Researchers at RMIT have found a new approach to remove microplastics from water sources. Could this be the solution to the notoriously hard to capture pollutants?

Why are microplastics so widely talked about as one of the most persistent and pervasive forms of pollution? Well, the clue is in the name. Defined as any plastic with a diameter less than 5 mm, they can reach sizes into the nanometre scales (1 nanometre is equivalent to a billionth of a metre). It doesn’t come as much of a surprise, then, that these tiny particles are not only difficult to remove from the environment but often hard to even detect.

Microplastics have been found everywhere humans have looked for them; from deep oceans and arctic snow, to floating in the air above mountains. Like common plastic, they do not readily break down so can persist in the environment for hundreds and even thousands of years.

Microplastics come in two forms: primary and secondary. Primary microplastics are made for commercial use such as in cosmetics, whilst secondary microplastics are formed from the degradation of larger plastic waste as a result of environmental exposure. Both types tend to accumulate in water sources, with research undertaken in 2021 estimating there were 24.4 trillion pieces of microplastic in the world’s oceans.

Primary types are washed down the drain and slip through water treatment processes, whereas secondary microplastics are washed directly into rivers and oceans. This not only affects the marine ecosystem and particularly animals within it, but also human consumption as microplastics pollute our water sources and food. Some research suggests humans consume more than 100,000 microplastics a year.

An estimated 24.4 trillion pieces of microplastics pollute the world’s oceans.

Whilst the effects on human health are still largely unknown, research into the impacts on marine animals has shown disruption in reproductive systems, growth, and digestive systems. Other toxic pollutants in the water tend to accumulate on microplastics, meaning animals also ingest these. As plastic pollution continues to increase, one thing is certain: the concerns around microplastics will only be growing.

A key way in which we can minimise the impacts of microplastics on humans and the environment is by removing them during water treatment processes. Wastewater and drinking water treatment commonly uses filtration systems to remove contaminants, but these are often insufficient for the trapping of smaller particles like microplastics. Photodegradation processes can also be used but suffer from slower and less effective removal rates. Both of these processes are also energy intensive, making their sustainability a challenge.

So, what’s the answer? Must we continue releasing microplastics into the environment and consequently ingesting them ourselves? Perhaps not.

Recent research published by the Royal Melbourne Institute of Technology (RMIT) has found a novel and innovative approach to the removal of microplastics from contaminated water. It comes in the form of a powder adsorbent and is made using a type of material called a metal-organic framework (MOF).

MOFs are widely used across science and technology due to their unique and useful properties. The nanoscale structure is made up of repeating hollow cage-like shapes consisting of metal ions and organic (carbon containing) linker molecules. This forms a 1D, 2D or 3D framework where the holes between molecules act as pores that can adsorb and store target molecules.

The millions of holes gives these structures the largest internal surface area of any material. To put into context, 1 gram of MOF has nearly the equivalent surface area to a football pitch. The composition of MOFs can be tuned depending on the required use to selectively adsorb the specific target molecules. These unprecedented qualities have meant MOFs have found use in a range of important technologies such as gas storage and separation, purification processes and catalysis.

In the removal of microplastics, the MOF powder is mixed into the polluted water where it will attract, adsorb and store the plastic particles. The MOF adsorbent can be removed using a magnet due to the magnetic iron component in its structure, making separation easy and cheap. This also makes the process low energy and therefore more sustainable compared to alternatives.

“The nano-pillar structured material was designed to attract microplastics without creating any secondary pollutants or carbon footprints,” said co-lead researcher Dr Nasir Mahmood.

This method of microplastic removal is not only innovative in design, but also works extremely well. Test results showed 100% of microplastic was removed in a matter of hours compared to current technologies which can take days. After being washed and dried, the MOF powder can be reused multiple times to remove further microplastics.

“Our powder additive can remove microplastics that are 1,000 times smaller than those that are currently detectable by existing wastewater treatment plants”

Professor Nicky Eshtiaghi, co-lead researcher.

The researchers are working with water utilities across Australia for its potential commercialisation in wastewater management. This discovery offers a promising route to removing microplastics industrially, tackling both the current difficulties in removing smaller particles and the sustainability of the processes.

Original research paper:

M. Haris, M. W. Khan, A. Zavabeti, N. Mahmood and N. Eshtiaghi, Chem. Eng. J., 2023, 455, 140390.