# Re: Slightly puzzled about an ammonia distribution between aqueous and gasphases

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I think your instincts were wrong (if it makes you feel better, I would
have expected a greater partition to the gas phase as well). As far as I
can see the calculation is correct. A little checking with a hand
calculation is also consistent. The K for NH3(g) -> NH3(aq) is 10^1.8 ~ 60.

So 60 ~ n(aq)/P(NH3), for a liter of water. From the ideal gas law P(NH3)=
n(g) RT/V, where V is 1000, R is .082, and T = 293. Therefore,
P(NH3) = n(g)*.024 and 60 ~ 40 * n(aq)/n(g) or 1.5 ~ n(aq)/n(g). In other
words the ratio of moles in the aqueous phase to moles in the gas phase is
on the order of 2 for 1000 liters of gas. There are some ionic strength
effects on the activity coefficients and some aqueous speciation, but the
PHREEQC calculation is pretty similar.

Looks like it would take 10-100 times more sparging to get most of the
ammonia out (if it is not oxidized along with the H2S).

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled

"Steve Short"
<steve@xxxxxxxxxx        To:       "David Parkhurst" <dlpark@xxxxxxxx>
rs.com.au>               cc:
Subject:  Slightly puzzled about an ammonia distribution between aqueous and gas phases
11/17/02 11:57 PM
steve

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
(See attached file: WGEPc)(See attached file: WGEPc.out)

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Attachment: WGEPc
Description: Binary data

Attachment: WGEPc.out
Description: Binary data

• References:

Complete Water Resources Division Software