Conceptual Models > Database selection and modification

methane carbon isotopes fractionation

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Mr.white:
I am new to PHREEQC and currently working on a project to study the impact of diffusion on carbon isotope fractionation. I am trying to understand how to implement carbon isotopes fractionation based on a 2021 paper by Diana B. Loomer and others. The paper mentions adding 13C as a new species in the database and using fractionation factors to simulate carbon isotope fractionation.
My questions are as follows:
Should I add both 13C and 12C as new species in the database?
If I am only focusing on diffusion and not considering any redox reactions, do I need to add any reactions for these isotopes in the solution species section, or should I set the logk of such reactions to zero?
Is there a need to include specific reactions in the phases section?
How can I set the fractionation factors in PHREEQC simulations to achieve isotopic fractionation?
Any guidance or step-by-step explanation on how to proceed with these settings in PHREEQC would be greatly appreciated.

dlparkhurst:
It sounds like you are suggesting the approach detailed in the following reports, where 12C and 13C are treated as separate "elements". Each isotope reacts independently from the other, and the equilibrium constants for carbonate aqueous, solid, and gas differ slightly. The slight differences result in fractionation.

Note that diffusion coefficients are not defined, and modeling diffusion processes would require additional work on your part.

Thorstenson, D. C., & Parkhurst, D. L. (2002). Calculation of individual isotope equilibrium constants for implementation in geochemical models. Water-resources investigations report, 2, 4172.

Thorstenson, D. C., & Parkhurst, D. L. (2004). Calculation of individual isotope equilibrium constants for geochemical reactions. Geochimica et cosmochimica acta, 68(11), 2449-2465.

The database iso.dat (distributed with PHREEQC programs) implements the approach of the two papers and allows calculation of isotopic effects. In it the element "C" represents 12C, whereas 13C is defined as the element "[13C]". Aqueous species CO2, HCO3-, CO3-2 and others have analogues [13C]O2, H[13C]O3-, [13C]O3-2, and so on, potentially with slightly different log Ks.

The following file calculates the gas isotopic composition that would be in equilibrium with a pH 2 water with -7 permil carbon. The gas composition is -6.2 permil.


--- Code: ---SOLUTION 1
        pH      2
        C       1       CO2(g)  -3.4
        [13C]   -7
        Cl      1       charge
END
RUN_CELLS
-cell 1
END
USE solution 1
GAS_PHASE
        -vol    .000001
        -fixed_volume
        CO2(g)                  0
        [13C]O2(g)              0
END

--- End code ---

It has been a long time since I worked with this approach. The file iso.dat is fairly long and involved with the way fractionation factors are specified and isotopic ratios are calculated. Take a look and if you don't simply ignore the approach as inappropriate or too complicated, I can respond if you have more questions.

Mr.white:
Thank you for your reply, but I did not clearly explain the type of isotope; it is the carbon isotope of methane, not carbon dioxide. The literature describes it like this:
Isotopic solubility fractionation was implemented in PHREEQC by adding 13C1to the database as a separate species. The differences in 12C1 and 13C1 log K were based on published fractionation factors (αlogK) in the form:αlogK=([δ13C1]/[δ12C1])aq/([δ13C1]/[δ12C1])g.However, solubility fractionation is temperature dependent, with α decreasing from 1.00062 to 1.00034 between 20℃and 80℃ (Harting et al., 1976). Therefore, the experimental data of Harting et al. (1976) were fit to Equation 1 with two fitting constraints: that α recalculated = α experimental and that log K for 13C1 was greater than or equal to that of 12C1(Equation 1 is temperature dependent equation for solubility: logk=A1+A2*T+A3/T+A4*logT+A5/T2A6*T2).And then the equation of state (EOS) parameters for 13C1 were set equal to those of 12C1. To track small differences in 13C1.
My understanding is that using the research data of predecessors, calculate the logK for 13C and 12C, and then add the data of 12C and 13C to the solution master species and phase in the PHREEQC database. Besides, do I need to add related reactions about 12C and 13C in the solution species? Because I only consider the impact of diffusion on isotopic fractionation, not considering other oxidation-reduction processes (only dissolution and diffusion). After adding the data of 13C and 12C, how do I implement isotopic fractionation in PHREEQC using the fractionation factor? The literature also mentions setting the equation parameters of 13C and 12C consistently, what does this mean? I don't quite understand, thank you very much for your answer, it is very helpful for my next research.

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