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Non Life Sciences => Chemistry => Topic started by: scientizscht on 12/01/2020 17:25:32

Title: What is the usual adsorption capacity?
Post by: scientizscht on 12/01/2020 17:25:32

Hello

I read some adsorption capacities like 1.5g/g or so. This is ridiculous, so you need a whole gram of an adsorbent to adsorb 1.5 grams of adsorbate?

This is extremely low capacity. So if you want to recover a substance via adsorption, you will need almost the same amount of adsorbent.

Is this true? If yes, it is very disappointing.

Title: Re: What is the usual adsorption capacity?
Post by: Bored chemist on 12/01/2020 17:47:31

It depends on the sorbent.
Ion exchange sorbents have capacities of the order of  5 meq/g
So, for example, 1 gram would sorb 5 meQ or 2.5 mMol of Cu++ ions
That's 2.5 * 63mg, which is about 160 mg
It would be even worse for magnesium which has a lower atomic mass
2.5 * 24 = 60 mg/g
So you would need about 17 grams of resin to trap 1 gram of magnesium.
I'm sorry that you find yourself living in a disappointing, ridiculous world.
Unfortunately, we only have access to science, rather than magic.

Title: Re: What is the usual adsorption capacity?
Post by: scientizscht on 12/01/2020 21:10:00

But it doesn't make sense. Most adsorbents have huge specific surface area, e.g. one gram can offer 600m2 of surface. Since molecules are adsorbed on surface, I would assume that one gram could adsorb a huge number of adsorbate molecules on that 600m2 surface.

How come this is not the case?

Title: Re: What is the usual adsorption capacity?
Post by: Bored chemist on 12/01/2020 21:51:07

The stuff sticks to the surface.
Ignore the fact that the surface isn't flat.
Imagine you have a small square of "substrate "- say gold leaf, because that's pretty thin. Anything much thinner is going to fall apart in use.
And imagine that you coat it with a monolayer of stuff that you want to absorb- say xenon.
The gold is about 100 to 200 nm thick
The atomic radius of gold is about 166 pm.
So the leaf is about 1000 atoms thick but the coating is 1 atom thick.
OK, xenon atoms are small, so they could pack closely, and you could get a monolayer on both sides of the gold, so things aren't quite that bad.

But still, why wouldn't you expect there to be much more absorbent than adsorbate?

Title: Re: What is the usual adsorption capacity?
Post by: Bored chemist on 12/01/2020 22:04:07

Work out what a layer of, for example, water 1 molecule think and 600 m^2 in area would be.
Water is about 0.25 nm in diameter
So the volume is 0.25 E-9 * 600 m^3
1.5E -7 m^3
The density is 10^6 g/m^3 So there's
about 0.15 grams of water.

Again, why do you think this is "wrong"?