Slightly puzzled about an ammonia distribution between aqueous and gasphases

["Steve Short" <steve@xxxxxxxxxxxxxxxxxxx>]

Hi David

I wonder if you would mind having a quick look over the attached input and
output files as I'm having a bit of difficulty understanding why so little
ammonia gas appears to transfer into the gas phase.

Briefly, I have an industrial waste water (from the washing of electrostatic
precipitator dusts) that is highly alkaline (due to dissolved lime and some
ammonia) from which I have previously removed most heavy metals and selenium
by addition of sodium sulfide (and a polymeric organic flocculant). Pilot
trials show that approach works well (and behaves in accord with Phreeqc
modeling).

Next, I want to destroy the residual dissolved sulfide (by oxidation) and
sparge ammonia from the waste water by passing air through it.

When I modeled this with Phreeqc (and carefully ensure that nitrogen in the
air behaves as an inert gas and that ammonia is not oxidized by the oxygen),
I get what, intuitively to me, seems to be too little ammonia being
transferred to the gas phase, even when I make the 'gas bubble' passing
through one liter of water as much as 1000 liters! I have checked the
Phreeqc databases for the Henry's law constant for ammonia and they are
apparently correct.

Am I just fooling myself (with my 'intuition') and is my model correct?

Sorry to bother you with this. But, as you know, I'm a very big fan of
Phreeqc and I've had a lot of fun with it in recent years.

Regards
Steve

Dr Steve Short
steve@xxxxxxxxxxxxxxxxxxx
Director
Ecoengineers Pty Ltd
www.ecoengineers.com.au
Tel: (61) 2 42944652
Fax: (61) 2 42941948
Mobile: (61) 0410 560766

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