Biodegradation of polyester polyurethane by Aspergillus flavus G10

Polyurethanes (PU) are integral to many aspects of our daily lives. Due to the extensive use of and difficulties in recycling or reusing PU, it mostly accumulates as waste. Various bacteria and fungi have been reported to degrade PU. We examined the fungus Aspergillus flavus G10 in that regard, after isolating it from the guts of Gryllus bimaculatus, a common cricket species. We observed surficial and chemical changes of PU with atomic force microscopy, scanning electron microscopy, and attenuated total reflectance Fourier-transform infrared spectroscopy. We measured physical changes as loss in tensile stress, stretching force, and weight of PU after incubations. Fungal hydrolysis of urethane bonds in the polymer backbone was demonstrated by detecting the formation of methylene di-aniline during incubations. Trapped CO2 during incubations equaled 52.6% of PU carbon. Biodegradation of PU was maximal by fungi cultured on a malt extract medium at 25 °C, pH 12, and 14:10 hrs light to dark ratio. Pretreating PU films with UV light or 1% FeSO4 or NaCl solutions further enhanced the rate of biodegradation. A range of techniques are needed to fully characterize the degradation of PU or other plastic polymers and to optimize conditions for their microbial degradation.


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Annual production of plastics increased to 355 million tones (Mt) in 2017 due to 45 societal and commercial benefits [1]. 9150 Mt of primary plastics have been produced 46 from 1950 to 2015, resulting in 6945 Mt of plastic waste on the surface of earth [2]. 47 However, not all plastic waste products are landfilled, incinerated, or recycled. 48 Environmental persistence of plastic waste can produce pollutants that have been 49 shown to be threatening to life in different ecosystems [3,4,5,6].  containing PU films with no fungal inoculum were used as controls.

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A. flavus G10 was tested for PU biodegradation in a broth culture of malt extract at 108 pH 7.6. Three Erlenmeyer flasks containing 300 ml of malt extract broth were 109 inoculated with 2 ml/flask spore suspension (1x 10 5 spores/ml) of A. flavus G10. Two 110 PU transparent films (90 mm) were cut longitudinally, sterilized with UV radiation    To characterize mechanical properties after biodegradation, transparent PU films 154 (n=11) were exposed to A. flavus G10 on a solid medium at 30 ºC for a period of four 155 weeks. After every week, three PU films were randomly selected and brushed with 156 sterilized water and stored at room temperature until analysis. This process was 157 continued until the end of 4 th week. A set of three transparent PU films that was not 158 exposed to A. flavus G10 were controls. where σ max is the tensile stress, F max is the maximum amount force on the PU film, 170 and A min is the area of the PU film exposed to applied maximum force. growing on foamy PU films (Fig 1D). Similarly, the SEM results of transparent and 243 foamy PU films exposed to the fungus showed that the fungus grew across PU film 244 surfaces, with the film broken into pieces. Cracks in transparent PU film, with 245 mycelia inside cracks, are shown in Fig 1 (D).

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Biodegradation efficiency of A. flavus G10 by percentage 247 The percentage weight of the PU films exposed to A. flavus G10 decreased over time, film surfaces remained smooth (Fig 2A), surfaces of transparent PU films exposed to 256 Aspergillus flavus G10 were rough, with deep, wide grooves (Fig 2B-D). The average 257 roughness (Ra) recorded for the PU films exposed to the fungus was 2.66, 2.72, and  .1157 for C 13 H 14 N 2 ) that appeared in both the positive control (MDA) and in PU 314 films exposed to A. flavus G10 (Figs 3A-a, c and e). This showed that the degradation

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Conversion of PU to CO 2 increased from 22.3% to 52.6% during the 12-day 319 incubation period ( Fig 3B). There were significant differences among CO 2 production 320 across days 3, 6, 9, and 12 (P = 0.01) (Fig 3B). culture media. Analysis of the surface of the PU exposed to A. flavus G10 revealed 332 that PU films had been broken down, were replete with numerous holes and cavities, 333 and that fungal spores and hyphae covered the surfaces of these PU films (Figs 1-3).

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Similar results, such as the formation of holes in the PU film, hyphal growth on its 335 surface, and discoloration, were previously reported for the biodegradation of PU by