Interactions of graphene-based nanomaterials with microwaves: Towards technology to regenerable nano-adsorbents

Inadequate access to clean water and increasing levels of pollution are among the most concerning global problems with expected increases in severity in coming decades. Therefore, transformative and ingenious solutions that can overcome the barriers presented by difficult-to-remove pollutants are needed in wastewater reuse or potable water treatment to protect the environment and address the clean water scarcity that are challenging for traditional approaches. In the past two decades, bottom-up fabricated carbon-based nanomaterials (e.g., carbon nanotubes, graphene nanosheets) have been shown to be superior and targeted adsorbents for organic pollutants (e.g., pharmaceuticals and personal care products, illicit drugs, perfluorinated chemicals, endocrine disrupting compounds, dyestuff, pesticides and herbicides) compared against traditional granular activated carbon produced from top-down methods starting from coal, peat, or other heterogeneous materials. Nanomaterial-based sorbents have higher selectivity towards adsorbates, greater adsorption capacity, and faster reaction kinetics. Despite the advantages of using nano-adsorbents in drinking water treatment, there remains a substantial cost barrier to applying them broadly. Nanomaterials are currently orders of magnitude more expensive to produce than the conventional carbon-based adsorbents, and may only be cost-effective if used multiple times before disposal. Therefore, regenerating and reusing carbon-based nano-adsorbents encompass an opportunity to help overcome the predominant cost limitation of applying carbon-based nano-adsorbents in drinking water treatment, enable their ubiquitous use, and sustainably improve current practice.

Recently, carbon-based nanoparticles were reported to generate extraordinary heating responses under microwave irradiation, which leads to pollutant oxidation or volatilization; however, the underlying microwave heating mechanisms are not fully understood. Materials with favorable dielectric properties such as silicon carbide, magnetic iron oxide, and other transition metal containing oxides can convert microwave energy into heat at common microwave frequency (i.e., 2.45 GHz). Nanomaterials exert more reactivity due to their exceptionally small sizes because there are more atoms/electrons on their surfaces that respond to the changes in the surrounding environment when compared to bulk materials. Carbon nanomaterials’ extraordinary ability to convert microwave energy into heat has the potential to shift the paradigm of carbon-additive enhanced microwave heating. Advancing in this research field can influence an array of industries including biomedicine (e.g., microwave-enabled imaging), food industry (e.g., food drying, pasteurization and other heat involved processing), telecommunication (e.g., microwave and radio wave communication), energy industry (e.g., efficient combustion of fuels), and environmental remediation (e.g., ex-situ soil remediation, spent carbon regeneration).