Microbial Biodegradation of Polyurethane Plastics

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Abstract

Plastic waste materials are an increasing concern when most of them are highly recalcitrant to degradation in the environment. Some plastics are however biodegradable such as polyurethane (PU). Biodegradable plastics are polymeric materials that are susceptible to microbial attack by enzymatic hydrolysis of the polymer’s chemical bonds. PU is a notable biodegradable synthetic polymer that is synthesised from the condensation between different combinations of polyols and polyisocyanates. PUs could be synthesised into various forms, ranging from soft elastomers to rigid plastics. Thus, they have many applications and are manufactured into many different products, contributing to the solid wastes accumulation in landfill sites. Biodegradation of plastics at landfill sites were thought to be more substantial when buried in the ground. However, some plastics may have been left exposed to the atmosphere instead. Therefore, they endured a different biodegradation process.In a longitudinal experiment, PU plastic coupons were hung and exposed to the environment for two years. The coupons were sampled and processed for microbial recovery and tested for PU tensile strength for deterioration assessment. PU-degrading microbial cells were highest recovered on the sixth coupons sample after 101 weeks of exposure which was 78.3 CFU/cm2. Those coupons also recorded the highest loss in tensile strength which was 39.1 MPa, a significant loss of 30.9% from the initial strength. The atmosphere surrounding the experiment were also sampled in order to understand the content of the air. The highest amount of PU-degrading microbial cells recovered was recorded to be 2220 CFU/m3 from the air sampling on week 70. The number of cells available in the atmosphere does coincide along with the number of cells recovered on the coupon samples. A total of 18 coupon samples alongside 21 air samples were successfully collected throughout the duration of the experiment. Isolates that were enumerated onto PUA while producing clearing in the media were considered to have the ability to utilise PU as a sole-carbon source, thus are regarded as PU-degraders.It was not uncommon to find PUA-clearing fungi in the atmosphere. Some of them even managed to attach themselves onto our exposed coupons. However, no enhanced microbial colonisation occurred on the coupons over the time they were exposed. Exposure to the atmosphere and several factors have led to the surface of the coupons to become very harsh for colonisation. UV light radiations were found to have much more significant effect on PU deterioration than microbial actions. Three fungi isolates were selected because of their prevalence and were successfully identified: Cladosporium sp., Penicillium sp. and Aspergillus fumigatus. These fungi could produce spores that helped them disperse in the environment. They also have wide range of enzymatic activities which could have allowed them to grow on PU coupons and PUA media. Further radial growth rates on these isolates showed that they are characteristically effective in utilising PU suspension.

Keyword(s)

biodegradation; microbiology; plastic; polyurethane

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