Page 281 - Materials Chemistry, Second Edition

264 Practical Design Calculations for Groundwater and Soil Remediation

(b) As shown in part (a), one mole of Mg(OH) is formed for each
2
mole of Mg removed. Since the molecular weight of Mg(OH) is
2+
2
equal to 58.3, the rate of Mg(OH) produced can be found as:
2
2+
Rate of Mg(OH) produced = (Rate of Mg removed)(58.3/24.3)
2
2+
= {[Mg ] − [Mg ] } × (Q) × (58.3/24.3)
2+
in
out
= [(100 − 0.22) mg/L] × [(150 gpm)(3.785 L/gal)] × (58.3/24.3)
= 136,000 mg/min = 136 g/min = 431 lb/day
(c) Since the solids are settled to 1% by weight, the rate of sludge
production can be found as:
Rate of sludge produced = Rate of Mg(OH) produced ÷ 10%
2
= 431 lb/day ÷ 10% = 4,310 lb/day

6.10 In Situ Chemical Oxidation

In situ chemical oxidation (ISCO) involves the introduction of a chemical oxi-
dant into the subsurface to transform COCs in soil or groundwater into less
harmful compounds. ISCO is predominantly used to address COCs in the
source area so that the mass flux to the groundwater plume can be reduced.
Consequently, it can shorten anticipated cleanup times for natural attenua-
tion and other remedial options [3].
The approaches of using ISCO for impacted aquifers are essentially the
same as those for the vadose zone. For the background information on types
of oxidants as well as oxidant demands, please refer to Section 5.6 for details.
The main difference between groundwater remediation versus vadose zone
remediation is that the applied oxidant needs to be delivered to the saturation
zone, in which all the pore space is filled with water. This section illustrates
one example that is related to application of ISCO to the saturated zone.

Example 6.21: Determine the Stoichiometric Amount of Oxidant

The underlying aquifer at a site is impacted by perchloroethylene (PCE). The
source area for the groundwater plume has been determined to be a zone with
an area of 20 m and a thickness of 2 m within the aquifer. The average PCE
2
concentration of groundwater samples taken from this zone is 400 mg/L.
The aquifer has the following characteristics:
• Porosity = 0.35
• Organic content = 0.02
• Subsurface temperature = 20°C
• Dry bulk density of aquifer materials = 1.6 g/cm 3