Equilibrium Assumption

ERDC TN-DOER-C16

July 2000

Equilibrium Assumption. In practice, a true equilibrium between dredged material solids and

pore water never exists because some of the processes shown in Figure 3 have very slow reaction

rates. However, a pseudo-steady-state can be reached between dredged material solids and water

if the water is moving past the solids slow enough, as discussed in a following section. Assuming

equilibrium between solid and aqueous phases eliminates the need for determining controlling

processes and the rate coefficients for these processes. Without the equilibrium assumption,

laboratory testing and mathematical modeling would require determination of controlling processes

and investigation of the kinetics for these processes. As is apparent from Figure 3, predictive

laboratory tests and mathematical models based on chemical and mass transfer kinetics would be

too complicated for routine application to dredged material leaching. Thus, application of the

equilibrium assumption is imperative for the development of predictive techniques suitable for

routine use.

Once equilibrium has been reached, only the relative distribution of contaminant between solid and

aqueous phases is needed to predict leachate quality.

Kd =

(1)

where

Kd = equilibrium distribution coefficient, L/kg

q = solid-phase contaminant concentration at equilibrium, mg/kg

C = aqueous phase contaminant concentration at equilibrium, mg/L

Equation 1 describes the equilibrium distribution of a single contaminant in a dredged material; that

is, equilibrium distribution coefficients are contaminant and dredged material specific. As will be

discussed in a later section, Kd is affected by various factors (sediment oxidation status, pH, and

ionic strength). Varying these factors during leaching can shift the equilibrium position of the

system and change Kd.

Laboratory Testing. Laboratory testing procedures have been developed to quantify the

dredged-material-specific equilibrium distribution coefficients considering changes in many of the

sediment geochemical factors. A sequential batch leach test (SBLT) has been recommended for

leachate testing of freshwater sediments (Brannon, Myers, and Tardy 1994). Because major

differences exist in the leaching characteristics of freshwater and estuarine sediments, prediction of

leachate quality for estuarine sediments using the SBLT is difficult.

A thin-layer column leach test has been developed to simulate contaminant leaching in CDFs

(Myers, Brannon, and Tardy 1996). This test is recommended for leachate testing of estuarine

sediments that are dredged and disposed in CDFs for which the primary source of water for leaching

is low in ionic strength (e.g., rainfall) relative to the dredged material initial pore water ionic strength.

Leaching of estuarine sediments and dredged materials with low-ionic-strength water results in

destabilization of the colloidal system as salt is washed out. Colloids are released and, along with

the colloids, colloid-bound contaminants.