Gas activities in methanogen medium

[jfholden]

I am interested in calculating the Gibbs energy available for hydrogenotrophic methanogenesis (4 H2(aq) + CO2(aq) -> CH4(aq) + 2 H2O) in my DSM 399 growth medium at the onset of growth. Do do this, I am trying to use GWB to calculate the activities of H2(aq) and CO2(aq) in my medium. I'll need to assume a small activity for CH4(aq) since we are not adding any to the medium. When I put the ionic species of my medium and the f H2(g) and f CO2(g) into REACT to get an equilibrium model and the activities, I get a message that says something like "N-R did not converge after 999th iteration". The problem seems to be the f CO2(g), since the model works if I replace it with a pH. Since this is a growth medium, the f H2(g) and f CO2(g) are both high, 0.49 and 1.86, respectively. Any suggestions or solutions? I'd also like to add S-- and NH4+ entries, but I cannot find anything to swap them with.

 

I am new to GWB and am using v.4, which may be outdated (I'm borrowing a colleague's computer with the software before purchasing my own new software). My entry into REACT is below.

 

Thanks in advance for any help or advice.

 

Jim

 

swap CO2(g) for H+

swap H2(g) for O2(aq)

f CO2(g) = 1.86

f H2(g) = 0.49

TDS = 25843.8

Na+ = 8.22832 g/l

K+ = 0.24116 g/l

Mg++ = 0.81840 g/l

Ca++ = 0.03817 g/l

Cl- = 12.54363 g/l

SO4-- = 3.17064 g/l

HPO4-- = 0.07715 g/l

HCO3- = 0.72633 g/l

 

The temperature is 85 C

[Brian Farrell]

Hi Jim,

 

One thing I would recommend is to decouple the redox couples in your redox reaction (H2(aq)/H+ and CH4(aq)/HCO3-) so that GWB can calculate the voltage for each half cell reaction. You can then solve for the deltaGr from the deltaEh. The decouple Carbon command will be useful to decouple all redox pairs with HCO3- (this prevents species like CH3COO- and O-phth from being considered in your calculation). After decoupling, you can add entries for both HCO3- and CH4(aq) (a very small amount). You'll want to get rid of the O2(aq) and add H2(aq), then swap in H2(g).

 

If you want to add S-- and NH4+, you'll probably want to decouple those redox pairs as well, not swap them.

 

What sort of pH value have you measured for your media? I find a pH of 5.848 if I swap in CO2(g) and balance on the HCO3-.

 

If your colleague has the Geochemical Reaction Modeling text, you should look at the chapter on Redox disequilibria for some helpful information.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC