Research: Graphene-based "nanosponge" can be an efficient filter for industrial wastewater

Engineers at the University of Vienna have developed a new composite material that can effectively filter out organic pollutants from water. The system uses ultra-porous "nanosponges" embedded in graphene sheets.

The key to the new filter is a class of materials called covalent organic frameworks (COFs). These structures are extremely porous, giving them a huge surface area in a small space, which means they can efficiently grab a large number of molecules. Related materials called metal-organic frameworks (MOFs) are being studied for use in carbon capture, desalination or extracting drinking water from thin air, and COFs could have a similar range of functions.

In the new effort, the researchers focused on using COFs to remove organic dyes from water. These chemicals are common contaminants of industrial wastewater and can be toxic and carcinogenic—not to mention difficult to remove.

The team tuned the COFs so that they selectively grab onto organic dye molecules. This involved making the shape and size of the pores just right — between 0.8 and 1.6 nanometers — and making their surfaces negatively charged to attract the positively charged dye molecules.

But there was still a hurdle to overcome. When the material is used in powdered form, the pores at the outer edges are filled with molecules first, leaving the pores in the middle empty and essentially useless. So the team developed a way to disperse COFs by growing them on graphene sheets.

The end result is a two-nanometer-thick layer of COFs on top of a single-atom layer of graphene, which increases the material's maximum capacity to hold organic dye molecules. Graphene itself has fairly large pores, allowing water to flow quickly through while the COFs do their work.

"Thus, the macropores of the graphene network in combination with the ultrathin COFs layer and its high number of adsorption sites enable particularly fast and efficient wastewater treatment," the researchers said.

The team says the technique should also be fairly cheap. Not much graphene needs to be used, and the COFs can be cleaned out and reused.

The research was published in the journal Angewandte Chemie.