![The acidic adsorbed layer enhances the nanoconfinement of nitrate anion impurities in single-walled carbon nanotubes (SWCNTs) due to the strong confinement by the pore and the strong interaction between the layer and the anion. When nitrate ions are adsorbed, hydroxide ions are desorbed from the nanospace. Thus, the aqueous solution exhibits alkaline properties. Credit: Takahiro Ohkubo of Okayama University Chemistry Department, Graduate School of Natural Science and Technology, Okayama University The acidic layer in single-walled carbon nanotubes facilitates containment of anionic impurities](https://oponame.com/wp-content/uploads/2022/10/The-acidic-layer-in-single-walled-carbon-nanotubes-facilitates-containment-of.jpg)
The acidic adsorbed layer enhances the nanoconfinement of nitrate anion impurities in single-walled carbon nanotubes (SWCNTs) due to the strong confinement by the pore and the strong interaction between the layer and the anion. When nitrate ions are adsorbed, hydroxide ions are desorbed from the nanospace. Thus, the aqueous solution exhibits alkaline properties. Credit: Takahiro Ohkubo of Okayama University Chemistry Department, Graduate School of Natural Science and Technology, Okayama University
Effective purification processes that separate impurities from air and water are necessary to sustain life on Earth. To this end, carbonaceous materials have long been used to deodorize, separate and remove harmful anionic impurities by adsorption. Until now, the detailed mechanism by which carbon purifies water has remained a mystery. Moreover, it is not known whether the aqueous solution adsorbed on the carbonaceous material is acidic, alkaline or neutral.
To fill these gaps, researchers led by Dr. Takahiro Ohkubo, Associate Professor in the Department of Chemistry, Faculty of Natural Sciences and Technology, University of Okayama, Japan, investigated the fundamental mechanism by which anions are adsorbed by carbon nanopores.
In a recent article posted online on September 16, 2022 and published in the Journal of Colloid and Interface Sciencethe researchers report using Raman spectroscopic tools to examine the adsorption of nitrate ions by the cylindrical pore of single-walled carbon nanotubes (SWCNTs).
Dr. Ohkubo and his colleagues succeeded in deciphering the mechanism of formation of the acid layer near the walls of the pores. It turns out that when an aqueous solution containing ions penetrates the carbonaceous material, even if the aqueous solution is neutral, an acidic aqueous layer containing protons is formed which maintains a stable state. Commenting on the novelty and fundamental nature of their work, Dr Ohkubo says: “To date, no reports have demonstrated the existence of acid adsorption layers formed in nanotubes of carbonaceous materials.
The research team, which also included Dr. Nobuyuki Takeyasu, an associate professor in the same faculty at Okayama University, found that the acid layer facilitates efficient adsorption of negatively charged nitrate anion impurities, where the amount adsorbed of nitrate ions is much larger than that of cations or positively charged groups. Additionally, hydroxide ions are generated as counter ions. The anions present in the bulk solution are exchanged with the hydroxide ions in the SWCNT, making the aqueous solution alkaline.
The team examined anion adsorption using several alkali metal nitrates, including solutions of lithium nitrate, sodium nitrate, rubidium nitrate and cesium nitrate. They discovered that more nitrate ions are adsorbed than metal ions. The amount of proton adsorption was almost the same regardless of the type of alkali metal ion used. Dr Ohkubo says: “The acidic layer in the pore can strongly adsorb nitrate anion species due to both the strong confinement by the pore and the strong interaction between the layer and the anion.
The results are indeed important steps towards the design and development of carbon nanotubes suitable for ion adsorption and water and air purification. The purification mechanism presented in this research is a new model that explains the alkalinity of aqueous solution medium, which has been a mystery until now. The researchers observe that the results of their study strongly emphasize the need to neutralize the water before use when ionic impurities are trapped by the carbonaceous materials.
Another remarkable contribution of this study is the demonstration that the interface of nanomaterials is a new field of chemical reaction, which could guide other experiments. Taken together, this work takes our understanding of the mechanism of anion adsorption by carbon to the next level, paving the way for new carbon nanotubes as efficient purifiers.
The subtle role of surfaces in the adhesiveness of ions
Takahiro Ohkubo et al, Nanoconfinement of acid-enhanced anions in a cylindrical pore of single-walled carbon nanotube, Journal of Colloid and Interface Science (2022). DOI: 10.1016/j.jcis.2022.09.070
Provided by Okayama University
Quote: Acidic Layer in Single-Walled Carbon Nanotubes Facilitates Containment of Anionic Impurities (October 20, 2022) Retrieved October 21, 2022 from https://phys.org/news/2022-10-acidic-layer-single-walled -carbon-nanotubes.html
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