Page 268 - Materials Chemistry, Second Edition

Groundwater Remediation 251

ozone. Each mole of hydrogen peroxide in water can dissociate into 0.5 mole
of oxygen and 1 mole of water, while 1 mole of ozone in water can dissociate
into 1.5 moles of oxygen as:

H O → H O + 0.5 O 2 (6.28)
2
2
2
O → 1.5 O 2 (6.29)
3
Ozone is 10 times more soluble in water than pure oxygen. Hydrogen per-
oxide and ozone can be added to the extracted groundwater before it is put
back to the impacted aquifer. It should be noted that hydrogen peroxide and
ozone are also strong oxidants. In addition to providing oxygen for biodegra-
dation, they can also generate radicals to oxidize COCs and other inorganic
and organic compounds present in the aquifer. However, at higher concen-
tration levels, they may become toxic to indigenous aerobic microorganisms
and suppress their biological activities [3].
Various enhanced in situ biodegradation approaches rely on oxygen-
releasing compounds. The common ORCs include calcium and magne-
sium peroxides that are introduced to the saturated zone in solid or slurry
phases. These peroxides release the oxygen to the aquifer when hydrated
by groundwater. Magnesium peroxide has been more commonly used
than calcium peroxide due to its lower solubility and prolonged release
of oxygen. Oxygen amounting to ≈10% of the mass of magnesium perox-
ide placed in the saturated zone is released to the aquifer over the active
period [3].

Example 6.14: Determine the Necessity of Oxygen Addition
for In Situ Groundwater Bioremediation

A subsurface is impacted by gasoline. The average dissolved-gasoline con-
centration of the groundwater samples is 20 mg/L. In situ bioremediation
is being considered for aquifer restoration. 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
• DO concentration in the aquifer = 4.0 mg/L

Illustrate that the addition of oxygen to the aquifer is necessary to support
biodegradation of the intruded gasoline.