Air and water

[gardenman]

The thread titled "Wherefore art thou Nitrogen" got me thinking. Can water absorb nitrogen from the air? The air we breathe is 78% nitrogen. We use air stones to inject oxygen into the water. CO2 can also be absorbed by water. I've never heard if nitrogen from the air can be absorbed by water though. How does the air entering our tanks vary from the air leaving our tanks? I would assume a deeper tank would alter the air more since the bubbles would have a longer exposure time to the water. Smaller bubbles would likely also result in more exchange. Does the exchange only work one way? Does tank water just gain oxygen and CO2 and the air gets nothing back? Or does the air absorb stuff from the water also? If I put a hundred air stones in a tank would the water absorb enough nitrogen to feed my plants? I'm starting to wonder if there might not be a lot more going on with that air/water interaction than we assume. Commercial fish farms often inject massive amounts of air as do many koi keepers. Is the air/water interaction part of the reason sponge filters are so effective? Hmm...must do some research. Off to Google I go.

[egruttum]

3 hours ago, gardenman said:

would the water absorb enough nitrogen to feed my plants?

Yes, nitrogen will dissolve in water no different than oxygen will dissolve in water.  The issue comes that atmospheric nitrogen (N2) is not in a form that plants can use.  The two nitrogen molecules are not reactive in the way that plants need.  This is why we need to fertilize plants (especially agricultural crops).  Nitrogen needs to be "fixed" into a form that plants can use.  The two main forms of nitrogen that plants use are ammonia/ammonium (NH3/NH4+) and nitrate (NO3).  Most plants don;t use nitrite (NO2) and its toxic to many plant species.  There are a few ways that nitrogen is "fixed" in nature.  There are bacteria and other micro-organisms that take N2 and "fix" them into a usable form.  There are also atmospheric methods of fixation such as lightning.  

One interesting thing to read about is the Haber-Bosch synthesis.  Fritz Haber was a scientist in Germany around 1900-1930's that invented many things.  His best positive creation was finding a way to create ammonia using hydrogen (H2) and nitrogen (N2).  Carl Bosch was a chemical engineer that worked with Haber.  Their method of creating fixed nitrogen changed the world (in my opinion probably the biggest man-made change in the world ever).  With their synthesis it was now incredibly cheap to make fixed nitrogen for crops.  Those two guys are the reason that we can produce enough food to feed the world today and into the future.  I would be remiss if I didn't state that Haber had some evil as well.  He was the father of chemical warfare in WWI and his science led to the creation of Zyklon B.  He is the embodiment of science being wonderful and evil.

[RockMongler]

There most certainly is Nitrogen gas (N2) from the atmosphere in your tank, but it's fairly inert, and not something your plants or fish make use of.  You specifically need nitrogen compounds (ammonia, nitrite and nitrate) for them to be readily available to your plants/beneficial bacteria.

It's like the same way you could grind up a chunk of iron to powder, and mix it into your food, but most of that iron is going to just come out the other end rather than ending up in your bloodstream and tissues.  For things to be bioavailable to most organisms, it has to be in a specific compound/form the organism 'knows' how to properly process.  

[OnlyGenusCaps]

I could not possibly add anything to the excellent reply by @egruttum but I wanted to touch upon a couple other points you raised.  The solubility of the largely inert N2 gas in water is about half of that as O2, and about 100 times less than CO2 (loads of rounding error there, and it all varies by temperature as well).  

6 hours ago, gardenman said:

Does the exchange only work one way? Does tank water just gain oxygen and CO2 and the air gets nothing back? Or does the air absorb stuff from the water also?

The main thing moving into the air is water itself, in the form  of water vapor.  But for a heavily stocked tank, CO2 likely exits the water also.  It all depends on what is being taken up or given off in relation to the equilibrium with the atmosphere.  If your plants are pearling, it is because O2 is forming bubbles and going into the air.

5 hours ago, gardenman said:

I would assume a deeper tank would alter the air more since the bubbles would have a longer exposure time to the water. Smaller bubbles would likely also result in more exchange.

Yes, smaller bubbles have larger impacts, and an interesting one is at the surface tension layer.  You can separate out certain molecules at the surface layer because of their state of polarity, particularly amphiphilic species.  This is the basis of how protein skimmers (properly called foam fractionation) work.  And they do work in freshwater as well, though it's not as efficiently.  But the same general principle applies.

6 hours ago, gardenman said:

I'm starting to wonder if there might not be a lot more going on with that air/water interaction than we assume.

You are absolutely correct that there is a lot going on with the interface of air and water.  It's a fascinating field of study in fact and controls everything form how a bubble forms that a child might delight in all the way up to how the oceans act as a sink for increasing carbon in the atmosphere. 

There is even a documentary about bubbles from 2015 that touches upon many of these topics, if you are interested, called "The Science of Bubbles".  A worthwhile hour.