Wastewater often contains a cocktail of organic pollutants, ranging from pesticides to pharmaceutical residues. These are difficult to remove using conventional purification methods.

However, a recent doctoral thesis presents a creative method to get rid of them.

“We can break down the harmful chemicals in the water using sunlight and small droplets of oil,” explained Zygimantas Gricius from the Norwegian University of Science and Technology (NTNU).

“Gricius and his colleagues have studied the purification of industrial wastewater. They looked at the breakdown of naphthenic acids, which can occur in wastewater from the petrochemical industry, chemical production and the textile industry,” said Professor Gisle Øye at the Department of Chemical Engineering, who was Gricius’ academic supervisor.

The small droplets of oil are called photocatalytic Pickering emulsions. An emulsion is a mixture of two liquids that are not completely soluble in each other. We are not going to delve deeper into the physics of emulsions here, but the droplets act as tiny chemical reactors that can break down organic pollution.

“The oil droplets are stabilized by nanoparticles that are activated by light, for example from the sun,” explained Gricius.

One nanoparticle the researchers tested was titanium dioxide (TiO₂). The nanoparticles position themselves between the oil and the water where they capture sunlight and initiate reactions that break down the pollutants.

The research focused on fine-tuning these mixtures so that they became more effective, stable and reusable. Gricius identified several useful factors.

• By combining the droplets with TiO₂, the researchers obtained droplets that could be used multiple times without losing their effectiveness.
• Polymers are large molecules composed of repeating structural units. Coatings of polymers, such as poloxamers, helped the droplets withstand changes in water composition. However, these coatings can slightly reduce purification efficiency.
• Silanes are a group of chemical compounds. Surface treatment with silanes provided better control over how the droplets form and interact with the pollutants.
• Adding gold to titanium dioxide improved light capture and efficiency. This was the case under both UV and normal daylight, where different light-harvesting mechanisms come into play. This process also produced more stable droplets.

The thesis not only provides new insights into photocatalytic emulsions, it also demonstrates how we can think innovatively about sustainable water purification.

“By combining sunlight, smart materials and chemical additives, we have found an inexpensive, reusable and scalable method to purify contaminated water,” said Gricius.

The technology is still in an early stage, so no direct contact with industry has yet been established. There are currently no commercial products based on Pickering emulsion technology. This is partly because the field of study has only recently experienced a renaissance and been recognized as industrially promising.

The project was a collaboration between the Ugelstad Laboratory, the Catalysis Group and the Particle Technology Centre at NTNU’s Department of Chemical Engineering.

Gricius worked with students Cippora Magagnin, Adriana Mina and Laurine Mroz. Magnus Rønning also contributed as project supervisor.