There are four media properties that must be specified for each species in the
model:
• Solubilities
in the Reference Fluid (and optionally, for other fluids);
• Partition
Coefficients for other Fluids and Solids;
• Relative
Diffusivities for Fluids; and
• Available
Porosities for Solids.
If you define multiple isotopes of the same element,
GoldSim treats these four properties in a special way: isotopes of the same
element are assumed to have equivalent properties. If the properties are
defined by element (the default), this is straightforward (as isotopes of the
same element are represented by a single entry). However, you can
optionally define these properties individually for each species. In this
case, it is possible that they could be entered inconsistently (i.e., isotopes
of the same element could be assigned different values).
In this case, within a Fluid or Solid element, GoldSim forces the
properties for isotopes of the same element to be equivalent. In
particular, all isotopes of the same element take on the properties of the
first of the element's isotopes in the species array label
set. For example, if the species array label set included U234,
U235, and U238 (in that order), all three species would use the properties
assigned to U234, regardless of what was specified for the other two
species.
Note: When specifying
solubilities, GoldSim requires that the molar solubilities of all
isotopes of an element are identical. By default, solubilities are
specified as molar solubilities. Note, however, that solubilities can optionally
be specified on a mass basis (e.g., mg/l). In this case, the mass-based
solubilities of isotopes of an element should be specified as being proportional
to the atomic weights of the isotopes. For example, if you have two
isotopes of an element with atomic weights 238 and 234, and the solubility of
the first isotope is specified as 1 mg/l, then the solubility of the second
isotope should be specified as (234/238) mg/l. If it is not, GoldSim will
automatically correct the problem (using the molar solubility defined by the
first isotope), and will write a warning message to the Run Log.
An important effect of defining multiple isotopes of the
same element is related to how solubilities are computed in Cell pathways (which are the only type of
transport
pathway where solubility constraints can be applied). In particular,
when computing solubility constraints in Cell pathways, the solubility is
"shared" by (split among) multiple isotopes of the same element.
Note: Because solubilities are
“shared” between isotopes of the same element, it is of critical importance that
you include all isotopes that may be present to a significant degree in the
system. This includes any stable isotopes (even if you are not interested in
their concentrations due to the lack of radiological impacts). This is because
if the stable isotope is present in significant quantities it could “claim” a
portion of the solubility. Ignoring the stable isotope in such a case
could significantly overestimate the amount of radioisotopes of that element in
solution.
Related Topics…
Learn more about:
Simulating Solubility Limits in
Cells