Background
The growing global population and advancements in science and technology have led to an increasing demand for plastics, which are now integral to various industries, including packaging, construction, furniture, electronics, transportation, and agriculture Annually, global plastic production reaches around 322 million tonnes, and in countries like the United States and Germany, plastics constitute approximately 20%–30% of the volume of municipal solid waste in landfills.
Australia (Adamcova & Vaverkova, 2014; Leja & Lewandowicz, 2010)
Plastic is a name given to synthetic organic polymers with high molecular weight such as low-density polyethylene (LDPE), high-density polyethylene
Synthetic polymers, including HDPE, PVC, PP, PS, polyurethane, and polyethylene terephthalate, are categorized into two main groups based on their manufacturing processes: thermoplastics, like polyethylene and polypropylene, which can be reshaped upon heating, and thermosetting plastics, such as polyurethane, which harden permanently after being shaped.
Thermoplastics are created by breaking the double bonds in olefins through additional polymerization, resulting in new carbon-carbon bonds and forming linear carbon-chain polymers Notably, polyethylene is a type of thermoplastic that can undergo cross-linking These materials can be repeatedly softened and hardened by heating and cooling, as their polymer chains are held together by intermolecular forces that diminish with temperature This property allows thermoplastics to be recycled multiple times.
Thermosetting plastics are formed by the elimination of water between a carboxylic acid and an alcohol or amine to form polyester or polyamide
Thermoset plastics feature a highly cross-linked structure that solidifies during processing, making them incapable of being melted or reformed Certain polymers, such as polyethylene and vinyl acetate-ethylene (VAE) copolymers, are utilized in both thermoplastic and thermoset forms While thermoplastics account for approximately 80% or more of total plastic consumption, thermosets represent about 12-20%.
Plastics are increasingly popular due to their numerous advantages, including low cost, lightweight nature, ease of manufacturing, versatility, thermal efficiency, durability, and moisture resistance As a result, they have replaced traditional materials like metal, glass, wood, stone, leather, paper, and ceramics in a wide array of products, showcasing their expanding role in various industries.
Figure 1.1 Types of popular plastics used in the market (Yang et al., 2018)
In 2012, the U.S Environmental Protection Agency reported that approximately 32 million tonnes of plastic waste were produced, making up 12.7% of total municipal solid waste This figure includes nearly 14 million tonnes from containers and packaging, around 11 million tonnes from durable goods like appliances, and close to 7 million tonnes from nondurable goods such as plates The consumer market for plastics is predominantly led by Asia, which accounts for about 35% of global consumption, followed by North America and Western Europe.
26% and 23% respectively China and India are the two biggest markets for plastic consumption in Asia with the growth rate of polymer consumption around 10% and 16% respectively
In Australia, total plastic consumption in 2009-2010 was 1,501,258 tonnes
In Australia, total plastic consumption reached 565,285 tons, with packaging plastics accounting for 37% of this figure Each year, billions of disposable cups, lids, and plates are discarded, predominantly ending up in landfills without being recycled (Drake, 2012).
Figure 1.2 The percentage of plastics used in Australia (A’Vard & O’Farrell,
Polystyrene (PS) is a popular plastic known for its excellent mechanical properties and affordability It is commonly utilized in various applications, including construction materials for insulation, packaging foam, food containers, disposable cups, plates, cutlery, cassette boxes, and compact discs In 2013, global production of polystyrene reached approximately 21 million tonnes.
The extensive use of plastics, particularly polystyrene (PS), has led to their accumulation in ecosystems, resulting in harmful changes due to their toxic and persistent compounds Although PS materials are recyclable, a significant portion of PS foam ultimately ends up in landfills.
PS foam is lightweight and bulky, transportation costs are a significant component of its recycling The recycling rate for PS foam in the United
States rose to 28% in 2010 from around 20% in 2008 (Rubio, 2018)
Figure 1.3 Estimated time range for plastic degradation in the marine environment (West, 2016)
The rising demand for plastic has led to a significant increase in plastic waste, exacerbating landfill space shortages and the dangers associated with waste incineration Consequently, scientists are exploring innovative waste management strategies, especially for plastic waste Under anaerobic conditions in landfills, the biodegradation of polystyrene offers a promising solution by conserving landfill space and enabling energy recovery from the biogas produced.
Future research must focus on identifying effective additives that can serve as carbon sources for microorganisms, similar to other hydrocarbons The high molecular weight of polystyrene, however, restricts its use in enzymatic reactions As demand for these polymers rises alongside stricter environmental regulations, there is a pressing need to convert them into biodegradable materials more rapidly One potential solution is the use of additives that enhance the reaction of plastic with atmospheric oxygen, facilitating the incorporation of oxygen atoms into carbon chains during the initial degradation phase Commonly used additives, including metal salts like iron, cobalt, and manganese, as well as copolymers, can significantly accelerate this biodegradation process.
(Ammala et al., 2011; Ojeda et al., 2009) However, the relation and interaction between additives and rate of biodegradation have still not been clarified
To mitigate environmental pollution from polystyrene (PS), various solutions have been implemented, including minimizing the use of PS products and adopting modified PS materials that are biodegradable Additionally, PS foam has been prohibited in certain areas, such as San Francisco, to further address these environmental concerns.
Washington DC, Paris, Toronto, and New York because of the enormous problem it causes in waterways (Daneman, 2013; McIlroy, 2015)
Researchers have been trying to modify polystyrene by blending with other materials to increase biodegradability of polystyrene in the environment
Recent studies have explored the potential of microbes to degrade polystyrene (PS) found in environments like soil, landfills, and activated sludge Two primary approaches—utilizing pure microbial strains and complex microbial communities—have demonstrated that polystyrene can be biodegradable, albeit at a slow rate Most existing research has been descriptive, with a focus on isolating microbes and enzymes capable of breaking down polystyrene chains to better understand the degradation mechanisms and the behavior of polystyrene within microorganisms However, the majority of these studies have been conducted in laboratory settings using synthesized polystyrene, often blended with more easily degradable materials like starch There is a notable lack of research on the degradation of commercial polystyrene products, which typically have a higher molecular weight and contain additives for enhanced durability Consequently, in situ studies on the biodegradation of commercial polystyrene in anaerobic environments, such as landfills, remain scarce.
Aim and objectives
The main aim of the present study was to evaluate the biodegradability of modified polystyrene The following specific objectives were chosen to elucidate this aim:
This study aims to assess the biodegradability of polystyrene, specifically focusing on beverage foam cups and HIPS lids, by measuring their biodegradation rates under real conditions found in managed landfills and soil environments.
• To determine the bacterial community in a managed landfill participating in the biodegradable process of polystyrene
• To isolate bacteria that can degrade polystyrene and investigate the end- products of the biodegradation process
All of the research works are summarised in figure 1.4
Polystyrene foam cups and High impact polystyrene lids
Biodegradation studies by isolated strain of microorganisms
Isolation of microorganisms Garden soil test
Polystyrene
Polystyrene (PS) is a high molecular weight synthetic aromatic polymer, represented by the formula (C8H8)n, derived from liquid styrene monomers This vinyl polymer can exist in both solid and foamed forms, characterized by long hydrocarbon backbones with benzene rings attached to alternating carbon atoms The production of polystyrene involves the process of free radical polymerization of styrene.
Figure 2.1 Polymerization of styrene to produce polystyrene
Polystyrene can be classified into three structural forms: isotactic, atactic, and syndiotactic Isotactic polystyrene features phenyl groups aligned on one side of the hydrocarbon backbone, while atactic polystyrene has these groups randomly distributed on both sides In syndiotactic polystyrene, the phenyl groups alternate sides along the chain Notably, isotactic polystyrene has received less research attention due to its low crystallization rate.
Syndiotactic polystyrene has a much faster crystallization rate than isotactic polystyrene However, the brittleness of syndiotactic polystyrene and the temperature above 290 o C required for its processing limits its industrial
This chapter, authored by Ho, Roberts, and Lucas in 2017, provides a comprehensive overview of the biodegradation processes of polystyrene and its modified forms, emphasizing microbial methods It highlights that atactic polystyrene is the most commercially significant variant, as noted by Laur, Kirillov, and Carpentier (2017).
Figure 2.2 Structure types of polystyrene
Polystyrene is primarily utilized in four main forms: General Purpose Polystyrene (GPPS), High Impact Polystyrene (HIPS), Polystyrene foam, and Expanded Polystyrene (EPS) foam GPPS is a clear, hard, and somewhat brittle material commonly found in food packaging, laboratory equipment, and electronics EPS is created by expanding polystyrene beads, which are then fused together, offering lightweight, strength, durability, thermal insulation, shock absorption, and moisture resistance These advantageous properties make EPS a popular choice in various applications, including building and construction (insulated panel systems), packaging, and disposable items like cups, cutlery, lids, and plates.
Polystyrene is an inexpensive resin characterized by a low melting point and low permeability to oxygen and water vapor Its outstanding physical and processing properties enable its use in a diverse array of applications, making it more versatile than other plastics For a comprehensive overview of polystyrene applications, refer to Table 2.1.
Table 2.1 Summary of industrial applications of polystyrene (American
(OPS) produce baskets pie containers cookie trays deli hinged take-out containers bakery cake domes cutlery (disposable serviceware) plates, bowls, platters (disposable serviceware) cups (disposable serviceware)
High Impact Polystyrene (HIPS) yogurt containers creamers cold drink cups lids single-service condiment containers plates (single-service and reusable) stirrers
Polystyrene foam products are widely used in the food service industry, including pre-packaged and store-packaged meat and poultry trays, cold and hot drink cups, single-service plates and bowls, hinged take-out containers, and school lunch trays Additionally, other foam sheet applications include egg cartons and trays for fruits and vegetables, highlighting the versatility of polystyrene foam in various food packaging solutions.
Foam cups and containers coolers (grape and fish boxes) insulated panel systems
History of polystyrene
Polystyrene was accidentally discovered in 1839 by German apothecary Edward Simon, who observed that an oily substance derived from the resin of the sweetgum tree, Liquidambar orientalis, thickened into a jelly-like form when exposed to air He named this substance styrol oxide In 1845, John Blyth and August Wilhelm von contributed further to its study.
Hofmann discovered that styrol undergoes changes in the absence of oxygen, naming it metastyrol In 1866, Marcelin Berthelot identified the polymerization process that transforms styrol into metastyrol Subsequently, Hermann Staudinger described a chain reaction that occurs when heating styrol, leading to the formation of macromolecules known as polystyrene In Germany, the I G Farben company began the production of polystyrene.
Ludwigshafen in 1931 Later, expanded polystyrene was produced in 1959 by the Koppers Company in Pittsburgh, Pennsylvania Polystyrene with syndiotactic conformation was synthesized for the first time in the early
Synthesis of polystyrene
The synthesis of polystyrene involves several key steps: it starts with hydrocarbon cracking of natural gas or crude oil to produce ethylene, with the cracking temperature influencing the yield Next, ethylene undergoes alkylation with benzene to create ethyl-benzene, which is then converted into styrene through dehydrogenation Finally, polystyrene is generated by the polymerization of styrene monomers.
Polystyrene products are manufactured using various techniques such as injection blow moulding, extrusion, injection stretch blow moulding, and thermoforming, depending on their intended applications General purpose polystyrene is primarily produced through extrusion and injection moulding, while thermoforming is commonly employed to create expanded polystyrene foam products.
Expanded polystyrene (EPS) is created by expanding polystyrene beads with a low boiling point blowing agent, such as pentane, resulting in a low-density foamed material The steam process softens the thermoplastic polystyrene, and the internal pressure from the blowing agent facilitates the expansion and adhesion of the beads Consequently, EPS consists of 95-98% air, making it an efficient lightweight material.
Figure 2.3 Production of EPS and HIPS pellets (Polystyrene packing Council, n.d.)
Other polystyrene blends and copolymers
General purpose polystyrene has a low melting point and limited resistance to chemicals, scratching, and impact, which restricts its use in high-performance applications To enhance its properties for specific uses, it is often blended with other monomers like butadiene and acrylonitrile, resulting in materials such as high-impact polystyrene (HIPS) and acrylonitrile butadiene styrene (ABS) These modified polystyrenes exhibit improved impact resistance, toughness, and heat resistance, making them suitable for electrical and electronic equipment.
HIPS, or high-impact polystyrene, is a thermoplastic elastomer created from poly (styrene-butadiene-styrene), where polybutadiene's double bonds facilitate polymerization with styrene to form a graft copolymer This rubber content enhances flexibility and reduces the softening point, enabling efficient thermoforming of HIPS Additionally, it offers improved impact resistance and barrier properties, although it has diminished transparency.
ABS, or acrylonitrile-butadiene-styrene, is a versatile blend polymer created from acrylonitrile, butadiene, and styrene, each derived from various chemical processes Acrylonitrile is synthesized from propylene and ammonia, while butadiene is obtained through the steam cracking of petroleum hydrocarbons The unique composition of ABS allows for flexibility in its structure and properties, making it suitable for a wide range of applications, particularly in the electronics and automotive industries Additionally, ABS can be blended with other polymers, such as polycarbonate and polyvinyl chloride, to enhance its performance for specific uses (Subramanian, 2017).
Styrene-acrylonitrile copolymer (SAN), a rigid, transparent plastic produced by the copolymerization of styrene and acrylonitrile in a ratio of approximately
SAN, a blend of 70% styrene and 30% acrylonitrile, has been utilized in various industries since the 1950s, including automotive parts, battery cases, kitchenware, appliances, furniture, and medical supplies This material offers superior clarity and rigidity akin to polystyrene, while also providing the hardness, strength, and resistance to heat and solvents characteristic of polyacrylonitrile As a result, SAN exhibits enhanced mechanical properties compared to both polystyrene and polyacrylonitrile, making it a preferred choice in automotive manufacturing, home wiring, and other applications (Wang et al., 2008).
Uses of polystyrene
Polystyrene is one of the most frequently used thermoplastic materials after polyolefin and polyvinyl chloride (Chemistry Research and Applications,
Polystyrene plays a crucial role in various aspects of daily life, particularly in the food service industry and protective packaging for electrical, pharmaceutical, and retail sectors due to its lightweight, shock-resistant, and cushioning properties Expanded polystyrene (EPS) is especially valued for its thermal insulation capabilities, making it ideal for applications in cold rooms, refrigeration, and building insulation This versatile material is marketed under several trade names, including Styrofoam™, Styropor®, and Depron XPS®.
In 2013, global polystyrene production reached approximately 21 million tonnes, with brands such as Styraclear®, Lustrex®, SABIC® PS, and INEOS® Styrenics leading the market (Yang et al., 2015a, b) Unfortunately, the majority of polystyrene waste was sent to landfills, with a minimal recycling rate; in the United States, less than 1% of polystyrene waste was recycled in 2012.
Table 2.2 Plastic waste generation and recovery in the United States, 2012
Treatment of polystyrene wastes and its effects on the environment
Polystyrene waste presents significant challenges for treatment, as methods like thermal decomposition can generate harmful pollutants, including dioxin precursors While recycling is viewed as a viable environmental solution, the lightweight and bulky nature of foamed polystyrene results in high transportation costs, making recycling often more expensive than producing new plastic As a result, a majority of polystyrene waste ends up in landfills, contributing to landfill space scarcity and escalating waste disposal costs.
Polystyrene waste has become a significant environmental concern due to ineffective treatment options, leading to pollution, health issues, and ecosystem alterations caused by its toxic compounds A large portion of floating marine debris consists of plastics, particularly Styrofoam, which severely impacts marine life and natural ecosystems Over time, environmental factors contribute to the breakdown of polystyrene into microplastics, tiny particles ranging from 5 mm to less than 1 micrometre These microplastics are ingested by various marine organisms, including zooplankton, mussels, and fish, potentially entering human food chains Recent studies have detected microplastics from polystyrene in ocean waters, highlighting their pervasive presence Additionally, research indicates that polystyrene can degrade relatively quickly into microplastic particles in salt marshes, occurring within eight weeks.
2016) Lambert and Wagner (2016) also reported that weathering of polystyrene generated small particles, especially in the nanometre range
Polystyrene is produced from styrene, a volatile and colorless liquid with a strong odor While styrene is generally safe in small amounts found in air or food, it can irritate the eyes and mucous membranes and lead to gastrointestinal issues upon contact Additionally, styrene and its metabolites pose serious health risks, including neurological impairment and liver toxicity (Mooney, Ward, & O’Connor, 2006) In natural environments, some microbes can metabolize styrene, converting it to styrene epoxide, a process influenced by factors such as fat content, temperature, and exposure duration (Khaksar and Ghazi-Khansari, 2009).
Additives used in polystyrene manufacturing can lead to significant adverse effects Research by Rani et al (2017) identified hazardous hexabromocyclododecanes, commonly found in expanded polystyrene buoys used in aquaculture, contaminating nearby marine sediments and mussels Additionally, decabromodiphenyl ether (decaBDE) and Antimony Trioxide (Sb2O3), often used in high-impact polystyrene (HIPS) as flame retardants, have been classified as potential human carcinogens (Sekhar et al., 2016).
Biodegradation of polystyrene and polystyrene blends
Polystyrene is a durable thermoplastic commonly regarded as non-biodegradable However, it does undergo biodegradation, albeit at a very slow rate in natural environments, leading to its persistence as solid waste for extended periods Research by Kaplan and colleagues in 1979 indicated that in cultivated soils rich in fungi, microbes, and invertebrates, less than 1% of polystyrene is degraded after 90 days, with no significant increase in the degradation rate thereafter.
Hartenstein, Sutter, 1979) Conversely, Otake and colleagues reported that a sheet of polystyrene buried in soil for 32 years had no sign of degradation (Otake et al., 1995)
The hydrophobic characteristics of thermoplastics diminish their resistance to hydrolysis, as the molecular structure of the plastics influences the hydrophobicity of the polymer surface This hydrophobicity, in turn, impacts the ease with which microorganisms can adhere to the material (Albertsson & Karlsson, 1993).
Thermoplastics, characterized by their high molecular weights and low water solubility, resist microbial metabolism as microorganisms struggle to transport them into their cells While biological processes can initiate outside microbial cells through the secretion of extracellular enzymes, these enzymes are too large to penetrate deeply into the polymer, limiting their action to the surface where they cleave polymer chains via hydrolytic mechanisms Generally, synthetic polymers composed solely of carbon and hydrogen are not degradable by microbes due to the absence of functional chemical groups, such as carbon-to-oxygen bonds, which are essential targets for microbial enzymes The degradation process can be enhanced by the introduction of functional groups within the polymer chain.
(Albertsson, Andersson, & Karlsson, 1987; Nagai, Matsunobe, & Imai, 2005)
The incorporation of anti-oxidants, flame-retardants, processing lubricants, and stabilizers in thermoplastic manufacturing enhances resistance to oxidation and biodegradation, ultimately improving the resin's quality and lifespan Bisphenol A is a commonly used antioxidant and stabilizer in polymer products, while other additives such as antimicrobial agents help extend the shelf-life of food packaging Additionally, dyes and pigments are frequently added to enhance the aesthetic appeal of materials Recently, silver nanoparticles have emerged as effective antimicrobial agents in plastic food packaging.
Nanosilver enhances food preservation by damaging bacterial cells, weakening their membranes, and disrupting nutrient transport enzymes (Silvestre, Duraccio, & Cimmino, 2011) Additionally, stabilizer technology aims to prolong the lifespan of plastics used outdoors, particularly in hot climates with extended summer seasons (Al-Salem, 2009) UV stabilizers, such as benzophenones, hindered amines, and benzotriazoles, protect plastics from sunlight damage, preventing issues like discoloration, cracking, and brittleness.
Table 2.3 Typical commercial additives used with polystyrene
Taylor, 2002 Alicylic bromine 4% Anti-oxidant Grossman &
Lutz, 2001 bis (2, 2, 6, 6-tetramethyl-4- piperidyl)sebacate (Tinuvin 770)
Grossman & Lutz, 2001 Decabromodiphenyl oxide 1-12% Flame retardant
Grossman & Lutz, 2001; Alaee et al.,
Grossman & Lutz, 2001 Ethylene bistetrabromonorbornane dicarboximide (Saytex BT-93)
Grossman & Lutz, 2001; Alaee et al.,
Taylor, 2002 Octadecyl 3,5-di-tert-butyl-4- hydroxycinnamate (Irganox 1076)
Since the advent of synthetic plastics, research has predominantly concentrated on creating durable and slowly degrading materials However, the growing issues of limited landfill space, the dangers associated with waste incineration, and rising disposal costs have prompted scientists to explore innovative waste management solutions.
The biodegradation of synthetic polymers presents a promising solution to environmental challenges, with early research by Sielicki et al (1978) focusing on the biodegradation of polystyrene and its derivatives in various environments Subsequent studies have explored the biodegradation of diverse polystyrene types using a range of analytical methods, with a comprehensive summary of this research documented in Table 2.4.
Table 2.4 Summary of studies on biodegradability of polystyrene and modified polystyrene
Blend of polystyrene and starch irradiated
Samples were buried for 6 months in soil including agricultural and desert soils
Fourier transform infrared (FTIR), swelling behaviour, mechanical properties, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to measure biodegradability
The Polystyrene was generally found to be more resistant to the biodegradation in the two types of soil
The degradation of irradiated Polystyrene/10 wt% Starch bio-blend at a dose of 5 kGy in agricultural soil was slightly higher than that in desert soil
Irradiated PSty/10 wt% Starch bio-blend at a dose of 5 kGy could be used as a potential candidate for packaging material due to the improvement in its mechanical and thermal properties
Polystyrene films subjected to pure
(a solution of hydrogen peroxide with
The films were incubated with brown-rot fungus Gloeophyllum trabeum up to 20 days
Biodegradation was analysed by gravimetry, water contact angle (CA) measurement and X-ray photoelectron spectroscopy
There was signal of superficial oxidation; however, the overall effects on the polymer were only slight
Polystyrene and poly(lactic acid)
PS:PLA filled with organically modified montmorillonite
The samples were incubated with fungus Phanerochaete chrysosporium up to 28 days
The scanning electron microscope was utilized to investigate microorganism growth and fractures within the polymer matrix Additionally, the study examined changes in extracellular protein content, biomass production, and the percentage of degradation over time in the incubated samples.
The PS:PLA:OMMT (at 5 phr OMMT content) and PS:PLA (at 30% PLA) composites show an increment in degradation
The presence of OMMT leads to faster degradation of PS:PLA:OMMT nanocomposites, which decreases in mechanical property by 30% of PLA and 5 wt% of OMMT content
Polystyrene Two bacterial strains TM1 and ZM1 (isolated from guts of two worms Tenebrio molitor and
Zophobas morio) were incubated with PS emulsion
Biodegradation was evaluated by turbidity assay
TM1 and ZM1 could utilize polystyrene as their carbon sources
Yeast extract was a very important co-factor for the TM1 and ZM1 with more efficient PS degrading ability
The samples were incubated with fungus
Penicillium variabile CCF3219 for 16 weeks
The samples were also pre-treated by ozonation to find its effect on the
The fungi mineralised both the labelled polymers, and that the [U-ring-14C]-PS with a lower molecular weight led to a higher mineralisation rate
Tian et al., 2017 on the ring ([U- ring- 14 C]-PS) and labelled at the β- carbon position of the alkyl chain
14CO2 was captured to calculate the mineralisation of 14 C-PS
Biodegradation was analyzed using scanning electron microscopy, Fourier transform infrared spectrometry, and gel-permeation chromatography The surface of the polymer became uneven and rough after incubation, indicating that the fungus had attacked the material.
Ozonation generated carbonyl groups on the [β- 14 C]-PS and the amount of the carbonyl groups decreased after incubation of the [β-
The molecular weights of the ozonated [β- 14 C]-
Ozonation pre-treatment could be useful for degradation of PS waste and remediation of PS-contaminated sites
High impact polystyrene with decabromodiphe nyl oxide and antimony trioxide
Enrichment medium containing the test samples was used to isolate microbial cultures
16S rRNA sequencing was used to identify isolated bacteria
Fourier transform infrared, thermogravimetric analysis, Nuclear magnetic resonance and scanning electron microscopy were used to measure biodegradability
Four bacterial strains were isolated and identified as Enterobacter sp., Citrobacter sedlakii, Alcaligenes sp and Brevundimonas diminuta
12.4% (w/w) of the test sample lost within 30 days using an isolate, Enterobacter sp
Polystyrene intermediates were detected in the degradation medium
High-impact Bacillus spp and Pseudomonas spp were identified through 16S rRNA sequencing as effective degraders of polystyrene, specifically high-impact polystyrene (HIPS), which was isolated from soil and used as the sole carbon source in the study conducted by Mohan et al.
These techniques of HPLC, NMR, FTIR, TGA and weight loss analysis were used to confirm biodegradation weight loss of 23% (w/w) of HIPS film in 30 days
Reduction in turbidity in four days incubation of HIPS emulsion with Bacillus spp and
Pseudomonas spp was 94% and 97%, respectively
PS degrading micro-organisms were isolated from five different soil samples collected at five different locations
The degradation rate of five strains of microorganism including Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, Streptococcus pyogenes, and
Aspergillus niger on plastic samples was observed separately by calculating percentage weight loss
The maximum percentage of biodegradation of
PS was by Gram negative cocci (in single) isolated from garbage soil after four months of incubation period
The percentage loss in weight of PS was highest by Bacillus subtilis
Mealworms (the larvae of Tenebrio molitor
Linnaeus) were fed with Styrofoam as a sole diet
Gel permeation chromatography, solid-state
13C cross-polarization/magic angle spinning
- 47.7% of the ingested Styrofoam carbon was converted into CO2
- 49.2% was egested as fecula with a limited fraction incorporated into biomass
Yang et al., 2015a spectroscopy, and thermogravimetric Fourier transform infra-red (TG−FTIR) spectroscopy were used to analyse fecula egested from Styrofoam-feeding larvae and incorporated into lipids
0.02 mm thick were synthesized in lab condition
Mealworms (the larvae of Tenebrio molitor
Linnaeus) were fed with the test samples
Exiguobacterium sp strain YT2 from guts of the mealworms that ate the polystyrene film was isolated and identified by 16S rDNA sequencing
PS biodegradation by the isolated was characterized by weight loss and molecular weight shifts after 60 days of incubation
Water contact angle (WCA), GC/MS, X-ray photoelectron spectroscopy (XPS) and SEM were used to confirm the biodegradation
Exiguobacterium sp could form biofilm on PS film over 28 days of incubation and made obvious pits and cavities (0.2−0.3 mm in width) on PS film surfaces
This strain was able to degrade 7.4 ± 0.4% of the PS pieces (2500 mg/L) over 60 days of incubation in suspension culture
Disc-shaped samples measuring 3 cm in radius and 0.5 cm in thickness were buried at a depth of 6 cm in a 1-liter vessel containing three different types of commercially available soils for a duration of 6 months.
Monitor mass degree The mass of samples
During biodegradation process in PS-graft- starch copolymers, only starch was degraded, while polystyrene remains intact
The highest degradation rate was achieved in soil for cactus growing (81.30 %)
Nikolic, Velickovic, & Popovic, 2014 was measured every 15 days
Fourier transform infrared spectroscopy and scanning electron microscopy were used as methods for characterization of grafted copolymers of polystyrene and starch
50 mg of PS/CaSO4 nanocomposites films in
50 ml of mineral salts basal were inoculated with 2 ml of a Gram-positive bacterium,
The biodegradation process was evaluated through various methods, including the quantitative estimation of bacterial biomass in biofilms, weight loss studies, FTIR and Raman spectroscopy, gel permeation chromatography, contact angle measurements, GC-MS analysis, and CO2 release assessments.
There is a steep increase in protein content over three weeks of incubation
A linear positive correlation was observed between the biomass attached on the polymer surfaces and weight loss over the whole incubation period studied
Grafted copolymers of corn starch and polystyrene (PS)
The synthesized copolymers and products of degradation were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy Biodegradation was
The starch–graft-poly(methacrylic acid) copolymers had completely degraded after 21 days, the starch–graft–polystyrene had partially degraded (45.8–93.1 % mass loss)
2013 and poly(methacrylic acid) of microorganisms by the Koch method
PS:PLA:organica lly modified montmorillonite
Confirmation of surface modification using FTIR
Put these polymers in broth medium containing pure Pseudomonas aeruginosa in shaking incubator in 28 days at room temperature
Determination of biomass, protein and degradation
SEM was used to view film surfaces
All composition supported to the degradation nature properly
The bacterial growth and extracellular protein concentration varies with various composition
PS: PLA and 2 phr (parts per hundred parts of resin) PS: PLA: OMMT nanocomposite showed maximum degradation efficiency
2012 loose-fill foams contain corn starch and polystyrene at ratios of 70:30 and 80:20
The structures and biodegradability of loose- fill foams were evaluated using a laboratory composting system, five 6 L chambers
Biodegradability was measured by determining the percentage of CO2 in the exhaust gas emitted from individual chambers A gas chromatograph was used to record the CO2 concentrations, and net CO2 production was calculated by subtracting the baseline levels.
The CO2 generation peaked after about 15 days of composting, and then decreased The rate and amount of CO2 eluted depended on the starch content in the foams
At the end of the composting tests, the remaining foam material had fibrous and crumbly textures, presumably consisting primarily of polystyrene
FTIR and NMR spectra of the foams, taken after 39 days of composting, did not reveal the
CO2 production in the control (chamber) SEM was used to view the microstructures of foams spectral features of starch, thereby confirming the decomposition of the starch
Polystyrene (PS) and Expanded polystyrene
(2%) in chloroform was casted on petri plates to get thin films (0.3 - 0.5 mm)
The films remained buried in garden soil for eight months
Bacteria were isolated and identified molecular characterise
FTIR spectroscopy was employed to study surface changes of polystyrene films
Biodegradation products were analysed by High pressure liquid chromatography
The bacterial isolated strains were identified as
NA26, Bacillus sp NB6, and Pseudomonas aeruginosa NB26 demonstrated the ability to extract carbon from complex polystyrene molecules; however, the process was notably slow and did not result in significant chemical alterations to the surface.
(films were 2mm in thickness)
The samples were evaluated by soil burial test under laboratory conditions for a period of 60 days
Weight loss of the specimens with time was used to evaluate degradation
The morphology of the samples was observed with a scanning electron microscope
The microbial activity inside the specimens was accelerated in the first 15 days of evaluation
There were significant differences in film images after 30 days of incubation in soil
Fractured surfaces covered with a heterogeneous microorganism community
2010 were mixed in different ratios
TPS was obtained by mixing starch powder, water and glycerol or buriti oil in
(mass/vol/vol) ratios box to allow the samples to be attacked by the microorganisms and moisture The box was buried at a depth of 7 - 9 inches beneath the soil surface
Thermogravimetry was used to determine the mass loss and decomposition temperature (Td) of the blends presented only one thermal degradation stage with a significant increase in mass loss al., 2009
Atactic polystyrene homopolymer samples (Mw) of
(without pro- oxidant additive) and 286,000 g
The samples underwent degradation through ultraviolet radiation and heat across three distinct time intervals The oxidized surface residues that detached from the samples were then incubated in stabilized urban waste compost at 58°C or in an aqueous mineral medium at 25°C, which was inoculated with urban waste compost.
Analytical techniques used in biodegradation studies
Visual observation is a crucial method for assessing the surface morphology changes in biodegraded polymers, providing an initial estimate of biodegradation Key indicators of this process include surface roughening, loss of smoothness, the emergence of holes and cracks, color alterations, and the presence of microbial colonies These visual changes serve as primary indicators of potential microbial activity on the material.
Microbial colonies are visible to the naked eye, and their quantities on Petri dishes can be estimated through various standardized tests (Lucas et al., 2008; Nikolic et al., 2014; Tang et al., 2017) For a deeper understanding of degradation mechanisms, advanced techniques such as scanning electron microscopy, photonic microscopy, polarization microscopy, electronic microscopy, atomic force microscopy, and scanning force microscopy are recommended While visual observation methods are simple, quick, and cost-effective, they primarily yield qualitative results, as microbial colonies may be consuming additives in the polymer rather than the polymer itself.
Furthermore, some structural differences may be due to physical/chemical degradation rather than biodegradation (Moore & Saunders, 1997)
2.8.2 Changes in mechanical properties and molar mass
Stress-strain tests (tensile strength, elongation at break, modulus, and yield stress) are used to measure mechanical changes during degradation
One limitation of relying on mechanical properties to estimate biodegradability is that they only reflect the initial stages of the biodegradation process Typically, these mechanical properties are utilized to complement findings from other testing methods.
A decrease in average molecular weight and a broader molecular weight distribution are initial indicators of polymer degradation, reflecting bulk deterioration While biodegradation starts on the polymer's surface, changes in molecular weight may not reveal significant weight loss, yet they can indicate where cleavage occurs in the polymer chain This measurement is a straightforward method for assessing biodegradation, and when combined with other techniques, it serves as a valuable indicator of a polymer's biodegradability Gel permeation chromatography is a reliable method for measuring the molecular weight of biodegradable polymers.
The measurement of weight loss is a commonly utilized method for estimating biodegradation, particularly when polymers are exposed to specific microbes in culture media where the polymer serves as the sole carbon source This technique is standardized for both in-field and simulation biodegradability tests, as outlined in ASTM D7473-12 (2012) However, challenges may occur regarding the proper cleaning of samples or excessive disintegration of the material, as noted by Muller (2005) It's important to recognize that weight loss measurements may not accurately reflect material biodegradability, since this loss can result from the evaporation of volatile and soluble impurities.
(Lucas et al., 2008) Furthermore, the method only addresses the early stages of the biodegradation process but gives no information on the extent of mineralization (Zee, 2005)
2.8.4 Determination of biogas (CO 2 /CH 4 ) evolution
Evolved CO2 or CH4 from biodegradation serves as key analytical parameters to assess the ultimate biodegradability of polymers Under aerobic conditions, microbes oxidize carbon compounds, primarily producing CO2, which quantifies the extent of biodegradation achieved This is expressed as a percentage of the theoretical total carbon conversion to CO2, with a 60% conversion within 28 days indicating ready degradability for single polymer resins The respirometer is the primary tool for measuring CO2 production, making CO2 evolution a widely used method for assessing biodegradation (Kundu et al., 2014; Pushpadass et al., 2014) Standardized public tests for aerobic biodegradation include the modified Sturm test and laboratory-controlled composting tests, as outlined in various ISO and EN standards (ISO 14852, 1999; ISO 14855-2, 2007; ISO 17556, 2012; EN 14047, 2002).
Anaerobic tests assess biodegradation by measuring pressure and volume increases from methane production, often paired with gas chromatographic analysis Standardized public tests for anaerobic biodegradation of polymers include the anaerobic sludge test and the anaerobic digestion test, as outlined in ISO 13975:2012 and ASTM 5526–12.
Oxygen consumption during biodegradation serves as a key indicator of the process, measured by comparing biological oxygen demand (BOD) to chemical oxygen demand (COD) A respirometer is utilized to assess oxygen consumption, as outlined by Moore & Saunders (1997) This measurement method is based on the MITI (Ministry of International Trade and Industry, Japan) test and is standardized under ISO 14851:1999, ISO 17556:2012, and EN standards.
The agar plate test involves dispersing fine particles of a polymer within a synthetic agar medium, leading to an opaque appearance Upon inoculation with microorganisms, a bright halo forms around the colony, indicating that the microorganisms can depolymerize the polymer, marking the initial stage of biodegradation.
This method is primarily utilized to identify microorganisms capable of degrading specific polymers, while also allowing for semi-quantitative analysis through the examination of the growth in the clear zone (Lucas et al., 2008).
Radiolabelling is an effective non-destructive method for assessing the biodegradation of polymeric materials across various microbial environments This technique utilizes carbon isotope 14 C to label the carbon in polymers, allowing for precise and consistent measurements of biodegradability when exposed to microbial conditions.
The duration of exposure can be assessed by comparing the levels of radioactive 14 CO2 or 14 CH4 to the original radioactivity of the labeled product, with the 14 CO2 measured using a scintillation counter This technique remains unaffected by additives or biodegradable impurities in the polymer However, challenges include the complexity and expense of preparing radiolabeled polymers, as well as the need for specialized laboratories and equipment Additionally, licensing requirements for trained technicians and the disposal of radioactive waste present further obstacles (Zee, 2005; Shah et al., 2008).
Techniques available for evaluating PS degradation are summarized in Table 2.5.
Table 2.5 Existing techniques for assessment of polystyrene biodegradation
No Changes in properties of polymer Type of techniques References
Morphology- Micro cracks Field emission scanning electron microscopy, scanning electron microscopy
Ali & Ghaffar 2017; Atiq et al., 2010; Kundu et al., 2014; Mohan et al., 2016; Naz et al., 2013; Nikolic et al., 2014; Mor
& Silvan, 2008; Shimpi et al., 2012 Density, Contact angle, Viscosity,
Molecular Weight Distribution High Temperature Gel Permeation
Kukut et al., 2013; Kundu et al., 2014; Yang et al., 2015b
Melting and Glass Transition temperature Thermogravimetric analysis, Differential
Scanning Calorimetry Mohan et al., 2016; Schlemmer et al.,
Residual polymer Weight loss Ali & Ghaffar 2017; Asmita et al., 2015;
Kukut et al., 2013; Mohan et al., 2016; Mor & Silvan, 2008; Naz et al., 2013; Nikolic et al., 2014; Nikolic et al., 2013; Shimpi et al., 2012; Yang et al., 2015b
2 Chemical properties Fourier Transformed Infra-red
Spectroscopy Atiq et al., 2010; Kukut et al., 2013;
Kundu et al., 2014; Mohan et al., 2016; Nikolic et al., 2014; Pushpadass et al., 2010; Sekhar et al., 2016; Yang et al., 2015b
3 Molecular Weight Gas Chromatography, Nuclear Magnetic
Resonance, Gas Chromatography-Mass Spectrometry, High-pressure liquid chromatography, cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy
Atiq et al., 2010; Kukut et al., 2013; Kundu et al., 2014; Mohan et al., 2016; Pushpadass et al., 2010; Sekhar et al., 2016; Schlemmer et al., 2009; Yang et al., 2015b
5 Others Protein analysis, Turbidity Assay, plate count Kundu et al., 2014; Mor & Silvan, 2008;
Naz et al., 2013; Nikolic et al., 2013; Shimpi et al., 2012; Tang et al., 2017
Standard tests for plastic biodegradation
Many countries have attempted to standardise test methods of plastic biodegradation and currently ASTM standards, ISO standards, and
European standards are the most commonly utilized globally, while ASTM International stands out as a leading voluntary standards development organization Renowned for its high technical quality and market relevance, ASTM has established numerous technical standards that are implemented worldwide.
International standards play a crucial role in shaping the information infrastructure that influences design, manufacturing, and trade within the global economy Initially implemented in 33 European countries, these standards have also been adopted by other nations to develop their own guidelines Specifically, standards for evaluating plastic biodegradability are designed to assess plastic materials in various environmental conditions, including municipal and industrial biological waste treatment facilities, such as aerobic composting and anaerobic digestion in managed landfill sites Key standards related to this topic are detailed in Table 2.6.
Table 2.6 Standard tests for biodegradation of plastic materials
ASTM D6400-04 Standard specification for compostable plastics
ASTM D5338-11 Standard Test Method for Determining Aerobic
Biodegradation of Plastic Materials under Controlled Composting Conditions, Incorporating Thermophilic Temperatures
ASTM D6691-09 Standard test method for determining aerobic biodegradation of plastic materials in the marine environment by defined microbial consortium or natural sea water inoculum
ASTM D7081-05 Standard specification for non-floating biodegradable plastics in the marine environment ASTM D5988-18 Standard Test Method for Determining Aerobic
Biodegradation of Plastic Materials in Soil
ASTM D5210-92 Standard Test Method for Determining the Anaerobic
(2007) Biodegradation of Plastic Materials in the Presence of
Standard Test Method for Determining the Aerobic Degradation and Anaerobic Biodegradation of Plastic Materials under Accelerated Bioreactor Landfill Conditions
ASTM D5511-12 Standard Test Method for Determining Anaerobic
Biodegradation of Plastic Materials under High-Solids Anaerobic-Digestion Conditions
ASTM D5526-12 Standard Test Method for Determining Anaerobic
Biodegradation of Plastic Materials under Accelerated Landfill Conditions
ASTM D6776-02 Standard test method for determining anaerobic biodegradability of radiolabelled plastic materials in a laboratory-scale simulated landfill environment
ISO 10210:2012 Plastics – Methods for the preparation of samples for biodegradation testing of plastic materials
ISO 13975:2012 Plastics – Determination of the ultimate anaerobic biodegradation of plastic materials in controlled slurry digestion systems – Method by measurement of biogas production
ISO 14851:1999 Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium – Method by measuring the oxygen demand in a closed respirometer
ISO 14852:2018 Determination of the ultimate aerobic biodegradability of plastic materials in an aqueous medium – Method by analysis of evolved carbon dioxide
ISO 14853:2005 Plastics - Determination of the ultimate anaerobic biodegradation of plastic materials in an aqueous system – Method by measurement of biogas production
Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions — Method by analysis of evolved carbon dioxide – Part 1: General method
ISO 14855- Determination of the ultimate aerobic biodegradability conditions – Method by analysis of evolved carbon dioxide – Part 2: Gravimetric measurement of carbon dioxide evolved in a laboratory-scale test
ISO 15985:2004 outlines a method for assessing the ultimate anaerobic biodegradation and disintegration of plastics under thermophilic conditions (52°C ± 2°C) by analyzing biogas releases during high-solids anaerobic digestion In contrast, ISO/DIS 17556 focuses on determining the ultimate aerobic biodegradability of plastic materials in soil by measuring oxygen demand in a respirometer or the carbon dioxide produced These standards are essential for evaluating the environmental impact of plastic waste management.
ISO 17088 Specifications for compostable plastics
EN 13432- 2000 Requirements for packaging recoverable through composting and biodegradation – Test scheme and evaluation criteria for the final acceptant of packaging
Plastics Evaluation of compost ability Test scheme and specifications.
Materials
There were three different plastic samples used in the research project:
1) Modified polystyrene (MPS) foam cups,
2) High-impact polystyrene (HIPS) lids,
3) Polystyrene (PS) cups, Dart ® (Dart Container Corporation, Michigan, US)
Steripak Pty Ltd, Australia supplied disposable beverage cups (MPS) and lids (HIPS) for a biodegradability assay, claiming that the foam cups are made of modified polystyrene with a 99.5:0.5 ratio of polystyrene to starch This biodegradable additive is electrostatically coated on pre-expanded polystyrene beads, as detailed in Lefebvre's Patent US 20120301648A1 The biodegradability of these products was tested in compost at Victoria University, Australia, revealing significant biodegradation under laboratory conditions, as reported by Marlene Cran in an internal report for Rema Industries & Services Pty Ltd (2011).
Dart foam cups (Stock Number: 8J8) are available in the Australian market, produced by Dart Container Corporation, the leading global manufacturer of foam cups and containers.
A photo of all test samples is shown in figure 3.1
Figure 3.1 Polystyrene samples used in this project
The basic laboratory apparatus used for the research is listed in table 3.1 Other equipment available in the laboratory was also used and is referred to in the relevant chapters
Table 3.1 List of general equipment used in the project
Analytical balance (0.001g) Sartorius Gottingen, Germany
Level Troll 700 Data logger In-Situ, USA
Variable temperature-Water baths Labec, Australia
Dry block heater Memmert Co., Germany
Vortex mixer Thermo Fisher Scientific, USA
Shaker UK Labs Direct, UK
CO2 Incubator Thermo Fisher Scientific, USA
Microflow advanced Bio safety cabinet
Micropipettes Thermo Fisher Scientific, USA
Thermometer Thermo Fisher Scientific, USA pH meter Hanna Instrument, Australia
Gas sampling bags Restek Corporation, U.S
NMR tubes Sigma-Aldrich, USA
Three-way valves (PVDF) Burkle, Deutschland
GC-MS system Agilent Technologies
FTIR system PerkinElmer UATR Two model-
NMR system Ascend TM 400, Bruker
Glassware (beakers, conical flasks, Petri dishes, test tubes, glass bottles, pipets, 2 ml vials, 10 mL vials, etc.,)
Mineral salts for microbial culture (Oxoid)
Styrene oxide (Styrene-7,8-oxide; Sigma-Aldrich)
Introduction
Polystyrene is known for its remarkable stability, potentially persisting in the environment for hundreds of years Research by Otake et al (1995) demonstrated that a polystyrene sheet buried in soil for 32 years showed no signs of degradation Similarly, Kaplan et al (1979) found that less than 1% of polystyrene degraded in cultivated soils rich in fungi, microbes, and invertebrates after 90 days, with no significant increase in degradation thereafter.
Recent research indicates that polystyrene (PS) can be degraded by environmental bacteria, although the degradation rate is relatively low Notably, the biodegradation of PS improves when it is blended with additives like corn or potato starch, as well as metal salts from carboxylic acids or dithiocarbamates containing cobalt, iron, manganese, or nickel Various companies are marketing biodegradation-promoting additives, claiming these products enhance the biodegradability of plastics under terms such as “degradable” and “oxo-degradable.”
Oxo-biodegradable, Oxo-green, and landfill degradable are terms used to describe certain types of plastics enhanced with additives Key producers of these additives include Add-X Biotech from Sweden, EKM Developments in Germany, EPI from Canada, Wells Plastics Ltd in Britain, Willow Ridge Plastics Inc in the US, d2w by Symphony International in England, and ENSO Plastics based in the US.
When applying findings from prior research to the commercial digestion of polystyrene, it is crucial to exercise caution Most studies were conducted under controlled laboratory conditions, which may not reflect real-world scenarios Key factors that significantly influence the growth of polystyrene-degrading microorganisms include pH, temperature, humidity, toxicity, oxygen levels, and interactions between different organisms.
The biodegradation rate of polystyrene is significantly influenced by uncontrollable environmental factors, while laboratory tests were conducted using pure polystyrene to assess its biodegradability.
Commercial polystyrenes used in industries such as food and construction contain various additives that impact their biodegradability The incorporation of antioxidants, flame retardants, processing lubricants, and stabilizers during manufacturing helps prevent oxidation and biodegradation, enhancing the quality and longevity of the resin Recently, silver nanoparticles have emerged as effective antimicrobial agents in plastic food packaging, as they damage bacterial cells by compromising cell membranes and disrupting nutrient transport enzymes, thereby extending the shelf life of food products.
Research indicates that the degradation rate of commercial polystyrene products is often lower than observed in laboratory tests (Silvestre, Duraccio, & Cimmino, 2011) Most polystyrene waste ultimately ends up in landfills, yet comprehensive studies on its biodegradation in such environments are lacking Previous research has been conducted under conditions that mimic landfills, but these studies fail to account for the unique anaerobic microbial communities present in actual dumps, which differ significantly from compost or soil environments Additionally, the duration of these experiments typically ranges from one to six months, a timeframe that is not representative of the decades-long processes occurring within a landfill.
Therefore, the research questions that were investigated in the work reported here were:
Could polystyrene be degraded by microbes in a landfill? And
Do the commercial biodegradation-promoting additives increase the rate of biodegradation of polystyrene?
Experimental procedure and materials
This experiment evaluated the biodegradability of three sample types in a landfill: modified polystyrene foam cups (MPS), standard polystyrene foam cups (PS), and high impact polystyrene lids (HIPS) Detailed information about these test samples can be found in Chapter 3.
The experiment was carried out at Summerhill Waste Management Centre, Wallsend city, New South Wales 2287, Australia (32°53'32.9"S
The Summerhill Waste Management Centre, located at 151°38'33.7"E, is a licensed solid waste landfill operated by The City of Newcastle, authorized by the NSW Environment Protection Authority to accept various types of domestic solid waste Positioned in an old open cut mining area, this managed landfill site utilizes evolving gas for electricity production, overseen by LMS Energy Pty Ltd, with necessary work health and safety approvals in place Additionally, leachate is collected in a holding dam at the landfill's lowest edge.
At the test site shown in the Fig 4.1, a hole (500 mm diameter) was drilled deep into the landfill to a depth of 11 meters by a third party (Goodman
Drilling & Piling Pty Ltd conducted a project involving the installation of six 12-meter PVC pipes with a 110 mm diameter, which were drilled into the ground and perforated with 10 mm holes for approximately 5 meters from the bottom to facilitate leachate movement Subsequently, five housing cases made of 90 mm diameter PVC pipes, each 750 mm in length and equipped with multiple holes for leachate interaction, were lowered into the piles using wire ropes to a depth of about 11 meters, ensuring the test samples were submerged in the leachate.
After inserting the housing cases into the pipes, the openings were securely sealed with screw caps To prevent gas from escaping the landfill into the surrounding environment, a gravel layer was placed at the bottom, followed by a two-meter clay layer on top (Fig 4.4).
Figure 4.2 Diagram of location of samples in the landfill test seen from above
Note: Number 1-5 represents test pipe containing housing cases; Level TROLL 700 data logger was installed in pipe number 6
Each of the 5 pipes was filled with the same set of samples so that one pipe could be sampled for a particular time period as shown in Table 4.1
Figure 4.3 Housing cases used for the test in the landfill
Figure 4.4 Longitudinal section of a sample in the landfill test (left) and diagram of all test samples after being installed into the landfill (right)
The degradation tests conducted in the landfill were repeated three times, with each sample weighed to ascertain its mass prior to being placed in a housing case A total of 45 sampling tests were performed, as detailed in Table 4.1, which outlines the types and quantities of testing samples used in the Summerhill landfill.
No of test samples and proposed sampling time Total
Monitoring of temperature and moisture
Over a 12-month testing period, temperature and moisture levels were continuously monitored using a Level TROLL 700 data logger, which was installed in the sixth pipe of the landfill, as illustrated in Fig 4.2 The data logger recorded daily measurements of temperature and moisture within the landfill.
Results and discussion
Initially, the plan was to analyze test samples at intervals of 30, 60, 90, 180, and 365 days to assess degradation in the landfill The evaluation aimed to include weight loss, visual observations, surface morphology changes, chemical structure alterations, and molecular weight variations However, due to logistical constraints, the analyses were conducted at 76, 165, 257, and 356 days instead Detailed methodologies for these analyses are outlined in Chapter 3.
4.3.1 Monitoring of temperature and water level
Over a 12-month testing period, the temperature within the landfill remained relatively stable, with hourly auto-recorded values ranging from 46 to 52 °C This stability was observed despite significant seasonal fluctuations in outside temperatures, which varied from 3 to 47 °C, as illustrated in Figure 4.5.
(http://www.weatherzone.com.au//nsw/hunter/wallsend)
During the second stage of anaerobic decomposition, the temperature stability of the test samples was anticipated due to the active thermophilic microorganisms, which generated significant heat This heightened microbial activity likely facilitated the decomposition of polystyrene.
During the test period, the level of leachate in the landfill changed significantly, with samples only submerged in leachate during the initial months At the Summerhill landfill, leachate is directed to a storage pool for treatment before discharge Although there was an initial proposal to pump leachate to the experiment area at the top of the landfill, this was not implemented Consequently, due to inadequate recycling of leachate, the samples did not remain in contact with it during the latter part of the field test Additionally, some leachate was trapped inside the cups after the first immersion, as the cups were consistently kept upright during installation.
Figure 4.5 Temperature ( o C) data from inside the Summerhill landfill at 11m depth from Nov 2015 to Oct 2016
Figure 4.6 Level of leachate data from inside the Summerhill landfill at 11 m depth from Nov 2015 to Oct 2016
There was no significant change in the colour of the lid samples during the test period (Fig 4.7), however compared to the control sample, the MPS and
The PS test samples exhibited dark spots due to dry sludge, which were easily removed using distilled water Remarkably, all samples retained their original shape, showing no signs of cracks or holes.
In both MPS and PS cup samples, the color changed from white to yellowish due to contact with landfill leachate, with no significant variation in color change over different test periods A thin dark layer formed inside the cups, resulting from the combination of sludge and solid particles in the leachate, which increased in intensity over time Notably, all test cups maintained their original shape without any holes or cracks, and there was no observable difference in the shape and surface between PS and MPS cups.
These changes on colour of the test samples were estimated on table 4.2 Table 4.2 Estimation of colour change of the test samples
Note: The more stars, the darker the colour; “-”: No change
Figure 4.7 HIPS lids after incubation inside the landfill for 356 days (left) and
Figure 4.8 MPS cups after incubation inside the landfill Photo a) & c): inside outside of the cup after 356 days; photo b) & d): inside and outside of the cup after 76 days a) b) c) d)
An investigation into the presence of starch as an additive in modified polystyrene cups revealed that the starch was completely lost after 76 days of burial in a landfill, while it remained visible in the control sample, which turned dark blue/black in iodine solution This complete disappearance of starch was also observed in samples tested after 165, 257, and 356 days The loss of starch is likely due to decomposition by microorganisms in the landfill, as starch is a carbohydrate that is easily degraded by these organisms.
Figure 4.9 MPS cups stained with iodine solution: test sample after 76 days inside the Summerhill landfill (left) control sample (right)
4.3.3 Surface imaging of test samples
Changes on the surface of the samples and adherence of microbes on the surface of the test samples were observed by FESEM In general, the
FESEM micrographs revealed that untreated foam cups (MPS & PS) exhibited smooth and uniform surfaces, while samples buried in the landfill displayed the presence of microorganisms.
(Fig 4.10) The bead boundaries of PS foam of the test samples seemed to be looser than those of the control sample (Fig 4.11)
Test samples of HIPS lids exhibited slight surface erosion compared to the control sample, which was not installed in the landfill (Fig 4.12) The high molecular weight and hydrophobic properties of HIPS may contribute to its reduced susceptibility to microbial effects A summary of the surface changes observed in the test samples is presented in Table 4.3.
Table 4.3 Estimation of surface change of the test samples
MPS scabrous scabrous scabrous scabrous
PS scabrous scabrous scabrous scabrous
HIPS slight scabrous slight scabrous slight scabrous slight scabrous
From table 4.3, there were two conclusions could be drawn from FESEM analysis The first, there was no significant difference of test samples (MPS,
The degradation of PS and HIPS samples over varying test durations of 76, 165, 257, and 356 days indicated that their breakdown was not influenced by the time spent in the landfill, as they were only submerged in landfill fluids for 8 weeks before drying out During the initial degradation phase, a thin film of suspended solids and microorganisms formed on the foam cup surfaces, which gradually thickened and hindered further microbial contact with the polystyrene In the unexpectedly dry conditions of the landfill, this film remained intact due to the lack of mechanical disturbances, such as leachate flow, preventing it from peeling off.
The observed changes in MPS and PS samples were not significantly different, despite MPS containing a low concentration of starch additive (0.5% w), which may have been insufficient to attract microorganisms Additionally, the additive was electrostatically coated onto the surface of pre-expanded polystyrene beads, resulting in only a minimal volume of the additive being present on the surface.
Figure 4.10 FESEM micrographs of modified polystyrene foam cups
(10,000X) Control samples (top), and the test sample after being buried in the landfill for 356 days (bottom)
Figure 4.11 FESEM micrographs of modified polystyrene foam cups (100X): sample after being buried in the landfill for 356 days (bottom) compared to control sample (top)
Note: The expanded junctions between individual polystyrene beads as
Figure 4.12 FESEM micrographs of HIPS lids (10,000X): control sample (top) and the test sample after being buried in the landfill for 356 days (bottom)
Field emission scanning electron micrographs revealed that all tested samples exhibited signs of degradation in the landfill, characterized by alterations in their microscopic morphology Additionally, bacterial cell adherence to the sample surfaces was observed, indicating microbial activity These morphological changes confirmed the degradation of starch by microorganisms, further validated by iodine solution tests conducted in section 4.3.2.
The findings of this study closely align with those reported in various previous research studies, including works by Adamcova et al (2018), Ali & Ghaffa (2017), Atiq et al (2010), Kundu et al (2014), Mohan et al (2016), Naz et al (2013), Nikolic et al (2014), Oliveira et al (2010), Pushpadass et al (2010), Shimpi et al (2012), and Yang et al (2015b).
Using scanning electron microscopy (SEM), Ali & Ghaffar (2017) observed that polystyrene-starch blend films exhibited a rough surface with numerous holes after soil burial tests, indicating microbial degradation of starch components Similarly, Mohan et al (2016) reported that high-impact polystyrene (HIPS) films without microbial treatment maintained a smooth surface, while treated samples displayed colonization of microorganisms around incisions, leading to the formation of pits and holes indicative of degradation Furthermore, Yang et al (2015b) studied the biodegradation of polystyrene by strain YT2, isolated from mealworm gut, and noted significant surface deterioration with visible pits and cavities, contrasting with the smooth surface of the control sample.
Analysis of treated foam cups and HIPS lids in the landfill using FTIR spectroscopy revealed alterations in the chemical structure of polystyrene The findings indicated changes in peak intensities across various spectral regions and the emergence of new peaks, suggesting significant digestive modifications in the polystyrene structure.