Geochemist's Workbench Support Forum

# About calculating Kd value

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Hello,

We have a question about Kd value as following:

In the known environment (EX: known granite composition, temperature, pressure, pH, Eh value, etc.), is there a way to calculate the Kd value of a particular nuclear species (EX: Tc)?

Thanks a lot.

John

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Hi John,

A distribution coefficient, or Kd, is simply the ratio under specific conditions of the sorbed to the dissolved mass of a contaminant. In other words, the Kd as originally described is an observation; it is determined from measurements at a specific field site or in an experiment.

A field or lab-measured Kd is sometimes used in a predictive sense. In the “Kd approach”, you solve, for example, for the amount of an ion sorbed to mineral surfaces given the Kd value and information about how much of the ion is dissolved in the fluid. Such an approach can attempt to predict the retardation of an ion in flowing groundwater. In other words, the Kd is used here as input to a geochemical model. From your description, it sounds like you want a Kd that is valid for a certain environment (known granite composition, temperature, pressure, pH, Eh, etc.) to use in a predictive sense. Unfortunately, the Kd must be measured for the site; it cannot be calculated solely from information about the composition and temperature of the system.

I have one final point that isn’t directly related to your question, but I should point it out in case you discover it while searching through the documentation. As I mentioned before, the GWB can create predictive models of sorption in geochemical environments. The predictive models need as input a measure Kd or measured constants for other surface models like the Freundlich or Langmuir isotherms, ion exchange model, or the two-layer surface complexation model (or some combination of multiple sorption models). Just like we can measure a Kd in a real environment, we can measure a Kd in a simulated environment, assuming we have completely described the mineral mass of the system. To have React calculate a Kd based upon the distribution of species and the mineral mass, go to the Config – Iteration dialog and check the “Kd” option. For more information, please see 6.51, Kd, in the GWB Reference Manual.

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

Edited by Brian Farrell

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