Graphene-based nano-sponges are effective industrial wastewater filters

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

Key to the new filters is a class of materials called covalent organic frameworks (COFs). These structures are extremely porous, giving them a massive surface area contained in a small space, which means they are effective at capturing large amounts of molecules. Related materials known as organometallic frameworks (MOFs) are being explored for use in carbon capture, desalination, or extracting potable water from the air, and COFs could have a range of similar 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 modified the COF to selectively grab organic dye molecules. This involves giving the pores the right shape and size – between 0.8 and 1.6 nanometers – and giving the surface a negative charge, to attract 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 first fill with molecules, leaving those at the center empty and essentially useless. So the team developed a way to spread the COF by growing it on a sheet of graphene.

A diagram comparing regular bulk COF to the more efficient version grown on graphene

Changxia Li, Freddy Kleitz et al

The end result was a two-nanometer-thick COF layer over a single-atom layer of graphene, which increased the material’s maximum ability to hold organic dye molecules. Graphene itself has fairly large pores, allowing water to flow through quickly while the COF does its job.

“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,” the researchers said.

The technique should also be fairly inexpensive, according to the team. Little graphene needs to be used and the COF can be cleaned and reused.

The research was published in the journal Angewandte Chemie.

Source: University of Vienna

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