Granulated activated carbon oxidation, a comparison between wet-chemistry and plasma approaches in dry and wet forms

Abstract

Activated carbon oxidation was explored by comparing two approaches: wet-chemistry (employing HNO₃ or H₂O₂ solutions) and glidarc plasma exposure, working with different reactor configuration modes and gases. The oxidized samples were characterized by XPS, FTIR, N₂-physisorption, SEM, Boehm titrations and compared in terms of oxidation efficiency, created functionalities and morphological changes. Excellent oxidation is accomplished by exposing activated carbon in solid form directly to the plasma plume and the oxidized material shows characteristics similar to HNO₃ oxidized carbon: high oxidation degree with abundance of carboxylic acids. In contrast, if the carbon is exposed to plasma while being in water suspension, the oxidation degree is lower and the main created functions are phenols, with equivalent characteristics to H₂O₂ oxidized carbon. Additionally, it was demonstrated that the carbon's oxidation degree and type of oxygen functionalities seem little influenced by the type of gas used for plasma generation, though the presence/absence of water could enhance/diminish the nitrates or O–H containing species present in the oxidized carbons. Plasma oxidation has advantageously a lower impact on the carbon's textural properties than HNO₃ oxidation, while obtaining high oxidation efficiency, and the introduced oxygen functions seem stable for at least 6 weeks after plasma exposure.