Environmental Engineering Reference
In-Depth Information
photo-Fenton reactions and the other processes that have been mentioned earlier in
chapter
“
Photoinduced Generation of Hydroxyl Radical in Natural Waters
”. The gen-
erated HO
•
•
+
can rapidly react with the DOM
, initially formed during H
2
O
2
pro-
•
+
•
)
*
(Eq.
2.3
) that is subsequently
transformed into low molecular weight DOM (LMWDOM), dissolved inorganic car-
bon (DIC), CO
2
, and other byproducts (Eq.
2.4
). It is noted that the sequential photoin-
duced degradation of functional groups in DOM will be elucidated in the next section.
The described mechanism might be applicable in merely DOM-rich natural
waters. However, in iron-rich waters the degradation of DOM might be caused by the
HO
duction (Eq.
2.1
), to form a complex (DOM
HO
•
radicals mostly generated from Fenton reaction (Fenton
1894
; Kang et al.
2000
),
photo-Fenton reaction (Zepp et al.
1992
; Southworth and Voelker
2003
; Voelker
et al.
1997
) and photo-ferrioxalate/H
2
O
2
reaction (Safarzadeh-Amiri et al.
1997
;
Safarzadeh-Amiri et al.
1996
; Jeong and Yoon
2005
) depending on the concentrations
of iron as well as oxalate ions in waters. The mechanisms for HO
•
production from
Fenton, photo-Fenton and photo-ferrioxalate/H
2
O
2
reaction have been discussed ear-
lier in chapter
“
Photoinduced Generation of Hydroxyl Radical in Natural Waters
”
.
Another most important pathway of HO
•
−
production is the photolysis of NO
2
and
−
−
−
NO
3
in
waters (Zafiriou and True
1979a
,
b
; Takeda et al.
2004
; Minella et al.
2011
; Arakaki et
al.
1999
; Mack and Bolton
1999
).
Therefore, the photoinduced transformation of DOM may undergo by two
major pathways depending on the production of free radicals (
, thereby causing the indirect photodegradation of DOM by NO
2
and NO
3
•
OR, R
=
H or
alkyl group) in aqueous solution. First, direct photoinduced reactions of DOM,
which take place by HO
•
or other reactive species that may be photolytically
generated from DOM components in natural waters. Second, indirect photoin-
duced reactions of DOM, which typically occur photolytically by HO
•
that may
be generated from Fenton reaction, photo-Fenton reaction, photo-ferrioxalate/
H
2
O
2
reaction, as well as NO
2
−
−
photolysis in natural waters. If
the direct photoinduced processes dominate, the rates of photoinduced deg-
radation as well as of product formation will be proportional to the amount
of light absorbed by DOM components such as FDOM or CDOM (Cooper
et al.
1989
; Blough and Zepp
1995
; Goldstone et al.
2002
). The indirect photoin-
duced processes induce the homogeneous production of HO
and/or NO
3
•
, which subsequently
leads to non-selective photoinduced degradation of all organic moieties in DOM in
natural waters (Haag and Hoigné
1985
; Zepp et al.
1987
; Zhou and Mopper
1990
;
Nakatani et al.
2004
). These results can suggest two important facts that occur in
photoinduced reactions: (a) Photobleaching can typically proceed with photopro-
duction of LMW organic substances via direct mechanisms, especially in waters
having high FDOM or CDOM content such as in river, lake and coastal waters,
and (b) Photoinduced generation of LMW organic substances can typically pro-
ceed via indirect mechanisms. The photoinduced generation rate of HO
•
in lake
water by CDOM/FDOM and other sources was too low to account for the pho-
toinduced mineralization of DOM. The latter process appears to be favoured in
Fe-rich waters, and possibly involves the photochemistry of Fe(III)-DOC com-
plexes (Vione et al.
2009
).
