Sciencemadness Discussion Board

Reverse osmosis through shirt fabric?

Heptylene - 24-2-2020 at 13:23

I saw this video on youtube about a guy who reportedly survived 49 days on a raft without fresh water and eating fish he caught himself.

Apparently he drank sea water filtered through a shirt. Ocean water has about 35 g NaCl per liter, so you die pretty quickly if you only drink that. So how did he do it? Was the water less saline than expected? Is "filtration" through a shirt able to remove any significant quantity of salt?

This got me thinking about reverese osmosis. Apparently the pore in typical membranes used in reverse osmosis are in the nanometers, but cotton fabric from a shirt would have pores in the micrometers I assume? Maybe by squeezing water through a shirt you could actually deplete it of some of its salt, and if you did that many (100?) times using the depleted water in the next step, you could obtain drinkable water? I really haven't looked much into this, just throwing around some ideas really

Or is there another simpler explanation for how he survived in the middle of the ocean without water? Maybe he got his water from eating fish, I assume their meat is less saline than the surrounding water? I'm just curious to see if anyone has ideas.

EDIT: Or maybe the story is bullshit, that also an option...

[Edited on 24-2-2020 by Heptylene]

SWIM - 24-2-2020 at 13:53

Certainly the fish would be much less salty than the seawater.

Low enough that a person sweating in the sun on a raft might even become salt depleted eventually if he had fish and only fish to survive.

As for drinking sea water, I've seen some 20th century assertions that sea water is less immediately toxic than generally believed. If these claims are accurate it appears that people can handle a lot more salt than was generally thought. At least in the short term.

I would imagine it does your kidney in eventually, but they do have some ability to maintain salt balance even when you're drinking something so hypertonic.

Consider this: You drink a glass of seawater and it is absorbed into your system. Your kidneys don't see any 3.5% saline, just the increased level in your body which is a fraction of that and much less of an osmotic gradient or whatever it is called.
If you drink your seawater slowly and steadily perhaps the body can deal with the load.

I can't remember exactly where these assertions came from, but it was either some guy who did experiments with survival at sea or perhaps some guy who was trying to circumnavigate the globe solo non-stop (Maybe in a nice little 35-40 foot ketch?) in the mid 20th century.

This does remind me of the old tale that blood and seawater have the same salinity.
Obviously nowhere near true, but I think it is a corruption of the observation that blood and seawater have similar compositions. Apparently the salt ratios in human blood are somewhat similar to those in seawater even though the concentrations are quite different.

Edit: I should've tried looking it up first. Dr Alain Bombard.
A medical researcher. The guy in the ketch was something else altogether.

For a refutation of Bombard see Hannes Lindemann, who tried the same thing and later said Bombard cheated.

[Edited on 24-2-2020 by SWIM]

[Edited on 24-2-2020 by SWIM]

Metacelsus - 24-2-2020 at 14:21

Shirt fabric can't be used for reverse osmosis, as it's permeable to sodium and chloride ions. (RO membranes, in the ideal case, are permeable only to water).

wg48temp9 - 24-2-2020 at 16:02

I think the key issue would be if the maximum possible concentration of salt in urine is less than the concentration in the sea water then the kidneys may dehydrate you in a futile attempt to reduce the salt in your blood.

I checked maximum physiological concentration of sodium in the urine is
probably in the range of 270–290 mEq/l and 6 g salt ≈ 2,400 mg sodium = 104 mEq sodium, therefore the max concentration of salt in urine is about 18g/l which is less than the concentration in sea water.

Heptylene - 25-2-2020 at 01:35

Quote: Originally posted by Metacelsus  
Shirt fabric can't be used for reverse osmosis, as it's permeable to sodium and chloride ions. (RO membranes, in the ideal case, are permeable only to water).

I didn't know that, I thought it only depended on pore size and molar mass of the species. Like gas effusion but for liquids. So that settles it, a shirt won't help.

EDIT: syntax

[Edited on 25-2-2020 by Heptylene]

Tsjerk - 25-2-2020 at 02:23

The idea behind osmosis is a bit different and a bit more complicated than only pore size.

Osmosis occurs when one solute is allowed to pass the barrier, while another is not.

Pore size is not really relevant, a Na+ ion is picometers in size, if not less, butt still osmosis, or more correctly osmotic forces, take place in our bodies with pores a lot bigger.

Charge is very relevant, if your positive charge can't pass the barrier while the negative can? Osmosis! Other way around? Osmosis!

Osmotic power comes from excluding one type from entering a body while excluding another which makes the first one incapable of going.