On: One Way to Fight California's Drought: Desalt the Ocean

[timdierks]

I've always wondered why desalination is so hard. From a thermodynamics perspective, dissolving salt into water isn't particularly enthalpic (the water doesn't get warmer when you do it), so it's never been clear what the fundamental challenges to reversing that dissolution are.

It's surprising there isn't some chemical reaction to precipitate out the sodium and chloride atoms bonded to something else, or whatever.

[gus_massa → timdierks]

It's almost impossible to use a chemical reaction to precipitate sodium and chloride. Almost all their compounds are soluble in water. [1]

The easier way to precipitate chloride is using silver, you mix the salted water with silver nitrate and the silver chloride goes to the bottom. But you now have a sodium nitrate solution. (It's safe to do this experiment at home, if you are careful and follow minimal laboratory safetely procedures. But don't drink the water!) https://en.wikipedia.org/wiki/Silver_chloride#Chemistry

Just assume that all the sodium salts are soluble. The only example that I remember is the sodium uranil acetate salt. But it's definitely not a good idea to drink that water later. https://en.wikipedia.org/wiki/Uranyl_acetate#Safety

[1] I think that this is the main reasons why most of the salt in the sea is sodium chloride. Related question: Why doesn't the sea have too much potassium?

[nanofortnight → timdierks]

There are [multiple desalination methods](https://en.wikipedia.org/wiki/Desalination). My favourite is [reverse osmosis](https://en.wikipedia.org/wiki/Reverse_osmosis), though it does cost about $1.2 per cubic meter of water.

To answer your question, ion exchangers do what you ask, using sulfuric acid and sodium hydroxide, and produce a ton of sodium sulfate in the resulting desalinated water.

[aqme28 → timdierks]

From a thermodynamic perspective, un-mixing two mixed things always requires energy.

[timdierks → aqme28]

From a first-law standpoint, how can it fundamentally require more energy than is released by mixing them?

[jcranmer → timdierks]

The alkali ions are all inherently very soluble. Sodium chloride has a solubility of ~6 mol/L (and most other sodium compounds are in the same range of solubility), whereas an insoluble compound like copper chloride is more like ~6·10¯⁴ mol/L.

[chadgeidel → timdierks]

I always assumed the big question was "where do you put the brine"? (forget the name of the by-product of desalination) Do you dump it back into the sea? I don't actually know.

[stephengillie → timdierks]

Electrolysis is a rather energy-intensive desalinization method.