Figure D-4. Desorption isotherms for slope-derived and single-point distribution coefficients

Figure D-4. Desorption isotherms for slope-derived and single-point distribution

coefficients

The general form of the q versus C relationship for classical desorption

isotherms is as follows:

q = Kd C + qr

(D-2)

where qr is contaminant concentration in solid phase resistant to leaching, mg/kg

Nonconstant distribution of contaminants between dredged material solids

and water is commonly observed during leaching of estuarine sediments

(Brannon et al. 1989; Brannon, Myers, and Price 1990; Brannon et al. 1991).

Nonconstant contaminant partitioning yields batch isotherms for which the

distribution coefficient changes as the solid phase concentration q decreases

during sequential leaching, until a turning point is reached (Figure D-5). At the

turning point, the distribution coefficient becomes constant and desorption

begins to follow the classical isotherm. The nonconstant distribution coefficient

portion of the desorption isotherm is related to elution of salt.

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

Appendix D Leachate Testing Procedures