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Author Topic: Can DC and AC electrolysis be combined to remove salt from water?  (Read 1316 times)

Offline knowledgequest

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I am currently doing a study on the effects of combining DC (actually pulsed AC), and AC electrolysis to dissociate salt from water.  Salt concentration is approximately 50,000 ppm.  AC electrodes are made of .375 dia. allthread rod.  DC electrodes are made of 2 inch aluminum L channel.  I am using between 9- 10 volts DC at 40-50 amps.  I am using approximately 14-15 volts AC at 40- 50 amps.  I am getting a white thick foam forming on the top of my vessel and when removed in a beaker it immediately settles to the bottom.  When the solution is poured through a coffee filter the white substance is in the form of a goo until it dries. 

Could this be the salt dissociating from the water or something else entirely

In addition in a second vessel I am using DC (pulsed AC) only, with 2 different types of electrodes.  One set of electrodes is .375 dia. all thread rod.  The other set of electrodes is 2 inch aluminum L channel.  In this vessel I am using  pulsed 8-9 volts DC at 40 - 50 amps.  The same white goo like foam is on the surface along with the traditional green goo like foam associated with Zinc or steel electrodes.  It appears there is a greater production of the white foam than the green..

I have read many post that state the green foam is the organic material being released. I have not seen much information as to what the white foam might be.  I am hoping it is indeed the salt being dissociated from the water.  when collected in a  vessel and poured off through a coffee filter is becomes more gelatinous and firm.  I am hoping when I return to my lab I find it to be a dry powder salt.

Any ideas???
« Last Edit: 12/04/2016 09:01:46 by chris »


 

Offline chiralSPO

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I would not recommend using aluminum electrodes for electrolysis, especially in salt water. I would guess that most of the white sludge was aluminum salts from the corrosion of your electrodes. Few metals are stable with such high voltages, with substantial amounts of chloride in solution. Titanium, platinum and gold would all work, but metals such as iron, zinc, aluminum, copper, silver, brass, and most grades of steel will corrode.

If you want to separate the salt from the water you will need more than just electricity. Ion-selective membranes can be used in combination with dc current (electrodialysis), or just semi-permeable membranes and pressure (reverse osmosis).
 

Offline knowledgequest

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Thanks for the information. I am trying to remove the majority of salts prior to any sort of R/O treatment.  I have seen studies where this is possible and simply trying to accomplish the same.  I have used other types of electrodes but produced chlorine but after switching to the aluminum the chlorine ceased.  Also, am I to assume, (bad choice in any situation), the green foam being produced in my second vessel is organics or something else?
 

Offline evan_au

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the green foam being produced in my second vessel is organics or something else?
I doubt that algae could survive being savagely electrocuted every few milliseconds.

Test: Algae only grow in sunlight. Run your electrolysis in the dark. (Ensure that you have good ventilation; a hydrogen explosion from electrolysis would really disrupt your day!) 

You are using iron in one of the electrodes, and there is chlorine in the salt. Some iron chloride compounds have a greenish tinge.
See: https://en.wikipedia.org/wiki/Iron(II)_chloride
 

Offline knowledgequest

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Come on folks you guys can do better than all this.  Give me something that will make me ponder the thought.  Your current answers are very inside the box.
 

Offline chiralSPO

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Of course you stopped creating chlorine when you switched to the aluminum electrodes! Aluminum is 3 volts easier to oxidize than chloride!

Al → Al3+ + 3 e  E0 = -1.66 V
2 Cl → Cl2 + 2 e E0 = +1.36 V

As for the green scum, did you get it when you used electrodes that contain copper (brass) or nickel (some types of steel), but not when you used aluminum? If so the color is due to either Cu2+ or Ni2+ compounds.

Evan, I doubt there would be much Fe2+ under these conditions, it is easily oxidized to FeIII, which is much less soluble and usually on the red-yellow-brown end of the spectrum (in contrast to, as you point out, Fe2+ compounds which are often greenish).
 

Offline knowledgequest

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If you folks read my original post you will see my voltages are much higher than you are describing and my current is between 40 and 50 amps.  Yes I am using both AC and DC, getting some very interesting reactions when I use the 2 together.  Again, I ask you to think outside the box on this one.  Looking at Platinized Titanium as an anode and graphite as a cathode for both AC and DC to see what happens next.  Who is brave enough to give any possibilities as to what you think might happen.
 

Offline chiralSPO

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If you folks read my original post you will see my voltages are much higher than you are describing and my current is between 40 and 50 amps.  Yes I am using both AC and DC, getting some very interesting reactions when I use the 2 together.  Again, I ask you to think outside the box on this one.  Looking at Platinized Titanium as an anode and graphite as a cathode for both AC and DC to see what happens next.  Who is brave enough to give any possibilities as to what you think might happen.

First, anode and cathode designations don't apply to AC, only DC.

Second, yes I saw that you are using very high voltages. 9 or 10 volts is thermodynamically enough to drive pretty much any electrochemical reaction, but it is still useful to compare the reduction and oxidation potentials of the species involved in the system--all of the aluminum metal in an electrode will be oxidized to aluminum salts before there is a significant amount of chlorine production from chloride. Similarly, as long as there is any trace of water around (or other source of protons) all current will go into producing hydrogen gas before any sodium metal is formed at the cathode (reducing electrode). Increasing the voltage only leads to increased rate (amperage), and the excess energy is dissipated as heat.

As to ideas for purifying salt water, please see my first post, where I note that just adding energy to the water won't do anything without some way of separating the contaminants (membranes are the current method of choice). Even putting heat into the water, and attempting to distill it requires a mechanism for capturing and condensing the vaporized water.

Platinized titanium will probably produce both chlorine gas and oxygen gas at the anode (oxidizing electrode) and hydrogen at the cathode.
 

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