105

but Sand 1 and 3 showed a concave non-linearity (i.e., 1/n>1). An example of
model fits using Method 1 and Freundlich model for Sand 10 is shown in Fig. 5.8.
The non-linearity is expected at low concentrations, but the isotherm is linear at
the equilibrium concentrations ranging from 1 mg/L to 17 mg/L. This type of
isotherm corresponds to sand with a high affinity for TCE at low concentrations,
and lower affinity at higher concentrations.
Moderate affinity for TCE at higher concentrations is characteristic of a
high surface-area carbonaceous material (HSACM), such as charcoal-like
substances. Chiou et al. (2000) found that HSACM was responsible for non-
linearity in TCE adsorption on a peat. The isotherms of the tests on foundry
sands are similar to those of Chiou et al. (2000), with high sorption affinity at low
concentrations and a moderate affinity at higher concentrations. The similarity of
the foundry sand isotherms and those reported by Chiou et al. is probably due to
the presence of "sea coal," a powdered coal additive in foundry sands, which can
be considered to be a HSACM.
The partition coefficients obtained from the batch sorption tests are plotted
against TOC in Fig. 5.9. A linear relationship is observed for TOC ranging from
1 % to 3.5%, which is characteristic of most foundry sands (Table 3.2). An
empirical equation can be expressed to describe the relationship between Kp and
TOC in the moderate TOC ranges (Fig. 5.9b):
Kp=4.76 TOO                     (5.2)
where Kp is in L/kg and TOO is in percent. The R2 for Eq. 5.2 is 0.93.