(583c) Chemical Functionalizations of Graphitic Structures for Effective Water Purification and Desalination

“Provide access to clean water” is one of the 14 “Grand Challenges” identified by the US National Academy of Engineering. By 2050, 57% of the world’s population will live in areas that suffer water scarcity (UN-WWDR 2018).

The Sustainable Energy and Environment Group (SEEG) of the University of Mississippi (UM) discovered the synergisms of activating biochar (BC) by ultrasound that induces rupture and exfoliation of BC’s graphitic structure and mineral leaching leading to a significant increase in BC’s internal surface area and porosity, as well as the creation of new pores and opening of the blocked mesopores. Such treatment also results in reductive fixation of CO₂ in the form of induced carboxylic functional groups. The scientific background of the project is based on the work done by Stankovic et al. (Nature 2006;442(7100): 282-6) who produced, for the first time, stable aqueous dispersions of single layer polymer-coated graphitic nanoplatelets via an exfoliation/in-situ reduction of graphite oxide under ultrasound irradiation. Biochar has a similar graphitic structure in its backbone.

Our studies suggested that acoustic exfoliation of biochar integrated with chemical functionalization of biochar (in aqueous solution or gaseous plasma) can unlock an array of applications across a wide spectrum of fields. For example, the conversion of a -COOH (carboxyl) group to a -CONHR (amide) results in 184-200% increase in CO₂ adsorption. Our group demonstrated that the covalent attachment of phosphorus-containing structures (such as P=O or P=OOH) followed by hydrogen bonding with a modifying agent (diethanolamine or urea) that creates N functional groups onto carbonaceous materials can increase the metal ion adsorption capacity from 11% (in pristine BC) to 72%. In the same manner, the quantity of oxygen-centered and carbon-centered persistent free radicals (PFRs) on biochar (or any carbonaceous structure) can be increased by using structural modification followed by carbon-metal complexation. This results in an increase in the catalytic activity of biochar for the generation of hydroxyl radicals (â‹…OH) in advanced oxidation process (AOP) and degradation of organic contaminants; such as phenol (in-situ degradation: 17% in H₂O₂-raw BC system vs 80.3% in H₂O₂-modified BC system). Another example of our works in this area focuses on carbon magnetization followed by 3-(triethoxysilyl) propylamine (TES) functionalization (known as magnetic-carbon composites), which has shown 139% higher metal ion adsorption capacity compared to raw biochar. Magnetically separable composites are of high importance to remove radioactive material from contaminated water; as reported in Case Study: Fukushima Nuclear Accident. Our studies demonstrated that the synergism created by the combined effects of ultrasound and chemical functionalization resulted in a faster adsorption rate, and far higher metal retention capacity with no leaching of adsorbed metals during adsorption with long durations.

We are exploring the potential of employing nonthermal plasma for tackling water desalination challenges through structural modification (perforation) and functionalization of graphene membrane. Such graphene membrane with regular defects will have ability to reject or repel the Na⁺ and Cl^- ions and make a contribution to millions of people around the world. Efficient plasma functionalization can also open new routes in other fields such as in selective ion sieving, the process of kidney dialysis and reusable masks, which will be our future targets.

Key words: Biochar, Graphene, Ultrasound, Plasma, Functionalization, Water Treatment, Desalination