# Extraction of elements from seawater...

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Good day.

Somewhere read that elements can be extracted from seawater by

- Adsorption by amidoxime-based polymers -- what is that and where to source such ?

- Electro chemical cells ---  What determines ignoring/rejecting sodium and chlorine ions from the process and collect only other than those ?  Any 'special' voltage to ignore Cl and Na ions ?

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Posted (edited)

Sodium and a lot of other elements cannot obtained from aqueous solutions. Its a matter of the redoxpotential. Mainly elements with positiv potential can be obtained. Below 0V hydrogen will be developed. So Metal will be plated at cathode. Nonmetal will be developed at Anode.  Chlorine will be developed on carbon or gold or platinum. By using of other metal, the Anode will be corode and no chlorine will be developed.

Amidoximes

Amidoximes such as polyacrylamidoxime can be used to capture trace amounts of uranium from sea water. In 2017 researchers announced a configuration that absorbed up to nine times as much uranyl as previous fibers without saturating.

Edited by chenbeier
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29 minutes ago, chenbeier said:

Sodium and a lot of other elements cannot obtained from aqueous solutions.

Now am more confused than before.  Common Na, K, Cl, Ca are not the desired elements to collect/deposit in the electrodes.

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Posted (edited)

Why more confused?

Na, K, Ca are innoble metals. During an electrolysis in aqueous solution these elements travel to the cathode, but instead of the element itself hydrogen  will be developed from the water. Cl is an anion and travel to the Anode. Depending of the used material of the electrode, it can be developed as chlorine or it react with the electrode Material and corrodes the electrode.

Alkali metal can be obtained by electrolysis of melted salt.

Edited by chenbeier
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Posted (edited)
11 hours ago, Externet said:

Now am more confused than before.  Common Na, K, Cl, Ca are not the desired elements to collect/deposit in the electrodes.

Do you have any particular elements in mind that you want to extract from seawater, by electrolysis?

As @chenbeier indicates, there is a general issue with electrolysis from aqueous solutions, in that cations of elements with -ve reduction potential relative to H (which is set at zero by convention), won't be reduced at the cathode, because H+ from the water will be preferentially reduced instead. So what you get is evolution of H2 gas - and a corresponding gradual accumulation of OH-.

This applies for example to lithium, which is of current interest as it is present at low concentrations in seawater and is in high demand for batteries.  There is a paper here about a technique for extraction of Li from seawater via electrolysis: https://www.sciencedirect.com/science/article/pii/S2542435118302927 You will see that a key feature of this concept is getting Li+ ions alone to pass through a selective membrane from the seawater into a different electrolyte that contains no H+, thereby avoiding the problem of competition at the cathode from H+ which is more easily reduced than Li+.

This is quite clever, as it has to leave Na+, H+ and other species behind and only allow Li+ to be transported across. I imagine this will be to do with the state of hydration of the ions (you never have "naked" cations in aqueous solution) and how to break the hydration shell around Li+, only, to get "naked" Li+ through the membrane.  But I'm speculating.

Edited by exchemist
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6 hours ago, exchemist said:

Do you have any particular elements in mind that you want to extract from seawater, by electrolysis?

Thanks.  Yes, the ones that yield a pay for the process and leave some profit.  Lithium is one good candidate.

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40 minutes ago, Externet said:

he ones that yield a pay for the process

So, not the cheap ones like Na Ca , K...

41 minutes ago, Externet said:

Lithium is one good candidate.

You have to shift about 5 tonnes of water to get a gram of Lithium.

That's on the market at about $17000 per tonne,$17 per Kg

1.2 cents per gram

Good luck.

Getting Mg is just about worth it at 1300 ppm.

So there's about 6500 times as much Mg as Li.

It's just possible that a plant that extracts the Mg could, without too much additional work, extract Br, Sr maybe others..

Since the Mg production is already paying for the pumps, and workforce, it might be possible.

The people who know about it either are already doing it, or know it can't be done.

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