Add base such as NaOH or KOH in acidic solution till nertrilization. However, required amount of  base is depends on the initial pH and quantity of acidic mixture,

Take care acid-base reaction is highly exothermic, should do at RT or low temperature in fume hood and slowly add the base under continous stirring.

Eventhough, after adding base when pH reach at 3-4 you can use more DD water for dilution then easily filter the product.

thanks Rahul, my concern is as you mentioned the reaction b/w strong acid and strong base will be highly exothermic and will it effect already fictionalized CNTS with caboxylic acid group???? 

I would NOT recommend to add a strong base to a strong acid solution. The risk of accidents due to the strong exothermic acid-base reaction is too high. Moreover, the acid-base  reaction will generate a lot of salt which is difficult to remove from your functionalized CNT sample.

Instead, I would recommend to follow Menna's procedure which afford selectively oxidized carbon nanotubes (https://www.researchgate.net/publication/258000808_Selectivity_of_chemical_oxidation_attack_of_single-wall_carbon_nanotubes_in_solution ).

Briefly,

1) Prepare a large beaker containing a tenfold volume of deionized water with respect to the amount of acidic solution you are using. Put the beaker in a large basin filled with ice. This is aimed to cool down the beaker and its content.

2) carefully pour the acidic solution in the deionized water contained on the beaker (1-5 mL at a time). You should start from a blackish homogeneous strongly acidic solution of functionalized nanotubes and you will obtained a diluted acidic solution in which the functionalized nanotubes will be aggregated

3) proceed with suction filtration using a suitable filter (e.g., PTFE filters with 45 micrometers pore). It may require some hours and at the end of the filtration you will obtained a wet cake of CNTs still containing trace of acids.

4) wash the CNTs with deionized water while still on the filter and using again vacuum filtration. Repeat this step till the water passed through the CNTs have a neutral pH.

5) Eventually, you can dry the CNT cake with an IR lamp. Characterization by IR, Raman, TGA to assess qualitatively and quantitatively the efficiency of the functionalization process.

6) Check the pH of the filtrate and dispose it accordingly to waste regulation.

By the way, if your aim is to obtain CNTs with well defined oxidized ends, you might want to follow the oxidation reaction with an etching procedure as described in the paper I mention, for example. 

http://www.chimica.unipd.it/lab205/pubblica/

Article Selectivity of chemical oxidation attack of single-wall carb...

Thanks Patrizio Salice for detail procedure

Instead of filtering the solution using vacuum pump, is there any other methods?

let the functionalized CNTs settle and filter it slowly and then start by washing it with weak nitrating solution and subsquently water. after 5 - 6 washes check pH and then just one wash with a 0.01M NaOH followed by 2 - 3 water washes should work. whole process takes atleast a couple of hours but its safe and does the job

Ist, pour the functionalized CNT solution in 5000ml beaker.Then, add DI water upto the mark and leave the solution  undisturbed for several hours for sedimentation of well dispersed functionalised CNT nanoparticles.finally,remove above acid solution cautiously.

Repeat the same process by checking the PH of the decanted liquid till it becomes neutral.

hi..

do you have detail mechanism on this functionalization reaction?

tq in advance