Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 372

E XAMPLE 6.30 N UMBER OF P ORE V OLUMES FOR I N S ITU F LUSHING OF
R ESIDUAL NAPL

Calculate the total number of pore volumes to remove 99% of the following pollutants
in porewater from an aquifer with porosity 0.3 and an organic carbon content of 1%.
Soil bulk density is 1.4 g/cm 3 : (a) benzene, (b) naphthalene, and (c) pyrene.
First calculate K oc from log K oc = 0.937 log K ow − 0.006 and K sw = f oc K oc . Then
calculate R F = ε + ρ b K sw . For 99% removal [A]/[A] 0 = 0.01.

Compound log K sw K sw R F N PV = ( − R F / ε ) ln ( 0.01 )

Benzene 2.13 1.6 2.5 38

Naphthalene 3.36 13.9 19.8 304

Pyrene 5.13 632.1 885 13,585

Extraction from the saturated zone is incapable of removing most contaminants
that have low solubility in water. These cases require excessively large volumes of
water to be pumped (see Example 6.30). As a result, techniques have been sought to
enhance P&T. In Chapter 4, we noted that the use of surfactants significantly lowers
the surface tension at oil-water interfaces and also increases the aqueous solubility of
organic compounds through the presence of surfactant micelles. Both of these aspects
have been utilized in enhancing the extent of removal using P&T methods.
Figure 6.62 identifies the different phases into which a pollutant can partition
when surfactants are present in the aqueous phase in the form of micelles. If a CSTR
is assumed for the aquifer, the overall mass balance with added surfactant in the

Micelle in water Water

Monomer

Pollutant in
micelle

Pollutant in
water

Adsorbed
surfactant

Adsorbed
pollutant

Water

Solid

FIGURE 6.62 Partitioning of a pollutant between various compartments in the groundwater
when surfactants are introduced into the subsurface.