Page 212 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 195

Discussion:
The cost of nutrients is relatively low compared to other project
expenses.

5.4.4 Oxygen Requirement
For soil bioremediation, the oxygen involved in the biological activity is
often supplied through the oxygen in the air. Oxygen is approximately 21%
by volume in the ambient air. On the other hand, oxygen is sparingly soluble
in water. At 20°C, the saturated dissolved oxygen (DO ) in water is only
sat
about 9 mg/L.
Let us use the following simplified scheme to demonstrate the oxygen
requirements:

C + O 2 → CO 2
moles: 1 1 1
mass (gram, kg, or lb): 12 32 44

This simplified scheme illustrates that each mole of carbon element requires
one mole of oxygen molecule, or every 12 g of carbon requires 32 g of oxygen,
a ratio of 2.67. Other elements in the COCs, such as hydrogen, nitrogen, and
sulfur, would also demand oxygen for bioremediation. For example, the the-
oretical amount of oxygen required to aerobically biodegrade benzene can
be found as:

C 6 H 6 + 7.5O 2 → 6CO 2 + 3H 2 O
moles: 1 7.5 6 3
mass (gram, kg, or lb): 78 240 264 54

This indicates that each mole of benzene requires 7.5 moles of oxygen mol-
ecule, or every 78 g of carbon requires 240 g of oxygen, a ratio of 3.08, which
is larger than 2.67 based on pure carbon. Using benzene as the basis, this
means that every gram of hydrocarbon requires approximately 3 grams of
oxygen for aerobic degradation. It should be noted that this is the theoretical
ratio based on the stoichiometric relationship. A larger amount of oxygen
would be needed. Using this ratio, the amount of oxygen in an aqueous solu-
tion can only support biodegradation of COCs at a concentration of 3 mg/L
or less, even if the water is saturated with dissolved oxygen. However, the
dissolved oxygen (DO) concentration in the soil moisture would be lower
than its saturation value.