Beginner to diffusion fractionation modelling

[AWI]

Hey,
I need to model some isotope fractionation due to diffusive transport like showing Chapter 27.3 of here: http://www.hydrochemistry.eu/pub/ap_br02.pdf

I'm absolutely new to Phreeqc and diffusive modelling and need some advice, how I can do this with Phreeqci.
I've got a fixed column with lenght z and diameter y, the starting isotope value of the source and the changing isotope concentrations of the sink over time.
The isotopes are for example 87Sr and 86Sr.

May anyone could explain it to me what I have to do with the programm?

Thanks!

[dlparkhurst]

Here is a pure diffusive calculation of a Sr Cl solution; constant boundary condition at the inflow; closed boundary condition at z = 1 m.

Note only one Sr species is defined 87Sr+2 and 86Sr+2. Multicomponent diffusion is used, so fractionation depends on the assigned diffusion coefficients of these Sr species, but also of Cl-l If you consider an carbonate and (or) sulfate system, you will have to deal with diffusion coefficients for additional aqueous species.

You don't give much information, but alternatively, you could assume that the fractionation is due to fractionation in the precipitation of a mineral (or both multicomponent diffusion and fractionation on precipitation). In the reference you give, I think Tony uses KINETICS to give different rates to the precipitation reactions for the different isotopes. If you assume equilibrium mineral precipitation, then fractionation can be modeled with slightly different log Ks for each major isotope; in either kinetics or equilibrium you probably need to consider 88Sr because it is the predominant Sr isotope and affects the saturation index for Sr minerals.

Code: [Select]

SOLUTION_MASTER_SPECIES
[Sr87] [Sr87]+2 0 Sr 87
[Sr86] [Sr86]+2 0 Sr 86
SOLUTION_SPECIES
[Sr87]+2 = [Sr87]+2
-gamma 5.260 0.121
-dw 0.794e-9  161
-Vm  -1.57e-2  -10.15  10.18  -2.36  0.860  5.26  0.859  -27.0  -4.1e-3  1.97 # ref. 1

[Sr86]+2 = [Sr86]+2
-gamma 5.260 0.121
-dw 7.9860E-10  161 # assuming D86 = (87/86)^0.5*D87
-Vm  -1.57e-2  -10.15  10.18  -2.36  0.860  5.26  0.859  -27.0  -4.1e-3  1.97 # ref. 1
END
SOLUTION 0
[Sr87] 0.7
[Sr86] 1.0
Cl     3.4 charge
END
SOLUTION 1-10
pH 7 charge
END
TRANSPORT
    -cells                 10
    -shifts                50
    -time_step             31557600 1 # seconds
    -flow_direction        diffusion_only
    -boundary_conditions   constant closed
    -lengths               10*0.1
    -dispersivities        10*0
    -print_cells           1-10
    -punch_cells           1-10
    -punch_frequency       10
    -print_frequency       100
    -multi_d               true 0 1 0.05 0
USER_GRAPH 1
    -axis_titles            "Distance, meters" "Sr87/Sr86" ""
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 graph_x dist
20 graph_y TOT("[Sr87]")/TOT("[Sr86]")
  -end
    -active                 true
END


[AWI]

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