biodegradable plastics and biodegradation of plastic wastes has assumed increasing importance in the last few years. Awareness of the waste

4. Standard testing methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

Available online at www.sciencedirect.com

Biotechnology Advances 26 (2008) 2462654.1. Visual observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2514.2. Weight loss measurements: determination of residual polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2514.3. Changes in mechanical properties and molar mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2524.4. CO2 evolution/O2 consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2534.5. Radiolabeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2534.6. Clear-zone formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2534.7. Enzymatic degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2534.8. Controlled composting test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

5. Biodegradation of natural plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2545.1. Biodegradation of polyhydroxyalkanoates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254mechanisms involved. This requires understanding of the interactions between materials and microorganisms and the biochemical changesinvolved. Widespread studies on the biodegradation of plastics have been carried out in order to overcome the environmental problems associatedwith synthetic plastic waste. This paper reviews the current research on the biodegradation of biodegradable and also the conventional syntheticplastics and also use of various techniques for the analysis of degradation in vitro. 2008 Elsevier Inc. All rights reserved.

Keywords: Biodegradation; Synthetic plastics; Biodegradable plastics; Analysis of degradation

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2472. Degradation of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2483. Biodegradation of plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250consider the microbial degradation of natural and synthetic polymerproblem and its impact on the environment has awakened new interest in the area of degradable polymers. The interest in environmental issues isgrowing and there are increasing demands to develop material which do not burden the environment significantly. Biodegradation is necessary forwater-soluble or water-immiscible polymers because they eventually enter streams which can neither be recycled nor incinerated. It is important to

s in order to understand what is necessary for biodegradation and theResearch review paper

Biological degradation of plastics: A comprehensive review

Aamer Ali Shah , Fariha Hasan, Abdul Hameed, Safia Ahmed

Department of Microbiology, Quaid-i-Azam University, Islamabad, Pakistan

Received 22 November 2007; received in revised form 31 December 2007; accepted 31 December 2007Available online 26 January 2008

Abstract

Lack of degradability and the closing of landfill sites as well as growing water and land pollution problems have led to concern about plastics.With the excessive use of plastics and increasing pressure being placed on capacities available for plastic waste disposal, the need for

www.elsevier.com/locate/biotechadv5.2. Enzymatic degradation of polyhydroxalkanoates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2556. Biodegradation of synthetic plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

Corresponding author. Tel.: +92 51 90643065; fax: +92 51 9219888.E-mail address: alishah_75@yahoo.com (A.A. Shah).

0734-9750/$ - see front matter 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.biotechadv.2007.12.005

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258sters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

the Greek word plastikos, which means able to be molded Pakistan is growing at an average annual growth rate of 15%.

247gy Advances 26 (2008) 246265into different shapes (Joel, 1995). The plastics we use today aremade from inorganic and organic raw materials, such as carbon,silicon, hydrogen, nitrogen, oxygen and chloride. The basicmaterials used for making plastics are extracted from oil, coaland natural gas (Seymour, 1989).

Plastics are resistant against microbial attack, since duringtheir short time of presence in nature evolution could not designnew enzyme structures capable of degrading synthetic polymers(Mueller, 2006). Nowadays, a wide variety of petroleum-basedsynthetic polymers are produced worldwide to the extent ofapproximately 140 million tons per year and remarkableamounts of these polymers are introduced in the ecosystem asindustrial waste products (Shimao, 2001).

Synthetic plastics are extensively used in packaging ofproducts like food, pharmaceuticals, cosmetics, detergents andchemicals.Approximately 30%of the plastics are usedworldwidefor packaging applications. This utilization is still expanding at ahigh rate of 12% per annum (Sabir, 2004). They have replacedpaper and other cellulose-based products for packaging becauseof their better physical and chemical properties, such as theirstrength, lightness, resistance to water and most water-borne

There are about 600700 medium sized plastic processing unitsscattered all over Pakistan (Sabir, 2004).6.1. Biodegradation of thermoplastic polyolefins . . . . . .6.1.1. Polyethylene . . . . . . . . . . . . . . . . . .6.1.2. Polyvinyl chloride . . . . . . . . . . . . . . .6.1.3. Polystyrene . . . . . . . . . . . . . . . . . .

7. Biodegradation of polymer blends . . . . . . . . . . . . . . .7.1. Starch/polyethylene blends . . . . . . . . . . . . . . .7.2. Starch/polyester blends . . . . . . . . . . . . . . . . .7.3. Starch/PVA blends . . . . . . . . . . . . . . . . . . .7.4. Polylactic acid (PLA) (renewable resource) polyesters .7.5. Polybutylene succinate (PBS) (synthetic aliphatic) polye7.6. Aliphaticaromatic (AAC) copolyesters . . . . . . . .7.7. Modified polyethylene terephthalate (PET) . . . . . . .

8. Other degradable polymers . . . . . . . . . . . . . . . . . . .8.1. Ethylene vinyl alcohol (EVOH) . . . . . . . . . . . .8.2. Photo-biodegradable plastics . . . . . . . . . . . . . .8.3. Biodegradation of thermoset plastics . . . . . . . . . .

8.3.1. Polyurethane . . . . . . . . . . . . . . . . . .9. Composting . . . . . . . . . . . . . . . . . . . . . . . . . .10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Plastics are man made long chain polymeric molecules(Scott, 1999). More than half a century ago synthetic polymersstarted to substitute natural materials in almost every area andnowadays plastics have become an indispensable part of ourlife. With time, stability and durability of plastics have beenimproved continuously, and hence this group of materials isnow considered as a synonym for materials being resistant tomany environmental influences. The word plastic comes from

A.A. Shah et al. / Biotechnolomicroorganisms. Themostwidely used plastics used in packagingare polyethylene (LDPE, MDPE, HDPE and LLDPE), poly-propylene (PP), polystyrene (PS), polyvinyl chloride (PVC),polyurethane (PUR), poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), nylons (Fig. 1 & Table 1). Thewidespread applications of plastics are not only due to theirfavorable mechanical and thermal properties but also mainly dueto the stability and durability (Rivard et al., 1995). Because oftheir durability and visibility in litter, plastics (polymers) haveattracted more public and media attention than any othercomponent of the solid waste stream. In 1993, the total worlddemand for plastics was over 107 million tones and it wasestimated about 146 million tones in 2000. The plastic industry inFig. 1. Structures of conventional petrochemical plastics (adapted from Paviaet al., 1988). Polyethylene (PE), Polyvinyl chloride (PVC), Polypropylene (PP),Polystyrene (PS), Polyethylene Terephthalate (PET), Polyurethane (PU).

gy ATable 1Uses of synthetic plastics

Plastic Use

Polyethylene Plastic bags, milk and water bottles, food packagingfilm, toys, irrigation and drainage pipes, motor oilbottles

Polystyrene Disposable cups, packaging materials, laboratory ware,certain electronic uses

Polyurethane Tyres, gaskets, bumpers, in refrigerator insulation,sponges, furniture cushioning, and life jackets

Polyvinyl chloride Automobile seat covers, shower curtains, raincoats,bottles, visors, shoe soles, garden hoses, and electricitypipes

Polypropylene Bottle caps, drinking straws, medicine bottles, car seats,car batteries, bumpers, disposable syringes, carpetbackings

Polyethyleneterephthalate (PET)

Used for carbonated soft drink bottles, processed meatpackages peanut butter jars pillow and sleeping bagfilling, textile fibers

Nylon Polyamides or Nylon are used in small bearings,speedometer gears, windshield wipers, water hosenozzels, football helmets, racehorse shoes, inks,clothing parachute fabrics, rainwear, and cellophane

Polycarbonate Used for making nozzles on paper making machinery,street lighting, safety visors, rear lights of cars, babybottles and for houseware. It is also used in sky-lightsand the roofs of greenhouses, sunrooms and verandahs.One important use is to make the lens in glasses

Polytetraflouro- PTFE is used in various industrial applications such

248 A.A. Shah et al. / BiotechnoloThe dramatic increase in production and lack of biodegrad-ability of commercial polymers, particularly commodityplastics used in packaging (e.g. fast food), industry andagriculture, focused public attention on a potentially hugeenvironmental accumulation and pollution problem that couldpersist for centuries (Albertsson et al., 1987). The plastic wasteis disposed off through landfilling, incineration and recycling.Because of their persistence in our environment, severalcommunities are now more sensitive to the impact of discardedplastic on the environment, including deleterious effects onwildlife and on the aesthetic qualities of cities and forests.Improperly disposed plastic materials are a significant source ofenvironmental pollution, potentially harming life. In addition,the burning of polyvinylchloride (PVC) plastics producespersistent organic pollutants (POPs) known as furans anddioxins (Jayasekara et al., 2005). The estimated figure of plasticwaste generation across the Pakistan is 1.32 million tons perannum. This considerable content of plastic in the solid wastegenerated in Pakistan is of great concern. Plastic waste isreleased during all stages of production and post consumptionevery plastic product is a waste (Sabir, 2004).

Some synthetic plastics like polyester polyurethane, poly-ethylene with starch blend, are biodegradable, although mostcommodity plastics used now are either non-biodegradable oreven take decades to degrade. This has raised growing concernabout degradable polymers and promoted research activityworld wide to either modify current products to promote

ethylene (PTFE) specialized chemical plant, electronics and bearings. Itis met with in the home as a coating on non-stickkitchen utensils, such as saucepans and frying pans

(Vona et al., 1965).degradability or to develop new alternatives that are degradableby any or all of the following mechanisms: biodegradation,photodegradation, environmental erosion and thermal degrada-tion (Kawai, 1995).

In 1980s, scientists started to look if plastics could bedesigned to become susceptible to microbial attack, makingthem degradable in a microbial active environment. Biodegrad-able plastics opened the way for new considerations of wastemanagement strategies since these materials are designed todegrade under environmental conditions or in municipal andindustrial biological waste treatment facilities (Augusta et al.,1992; Witt et al., 1997).

Due to similar material properties to conventional plastics(Hocking and Marchessault, 1994; Steinbuchel and Fuchten-busch, 1998), the biodegradable plastics (polyesters), namelypolyhydroxyalkanoates (PHA), polylactides, polycaprolactone,aliphatic polyesters, polysaccharides and copolymer or blend ofthese, and have been developed successfully over the last fewyears (Fig. 2). The most important are poly(3-hydroxybutyrate)and poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Bioplastics(Biopolymers) obtained from growth of microorganisms orfrom plants which are genetically-engineered to produce suchpolymers are likely to replace currently used plastics at least insome of the fields (Lee, 1996). PHA's key properties are itsbiodegradability, apparent biocompatibility, and its manufacturefrom renewable resources. The global interest in PHAs is highas it is used in different packaging materials, medical devices,disposable personal hygiene and also agricultural applicationsas a substitute for synthetic polymers like polypropylene,polyethylene etc. (Ojumu et al., 2004) (Table 2) (Lee, 1996).

In the past 10 years, several biodegradable plastics have beenintroduced into the market. However, none of them is efficientlybiodegradable in landfills. For this reason, none of the productshas gained widespread use (Anonymous, 1999). At present,biodegradable plastic represents just a tiny market as comparedwith the conventional petrochemical material. Bioplastics willcomparatively prove cheaper when oil prices will continue tohike up. Although not in use today, plastic shopping bags couldbe made from Polylactic acid (PLA) a biodegradable polymerderived from lactic acid. This is one form of vegetable-basedbioplastic. This material biodegrades quickly under compostingconditions and does not leave toxic residue. However, bioplasticcan have its own environmental impacts, depending on the wayit is produced (http://en.wikipedia.org). There is an urgent needto develop efficient microorganisms and their products to solvethis global issue (Kathiresan, 2003).

This paper reviews the current research on the degradation ofnot only the biodegradable plastics but also the conventionalsynthetic (commodity) plastics and their blends, and also use ofvarious techniques for the analysis of degradation in vitro.

2. Degradation of plastics

Any physical or chemical change in polymer as a result of

dvances 26 (2008) 246265environmental factors, such as light, heat, moisture, chemicalconditions or biological activity. Processes inducing changes inpolymer properties (deterioration of functionality) due to

249gy Advances 26 (2008) 246265A.A. Shah et al. / Biotechnolochemical, physical or biological reactions resulting in bondscission and subsequent chemical transformations (formation ofstructural in homogeneities) have been categorized as polymerdegradation. Degradation has been reflected in changes ofmaterial properties such as mechanical, optical or electricalcharacteristics, in crazing, cracking, erosion, discoloration,phase separation or delamination. The changes include bondscission, chemical transformation and formation of newfunctional groups (Pospisil and Nespurek, 1997). The degrada-tion will either be photo, thermal or biological (Table 3).

Sensitivity of polymers to photodegradation is related to theability to absorb the harmful part of the tropospheric solarradiation. This includes the UV-B terrestrial radiation (~295315 nm) and UV-A radiation (~315400 nm) responsible forthe direct photodegradation (photolysis, initiated photooxida-tion). Visible part of sunlight (400760 nm) acceleratespolymeric degradation by heating. Infrared radiation (7602500 nm) accelerates thermal oxidation (Gugumus, 1990;Pospisil and Nespurek, 1997). Most plastics tend to absorbhigh-energy radiation in the ultraviolet portion of the spectrum,which activates their electrons to higher reactivity and causesoxidation, cleavage, and other degradation (http://composite.about.com/library/glossary/p/bldef-p3928.htm, Photodegrada-tion, 1989).

Thermal degradation of polymers is molecular deteriora-tion as a result of overheating. At high temperatures thecomponents of the long chain backbone of the polymer can

Fig. 2. Chemical structure of poly(lactide)(PLA), poly(3-hydroxybutyrate)(PHB), poly(propiolactone)(PPL), poly(-caprolactone)(PCL), poly(ethylene succinate)(PES), poly(butylenes succinate)(PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV) and poly(ester carbonate)(PEC) (Tokiwa and Calabia 2004).

Table 2Uses of biodegradable plastics

Plastics Uses

Polyglycolic acid(PGA)

Specialized applications; controlled drug releases;implantable composites; bone fixation parts

Polylactic acid(PLA)

Packaging and paper coatings; other possible marketsinclude sustained release systems for pesticides andfertilizers, mulch films, and compost bags

Polycaprolactone(PCL)

Long-term items; mulch and other agricultural films;fibers containing herbicides to control aquatic weeds;seedling containers; slow release systems for drugs

Polyhydroxybutyrate(PHB)

Products like bottles, bags, wrapping film anddisposable nappies, as a material for tissueengineering scaffolds and for controlled drugrelease carriers

Polyhydroxyvalerate(PHBV)

Films and paper coatings; other possible markets includebiomedical applications, therapeutic delivery of wormmedicine for cattle, and sustained release systems forpharmaceutical drugs and insecticides

Polyvinyl alcohol(PVOH)

Packaging and bagging applications which dissolve inwater to release Products such as laundry detergent,pesticides, and hospital washables

Polyvinyl acetate(PVAc)

Adhesives, the packaging applications includeboxboard manufacture, paper bags, paper lamination,tube winding and remoistenable labels

(http://www.envis-icpe.com, Plastics recycling-Economic and EcologicalOptions. ICPE 2006;4(4):112).http://en.wikipedia.org/w/index.php?title=Polyhydroxybutyrate&oldid=189442759. Accessed February 14, 2008.http://www.chemquest.com/store/polyvinyl-acetate-european-adhesives.html,2006.

gy Abegin to separate (molecular scission) and react with oneanother to change the properties of the polymer. The chemicalreactions involved in thermal degradation lead to physical andoptical property changes relative to the initially specifiedproperties. Thermal degradation generally involves changes tothe molecular weight (and molecular weight distribution) ofthe polymer and typical property changes include; reducedductility and embrittlement, chalking, color changes, crackingand general reduction in most other desirable physicalproperties (Olayan et al., 1996).

Oxo-biodegradation process uses two methods to start thebiodegradation. These methods are photodegradation (UV)

Table 3Various polymer degradation routes

Factors(requirement/activity)

Photo-degradation Thermo-oxidativedegradation

Biodegradation

Active agent UV-light orhigh-energyradiation

Heat and oxygen Microbialagents

Requirementof heat

Not required Higher thanambienttemperaturerequired

Not required

Rate ofdegradation

Initiation is slow.But propagation is fast

Fast Moderate

Otherconsideration

Environment friendlyif high-energyradiation is not used

Environmentallynot acceptable

Environmentfriendly

Overallacceptance

Acceptablebut costly

Not acceptable Cheap and verymuch acceptable

(http://www.envis-icpe.com, Plastics recycling-Economic and EcologicalOptions. ICPE 2006;4(4):112).

250 A.A. Shah et al. / Biotechnoloand oxidation. The UV degradation uses UV light to degradethe end product. The oxidation process uses time, and heat tobreak down the plastic. Both methods reduce the molecularweight of the plastic and allow it to biodegrade (http://www.willowridgeplastics.com/faqs.html, 2005).

Biodegradation is the process by which organic substancesare broken down by living organisms. The term is often usedin relation to ecology, waste management, environmentalremediation (bioremediation) and to plastic materials, due totheir long life span. Organic material can be degradedaerobically, with oxygen, or anaerobically, without oxygen.A term related to biodegradation is biomineralisation, inwhich organic matter is converted into minerals (http://en.wikipedia.org/wiki/Biodegradation, Biodegradation, 2007).Plastics are biodegraded aerobically in wild nature, anaero-bically in sediments and landfills and partly aerobically andpartly anaerobically in composts and soil. Carbon dioxide andwater are produced during aerobic biodegradation and carbondioxide, water and methane are produced during anaerobicbiodegradation (Gu et al., 2000a). Generally, the breakdownof large polymers to carbon dioxide (mineralization) requiresseveral different organisms, with one breaking down thepolymer into its constituent monomers, one able to use themonomers and excreting simpler waste compounds as by-products and one able to use the excreted wastes (http://en.wikipedia.org/wiki/Microbial_metabolism, Microbial metabo-lism, 2007).

3. Biodegradation of plastics

Microorganisms such as bacteria and fungi are involved inthe degradation of both natural and synthetic plastics (Gu et al.,2000a). The biodegradation of plastics proceeds actively underdifferent soil conditions according to their properties, becausethe microorganisms responsible for the degradation differ fromeach other and they have their own optimal growth conditions inthe soil. Polymers especially plastics are potential substrates forheterotrophic microorganisms (Glass and Swift, 1989).

Biodegradation is governed by different factors that includepolymer characteristics, type of organism, and nature ofpretreatment. The polymer characteristics such as its mobility,tacticity, crystallinity, molecular weight, the type of functionalgroups and substituents present in its structure, and plasticizersor additives added to the polymer all play an important role in itsdegradation (Artham and Doble, 2008; Gu et al., 2000b).

During degradation the polymer is first converted to itsmonomers, then these monomers are mineralized. Mostpolymers are too large to pass through cellular membranes, sothey must first be depolymerized to smaller monomers beforethey can be absorbed and biodegraded within microbial cells.The initial breakdown of a polymer can result from a variety ofphysical and biological forces (Swift, 1997). Physical forces,such as heating/cooling, freezing/thawing, or wetting/drying,can cause mechanical damage such as the cracking of polymericmaterials (Kamal and Huang, 1992). The growth of many fungican also cause small-scale swelling and bursting, as the fungipenetrate the polymer solids (Griffin, 1980). Synthetic poly-mers, such as poly(caprolactone) (Toncheva et al., 1996; Junet al., 1994), are also depolymerized by microbial enzymes,after which the monomers are absorbed into microbial cells andbiodegraded (Goldberg, 1995). Abiotic hydrolysis is the mostimportant reaction for initiating the environmental degradationof synthetic polymers (Gpferich, 1997) like polycarboxylates(Winursito and Matsumura, 1996), poly(ethylene terephthalate)(Heidary and Gordon, 1994), polylactic acids and theircopolymers (Hiltunen et al., 1997; Nakayama et al., 1996),poly (-glutamic acids) (Fan et al., 1996), and polydimethylsi-loxanes, or silicones (Lehmann et al., 1995; Xu et al., 1998).

Generally, an increase in molecular weight results in adecline of polymer degradability by microorganisms. Incontrast, monomers, dimers, and oligomers of a polymer'srepeating units are much easily degraded and mineralized. Highmolecular weights result in a sharp decrease in solubilitymaking them unfavorable for microbial attack because bacteriarequire the substrate to be assimilated through the cellularmembrane and then further degraded by cellular enzymes. Atleast two categories of enzymes are actively involved inbiological degradation of polymers: extracellular and intracel-

dvances 26 (2008) 246265lular depolymerases (Doi, 1990; Gu et al., 2000b). Duringdegradation, exoenzymes from microorganisms break downcomplex polymers yielding smaller molecules of short chains,

e.g., oligomers, dimers, and monomers, that are smaller enoughto pass the semi-permeable outer bacterial membranes, and thento be utilized as carbon and energy sources. The process iscalled depolymerization. When the end products are CO2, H2O,or CH4, the degradation is called mineralization (Frazer, 1994;Hamilton et al., 1995). It is important to note that biodeteriora-tion and degradation of polymer substrate can rarely reach100% and the reason is that a small portion of the polymer willbe incorporated into microbial biomass, humus and othernatural products (Atlas and Bartha, 1997; Narayan, 1993).Dominant groups of microorganisms and the degradativepathways associated with polymer degradation are oftendetermined by the environmental conditions. When O2 isavailable, aerobic microorganisms are mostly responsible fordestruction of complex materials, with microbial biomass, CO2,and H2O as the final products. In contrast, under anoxicconditions, anaerobic consortia of microorganisms are respon-sible for polymer deterioration. The primary products will bemicrobial biomass, CO2, CH4 and H2O under methanogenic

a first indication of any microbial attack. To obtain informationabout the degradation mechanism, more sophisticated observa-tions can be made using either scanning electron microscopy(SEM) or atomic force microscopy (AFM) (Ikada, 1999). Afteran initial degradation, crystalline spherolites appear on thesurface; that can be explained by a preferential degradation ofthe amorphous polymer fraction, etching the slower-degradingcrystalline parts out of the material. In another investigation,(Kikkawa et al., 2002) used AFMmicrographs of enzymaticallydegraded PHB films to investigate the mechanism of surfaceerosion. A number of other techniques can also be used to assessthe biodegradability of polymeric material. These include;Fourier transform infrared spectroscopy (FTIR), differentialscanning colorimetry (DSC), nuclear magnetic resonancespectroscopy (NMR), X-ray photoelectron spectroscopy(XPS), X-ray Diffraction (XRD), contact angle measurementsand water uptake. Use of these techniques is generally beyondthe scope of this review, although some are mentioned in thetext.

251A.A. Shah et al. / Biotechnology Advances 26 (2008) 246265(anaerobic) conditions (Barlaz et al., 1989) (e.g. landfills/compost) (Fig. 3).

The list of different microorganisms degrading differentgroups of plastics is given in Table 4.

4. Standard testing methods

4.1. Visual observations

The evaluation of visible changes in plastics can beperformed in almost all tests. Effects used to describedegradation include roughening of the surface, formation ofholes or cracks, de-fragmentation, changes in color, orformation of bio-films on the surface. These changes do notprove the presence of a biodegradation process in terms ofmetabolism, but the parameter of visual changes can be used asFig. 3. General mechanism of plastic biodegradati4.2. Weight loss measurements: determination of residualpolymer

The mass loss of test specimens such as films or test bars iswidely applied in degradation tests (especially in field- andsimulation tests), although again no direct proof of biodegrada-tion is obtained. Problems can arise with correct cleaning of thespecimen, or if the material disintegrates excessively. In thelatter case, the samples can be placed into small nets to facilitaterecovery; this method is used in the full-scale compostingprocedure of DIN V 54900. A sieving analysis of the matrixsurrounding the plastic samples allows a better quantitativedetermination of the disintegration characteristics. For finelydistributed polymer samples (e.g., powders), the decrease inresidual polymer can be determined by an adequate separationon under aerobic conditions (Mueller, 2003).

gy ATable 4List of different microorganisms reported to degrade different types of plastics

Synthetic Plastics

252 A.A. Shah et al. / Biotechnoloor extraction technique (polymer separated from biomass, orpolymer extracted from soil or compost). By combining astructural analysis of the residual material and the lowmolecular weight intermediates, detailed information regardingthe degradation process can be obtained, especially if a definedsynthetic test medium is used (Witt et al., 2001).

Plastic

Polyethylene

Polyurethane

Polyvinyl chloride

PlasticizedPolyvinylchloride

BTA-copolyester

Natural plastics

Poly(3-hydroxybutyrate-co-3-mercaptopropionate)

Poly(3-hydroxybutyrate)Poly(3-hydroxybutyrate-co-3-mercaptopropionate)Poly(3-hydroxybutyrate) Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)Poly(3-hydroxybutyrate-co-3-hydroxypropionate)Poly(3-hydroxybutyrate-co-3-hydroxypropionate)Poly(3-hydroxybutyrate) poly(3-hydroxypropionate) poly(4-hydroxybutyrate)poly(ethylene succinate) poly(ethylene adipate)

Poly(3-hydroxybutyrate)Poly(3-hydroxybutyrate)

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Polycaprolactone

PolycaprolactonePolylactic acid

Polymer blends

Starch/polyethylene

Starch/polyesterdvances 26 (2008) 2462654.3. Changes in mechanical properties and molar mass

As with visual observations, changes in material propertiescannot be proved directly due to metabolism of the polymermaterial. However, changes in mechanical properties are oftenused when only minor changes in the mass of the test specimen

Microorganism Reference

Brevibacillus borstelensis Hadad et al. (2005)Rhodococcus rubber Sivan et al. (2006); Gilan et al.,2004Penicillium simplicissimum YK Yamada-Onodera et al., 2001Comamonas acidovorans TB-35 Akutsu et al., 1998Curvularia senegalensis Howard (2002)Fusarium solaniAureobasidium pullulansCladosporium sp.Pseudomonas chlororaphis Zheng et al. (2005)Pseudomonas putida AJ Anthony et al. (2004)Ochrobactrum TDPseudomonas fluorescens B-22 Mogil`nitskii et al. (1987)Aspergillus nigervan Tieghem F-1119Aureobasidiumpullulans

Webb et al. (2000)

Thermomonsporafusca

Kleeberg et al. (1998)

Schlegelellathermodepolymerans

Elbanna et al. (2004)

Pseudomonas lemoignei Jendrossek et al. (1995)Pseudomonas indica K2 Elbanna et al. (2004)Streptomyces sp. SNG9 Mabrouk and Sabry (2001)Ralstonia pikettii T1Acidovorax sp. TP4 Wang et al. (2002)Alcaligenes faecalis Kasuya et al. (1999)Pseudomonas stutzeriComamonas acidovoransAlcaligenes faecalis Kita et al. (1997)Schlegelellathermodepolymerans

Romen et al. (2004)

Caenibacterium thermophilumClostridium botulinum Abou-Zeid et al. (2001)Clostridium acetobutylicumClostridium botulinum Abou-Zeid et al. (2001)Clostridium acetobutylicumFusarium solani Benedict et al., 1983Fusarium moniliforme Torres et al., 1996Penicillium roquefortAmycolatopsis sp.

Pranamuda et al., 1997;Pranamuda and Tokiwa, 1999

Bacillus brevis Tomita et al., 1999Rhizopus delemer Fukuzaki et al., 1989

Aspergillus niger Lee et al., 1991Penicillium funiculosmPhanerochaetechrysosporiumStreptomyces Lee et al., 1991Phanerochaete chyrsosporium

gy Aare observed. Properties such as tensile strength are very sensitiveto changes in the molar mass of polymers, which is also oftentaken directly as an indicator of degradation (Erlandsson et al.,1997). Whilst, for an enzyme-induced depolymerization thematerial properties only change if a significant loss of mass isobserved (the specimen become thinner because of the surfaceerosion process; the inner part of thematerial is not affected by thedegradation process), for abiotic degradation processes (whichoften take place in the entirematerial and include the hydrolysis ofpolyesters or oxidation of polyethylenes) the mechanical proper-ties may change significantly, though almost no loss of mass dueto solubilization of degradation intermediates occur at this stage.As a consequence, this type of measurement is often used formaterials where abiotic processes are responsible for the firstdegradation step (Breslin, 1993; Tsuji and Suzuyoshi, 2002).

4.4. CO2 evolution/O2 consumption

Under aerobic conditions, microbes use oxygen to oxidizecarbon and form carbon dioxide as one of themajormetabolic endproduct. Consequently, the consumption of oxygen (respirometrictest) (Hoffmann et al., 1997) or the formation of carbon dioxide(Sturm test) are good indicators for polymer degradation, and arethe most often used methods to measure biodegradation inlaboratory tests. Due to the normally low amount of other carbonsources present in addition to the polymer itself when usingsynthetic mineral media, only a relatively low backgroundrespiration must be identified, and the accuracy of the tests isusually good. In particular, the type of analytical methods,especially for the determination of CO2 have been modified.Besides conventional trapping of CO2 in Ba(OH)2 solution,followed by manual titration, infrared and paramagnetic O2detectors can also be used to monitor O2 and CO2 concentrationsin the air stream. Although, the automated and continuousmeasurements have advantages, they also have disadvantages.For example, the exact air flow must be measured, the signals ofthe detectors must be stable for long periods of time and, if slowdegradation processes are to be determined, the CO2 concentra-tion or fall in O2 concentration to be detected is very small,thereby, increasing the likelihood of systematic errors. Underthese circumstances, other concepts (e.g., trapping CO2 in a basicsolution, pH 11.5) with continuous titration or detection of thedissolved inorganic carbon (Pagga et al., 2001) may be usefulalternatives. Other attempts to overcome problems with CO2detection are based on non-continuously aerated, closed systems.Here, either a sampling technique in combination with aninfrared-gas analyzer (Calmon et al., 2000) or a titration system(Mueller, 1999) was applied. Another closed system with adiscontinuous titration method has been described by (Solaro etal., 1998). Tests using small closed bottles as degradation reactorsand analyzing the CO2 in the headspace (Itavaara and Vikman,1995) or by the decrease in dissolved oxygen (closed-bottle test)(Richterich et al., 1998) are simple and relatively insensitive toleakages, but may cause problems due to the low amounts of

A.A. Shah et al. / Biotechnolomaterial and inoculums used.Although used originally in aqueous test systems for

polymer degradation, CO2 analysis was also adapted for testsin solid matrices such as compost (Pagga, 1998), and thismethod has now been standardized under the name, controlledcomposting test (ASTM, 1998; DIN, 1998; ISO 14855, 1999;JIS, 2000). For polymer degradation in soil, CO2 detectionproved to be more complicated than in compost because ofslower degradation rates that led not only to long test durations(up to 2 years) but also low CO2 evolution as compared to thatfrom the carbon present in soil. One means of overcomingproblems with background CO2 evolution from the naturalmatrices compost or soil is to use an inert, carbon-free andporous matrix, wetted with a synthetic medium and inoculatedwith a mixed microbial population. This method provedpracticable for simulating compost conditions (degradation at~60 C) (Bellina et al., 2000), but has not yet been optimized forsoil conditions.

4.5. Radiolabeling

In contrast to residue analysis, net CO2 and14CO2 evolution

measurements are simple, non-destructive and measure ultimatebiodegradation. If appropriately 14C labelled test material isavailable, the measurements and their interpretations arerelatively straightforward. Materials containing a randomlydistributed 14C marker can be exposed to selected microbialenvironments. The amount of 14C carbon dioxide evolved isestimated using a scintillation counter. This method is notsubject to interference by biodegradable impurities or additivesin the polymer. Biodegradability investigations using thistechnique for polymeric materials in different microbialenvironments show a high degree of precision and consistency(Sharabi and Bartha, 1993). However, labeled materials areexpensive and not always available. The licensing and the wastedisposal problems connected with radioactive work may also bea drawback.

4.6. Clear-zone formation

A very simple semi-quantitative method is the so-calledclear-zone test. This is an agar plate test in which the polymer isdispersed as very fine particles within the synthetic mediumagar; this results in the agar having an opaque appearance. Afterinoculation with microorganisms, the formation of a clear haloaround the colony indicates that the organisms are at least ableto depolymerize the polymer, which is the first step ofbiodegradation. This method is usually applied to screenorganisms that can degrade a certain polymer (Nishida andTokiwa, 1993; Abou-Zeid, 2001), but it can also be used toobtain semi-quantitative results by analyzing the growth of clearzones (Augusta et al., 1993).

4.7. Enzymatic degradation

The enzymatic degradation of polymers by hydrolysis is a two-step process: first, the enzyme binds to the polymer substrate then

253dvances 26 (2008) 246265subsequently catalyzes a hydrolytic cleavage. PHB can bedegraded either by the action of intracellular and extracellulardepolymerases in PHB-degrading bacteria and fungi. Intracellular

gy Adegradation is the hydrolysis of an endogenous carbon reservoirby the accumulating bacteria themselves while extracellulardegradation is the utilization of an exogenous carbon source notnecessarily by the accumulating microorganisms (Tokiwa andCalabia 2004). During degradation, extracellular enzymes frommicroorganisms break down complex polymers yielding shortchains or smaller molecules, e.g., oligomers, dimers, andmonomers, that are smaller enough to pass the semi-permeableouter bacterial membranes. The process is called depolymeriza-tion. These short chain lengthmolecules are thenmineralized intoend products e.g. CO2, H2O, or CH4, the degradation is calledmineralization, which are utilized as carbon and energy source(Gu, 2003).

4.8. Controlled composting test

The treatment of solid waste in controlled compostingfacilities or anaerobic digesters is a valuable method for treatingand recycling organic waste material (Biological WasteManagement Symposium, 1995; OECD, 1994). Compostingof biodegradable packaging and biodegradable plastics is a formof recovery of waste which can cut the increasing need of newlandfilling sites. Only compostable materials can be recycledthrough biological treatment, since materials not compatiblewith composting could decrease the compost quality and impairits commercial value. The environmental conditions of thecomposting test are the following: high temperature (58 C);aerobic conditions; proper water content (about 50%). Maturecompost is used as a solid matrix, as a source of thermophilicmicroorganisms (inoculum), and as a source of nutrients. Thetest method is based on the determination of the net CO2evolution, i.e. the CO2 evolved from the mixture of polymer-compost minus the CO2 evolved from the unamended compost(blank) tested in a different reactor (Bellina et al., 1999). A veryimportant requisite is that the packaging material under studymust not release, during degradation, toxic compounds into thecompost which could hinder plants, animals, and human beingsby entering the food chain (Tosin et al., 1998).

5. Biodegradation of natural plastics

5.1. Biodegradation of polyhydroxyalkanoates

Microorganisms that produce and store PHA under nutrientlimited conditions may degrade and metabolize it when thelimitation is removed (Williams and Peoples, 1996). However,the ability to store PHA does not necessarily guarantee theability to degrade it in the environment (Gilmore et al., 1990).Individual polymers are much too large to be transporteddirectly across the bacterial cell wall. Therefore, bacteria musthave evolved extracellular hydrolases capable of converting thepolymers into corresponding hydroxyl acid monomers (Gilmoreet al., 1990). The product of PHB hydrolysis is R-3-hydroxybutyric acid (Doi et al., 1992), while extracellular

254 A.A. Shah et al. / Biotechnolodegradation of PHBV yields both 3-hydroxybutyrate and 3-hydroxyvalerate (Luzier, 1992). The monomers are watersoluble but small enough to passively diffuse through the cellwall, where they are metabolized by -oxidation and tricar-boxylic acid cycle (TCA) to produce carbon dioxide and waterunder aerobic conditions (Scott, 1999). Under anaerobicconditions, methane is also produced (Luzier, 1992). In general,no harmful intermediates or by-products are generated duringPHA degradation. In fact, 3-hydroxybutyrate is found in allhigher animals as blood plasma (Lee, 1996). For this reason,PHAs have been considered for medical applications, includinglong-term controlled drug release, surgical pins, sutures, andbone and blood vessel replacement.

A number of aerobic and anaerobic microorganisms thatdegrade PHA, particularly bacteria and fungi, have beenisolated from various environments (Lee, 1996). Acidovoraxfaecilis, Aspergillus fumigatus, Comamonas sp., Pseudomonaslemoignei and Variovorax paradoxus are among those found insoil, while in activated sludge Alcaligenes faecalis and Pseu-domonas have been isolated. Comamonas testosteroni has beenfound in seawater, Ilyobacter delafieldii is present in theanaerobic sludge. PHA degradation by Pseudomonas stutzeri inlake water has also been observed. Because a microbialenvironment is required for degradation, PHA is not affectedby moisture alone and is indefinitely stable in air (Luzier, 1992).PHAs have attracted industrial attention for use in theproduction of biodegradable and biocompatible thermoplastics(Takaku et al., 2006). Poly (3-hydroxybutyrate) (PHB) and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) degrading organ-isms were isolated from sewage sludge and evaluated theirefficiency to degrade PHB and PHBV in solid-plate assay. Inour study we have isolated a Streptomyces strain, identified asStreptoverticillium kashmeriense AF1, capable of degradingPHB and PHBV, and a bacterial strain Bacillus megateriumAF3, capable of degrading PHBV, from the soil mixed withactive sewage sludge on the basis of producing clear zones ofhydrolysis on PHB and PHBV containing mineral salt agarplates (Shah et al., 2007; Shah, 2007).

The microbial degradation rate of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) films in soil appeared to bedependent on the microbial population and distribution, and thedegradation ability of the PHBV-degrading microorganismscolonizing the surface of incubated PHBV films (Sang et al.,2000). In one of our studies, the scanning electron micrographsof the PHBV film buried in soil mixed with sewage sludge for120 days showed clear evidences of degradation with pits,surface roughening, grooves, cavities and disintegration(Fig. 4). Later on S. kashmirense AF1 was recovered from thesurface of PHBV film (Shah, 2007). Sang et al. (2002) alsoreported various traces, cavities, and grooves as observed on thedented surface of PHBV films demonstrating that the degrada-tion was a concerted effect of a microbial consortium colonizingthe film surface, including fungi, bacteria, and actinomycetes.Numerous irregular erosion pits have also been observed byMolitoris et al. (1996) on the surface of poly polyhydroxyalk-anoate by Comamonas sp.

Sturm test has been used by many researchers to study the

dvances 26 (2008) 246265biodegradation of biodegradable polymers (Whitchurch andTerence, 2006), the aliphatic and aromatic compounds (Kimet al., 2001). We have gravimetrically calculated CO2 evolved

gy AA.A. Shah et al. / Biotechnoloas a result of PHB and PHBV biodegradation by S. kashmirenseAF1 and B. megaterium AF3, through Sturm Test. The resultsshowed that in all the cases the amount of CO2 evolved in testwas more than in control.

5.2. Enzymatic degradation of polyhydroxalkanoates

At least two categories of enzymes are actively involved inbiological degradation of polymer: extracellular and intracellulardepolymerases (Doi, 1990; Gu et al., 2000b) Extracellular PHBdepolymerases are secreted from various microorganisms andplay an important role in the metabolism of PHB in theenvironment. Several PHB depolymerases have been isolatedand purified from various microorganisms belonging to the Al-caligenes (Bachmann and Seebach, 1999), Comamonas (Jen-drossek et al., 1993; Kasuya et al., 1994) and Pseudomonasspecies (Nakayama et al., 1985; Mukai et al., 1994; Schober et al.,2000). This shows that extracellular PHB depolymerases areubiquitous in the environment.Analysis of their primary structuresrevealed that the enzymes are composed of substrate-bindingdomain, catalytic domain, and a linker region connecting the twodomains. The substrate-binding domain plays a role in binding tothe solid PHB. The catalytic domain contains the catalytic

Fig. 4. Scanning Electron Micrograph of poly(3-hydroxybutyrate-co-3-hydro-xyvalerate) film (a) Before soil burial; (b) After soil burial for 120 days.machinery composed of a catalytic triad (Ser-His-Asp). The serineis part of a lipase box pentapeptide Gly-X-Ser-X-Gly, which hasbeen found in all known hydrolases such as lipases, esterases andserine proteases (Jaeger et al., 1994). The properties of PHBdepolymerases have been studied extensively and share severalbiochemical properties such as: relatively small molecular weight,below 100 kDa andmost PHA depolymerases are between 40 and50 kDa; do not bind to anion exchangers such as DEAE but havestrong affinity to hydrophobic materials such as butyl-Toyopearland phenyl-Toyopearl; optimum pH is between 7.59.8, only thedepolymerase of Pseudomonas picketti and Penicillium funiculo-sum have pH optima between 5.5 and 7; highly stable at a widerange of pH, temperature, ionic strength, etc; most PHAdepolymerases are inhibited by serine esterase inhibitors such asdiidopropyl-fluorylphosphate or acylsulfonyl compounds, whichhave been shown to bind covalently to the active site serine ofserine hydrolases (Jendrossek, 1998). In our study we haveisolated a PHBV depolymerase from Bacillus sp. AF3, withmolecular size 37 kDa, at pH 7 and temperature 37 C (Shah et al.,2007).

Apparently, most PHA-degrading bacteria that have beenanalyzed produce only one PHA depolymerase. P. lemoignei,one of the best-studied PHA-degrading bacteria, however,produces at least seven different extracellular PHA-depoly-merases which differ slightly in their biochemical properties.The three PHA-depolymerases (PHB depolymerases A, B andD) are specific for PHB and P(3HB-co-3HV) with a low 3-HVcontent. The other two PHA depolymerases (PHB depolymer-ase C and poly(D-3-hydroxyvalerate depolymerase) alsodegrade both PHB and PHV (Jendrossek and Handrick,2002). A strain of A. faecalis T1, isolated from activatedsludge, excreted D-3-hydroxybutyrate oligomer hydrolase andan extracellular PHB depolymerase (Shirakura et al., 1983). Wehave isolated two different types of PHB and PHBVdepolymerases from S. kashmirense AF1, having molecularsizes 3537 and 45 kDa (Shah, 2007).

In 1990 the British-based company Imperial ChemicalIndustries (ICI) released a material called Biopol. Biopol ismade from PHBV. When thrown away, this material is brokendown by the microorganisms present in waste and decomposedcompletely within a couple of months. Other versions ofbiodegradable plastics have been developed in the UnitedStates. In 1991 Procter and Gamble, Du Pont, and Exxonfunded bacteria-based plastic research at the University ofMassachusetts at Amherst. In addition, Battelle, a privateresearch company, produced a completely biodegradable plasticfrom vegetable oils. Plastics can be made from other glucose-intensive materials such as potato scraps, corn, molasses, andbeets (Kings et al., 1992; Sharpley and Kaplan, 1976).

By 1997, compostable bags still sold for more than non-degradable bags. One synthetic polymer used in biodegradableplastic bags was aliphatic polyester called polycaprolactone,marketed by Union Carbide. Another biodegradable plastic filmcalled MaterBi contained corn starch and other proprietary

255dvances 26 (2008) 246265ingredients. Corn was also used to make polylactic acid forCargill's EcoPLA biopolymer. University of Michigan workedon a material called Envar consisting of an alloy of caprolactone

gy Aand a thermoplastic starch. One research group at the Universityof Utah at Salt Lake City in 1997, for instance, synthesized aninjectable polymer that forms a non-toxic biodegradable hydrogel that acts as a sustained release matrix for drugs (Kings et al.,1992; Sharpley and Kaplan, 1976).

6. Biodegradation of synthetic plastics

The degradation of most synthetic plastics in nature is a veryslow process that involves environmental factors, followed bythe action of wild microorganisms (Albertsson, 1980; Cruz-Pinto et al., 1994; Albertsson et al., 1994). The primarymechanism for the biodegradation of high molecular weightpolymer is the oxidation or hydrolysis by enzyme to createfunctional groups that improves its hydrophylicity. Conse-quently, the main chains of polymer are degraded resulting inpolymer of low molecular weight and feeble mechanicalproperties, thus, making it more accessible for further microbialassimilation (Albertsson and Karlsson, 1990; Albertsson et al.,1987; Huang et al., 1990). Examples of synthetic polymers thatbiodegrade include poly(vinyl alcohol), poly(lactic acid),aliphatic polyesters, polycaprolactone, and polyamides. Severaloligomeric structures which biodegrade are known: oligomericethylene, styrene, isoprene, butadiene, acrylonitrile, andacrylate. Physical properties such as crystallinity, orientationand morphological properties such as surface area, affect therate of degradation (Huang et al., 1992).

6.1. Biodegradation of thermoplastic polyolefins

6.1.1. PolyethyleneSynthetic polyolefins are inert materials whose backbones

consist of only long carbon chains. The characteristic structuremakes polyolefins non-susceptible to degradation by micro-organisms. However, a comprehensive study of polyolefinbiodegradation has shown that some microorganisms couldutilize polyolefins with low molecular weight (Yamada-Onodera et al., 2001). The biodegradation always followsphotodegradation and chemical degradation.

Polyethylene is one of the synthetic polymers of highhydrophobic level and high molecular weight. In natural form, itis not biodegradable. Thus, their use in the production ofdisposal or packing materials causes dangerous environmentalproblems (Kwpp and Jewell, 1992). To make PE biodegradablerequires modifying its crystalline level, molecular weight andmechanical properties that are responsible for PE resistancetowards degradation (Albertsson et al., 1994). This can beachieved by improving PE hydrophilic level and/or reducing itspolymer chain length by oxidation to be accessible for microbialdegradation (Bikiaris et al., 1999).

The degradation of polyethylene can occur by differentmolecular mechanisms; chemical, thermal, photo and biodegrada-tion. Some studies (Lee et al., 1991; Glass and Swift, 1989; Imamet al., 1992; Gu, 2003) have assessed the biodegradability of some

256 A.A. Shah et al. / Biotechnoloof these new films by measuring changes in physical properties orby observation of microbial growth after exposure to biological orenzymatic environments, but mostly by CO2 evolution.Since polyethylene (PE) is widely used as packaging material,considerable work not only on biodegradable polyethylene butalso on biodegradation of polyethylene has been conducted(Bonhomme et al., 2003; Wang et al., 2004). The results of thesestudies indicated that polyethylene was biodegraded followingphotodegradation and/or chemical degradation.

Biodegradation of polyethylene is known to occur by twomechanisms: hydro-biodegradation and oxo-biodegradation(Bonhomme et al., 2003). These two mechanisms agree withthe modifications due to the two additives, starch and pro-oxidant, used in the synthesis of biodegradable polyethylene.Starch blend polyethylene has a continuous starch phase thatmakes the material hydrophilic and therefore, catalyzed byamylase enzymes. Microorganisms can easily access, attack andremove this part. Thus the hydrophilic polyethylene with matrixcontinues to be hydro-biodegraded. In case of pro-oxidantadditive, biodegradation occur following photodegradation andchemical degradation. If the pro-oxidant is a metal combination,after transition, metal catalyzed thermal peroxidation, biode-gradation of low molecular weight oxidation products occurssequentially (Bonhomme et al., 2003; EI-Shafei et al., 1998;Yamada-Onodera et al., 2001). EI-Shafei et al. (1998)investigated the ability of fungi and Streptomyces strains toattack degradable polyethylene consisting of disposed poly-ethylene bags containing 6% starch. He has isolated 8 differentstrains of Streptomyces and two fungi Mucor rouxii NRRL1835 and Aspergillus flavus. In a study, Low densitypolyethylene pieces buried in soil mixed with sewage sludgewere examined microscopically after 10 months, fungalattachment was found on the surface of the plastic, indicatingpossible utilization of plastic as a source of nutrient (Shah,2007) The isolated fungal strains were identified as Fusariumsp. AF4, Aspergillus terreus AF5 and Penicillum sp. AF6. Theability of the fungal strains to form a biofilm on polyethylenewas attributed to the gradual decrease in hydrophobicity of itssurface (Gilan et al., 2004).

The structural changes in the form of pits and erosionsobserved through scanning electronmicroscopy indicated surfacedamage of PE incubated with Fusarium sp. AF4. That suggestedthat the fungal strains, especially Fusarium sp. AF4, was able toadhere to the surface of LDPE and can cause surface damage(Shah, 2007). In a study by Bonhomme et al. (2003), SEMevidence confirmed that microorganisms (fungi) build up on thesurface of the polymer (polyethylene) and after removal of themicroorganisms, the surface became physically pitted and eroded.The surface of the polymer after biological attack was physicallyweak and readily disintegrates under mild pressure. Otake et al.(1995) reported the changes like whitening of the degraded areaand small holes on the surface of PE film after soil burial for32 years. Biodegradation of LDPE filmwas also reported as 0.2%weight loss in 10 years (Albertsson, 1980). Ohtaki et al. (1998)tested LDPE bottles exposed in aerobic soil for over 30 years, andobserved some evidences of biodegradation using SEM of thedegraded parts and as reduction in molecular weight by Time of

dvances 26 (2008) 246265Flight Mass Spectrometry (TOF-MS).Yamada-Onodera et al. (2001) isolated a strain of fungus

Penicillium simplicissimum YK to biodegrade polyethylene

gy Awithout additives. UV light or oxidizing agents, such as UVsensitizer, were used at the beginning of the process to activatean inert material, polyethylene. Polyethylene was also treatedwith nitric acid at 80 C for 6 days before cultivation withinserted functional groups that were susceptible to microorgan-isms. Bonhomme et al. (2003) has reported that with the fungalactivity, polyethylene with a starting molecular weight in therange of 4000 to 28,000 was degraded to units with a lowermolecular weight of 500 after three months of liquid cultivation,which indicated the biodegradation of that polyethylene. In asimilar study the Polyethylene pieces were treated by exposingit to UV light and also nitric acid. That pretreated polymer wasthen applied to microbial treatment using Fusarium sp. AF4 in amineral salt medium containing treated plastic as a sole sourceof carbon and energy. An increase in the growth of fungus andsome structural changes as observed by FTIR, were observed incase of treated PE which according to Jacinto indicated thebreak down of polymer chain and presence of oxidationproducts of PE. Non-degraded polyethylene exhibits almostzero absorbancy at those wave numbers (http://www.dasma.dlsu.edu.ph/offices/ufro/sinag/Jacinto.htm). Absorbance at17101715 cm1 (corresponding to carbonyl compound),1640 cm1 and 830880 cm1 (corresponding to C=C),which appeared after UV and nitric acid treatment, decreasedduring cultivation with microbial consortia (Hasan et al., 2007).Typical degradation of PE and formation of bands at 16201640 cm1 and 840880 cm1 was also reported by Yamada-Onodera et al. (2001), attributed to oxidation of polyethylene.Overall, polyethylene degradation is a combined photo- andbio-degradation process. First, either by abiotic oxidation (UVlight exposure) or heat treatment, essential abiotic precursors areobtained. Secondly, selected thermophilic microorganismsdegrade the low molar mass oxidation products to completethe biodegradation (Bonhomme et al., 2003).

6.1.2. Polyvinyl chloridePolyvinyl chloride (PVC) is a strong plastic that resists

abrasion and chemicals and has low moisture absorption.Mostly, PVC is used in buildings for pipes and fittings, electricalwire insulation, floor coverings and synthetic leather products. Itis also used to make shoe soles, rigid pipes, textiles and gardenhoses. There are many studies about thermal and photodegrada-tion of PVC (Braun and Bazdadea, 1986; Owen, 1984) but thereonly few reports available on biodegradation of PVC. Accordingto Kirbas et al. (1999) PVC having low molecular weight can beexposed to biodegradation by the use of white-rot fungi.

6.1.3. PolystyrenePolystyrene (PS) is a synthetic plastic used in the production

of disposable cups, packaging materials, in laboratory ware, incertain electronic uses. PS is used for its lightweight, stiffnessand excellent thermal insulation. When it is degraded bythermal or chemical means it releases products like; styrene,benzene, toluene and acrolein. There are very few reports on the

A.A. Shah et al. / Biotechnolobiodegradation of polystyrene but the microbial decompositionof its monomer; styrene, have been reported by few researchers(Tsuchii et al., 1977).7. Biodegradation of polymer blends

The rate of degradation of polymer blends is initiallycontrolled by the degradation of the more readily biodegradablecomponent. The initial degradation process interferes with thestructural integrity of the polymer and increases the surface areaconsiderably for enzyme attack on the less affected area. Theexposure of the remaining polymer to microbes and secreteddegradative enzymes is then enhanced.Work has been carried outto understand the degradation mechanisms of different polymerblends and their degradation products by microorganisms.

There are several categories of biodegradable starch-basedpolymers including:

1. Thermoplastic starch products;2. Starch synthetic aliphatic polyester blends;3. Starch PBS/PBSA polyester blends; and4. Starch PVOH blends.

7.1. Starch/polyethylene blends

Polyethylene is reported to be an inert polymer with strongresistance to microbial breakdown (Weiland et al., 1995).Biodegradation is decreased with an increase in molecular size(Tung et al., 1999). Linear paraffin molecules below amolecular size of about 500 Da can be utilised by severalmicroorganisms (Albertsson and Karlsson, 1993). Scott (1990)concluded that microbial attack on polyethylene is a secondaryprocess where an initial oxidation step results in the reduction ofmolecular size to a size sufficiently small for biodegradation tooccur. Addition of readily biodegradable compounds, such asstarch, to a low-density polyethylene matrix may enhance thedegradation of the carboncarbon backbone (Griffin, 1977).The biodegradability of starch/polyethylene blends, andchemically modified samples of blends has been investigated(Johnson et al., 1993; Bikiaris et al., 1998). The aerobic andanaerobic biodegradability of starch-filled polymer blends hasbeen evaluated. Carbon removal from a starch polyethyleneblend was low compared to pure starch and the rate of removalwas higher under aerobic conditions. Similar results wereobtained by Chandra and Rustgi (1997) for the biodegradationof maleated linear low-density polyethylene starch blends in asoil environment composed of a mixed fungal inoculumconsisting of Aspergillus niger, Penicillium funiculom, Chae-tomium globosum,Gliocladium virens and Pullularia pullulans.Biodegradation of starch polyethylene films containing a pro-oxidant and 6% starch showed evidence of polyethylenedegradation in the presence of lignin degrading bacteria of thegenus Streptomyces and also in the presence of the white-rotfungus Phanerochaete chrysosporium (Lee et al., 1991). Therate of degradation of starch-filled polyethylene depended onthe starch content, and was very sensitive to the environmentalconditions and other ingredients in the formulation (Albertssonand Karlsson, 1993). Oxidation of impurities, such as fats and

257dvances 26 (2008) 246265oils, would seem to be an important trigger for the biodegrada-tion of polyethylene. The production of reactive oxygen speciessuch as peroxides is a likely source of initiators of molecular

gy Abreakdown although other radicals may be involved (Cornellet al., 1984).

The degradation of 11 commercially produced degradablestarch/polyethylene blended compost bags was evaluated in amunicipal yard waste compost site (Johnson et al., 1993) bychemical degradation, photodegradation and biodegradation ofeach product. The oxygen concentration at the surface of thefilms appeared to be the rate-limiting component. A pro-oxidantadditive (transition metal) was critical in promoting theoxidative degradation of the polyethylene. Several researchershave investigated the use of modified starch in starch/low-density polyethylene blends (Evangelista et al., 1991). Themodified starches enhanced the miscibility and adhesion ofstarch in those blends. However, the poor biodegradability ofmodified starches leads to a very slow rate of biodegradationcompared with unmodified starch/polyethylene blends. Otherinvestigations have been carried out to evaluate the biodegrad-ability of films containing starch, polyethylene and EAA(Shogren et al., 1992). The results indicated that rapid andappreciable starch depletion led to deterioration of themechanical properties of the films leaving a rather weak matrixwhich was prone to further physical disruption by biotic andabiotic factors.

7.2. Starch/polyester blends

Blends of starch and Polycaprolactone (PCL) are assumed tobe completely biodegradable since each component in the blendis readily biodegradable as well as compostable (Tokiwa et al.,1994; Bastioli, 1994). Biodegradability of different grades ofthe commercial polyester Bionolle has been studied inactivated sludge, soils and compost (Nishioka et al., 1994).Bionolle 3000 was degraded more easily than Bionolle1000 and Bionolle 6000. Moulds did not degrade Bionolle6000 and some Gram-negative bacteria did not degradeBionolle 1000. The degree of degradation mainly dependedon the type of microorganism and their population. It is knownthat poly-(hydroxybutyrate) depolymerases and lipases are bothcapable of cleaving the ester bonds 4 poly-(hydroxyalkanoate)(Doi et al., 1992). Because of structural similarity, theseenzymes are expected to degrade Bionolle. Biodegradation ofBionolle by compost has been studied and shown to becomplete (Jayasekara et al., 2005). Furthermore, the productswere shown to be non-toxic to the earthworm, Eisena fetida.Blending of Bionolle with low cost starch has been investigatedin order to improve its cost competitiveness whilst maintainingother properties at an acceptable level. It has been shown thatthe addition of starch filler significantly improved the rate ofdegradation of the Bionolle component (Ratto et al., 1999). Astudy of the biodegradation of cellulose, a poly(-caprolactone)starch blend and an aliphaticaliphatic copolyester was under-taken in a ring test (Pagga et al., 2001) involving severallaboratories using standards like; ISO 14852 (1999), and ISO14852. According to both criteria (the biological oxygen

258 A.A. Shah et al. / Biotechnolodemand (BOD) and CO2 released) the cellulose was moredegraded than the poly(-caprolactone)-starch blend which inturn was more degraded (on average) than the copolyester.7.3. Starch/PVA blends

The water-soluble synthetic polymer, polyvinyl alcohol(PVA) has excellent compatibility with starch and blends areexpected to have good film properties. Several such blends havebeen developed and tested for biodegradable packagingapplications and appear to have potential for use (Tudorachiet al., 2000). PVA and starch blends are assumed to bebiodegradable since both components are biodegradable invarious microbial environments. The processing and mechan-ical properties of starch/PVA blends have been well investigated(Park et al., 1994; Bastioli, 1994; Haschke et al., 1998) but onlya limited number of publications are available regarding thebiodegradability of such blends. Data on the biodegradability ofstarch, PVA, glycerol and urea blends by bacteria and fungiisolated from the activated sludge of a municipal sewage plantand landfill indicated that microorganisms consumed starch andthe amorphous region of PVA as well as the glycerol and ureaplasticisers (Tudorachi et al., 2000). The crystalline region ofPVA was unaffected. A biodegradability study of starch, PVAand glycerol blends has also been carried out by Park et al.(1994). ASTM D 5271-92 (1992) was used to evaluate the rateand degree of biodegradation of PVA blends which werereported as biodegradable. In a separate study of PVA and poly-(3-hydroxybutyric acid) blended films, the solubility of the PVAcomponent in water was found to effect the biodegradability ofthe blend (Ikejima et al., 1999).

7.4. Polylactic acid (PLA) (renewable resource) polyesters

Polylactic acid (PLA) is linear aliphatic polyester producedby poly-condensation of naturally produced lactic acid or by thecatalytic ring opening of the lactide group. Lactic acid isproduced (via starch fermentation) as a co-product of corn wetmilling. The ester linkages in PLA are sensitive to bothchemical hydrolysis and enzymatic chain cleavage. PLA isoften blended with starch to increase biodegradability andreduce costs. However, the brittleness of the starch-PLA blendis a major drawback in many applications. To remedy thislimitation, a number of low molecular weight plasticisers suchas glycerol, sorbitol and triethyl citrate are used. A number ofcompanies produce PLA, such as Cargill Dow LLC. PLAproduced by Cargill Dow was originally sold under the nameEcoPLA, but now is known as NatureWorks PLA, which isactually a family of PLA polymers that can be used alone orblended with other natural-based polymers (DevelopingProducts that Protect the Environment, 2007).

The applications for PLA are thermoformed products such asdrink cups, take-away food trays, containers and planter boxes.The material has good rigidity characteristics, allowing it toreplace polystryene and PET in some applications. PLA is fullybiodegradable when composted in a large-scale operation withtemperatures of 60 C and above. The first stage of degradationof PLA (two weeks) is via hydrolysis to water-soluble

dvances 26 (2008) 246265compounds and lactic acid. Rapid metabolisation of theseproducts into CO2, water and biomass by a variety ofmicroorganisms. There have been reports on the degradation

gy Aof PLA oligomers (molecular weight ~1000) by Fusariummoniliforme and Penicillium Roquefort (Torres et al., 1996) andthe degradation of PLA by Amycolatopsis sp. (Pranamuda et al.,1997; Pranamuda and Tokiwa, 1999) and by Bacillus brevis(Tomita et al., 1999. In addition, enzymatic degradation of lowmolecular weight PLA (molecular weight ~2000) has beenshown using esterase-type enzymes such as Rhizopus delemerlipase (Fukuzaki et al., 1989). McCarthy (1999) showed that A-PLA presents a soil degradation rate much slower compared toPBSA.

7.5. Polybutylene succinate (PBS) (synthetic aliphatic)polyesters

Polybutylene succinate (PBS) is a biodegradable syntheticaliphatic polyester with similar properties to PET. PBS isgenerally blended with other compounds, such as starch (TPS)and adipate copolymers (to form PBSA), to make its useeconomical. PBS has excellent mechanical properties and canbe applied to a range of end applications via conventional meltprocessing techniques. Applications include mulch film,packaging film, bags and flushable hygiene products. PBS ishydro-biodegradable and begins to biodegrade via a hydrolysismechanism. Hydrolysis occurs at the ester linkages and thisresults in a lowering of the polymer's molecular weight,allowing for further degradation by microorganisms. Data fromSK Chemicals (Korea), a leading manufacturer of PBSpolymers, quotes a degradation rate of 1 month for 50%degradation for 40 m thick film in garden soil.

7.6. Aliphaticaromatic (AAC) copolyesters

Aliphaticaromatic (AAC) copolyesters combine the biode-gradable properties of aliphatic polyesters with the strength andperformance properties of aromatic polyesters. This class ofbiodegradable plastics is seen by many to be the answer tomaking fully biodegradable plastics with property profilessimilar to those of commodity polymers such as polyethylene.To reduce cost AACs are often blended with TPS. AlthoughAACs have obvious benefits, their market potential may beaffected by legislation, such as that in Germany, whichdistinguishes between biodegradable plastics made fromrenewable resources and those, like AAC, which use basicallythe same raw materials as commodity plastics and petrochem-icals. Currently in Germany, biodegradable plastics mustcontain greater than 50% renewable resources to be accepted(Biodegradable Plastics-Developments and EnvironmentalImpacts, 2002).

The two main types of commercial AAC plastics areEcoflex produced by BASF and Eastar Bio produced byEastman. Under each trade name are a number of specificgrades. Each grade of polymer has been designed withcontrolled branching and chain lengthening to match itsparticular application. AACs come closer than any other

A.A. Shah et al. / Biotechnolobiodegradable plastics to equalling the properties of low densitypolyethylene, especially for blown film extrusion. AACs alsocan meet all the functional requirements for cling film such astransparency, flexibility and anti-fogging performance, andtherefore this material has great promise for use in commercialfood wrap for fruit and vegetables, with the added advantage ofbeing compostable. Whilst being fossil fuel-based, AACs arebiodegradable and compostable. AACs fully biodegrade tocarbon dioxide, water and biomass. Typically, in an activemicrobial environment the polymer becomes invisible to thenaked eye within 12 weeks (Biodegradable Plastics-Develop-ments and Environmental Impacts, 2002).

7.7. Modified polyethylene terephthalate (PET)

Modified PET (polyethylene terephthalate) is PET whichcontains co-monomers, such as ether, amide or aliphaticmonomers that provide weak linkages that are susceptible tobiodegradation through hydrolysis. The mechanism involves acombination of hydrolysis of the ester linkages and enzymaticattack on ether and amide bonds. With modified PET it ispossible to adjust and control degradation rates by varying theco-monomers used. Depending on the application, up to threealiphatic monomers are incorporated into the PET structure.Typical modified PET materials include PBAT (polybutyleneadipate/terephthalate) and PTMAT (polytetramethylene adipate/terephthalate). DuPont have commercialised Biomax whichis a hydro-biodegradable modified PET polyester. CertainBiomax grades also contain degradation promoters to providetailored combinations of performance properties and degrada-tion rates. The options available for modified PET provide theopportunity to produce polymers which specifically match arange of application physical properties whilst maintaining theability to adjust the degradation rate by the use of copolyesters.

8. Other degradable polymers

8.1. Ethylene vinyl alcohol (EVOH)

EVOH is another water-soluble synthetic plastic, and is usedas an oxygen barrier layer in multilayer film packaging. Thehigh cost of EVOH is a significant barrier to its widespread usein other biodegradable plastics applications.

8.2. Photo-biodegradable plastics

Photodegradable plastics are thermoplastic synthetic poly-mers into which have been incorporated light-sensitivechemical additives or copolymers for the purposes of weaken-ing the bonds of the polymer in the presence of ultravioletradiation. Photodegradable plastics are designed to becomeweak and brittle when exposed to sunlight for prolongedperiods. Photosensitisers used include diketones, ferrocenederivatives (aminoalkyferrocene) and carbonyl-containing spe-cies. These plastics degrade in a two-stage process, with UVlight initially breaking some bonds leaving more brittle lowermolecular weight compounds that can further degrade from

259dvances 26 (2008) 246265physical stresses such as wave action or scarification on rocks.The effectiveness is dependent on exposure intensity and willvary with factors such as the season, geography, dirt or water

PUR is postulated to be the mechanism of PUR biodegradation.

d ( al.ree ra haure tio riaat se esle ng las m s a

polyurethane degrading bacteria and also to screen andcharacterize PU degrading enzymes. The detection of poly-urethanase in a PU gel is based on the ability of enzymes todepolymerise the substrate. Thus by hydrolyzing the substrate,the interaction of the Coomassie blue with the polyurethane isdiminished resulting in a zone of clearing within a bluebackground (Howard et al., 1999).

FTIR spectroscopy was used to confirm that the mechanism ofpolyurethane biodegradation was the hydrolysis of the ester bondin polyurethane. Results obtained by Nakajima-Kambe et al.(1995) and Howard et al. (1999) indicated that the polyurethanebiodegradation was due to the hydrolysis of ester bonds. Thedecrease of the ratio of ester bond over ether bond was alsoapproximately 50%, which agreed with the measured amount ofpolyurethane degraded. In our study the PUR plastic films wereburied in soil for about 6 months. FTIR analysis of these PURfilms showed a slight decrease in the peak from wavelength2963 cm1(control) to 2957 cm1(test) indicating the cleavage ofC\H bonds and formation of C=C at the region of 14001600 cm1. Pettit and Abbott (1975) were also of the opinion thatthe decomposition of urea units by release of ammoniacontributes to the degradation of polyurethane. Sequentially theester bonds of the urethane groups (H2N-CO-OR) at 1715 cm

1

gy Advances 26 (2008) 246265were obtained from soil and found to degrade ester-basedpolyurethane. Kay et al. (1991) isolated and investigated 16different bacteria with their ability to degrade PUR. In anothercomprehensive study in Japan, PUR was biodegraded as a solecarbon and nitrogen source by Comamonas acidovorans(Nakajima-Kambe et al., 1999; Akutsu et al., 1998). Shah(2007) has reported 5 bacterial strains that were isolated aftersoil burial of Polyurethane film for 6 months. Those bacteriawere identified as Bacillus sp. AF8, Pseudomonas sp. AF9,Micrococcus sp. AF10, Arthrobacter sp. AF11 and Coryne-bacterium sp. AF12 (Table 4). The bacterial isolates were listedfor polyurethanolytic activity by clear zones formation aroundthe bacterial colonies when Coomassie blue R-250 was added tosariumbacterialThis is alytic baolani, Aureobasidiucultures in mineralrapid and sensitive sccteria. The method wpullulanmedia conreening asas usednd Cladosporium sp,

examp , four species of fu i, Curvu ria senegalensis, Fu-

degrad ion by polyurethana enzym (Howard, 2002). For

literat : fungal biodegrada n, bacte l biodegradation andTh

Nakajima-Kambe ettypes of PUR deg dations ve been identified inpolyescleavekdown products of tsegment in PUR wh ester b

, 1999).tion are derived fromds are hydrolyzed andThe brea he biodegradater en oncover, and shading. A new approach to making photodegrad-able plastics involves adding catalytic metal salts or chelates toinitiate the breakdown process. In photodegradable systems,biodegradation occurs only after an initial photodegradationstage. Degradation of the polymer is triggered by UV light, andassisted by the presence of UV sensitizers in the polymer. Thepolymer is initially converted to low molecular weight material(i.e. waxes), and then converted to carbon dioxide and water bybacterial action.

Biodegradable polyesters which have been developedcommercially and are in commercial development are asfollows:

PHA Polyhydroxyalkanoates PHB PolyhydroxybutyratePHH Polyhydroxyhexanoate PHV PolyhydroxyvaleratePLA Polylactic acid PCL PolycaprolactonePBS Polybutylene succinate PBSA Polybutylene succinate

adipateAAC AliphaticAromatic

copolyestersPET Polyethylene terephthalate

PBAT Polybutylene adipate/terephthalate

PTMAT Polymethylene adipate/terephthalate

8.3. Biodegradation of thermoset plastics

8.3.1. PolyurethanePolyurethane (PUR) is commonly utilized as a constituent

material inmany products including furniture, coating, constructionmaterials, fibers, and paints. Structurally, PUR is the condensationproduct of polyisocyanate and polyol having intramolecularurethane bonds (carbonate ester bond, NHCOO) (Sauders andFrisch, 1964). The urethane bond in PUR has been reported to besusceptible to microbial attack. The hydrolysis of ester bonds in

260 A.A. Shah et al. / Biotechnolotaining polyurethanesay for polyurethano-to count the viableFig. 5. Scanning Electron Micrographs of Polyurethane film treated withbacterial consortia, (a) Control; (b) Test.

newer polymers, either alone or in blended films.

gy Acould be broken by the hydrolytic effects of microbial esterases.PUR breakdown products were analyzed by FTIR using Cory-nebacterium sp. and reported that PUR degradation was causedby the hydrolysis of ester bonds (Fig. 5) (Kay et al., 1993).

Two kinds of PUase enzymes were isolated and char-acterized by Howard et al. (1999), Allen et al. (1999) andVega et al. (1999). These were shown to be a cell-associatedmembrane bound PU-esterase and an extracellular PU-esterase. These two enzymes play different roles inpolyurethane biodegradation. The membrane bound PU-esterase provides cell-mediated access to the hydrophobicpolyurethane surface. Then the extracellular PU-esterasesticks on the surface of the polyurethane. Under theseenzymatic actions, bacteria could adhere to the surface ofpolyurethane and hydrolyze PU substrate to metabolites. Theenzyme esterase can hydrolyze polyester chains in PU todiethylene glycol and adipic acid (Akutsu et al., 1998).Results obtained by Nakajima-Kambe et al. (1995) andHoward et al. (1999) indicated that the polyurethanebiodegradation was due to the hydrolysis of ester bonds.Pathirana and Seal (1985) reported that some polyester-PURdegrading fungi produce extracellular esterases, proteases orureases in the presence of PUR. Esterase activity has beendetermined in the culture supernatant of Corynebacterium sp.(Kay et al., 1993) C. acidovorans (Nakajima-Kambe et al.,1997) and in fungus like C. senegalensis (Crabbe et al.,1994).

Bacteria known to degrade polyester PU also produce PUdegrading enzymes such as polyurethanases (Ruiz et al., 1999;Rowe and Howard, 2002). Purified polyurethanase enzymes,PueA and PueB, were used to assay enzyme activity towardspolyurethane-rhodamine agar plates, poly(ester-urethane)(PEU) and poly(carbonate-urethane) (PCU). The lipase-cata-lyzed polymerization of low molecular weight and biodegrad-able urethanediols with short chain dialkyl carbonate andalkanedioates produced PCU and PEU, respectively. They werereadily degraded in an organic solvent into the repolymerizablecyclic oligomers by lipase as a novel chemical recycling (Degli-Innocenti et al., 1998).

9. Composting

Composting is a managed process that controls the biologicaldecomposition and transformation of biodegradable materials intoa humus-like substance called compost. The controlled biooxida-tion process proceeds throughmesophilic and thermophilic phasesand results in the production of carbon dioxide, water, mineralsand stabilized organicmatter (compost or humus). Another benefitof this process is that the heat produced can result in destruction ofpathogens that may be present in the waste stream. For mostoperations, including plastic biodegradation, a combination ofsludge and solid waste provides the best operation.

The plastic product or material must disintegrate duringcomposting such that the residual plastic is not readily

A.A. Shah et al. / Biotechnolodistinguishable from other organic materials in the finishedcompost. A plastic material or product is considered to havedemonstrated satisfactory disintegration if after lab-scaleThere are a large number of tests which are used todetermine the extent of degradation of polymers either aloneor in blended forms. Many are respirometric, determining theamount of carbon dioxide released on exposure to fungi,bacteria, activated sludge (aerobically or anaerobically),compost or soil. Some tests use loss of weight or change inphysical properties such as tensile strength and comparison ofspectroscopic (FTIR, DSC, NMR, SEM, AFM, XRD) data. Itis important to have comparable international standardmethods of determining the extent of biodegradation.Unfortunately, the current standards have not, so far, beenequated to each other and tend to be used in the countrieswhere they originated [e.g. ASTM (USA), DIN (Germany),JIS (Japan), ISO (international standards), CEN (Europe)].Many, which are otherwise harmonious, differ in the finedetails of the testing. There is an urgent need to standardize alldetails so that researchers may know that they have all workedto the same parameters. It is clear that most recalcitrantpolymers can be degraded to some extent in the appropriateenvironment at the right concentration. By judicious blendingtheir persistence may be minimised environmentally. Screen-ing of organisms which degrade polymers, or produceenzymes or enzyme systems that degrade polymers, mayprove as environmentally profitable in the 21st century, as thescreening program for antibiotics in the 1950 s and 1960 s. Ascreening program for such organisms and enzymes isrequired but will require more universally uniform standardsfor assessment of their degradative ability.

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10. Conclusions

This review has covered the major concerns about the naturaland synthetic polymers, their types, uses and degradability. Ithas looked at the disposal methods and the standards used inassessing polymer degradation. Another area examined hasbeen the developments in the biodegradation of some of the

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