Chemically, how does a water softener recharge work?

Factual Questions
1

I understand the basics of how my water softener works.

There’s a bunch of little plastic things in the tank that are (shaped? chemically weakly bonded?) to salt, and when water with dissolved minerals passes through them, the salt comes off and the minerals go on. And every once in while, the softener recharges the little plastic things by bathing them in really salty water, which makes the other minerals come off, more salt go on, and the whole thing is dumped down the drain.

Is that basically right?

So, somehow the little plastic things have a stronger affinity for the non-salt minerals in normal water, but in really salty water, it goes the other way. Is this just an equalizing of concentrations thing?

2

The tank contains Zeolites, either natural or manmade.

From the Zeolite Wiki: https://en.wikipedia.org/wiki/Zeolite
“Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite. An example of the mineral formula of a zeolite is: Na2Al2Si3O
10·2H2O, the formula for natrolite. These cation exchanged zeolites possess different acidity and catalyse several acid catalysis.”

Not a chemist, but the simple answer seems to be that the zeolite would rather grab magnesium etc. a

3

Battery went dead on my keyboard! :o

Not a chemist, but the simple answer seems to be that the zeolite would rather grab magnesium etc. and release the sodium.

There’s a good video here

4

Right, I understand that part. But how does the opposite part work? What makes it later release the magnesium etc. and release the sodium? That’s the part I’m not understanding.

I understand the basic process, I just don’t understand the chemistry of the recharge step.

5

That was supposed to be “and regrab the sodium”. Missed edit window.

6

It’s not impossible for the grabbers to grab sodium and release calcium/magnesium, it’s just less likely. Let’s say a grabber has a 99.99% chance of releasing sodium when bumped by calcium/magnesium and a 0.01% chance of releasing calcium/magnesium when bumped by sodium.

If you want the grabber to drop its calcium/magnesium, you just need to vastly increase the number of times the grabber is bumped by sodium. The number of swaps in any direction is a matter of the affinity % times the opportunity #. Even a 0.01% chance per bump becomes a sure thing with enough bumps.

I’m assuming that the salt doesn’t cause the calcium/magnesium to precipitate from the water, becoming unavailable for bumping the grabbers, so there will still be grabbers holding c/m after the flush. There will just be enough grabbers holding sodium to make the matrix useful again.

7

My guess is it’s a matter of concentration. If the zeolite or ion exchange polymer has a greater affinity for Mg2+ or Ca2+, but you subject it to concentrated Na+, you’ll still displace the former. If you have a high school chemistry textbook handy, see the chapters on equilibria. Also see: Ion exchange - Wikipedia

Or what the above poster wrote.

8

Yllaria and Ruken, thank you, that explanation totally makes sense to me. It’s probabilistic, so with a high enough concentration of sodium enough spots will be holding sodium to be useful.

9

Good explanation. It’s concentration. Like in real estate it’s Location.