Nano-sponges with potential for rapid wastewater treatment

Effective adsorbents for industrial wastewater treatment are important to minimize potential damage to the environment. In particular, organic dyes, as a significant group of industrial pollutants, are usually well soluble in water, do not decompose, and many of them are toxic to carcinogenic. Changxia Li and Freddy Kleitz from the Department of Chemistry at the University of Vienna and colleagues presented a new approach to develop an innovative composite material consisting of a nanoporous ultrafine covalent organic framework (COF) anchored on graphene, which is highly effective in filtering organic pollutants from water. The study was published in Angewandte Chemie.

“Today, there are several ways, including activated carbon filters, to purify water, but there is still room to improve the efficiency or adsorptive capacity of the application,” says first author and doctoral student Changxia Li.

Freddy Kleitz’s group at the Institute of Inorganic Chemistry – Functional Materials is developing new nanoporous materials. Porous materials have a much larger total surface area than non-porous materials for the same volume and, therefore, can accumulate a particularly large number of molecules on surfaces during the adsorption process.

Highly porous COF as a new class of materials

Covalent organic frameworks (COFs) are a relatively new class of materials. They are particularly porous, at the same time low density and light. Covalent means that their chemical bonds are formed by electron pairs between atoms.

The dyes the researchers studied in their aqueous model solution ranged in size from 0.8 to 1.6 nanometers. “We have developed a new method to form COF in a relatively environmentally friendly way using water. Thus, we were able to develop small “sponges” with nanometer-scale pore sizes and shapes, and a tuned negative surface charge that highly selectively pulled out positively charged target molecules, ie. our dyes, from water,” the researchers said. “Just like a sponge soaks up water, only in our case it’s pollutants.”

The backbone is made of graphene

When using bulk COF powder, the internal pores of the material often become inaccessible to contaminants due to plugging of the pores at the outer edge, especially for large a pollutant molecules. The new composite material developed by the researchers offers a fully permeable structure: for this purpose, the researchers grew COFs on thin-layered graphene nanosheets. The combination of graphene, which itself is already a 2D layer of carbon atoms, and a COF layer up to two nanometers thick resulted in a compact, open 3D structure. An ultrathin COF layer can open more adsorption sites than a bulk COF powder.

On the other hand, the larger honeycomb-like pores of the graphene network support transport water through the filter material. “Thus, the large pores of the graphene network combined with an ultra-thin COF layer with a large number of adsorption sites provide particularly fast and efficient wastewater treatment,” the researchers said. Due to the relatively low material content of graphene as well as reusability composite materialAfter washing out pollutants as a filter, the development is also relatively cost-effective, they said.