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Home » Featured Research » Recent Research Topic from Inorganic Chemistry Laboratory: Activated carbons hate to remove metal ions in water?

Recent Research Topic from Inorganic Chemistry Laboratory: Activated carbons hate to remove metal ions in water?

Posted on : September 30, 2019

 

Various kinds of materials for adsorption and separation are being developed for the purpose of removing unpleasant odor components in the air and harmful substances in water. Activated carbon (AC) is one of the materials that is frequently used in various purposes such as water purifiers and air conditioner filters. AC has a myriad of pores whose size is comparable to molecules and ions so that we can remove and confine the desired substance into these pores. In fact, when added activated carbon in an aqueous solution in which indigo carmine (a substance made from indigo, which is a blue dye of jeans) can be adsorbed on a AC to obtain transparent water (Figure 1).


 

Fig. 1 Indigo carmine aqueous solution (left) and indigo carmine aqueous solution with activated carbon (right). 

Here is a question: what happens when you put a porous carbon material in an aqueous solution dissolved cation and anion such as sodium chloride (NaCl) aqueous solution? You can imagine that NaCl adsorbs like indigo carmine to reduce salinity in the water, but our experiments have shown that something a little more complicated happens. 

 

In our study, we used single-walled carbon nanotubes (hereinafter referred to as nanotube) that clearly show the size of pores rather than AC. It has a very narrow space with a tube diameter of about 1 to 2 nm. On the other hand, for the convenience of the experiment, we used rubidium bromide (RbBr) instead of NaCl. NaCl and RbBr are similar in terms of alkali metal halides, and the size of ions when dissolved in water is different. 

 

Now, even if the end of the nanotube is closed and dispersed in an aqueous solution, ions are not adsorbed. However, the ions can be adsorbed into the pores of open-ended nanotube. At that time, it was found that 10 to 100 times more Br- adsorbed than Rb+ in water (Figure 2). It is very strange that such a phenomenon occurs because the charge balance cannot be balanced if the cation and the anion are not adsorbed as much as the same number. Furthermore, since the surface of the nanotube is rich in electrons (called p electrons) used to form bonds between carbon atoms, Br- repels the surface of the nanotube.


Fig. 2 Conceptual diagram showing abnormal adsorption phenomena of anion species.   

  

When we investigated the cause of such abnormal adsorption behavior, we found that H+ (proton) in the water were actually adsorbed to the nanotubes. The water is ionizing some of it to H+ and OH- but such slight amount of H+ adsorbs instead of Rb+. In fact, when measuring the pH before and after dispersing the nanotube in RbBr aqueous solution, it has increased from around 6 to near 10 showing that the concentration of H+ in the solution decreased.  The pH of the aqueous solution can be controlled by activated carbon if we can apply the phenomena. AC can be easily recovered later by filtration of an aqueous solution, so it may be used as an unprecedented pH adjuster.

 

It has not been discussed at all that such water-derived protons are greatly related to the adsorption of ions from aqueous solutions. We are continuing our research to elucidate various mysteries related to this phenomenon, including why protons are adsorbed on nanotubes instead of cations in the water.

 

References

“Surplus adsorption of bromide ion into p-conjugated carbon nanospaces assisted by proton coadsorption” M. Nishi, T. Ohkubo, M. Yamasaki, H. Takagi, and Y. Kuroda, J. Colloid Interface Sci., 508, 415-418 (2017).

“ Experimental information on the adsorbed phase of water formed in the inner pore of single-walled carbon nanotube itself” M. Nishi, T. Ohkubo, K. Urita, I. Moriguchi, and Y. Kuroda, Langmuir, 32, 1058-1064 (2016). 

 

 

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