You could try to add to your suspension a solvent where oxidized cnt are not soluble, mix or sonicating everything and then centrifuge at 12000. I know it works for other dispersed nanoparticles but i don't have idea about cnt. I usually centrifuge them at 15000 rpm for washing.

What is the pH of the suspension?. Try to change the pH close to the PZC of the mix.

Very good job on the oxidation process there. The solubility of the oxidized MWCNTs relies on the formation of carboxylic acid groups which have an estimated pKs value of 4.7. So at neutral pH values most of them will have deprotonated and hence establish alot of negativly charged carboxylates on the carbon scaffolds. These carboxylates are soluble in the polar medium as long as they are charged or something else is more attractive for the medium to solvate - or the medium is getting less polar. Hence you could try the following to precipitate the oxidized tubes:

1.) acidify the solution to very low pH values using non oxidizing but strong acids (like HI, HBF4, etc.) - maybe you can protonate enough carboxylates to make the tubes precipitate

2.) Add another solute to the solution, e.g. use the "Aussaltz"-Effekt - try with NaCl for example and see if it works

3.) Pour the whole thing into a less polar but water miscible solvent like ethanol, isopropanol, tetrahydrofuran etc. until they precipitate. Beware though that most of these organic solvents will be flammable at a certain volume ratio even though water is present and centrifugation of flammable solvents in maybe unstable vessels is not a very clever idea ...

If nothing helps - check the "solution" for tubes by Raman & AFM or very rapidly by optical Microscopy (see attached paper) - maybe you produced something closer to carbon nanodots/CNPs that you wanted and oxidation worked too well ...

Let us know if you succeeded!

Best Regards,

JME

Article Optical Visualization of Carbon Nanotubes-a Unifying Linkage...

Good suggestion by Englert.... simple 'salt' test will work.

Another possibility is, perhaps you have unzipped MWCNTs into graphene nano-ribbons..

Thanks Dr. Carlos Enrique Daza:

pH of my solution is 3.2. I reduce conc. of CNTs taken for washing; but it is still stabilizing. It took a quite lot of time to filter it out...

Die you already find the time to add a good portion of brine?

No success in salting-out?

No Dr. Jan M Englert. I want to pertain the oxidized form of CNTs as such but sediment them . So I would not like those CNTs to react with some solutions so as to form a salt....

Oftentimes when we had suspensions of O-MWCNTs that would not centrifuge down while cleaning and continued to autosuspend we had to take an alternate approach. We would pipette the CNTs into a petri dish over low heat (~80C) and evaporate the water off slowly. Once we removed the water, and the CNTs were gently stuck to the bottom of the dish, we would add fresh DI water to rinse the CNTs (as this will pull off any loose ions from your various oxidation and rinsing steps). Then we pipetted off this new aliquot of water, allowed the CNTs to dry down again, then added fresh DI and repeat. We did this until our DI water was no longer pulling off a considerable amount of ions, checking this by measuring the resistivity of the aliquot.

I would also suggest reading the experimental section of my colleagues' paper on O-MWCNTs (Billy Smith et. al. Langmuir), he goes into detail about the washing and rinsing steps taken to clean our oxidized CNTs. I do not remember if the above mentioned petri dish part was included in the experimental details. I hope this helps!

Dear Anas,

Adding brine (saturated NaCl solution in water) to your solution will not induce and reaction to your oxidized nanotubes. It will just lead to a preferential solubilization of the Na and Cl ions in contrast to the polycarboxylates (which are accompanied by sodium ions anyway if you neutralized with NaOH beforehand) which will eventually induce their precipitation. If it is working with a small aliquot you can still try to scale up.

I am 100% sure it will not induce any chemical reaction.

Best,

JME

Thank U Dr. Jan. It really worked. But I would like to know the exact chemistry of that sedimentation, since I had not done neutralization with NaOH before.

Thanks Dr. Julie Bitter for your suggestion. Can U please provide me the literature of O-MWCNTs (Billy Smith et. al. Langmuir) mentioned above.

Dear Anas,

Thank you for your question.I have the same problem. Did you apply the suggestion of Dr. Englert?  Could you please tell me the ratio of NaCl solution to CNT solution that it worked ? 

Maryam Anjidani: I added saturated salt solution to CNT dispersed one

Thank you, it's work.

Hello Larry Kwesi Sarpong,

The sedimentation will start once we add saturated salt solution. So centrifuging is not required at all....

Helpfull text

http://centrifuges.hitachi-koki.com/application/pdf/red150e.pdf

Very interesting discussion on the solubility of MWCNT. I have similar problem to Anas. After oxidative treatment I want to fix certain functional group for metal extraction. 

So do you asuggest that I add plenty of salt to precipitate the treated tubes? 

Thanks in advance 

Thabk for your valuable suggestions dr. Jan M Englert. But can u suggest me the quantity brine solution? I have 1 ltr of cnt solution dissolved in water.

Thank u

Dear Anas and Maryam

It didnt work in my case... what could be the reason?

Dear Anas N S

I'm also trying to functionalize MWNTs by H2SO4/HNO3 mixture (3:1 v/v) now.

Could you please suggest me a suitable volume of the acid mixture to treat a given amount of MWNTs? And how long did you treat the MWNTs?

In my  experiment, I used 240ml of H2SO4/HNO3 (3:1 v/v) to treat 3 g of MWNTs. The experiments lasted for 8 hours in 80oC, but it seems that the MWNTs were not oxidized so well

Dear Bui Tan, 

The oxidation of CNTs was carried out by treating 1 wt% CNT in  H2SO4/HNO3 mixture (3:1) by volume and stirred at 100-140 C for 8 h. The oxidation can be confirmed by the stabilization of CNTs in the solution and there won't be any sedimentation of CNTs at the bottom.

Anas

Dear Salah Selim,

I've oxidized MWCNTs in Conc.H2SO4 and HNO3 in the ratio 3:1 by volume (75 ml:25 ml)

Anas

Jan M Englert Thanks for your suggestion. But now CNTs have salt (NaCl) impurities. How to remove that salt from CNTs after sedimentation.