Chemistry Reference
In-Depth Information
Initiating reactions (4.41) and (4.42) suggest that the decomposition of
ozone can be increased by raising the pH or by adding hydrogen peroxide.
Recent studies on the decomposition of O
3
have estimated the value of
k
41
to
be (1.7-1.8) × 10
2
/M/s [220, 221]. The O
3
decomposition produces
•
OH by a
rapid reaction (4.45). Reaction (4.47) produces other oxidant species, super-
oxides. The generated
•
OH reacts with carbonate species (
HCO
−
and
CO
2−
)
and other constituents of water. A half-life of O
3
has thus a range of seconds
to hours depending on the pH and concentrations of carbonate species and
organics in water [211].
4.3.1 Reactivity
The reactivity of O
3
with a wide range of inorganic and organic compounds
has been reported [222]. The second-order rate constants for the reactions vary
widely between <0.1 and 7 × 10
9
/M/s. The inorganic compounds such as Fe(II),
Mn(II), H
2
S, cyanide, and nitrite reacted rapidly with O
3
through an oxygen
transfer mechanism. The reactions with organic compounds proceeded through
well-defined mechanisms involving double bonds, activated aromatic systems,
and neutral amines [213] In the water treatment process, the reactions of O
3
with components of water matrix compete with the reactions of ozone with
desired pollutants. Therefore, only the pollutants resulting in high rate con-
stants with O
3
can be removed by direct direction [212]. The kinetics of the
oxidation of amino acids by O
3
in aqueous solution have been determined as
a function of pH at 22-24°c [215, 223-229]. Second-order rate constants for
the oxidation of selected amino acids by O
3
at different pH values are provided
in Figure 4.14, which show an increase in rate constants with an increase in
pH. Since carboxylic acids and protonated forms of amino acids reacted slowly,
the variation in rates with pH was analyzed using the dissociation of the amino
group of amino acids (HA
H
+
+ A
−
;
K
a2
) [15]. The mathematical interpreta-
tion of the data considers the reactions of O
3
with undissociated (HA) and
dissociated (A
−
) forms of amino acids separately [223, 228]. The rate constants
for the reaction of O
3
with HA (
k
HA
) and A
−
(
k
A
) were obtained by plotting
k
versus (
K
a2
/[H
+
]). The results of the oxidation of gly and Pro by O
3
are
shown in Figure 4.15. The slope of the plot gave the value of
k
A
while the
intercept gave the value of
k
HA
. Values of
k
HA
near zero indicated the undis-
sociated amino group of amino acids did not react significantly with O
3
in
aqueous solutions. Therefore, the dependence of rate with pH was largely
related to the reaction of the free amine form of amino acids with O
3
.
The rate constants for various amino acids, except cys, Met, and Trp, which
reacted with rate constants, varied from 2.6 × 10
4
/M/s to 4.4 × 10
6
/M/s [223].
The order of reactivity was determined as glu ≈ gln ≈ Lys < Asp ≈ Asn ≈
Thr < Leu ≈ Ile ≈ Arg < Val < Ala < Ser < gly ≈ His < Phe < Pro. The high
reactivity of cys with O
3
(Fig. 4.14) indicates the possible site of reaction was
at the sulfhydryl functional group rather than the amino group. The measured
rate constants shown in Figure 4.14 involved a change of the neutral form of
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