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Degradation of unpretreated and thermally pretreated polypropylene by soil consortia

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Abstract

Unpretreated (PP-UT) and thermally pretreated (at 80 °C for 10 days) polypropylene (PP-TT) films of 0.05 mm thickness were subjected to in vitro biodegradation in minimal medium with mixed soil culture for 12 months. In this period 10.7 and 0.4% weight loss was observed with PP-TT and PP-UT, respectively. The tensile strength decreased by 51.8 and 28.3%, the crystallinity increased by 28 and 33% and isotacticity increased by 3 and 9%, respectively, over the same time period. The ester carbonyl index in PP-TT increased up to 9 months and later decreased indicating abiotic followed by biotic process. No such changes were observed with PP-UT. Methyl group index decreased in both the cases indicating oxidation at the primary carbon. Increase in surface energy indicated that the polymer became hydrophilic. Surface changes were observed by SEM and AFM. A single culture was isolated at the end of 12 months and it was identified as Bacillus flexus. The morphology of the organism was rods in a chain and it was present in the form of an endospore.

Introduction

The consumption of plastics in the year 2006 in India was around 4545 KT, and out of this polypropylene (PP) accounted for 1200 KT. The consumption of thermoplastics was 40 million tonnes in European countries (Plastemart.com website, 2004). These figures give the scenario of the alarming rate at which plastic will be dumped in the environment after its useful life. Natural weathering, which includes solar radiation, UV rays and ambient temperature, affect the properties of the polymer to some extent, but generally these changes take place at a slow rate (Suits and Hsuan, 2003). Biodegradation is the result of the utilisation of the polymer as a carbon source by the microorganism. This process is facilitated if the microorganism initially forms a biofilm over the polymer surface (Hadad et al., 2005). In the case of polymers such as PP, a continuous chain of repetitive methylene units makes it resistant to degradation. The hydrophobic nature of PP hinders the attachment of microorganism on its surface.

Physical treatments which include UV, thermal and chemical lead to oxidation of the polymer surface. This in turn leads to the formation of carbonyl, carboxyl and ester functional groups (Iring and Tudos, 1990, Graeme and Mathew, 2000) and also decreases the hydrophobicity of the surface (Sudhakar et al., 2008) that ultimately helps in the formation of microbial biofilm on its surface (Gilan et al., 2004). Thus treatments leading to oxidation of the polymer can be effectively used as a pretreatment strategy before subjecting it to biodegradation. It is reported that there is a synergistic effect between photo-oxidation and biodegradation of polyethylene (Albertsson et al., 1987). An increase in the biodegradation of polyethylene was observed with increase in the time of exposure to UV (Hadad et al., 2005). Reports on biodegradation of PP are very scarce, although considerable literature addresses the biodegradation of low and high density polyethylenes (Arutchelvi et al., 2008). Fungal species (Aspergillus niger) and microbial communities such as Pseudomonas and Vibrio species have been reported to biodegrade PP (Cacciari et al., 1993). Isotactic polypropylene exposed to bacterial consortia for 175 days had 40% methylene chloride extractable compounds, and this extract was identified to be a mixture of hydrocarbons (between C10H22 and C31H64) (Cacciari et al., 1993). Thirty to sixty percent growth of A. niger was observed on gamma irradiated PP films at the end of 6 weeks, which indicated that the fungus was able to utilise this polymer as its sole carbon source (Alariqi et al., 2006).

In this paper, the in vitro biodegradation of unpretreated (PP-UT) and thermally pretreated polypropylene (PP-TT) films by mixed soil culture from a local plastic dumping site is reported. Here the polymer is supplied to the organism as the sole carbon source. The changes in its physical, chemical and mechanical properties at different time intervals were monitored. A pure culture, isolated at the end of 12-months of experiment was characterised using biochemical tests and the nucleotide sequence of the conserved region was determined from 16S rRNA.

Section snippets

Polypropylene

Commercial PP films (Reliance Industries Ltd., Mumbai, India) of size 8 × 2.5 cm and 0.05 mm thickness were used for the present experiments. The films were thermally pretreated at 80 °C for 10 days in a hot air oven (Sigma instruments, Chennai, India) (Achimsky et al., 1997). Unpretreated films of the same size and thickness were subjected to similar study to determine the effect of thermal pretreatment on the process. These polypropylene films had additives such as Irganox 1010, Irgafos 126,

Biodegradation of PP subjected to mixed culture

The formation of a biofilm on the polymer surface is the first crucial step prior to the onset of biodegradation. Since the biodegradation proceeds at a slow rate, the biofilm on the surface should remain active having live microorganisms for a long period of time. After 12 months of treatment with mixed culture, both live and dead organisms were observed on the PP surface as shown in Fig. 1. More live organisms were observed on PP-TT surface when compared to that on the PP-UT surface.

The

Acknowledgements

The author thanks Mr. Tej Pandit (Polymer Research Center, Reliance Industry, Mumbai) for DSC analysis and Miss. Shridevi (Sophisticated Analytical Instruments Facility, IIT Madras, Chennai) for FTIR analysis.

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