Figure 6. Desorption isotherm illustrating constant and nonconstant partitioning

ERDC TN-DOER-C16

July 2000

As salt is eluted from estuarine sediments, the ionic strength of the aqueous phase is reduced.

According to the Gouy-Chapman model of charge distribution in double layers, decreasing the ionic

strength increases repulsive forces (Stumm and Morgan 1981) and causes the double-layer thickness

between colloids to increase. Flocculated colloidal matter becomes increasingly deflocculated and

more easily entrained in flow. The overall effect is an increase in dissolved organic carbon (DOC)

concentrations in the aqueous phase, mobilizing metals and organic contaminants bound to the

colloidal matter (Brannon et al. 1991). For these reasons, the type of desorption isotherm shown

in Figure 6 is referred to as a DOC-facilitated desorption isotherm. Since the relationship of q versus