Determining species, concentrations, and physicochemical
parameters
in natural waters is key to improve our understanding of the functioning
of these ecosystems. Diffusive Gradients in Thin-films (DGT) devices
with different thicknesses of the resin or of the diffusive disc can
be used to collect independent information on relevant parameters.
In particular, DGT devices with a stack of two resin discs offer a
simple way to determine dissociation rate constants of metal complexes
from the accumulation of the target metal in the back resin disc.
In this work, simple approximate expressions for the determination
of the dissociation rate constant are reported and applied to a model
Ni nitrilotriacetic complex as well as to Zn complexes in the Mediterranean
Osor stream. Once the physicochemical parameters are known, one can
plot the labile fraction of the metal complexes in terms of the thickness
of the diffusion domain. These plots reveal a strong dependence on
the nature of complexes as well as on the characteristics of the diffusion
domain, and they are of high interest as predictors of availability
to biota whose uptake is limited by diffusion.