two polymer films and strong UV light irradiated the assembly from the side transparent to UV light, a fast surface hydrophilic modification method for most of commercial polymeric materials was developed. For example, irradiating for 90 s and using 30 wt% ammonium persulfate, the static surface water contact angles of polymeric substrates decreased from 100 to 448 for LDPE, from 107 to 348 for HDPE. The increases in surface hydrophilicity came from the formation of a sulfate salt group (SO4¯2NH4+) -ionized surface, which was characterized by XPS and ATR-FTIR.120. Contact angle measurements demonstrated an enhancement of the surface hydrophilicity with the increase of the plasma power121. The effect of fuming conc. H2SO4 decreases the contact angle of LDPE to 63°. Without adding reactive gases, plasma treatment using low-temperature cascade arc plasma torch (LTCAT) of only Ar significantly improved the LDPE surface wettability to 40° within a very short treatment time of 2.0 s122. The degradation of abiotically aged low density polyethylene (LDPE) films containing trace quantities of a representative pro-oxidant (cobalt stearate) was investigated in the presence of well-defined enriched microbial strains namely, Bacillus pumilus, Bacillus halodenitrificans and Bacillus cereus in Basal salt medium. The films were initially subjected to an abiotic treatment comprising UV-B irradiation, and subsequently inoculated with the bacterial strains. The initial contact angle of the PLD film was 99.6± 3.5°, which decreased to 92.5± 3.5° as a result of abiotic degradation. This lowering of the contact angle is an indication of the increase in the hydrophilicity of the polymer surface. After this abiotically treated film was exposed to the biotic environment for 5 weeks, the wettability and the associated hydrophilicity of the polymer surface increased further, with the contact angle decreasing to 68±2.6°. There was, however, no such decrease in the contact angle for the control set of sample60. The contact angle of LDPE film without treatment was 98.6 + 3.5 which remained to 91.5 + 3.5 after abiotic degradation97.

Consumption of polymer

Various methods have been employed for determination of CO2 evolved during biodegradation. The effective biodegradation was assessed by monitoring the amount of CO2 developed over time in a Respirometer apparatus as per ASTM D5988-03107 . The rate of biodegradation of polyethylene, even under prolonged exposure time (10-32 years) to microbial consortia of soil, was found to be very low, thus accounting for less than 1% carbon mineralization59. The biodegradation of plastics in soils takes place partly aerobically by producing H2O and CO2 followed by partly anaerobically with producing CH4123. C-labelled polyethylene subjected to 26 days of artificial UV irradiation before buried in to soil evolved <0.5% carbon dioxide (CO2) by weight after 10 years which was reported by another researcher without prior UV treatment, <0.2% CO2 was produced93. Strum test was followed for degradation of metabolic carbon dioxide evolved during the growth period of fungus. LDPE was incubated for 4 weeks, along with A. clavatus sp. resulting in 2.32 g l-1 production of CO218. In another experiment, CO2 evolution as a result of PE biodegradation was calculated gravimetrically by Sturm Test at 30 °C for 4 weeks. About 1.85 g/l of CO2 was produced by fungus strain Fusarium sp. AF4 (isolated from sewage sludge) in case of test whereas, 0.45 g/l in case of control104. In another experiment, for measurement of CO2, the bioreactor used in the degradation studies with an aerator for supply of air and an outlet for collection of CO2 was used. In this reactor also the release of CO2 increased with time. There was no decrease in CO2 release anytime during incubation. This study showed that with increase in starch content, the CO2 release also increased78.

Benefits and challenges of biodegradation

Plastics waste leads to many problems like health issues, environmental pollutions, land pollution and greenhouse effect etc. To resolve effect of greenhouse which include land filling with landfill gas recovery, post-consumer recycling, composting of selected waste fractions and processes that reduce gases generation compared to landfilling. Many developed and developing countries converting waste to composting and anaerobic digestion of mixed waste or biodegradable waste fractions (kitchen or restaurant wastes, garden waste and sewage sludge)36. Bio-degradable plastics and Oxo-degradable plastics were developed to reduce the pollutions up to some extent. Bio-degradable plastics were originally developed in order to solve specific waste issues related either to agricultural films or collection and separation of food waste. Now a days, biodegradable polymers has been used which increased at a rate of 30% per year in some markets worldwide. Oxo-degradable LDPEs are claimed to provide a potential solution to littering issues. There is very little evidence for the fate of oxo-degradable fragments and this is an area identified as requiring further research40.

Polyethylene is used in many applications including carrier bags, packaging film, agriculture mulching film13. As it was already mentioned that the onetime use of plastics creates pollution to the environment, so awareness regarding use of plastics should be provided. Plastics products used in packaging application in the recent years have increased the quantity of plastics in the form of solid waste to a great extent. Thus, there is a proper systematic way of waste collection in urban areas, however, informal sectors i.e. rag pickers, collect only value added plastics waste such as pet bottles etc. But plastic carry bags and low quality plastic less than 20 micron do not figure in their priorities, because collecting them is not profitable which need huge man-power. The methods of recycling and the technology used for the same at present are quite outmoded and are in need of upgradation. It was observed that some of industries recycle the plastic waste/scrap which is totally unhygienic. This type of recycled plastics may create health hazard for persons who use items made from such plastics and even used at times for packaging of foodstuff and medicines36. According to Plastic Waste Management Rules in India, 2016124, following actions have been taken to reduce the plastics waste.

  1. Minimum Thickness of Plastic Carry Bags Increased from 40 to 50 Microns for easy collection and recycling. This can increase the cost of plastics for which the tendency to give free plastics bags can be avoided.
  2. The Plastic Waste Management Rules should reach rural areas too.
  3. The producers, importers, brand owners who are endorsing the use of plastic products shall also make an arrangement for collecting back all the waste material generated by their products.
  4. All institutional generators of plastic waste shall segregate their waste at the source itself and handover them to the authorized persons
  5. Reuse the plastic waste in many ways, such as in road construction, waste to oil and waste to energy so that it enhances the plastic waste recycling.
  6. Only the registered shopkeepers, or street vendors shall be eligible to provide plastic carry bags for dispensing the commodities. So there is a huge challenge to improve the above system but awareness about the recent threat may help for minimum usage. Plastics can dissolved in several solvents or suitable chemicals. Mainly three types of LDPE samples, namely virgin LDPE, LDPE waste, and LDPE powder which can be dissolved in different solvents. To determine the best solvent to be used for dissolution of LDPE, experiments have been conducted. In all the experiments, benzene was found to be able to dissolve all types of LDPE samples in the shortest time when compared to other solvents. However, when considering the safety factor, hazardous properties of benzene makes it less suitable to be used in this application. Toluene, xylene and trichloroethylene were more preferable to be used, as they have similar performance to benzene, as well as less hazardous compared to benzene125.

Conclusion

This review has discussed major concerns regarding various types of polyethylene, their impact on environment and degradability. Oxo-degradation of polyethylene mainly used as a pretreatment method for polyethylene which helps microorganism to attach with polymer. Effect of bacterial and fungal strain on HDPE, LDPE and LLDPE over degradation have been analyzed. Various parameters used for degradation studies which confirms degradation of polyethylene have been elaborated. Another approach has been made for the developments in the biodegradation of some of the new polymers, either alone or in blended films. The rate of this process is modulated by the intensity and presence of abiotic factors such as UV light or other oxidizing agents as well as by the physical and chemical properties. Though bacteria and fungus are capable to degrade polyethylene, but there is no such research for fully biodegradation of polyethylene. Polyethylene modified with natural additives and reinforcement have been discussed but microorganism cannot degrade the full material (composite, blend of polyethylene).

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Author Details

Kamalakanta Maikap and Prakash Anna Mahanwar

Department of Polymer and Surface Engineering, Institute Of Chemical Technology, Matunga, Mumbai, India