Microbial degradation of low density polyethylene

Journal of Environmental Chemical Engineering 3 (2015) 462–473

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

Review

Microbial degradation of low density polyethylene (LDPE): A review Sudip Kumar Sen, Sangeeta Raut * Department of Biotechnology, Gandhi Institute of Engineering and Technology, Gunupur, Rayagada, Odisha 765 022, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 July 2014 Accepted 8 January 2015

Biodegradation is considered to take place throughout three stages: biodeterioration, biofragmentation and assimilation, without neglect the participation of abiotic factors. However, most of the techniques used by researchers in this area are inadequate to provide evidence of the final stage: assimilation. In this review, we describe the different stages of biodegradation and we state several techniques used by some authors working in this domain. Validate assimilation (including mineralization) is an important aspect to guarantee the real biodegradability of items of consumption (in particular friendly environmental new materials). Since LDPE is considered to be practically inert, efforts were made to isolate unique microorganisms capable of utilizing LDPEs. Recent data showed that biodegradation of LDPE waste with selected microbial strains became a viable solution. Among biological agents, microbial enzymes are one of the most powerful tools for the biodegradation of LDPEs. Activity of biodegradation of most enzymes is higher in fungi than in bacteria. It is important to consider fungal degradation of LDPE in order to understand what is necessary for biodegradation and the mechanisms involved. This requires understanding of the interactions between materials and microorganisms and the biochemical changes involved. Widespread studies on the biodegradation of LDPEs have been carried out in order to overcome the environmental problems associated with LDPE waste. This paper reviews the current research on the biodegradation of LDPEs and also use of various techniques for the analysis of degradation in vitro. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Microorganisms Plastics Waste Oxodegradable Biodegradable

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benefits and challenges of bio- and oxo-degradable LDPEs . . Biodegradable LDPEs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxo-degradable LDPEs . . . . . . . . . . . . . . . . . . . . . . . . . . . Microorganisms involved in LDPE degradation . . . . . . . . . . . Isolation and screening of LDPE degrading fungal strains Effect of fungal activity on LDPE . . . . . . . . . . . . . . . . . . . . . . . Functional groups on the surface . . . . . . . . . . . . . . . . . . . Hydrophobicity/hydrophilicity . . . . . . . . . . . . . . . . . . . . . Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular weight distribution . . . . . . . . . . . . . . . . . . . . . Surface topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . Consumption of the polymer . . . . . . . . . . . . . . . . . . . . . . Mechanisms of LDPE biodegradation . . . . . . . . . . . . . . . . . . . Conclusion and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. Tel.: +91 9438450789. E-mail address: [email protected] (S. Raut). http://dx.doi.org/10.1016/j.jece.2015.01.003 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

463 464 464 464 464 466 467 467 468 468 468 468 469 469 469 470 471

S. Kumar Sen, S. Raut / Journal of Environmental Chemical Engineering 3 (2015) 462–473

Introduction The discovery of the chemical process for developed synthetic polymers (plastics) from crude oil was a breakthrough, in chemistry and in material sciences, and paved the way to the production of one of the most resourceful group of materials ever produced. Low density polyethylene (LDPE) was originally prepared by the high pressure polymerization of ethylene. Its comparatively low density arises from the presence of a small amount of branching in the chain (on about 2% of the carbon atoms) (Fig. 1). Chemically LDPE is un-reactive at room temperature although it is slowly attacked by strong oxidizing agents and some solvents will cause softening or swelling. It may be used at temperatures up to 95 C for short periods and at 80 C, continuously. Low-density polyethylene is an incompletely crystalline solid with a degree of crystallinity in the 50–60% range that leads to several properties such as opacity, tensile strength, tear strength, rigidity and chemical resistance, flexibility even at a low temperature [1,2]. Typical properties of LDPE are given in Table 1. The most common LDPE types are: linear low density polyethylene (LLDPE) and branched low density polyethylene (BLDPE). They differ in their density, degree of branching and availability of functional groups on the surface. Low density polyethylene (LDPE) has been commonly used due to its versatile characteristics and usefulness. These traits facilitated the application of plastics to almost all industrial, agricultural or domestic market. Each year, an estimated 500 billion to 1 trillion plastic bags are consumed worldwide [3]. In Japan, the percentage of municipal plastic wastes, as a fraction of municipal solid waste (MSW), that was landfilled in the early 1980s was estimated to be 45%, incineration was 50%, and the other 5% was subjected to separation and recycling (Plastic Waste Management Institute, 1985). In USA, more than 15% of the total MSW was incinerated in 1990; only about 1% of post-consumer plastics were recycled [4–6]. Since the use of plastics is gradually increasing, the problem of post-consumer recycling of these materials has become a pivotal issue for economic and environmental reasons [7]. With the excessive use of plastics, an increasing pressure is being placed on capacities available for plastic waste disposal, the need for biodegradable plastics and biodegradation of plastic wastes has gained considerable importance in the last few years [8]. As per a recent estimate of the Central Pollution Control Board, New Delhi, India, 8 million tons of plastic products are consumed every year in India alone. A study on plastic waste generation in 60 major Indian cities revealed that approximately 15,340 tons/day of plastic waste is generated in the country [9]. Reasonable transparency of thin films, free from odor and toxicity, better ductility, low water vapor permeability and heat seal ability are also the peculiarities of LDPE [10,11]. It is used for packaging applications, making trays and plastic bags for food and non-food items. It is also used as a protective coating on

Fig. 1. (a) Low density polyethylene (b) structure of polyethylene.

463

Table 1 Properties of low density polyethylene. Property

Value

Density, g/cc Hardness, shore D Tensile strength, yield MPa Tensile strength, ultiMate MPa Modulus of elasticity, GPa

0.91 44 10 25 0.2

Flexural modulus, GPa Coefficient of thermal expansion, linear 20 C, pm/m- C Melting point, C

0.4 30

Range/comments 0.910–0.925 glee 41–46 Shore D 4–16 MPa: ASTM D638 7–40 MPa 0.07–0.3 GPa; in tension; ASTM D638 0–0.7 GPa: ASTM D790 20–40 mm/m 1C; ASTM D696

115

paper, textiles and other plastics. However, LDPE is hardly degraded after disposal, which pollutes the environment and disturbs the ecosystem [12]. Steady with its inert nature, a polyethylene sheet that was kept back in moist soil for a time period of 12 years showed no confirmation of weight loss [13]. In other study, only partial degradation of polyethylene film has been observed after a long incubation of 32 years inside the soil [14]. It also poses an ever increasing ecological threat to terrestrial and marine wild life [15,16]. There are reports that suggest that polyethylene causes blockages in the intestines of fish, birds and marine mammals. In addition, entanglement in or ingestion of this waste has endangered hundreds of different species [17,18]. The increasing levels of LDPE waste, decreasing landfill capacity and very slow rate of LDPE degradation in the environment have caused the research tendency to decrease the amount of waste. Degradation of polyethylene can be classified as abiotic or biotic, the previous being defined as deterioration caused by natural factors such as temperature, UV irradiation, while the latter is defined as biodegradation caused by the participation of microorganisms that modify and consume the polymer leading to changes in its properties. It is evident from the recent studies that the speed of biotic degradation of low-density polyethylene (LDPE) can be enhanced by its prior oxidation [19,20,3,21]. It is probable that the oxidation of polyethylene generates carbonyl groups that can be used by microorganisms for its degradation [22–24]. All these reports signify that polyethylene is very much recalcitrant in comparison to natural degradation. Biodegradation of LDPE/ cellulose blends, starch–PE, plastic–PE have also been established by several workers [25–28]. During degradation process, LDPE provided carbon which is the sole energy source for microorganisms specifically, showed that small fragments were consumed faster than larger ones [29]. Recently, efforts have focused on the biodegradation of LDPE wastes due to the disadvantages of other methods such as cost and pollution. Biodegradation is the ability of microorganism to influence abiotic degradation through physical, chemical or enzymatic action [30–32]. This problem can be overcome by proper introduction of natural polymers such as starch, chitosan or cellulose in the matrix of polyethylene [33]. It is important to highlight that although the damage to LDPE is classified by only one of these two damage modes, in nature it is typical that both act cooperatively [34]. In practice, biodegradation is seldom due to a single cause, but a combined effect of heat, UV light, microorganisms, stress and water [30]. More recently it has been demonstrated in soil burial tests that the use of suitable additives in polyethylene films induced substantial oxidation with consequent fragmentation, drop in molecular weight, increase in wet ability, ultimately followed by high mineralization (60–70%) and fixation of about 8–10% of carbon into cell biomass [32,35]. The low rate of biodegradation of plastics is usually due to lack of water solubility and due to the size of the polymer molecules which prevents it to get transported directly into the cells [36,37]. The

464


two major problems with polyethylene are its high hydrophobicity (due to the presence of only —CH2 groups) and its high molecular weight (more than 30 kDa). The biotic mechanism reported for the degradation of high molecular weight polymers are due to the extra cellular enzymes produced by microorganisms which degrade the main polymeric chain and result in intermediates of lower molecular weight with modified mechanical properties, making it more accessible for the microbial assimilation [38]. Thermal or radiation treatments on polyethylene reduce the polymeric chain size and form oxidized groups such as carboxyl, carbonyl and hydroxyl. These treatments modify the properties (crystallinity level, morphological changes) of the original polymer and facilitate the polymer biodegradation [25]. The biodegradation of polyethylene has been reported in a number of research studies published over the last 30 years; however, there is general agreement that the process under normal conditions is extremely slow [39–43]. But, LDPE biodegradation is less focused in previous reviews. Till date, most of the published reviews on LDPE degradation having focuses on abiotic mechanisms of deterioration of polyethylene have been described extensively [40], thus this review will be focusing on the biodegradation of LDPE and mechanisms associated with this process. Although there is enough evidence that proves biodegradation of LDPE, there is still a lack of knowledge on the complete metabolic pathways involved in the process and in the structure and identity of all the enzymes involved. Only some advances have been made in this regard and even then the conclusions outlined require verification [44–47]. Taking into account the above mentioned facts, present communication is an attempt to focus current and last 30 years of achievement in the field of LDPE biodegradation. The present review describes three different topics, the first being a comprehensive summary of the microorganisms reportedly involved with LDPE biodegradation; second, the effects of these microorganisms on LDPE properties will be presented; and third, an outline of the degradation process of LDPE based on published literature will be discussed. Benefits and challenges of bio- and oxo-degradable LDPEs Biodegradable LDPEs 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. The majority of biodegradable LDPEs meet the requirements of well-recognized standards of industrial composting. Solid proof of biodegradation is available through certification by accredited laboratories and institutes for biodegradable plastics. One of the major strategy to facilitate dissolution and subsequent degradation is by direct degradation of LDPE by microorganisms using only the polymer as sole carbon source [3]. Previous studies have reported on biodegradation of polyethylene by bacterial [48,37] and fungal species [48–50]. There are few reports that deal with microbial degradation of polyethylene materials [31,51,52,8]. Fungi survive environments with low nutrient availability, low pH and low moisture as well. The use of biodegradable polymers is increasing at a rate of 30% per year in some markets worldwide.

as initiators of thermal and photo oxidation of polyethylene films promote the fragmentation of the tested samples eventually followed by microbial attack. Oxo-degradable LDPEs are claimed to provide a potential solution to littering issues. The very few positive biodegradation results obtained with oxo-degradable plastics were achieved in unrealistic testing environments and could not be repeated. Oxo-degradable plastics do not meet the requirements of industrial and/or home compostability set out in various established standards. Oxo-degradable plastic bags are also available today to substitute traditional LDPE plastic bags. The oxodegradable plastic bags are not biodegradable but are designed to break down into small pieces after exposure to oxygen. The smaller pieces may lead to environmental problems if they are consumed by animals or if the small pieces are scattered over the ground. The “oxo-biodegradable” additives are usually incorporated in conventional plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephtalate (PET) and sometimes also polyvinyl chloride (PVC) at the moment of conversion into final products. Narayan [53] has pointed out that oxo-degradable fragments may act to concentrate pesticide residues in the soil, as has been shown for PE and polypropylene (PP) remains found in the marine environment [54,55]. There are also concern that degraded fragments may become cross-linked and hence persist in the environment [56]. Research into the toxicological impact of oxo-degradable additives [57] found no evidence of toxicity to tomato, cucumber or cress seeds. There is evidence that plastic debris in the marine environment can degrade to give fine particles that then become ingested and accumulate in marine organisms [58,59]. No evidence was found that oxo-degradable fragments have a harmful bio-accumulative effect but neither was there evidence that they do not. There is very little evidence for the fate of oxo-degradable fragments and this is an area identified as requiring further research. Microorganisms involved in LDPE degradation Biodegradation of LDPE is complex and not fully understood. In order to elucidate the potential mechanisms, two different strategies have been followed in the literature. In the first approach, degradation studies have been performed using pure strains able to degrade LDPE [20,36,37,45–47,60–67]. This approach has the advantage of using pure strains, which is a convenient way to investigate metabolic pathways or to evaluate the effect of different environmental conditions on LDPE degradation. A disadvantage of this approach is that it ignores the possibility that LDPE biodegradation can be the result of a cooperative process between different species. These limitations are avoided by the second approach, in which the use of complex environments and microbial communities are applied [68–70,23,71,32,72–75]. Table 2 summarizes some of the different microenvironments that have been employed to study LDPE biodegradation using mixed and complex microbial communities. Marine water, soil or compost are examples of the environments whereby LDPE has been investigated under the second approach. The structure of a microbial community isolated on an LDPE surface during biodegradation experiments can also be influenced by the type of polymer used as the substrate. In several studies it has been proven that the physicochemical nature of a surface

Oxo-degradable LDPEs Oxo-degradable plastics are made of petroleum-based polymers (usually polyethylene (PE)) and contain special additives that cause them to degrade. The very low propensity to oxidation and further degradation followed by biodegradation of conventional polyethylene is widely accepted. UV, heat exposure, oxidation with nitric acid as well as the addition of pro degrading systems acting

Table 2 Different experimental environments used in the study of LDPE biodegradation. Experimental environment

References

Marine exposure conditions Composting conditions Soil burial conditions

[23,30,69–71,73,74] [32] [68,72,75]


determines the ability of microorganisms to form biofilm structures [76–79]. LDPE has more branching (on about 2% of the carbon atoms) than HDPE, so its intermolecular forces (instantaneous-dipole induced-dipole attraction) are weaker, its tensile strength is lower, and its resilience is higher. Also, since its molecules are less tightly packed and less crystalline (because of the side branches), its density is lower. LDPE contains the chemical elements carbon and hydrogen. It is important to highlight that LDPE can be also found mixed with additives such as pro-oxidants or starch [80,41], both of these being applied to improve the biodegradability of the polymer. The presence of these additives can affect the types of microorganisms colonizing the surfaces of these polymers. Over the past 50 years, a number of strains have been identified for their ability to interact with LDPE causing some kind of deterioration; this has been done based on the two approaches mentioned before, and using different kinds of LDPE. The richness of microorganisms able to degrade LDPE is so far limited to 19 genera of bacteria and 12 genera of fungi; however, these numbers are likely to increase based on the more sensitive isolation and characterization techniques based on sequencing of rDNA. This technology allows a broader approach to assessing the composition of a community, including the non-culturable fraction of microorganisms that is invisible by traditional microbiology methods yet that constitutes up to the 90% of the real biodiversity in an ecosystem [81]. Several studies have suggested partial biodegradation of polyethylene after UV irradiation [24], thermal treatment [82,83] or oxidation with nitric acid [84]. It is also reported that there is a synergistic effect between photo-oxidation and biodegradation of polyethylene [30]. An increase in the biodegradation of polyethylene was observed with increase in the time of exposure to UV [62]. Biodegradation in the un-inoculated treatments were slow and were about 7.6% and 8.6% of mineralization for the non-UV-irradiated and UV-irradiated LDPE respectively after 126 days. In contrast, in the presence of the selected microorganisms, the biodegradation was much more efficient and the percentages of biodegradation were 29.5% and 15.8% for the UV-irradiated and non-UV-irradiated films, respectively. The percentage decrease in the carbonyl index was higher for the UV-irradiated LDPE when the biodegradation was performed in soil inoculated with the selected fungal and bacterial isolates [85]. Tables 3 and 4 present an extensive list of the microorganisms that in some way have been related with LDPE colonization, biodegradation or both. This list has to be approached carefully because in some studies not all the tests required to prove LDPE biodegradation were performed. As the microorganisms possess different characteristics, so the degradation varies from one microorganism to another [86]. Recently, several microorganisms have been reported for degradation of plastics. The bacterial species identified from the polyethylene bags tested were Bacillus sp., Staphylococcus sp., Streptococcus sp., Diplococcus sp., Micrococcus sp., Pseudomonas sp. and Moraxella sp. Among the fungal species identified, Aspergillus niger,Amauroclopius ornatus, Aspergillus nidulans, Janibacter cremeus, Aspergillus flavus, Aspergillus candidus and Aspergillus glaucus were the predominant species [48]. Brevibacillus borstelensis strain isolated from soil, a thermophilic bacterium, recovered for the degradation of branched low-density polyethylene by utilizing it as the sole carbon source and energy source. The incubation of polyethylene film with B. borstelensis revealed the reduction in molecular weight of polyethylene by 30% [62]. A maximum biodegradation of 17–24% reduction in gravimetric weight after irradiation of polyethylene containing pro-oxidants followed by 90 days of incubation with the bacterium B. borstelensis at 50 C was observed. The lower value of mineralization found in this study was in the case of photodegraded LDPE-Mn

465

Table 3 Bacterial strains associated with polyethylene biodegradation. Genus

Species

References

Pseudomonas

sp. aeruginosa fluorescens macerans aquatilis sp. erythropolis rhodochrous ruber cohnii epidermidis xylosus sp. badius setonii viridosporus amyloliquefaciens brevies cereus circulans halodenitrificans mycoides pumilus sphericus thuringiensis borstelensis acidovorans sp. luteous lylae paraoxydans asteroides baumannii sp. paraffineus viscosus

[67,47,65] [66,143] [74] [74] [74] [143] [143] [64,63,116] [46,37,36] [74] [21] [74] [143] [45] [45] [45] [74] [122] [74,3,150,118] [122] [3] [74,100] [74,3,150] [118,29] [74] [62] [143] [143] [74] [74] [66] [63,116] [74] [65,150] [19,60] [74]

Paenibacillus Rahnella Ralstonia Rhodococcus

Staphylococcus

Stenotrophomonas Streptomyces

Bacillus

Brevibacillus Delftia Flavobacterium Micrococcus Microbacterium Nocardia Acinetobacter Arthrobacter

with B. borstelensis, 15.7%, and similar to that obtained by Fontanella et al. [64], 16–24% using Rhodococcus rhodochrous at 58 C on photo- and thermo-degraded LDPE containing a mixture of Mn/Fe pro-oxidants [87]. The ability of Bacillus species to utilize PE, with and without pro-oxidant additives, was also evaluated [87]. Biodegradation resulting from the utilization of polyethylene as a nutrient (carbon source) may be more efficient if the degrading microorganism forms a biofilm on the polyethylene surface. However, the hydrophobicity of the polyethylene interferes with the formation of a microbial biofilm. Attempts to facilitate colonization of polyethylene by adding non-ionic surfactants to the culture medium promoted the biodegradation of polyethylene [88,89]. Gilan et al. [36] isolated a strain Rhodococcus ruber that was found to colonize and degrade polyethylene. The ability of this

Table 4 Fungal strains associated with polyethylene biodegradation. Genus

Species

References

Aspergillus

niger versicolor flavus cladosporioides sp. redolens virens circinelloides simplicissimum pinophilum frequentans chrysosporium lecanii

[91,20,119] [149,69] [121,63] [63,116] [152] [70,69,23] [91] [121] [61] [91,20] [100] [91,71,144] [69]

Cladosporium Chaetomium Fusarium Glioclodium Mucor Penicillum

Phanerochaete Verticillium

466


bacterium to form biofilm on polyethylene was attributed to the hydrophobicity of its cell surface. Addition of a small amount of mineral oil to the culture medium increased biofilm formation and the subsequent biodegradation of the polyethylene, presumably by increasing the hydrophobic interactions between the bacterial biofilm and the polymer. Gilan et al. (2004) and Sivan et al. (2006) [36,37] isolated a biofilm producing strain of R. ruber (C208) that degraded PE at a rate of 0.86% per week. Kathiresan and Bingham [90], reported that bacteria caused the biodegradation ranging from 2.19 to 20.54% for polyethylene and 0.56–8.16% for plastics. A. glaucus was more active in degrading 28.8% of polyethylene and 7.26% of plastics within a month. This may be documented to the thickness of the polyethylene that is 5-times thinner than the plastics. Some studies have investigated the PE biodegradation process using fungal isolates, such as Phanerochaete chrysosporium [71], A. niger [20,91], and other strains of the Aspergillus genus including A. terreus, A. fumigatus [92] and A. flavus [27]. The most convenient method to determine the degradation is to measure the weight loss. The microbial enzymes catalyzed the depolymerization and thus there was weight reduction of polyethylene. Among the fungi strains, identified as Aspergillus sp. (FSM-3, 5, 6, 8) and the rest one was Fusarium sp. (FSM-10), FSM-10 (9%) and FSM-3 showed maximum (8%), both FSM-6, 8 exhibited moderate (7%) and FSM-5 showed less (5%) LDPE weight reduction and thus degradation after 60 days of incubation [93]. A. niger showed degradation of LDPE up to 5.8% in 1 month while A. japonicas showed more capability to degrade LDPE up to 11.11% in 1 month under laboratory conditions [94]. These findings corroborate the fact that fungi have better biodegradation efficiency than bacteria. Research works have shown that most of the constituents of plastics can be degraded by microbes and the plastic films can be treated by microbial systems. Acrylonitrile fibers are attacked by species of Aspergillus, Penicillium, Stachybotrys, and Nigrespora. Pullularia pullulans can degrade polycaprolactone and other aliphatic polyesters. N-alkenes, alkenes and other aliphatic hydrocarbons are readily utilized by yeasts and fungi. Since a wide variety of fungi grow and degrade plastics and their polymers, only they have to be upgraded [95]. It has been shown that the members of order Xylariales belonging to class Ascomycetae such as Xylaria also grow on the plastic strips (as a source of carbon) [96]. Microorganism for biological decomposition of polyethylene and plastics are isolated and tested for their ability in in-vivo and in-vitro condition by Nayak et al. [97]. The faster growth of fungal biomass in soil compared to bacteria [98], and the growth extension and penetration into other locations through the distribution of hyphae makes them more preferable than bacteria for degradation of LDPE. In most studies, fungi were considered for the degradation of LDPE due to their ability to form hydrophobic proteins that can attach to the polymer surface [99,100], their generation of degrading enzymes that are well-matched to the insoluble LDPE [8] and survive in environments with low nutrient, pH and moisture availability. Since a wide variety of fungi grow and degrade plastics and their polymers, only they have to be upgraded [101]. It has been recently shown that the members of order Xylariales belonging to class Ascomycetae such as Xylaria also grow on the plastic strips (as a source of carbon) [96]. Microorganisms for biological decomposition of polyethylene and plastics are isolated and tested for their ability in in-vivo and in-vitro condition by Nayak et al. [97]. Thus, characterization of LDPE degradation by wastesource fungi is major objective of this review because of their compatibility with a waste-rich environment (such as landfill and composting) that contains a variety of discarded polymers. Still a lot has to be done to isolate the right kind of microbial strain that could promote degradation of LDPE in a shorter period of time since all the previous reports showed activity only after a minimum period of 3– 4 months.

Isolation and screening of LDPE degrading fungal strains Although several microorganisms are involved in degradation of LDPE, it remains a challenging task to obtain a strain for commercial and eco-friendly degradation of LDPE. Over the years, culturable, LDPE degrading microbes have been isolated from a wide variety of sources such as soil, waste water, compost, garbage, etc. Moreover, efficient screening techniques are a prerequisite for isolation of novel strains. Kumar et al. (2010) [102] reported the techniques for isolation and screening of potential fungal strains to degrade LDPE in-vitro. Identification of microorganisms is based on their cellular fatty acid methyl ester (FAME) profiles. A very simple semi-quantitative method is the so-called clearzone test. This is an agar plate test in which the polymer is dispersed as very fine particles within the synthetic medium agar; this results in the agar having an opaque appearance. After inoculation with microorganisms, the formation of a clear halo around the colony indicates that the organisms are at least able to depolymerize the polymer, which is the first step of biodegradation. This method is usually applied to screen organisms that can degrade a certain polymer [103,104] but it can also be used to obtain semi-quantitative results by analyzing the growth of clear zones [105]. Usha et al. (2011) [106] isolated Aspergillus nidulans and A. flavus by enrichment culture using sabouraud dextrose agar medium having polyethylene powder at a final concentration of 0.1% (w/v) and the mixture was sonicated for 1 h at 120 rpm in shaker. After sonication the medium was sterilized at 121 C and pressure for 15 lbs/inch2 for 20 min. The organisms, producing zone of clearance around their colonies were selected for further analysis. The fungus was identified after staining them with cotton blue by following the keys Raper and Fennell (1987) [107]. Webb et al. (2000) [108] studied the fungal colonization and biodeterioration of plasticized polyvinyl chloride in in situ and ex situ conditions. These results suggested that microbial succession may occur during the long periods of exposure in in situ. They have identified Aureobasidium pullulans as the principal colonizing fungus and a group of yeasts and yeast-like fungi, including Rhodotorula aurantiaca and Kluyveromyces spp. To test the ability of fungal strains to attack polyethylene films, strains were cultured on mineral base agar (MBA) medium described by ASTM G21-90 (1996). This carbon free synthetic medium was supplemented with 0.02% yeast extract. pH was adjusted to 6.5. Degradative ability of the fungal isolates was examined by the quantitative assessment of growth on polyethylene sample as per ASTM G21-90 (1996) specifications. A standard rating system by ASTM for the evaluation of fungal growth on polymeric materials, for biodegradation is 0 = no visible growth, l = less than 10% growth, 2 = 10–30% growth, 3 = 30–60% growth, 4 = 60–100% growth covering surface of polymer film. However, these methods are mainly limited to culturable microbes and the full LDPE degrading potential of the microbes (culturable and nonculturable microorganisms) is not being fully examined. Researchers are now taking the efforts on identification and exploitation of LDPE degrading genes from unculturable microbes using metagenomic approach. Kathiresan (2003) [48] has reported isolating fungi from the mangrove soil which has the potential to degrade polyethylene materials. Yamada-Onodera et al. (2001) [61] isolated a strain of fungus Penicillium simplicissimum YK to bio-degrade polyethylene without additives. El-Shafei et al. (1998) [27] investigated the ability of fungi to attack degradable polyethylene consisting of disposed polyethylene bags containing 6% starch. Paul Das and Kumar (2014) [109] isolated four Aspergillus sp. (FSM-3, 5, 6, 8) and one Fusarium sp. (FSM-10) from soil samples by spread plate technique using mineral salt medium (g/l: K2HPO4 1.0, KH2PO4 0.2, NaCl 1.0, CaCl22H2O 0.002, H3BO3 0.005, NH4 (SO4)2 1.0, MgSO47H2O 0.5, CuSO45H2O 0.001, ZnSO4H2O 0.001,


MnSO4H2O 0.001, Fe2(SO4)3.6H2O 0.01, Agar 15) supplemented with 3% LDPE powder as carbon source. The developed fungal mats were subcultured on Saboraud’s dextrose agar to get pure culture and preserved in slant at 4 C. The identification of the fungal isolate was performed by recognizing the diagnostic morphological features of genera using macroscopic and microscopic examinations [85]. In addition, the molecular identification methods using PCR to amplify a segment of the rRNA operon encompassing the 5.8S rRNA gene and flanking internal transcribed spacers (ITS) is now in progress at the Iranian Biological Resource Center (IBRC). Esmaeili et al. (2013) [85] isolated A. niger from soils of landfills (in which PE wastes had been buried for different periods) on mineral medium containing PE powder (5% ethylene oligomer) as the sole source of carbon. Mishra et al. (2013) [110] isolated, Chrysonilia, Aspergillus and Penicillium using synthetic medium on the basis of microscopic examination and morphological characteristics. The fungal strain was identified with the help of “Manual of soil Fungi” [111]”. Further this taxonomic identification was confirmed to Agharkar Research Institute, Pune. During second set of experiment more fungal forms were isolated by using same synthetic medium where plastic was sole carbon source instead of glucose. About 4 different forms were found growing on powder of PVC and granules of LDPE and HDPE. These forms were species of genus, Aspergillus, Penicillium. Fusarium and Chaetomium. LDPE pieces buried in soil mixed with sewage sludge were examined microscopically after 10 months, fungal attachment was found on the surface of the plastic, indicating possible utilization of plastic as a source of nutrient [112] (Shah, 2007). The isolated fungal strains were identified as Fusarium sp. AF4, Aspergillus terreus AF5 and Penicillum sp. AF6. The ability of the fungal strains to form a biofilm on polyethylene was attributed to the gradual decrease in hydrophobicity of its surface [36]. Effect of fungal activity on LDPE Once the organisms get attached to the surface, it starts growing by using the polymer as the carbon source. In the primary degradation, the main chain cleaves leading to the formation of low-molecular weight fragments (oligomers), dimers or monomers [113]. The degradation is due to the extra cellular enzyme secreted by the organism (Table 5). These low molecular weight compounds are further utilized by the microbes as carbon and energy sources. The resultant breakdown fragments must be completely used by the microorganisms, otherwise there is the potential for environmental and health consequences [113]. In most studies, fungi have been investigated for the biodegradation of LDPE because these organisms produce degrading enzymes [8]

467

and, extracellular polymers, such as polysaccharides, which can help to colonize the polymer surface [20], and the distribution and penetration ability of the fungal hyphae is an advantage. Some studies have investigated the polyethylene biodegradation process using fungal isolates, such as Phanerochaete chrysosporium [71], A. niger [20,91], and other strains of the Aspergillus genus including A. terreus, A. fumigatus [92] and A. flavus [27]. In order to establish the extent of biodegradation of the polymer, seven different characteristics are usually monitored: functional groups on the surface, hydrophobicity/hydrophilicity, crystallinity, surface topography, mechanical properties, molecular weight distribution and consumption of the polymer. So far there have been no studies in the literature that prove incorporation of LDPE’s carbon into a microorganism’s macromolecular structure such as its DNA or polysaccharides. Functional groups on the surface An increase in the growth of fungus and some structural changes as observed by FTIR, were observed in case of treated PE which according to Jacinto indicated the breakdown of polymer chain and presence of oxidation products of PE. Non-degraded polyethylene exhibits almost zero absorbancy at those wave numbers (http://www.dasma.dlsu.edu.ph/offices/ufro/sinag/ Jacinto.htm). Absorbance at 1710–1715 cm1 (corresponding to carbonyl compound), 1640 cm1 and 830–880 cm1 (corresponding to —C¼C—), which appeared after UV and nitric acid treatment, decreased during cultivation with microbial consortia [115] (Hasan et al., 2007). Typical degradation of modified LDPE-starch mixed and formation of bands at 1620–1640 cm1 and 840–880 cm1 was also reported by [61] Yamada-Onodera et al. (2001), attributed to oxidation of polyethylene (Fig. 2b). Overall, polyethylene degradation is a combined photo- and bio-degradation process. First, either by abiotic oxidation (UV light exposure) or heat treatment, essential abiotic precursors are obtained. Secondly, selected thermophilic microorganisms degrade the low molar mass oxidation products to complete the biodegradation [116,91]. Manzur et al. (2004) [91] reported that the segments formed by the rupturing of the chains because of the biological treatment could cause the formation of the vinyl group and the increase in the double bond index (DBI). In addition, the increase in the DBI can be attributed to biotic dehydrogenation [32]. Kumar and Jha (2013) [117] reported that polyethylene biodegradation was further confirmed by an increase in the keto carbonyl bond index, the ester carbonyl bond index and the vinyl bond index, which were calculated from FT-IR spectra. Although the FTIR findings discussed might seem contradictory at first glance they reveal the degradation of LDPE to be a complex process that can differ for different

Table 5 Some of the useful fungal and bacterial enzymes for biodegradation of plastics. Source

Enzyme

Microorganisms

Plastic act as substrate

References

Fungal

Glucosidases Unknown

Aspergillus flavus Penicillium funiculosum Amycolaptosis sp. Streptomyces sp. Aspergillus oryzae Fusarium sp. Aspergillus niger Trichoderma sp. Phanerochaete chrysosporium Pestalotiopsis microspora Rhizopus delemar Rhizopus arrizus Firmicutes sp. Protobacteria sp. Pseudomonas stutzeri

Polycaprolactone (PCL) Polyhydroxybutyrate (PHB) Polylactic acid (PLA) PHB, PCL Polybutylene succinate (PBS) PCL PCL Polyurethane Polyethylene Polyurethane PCL Polyethylene adipate (PEA), PBS, PCL PHB, PCL, and PBS

[158] [158] [159] [158] [160] [159] [158] [161] [59] [161] [158] [158] [158]

Polyhydroxyalkanoate (PHA)

[159]

Cutinase

Bacterial

Catalase, Protease Urease Manganese peroxidase Serine hydrolase Lipase Unknown Serine hydrolase

468


Fig. 2. (a) Tensile strength depending on the time of exposure to UV radiation from LDPE-modified starch mixed (b) FTIR spectrum of LDPE-modified starch mixed.

microorganisms and different communities. What is certainly true is that incubation with microorganisms generates changes in the concentrations of functional groups at the surface of an LDPE substrate either because of their consumption or production. In a complex microbial community in which abiotic factors are also affecting the chemistry of the polymer the net effect observed (accumulation or consumption of functional groups) will depend on the balance of rates of oxidation and degradation, which in turn will depend on the nature of the microorganisms present. The study of the chemistry of LDPE surface turns out to be very important, because oxidized groups are more easily degraded by microorganisms [60] and because oxidized groups modulate microbial attachment by increasing the hydrophilicity of the surface [67], which implies that LDPE degradation will be boosted if a more oxidized surface is used as substrate. Hydrophobicity/hydrophilicity Hydrophobicity is an important property of the surface in biodegradation studies, because the relation between surface hydrophobicity and microorganisms will determine the extent of colonization on the polymer substrate. In general, it is accepted that more hydrophilic surfaces are more easily colonized by microorganisms [76–79]. If the extent of the oxidation process due to the action of abiotic factors such as UV light or activity of enzymes is higher than the extent of consumption of functional groups then an increase in the hydrophilicity will be observed. The viability of the isolates growing on the LDPE surface is confirmed using a triphenyl tetrazolium chloride reduction test. The viability is also correlated with a concomitant increase in the protein density of the biomass. Hydrophobicity is usually determined by the contact angle of the surface with a probe liquid such as water, the more hydrophilic the surface the smaller the contact angle with water [118,3]. A more advance approach to study hydrophilicity of surfaces is the use of YoungeDupré equation (Eq. (1)), which allows the estimation of the energy of adhesion to the solid as well as its acid (g S+), basic (g S) and Vander Waals (g SLW) components [72]. Wsl ¼ Ylv ð1 þ COS ;Þ

(1)

Crystallinity LDPE molecules are less tightly packed and less crystalline because of the side branches. It has been corroborated experimentally that, first the amorphous regions are consumed because it is

the thought they are more accessible to microorganisms. Due to consumption of amorphous portions there is an initial increase in percentage crystallinity [119,60,82,20,118,46]. Once the accessible amorphous regions have been depleted, microorganisms will start consuming the smaller crystals present [82], resulting in an increase in the proportion of larger crystals [60,82,118]. Yet there is insufficient research to date to state definitively what happens after the amorphous regions are consumed. Molecular weight distribution As a result of consumption of the lower molecular weight chains, an increase in the average molecular weight is observed after microbial attack [45,61,62,46] (Table 6). However, this result is not universal, with some authors observing only a slight change in the molecular weight distribution [116,64]. Some others have concluded that the main factor affecting the molecular weight is the effect of abiotic factors such as UV irradiation rather than direct microbial attack [64]. Ohtaki et al. (1998) [120] tested LDPE bottles exposed in aerobic soil for over 30 years, and observed some evidences of biodegradation as reduction in molecular weight by time of flight mass spectrometry (TOF-MS). For the determination of molecular weight distribution, two different approaches have been used: the most common one is the use of size exclusion chromatography techniques at high temperature [45,60,19,61,32,116,63,64]. The other possibility is the use of rheological measurements that correlate indirectly with the molecular weight distribution [62]. Some results showing the extent of reduction based on the number-average molecular weight (Mn) of polyethylene samples are presented in Table 6. Surface topography Development of microcolonies of different microorganisms on the surface of the polymer [116,36,63,37,64,121,67] as well as penetration of hyphal structures [119,82,20] have been reported as common features after microbial attack. There is enough evidence which proves that some superficial damage will be observed after LDPE surfaces have been exposed to biodegradation [122,73,74]. The structural changes in the form of pits and erosions observed Table 6 Molecular number changes in different biodegradation studies. Substrate

%D Molecular number (Mn)

References

LDPE UV irradiated LDPE LDPE + starch

34 15 17

[62] [46] [45]


through scanning electron microscopy indicated surface damage of PE incubated with Fusarium sp. AF4. That suggested that the fungal strains, especially Fusarium sp. AF4, was able to adhere to the surface of LDPE and can cause surface damage [52]. In a study by Bonhomme et al. (2003) [116], SEM evidence confirmed that microorganisms (fungi) build up on the surface of the polymer (polyethylene) and after removal of the microorganisms; the surface became physically pitted and eroded. The surface of the polymer after biological attack was physically weak and readily disintegrates under mild pressure. Otake et al. (1995) [14] reported the changes like whitening of the degraded area and small holes on the surface of PE film after soil burial for 32 years.

469

Since CO2 evolution out of the system can be monitored continuously it allow determining not only the total consumption of the polymer but also the rate of degradation [23,69,100,121]. Some authors have used this technique successfully to verify the ability of some strains to degrade polyethylene of very low molecular weight [47]. Results in weight reduction have to be read with special care when LDPE mixed with starch is used; in this case initial reduction in weight can be due to starch consumption rather than LDPE usage. Table 8 presents the main results obtained for the extent of biodegradation found in different LDPE types prepared without any oxidative treatment. Mechanisms of LDPE biodegradation

Mechanical properties Results of most of the studies on LDPE biodegradation, shows that in this form of the substrate deterioration of the mechanical properties such as breaking load, is common (Fig. 2a). Oxidationinduced changes in crystallinity and in the average molecular weight cause modification of the mechanical properties. Table 7 presents results showing changes to different mechanical properties for polyethylene after biodegradation. Biodegradation of modified polyethylene films in soils led to significant changes (i.e., elongation at brake of 98%) in their mechanical properties that are caused by biochemical modifications of the polyethylene. Compared to waste coal soil, films underwent rapid biodegradation in soils that were rich in organic matter. Fungi belonging to the genus, Gliocladium viride, Aspergillus awamori and Mortierella subtilissima, are easily able to colonize polyethylene and polyethylene modified with Bionolle [74]. Although rheological analysis can be performed to determine the storage and loss modulus of the polymer, in biodegradation studies authors have been preferred the use of a universal mechanical testing system (UMTS) for determination of mechanical properties of a polymer specimen [68,118,74]. However, it is likely that a microorganism’s effect will only be superficial in that case since the resolution of bulk testing methods commonly employed for such measurements diminish as the local surface related damage influences less of the overall sample. Consumption of the polymer Biodegradation of LDPE film was also reported as 0.2% weight loss in 10 years [23]. It is important to note that the rate and extent of polymer consumption can be extensively influenced by abiotic factors that promote oxidation. Albertsson and Karlsson (1990) [70] proved that biodegradation rate can increase from 0.2% to 8.4% by irradiating the samples with UV light before a biotic treatment. Nevertheless, some studies have reported a reduction in the weight of samples determined either by gravimetric measurements [62,37,118,72,74,67] or by CO2 evolution from the samples [70,23,69,100,121]. It is assumed that LDPE used by microorganisms as a carbon source will be finally converted to CO2 during respiration and can therefore be used as an indirect measurement of the amount of LDPE that has been used by microorganisms.

Biodegradation is governed by different factors that include LDPE characteristics, type of organism, and nature of pretreatment. The polymer characteristics such as its mobility, tacticity, crystallinity, molecular weight, the type of functional groups and substituents present in its structure, and plasticizers or additives added to the polymer all play an important role in its degradation [123,124]. During degradation, LDPE is first converted to its monomers, then these monomers are mineralized. LDPEs are too large to pass through cellular membranes, so they must first be depolymerized to smaller monomers before they can be absorbed and biodegraded within microbial cells. The initial breakdown of an LDPE can result from a variety of physical and biological forces [125]. Physical forces, such as heating/cooling, freezing/thawing, or wetting/drying, can cause mechanical damage such as the cracking of polymeric materials [126]. The growth of many fungi can also cause small-scale swelling and bursting, as the fungi penetrate the polymer solids [127]. LDPEs are also depolymerized by microbial enzymes, after which the monomers are absorbed into microbial cells and biodegraded [128]. The enzymatic degradation of LDPEs by hydrolysis is a two step process: first, the enzyme binds to the LDPE substrate then subsequently catalyzes a hydrolytic cleavage. LDPE can be degraded either by the action of intracellular and extracellular depolymerases in LDPE-degrading fungi. Intracellular degradation is the hydrolysis of an endogenous carbon reservoir by the accumulating microbes themselves while extracellular degradation is the utilization of an exogenous carbon source not necessarily by the accumulating microorganisms [129]. During degradation, extracellular enzymes from microorganisms break down complex polymers yielding short chains or smaller molecules, e.g., oligomers, dimers, and monomers that are smaller enough to pass the semi-permeable membranes. The process is called depolymerization. These short chain molecules are then mineralized into end products, e.g., CO2, H2O, or CH4, the degradation is called mineralization, which are utilized as carbon and energy source [39]. There are several papers reporting both the formation of carbonyl groups (oxidation) and reduction of molecular weight after treatment with UV light [30,69,70,63,64]. Various methods are available to estimate the biodegradability of LDPE. It is desirable to estimate the biodegradability of plastic wastes under natural condition such as soil [71]. A standard test to

Table 7 Changes in mechanical properties due to microbial activity in different polyethylene samples. Substrate

Environment

Time

%D Elongation

%D Tensile strength

References

LDPE

Waste coal Sea water Sterile seawater + B. sphericus Mineral media + Pseudomonas sp. Forest soil Cater soil Sterile seawater + B. sphericus

225 365 365 45 225 225 365

+4% 12% +2.7% NR 4% 1.5% +8.9

16.4 15 3.8 30 16.4 19.5 9.7

[74] [68] [118] [118] [74] [74] [118]

HDPE

470


Table 8 A brief summary of biodegradation of polymers. Material

Reported result

Conditions

Rate or duration of degradation

LDPE

Bioactive soil (rather shallow)

32–37 years in soil

LDPE

Partially biodegradable Biodegradable

LDPE

Biodegradable

LDPE containing totally degradable plastic additives (TDPA) and prooxidants LDPE/starch

Biodegradable

Physicochemical treatments thermal treatment at 105 and 150 C or accelerated aging treatment Polyethylene-degrading microorganism Brevibaccillus borstelensis strain 707 (isolated from soil) 1. Pre-thermally-oxidized at 55 C 2. Fragmented

LDPE with prooxidants LLDPE LDPE/LLDPE

Partially biodegradable Environmentally degradable Biodegradable

LDPE, LLDPE, HDPE, UHMWPE

Thermally degradable

LDPE, HDPE, LLDPE

Degradable

PE

Biodegradable

PAH

Biodegradable

LDPE/starch (12%)

Biodegradable

Usage of activated sludge

Reported degradation

1. About 2/3 decrease of thickness 2. Slow rate of oxidative degradation Subjected to biodegradation Morphological, structural, surface by a consortium of four fungi changes and mineralization. during 9 months Incubation of polyethylene Degradation of polyethylene in the with B. borstelensis (30 days, presence of mannitol. Maximal 50 C) biodegradation was obtained in combination with photo-oxidation. 800 weeks (600 days) in 1. Cumulative CO2 emissions inoculum 1700 mg CO2 (70 mg/g soil) 2. 44% Mineralization

References [147] [91]

[62]

[32]

1 month in inoculum

Not reported

[146]

Thermally-oxidized at 100 C

14 days

[141]

Pre-thermally oxidized in an oven 40– 70 and then in compost Addition of metals

140 days

1. Drop in molecular weight 2. Carbonyl formation 17–27% O2 consumption 1. Decrease in chemiluminensence intensity 2. Increase in oxidationrates 1. Reported density 0.96 g/cm3 2. 35% weight loss for HDPE, 5% for LDPE, >5% for NP

[138]

1. Microbial growth

[116]

Accelerated aging. Thermal degradation caused by contact with metals Artificial accelerated weathering (UV1600 h using a xenon lamp and xenon arc radiation) of 65,000 W and 800 h using UVB lamp 0.60 W/m2 irradiance at 313 nm Pre-heated at 60 C in an air oven to 1. Sterilized by UV/ simulate the effect of the compost inoculated 30 min] environment. Incubated in the presence 2. Incubated for 6 months at of selected microorganisms 27 C in soil containing 85% of water Buried in extremely acidic environment 28 days (coal runoff basin) Controlled biologically active soil

determine the biodegradation of plastic materials when exposed to soil was developed by the ASTM (2003) [133]. The microbial degradation process of polymers is initiated by the secretion of enzymes which cause a chain cleavage of the polymer into monomers. Metabolism of the split portions leads to progressive enzymatic dissimilation of the macromolecules from the chainends; eventually, the chain fragments become short enough to be consumed by microorganisms [130]. By rapid and sensitive analytical technique using Fourier transform infrared coupled attenuated total reflectance (FTIR-ATR) spectroscopy we can quantify LDPE biodegradation. Biodegradation quantification can be performed with FTIR-ATR spectroscopy using various concentrations of LDPE standards for comparison. It was observed that the percentage of transmittance at 2920 cm1 was directly proportional to the concentration of LDPE. Results indicated that M. paraoxydans degraded 61.0% of LDPE while P. aeruginosa degraded 50.5% in 2 months [66]. Breaking down large polyethylene molecules can be accomplished by enzymatic action, as proven by Santo et al. (2012) [46], who found that by incubation with the enzyme laccase the molecular weight of a polyethylene was reduced and its keto carbonyl index increased. These two factors were felt to indicate that both scission and oxidation reactions were taking place by the same enzyme. In regards to the oxidation process, Yoon et al. (2012) [47] isolated an alkane hydroxylase from the AlkB family that was active to polyethylene samples with molecular weights up to 27,000 Da. It is interesting to note that enzymes of this family have been described as microorganisms that are able to degrade hydrocarbons. In general, it is accepted

7 months

[156]

[139]

2. Erosion of the film surface

1. 60% mineralization [153] 2. CO2 production from 0–10% depending on the hydrocarbon Produced biomass 300 lg/l 7 g CO2/ [156] 50 ml 17–27% O2 consumption

that alkane hydroxylase performs the first oxidation that leads to the subsequent degradation of a hydrocarbon [131]. Kumar and Jha (2013) [117] reported that the viability of the isolates growing on the polyethylene surface can be confirmed using a triphenyltetrazolium chloride reduction test. The viability is also correlated with a concomitant increase in the protein density of the biomass. Name of the isolate Total CO2 evolved (g/L) by Modified Sturm test Aspergillus flavus 4.4507 Mucor circinelloides 5.9328 Conclusion and perspectives This review has covered the major concerns about LDPE, their types, uses and degradability. It has looked at the disposal methods and the standards used in assessing polymer degradation. Another area examined has been the developments in the biodegradation of some of the new polymers, either alone or in blended films. The above discussion illustrates conceptually different approaches to LDPE biodegradation where many groups have drawn straight forward portrait on bacterial degradation of LDPE rather than fungal. LDPE biodegradation in solid waste environments, show that isolated fungi have great potential for LDPE biodegradation in the composting process. There is great potential for the development of a process for degrading LDPE in a composting environment using fungi in the near future. Characterization of genes responsible for the production of degrading enzymes and its regulation by using genetic engineering tools, one


can genetically modify the microorganisms and use them as a superbug for degrading the recalcitrant LDPEs.There are a large number of tests which are used to determine the extent of degradation of polymers either alone or in blended forms. Many are respirometric, determining the amount of carbon dioxide released on exposure to fungi, activated sludge (aerobically or anaerobically), compost or soil. Some tests use loss of weight or change in physical properties such as tensile strength and comparison of spectroscopic (FTIR, SEM, XRD) data. It is important to have comparable international standard methods of determining the extent of biodegradation. Unfortunately, the current standards have not, so far, been equated to each other and tend to be used in the countries where they originated (e.g., ASTM (USA), DIN (Germany), JIS (Japan), ISO (international standards), CEN (Europe)). Many, which are otherwise harmonious, differ in the fine details of the testing. There is an urgent need to standardize all details so that researchers may know that they have all worked to the same parameters. It is clear that most recalcitrant polymers can be degraded to some extent in the appropriate environment at the right concentration. By judicious blending their persistence may be minimized environmentally. Screening of organisms which degrade LDPEs, or produce enzymes or enzyme systems that degrade LDPEs, may prove as environmentally profitable in the 21st century. A screening program for such organisms and enzymes is required but will require more universally uniform standards for assessment of their degradative ability. References [1] F.W. Billmeyer, Textbook of Polymer Science, second ed., John Wiley & Sons, Inc., 1971. [2] L.M. Ferreira, A.N. Falcão, M.H. Gil, Modification of LDPE molecular structure by gamma irradiation for bio applications, Nucl. Instrum. Methods B 236 (1–4) (2005) 513–520, doi:http://dx.doi.org/10.1016/j.nimb.2005.04.030. [3] P.K. Roy, S. Titus, P. Surekha, E. Tulsi, C. Deshmukh, C. Rajagopal, Degradation of abiotically aged LDPE films containing pro-oxidant by bacterial consortium, Polym. Degrad. Stab. 93 (10) (2008) 1917–1922, doi:http://dx.doi.org/ 10.1016/j.polymdegradstab.2008.07.016. [4] H. Yakowitz, Incineration of municipal solid waste: scientific and technical evaluation of the state-of-the-art by an expert panel, Resour. Conserv. Recycl. 4 (3) (1990) 241–251, doi:http://dx.doi.org/10.1016/0921-3449(90)90005-O. [5] T. Curlee, S. Das, Identifying and assessing targets of opportunity for plastics recycling, Resour. Conserv. Recycl. 5 (4) (1991) 343–363, doi:http://dx.doi. org/10.1016/0921-3449(91)90012-D. [6] G.D. Andrews, P.M. Subramanian, Emerging technologies in plastics recycling, ACS Symposium Series, 513, American Chemical Society, Washington, D.C, 1992. [7] U.S. Ishiaku, K.W. Pang, W.S. Lee, I.Z.A. Mohamad, Mechanical properties and enzymic degradation of thermoplastic and granular sago starch filled polycaprolactone, Eur. Polym. J. 38 (2) (2002) 393–401, doi:http://dx.doi.org/ 10.1016/S0014-3057(01)00125-2. [8] A.A. Shah, F. Hasan, A. Hameed, S. Ahmed, Biological degradation of plastics: a comprehensive review, Biotechnol. Adv. 26 (3) (2008) 246–265, doi:http:// dx.doi.org/10.1016/j.biotechadv.2007.12.005. 18337047. [9] Central Pollution Control Board (CPCB), Material on Plastic Waste Management, CPCB, New Delhi, India, 2013. [10] R.N. Tharanathan, Biodegradable films and composite coatings: past, present and future, Trends Food Sci. Technol. 14 (3) (2003) 71–82, doi:http://dx.doi. org/10.1016/S0924-2244(02)00280-7. [11] M. Ozdemir, J.D. Floros, Active food packaging technologies, Crit. Rev. Food Sci. Nutr. 44 (3) (2004) 185–193, doi:http://dx.doi.org/10.1080/ 10408690490441578. 15239372. [12] C. Bastioli, Handbook of Biodegradable Polymers, Smithers Rapra Technology, 2005. [13] J.E. Potts, H.H.G. Jelink, Biodegradation Aspects of Biodegradation & Stabilization of Polymers, Elsevier, 1978, pp. 617–658. [14] Y. Otake, T. Kobayashi, H. Asabe, N. Murakami, K. Ono, Biodegradation of lowdensity polyethylene, polystyrene, polyvinyl chloride, and urea formaldehyde resin buried under soil for over 32 years, J. Appl. Polym. Sci. 56 (13) (1995) 1789–1796, doi:http://dx.doi.org/10.1002/app.1995.070561309. [15] M. Shimao, Biodegradation of plastics, Curr. Opin. Biotechnol. 12 (3) (2001) 242–247, doi:http://dx.doi.org/10.1016/S0958-1669(00)00206-8. 11404101. [16] D.K. Barnes, F. Galgani, R.C. Thompson, M. Barlaz, Accumulation and fragmentation of plastic debris in global environments, Philos. Trans. R. Soc. Lond. B Biol. Sci. 364 (2009) 1985–1998, doi:http://dx.doi.org/10.1098/ rstb.2008.0205. 19528051.

471

[17] E.R. Secchi, S. Zarzur, Plastic debris ingested by a Blainville’s beaked whale, Mesoplodon densirostris, washed ashore in Brazil, Aquat. Mamm. 25 (1) (1999) 21–24. [18] L.B. Spear, D.G. Ainley, C.A. Ribic, Incidence of plastic in seabirds from the tropical pacific, 1984–1991: relation with distribution of species, sex, age, season, year and body weight, Mar. Environ. Res. 40 (2) (1995) 123–146, doi: http://dx.doi.org/10.1016/0141-1136(94)00140-K. [19] A.C. Albertsson, B. Erlandsson, M. Hakkarainen, S. Karlsson, Molecular weight changes and polymeric matrix changes correlated with the formation of degradation products in biodegraded polyethylene, J. Environ. Polym. Degrad. 6 (1998) 187–195. [20] T. Volke-Sepúlveda, G. Saucedo-Castañeda, M. Gutiérrez-Rojas, A. Manzur, E. Favela-Torres, Thermally treated low density polyethylene biodegradation by Penicillium pinophilum and Aspergillus niger, J. Appl. Polym. Sci. 83 (2) (2002) 305–314, doi:http://dx.doi.org/10.1002/app.2245. [21] S. Chatterjee, B. Roy, D. Roy, R. Banerjee, Enzyme-mediated biodegradation of heat treated commercial polyethylene by Staphylococcal species, Polym. Degrad. Stab. 95 (2) (2010) 195–200, doi:http://dx.doi.org/10.1016/j.polymdegradstab.2009.11.025. [22] A.-C. Albertsson, Biodegradation of synthetic polymers. II. A limited microbial conversion of 14C in polyethylene to 14CO2 by some soil fungi, J. Appl. Polym. Sci. 22 (12) (1978) 3419–3433, doi:http://dx.doi.org/10.1002/ app.1978.070221207. [23] A.-C. Albertsson, The shape of the biodegradation curve for low and high density polyethenes in prolonged series of experiments, Eur. Polym. J. 16 (7) (1980) 623–630, doi:http://dx.doi.org/10.1016/0014-3057(80)90100-7. [24] J.H. Cornell, A.M. Kaplan, M.R. Rogers, Biodegradability of photooxidized polyalkylenes, J. Appl. Polym. Sci. 29 (8) (1984) 2581–2597, doi:http://dx.doi. org/10.1002/app.1984.070290814. [25] B. Lee, A.L. Pometto, A. Fratzke, T.B. Bailey, Biodegradation of degradable plastic polyethylene by Phanerochaete and Streptomyces species, Appl. Environ. Microbiol. 57 (3) (1991) 678–685. 16348434. [26] C.J. Foust, R.R. Mills, T. Haas, C. Castevens, R.M. Ottenbrite, Biodegradation of LDPE/cellulose blends by common fungi, Macromol. Symp. 123 (1) (1997) 1–8, doi:http://dx.doi.org/10.1002/masy.19971230103. [27] H.A. El-Shafei, N.H. Abd El-Nasser, A.L. Kansoh, A.M. Ali, Biodegradation of disposable polyethylene by fungi and Streptomyces species, Polym. Degrad. Stab. 62 (2) (1998) 361–365, doi:http://dx.doi.org/10.1016/S0141-3910(98) 00019-6. [28] N.C. Billingham, M. Bonora, D. de Corte, Environmentally degradable plastics based on oxo-degradation of conventional polyolefins, Biodegrad. Polym. Plast. (2003) 313–325. [29] F. Kawai, M. Watanabe, M. Shibata, S. Yokoyama, Y. Sudate, S. Hayashi, Comparative study on biodegradability of polyethylene wax by bacteria and fungi, Polym. Degrad. Stab. 86 (1) (2004) 105–114, doi:http://dx.doi.org/ 10.1016/j.polymdegradstab.2004.03.015. [30] A.-C. Albertsson, S.O. Andersson, S. Karlsson, The mechanism of biodegradation of polyethylene, Polym. Degrad. Stab. 18 (1) (1987) 73–87 , doi:http://dx.doi.org/10.1016/0141-3910(87)90084-X. [31] B. Lee, A.L. Pometto III, A. Fratzke, T.B. Bailey, Biodegradation of degradable plastic polyethylene by Phanerochaete and Streptomyces species, Appl. Environ. Microbiol. 57 (3) (1991) 678–685. 16348434. [32] E. Chiellini, A. Corti, G. Swift, Biodegradation of thermally-oxidized, fragmented low-density polyethylenes, Polym. Degrad. Stab. 81 (2) (2003) 341–351, doi:http://dx.doi.org/10.1016/S0141-3910(03)00105-8. [33] D. Le Corre, J. Bras, A. Dufresne, Starch nanoparticles: a review, Biomacromolecules 11 (5) (2010) 1139–1151, doi:http://dx.doi.org/10.1021/ bm901428y. 20405913. [34] M. Hakkarainen, A. Albertsson, Environmental degradation of polyethylene, Adv. Polym. Sci. 169 (2004) 177–199, doi:http://dx.doi.org/10.1007/b13523. [35] I. Jakubowicz, Evaluation of degradability of biodegradable polyethylene (PE), Polym. Degrad. Stab. 80 (1) (2003) 39–43, doi:http://dx.doi.org/10.1016/ S0141-3910(02)00380-4. [36] I.G. Orr, Y. Hadar, A. Sivan, Colonization, biofilm formation and biodegradation of polyethylene by a strain of Rhodococcus ruber, Appl. Microbiol. Biotechnol. 65 (1) (2004) 97–104, doi:http://dx.doi.org/10.1007/s00253004-1584-8. 15221232. [37] A. Sivan, M. Szanto, V. Pavlov, Biofilm development of the polyethylenedegrading bacterium Rhodococcus ruber, Appl. Microbiol. Biotechnol. 72 (2) (2006) 346–352, doi:http://dx.doi.org/10.1007/s00253-005-0259-4. 16534612. [38] A.C. Palmisano, C.A. Pettigrew, Biodegradability of plastics, BioScience 42 (9) (1992) 680–685, doi:http://dx.doi.org/10.2307/1312174. [39] J.-D. Gu, Microbiological deterioration and degradation of synthetic polymeric materials: recent research advances, Int. Biodeterior. Biodegrad. 52 (2) (2003) 69–91, doi:http://dx.doi.org/10.1016/S0964-8305 (02)00177-4. [40] M. Hakkarainen, A. Albertsson, Environmental degradation of polyethylene, Adv. Polym. Sci. 169 (2004) 177–199, doi:http://dx.doi.org/10.1007/b13523. [41] M. Koutny, J. Lemaire, A.M. Delort, Biodegradation of polyethylene films with prooxidant additives, Chemosphere 64 (8) (2006) 1243–1252, doi:http://dx. doi.org/10.1016/j.chemosphere.2005.12.060. 16487569. [42] J. Arutchelvi, M. Sudhakar, A. Arkatkar, M. Doble, S. Bhaduri, P. Uppara, Biodegradation of polyethylene and polypropylene, Indian J. Biotechnol. 7 (2008) 9–22.

472


[43] J.P. Eubeler, M. Bernhard, T.P. Knepper, Environmental biodegradation of synthetic polymers II. biodegradation of different polymer groups, TrAC Trends Anal. Chem. 29 (1) (2010) 84–100, doi:http://dx.doi.org/10.1016/j. trac.2009.09.005. [44] R. Wasserbauer, M. Beranová, D. Vancurová, B. Dolezel, Biodegradation of polyethylene foils by bacterial and liver homogenates, Biomaterials 11 (1) (1990) 36–40, doi:http://dx.doi.org/10.1016/0142-9612(90)90049-V. 2302448. [45] A.L. Pometto, B.T. Lee, K.E. Johnson, Production of an extracellular polyethylene-degrading enzyme(s) by Streptomyces species, Appl. Environ. Microbiol. 58 (2) (1992) 731–733. 1610196. [46] M. Santo, R. Weitsman, A. Sivan, The role of the copper-binding enzyme – laccase – in the biodegradation of polyethylene by the actinomycete Rhodococcus ruber, Int. Biodeterior. Biodegrad. 208 (2012) 1–7. [47] M.G. Yoon, H.J. Jeon, M.N. Kim, Biodegradation of polyethylene by a soil bacterium and AlkB cloned recombinant cell, J. Bioremed Biodegrad. 3 (04) (2012) 145–154, doi:http://dx.doi.org/10.4172/2155-6199.1000145. [48] K. Kathiresan, Polythene and plastic-degrading microbes in an Indian mangrove soil, Rev. Biol. Trop. 51 (3–4) (2003) 629–633. 15162769. [49] M. Kim, Evaluation of degradability of hydroxypropylated potato starch/ polyethylene blend films, Carbohydr. Polym. 54 (2) (2003) 173–181, doi: http://dx.doi.org/10.1016/S0144-8617(03)00169-3. [50] M.C. Upreti, R.C. Srivastava, A potential Aspergillus species for biodegradation of polymeric materials, Curr. Sci. 84 (2003) 1399–1402. [51] R.J. Muller, J. Augusta, M. Pantke, An interlaboratory investigation into the biodeterioration of plastics, with special reference to polyurethanes; Part 1: a modified sturm test, Mater. Org. 27 (1992) 179–189. [52] A. Shah, Role of microorganisms in biodegradation of plastics, PhD Thesis, Quaid-i-Azam University, Islamabad, Pakistan, 2007. [53] R. Narayan, Fundamental principles and claims of biodegradability – sorting through the facts, hypes and claims of biodegradable plastics in the marketplace, Bioplast. Mag. 4 (2009) 28–31. [54] E.L. Teuten, S.J. Rowland, T.S. Galloway, R.C. Thompson, Potential for plastics to transport hydrophobic contaminants, Environ. Sci. Technol. 41 (22) (2007) 7759–7764, doi:http://dx.doi.org/10.1021/es071737s. 18075085. [55] R.C. Thompson, C.J. Moore, F.S. Vom Saal, S.H. Swan, Plastics, the environment and human health: current consensus and future trends, Philos. Trans. R. Soc. Lond. B Biol. Sci. 364 (1526) (2009) 2153–2166, doi:http://dx.doi.org/10.1098/ rstb.2009.0053. 19528062. [56] P. Feuilloley, G. César, L. Benguigui, Y. Grohens, I. Pillin, H. Bewa, S. Lefaux, M. Jamal, Degradation of polyethylene designed for agricultural purposes, J. Polym. Environ. 13 (4) (2005) 349–355, doi:http://dx.doi.org/10.1007/ s10924-005-5529-9. [57] E. Rojas, J. Greene, Performance Evaluation of Environmentally Degradable Plastic Packaging and Disposable Food Service Ware – Final Report, Integrated Waste Management Board, State of California, USA, 2007, pp. 1–70. [58] R.C. Thompson, Y. Olsen, R.P. Mitchell, A. Davis, S.J. Rowland, A.W. John, D. McGonigle, A.E. Russell, Lost at sea: where is all the plastic? Science 304 (5672) (2004) 838–844, doi:http://dx.doi.org/10.1126/science.1094559. 15131299. [59] M.A. Browne, A. Dissanayake, T.S. Galloway, D.M. Lowe, R.C. Thompson, Ingested microscopic plastic translocates to the circulatory system of the mussel, Mytilus edulis (L), Environ. Sci. Technol. 42 (13) (2008) 5026–5031, doi:http://dx.doi.org/10.1021/es800249a. 18678044. [60] A.-C. Albertsson, C. Barenstedt, S. Karlsson, T. Lindberg, Degradation product pattern and morphology changes as means to differentiate abiotically and biotically aged degradable polyethylene, Polymer 36 (16) (1995) 3075–3083, doi:http://dx.doi.org/10.1016/0032-3861(95)97868-G. [61] K. Yamada-Onodera, H. Mukumoto, Y. Katsuyaya, A. Saiganji, Y. Tani, Degradation of polyethylene by a fungus, Penicillium simplicissimum YK, Polym. Degrad. Stab. 72 (2) (2001) 323–327, doi:http://dx.doi.org/10.1016/S01413910(01)00027-1. [62] D. Hadad, S. Geresh, A. Sivan, Biodegradation of polyethylene by the thermophilic bacterium Brevibacillus borstelensis, J. Appl. Microbiol. 98 (5) (2005) 1093–1100, doi:http://dx.doi.org/10.1111/j.1365-2672.2005.02553.x. 15836478. [63] M. Koutny, M. Sancelme, C. Dabin, N. Pichon, A. Delort, J. Lemaire, Acquired biodegradability of polyethylenes containing pro-oxidant additives, Polym. Degrad. Stab. 91 (7) (2006) 1495–1503, doi:http://dx.doi.org/10.1016/ j.polymdegradstab.2005.10.007. [64] S. Fontanella, S. Bonhomme, M. Koutny, L. Husarova, J.M. Brusson, J.P. Courdavault, S. Pitteri, G. Samuel, G. Pichon, J. Lemaire, A. Delort, Comparison of the biodegradability of various polyethylene films containing pro-oxidant additives, Polym. Degrad. Stab. 95 (2010) 1011–1021. [65] V. Balasubramanian, K. Natarajan, B. Hemambika, N. Ramesh, C.S. Sumathi, R. Kottaimuthu, V. Rajesh Kannan, High-density polyethylene (HDPE)degrading potential bacteria from marine ecosystem of Gulf of Mannar, India, Lett. Appl. Microbiol. 51 (2) (2010) 205–211, doi:http://dx.doi.org/10.1111/ j.1472-765X.2010.02883.x. 20586938. [66] H. Rajandas, S. Parimannan, K. Sathasivam, M. Ravichandran, L. Su Yin, A novel FTIR-ATR spectroscopy based technique for the estimation of lowdensity polyethylene biodegradation, Polym. Test. 31 (8) (2012) 1094–1099, doi:http://dx.doi.org/10.1016/j.polymertesting.2012.07.015. [67] P. Tribedi, A.K. Sil, Low-density polyethylene degradation by Pseudomonas sp. AKS2 biofilm, Environ. Sci. Pollut. Res. Int. 20 (6) (2013) 4146–4153, doi: http://dx.doi.org/10.1007/s11356-012-1378-y. 23242625.

[68] J.E. Pegram, A.L. Andrady, Outdoor weathering of selected polymeric materials under marine exposure conditions, Polym. Degrad. Stab. 26 (4) (1989) 333–345, doi:http://dx.doi.org/10.1016/0141-3910(89)90112-2. [69] S. Karlsson, O. Ljungquist, A. Albertsson, Biodegradation of polyethylene and the influence of surfactants, Polym. Degrad. Stab. 21 (3) (1988) 237–250, doi: http://dx.doi.org/10.1016/0141-3910(88)90030-4. [70] A.-C. Albertsson, S. Karlsson, The influence of biotic and abiotic environments on the degradation of polyethylene, Prog. Polym. Sci. 15 (2) (1990) 177–192, doi:http://dx.doi.org/10.1016/0079-6700(90)90027-X. [71] Y. Orhan, H. Büyükgüngör, Enhancement of biodegradability of disposable polyethylene in controlled biological soil, Int. Biodeterior. Biodegrad. 45 (1–2) (2000) 49–55, doi:http://dx.doi.org/10.1016/S0964-8305(00)00048-2. [72] T. Artham, M. Sudhakar, R. Venkatesan, C. Madhavan Nair, K.V.G.K. Murty, M. Doble, Biofouling and stability of synthetic polymers in seawater, Int. Biodeterior. Biodegrad. 63 (7) (2009) 884–890, doi:http://dx.doi.org/ 10.1016/j.ibiod.2009.03.003. [73] T. Mumtaz, M.R. Khan, M.A. Hassan, Study of environmental biodegradation of LDPE films in soil using optical and scanning electron microscopy, Micron 41 (5) (2010) 430–438, doi:http://dx.doi.org/10.1016/j.micron.2010.02.008. 20207547. [74] B. Nowak, J. Paja˛k, M. Drozd-Bratkowicz, G. Rymarz, Microorganisms participating in the biodegradation of modified polyethylene films in different soils under laboratory conditions, Int. Biodeterior. Biodegrad. 65 (6) (2011) 757–767, doi:http://dx.doi.org/10.1016/j.ibiod.2011.04.007. [75] D. Lobelle, M. Cunliffe, Early microbial biofilm formation on marine plastic debris, Mar. Pollut. Bull. 62 (1) (2011) 197–200, doi:http://dx.doi.org/10.1016/ j.marpolbul.2010.10.013. 21093883. [76] H. Al-Makhlafi, J. McGuire, M. Daeschel, Influence of preadsorbed milk proteins on adhesion of Listeria monocytogenes to hydrophobic and hydrophilic silica surfaces, Appl. Environ. Microbiol. 60 (10) (1994) 3560–3565. 7986033. [77] D. Cunliffe, C.A. Smart, C. Alexander, E.N. Vulfson, Bacterial adhesion at synthetic surfaces, Appl. Environ. Microbiol. 65 (11) (1999) 4995–5002. 10543814. [78] R.M. Donlan, Biofilms: microbial life on surfaces, Emerg. Infect. Dis. 8 (9) (2002) 881–890, doi:http://dx.doi.org/10.3201/eid0809.020063. 12194761. [79] R. Wang, K.G. Neoh, Z. Shi, E.T. Kang, P.A. Tambyah, E. Chiong, Inhibition of Escherichia coli and Proteus mirabilis adhesion and biofilm formation on medical grade silicone surface, Biotechnol. Bioeng. 109 (2) (2012) 336–345, doi:http://dx.doi.org/10.1002/bit.23342. 21956834. [80] Y. Zheng, E.K. Yanful, A.S. Bassi, A review of plastic waste biodegradation, Crit. Rev. Biotechnol. 25 (4) (2005) 243–250, doi:http://dx.doi.org/10.1080/ 07388550500346359. 16419620. [81] P. Hugenholtz, B.M. Goebel, N.R. Pace, Impact of culture-independent studies on the emerging phylogenetic view of bacterial diversity, J. Bacteriol. 180 (18) (1998) 4765–4774. 9733676. [82] J. Huang, A.S. Shetty, M. Wang, Biodegradable plastics: a review, Adv. Polym. Technol. 10 (1) (1990) 23–30, doi:http://dx.doi.org/10.1002/ adv.1990.060100103. [83] L.R. Krupp, W.J. Jewell, Biodegradability of modified plastic films in controlled biological environments, Environ. Sci. Technol. 26 (1) (1992) 193–198, doi: http://dx.doi.org/10.1021/es00025a024. [84] M. Mochizuki, T. Hayashi, K. Nakayama, T. Masuda, Studies on biodegradable poly(hexane-6-lactone) fibers. Part 2. Environmental degradation (technical report), Pure Appl. Chem. 71 (1999) 2177–2188. [85] A. Esmaeili, A.A. Pourbabaee, H.A. Alikhani, F. Shabani, E. Esmaeili, Biodegradation of low-density polyethylene (LDPE) by mixed culture of Lysinibacillus xylanilyticus and Aspergillus niger in soil, PLoS One 8 (9) (2013) 717–720, doi:http://dx.doi.org/10.1371/journal.pone.0071720. 24086254. [86] H. Bhardwaj, R. Gupt, A. Tiwari, Microbial Population Associated with Plastic Degradation, Open Access Scientific Reports, 2012. [87] C. Abrusci, J.L. Pablos, T. Corrales, J. López-Marín, I. Marín, F. Catalina, Biodegradation of photo-degraded mulching films based on polyethylenes and stearates of calcium and iron as pro-oxidant additives, Int. Biodeterior. Biodegrad. 65 (3) (2011) 451–459, doi:http://dx.doi.org/10.1016/j. ibiod.2010.10.012. [88] A.C. Albertsson, C. Sares, S. Karlsson, Increased biodegradation of LDPE with non-ionic surfactants, Acta Polym. 44 (5) (1993) 243–246, doi:http://dx.doi. org/10.1002/actp.1993.010440506. [89] K. Ehara, Y. Iiyoshi, Y. Tsutsumi, T. Nishida, Polyethylene degradation by manganese peroxidase in the absence of hydrogen peroxide, J. Wood Sci. 46 (2) (2000) 180–183, doi:http://dx.doi.org/10.1007/BF00777369. [90] K. Kathiresan, B.L. Bingham, Biology of mangroves and mangrove ecosystems, Adv. Mar. Biol. 40 (2001) 81–251, doi:http://dx.doi.org/10.1016/S0065-2881 (01)40003-4. [91] A. Manzur, M. Limón-González, E. Favela-Torres, Biodegradation of physicochemically treated LDPE by a consortium of filamentous fungi, J. Appl. Polym. Sci. 92 (1) (2004) 265–271, doi:http://dx.doi.org/10.1002/app.13644. [92] S. Zahra, S.S. Abbas, M.T. Mahsa, N. Mohsen, Biodegradation of low-density polyethylene (LDPE) by isolated fungi in solid waste medium, Waste Manag. 30 (3) (2010) 396–401, doi:http://dx.doi.org/10.1016/j.wasman.2009.09.027. [93] P.D. Merina, Microbial deterioration of low density polyethylene by Aspergillus and Fusarium sp, Int. J. ChemTech Res. 6 (2014) 299–305. [94] N. Raaman, N. Rajitha, A. Jayshree, R. Jegadeesh, Biodegradation of plastic by Aspergillus spp. isolated from polythene polluted sites around Chennai, J. Acad. Ind. Res. 1 (6) (2012) 313–316.

S. Kumar Sen, S. Raut / Journal of Environmental Chemical Engineering 3 (2015) 462–473 _ [95] L. Katarzyna, L. Grazyna, Polymer biodegradation and biodegradable polymers – a review, Pol. J. Environ. Stud. 19 (2) (2010) 255–266. [96] N. Sombatsompop, K. Sungsanit, C. Thongpin, Analysis of low- density polyethylene-g-poly(vinyl chloride) copolymers formed in poly(vinyl chloride)/low-density polyethylene melt blends with gel permeation chromatography and solid-state 13C-NMR, J. Appl. Polym. Sci. 92 (2003) 3167–3172. [97] P. Nayak, A. Tiwari, Biodegradation of polythene and plastic by the help of microbial tools: a recent approach, Int. J. Biomed. Adv. Res. 2 (9) (2011) 344–355, doi:http://dx.doi.org/10.7439/ijbar.v2i9.47. [98] D.Y. Kim, Y.H. Rhee, Biodegradation of microbial and synthetic polyesters by fungi, Appl. Microbiol. Biotechnol. 61 (4) (2003) 300–308, doi:http://dx.doi. org/10.1007/s00253-002-1205-3. 12743758. [99] M.J. Kershaw, N.J. Talbot, Hydrophobins and repellents: proteins with fundamental roles in fungal morphogenesis, Fungal Genet. Biol. 23 (1) (1998) 18–33, doi:http://dx.doi.org/10.1006/fgbi.1997.1022. 9501475. [100] G. Seneviratne, N. Tennakoon, K. Nandasena, Polyethylene biodegradation by a developed Penicillium–Bacillus biofilm, Curr. Sci. 90 (2006) 20–21. [101] K. Leja, G. Lewandowicz, Polymer biodegradation and biodegradable polymers – a review, Pol. J. Environ. Stud. 19 (2009) 225–266. [102] M. Kumar, S. Mohanty, S.K. Nayak, M. Rahail Parvaiz, Effect of glycidyl methacrylate (GMA) on the thermal, mechanical and morphological property of biodegradable PLA/PBAT blend and its nanocomposites, Bioresour. Technol. 101 (21) (2010) 8406–8415, doi:http://dx.doi.org/10.1016/j.biortech.2010.05.075. 20573502. [103] H. Nishida, Y. Tokiwa, Distribution of poly(b-hydroxybutyrate) and poly (e-caprolactone) aerobic degrading microorganisms in different environments, J. Polym. Environ. 1 (3) (1993) 227–233. [104] D.M. Abou-Zeid, R.J. Müller, W.D. Deckwer, Anaerobic biodegradation of natural and synthetic polyesters, Dissertation, Technical University Braunschweig, Germany, 2001, pp. 246. [105] J. Augusta, R. Müller, H. Widdecke, A rapid evaluation plate-test for the biodegradability of plastics, Appl. Microbiol. Biotechnol. 39 (4–5) (1993) 673–678, doi:http://dx.doi.org/10.1007/BF00205073. [106] R. Usha, T. Sangeetha, M. Palaniswamy, Screening of polyethylene degrading microorganisms from garbage soil, Libyan Agric. Res. Cent. J. Int. 2 (4) (2011) 200–204. [107] K.B. Raper, D.I. Fennell, in: R.E. Krieger (Ed.), The genus Aspergillus, Huntington, New York, 1987, pp. 686. [108] J.S. Webb, M. Nixon, I.M. Eastwood, M. Greenhalgh, G.D. Robson, P.S. Handley, Fungal colonization and biodeterioration of plasticized polyvinyl chloride, Appl. Environ. Microbiol. 66 (8) (2000) 3194–3200, doi:http://dx.doi.org/ 10.1128/AEM.66.8.3194-3200.2000. 10919769. [109] M. Paul Das, S. Kumar, Microbial deterioration of low density polyethylene by Aspergillus and Fusarium sp, Int. J. ChemTech Res. 6 (1) (2014) 299–305. [110] I.G. Mishra, S. Sasidharan, S. Tiwari, Aspergillus candidus: a phytase producing fungi, Int. J. Curr. Sci. 12 (2014) 1–7. [111] J.C. Gilman, A Manual of Soil Fungi, Biotech Books, New Dehli, India, 2012. [112] A.A. Shah, F. Hasan, A. Hameed, S. Ahmed, Isolation and characterization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) degrading bacteria and purification of PHBV depolymerase from newly isolated Bacillus sp. AF3, Int. Biodeterior. Biodegrad. 60 (2) (2007) 109–115, doi:http://dx.doi.org/10.1016/ j.ibiod.2007.01.004. [113] C. Vasile, Degradation and decomposition, in: C. Vasile, R.B. Seymour (Eds.), Handbook of Polyolefins Synthesis and Properties, Marcel Dekker Inc., New York, 1993, pp. 479–509. [115] F. Hasan, A.A. Shah, A. Hameed, S. Ahmed, Synergistic effect of photo and chemical treatment on the rate of biodegradation of low density polyethylene by Fusarium sp. AF4, J. Appl. Polym. Sci. 105 (3) (2007) 1466–1470, doi: http://dx.doi.org/10.1002/app.26328. [116] S. Bonhomme, A. Cuer, A. Delort, J. Lemaire, M. Sancelme, G. Scott, Environmental biodegradation of polyethylene, Polym. Degrad. Stab. 81 (3) (2003) 441–452, doi:http://dx.doi.org/10.1016/S0141-3910(03)00129-0. [117] K. Harshvardhan, B. Jha, Biodegradation of low-density polyethylene by marine bacteria from pelagic waters, Arabian Sea, India, Mar. Pollut. Bull. 77 (1–2) (2013) 100–106, doi:http://dx.doi.org/10.1016/j.marpolbul.2013.10.025. 24210946. [118] M. Sudhakar, M. Doble, P.S. Murthy, R. Venkatesan, Marine microbe-mediated biodegradation of low- and high-density polyethylenes, Int. Biodeterior. Biodegrad. 61 (3) (2008) 203–213, doi:http://dx.doi.org/10.1016/j. ibiod.2007.07.011. [119] D. Raghavan, A.E. Torma, DSC and FTIR characterization of biodegradation of polyethylene, Polym. Eng. Sci. 32 (6) (1992) 438–442, doi:http://dx.doi.org/ 10.1002/pen.760320609. [120] A. Ohtaki, N. Sato, K. Nakasaki, Biodegradation of poly-e-caprolactone under controlled composting conditions, Polym. Degrad. Stab. 61 (3) (1998) 499–505, doi:http://dx.doi.org/10.1016/S0141-3910(97)00238-3. [121] R. Pramila, K. Ramesh, Biodegradation of low density polyethylene (LDPE) by fungi isolated from marine water e a SEM analysis, Afr. J. Microbiol. Res. 5 (28) (2011) 5013–5018. [122] T. Watanabe, Y. Ohtake, H. Asabe, N. Murakami, M. Furukawa, Biodegradability and degrading microbes of low-density polyethylene, J. Appl. Polym. Sci. 111 (1) (2009) 551–559, doi:http://dx.doi.org/10.1002/app.29102.

473

[123] J.D. Gu, T.E. Ford, D.B. Mitton, R. Mitchel, Microbial degradation and deterioration of polymeric materials, in: W. Revie (Ed.), The Uhlig Corrosion Handbook, second ed., Wiley, New York, 2000, pp. 439–460. [124] T. Artham, M. Doble, Biodegradation of aliphatic and aromatic polycarbonates, Macromol. Biosci. 8 (1) (2008) 14–24, doi:http://dx.doi.org/ 10.1002/mabi.200700106. 17849431. [125] G. Swift, Non-medical biodegradable polymers: environmentally degradable polymers, in: A.J. Domb, J. Kost, D.M. Wiseman (Eds.), Handbook of Biodegradable Polymers, Harwood Academic, Amsterdam, 1997, pp. 473–511. [126] M.R. Kamal, B. Huang, Natural and artificial weathering of polymers, in: S.H. Hamid, M.B. Ami, A.G. Maadhan (Eds.), Handbook of Polymer Degradation, Marcel Dekker, New York, 1992, pp. 127–168. [127] G.J.L. Griffin, Synthetic polymers and the living environment, Pure Appl. Chem. 52 (2) (1980) 399–407, doi:http://dx.doi.org/10.1351/ pac198052020399. [128] D. Goldberg, A review of the biodegradability and utility of poly(caprolactone), J. Environ. Polym. Degrad. 3 (2) (1995) 61–68, doi:http://dx.doi.org/ 10.1007/BF02067481. [129] Y. Tokiwa, B.P. Calabia, Degradation of microbial polyesters, Biotechnol. Lett. 26 (15) (2004) 1181–1189, doi:http://dx.doi.org/10.1023/B: BILE.0000036599.15302.e5. 15289671. [130] A.K. Lau, W.W. Cheuk, K.V. Lo, Degradation of greenhouse twines derived from natural fibers and biodegradable polymer during composting, J. Environ. Manag. 90 (1) (2009) 668–671, doi:http://dx.doi.org/10.1016/j.jenvman.2008.03.001. 18440121. [131] F. Rojo, Handbook of Hydrocarbon and Lipid Microbiology, first ed., Springer, Heidelberg, Berlin, 2010. [133] ASTM, Standard Test Method for Determining Aerobic Biodegradation in Soil of Plastic Material or Residual Plastic Materials after Composting, ASTM, 2003. [138] L.M. Gorghiu, S. Jipa, T. Zaharescu, R. Setnescu, I. Mihalcea, The effect of metals on thermal degradation of polyethylenes, Polym. Degrad. Stab. 84 (1) (2004) 7–11, doi:http://dx.doi.org/10.1016/S0141-3910(03)00265-9. [139] J.V. Gulmine, P.R. Janissek, H.M. Heise, L. Akcelrud, Degradation profile of polyethylene after artificial accelerated weathering, Polym. Degrad. Stab. 79 (3) (2003) 385–397, doi:http://dx.doi.org/10.1016/S0141-3910(02)00338-5. [141] F. Khabbaz, A. Albertsson, S. Karlsson, Chemical and morphological changes of environmentally degradable polyethylene films exposed to thermo-oxidation, Polym. Degrad. Stab. 63 (1) (1999) 127–138, doi:http://dx.doi.org/ 10.1016/S0141-3910(98)00082-2. [143] M. Koutny, P. Amato, M. Muchova, J. Ruzicka, A. Delort, Soil bacterial strains able to grow on the surface of oxidized polyethylene film containing prooxidant additives, Int. Biodeterior. Biodegrad. 63 (3) (2009) 354–357, doi: http://dx.doi.org/10.1016/j.ibiod.2008.11.003. [144] A. Manzur, F. Cuamatzi, E. Favela, Effect of the growth of Phanerochaete chrysosporium in a blend of low density polyethylene and sugar cane bagasse, J. Appl. Polym. Sci. 66 (1) (1997) 105–111. [146] E.M. Nakamura, L. Cordi, G.S.G. Almeida, N. Duran, L.H.I. Mei, Study and development of LDPE/starch partially biodegradable compounds, J. Mater. Process. Technol. 162–163 (2005) 236–241, doi:http://dx.doi.org/10.1016/j. jmatprotec.2005.02.007. [147] Y. Ohtake, T. Kobayashi, H. Asabe, N. Murakami, K. Ono, Oxidative degradation and molecular weight change of LDPE buried under bioactive soil for 32–37 years, J. Appl. Polym. Sci. 70 (9) (1998) 1643–1648, doi:http://dx.doi. org/10.1002/(SICI)1097-4628(19981128)70:93.0.CO;2-V. [149] R. Pramila, K.V. Ramesh, Biodegradation of low density polyethylene (LDPE) by fungi isolated from municipal landfill area, J. Microbiol. Biotechnol. Res. 1 (4) (2011) 131–136. [150] A. Satlewal, R. Soni, M. Zaidi, Y. Shouche, R. Goel, Comparative biodegradation of HDPE and LDPE using an indigenously developed microbial consortium, J. Microbiol. Biotechnol. 18 (3) (2008) 477–482. 18388465. [152] H.V. Sowmya, M. Ramalingappa, M. Krishnappa, Degradation of polyethylene by Chaetomium sp. and Aspergillus flavus, Int. J. Recent Sci. Res. 3 (2012) 513–517. [153] R.D. Stapleton, D.C. Savage, G.S. Sayler, G. Stacey, Biodegradation of aromatic hydrocarbons in an extremely acidic environment, Appl. Environ. Microbiol. 64 (11) (1998) 4180–4184. 9797263. [156] M. Weiland, A. Daro, C. David, Biodegradation of thermally oxidized polyethylene, Polym. Degrad. Stab. 48 (1995) 275–289. [158] M. Shimao, Biodegradation of plastics, Curr. Opin. Biotechnol. 12 (3) (2001) 242–247, doi:http://dx.doi.org/10.1016/S0958-1669(00)00206-8. 11404101. [159] M. Dashtban, H. Schraft, T.A. Syed, W. Qin, Fungal biodegradation and enzymatic modification of lignin, Int. J. Biochem. Mol. Biol. 1 (1) (2010) 36–50. 21968746. [160] G.T. Howard, Biodegradation of polyurethane: a review, Int. Biodeterior. Biodegrad. 49 (4) (2002) 245–252, doi:http://dx.doi.org/10.1016/S0964-8305 (02)00051-3. [161] J.R. Russell, J. Huang, P. Anand, K. Kucera, A.G. Sandoval, et al., Biodegradation of polyester polyurethane by endophytic fungi, Appl. Environ. Microbiol. 77 (17) (2011) 6076–6084, doi:http://dx.doi.org/10.1128/AEM.00521-11. 21764951.