[Gas activities in methanogen medium]

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I am new to GWB and am using v.4, which may be outdated (I'm borrowing a colleague's computer with the software before purchasing my own new software).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