Daily disposal of mountains of used plastic bottles by cremation or burial has negative environmental repercussions (Getty Images)

We dispose of mountains of used plastic bottles daily using methods of landfill or incineration, which can have negative environmental repercussions. But a British research team claims to have solved the problem using microbes.

In the study, published in the latest issue of the journal ACS Central Science, researchers from the University of Edinburgh's School of Biological Sciences reported successfully genetically engineering the gut coliform bacteria to efficiently convert polyethylene terephthalate (PET) waste into adipic acid, which in turn is used to make medicines, perfumes and nylon.

Polyethylene terephthalate is a thermoplastic used in all modern chemical industries, and the global demand for this material exceeds about 30 million tons per year, and more than 80% of it is designed for single use, which leads to 25 million tons per year of waste, and contributes to a global waste crisis, the researchers say in the introduction to their study.

Although there are well-established chemical and mechanical methods for recycling such waste, these methods are not environmentally friendly enough. In contrast, biorecycling techniques, based on enzymatic processes and microorganisms that address the problem, are still less entrenched because of obstacles the researchers claim to have overcome during the study.

Using bacteria to convert plastic to adipic acid brings environmental and economic benefits (Shutterstock)

Recycling with environmental
As a study published in the journal Polymers in June last year explains, the mechanical recycling of polyethylene terephthalate waste relies on collecting it, cleaning it, cutting it into small pieces or chips, and then melting the shredded chips and thermally extruded to form pellets or other shapes that can be used to manufacture new products.

Thermal extrusion is a manufacturing process that involves heating a material, usually a polymer or metal, to a soft or molten state, and then pushed through a mold or nozzle to create shapes.

As for chemical recycling, it relies on three popular methods:

  • First, the breakdown of polyethylene terephthalate using glycol (a material with good thermal conductivity properties) to produce small particles of ethylene glycol and terephthalic acid, which can then be used to create a new plastic material.
  • Second: The use of methanol to decompose polyethylene terephthalate into its constituent monomers for reuse in the production of the material.
  • Third: Exposing waste polyethylene terephthalate to high temperatures in the absence of oxygen, which leads to its decomposition into smaller hydrocarbons, so that these hydrocarbons are used as raw materials for the production of new plastics or as fuel.

These methods seem better than burying and burning waste, but they still raise some environmental reservations, including that they are energy-consuming methods, and produce greenhouse gas emissions during the production process, which prompted researchers to turn to biorecycling methods that use enzymatic processes and microorganisms, which in turn overcome these environmental effects, but they do not give large production that is economically feasible, for reasons related to not finding the most efficient enzymes in analysis, and the decomposition process They are very slow.

The researchers of the new study succeeded in overcoming these obstacles to establish the feet of biodegradation methods as an environmentally friendly and economically viable option at the same time, by finding the appropriate enzymes that engineered the coliform bacteria to produce and use in the decomposition process, leading to the production of an important economic compound, adipic acid.

Beads containing genetically engineered coliform to convert plastic waste into a valuable compound (ACS Central Science)

From vanilla flavor to versatile acid
A report published on the official website of the American Chemical Society explains that the new study builds on an earlier effort by researchers at the University of Edinburgh and others, aimed at maximizing the economic value of the biorecycling process of polyethylene terephthalate.

Previously, researchers published a study announcing the design of a strain of Escherichia coli to transform the main ingredient in old polyethylene terephthalate bottles — terephthalic acid — into something tastier and more valuable: a vanilla flavor compound. Meanwhile, other researchers engineered microbes to metabolize terephthalic acid into a variety of small molecules, including short acids.

In the new study, researchers sought to expand the biosynthesis pathways of coliform to metabolize terephthalic acid into adipic acid.

Also known as dicarboxylic acid, this acid is a white crystalline powder with a slightly acidic taste and is used in many industries:

  • Textile industry: It is used as a raw material in the production of nylon, a type of synthetic polymer that is widely used in the manufacture of fibers, fabrics and other materials.
  • Pharmaceutical industry: Excipients are included in the composition of medicines, and "excipients" are inactive substances added to pharmaceutical formulations to aid in the manufacturing process, improve stability, or enhance the delivery of the active pharmaceutical ingredient.
  • Cosmetics: It can be found in cosmetics and personal care products, where it can be used in formulations such as creams, lotions and shampoos.
  • Food industry: It is used as a food additive to give a slightly pungent taste, and is used in some gelatinous sweets and gelling aids, as well as in some drinks and bakery products.
  • Automotive industry: It is used in the production of "polyurethane" foams, and other materials used in the interior design of cars.

This acid is synthesized in quantities of up to 2.6 million tons/year, using energy-intensive processes that release nitrous oxide into the atmosphere, which contributes significantly to rising global greenhouse gas levels, equivalent to 298 kilograms of carbon dioxide equivalent (a measure that allows the amount of greenhouse gases emitted to be compared to the ability of these emissions to cause global warming).

The most prominent advantages of coliform involvement are that it can be easily genetically modified to carry out biodegradation tasks (Shutterstock)

Two birds with one stone Therefore, the production of multi-use adipic acid from the waste of polyethylene terephthalate using the biorecycling method, based on the bacteria "E. coli", kills two birds with one stone, on the one
hand it rids the environment of the waste of that material, and on the other hand allows the production of acid in a biologically non-polluting way.

In their study, the researchers say that to access adipic acid from polyethylene terephthalate residues, two genetically engineered strains of bacteria that produce 8 important enzymes are used in the biodegradation process, where one strain produces enzymes that can convert terephthalic acid (the main ingredient in polyethylene terephthalate bottles) into compounds such as moconic acid, and then to convert moconic acid into adipic acid.

They also used a second, genetically engineered strain of Escherichia coli whose enzymes help produce hydrogen gas with the use of a type of catalyst called a palladium catalyst.

In laboratory experiments, the team found that in a short time the engineered bacteria converted up to 79% terephthalic acid into adipic acid.

Stephen Wallace, a professor at the Institute of Quantum Biology, Biochemistry and Biotechnology at the University of Edinburgh's School of Biological Sciences and the study's principal investigator, said: "Escherichia coli is an ideal microorganism for this action, as it can be genetically modified easily, enabling us to test new pathways for adipic acid production very quickly."

Wallace expects the technology, when widely applied, to use waste plastics directly in large-scale fermentation processes, where adipic acid will be isolated directly from the bioreactor.

He asserts that the production process takes place under moderate conditions and does not emit any greenhouse gases, compared to the current largely unsustainable manufacturing process of adipic acid, which uses fossil fuels and emits large amounts of greenhouse gases.

"We are now working closely with industrial partners to assess the commercial viability of this process over the coming years."

Source : Al Jazeera + Websites