Warning: Sodium hydroxide and hydrochloric acid are corrosive.
Wear gloves when handling them.
Greetings fellow nerds.
In this video we're going to regenerate depleted deionization resin so it can be used again in reverse osmosis deionization systems.
If you're like me you're using a reverse osmosis deionization system to produce purified water low in minerals for your amateur chemistry experiments.
Alternatively you might be one of those weird people that actually drink water.
Anyway, the reverse osmosis deionization system uses a reverse osmosis membrane to reduce the level of minerals in the incoming water
and then passes the water through a column of deionization resin to remove any remaining ions.
This resin has a limited capacity and will eventually saturate.
At that point it will need to be replaced.
Most individuals will simply buy new resin and discard the old resin in the trash.
If you don't use deionized water very much,
and/or have a very efficient reverse osmosis system that produces water very low in minerals so your resin depletes very slowly,
then buying new resin may not be that much of an issue.
But if you're like me and you use a lot of deionized water and have a very old and inefficient system that is not quite up to modern standards,
then buying new resin can become prohibitively expensive.
Fortunately, the used resin can be regenerated by removing the absorbed minerals
and replacing them with the hydroxide or hydronium ions the resin is normally loaded with when new.
Before we can do that though we need to first understand what the resin consists of and how it works.
Mixed bed deionization resin is actually a mixture of two types of highly specialized plastic formed into tiny beads.
One type is made using negatively charged functional groups and this gives the plastic an inherent negative charge.
This plastic will absorb and hold positively charged ions.
In chemistry we call this a cation exchange resin.
The other type of plastic is made with positively charged functional groups and so it will absorb and hold negatively charged ions.
We call this an anion exchange resin.
When they are first made, the cation exchange resin is loaded with hydronium ions, and the anion exchange resin is loaded with hydroxide ions.
When they are used to remove minerals from water, the ions are swapped out,
resulting in the minerals being absorbed into the resin and the hydronium and hydroxide ions being released.
Now the interesting part about these reactions is that they are actually reversible equilibrium reactions.
This means that we can force them backward by using highly concentrated solutions of hydronium or hydroxide ions.
This serves as the basis for our regeneration procedure.
So let's get started.
I have here some completely depleted mixed bed deionization resin, about 80 milliliters in volume.
I've also poured off most of the water, it doesn't need to be dry but it should not be submerged in water either.
Now I'm doing just a small amount because it looks better on camera
but i recommend saving the depleted resin from a few full cartridges and doing the regeneration all at once in bulk to save time and labor.
Now as is, this is very hard to regenerate because we have a mixture of two oppositely charged resins.
Any regenerant that we add will regenerate one type of resin, but completely saturate the other,
so we're going to need to separate them and regenerate them individually.
Fortunately, we can take advantage of the different densities of the two resins and separate them that way.
To do so, we need to make a liquid that is of an intermediate density between the two resins
so one resin will sink and the other will float.
First look at the approximate volume of resin you have and in a separate container get about twice that amount of water.
I have about 80mL so i'm going to get 170mL of water.
I'm going a little over because I wanted to keep the numbers simple for the next step.
Now to the water we add 3/17ths by mass of sodium hydroxide.
So with 170g of water, we add 30g of sodium hydroxide.
Stir the sodium hydroxide until dissolved.
I'm using sodium hydroxide because it's very easy to make solutions in the density range we need.
Now I know that 3/17ths is a very weird fraction but there is a good reason for that.
This produces a solution that is 15% sodium hydroxide by mass.
At room temperature this solution has a density of about 1.168g/mL.
This density will separate the two types of resin.
Now once the sodium hydroxide solution is fully dissolved, we pour it directly into the deionization resin.
I'm using indicator resin because it looks good on camera,
so the anion exchange resin changes color as the ions are replaced and the resin is regenerated back into its fresh state.
This is another reason why I used sodium hydroxide, it's actually a regenerant for anion exchange resin,
so we're doing two things at once in this step.
Now as you can see, the anion exchange resin is floating upward.
Most anion exchange resin used in deionization systems has a density of 1.07g/mL when in its hydroxide form.
Since the solution is 1.168g/mL the resin floats being less dense.
I used a bit more sodium hydroxide to create a higher density than necessary for resin separation
to account for manufacturing differences in the resin and possible extra water the resin contains that might dilute the solution.
Anyway, the cation exchange resin has a typical density of 1.2g/mL when in its unused hydronium form,
but since we're using sodium hydroxide, we're exchanging out all the absorbed minerals and hydronium ions for sodium.
The sodium form is even denser at 1.28g/mL, and helps our separation along.
Now I recommend agitating the container a little to dislodge any beads of resin trapped in the opposite type of resin.
Try and get most out but don't worry about getting every single possible bead.
A bit of cross contamination, while undesirable, will not ruin the resulting water quality after regeneration.
Don't agitate the resin too much or it will mix again.
Once most of the beads are separated, pour off the anion exchange resin into a separate container.
I used twice the volume of solution to get good separation between the two resins.
But you can use more or less solution to balance ease of separation versus chemical wastage.
Just recalculate your proportions so you're always making a 15% solution of sodium hydroxide by mass, or a 3/17 ratio.
Now pouring is actually pretty crude for separation
and some chemists and aquarium reefers have been successful scooping the resin directly off the top using an ultrafine mesh sieve or a scoop for better recovery.
Others have been successful siphoning out the bottom layer of resin since it actually behaves quite fluidly.
Another very popular separation method is to use a separatory funnel.
If you're an amateur chemist then you likely already have one.
Otherwise, you can make a crude one out of a sport's drink bottle with a push-pull cap.
Just cut off the bottom and after thoroughly washing the bottle you can add the slurry of sodium hydroxide and resin to the upside down bottle.
Then just open the bottom and let the cation exchange resin drain out.
Remember to wear gloves since this concentration sodium hydroxide solution is somewhat corrosive to human flesh.
Anyway, back to my original run, I still have a few beads of anion exchange resin in the cation exchange resin.
So to get it I'm going to first recover the sodium hydroxide solution by filtration.
And let me get my stir bar.
Then we pour the recovered sodium hydroxide into the cation exchange resin and separate the remaining anion exchange resin.
I'm now going to mix that in with the rest of anion exchange resin and filter off the sodium hydroxide solution to be discarded.
Now we have our fully recovered and regenerated anion exchange resin.
But it's not ready for use yet.
We need to wash away all remaining traces of sodium hydroxide.
So we use deionized water and continually wash and filter the resin.
We have to do this because sodium hydroxide would get absorbed by the opposing cation exchange resin and deplete its capacity.
So we have to thoroughly clean the anionic exchange resin now.
This is a very slow process and ideally you keep washing and filtering until the filtrate has a TDS reading of zero like the deionized water you're washing the resin with.
However as an amateur this is not strictly necessary and rather wasteful of deionized water,
so as long as your filtrate is below the TDS reading of your reverse osmosis water
then your resin is good enough and a tiny loss of capacity will be imperceptible.
A lot of aquarium enthusiasts just wash a couple of times with water and don't waste any more time or water than that.
Anyway, when the resin is as clean as you need it to be, scoop it out and set it aside.
And there is our fully regenerated and cleaned anion exchange resin under deionized water, ready for use.
Now for the cation exchange resin.
Because we used sodium hydroxide to separate the two resins earlier,
this resin is now fully loaded with sodium ions and maybe insoluble metal hydroxides.
The anion exchange resin was separated and regenerated in one step but we need a separate regeneration step for the cation exchange resin.
First we filter off the cation exchange resin and wash it a couple of times with water.
We don't need to be completely clean, we just want to get rid of the bulk sodium hydroxide clinging to it.
While that's filtering, in a separate container we get 400mL of deionized water and add to it 100mL of 30% hydrochloric acid.
The objective is get around 5%-7% acid concentration.
Looking back I only needed about a fifth as much total solution but you get the idea.
Anyway, to our washed but used cation exchange resin, we mix in our acid solution.
This will react with the resin and replace all the other cations with hydronium ions.
You can see a slight color change as this happens.
I used diluted acid to save on acid but you can use straight concentrated acid if that's more convenient.
Now my filter is very slow so i'm doing the regeneration right on the filter.
But if you have a fast filter then it might be worth it to pour the washed resin into a separate container and add acid there.
Letting it sit for an hour with occasional stirring every ten minutes.
You need some time for the regeneration to happen and reach completely into the resin beads.
Anyway, once all the acid is filtered off, we now have regenerated resin that we need to wash clean just like the anion exchange resin.
So once again we're going to add deionized water and filter until the filtrate is clean.
Similar to the anion exchange resin, our objective is to reach zero TDS,
but you can stop a bit earlier if you accept the decreased capacity. I would.
Once it's reached acceptable cleanliness, scoop it off the filter.
And there we have it, fully regenerated cation and anion exchange resins washed and ready for use.
Now we just mix them back together to create our mixed bed deionization resin.
Be sure to thoroughly mix them so the consistency is uniform.
I have too much water here so it's starting to separate a bit due to differences in density
so i recommend pouring off or filtering the mixture and then mixing to prevent this.
Anyway, the resin is now ready for use or storage.
If you're going to put it into storage I recommend using an airtight container
because the resin will absorb carbon dioxide in the air and be depleted.
Personally I store my regenerated resin in thoroughly cleaned and washed soft drink bottles.
So there you have it.
How to regenerate used mixed deionization resin for reuse into deionization systems.
You can stop the video now but i'm going to answer a few common questions that may come up.
First, how many times can resin be regenerated?
Resin can last many years through hundreds of cycles.
However, it will break down or become fouled as contaminants build up that aren't removed by regeneration.
Organic contaminants can dissolve the resin and react with the functional groups destroying its properties.
Indicator dyes may breakdown sooner though so you'll gradually lose the color distinction between fresh and depleted resin.
But the resin should retain capacity.
It's best to keep a log book or record of how much water the resin processes between changes when the TDS readings become too high.
If you notice the resin drastically losing effectiveness then it's time to discard it and buy new resin.
Can you use different regenerants than sodium hydroxide or hydrochloric acid?
Actually you can, as long as you provide a high concentration of your desired ions,
you can exchange out the undesired ions currently absorbed into your resin.
So for anion exchange then any strong source of hydroxide ions can be used.
Sodium hydroxide is cheapest and popular but you can also use potassium hydroxide if that's all you have.
Hydroxides that have low solubility like calcium hydroxide cannot be used
because they cannot produce solutions of high enough hydroxide concentration to effectively regenerate anion ion exchange resins.
So try and stick to potassium or sodium hydroxides.
Cation exchange resins require hydrogen or hydronium ions so you need a strong acid.
Hydrochloric acid is most used but sulfuric acid is also a popular alternative.
However it can precipitate calcium sulfate.
While this problem can be accommodated by slowly ramping up acid concentration in a continuous flow system,
this is only really viable in professional systems.
Thus for us amateurs it's recommended to use hydrochloric acid.
Next question, can you use a different solution to separate the resin by density?
Yes you can, highly concentrated sodium chloride or salt solutions can have enough density to separate the resin.
This is sometimes done in professional systems as sodium chloride is considerably cheaper than sodium hydroxide.
So it's used for separation and after filtering a much smaller amount of sodium hydroxide can be used for actual regeneration.
For an amateur the cost savings are minimal compared to labor.
But for professionals regenerating metric tons of resin the cost savings can be considerable.
Some professional systems don't even use any special solution and just use water.
Carefully agitating the resin will still cause the anion exchange resin to settle on top of the cation exchange resin so it can be scooped or siphoned off.
The separation quality is bad but the deionized water produced from such a system is still good enough for their purposes.
So that's how you regenerate deionization resin for water purification.
Thanks for watching.
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