[BryanC]

I am a chemical engineering first year student developing a student research project around the downstream processing of fermented succinic acid (SA), an industrial building block chemical. Downstream processing is responsible for 50-80% of the current overall cost of making this type of SA because of the low titer, requiring many by-products to be filtered out. Excellent yield and productivity rates have been derived from the usage of electrodialysis and ion exchange systems according to several experiments and patents. However, a few questions have surfaced in the process of understanding these two unit operations.

Question 1: Since an ion-exchanger relies on a stoichiometric exchange of ions between the ion exchange resin containing H+/OH-/salt ions and the aqueous solution containing the target ions, is it true that H+ or OH- ions will consistently be present in the final product?

Question 2: Since purification by electrodialysis relies on the movement of cations/anions through electrically charged membrane stacks, is it right to assume that impurities from the fermentation broth which are not ions, like remaining carbohydrates, amino acids and coenzymes cannot be separated or recycled back to the fermentor after using electrodialysis?
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Thank you

[chenbeier]

1. An ionexchanger has sometime to be regenerated to get a constant good quality of the product.
2. Also amino acids , coenzyms and others have some charges and will be influenced by an electrical field.

[Babcock_Hall]

When one ion binds to an exchanger, another is replaced.  I think a great deal depends on how you will elute the desired product.

[BryanC]

When one ion binds to an exchanger, another is replaced.  I think a great deal depends on how you will elute the desired product.

Could you explain the difference between elution and regeneration?

1. An ionexchanger has sometime to be regenerated to get a constant good quality of the product.
2. Also amino acids , coenzyms and others have some charges and will be influenced by an electrical field.

Thanks.

[Babcock_Hall]

@OP, Suppose I have Dowex-1 (a quaternary ammonium resin) in the acetate counterion form, and I bind an amino acid to it.  Now I elute the amino acid with 1 N HCl and collect it.  The HCl protonates the amino acid, which now has no attraction for the column.  The chloride ion binds to the column in its place.  This elution will convert the column into the chloride form.  Now I have to regenerate the column for the next reaction, meaning putting it back into the acetate form.  In this example, the regeneration would be done in two steps to save money:  convert the resin into the hydroxide form with a large excess of NaOH, then add a few column volumes of 1 N acetic acid. 

[BryanC]

@OP, Suppose I have Dowex-1 (a quaternary ammonium resin) in the acetate counterion form, and I bind an amino acid to it.  Now I elute the amino acid with 1 N HCl and collect it.  The HCl protonates the amino acid, which now has no attraction for the column.  The chloride ion binds to the column in its place.  This elution will convert the column into the chloride form.  Now I have to regenerate the column for the next reaction, meaning putting it back into the acetate form.  In this example, the regeneration would be done in two steps to save money:  convert the resin into the hydroxide form with a large excess of NaOH, then add a few column volumes of 1 N acetic acid.

Thanks for your replies Babcock_Hall.

Regarding what you have mentioned:

I took to understanding acetate counterion as acetate anions. Please enlighten me if I am wrong..

1) I understand that in ion exchange, the ions in the aqueous medium are directly "swapped" with the ions in the ion exchange material (in this case, the resin). Why then would we want acetate ions in our purified solution?

2) At the elution stage, we used HCL to elute the amino acid ions. I noticed that HCL is a strong acid -> Is the reason for using a strong acid because of it's 100% proton donation in the acid-base reaction and hence ensure that the amino acid will be completely protonated? Because my initial thought was to use acetic acid itself to elute, but then I realised that it is a weak acid.

[Babcock_Hall]

By counterion in this context I meant the ion that is bound to the resin.  I took an example from my own experience in purifying certain amino acids for synthetic purposes, which is one of many ways in which ion exchange is used, and not necessarily the easiest one to understand.  In this example, one reason to use HCl is that after one collects the amino acid, one can use rotary evaporation to remove excess HCl or HOAc and water.  The amino acid will be positively charged because of the ammonium group on the amino acid, and will remain behind as the chloride salt (we would say that chloride is the counterion of the amino acid in this context).  It is often the case when using ion exchange to purify something that one wants to separate the compound of interest from the chemical species that brought about elution.

It might be possible to use HOAc (acetic acid) instead because the large concentration present might be enough to protonate the amino acid by mass action.