Ion Exchange Study

Alright, here are some of my study notes on Ion Exchange! I didn’t see many books on it since the 1970s, so I don’t know if this is an unpopular area of chem or just that we’ve had our book buying budget crash since then but I thought I’d post my notes in case anyone wants to read along.

Title: Ion-exchange : introduction to theory and practice / [by] R. W. Grimshaw and C. E. Harland. —
By: Harland, C. E.. Grimshaw, Rex W.
Published: London : Chemical Society, 1975.
ISBN: 0851869696

MA(solid) + B(sol) ⇋ MB(Solid) + A(sol) Where A&B share a charge and M is the opposite charge.

The most important features of a good ion exchanger are:

  1. Hydrophilic structure of regular & reproducible form Makes sense, you need to be able to reproduce it to be useful
  2. Controlled & Effective ion exchange Makes sense again, you want it to exchange the ions that you desire and you don’t want to have to wait a long time for it to doso
  3. Rapid rate of exchange Makes sense, you want to just pour water through a pipe or something, not have to loop it through a filter a dozen times
  4. Physical stability in terms of mechanical strength & resistance to attrition Makes it last longer, you don’t want holes forming in your filter
  5. Thermal stability Don’t see why you need this all the time, couldn’t you control the water temp instead?
  6. Consistent particle size, effective surface area compatible with scaling up to a larger plant What if I’m making a Brita water filter or such? Then I don’t need to scale it up

As of the publishing of this book “All ion-exchange resins suffer some breakdown on being subjected to frequent drying and wetting cycles” so they are stored wet.

While increasing the affinity to an ion means the resin will bind to it more easily it makes the resin harder to regenerate once it is used

You don’t have to completely regenerate a resin, and this is in fact impractical, just get it most of the way and it will work.

To remove most dissolved salts from water:

1) Split Stream or Hydrogen-Sodium Blend method:

In 2 streams run water through a resin. In one stream it is in the Na form, the other the H+ form. The water in the first stream will be filled with NaHCO3, whereas in the other it will get HCl. Mix this two streams and NaCl, water, and carbon dioxide is generated, with a net loss of minerals, and the CO2 is easily removed.

2) ‘Starvation’ or ‘dealkalysing’ method: Has on 1975 had mostly replaced the split stream method: Uses carboxylic acid bead resins to remove the minerals: 2RH + Ca(HCO3)2 => R2Ca + 2H2O +2CO2, thus easily removing the minerals.

To remove all the dissolved salts from water:

1) Two-bed systems: Run the water through an acid-resin to replace any cations with H+, then a base-resin to remove any anions and the H+. This can also be done in reverse. Any acids formed in step 1 will be absorbed in step 2.

(April 8th starts here)

2) Mixed-bed systems: One column of mixed strong acid-base resins. This means that as soon as the cation or anion is removed and H+ or OH- is released it will react. This very rapidly removes all the ions from the water. The best of the listed systems for removing ions however the bed is very difficult to regenerate as you must separate the two resins, regenerate each and then remix them.

3) Combination systems: This involves one or more of the above systems used together. For example a weak-base bed to remove most of the ions followed by a harder to regenerate strong base bed to remove the rest.

4) Condensate polishing (Powdex process): A powdered strong acid/base mixture is placed on a candle filter in a thin layer. I don’t understand this one.

Saline Water treatment:

Salt water is contains too much salt to remove on a large scale via traditional methods (as of 1975). Softening it is practical (exchanging harmful ions for less/non-harmful ones) however. As of 1975 there was work being done on this however:

1) Electrodialysis: This uses layered membrians of which half allows only anions to pass and the other half only cations. This creates cells with almost no ions and cells with very large amounts if ions (See diagram, R. W. Grimshaw and C. E. Harland pg 43). The feeds from these cells is kept separate. The low concentration cells can be fed through a mixed-bed filter to remove more of the ions.

2) Ion-exchange methods: Three processes are listed as being suitable due to lost cost of regeneration of the resin.

A) Sirotherm Process: Uses heat to regenerate the resin from a mixed weak acid/base bed. It can take brackish water to a drinkable level, but no further. This uses acids & bases that dissociate much more readily at high temperatures and that have flat (highly buffered) titration curves so that a small change in pH will cause a large change in composition. Passing hot (80C and 20C) streams of water through the bed in turn will give a concentrated stream of water and a dilute stream which can be separated.

B) The Desal process: This uses a ‘new’ resin called Amberlite IRA-68 and 3 coloums. It is very cheap to regenerate and works very well. If I find newer references to it I’ll study it more

C) The sul-bi Sul process: A two-column demineralization that uses HSO4-. Another specific example I may return to.

Ok, I’m skipping ahead a bit as this is going way out into chemical engineering, when I’m pretty sure I’m doing more chemist type work, so I’m looking at Ion Exchange Equilibria now

Ion-exchange capacity: The number of equivalents of exchangeable ions per unit weight or volume of the exchanger.
Normally done with the number of equiv or exchangeable counter0ions per dry kg (mili-equiv /g) of the resin in a chosen ionic form. (Normally H+ for for the cation exchanger & Cl form for the anion exchanger)

You will have to differentiate between how much you can theoretically excess and how much can be practically be accessed. For example taking into account the size of a displacing ion in structures where not all the sites are going to be easily accessible. In weak  acid & base resins this will change with each pH due to buffering.

Ion exchange capacity = (Molarity)(vol of base)*100 / ([Mass of resin][100- {% of resin that is water}])

Simplified: Q= mols of base * 100 / (Mass of resin * (100-%water)

Q is the specific ion-exchange capacity in mol/kg or mmol/g of dry protonated form.

The breakthrough capacity is more useful in columns and is always less then the specific ion-exchange capacity. This is the point at which a measurable number of ions from the solute begin to ‘leak’ through the resin.