Page 220 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 203

reaction can occur. All the oxidants in this table have a stronger oxidation
power than that of oxygen.
To come up with a reaction equation for oxidation of a COC, we need to
have another half-reaction to describe the oxidation of the COC. Let us use
perchloroethylene (PCE) (C Cl ) as an example:
2
4
+
C Cl + 4H O → 2CO + 4Cl + 8H + 4e − (5.41)
−
4
2
2
2
We then multiply Equation (5.34) by 4 and Equation (5.41) by 3, and add them
up to get
3C Cl + 4MnO + 4H O → 6CO + 12Cl + 4MnO + 8H + (5.42)
−
−
2
2
2
2
4
4
Equation (5.42) shows that the stoichiometric requirement to oxidize PCE is
4/3 mole permanganate per mole of PCE. Using the same approach, the oxi-
dation of trichloroethylene (TCE) (C HCl ), dichloroethene (DCE) (C H Cl ),
2
3
2
2
2
and vinyl chloride (C H Cl) can be derived as [8]:
3
2
CHCl + 2MnO → 2CO + 3Cl + 2MnO + H + (5.43)
−
−
3
2
2
4
3C H Cl + 8MnO → 6CO + 6Cl + 8MnO + 2OH + 2H O (5.44)
−
−
−
2
2
2
4
2
2
2
3C H Cl + 10MnO → 6CO + 3Cl + 10MnO + 7OH + H O (5.45)
−
−
−
2
2
4
2
3
2
As shown, the stoichiometric requirements for TCE, DCE, and vinyl chlo-
ride are 2, 8/3, and 10/3 moles permanganate per mole of COC, respectively. If
other oxidants are used, the stoichiometric requirements would be inversely
proportional to the ratio of the “electrons accepted” of two oxidants listed in
Table 5.3. For example, the stoichiometric requirement of sodium persulfate
will be 1.5 times that of potassium permanganate because 1 mole of perman-
ganate can accept 3 moles of electrons, while 1 mole of persulfate can only
accept 2 moles of electrons.
In addition to the oxidant demand from COCs, the added oxidants will
also be lost due to subsurface reactions unrelated to oxidation of COCs, often
referred to as the natural oxidant demand (NOD). NOD stems from reactions
with organic and inorganic chemical species that are naturally present in the
subsurface. Consequently, the total oxidant demand should be the sum of the
NOD and the demand from target COCs as:
Total oxidant demand = natural oxidant demand
+ demand from target COCs (5.46)
NOD almost always exceeds the oxidant demand from target COCs. NOD
has a significant impact in determining if the ISCO is economically feasible
and in engineering the applied oxidant dose. Bench- and/or pilot-scale test-
ing should be conducted to determine the NOD for a project.