C-14 sparging by CO2 injection

[DivyeshTrivedi]

Hi all,

I am trying to put together a model for a laboratory experiments where CO2 sparging is being used to remove C-14 from solution. The problem in trying to do this in PHREEQC is that I think there might be no significant chemical driving force; pH 7 solution sparged with CO2(g) but the C-14 is also present as inorganic carbonate so there is only a limited chemical effect due to acidification of the solution at the higher injected CO2(g) partial pressure.

So far my thoughts are along the lines of a RATE/KINETICS mass-transfer reaction that assumes that the amount of the C-14 dissolved inventory that is released into the (expanding) gas headspace is proportional to a kinetic rate of gas sparging (in litres/sec of gas). I would define the C-14 as a new master species as I don't think the isotope fractionation capability will help here. I have toyed with the fixed pressure/expanding gas volume capability in GAS_PHASES keyword but I think that will only help me if I want to calculate the post-sparging concentration of C-14 accumulating in the expanding gas volume in the off-gas collection reservoir. 

Is it even appropriate to model such a process in PHREEQC? Is the RATE/KINERICS approach the way to go?

Any thoughts appreciated,
Divyesh 

   

[dlparkhurst]

My first thought is to use a sequence of GAS_PHASE reactions. Define C-14 in SOLUTION using iso.dat, and then add say a liter of 13CO2 GAS_PHASE (but including zero 14CO2 as a component) and let it equilibrate. 13CO2 and 14CO2 will equilibrate with the solution. Then repeat with the 13CO2 GAS_PHASE. Don't know if this is close enough or not.

[DivyeshTrivedi]

Thank you very much: I will have a try at this approach, Divyesh

[dlparkhurst]

If you need the Pitzer formulation for higher ionic strength, you would need to define a new element for 14CO2. You could probably get by with simply duplicating the definitions for C(4) from pitzer.dat.