Doing metathesis reaction for the production of aromatic ionic liquids upon changing the counter ion(iodine) from the precursor, I am still getting tresses of iodine in my final product. What is the best and most economical way to remove iodine present as an impurity in my ionic liquid?
I don't know if best or most economical, but adding silver nitrate will precipitate silver iodide from aqueous solution.
I assume what you have is traces of iodide, not elemental iodine (or do you have both? in that case you should improve control over oxygen in your process).
The question is then, what other ion can you afford to replace iodide with, without creating a new contamination problem? Obviously the best would be the anion of the salt you have formed. If you can find a cuprous or silver (or lead) insoluble salt of the same ion in powder form, you can stir it with your ionic liquid and hope for an ion exchange on the surface of the powder; then you can filter it out. Caveats: Not only should the Cu/Ag/Pb silver salt be insoluble in the ionic liquid, but the same should be true of the CuI/AgI/PbI2 that forms by the exchange (likely but not guaranteed). Therefore you should be prepared to analyze for traces, not only of iodide but also of the metal ion that you may have added to the solution in the exchange process.
If you know fairly accurately how much iodide you have and you can afford just slightly less than quantitative removal, then you can add a "just slightly less than" stoichiometric amount of the salt of silver with the anion of your target salt. Give it time to react, and filter out the precipitate, and/or centrifugate it out (AgI is surely much heavier than your ionic liquid). Again, the assumption is that AgI is insoluble in your liquid. Otherwise, you can dilute it with water and do the precipitation and filtration in aqueous medium, then remove the water; but with ionic liquids this is often not fun.
If you really have iodine in your final product, perhaps the best way is washing the ionic liquid with chloform or carbon tetrachloride, both can easily dissolve iodine and can be extracted
Good luck.
Mohammad Kooti has a good suggestion (although I would substitute toluene for CCl4 or CHCl3 for a variety of reasons). It can be useful even if much of your iodine is in the form of iodide, if you can find a suitable oxidizing reagent with which to convert all iodide to iodine. For instance, hydrogen peroxide should work and the excess H2O2 can probably be destroyed with a platinum metal mesh (takes time). However, with H2O2 you'd be adding some water that has to be removed.
An ideal water-free oxidant would be one for which both the oxidized and the reduced forms are extractable by the same solvent you use to extract the iodine. Some organic peroxides may qualify, but I don't know enough to recommend one in particular.