Page 67 - Materials Chemistry, Second Edition

50 Practical Design Calculations for Groundwater and Soil Remediation

Use Equation (2.21):

G = HC
8.62 mg/L = (1.08)C
So, C = 8 mg/L = 8 ppm

Discussion:
1. These two approaches yield identical results.
2. Henry’s constant of PCE is relatively high (five times higher than
that of benzene, 1.08 vs. 0.227).
3. A concentration of 8 mg/L of PCE in soil moisture is in equilib-
rium with a vapor concentration of 1,250 ppmV.
4. The numeric value of the vapor concentration (1,250 ppm) is much
higher than that of the corresponding liquid concentration (8 ppm).

2.4.3 Solid–Liquid Equilibrium
Adsorption. Adsorption is the process in which a compound moves from
liquid phase onto the surface of the solid across the interfacial boundary.
Adsorption is caused by interactions among three distinct components:

• Adsorbent (e.g., vadose zone soil, aquifer matrix, and activated carbon)
• Adsorbate (e.g., the COC)
• Solvent (e.g., soil moisture and groundwater)
In adsorption, the adsorbate is removed from the solvent and taken by the
adsorbent. Adsorption is an important mechanism governing the COC’s fate
and transport in the environment.
Adsorption Isotherms. For a system where solid phase and liquid phase
coexist, an adsorption isotherm describes the equilibrium relationship
between the liquid and solid phases. The isotherm indicates that the rela-
tionship is for a constant temperature.
The most popular isotherms are the Langmuir isotherm and the Freundlich
isotherm. Both were derived in the early 1900s. The Langmuir isotherm has
a theoretical basis that assumes monolayer coverage of the adsorbent surface
by the adsorbates, while the Freundlich isotherm is a semi-empirical rela-
tionship. For a Langmuir isotherm, the concentration on the soil increases
with increasing concentration in liquid until a maximum concentration on
the solid is reached. The Langmuir isotherm can be expressed as follows:
KC
=
SS max (2.22)
1 + KC
where S is the adsorbed concentration on the solid surface, C is the dis-
solved concentration in liquid, K is the equilibrium constant, and S max is the