BACTERIAL REMEDIATION OF POLYETHYLENE

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ABSTRACT


Improper disposal of commodity plastics e.g. polyethylene (PE) in the environment causes land pollution, soil infertility, is unsightly and poses dangers to plant and animal life. The current effort describes the bacteria mediated biodegradation of commodity plastics by Serratia marcescens marcescens without first subjecting the plastics to thermo-oxidative ageing. It further elaborates on the mechanism and breakdown that are involved in the biodegradation of PE. 90 ml of carbonless medium containing essential minerals and vitamins (minus carbon source) was measured into seven conical flasks and 2 g of powdered PE was poured into each flask. 5 ml of three overnight cultures of Serratia marcescens marcescens was measured into six flasks. The other flask served as the control. The samples were incubated at 30o C, 141 revolutions per minute (rpm) in a rotary shaker for four weeks to observe the degradation incidence. After 4 weeks, 5 ml of cell-free supernatant from Serratia marcescens marcescens culture was measured into a sterile conical flask to which 2 g of sterile PE was added. This was incubated at 30oC & 141 rpm in a shaker for four weeks. It was discovered that, the supernatants from Serratia marcescens marcescens degrade PE faster than the bacteria with a percent of degradation of 37.5 in a month. The SEM micrographs revealed that the biodegradation of polyethylene occurs due to the presence of voids and pits, which indicates the bacteria feeding on the PE.


CHAPTER ONE

1.0 INTRODUCTION


1.1 Background

The world has an abundance of land and resources [1]. However, due to human carelessness and negligence, we have contaminated the environment. Industrial activities generated from the past have resulted in polluted lands and landfills that are potentially harmful to the health and wellbeing of people [1]. There is, therefore, a need for improved methods of waste disposal that could limit the long term effects on human wellbeing.




Plastics are artificially made long chain ―polymeric molecules‖ and come from the Greek word ―Plastikos‖, indicating that they are able to form different shapes [2]. Nowadays, plastics are composed of organic and inorganic raw materials comprising Hydrogen, Carbon, Oxygen, Silicon, Nitrogen and chloride. They are often obtained from oil, coal and natural gas [3].

In most cases, plastics cannot be degraded easily by enzymes and microbes [4]. There are therefore environmental concerns and about the 30% of plastics that are used worldwide in the packaging of food, detergents, chemicals etc. occurs at an annual rate of 12% [5]. These include plastics such as; polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) among others are used in packaging products.


Plastics have been used increasingly due to their attractive combination of stability as well as their balance of thermal and mechanical properties [6]. However, the pile-up of non-biodegradable plastics has been a source of increasing environmental concerns [7]. Furthermore, since plastics are typically disposed of in landfills, there are serious concerns about the pile-up of plastic wastes that are non-biodegradable. There are also environmental concerns about the toxic fumes [8].

Polyethylene (PE) is a synthetic polymer that comprises repeating units of smaller molecules (monomers) of ethylene in a long chain. The annual production of synthetic polymers worldwide is approximately 140 million of tones with a utility rate of 12% [9]. Thus makes it difficult to develop an efficient way of disposing of it. The resulting landfills pose a serious threat to the environment [10].

However, PE can be degraded by chemical, thermal, photo and biodegradation. The two mechanisms that aid in the biodegradation of polyethylene are hydro-biodegradation and oxo-biodegardation. Hydro biodegradation is the reaction that involves the degradation of plastics by breaking down water molecules into protons and hydroxide anions. While Oxo-biodegradation involves the reaction of plastic with oxygen to molecular fragments and these smaller molecules are then biodegraded by microorganisms and converted into carbon dioxide, water and biomass [11].

Recent work [12, 13, and 14] has shown that some microbes such as rhodococcus spp, Penicillium simplicissimum, Brevibacillus and pseudomonas spp among others are able to biodegrade PE and use it as a source of carbon and energy.

The studies have revealed that microorganisms found it difficult to degrade PE with large molecular weight and vice versa. PE with small molecular weights are preferred in biodegradation since microorganisms easily degrade and use them as sources of carbon and energy a process called mineralization....

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