Efficient cleaning methods that can remove contaminants from water and air are needed to sustain life on earth. Therefore, carbon materials have long been used for deodorizing, separating and eliminating hazardous anion impurities by adsorption.
The acid-adsorbed layer enhances the nano-confinement of nitrate anion impurities in single-walled carbon nanotubes (SWCNT) due to the strong confinement by the pore and the strong interaction between the layer and the anion. When the nitrate ions are adsorbed, the hydroxide ions are desorbed from the nanospace. Thus, the aqueous solution exhibits alkaline properties. Credit: Takahiro Ohkubo from the Okayama University Department of Chemistry, Graduate School of Natural Science and Technology, Okayama University.
Until now, the full mechanism underlying the purification of water by carbon has remained a mystery. In addition, it is not determined whether the aqueous solution adsorbed on the carbon material is alkaline, acidic or neutral.
To fill these gaps, scientists led by Dr. Takahiro Ohkubo, Associate Professor at the Department of Chemistry, Faculty of Natural Science and Technology, Okayama University, Japan, the central mechanism by which carbon nanopores adsorb anions.
The scientists mention how they used Raman spectroscopy tools to analyze the adsorption of nitrate ions through the cylindrical pore of single-walled carbon nanotubes (SWCNT).
Details of the research were first made available online on September 16th2022, and then published in the Journal of Colloid and Interface Sciences (Volume 629 Part B).
dr Ohkubo and his contemporaries successfully deciphered the mechanism of acid layer formation near the pore walls. Thus, when an aqueous solution including ions enters the carbon material, even if the aqueous solution is neutral, an acidic aqueous layer including protons is generated that keeps a stable state.
To date, there have been no reports showing the existence of acidic adsorption layers formed within nanotubes made of carbon materials.
dr Takahiro Ohkubo, Associate Professor, Department of Chemistry, Faculty of Science and Technology, Okayama University
dr Nobuyuki Takeyasu, part of the research team and associate professor at the same faculty at Okayama University, discovered that the acidic layer allows excellent adsorption of the negatively charged nitrate anion impurities, with the amount of nitrate ions adsorbed being much larger than that of the cations or the positive ones loaded groups.
Furthermore, hydroxide ions are generated as counter ions. The anions in the bulk solution are exchanged with the hydroxide ions in the SWCNT, making the aqueous solution alkaline.
Researchers analyzed anion adsorption using a variety of alkali metal nitrates, including rubidium nitrate, lithium nitrate, sodium nitrate, and cesium nitrate solutions. They learned that more nitrate ions are adsorbed compared to metal ions. The amount of proton adsorption was almost the same regardless of the type of alkali metal ion used.
The acidic layer in the pore can strongly adsorb the nitrate anion species due to both the strong confinement by the pore and the strong interaction between the layer and the anion.
dr Takahiro Ohkubo, Associate Professor, Department of Chemistry, Faculty of Science and Technology, Okayama University
The discoveries are critical steps toward the design and development of carbon nanotubes that are ideal for ion adsorption and decontamination of air and water. The purification mechanism formulated in this study is a unique model that elucidates the hitherto unexplained alkalinity of the aqueous dissolution medium.
The scientists note that the results of their study strongly suggest the need to neutralize water before use when carbon materials trap ionic impurities.
Another important contribution of this research is the illustration that the interface of nanomaterials is a unique chemical reaction field that could guide further experiments. Based on the positive contributions, this study takes the understanding of the mechanism of anion adsorption by carbon to the next level and paves the way for unique carbon nanotubes as effective purifiers.
Magazine reference:
Ohkubo, T. et al. (2022) Acid layer-enhanced nano-encapsulation of anions in cylindrical pore of single-walled carbon nanotubes. Journal of Colloid and Interface Sciences. doi.org/10.1016/j.jcis.2022.09.070.
Source: http://www.okayama-u.ac.jp/index_e.html
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