Page 77 - Materials Chemistry, Second Edition

60 Practical Design Calculations for Groundwater and Soil Remediation

Discussion:
1. Although the sample volume is only 40 mL, the calculation basis
was 1 L to simplify the calculation.
2. With the presence of the headspace in the sample bottle, the appar-
ent liquid concentration was lower than the actual concentration.

Example 2.35: Mass Partition between Solid and
Liquid Phases in an Aquifer
The aquifer underneath a site is impacted by tetrachloroethylene (PCE). The
aquifer porosity is 0.4, and the (dry) bulk density of the aquifer material is 1.6
g/cm . A groundwater sample contains 200 ppb of PCE.
3
Assuming that the adsorption follows a linear model, estimate:

(a) The PCE concentration adsorbed on the aquifer material, which con-
tains 1% by weight of organic carbon.
(b) The partition of PCE in two phases, i.e., dissolved phase and
adsorbed onto the solid phase.

Solution:
(a) The PCE concentration adsorbed onto the solid has been deter-
mined in Example 2.31 as 0.50 mg/kg.
(b) Basis: 1-L aquifer formation
Mass of PCE in the liquid phase
= (C)[(V)(ϕ)] = (0.2)[(1)(0.4)] = 0.08 mg
Mass of PCE adsorbed on the solid
= (S)[(V)(ρ )] = (0.5)[(1)(1.6)] = 0.8 mg
b
Total mass of PCE = mass in the liquid + mass on the solid
= 0.08 + 0.8 = 0.88 mg
Percentage of total PCE mass in the aqueous phase
= 0.08/0.88 = 9.1%

Discussion:
Most of the PCE, 90.9%, in the impacted aquifer is adsorbed onto the
aquifer materials. This partially explains why the cleanup of aqui-
fers takes a long time using the pump-and-treat method.

Example 2.36: Mass Partition between Liquid and Solid Phases
A wastewater contains 500 mg/L of suspended solids. The fraction of organ-
ics of the solids is 1% by weight. The benzene concentration of the filtered
wastewater is determined to be 5 mg/L. K of benzene is 85 mL/g.
oc