Commercial Scale Desalination via Electrolysis

[GK12]

Gday all,

ive been trying to find out why electrolysis isn't used / less feasible than RO in sea water desalination?

The chemistry for NaCl electrolysis is well understood and thus works, so why is it not feasible/less feasible than say RO whereby we are essentially pumping water through a rock?

Im guessing it has something to do with power requirements -> more expensive than R.O ?

 

[SteamKing]

Desalination by electrolysis is not used because it destroys the water molecule and makes some nasty compounds in the process:

http://en.wikipedia.org/wiki/Electrolysis_of_water

See the section entitled 'Applications'.

In addition, there are some hefty energy inputs required for electrolysis of salt water, not to mention the efficiency of the process.

 

[GK12]

But isn't that for electrolysis of pure water?
Sea water (water containing NaCl) would follow the following wouldn't it?:

(at cathode) 2H+ + 2e- -----> H2(g)
(at anode) 2Cl- -----> Cl2(g) + 2e-
(in solution) Na+ + OH- -----> NaOH(aq)
The full equation:
2NaCl(aq) + 2H2O(l) -----> H2(g) + Cl2(g) + 2NaOH(aq)

Given the charged ions, they would be the first to interact with the electrode rather than H2O wouldn't they?

It wouldn't be that difficult to regulate the amount of input current (thus regulate the electroylsis process and prevent any unnecessary reactions with the H2O) by monitoring the conductivity characteristics of the input water.

I just found this which so far is the only reason I can potentially see and relates to the low efficiency and high energy input as you mentioned:
http://en.wikipedia.org/wiki/Overpotential#Bubble_overpotential

In other words, bubbles forming on the electrodes decreases available surface area thus requiring greater input current.

Seems to be a universal problem to electrolysis in general?

 

[chill_factor]

Is it really desalinizing though? The net reaction is just a 1:1 exchange of NaCl with NaOH which is worse than NaCl, since NaOH is a strong base.

Also, seawater is not pure NaCl in solution. It has many other dissolved salts that may interfere with electrolysis.