Is this possible? [filtering oxygen from water to breathe]

Factual Questions
1

Hi.

My daughter loves scuba diving, and asked me if it would be possible to “filter-out” the dissolved Oxygen in seawater to allow tank-less diving, just like something she saw on the internet. Some further googling as well as some very basic calculations quickly led me to believe that this approach is not practical, and what she saw was a scam.

However, it seems to me that there is a lot of Oxygen locked up in the water molecules themselves, so I wondered why nobody has tried electrolysis to liberate O[sub]2[/sub]. I mean, it’s been demonstrated a million times in science demos for middle-schoolers.

To make things easier, let’s not worry about issues of pressure and electrode efficiency.

Assumptions:
1 liter/breath
40 breaths/min

This gives us (1L/breath) x (40 breaths/minute) x (1 minute/60 seconds)=
0.667 liters of air/second

Air is roughly 20% oxygen (O[sub]2[/sub]), so we will need to generate
(0.667 liters/sec) x (0.2) = 0.133 liters/second of O[sub]2[/sub]

Using the universal gas law to calculate the number of moles of O[sub]2[/sub], I get
N=(PV)/(RT)
= (1 atm x 0.133 liters)/(0.082057 Liter atmosphere mol[sup]-1[/sup] K[sup]-1[/sup]) x 293 K
=5.5 x 10[sup]-3[/sup] moles of O[sub]2[/sub]/sec

Now, we’ll need to reduce two water molecules in order to liberate a molecule of O[sub]2[/sub]. Also, this is a 2-electron process, so we’ll actually need to provide
2.2 x10[sup]-2[/sup] moles of electrons/second

Using Avogadro’s number, this works out to
(2.2 x 10[sup]-2 [/sup]moles) x (6.02 x 10[sup]23[/sup]mole)
= 1.32 x 10[sup]22[/sup] electrons/sec

So far, so good. Let’s convert to coulombs:

(1.32 x 10[sup]22[/sup] electrons/second) x (1 coulomb/6.242 x 10[sup]18[/sup] electrons)
= **2.11 x 10[sup]3[/sup]coulombs/sec. = 2110 Amps **

If my math and reasoning are correct (are they?), I think I know why this approach hasn’t been tried; 2000 amps + seawater is probably a very bad idea.

Any thoughts? Did I miss something?

2

I have no knowledge in this, but I’m going to suggest that a moderator give your thread a title that’s more descriptive than “Is this possible?”, in hopes that it’ll help to get the attention of those who know the topic area.

3

Nuclear submarines use electrolysis to provide oxygen for their sailors. Carrying a nuclear reactor to convert water (also, you have to desalinize it first…) would make it difficult to swim.

Fish, on the other hands, use gills, which are lighter and don’t require much electricity, but with gills you got other problems…

4

Moderator Note

Thread title changed to more clearly indicate the topic.

Please use descriptive thread titles.

5

Looks to be somewhat impractical.

6

One thing is that you don’t use all the oxygen in a breath. Far from it. Systems that remove the CO[sub]2[/sub] and only add enough oxygen to make up the used proportion (generically called rebreathers) exist.

Your figure of 0.133 liters/second of O[sub]2[/sub] is about 30 times too high for a base rate, and still 4 times high for a fit person at full exertion. (Based on numbers from the above link Wikipedea page.) This will help with submarines rather a lot.

But still not enough to make a self contained system viable. The added complexity versus a simple Scuba system of any rebreather limits its utility from both a cost and reliability point of view. And you really want simplicity and reliability if a failure can kill you in a few moments. Further, the market for anything that allows you stay underwater for longer is constrained by nitrogen adsorption, which either gets you to mixed gasses and/or puts a hard limit on your dive times anyway.

The electricity is best used compressing air into a bottle.

7

Oh yeah, right…chlorine. I forgot about that.

Also, my apologies for the vague headline; I’ve been lurking long enough to know better.

8

[As I was typing this bizzwire posted a link which is probably more accurate. But I’m in the same order of magnitude as them, so I’ll leave it here]

And, take it a step further (though I’m not carefully checking the OP calcs). Looks like electrolysis needs 1.23 Volts, call it 1.3 with current losses.
1.3 Volts times 2110 Amps = 2.7 KiloWatts of power. Which is enough to power a small house.

Or, to put it another way, for a 20 minute dive, you’d need about 1 kW-hr. A car battery is it looks like maybe 500 W-hrs. So carry two car batteries or a beefy extension cord on your dive.

The good news is that 1.3 volts isn’t going to be particularly dangerous by itself. And you could probably get rid of the hydrogen in a reasonably safe way that keeps it from re-combining with the oxygen in a very exciting fashion. The massive array of electrodes and oxygen collectors you’d need to get a liter every ten seconds is going to be kind of bulky, though. And the batteries, of course.

9

Pretty good debunk of the Triton video/product here: Triton artificial gill: BUSTED! - YouTube

10

You don’t even need to do any calculations to see that this is impractical. To split water, you need an energy source. For this to be practical, the energy source would have to be more compact and/or lighter than the tank of oxygen you’re carrying. Short of nuclear reactions, the most compact and lightweight way we have of storing energy is in fuel combined with oxygen. So you’d need to carry fuel and oxygen with you to power your electrolysis device. But by the conservation of energy, even if you had 100% efficiency both in burning the fuel and in electrolyzing the water, you’d have to carry as much oxygen to fuel the system as you’re getting out of it. Plus the fuel, plus however much more to account for inefficiencies.

11

FWIW, Normal breathing rate for a relaxed person is less than half this. 12-18 breaths per minute is average. Also, a normal breath is about .5 liters.

12

Others factor at play here: Even if there was a sufficiently compact energy source and mechanism to extract oxygen from water in sufficient quantity for a human to breathe, there are still a few other problems:

Dilution: Humans ideally need their oxygen diluted about 1:4 with something else - if the oxygen is delivered in pure form at one-fifth of the normal volume, the lungs won’t inflate properly and the oxygen probably won’t be absorbed (also, elimination of CO2 is probably impaired by the same constraint) - not to mention that breathing pure O2 is probably not a brilliant idea.

Timing: Does this device supposedly extract oxygen fast enough for it to be inhaled directly?
If not, then where is it stored awaiting inhalation?
If so, then multiply any efficiency requirement by a factor of 5 or more - because the implication is that the machine has to work incredibly hard for the short duration of inhaling, then stop for a while

Thrust: You can’t pump that much water through a small space without generating some thrust - in fact, you have to do that, or else you’ll just deplete the oxygen in a local pocket of water.

Short of some magical SF technologies that aren’t even on our radar, it’s impossible to do the things that this device claims.

13

Yes, that matches some googling I did a while back on a tangentially related issue (humans being genetically modified to breathe underwater.)

14

As I noted earlier - there are existing systems that manage all of this. These are the rebreathers. They maintain the needed mix of gasses - replenishing the O[sub]2[/sub] and scrubbing the CO[sub]2[/sub] in an appropriate gas mix.

The point about pure O[sub]2[/sub] is critical. Oxygen toxicity is a significant problem and will cause convulsions in divers (usually with fatal consequences.) A partial pressure of O[sub]2[/sub] of only 1.6 bar limits a diver to 45 minutes dive time. Usually that is an upper limit of O[sub]2[/sub]. Diving on pure oxygen that would only be 20 feet down. So gas mix management is a given requirement for any system.

15

I feel that I have to point out that people seem to be addressing a different problem than the one the thread title seems to be stating.

You’re all talking about electrolytically splitting water to remove the oxygen. Fish don’t do that. They “filter out” the dissolved oxygen in the water, something that’s far less energy-demanding. There certainly is dissolved oxygen in water, and enough to support some pretty big lifeforms, like whale sharks:

http://omp.gso.uri.edu/ompweb/doee/science/physical/choxy1.htm

I haven’t got the time right now to do this analysis, but I suspect it’s not trivial to remove enough oxygen to support a person. I’m not sure that I’d trust a device doing so, without several warning systems to tell me that the oxygen concentration was getting too low and the cCO2 too high – it’s very easy to get into a situation where the gas combinations can make you pass out.

16

Nope, you’re the one who missed the point here. The OP is very explicitly about electrolysis of water. (And my link in post #13 above is about the feasibility of humans using dissolved oxygen in water: tl;dr is that there isn’t enough.)

17

Nope – it explicitly asks for what I said:

You can’t “filter out” water that’s locked up in the form of molecules. And sea water does contain dissolved water – it’s what fish breathe.

18

The thread is a little of both. The OP’s question touches on splitting water molecules to get at the O2, but the linked device (supposedly) filters out dissolved oxygen.

19

Sure, but you need a reservoir in which to store the scrubbed exhaled gases before adding O2 and allowing them to be re-inhaled - the linked device has none of that (unless it were proposing to compress them into a tiny tank on each exhalation, which it isn’t, and that would consume even more power)

20

And then he says

And the rest of his post is about figuring out the amount of energy needed to use electrolysis to provide enough oxygen for a human to survive.

And since you don’t seem to be interested in reading my link in post #13–where I actually did the work on how much dissolved oxygen is in water and how useful that would be to humans–I’ll copy-paste it here: