Engineers at the University of Vienna have developed a new composite material that makes an efficient filter for removing organic pollutants from water. The system uses super-porous “nano-sponges” embedded on a sheet of graphene.

The key to the new filters is a class of material called covalent organic frameworks (COFs). These structures are extremely porous, giving them a massive surface area contained within a small space, which means they’re effective at grabbing onto large amounts of molecules. Related materials known as metal-organic frameworks (MOFs) are being investigated for use in carbon capture, desalination or pulling drinking water from thin air, and COFs could have a similar set of functions.

For the new study, the researchers focused on using a COF to remove organic dyes from water. These chemicals are a common pollutant of industrial wastewater, and can be toxic and carcinogenic – not to mention difficult to remove.

The team tweaked the COF to make it selectively grab hold of organic dye molecules. That involves making the pores the right shape and size – between 0.8 and 1.6 nanometers – and giving the surface a negative charge, to attract the positively charged dye molecules.

But there was another hurdle to overcome. When the material is used in its powdered form, the pores at the outer edges fill up with molecules first, leaving those in the center empty and essentially useless. So the team developed a way to spread out the COF by growing it on a sheet of graphene.

The end result was a two-nanometer-thick layer of COF on a single-atom layer of graphene, which increased the maximum capacity of the material for holding organic dye molecules. The graphene itself has fairly large pores, allowing the water to flow through quickly while the COF does its work.

"The large pores of the graphene network in combination with the ultra-thin COF layer and its large number of adsorption sites therefore enable particularly fast and efficient wastewater treatment," said the researchers.

The technique should also be fairly inexpensive, according to the team. Not much graphene needs to be used, and the COF can be cleaned out and reused.

The research was published in the journal Angewandte Chemie.