Consider competitive langmuir sorption with IFM-Plugin

I am new to IFM programming ... Does anyone have an idea how to implement the competitive Langmuir isotherm in Feflow via IFM-Plugin or dealt with a similar problem? At equilibrium the competitive Langmuir isotherm describes the concentration at the solid phase for two species in the following form:

[b]Cs(species 1) = k1*Cf(species 1)/(1+k2*Cf(species 1)+ k3*Cf(species 2))
Cs(species 2) = k4*Cf(species 2)/(1+k2*Cf(species 1)+ k3*Cf(species 2))
[/b]
with k1 to k4 being the parameters of the Langmuir isotherm and Cs and Cf being the concentrations in the solid and in the fluid phase, respectively.

Thanks a lot for your help and best regards.

The formulation you indicated differs from the formulation implemented in FEFLOW. Unfortunately, IFM does not provide an option to introduce the formulation you are interested in. A possible alternative is the two-site equilibrium/kinetic sorption with degradation as described in
<a href="https://www.mikepoweredbydhi.com/-/media/shared%20content/mike%20by%20dhi/flyers%20and%20pdf/product-documentation/feflow%20white%20papers%20vol.4.pdf">White Paper IV (Chapter 1.8.3 page 44)</a>