Reef Chemistry Question of the Day #86 Deionizing resins

Randy Holmes-Farley · Dec 14, 2014

Reef Chemistry Question of the Day #86

The last stage of an RO/DI system consists of the deionizing resin. All systems actually have two resins, either mixed together or separate (to allow recharging). One of the resins binds ions with a positive charge and the other binds ions with a negative charge.

Suppose you had the bright idea that most of what you actually want to remove has a negative charge, such as nitrate, phosphate, and silicate, and to save money, you'll only use the resin designed to bind the negatively charged ions.

What would happen to the effluent pH (not the tank pH) if you only used the resin designed to bind negatively charge ions?

A. It would be higher than before the DI
C. It would be lower than before the DI
D. It would be about the same as before the DI
E. It could be higher or lower than before the DI, depending on what is in the source water

Good luck!

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DFW · Dec 14, 2014

Well, in my case the pH would go down (C), I believe. A lot of calcium in the tap water would be added to my tank, and I would no longer be able to dose as much lime water as I do now, since there would not be as much need for calcium. I would be adding baked baking soda to make up the difference in alkalinity, but I think the pH would still be lower than when using only lime. This is likely not what you were asking for. I think that it would leave the effluent with lots of calcium, which I read some place that it raises pH, so it might be A that you are asking for, but I can't prove it.

Also, did you see the post about using lots of KCl as an in tank treatment for AEFW? Looking forward to your take on that, and your improvements on the treatment!:focus:

Randy Holmes-Farley · Dec 14, 2014

OK, let me clarify and edit above, the pH I'm asking about is the pH of the RO/DI effluent.

I've not heard of the KCl treatment. I'll look into it.

DFW · Dec 14, 2014

Good deal! And thanks!

ahmed.boomer · Dec 15, 2014

C. The OH- ion has a negative charge, so you'd be removing that too. The H+ would not become binded. In neutral water OH ions are equal to the H ions, so with the DI resin, the pH would decrease to below 7. Even in acidic water there still tends to be some OH-, so even in that case the pH would decrease(though very minutely).

beaslbob · Dec 15, 2014

wild guess:
A

Randy Holmes-Farley · Dec 16, 2014

Here's a hint:

A DI resin is actually a type of ion exchange resin.

ahmed.boomer · Dec 16, 2014

So you're saying the negative ions are replaced by negative OH pins correct? A phosphate ion would then cause 1,2,3 OH ions to be released depending on how my +H's it has. Causing the water to remain relatively unchanged in this case or with slightly higher pH?

beaslbob · Dec 16, 2014

with my luck it's B

Randy Holmes-Farley · Dec 22, 2014

And the answer is...A. It would be higher than before the DI

How much higher depends on how many ions are in the water entering it.

My reasons for posting this question is to get at the way that DI resins work.

Let's just look at a resin designed to bind negative charges. It consists of fixed positive charges in a porous polymer. But to balance the charge, each positive charge has a hydroxyl group (OH^-) associated with it. When a negatively charged ion comes along in the source water, the OH- is bumped off and the negatively charged ion binds to the polymer.

Hence, you have SWAPPED a hydroxyl group for the negatively charged ion. Hydroxyl, of course, is a very strong base, and it raises pH a lot.

The reason this works out when used in a DI is that the resin designed to bind positive charges does just the opposite: it swaps a proton (H+) for every positively charged ion that comes along. So the resin essentially swaps out all charged ions for H⁺ or OH^-.

Since there is an exact charge balance in the source water (total positive charges equals total negative charges), then in the resin effluent there is an exact balance between H⁺ and OH^-. These combine to form water, leaving only very low amounts of each:

H⁺ + OH^- --> H₂O

This result has some implications for reefers.

For example, if there is not a good balance between when one type of resin depletes and the other depletes (for example, you use too much of one relative to the other), the effluent can have a very high or low pH when the resin is getting depleted.

That is not usually a problem in a mixed bed since manufacturers put in balanced amounts, but in a DIY mix it might be an issue, and could be an issue in even a mixed bed as the TDS rises since event he manufacturer is unlikely to have put in exactly the right balance between the two.

Happy Reefing.