Environmental Engineering Reference
In-Depth Information
humic acid), amino acids, fluorescence whitening agents (FWAs) or components of
detergents such as diaminostilbene (DAS1) and distyryl biphenyl (DSBP), phenolic
compounds, various kinds of autochthonous DOM components and unknown ones
(Page et al.
2011
; Mostofa et al.
2011
). The HO
•
production greatly depends on
the DOM components, an increase of which can enhance the HO
•
formation rate
in the aqueous solution (Fig.
1
) (Page et al.
2011
; Mostofa KMG and Sakugawa H,
unpublished data). Study shows that DOM isolates from the Suwannee River photo-
lytically produce free HO
•
, while Elliot Soil Humic Acid (ESHA), and Pony Lake
Fulvic Acid (PLFA) hydroxylate arenes, at least in part, through a lower-energy
hydroxylating species (Page et al.
2011
). The photoinduced generation of HO
•
from various standard organic substances (1 mg L
-1
aqueous solutions) has been
studied using a solar simulator (Table
2
; Fig.
1
) (Mostofa KMG and Sakugawa H
unpublished data). The differences in production rates among different substances
are attributed to the variation of the fluorophores or functional groups on the highly
unsaturated aliphatic carbon chains present in various DOM components (FDOM
or CDOM) (Mostofa et al.
2011
; Senesi
1990
). The electronic transitions involving
illuminated DOM can lead to the release of free electrons, which can induce the
production of H
2
O
2
in natural waters. It would then follow HO
•
production upon
H
2
O
2
photolysis, and the photogenerated hydroxyl radicals can contribute to the
transformation of DOM or of the pollutants present in natural waters. That would
lead to photoinduced self-transformation of DOM via photogenerated HO
•
. Table
2
•
reports the main HO
sources that are operational in the natural environment.
In polluted sewage waters, the HO
•
formation is greatly enhanced after 60 min
of irradiation (Fig.
3
a). This finding could be compatible with HO
•
production by
photogenerated H
2
O
2
, which would not be present in the system before irradiation
and would undergo accumulation at earlier irradiation times. The HO
•
generation
being proportional to [H
2
O
2
], the accumulation of hydrogen peroxide would lead
to an increase of the formation rate of the HO
•
.
•
is rapidly consumed in natural waters upon reaction with
dissolved organic compounds (Brezonik and Fulkerson-Brekken
1998
; Southworth
and Voelker
2003
; Westerhoff et al.
1999
; Goldstone et al.
2002
; Miller and Chin
2002
;
Miller et al.
2002
; Moran and Zepp
1997
). A rough estimation showed that the con-
sumption of HO
The photogenerated HO
•
by DOM is 12-56 % in rivers (Nakatani et al.
2004
). Considering
the DOM as a major sink, the maximum steady-state concentration of HO
•
is equal to
the production rate divided by the decay rate constant, and can be depicted by (Eq.
4.1
)
(Schwarzenbach et al.
1993
):
I
(λ)ϕ(λ)[
1
−
10
−ε(λ)
C
Z
]
zK
DOM
C
[
HO
]
SS,Fenton
=
(4.1)
λ
where the 1
−
10
−
(
λ
)
C
z
is the light attenuation at the depth
z
(cm),
ϕ
is the appar-
ent quantum yield for the generation of HO
Є
•
from DOM (mole Einstein
-1
),
Є
is the absorption coefficient of DOM [(mg C L
-1
)
-1
cm
-1
],
C
its concentration
of DOM (mg C L
-1
), and
k
DOM
(M
-1
s
-1
) is the second-order reaction rate con-
stant between HO
•
•
and DOM. The rate constant for the reaction of HO
with
