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: In-Reply-To: <LHEFKENKAKOGPEDEAMADMEFOCDAA.steve@xxxxxxxxxxxxxxxxxxx> Subject: Slightly puzzled about an ammonia distribution between aqueous and gas phases 11/17/02 11:57 PM Please respond to 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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