Environmental Engineering Reference
In-Depth Information
As far as NO
3
-
photolysis is concerned (Eq.
3.8
), the process can be expressed
as follows (Zepp et al.
1987
; Mack and Bolton
1999
):
(3.8)
NO
3
+
h
υ →
NO
2
+
1
2O
2
•
scavengers, over the
entire pH range and at irradiation wavelength around 200 nm (Shuali et al.
1969
;
Wagner et al.
1980
). However, irradiation above 280 nm results into two primary
photoinduced pathways (Zepp et al.
1987
; Mack and Bolton
1999
):
This stoichiometry can be followed in the absence of HO
NO
3
+
h
υ →
NO
3
∗
(3.9)
(3.10)
[NO
3
-
]
*
NO
2
-
+ O(
3
P)
H
2
O
NO
2
•
+ O
•
-
HO
•
+ HO
-
+ NO
2
•
(3.11)
In this mechanism, NO
3
-
absorbs a UVB photon yielding an excited state,
[NO
3
-
]
*
(Eq.
3.9
), which undergoes disintegration following two pathways: the
first one produces the nitrite ion (NO
2
-
) and atomic oxygen, O(
3
P) (Eq.
3.10
).
The second pathway produces nitrogen dioxide (NO
2
•
•
-
. The latter is
) and O
•
(Eq.
3.11
). The formation of NO
2
-
in Eq.
3.10
can be followed by nitrite photolysis to give HO
rapidly protonated to form HO
•
, as shown in (Eqs.
3.3
-
3.5
).
•
It can be noted that HO
is a strong oxidant that can react with DOM more
quickly than does atomic oxygen, O(
3
P). Indeed, the main fate of O(
3
P)
(Eq.
3.4
) would be the reaction with oxygen to form ozone, which is rapidly
consumed in natural waters by reaction with NO
2
-
or decomposition to HO
•
(Zepp et al.
1987
).
3 HO
•
Production from the Fenton Reaction
The ferrous ions (Fe
2
+
) catalyzes the formation of HO
•
in the presence of H
2
O
2
(Fenton
1894
). An aqueous solution of H
2
O
2
and ferrous or ferric salts is termed
as Fenton's reagent. The oxidation efficiency of the Fenton reaction is the high-
est at pH values ranging from 2 to 5 and at a 1:1 molar ratio of H
2
O
2
and Fe
2
+
(Walling
1975
). The reactivity of the Fenton's reagent is the effect of the gen-
eration of HO
•
in the reaction media (Haber and Weiss
1934
). The mechanism
for the chain Fenton reaction was initially depicted as follows (Eqs.
3.12
-
3.16
)
(Barb et al.
1951
):
Fe
2
+
+
H
2
O
2
→
Fe
3
+
+
HO
•
+
HO
−
(3.12)
