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HPO
2
−
+
•
OH HPO /PO
→
•−
• −
2
+
OH /H O
−
k
=
8 0 10
.
×
5
/M/s
(5.68)
4
4
4
2
68
HPO
2
−
+
SO
•−
→
HPO
•−
+
SO
2
−
k
=
1 2 10
.
×
6
/M/s
.
(5.69)
4
4
4
4
69
The photolysis of the phosphate ion also yields phosphate radicals (Eq.
5.70) [387, 388]:
HPO
2
−
+
h
ν λ
(
<
200
nm
)
→
HPO
•−
+
e
−
.
(5.70)
4
exc
4
aq
Phosphate radicals exist in three acid-base forms (Eqs. 5.71, 5.72) [386]:
•
+
•−
(5.71)
H PO
H HPO
+
p
a
K
=
5 7
.
2
4
4
1
•−
+
• −
4
2
(5.72)
HPO
H PO
+
.
p
a
K
=
8 9
.
4
2
Phosphate radicals have broad absorptions in the 400- to 600-nm range:
H PO
2
•
(λ
max
520 nm, ε
max
1850/M/cm),
HPO
•−
(λ
max
510 nm, ε
max
1550/M/cm),
and
PO
4
• −
(λ
max
530 nm, ε
max
2150/M/cm) [386].
The decay kinetics of the three acid-base forms of phosphate radicals have
been studied [389]. The kinetics results were independent of the dissolved
oxygen in the solution. Bimolecular rate constants (2
k
/ε
500 nm
) for the decay of
HPO
•−
and
PO
4
• −
radical ions were determined as 1.3 × 10
5
and 2.8 × 10
5
cm/s,
respectively [388, 389]. The kinetics of
H PO
2
4
•
was determined to be mixed first
and second orders with rate constants of 5 × 10
3
/second and 1.0 × 10
6
cm/s,
respectively.
4
5.4.1.2 Reactivity.
The kinetics of the reactions of phosphate radicals with
a number of organic compounds has been performed [363]. The reactivity of
H PO
2
•
radicals were determined to be higher than that of
HPO
•−
and
PO
4
• −
by a factor of ∼4 to 10. The reactivities of
HPO
•−
and
PO
4
• −
were similar. Phos-
phate radicals reacted with compounds by the abstraction of hydrogen atoms,
and the rate constants ranged from ∼10
5
/M/s for acetic acid and 2-methyl-2-
propanol to ∼10
8
/M/s for 2-propanol and formate. The rate constants for the
reactivity of phosphate radicals with aromatic compounds were in the range
of ∼10
8
to 10
9
/M/s [382, 385, 386, 390]. Similar to
SO
•−
radicals, the
H PO
2
4
•
radical reacted with the aromatic ring by a one-electron transfer to the inor-
ganic radical and also produced phenoxy radicals in reactions with phenolic
derivatives of α, α, and α-trifluorotoluene and flavanoid [382, 390]. Compara-
tively, the singlet oxygen (
1
O
2
) involved the charge transfer mechanism and
was found to be much less reactive with undissociated trifluoromethylphenols
than the
HPO
•−
. However, products obtained using
1
O
2
were more oxidized
than those observed (e.g., biphenyl) in the oxidation by
HPO
•−
. These results
are significant in wastewater treatment of phenol in which advanced oxidation
process is combined with biological mineralization [385].
The rates of hydrogen abstraction reactions of
SO
•−
and
H PO
2
4
•
radicals
with aromatic compounds were ∼10 to 100 times lower than those of the
•
OH radical. The general reactivity of radicals can be expressed as
4
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