Environmental Engineering Reference
In-Depth Information
•
(2) Upon oxidation of carbonate and bicarbonate by HO
, the carbonate radi-
•
-
) is produced (Eqs.
4.3
,
4.4
) in surface waters (Minella et al.
2011
) and
DOM is the key component to sink CO
3
cal (CO
3
•
-
by the reaction between them (Eq.
4.5
)
in surface waters (Buxton et al.
1988
; Canonica et al.
2005
).
K
4.3
=
3. 9
×
10
8
M
−
1
S
−
1
HO
•
+
CO
3
2
−
→
OH
−
+
CO
3
•−
(4.3)
HO
•
+
HCO
3
→
H
2
O
+
CO
3
•−
K
4.4
=
8. 5
×
10
6
M
−
1
S
−
1
(4.4)
DOM
+
CO
3
•−
→
DOM
+•
+
CO
3
2
−
K
4.5
=
10
2
(
MG C
)
−
1
S
−
1
(4.5)
These reactions are all dependent on pH since pK
a
of carbonate and bicarbo-
nate has 10.3 and 6.3 (Tossell
2005
). At around neutral pH, bicarbonate is domi-
nant over carbonate so HO radical scavenging is not so significant as to higher pH
(i.e., carbonate is more dominant).
(3) In seawater, HO
•
is mainly consumed by Br
−
ions (Zafiriou et al.
1984
;
Neta et al.
1988
; Goldstone et al.
2002
; Song et al.
1996
; Zafiriou et al.
1987
; Das
et al.
2009
). The reaction of HO
•
with Br
-
generates the oxidized bromine radical
•
-
(Eq.
4.7
)
(Goldstone et al.
2002
; Song et al.
1996
; Zafiriou et al.
1987
; von Gunten and
Oliveras
1997
; Grebel et al.
2009
).
•
-
) (Eq.
4.6
), which can be transformed into Br
.
and Br
2
species (BrOH
(4.6)
HO
•
+
Br
−
→
BrOH
•−
BR
−
(4.7)
BROH
−
+
H
+
→
BR
•
+
H
2
O
−→
BR
2
•−
+
H
2
O
Moreover, the bromine radical species could also react with DOM and induce
transformation reactions. The half-life of BrOH
•
-
is less than a few seconds, for
a H
2
O
2
concentration of 0.1 mg L
-1
in cloud water at pH 8. However, at pH 5 the
half-life of BrOH
•
-
is several hours for the same H
2
O
2
concentration (von Gunten
and Oliveras
1997
). A recent study shows that bromide scavenges the HO
•
radi-
cals formed upon photolysis of nitrate, before they leave the solvent cage, thereby
inhibiting the geminate recombination between the photogenerated HO
•
and
•
•
•
-
with nitrate
+
bromide
NO
2
. The result is that the formation rate of HO
+
Br
2
•
is higher than that of HO
with nitrate alone. Such an effect compensates for the
•
-
compared to HO
•
lower reactivity of Br
2
toward certain organic substrates, e.g.
phenol and tryptophan (Das et al.
2009
). It has also been shown that the radical
Br
2
•
-
is an effective brominating agent for phenol (Vione et al.
2008
).
(4) In stimulated polymorphonuclear leukocyte systems, a water extract from
green tea and green tea polyphenols had the strongest scavenging effect on HO
•
,
much stronger than vitamin C or vitamin E. Moreover, rosemary antioxidants and
curcumin have weaker scavenging effects than vitamin C, but a stronger one than
vitamin E (Zhao et al.
1989
).
