Environmental Engineering Reference
In-Depth Information
Suwannee River Fulvic Acid is (3.8-4.4)
×
10
4
(mg C)
-1
L s
-1
(Southworth
and Voelker
2003
; Westerhoff et al.
2007
). Rate constants in the same ranges,
(1-7)
×
10
4
(mg C)
-1
L s
-1
, have been determined for the reaction between HO
•
and DOM present in natural water samples or extracted from them (Vione et al.
2006
; Westerhoff et al.
1999
; Goldstone et al.
2002
; Gao and Zepp
1998
). The
half-life of a model pollutant can be estimated as
t
1/2
=
ln 2 (
k
HO
[HO
•
]
SS
)
-1
,
•
•
where
k
HO
is the second-order reaction rate constant with HO
]
ss
is
given by Eq. (
4.1
). Depending on the mixed layer depths that influence [HO
, and [HO
•
]
ss
,
and for
k
HO
values of the order of 10
9
-10
10
M
-1
s
-1
, t
1/2
can vary from some days
to some months.
•
4.2.1 Fulvic Acid as a Producer and Scavenger of HO
in Natural Waters
•
Fulvic acid (FA) can produce HO
photolytically in aqueous solution (Table
2
)
(Vaughn and Blough
1998
; Goldstone et al.
2002
). FA can account for approxi-
mately 23-70 % of H
2
O
2
production in rivers (Mostofa and Sakugawa
2009
).
A general reaction of FA that leads to the formation of H
2
O
2
(Eq.
4.2
) can
be depicted on the basis of Eqs. (3.13-3.18) (see chapter
“
Photoinduced and
h
υ
−→
(4.2)
FA
•+
+
H
2
O
2
+
O
2
+
OH
−
FA + O
2
+
H
2
O
•
•
) that
could further react with FA, at the same time being consumed and causing trans-
formation of FA (Voelker and Sulzberger
1996
). Recent experimental studies indi-
cate that at least 50 % of the hydroxylation reactions photosensitized by DOM
isolates would be a result of a pathway that is independent of hydrogen peroxide
(Page et al.
2011
). Recently, the photo-degradation of various functional groups
in DOM by HO
The generation of H
2
O
2
from FA can lead to HO
(H
2
O
2
+
h
υ
→
2HO
•
has been observed, and the rates determined in aqueous solution
(Minakata et al.
2009
). The results suggest that DOM or FA is important scaven-
gers of photolytically generated HO
•
in aqueous solution.
4.3 Other Chemical Species or Processes as HO
•
Sinks
•
There are several processes that can inhibit HO
formation or consume these radi-
cals in the aquatic environments, which can be distinguished as:
(1) Decrease in light intensity in deeper waters, which reduces the formation
rate of H
2
O
2
and of Fe(II). Photo-generated H
2
O
2
and Fe(II) at the surface could
be moved downward through vertical mixing processes, thereby reducing their
concentration in the surface layer (Southworth and Voelker
2003
; Pullin et al.
2004
). Such an effect can greatly decrease the HO
•
production in water.
