"We’ve developed several methods to capture and break-down PFAS. Now we’re bringing the technologies together in one unified design. We’re the only team in Denmark working on this one-step solution to capture, concentrate and break-down PFAS in a single process."

These are the words of Associate Professor Zongsu Wei, who is heading the Water Engineering Innovation research group at the Department of Biological and Chemical Engineering at Aarhus University. Sixteen of the university's researchers are trying to develop an environmentally friendly, circular technology that, using residual biomass from the agricultural sector and sunlight, can capture and then break down PFAS in one consolidated system.

And it’s no easy task. PFAS is a group of fluorinated substances that have been used since the 1940s in a myriad of products, ranging from raincoats and building materials to furniture, fire extinguishers, solar panels, saucepans, packaging and paints.

The substances are extremely stable chemically, and therefore they accumulate continuously in humans, animals, and elsewhere in nature.

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Strong chemical bonds

Chemically, PFAS consists of alkyl compounds (carbon chains) in which hydrogen atoms have been replaced by fluorine atoms. The chemical bonds between the carbon and fluorine atoms are at the crux of the problem when it comes to breaking down PFAS, because they are some of the strongest bonds in chemistry.

The strong carbon-fluoride bonds mean that fluorinated substances accumulate in nature. The bonds are so strong that they can last for many years. Unfortunately, many of the substances have a number of harmful effects on humans and the environment, and this presents an environmental problem.

In Denmark, PFAS substances have been found in drinking water wells, in surface foam on the sea, in the soil at sites for fire-fighting drills, and in many places elsewhere, for example in organic eggs. It is not possible to remove PFAS from everything, but work is underway to remove PFAS from the groundwater in drinking water wells that have been contaminated with the substances.

Permanent, self-cleaning solutions

In 2022, the Danish Environmental Protection Agency set a threshold value of 2 nanograms per litre for groundwater for four selected, particularly problematic PFAS substances. This meant that all the wells in the municipality of Fanø failed to comply with the threshold values, so the municipality had to install a temporary filter.

Currently, the most common method to filter drinking water for PFAS is via an active carbon filter, an ion-exchange filter, or using a specially designed membrane. All of these possibilities filter PFAS from the water, but they do not destroy the PFAS. The filters are therefore all temporary, as they have to be sent for incineration to destroy the accumulated PFAS or end up in the landfills.

The solution AU’s Water Engineering Innovation research group is looking at is not temporary, but permanent. The aim is a filter that constantly captures and breaks down PFAS and then regenerates itself. The filter can be installed in drinking water wells or at treatment plants, for example.

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Catalytic converter and filter

The research team has proven the technology at lab scale. Their filter can capture PFAS and then break down the substances via a metallic photocatalytic converter consisting of titanium dioxide and a number of transition metals. The metal photocatalytic converters are fixed on a membrane filter, and the process starts when they are exposed to UV light.

"This excites the electrons in the metals. They jump up to a higher energy level. And when this happens, we suddenly generate chemical reactions that would otherwise not normally happen. We create what’s called free radicals. These are super-reactive ions that can attack the carbon-fluorine bonds that are otherwise so hard to break down. At the same time, the process creates reductive conditions that contribute to the overall degradation of PFAS," says Allyson Leigh Junker, a PhD student in the Water Engineering Innovation group, who is currently working on the photocatalytic part of the technology.

PhD student Charlotte Skjold Qvist Christensen is also part of the group, working on the filter itself. Today, activated carbon filters are often used, but Charlotte is developing a biochar filter.

"Biochar is an analogue for activated carbon, but it is produced from residual biomass, usually from agriculture. For example, it could be produced from straw. The biomass undergoes a thermochemical transformation in an oxygen-poor environment that transforms it into a kind of carbon powder. The process transforms the structure of the lignocellulose in the biomass, and makes it a kind of sustainable version of activated carbon because the carbon in the product is the same as the carbon previously captured by the plants from the atmosphere," says Charlotte.

Process optimisation

The important difference between activated carbon and biochar, however, is that activated carbon has a huge surface area, which gives the substance extremely good adsorption properties. Charlotte is working on increasing the surface area of the biochar to give the material the same favourable properties as activated carbon.

"We're up to approx. 600 square metres of surface area per gram, and that means we’re getting very close to activated carbon. The greater the area the better, because that means more PFAS can bind to the surface," she says.

PFAS binds to the surface via hydrophobic interactions and through electrostatic attraction. Charlotte aims to further improve binding by these particular mechanisms by modifying the surface chemistry of the biochar.

Together, the filter with fixed photocatalysts should be able to remove all the PFAS from the water flowing through it. At lab scale, the team have so far demonstrated that the filter can remove over 99 percent of PFAS and that photocatalysts can break down 53 percent.

"So now we're working to optimise the photocatalytic converter by finding out which metals work best and under what conditions," says Allyson Leigh Junker.

Zongsu Wei adds:

"Then we have to test the technology on treated wastewater. After that we will design a flow reactor and test the equipment at pilot scale. We hope it can all be integrated into full scale in three to four years," he says.

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Contact

The Water Engineering Innovation research group

Associate Professor Zongsu Wei
Aarhus University, Department of Biological and Chemical Engineering
Mail: zwei@bce.au.dk
Tel.: +4593522047