Larvae are used to identify plastics in less time

At the Environmental Engineering Coordination of UNAM's Institute of Engineering (II), scientists discovered a new method based on the use of two species of coleopteran larvae of the Tenebrionidae family, to easily identify biodegradable and compostable plastics, in less time than conventional methods. The patent was applied for.

The goal is for manufacturers of bags made from these materials to have quick results, a maximum of three months, instead of a year and a half. "There is no test that confirms 100 percent biodegradability, and those that exist cannot be considered fast, since they take more than a year," explained the scientist in charge of the project, María Neftalí Rojas Valencia.

It is necessary to develop new methods that consider the criteria already established by the regulations, but that adds more evidence and reduces the time needed to determine whether a bioplastic is compostable or biodegradable.

These larvae, known as weevils, had already been studied, but the tests focused on unicel. After two years of research, it was proven that they give excellent results in plastics. "No one had reported working with bags or with the techniques and results we have obtained".

The fungus Pestalotiopsis sp was used -purchased abroad, which had also been used for unicel and with which they were able to degrade plastic bags. But not only that: in the research, they also discovered a new fungus, of a particular strain, which also feeds on this biodegradable material; this organism, which develops easily in the temperature and climate conditions of our country, has not yet been named, said Alberto López Juárez, a master's student.

When a bag is not biodegradable, but of hydrocarbon, the fungus does not cause any effect on it; but it "breaks" those that are biodegradable in a matter of days. "The problem is that after 11 days the life of the fungus begins to decay. I am working to make it last longer and complete the biodegradation process," said the young student.

Plastic invasion

Polyethylene or "plastic" bags appeared in the 1970s and immediately became the instrument most used by millions of people to transport all kinds of products. A decade later, they were already considered a problem, recalled Rojas Valencia.

It is a material that affects the environment and also health. Before the pandemic, the second phase of the ban on single-use plastics, including bags, went into effect in Mexico City. However, with the health emergency, their use was again permitted on a global scale, including the manufacture of special bags for corpses infected with the SARS-CoV-2 coronavirus.

Although there was a significant decrease in their use, especially in supermarkets, they did not disappear. In the world, said the scientist, there are 500 billion plastic bags in circulation; the minute they are used, one million of them are thrown away and only about one percent are sent for recycling. Their average time of use is 15 minutes and, in general, they are not usually given more than two uses.

We are also currently facing the problem of microplastics. There are studies that not only show their presence in places as far away as Antarctica but also in human placenta and blood, which is why they have become a threat to health, warned the expert.

In the search for solutions, bioplastics have been produced that are divided into two large groups: biopolymers, and biodegradable plastics of petrochemical origin, which contain cellulose or bacteria, but most of their components are hydrocarbons.

Biopolymers can be made of different materials such as polylactic acid, starch, cellulose, chitosan or proteins such as gelatin, collagen, serum, soy, or corn, or lipids such as triglycerides. However, one has to ask oneself what is available on the market, the university professor questioned.

Plastics are biodegradable because of their carbon and hydrogen composition, which makes them organic compounds; the problem is that they decompose very slowly, some in more than 150 years. What has been done to prevent them from lasting so long is to add an additive; this component 'helps' the bag to degrade, but also to break into fractions; this has caused the increase of microplastics.

Degradation is a gradual process and can be the result of simultaneous phenomena. Its speed depends on the material and its environment, and there is no universal scale to evaluate it. The Mexican standard says that the bags must be "biodegradable or compostable", but it is not so easy to prove that they are. The main difference between the two is that the former has an additive and the latter a resin. To certify them, the former must demonstrate their ability to decompose in the natural environment, speed of disintegration, eco-toxicity, and heavy metal content.

Compostables are made from corn, potato, wheat, or rice starch; from wood or cotton cellulose, cereals or vegetable waste; orange waste, shrimp peel, etcetera. To obtain the corresponding certificate, it must demonstrate that it can degrade in a composting plant, with certain humidity, temperature, and a number of organisms. However, it is common for bags to circulate, especially in markets on wheels, that have the "compostable" and "biodegradable" seal, without being so.

To determine which comply with the characteristics of each category and help manufacturers obtain their certification seals, the II tests the plastics based on four national and international biotoxicity standards to determine the presence of metals such as arsenic, cadmium, chromium, cobalt, copper, fluorine, lead, etc., which are contained in the dyes used in the advertising printed on the bags.

Likewise, aerobic degradation is analyzed in landfills and controlled composting plants; and anaerobic degradation, such as that which occurs in sanitary landfills.

Some standards require the cultivation of seeds to demonstrate that the bag does not have any toxic component, in this case, green tomato, grass, or barley; in theory, if any of these elements are present, the seeds should not germinate; however, the scientist has proven that all seeds germinate in any type of bag. Therefore, the scientist does not consider this to be a reliable test.

Earthworms are also used; however, these annelids do not ingest the plastic as such but must be fed with microplastics mixed with organic matter. On the other hand, weevils do eat it directly and their feces, similar to earthworm humus, can be used for compost or bird food and marketed.

Besides carrying out fieldwork in Acapulco and Isla Mujeres, the II laboratory simulates marine conditions to determine how the different types of bags biodegrade, or not, in the seas and oceans. The results will be obtained in the medium term.

The possibility of obtaining oil from the bags, or using them in co-processing, is being investigated because there are plants that require this material; thus, for example, they would be taken to be burned by cement factories so that they can develop their production process. "That would be a great solution: to create collection centers for bags that would be taken to be melted at the co-processing plants, without generating waste," concluded Rojas Valencia.