Environmental Engineering Reference
In-Depth Information
2.4 Mechanisms for Photoinduced Degradation of DOM
Functional Groups by HO
•
•
It has been shown that the photoinduced generation of HO
from DOM occurs pri-
marily through H
2
O
2
that is produced via photoionization of the most electron-
rich organic compounds. The process yields DOM
•
+
, which initiates several other
•
•
+
reactions (Eqs.
2.1
,
2.2
). The HO
radical can subsequently react with DOM
•
and initiate complex chain reactions (Eq.
2.3
). The sequential reactions with HO
yields various end products such as LMWDOM, CO
2
, DIC and CO (Eq.
2.4
).
The group contribution method (GCM) by Minakata and his co-workers
(Minakata et al.
2009
) allows the prediction of the aqueous phase HO
•
rate con-
stants for various functional groups of a given organic compound. It may pave the
way to understand the mechanism for the degradation of organic compounds in
Advanced Oxidation Processes (AOPs) such as O
3
/H
2
O
2
, UV/H
2
O
2
and UV/TiO
2
,
as well as in natural water photochemistry (Mostofa et al.
2007
,
2011
; Moran
et al.
2000
; Minakata et al.
2009
; Huber et al.
2003
; Rosenfeldt and Linden
2004
;
Westerhoff et al.
2005
; Minakata et al.
2011
). The rate constants are discussed in
the following section. Photodecarboxylation (RCOOH
+
1/2O
2
+
h
υ
→
ROH
+
CO
2
) is one of the important reactions for generating CO
2
by degradation of
LMWDOM such as RCOOH in aqueous media (Xie et al.
2004
). Another possible
pathway could involve phosgene, which is generated photolytically from photo-
sensitive chloroform (CHCl
3
+
O
2
+
h
υ
→
COCl
2
+
HCl) (Shriner and Cox
1943
). The phosgene (COCl
2
) is highly photosensitive and highly reactive, and
it could degrade fluorophores such as the amino groups (RNH
2
+
COCl
2
→
RN
=
CO
+
2 HCl) or carboxylic functional groups (RCO
2
H
+
COCl
2
→
RC(O)Cl
+
HCl
+
CO
2
) (Mostofa et al.
2011
; Shriner and Cox
1943
). It is noted that mac-
romolecules such as stream fulvic acid and humic acid of vascular plant origin are
composed of various functional groups such as -COOH, methoxyl, alcoholic OH,
phenolic OH, carbohydrate OH, -C
=
C-, C
=
O, aromatic carbon (17-30 %), ali-
phatic carbon (47-63 %) as well as N, S, and P-atom-containing functional groups
(Malcolm
1985
; Steelink
2002
). Marine DOM of biological origin is composed of
amino group in its molecular structure (Midorikawa and Tanoue
1998
,
1996
). The
photoinduced reactions of natural organic matter can lead to the sequential degra-
dation of various functional groups (Mostofa et al.
2011
; Xie et al.
2004
; Minakata
et al.
2009
,
2011
; Shriner and Cox
1943
; Li and Crittenden
2009
), of organic mol-
ecules bound to fulvic and humic acids (Allard et al.
1994
; Amador et al.
1989
),
and to the sequential decrease in fluorescence intensity of fulvic acid-like sub-
stances (peak C and A) with irradiation time (Mostofa et al.
2007
,
2011
; Moran
et al.
2000
). The general reaction mechanisms that HO radicals induce are a parent
compound
→
aldehydes and ketones
→
carboxylic compounds
→
carbon dioxide
and minerals (Bolton and Carter
1994
). This reaction mechanism is mostly appli-
cable to fulvic acid and humic acid of plant origin, autochthonous fulvic acid of
algal origin, proteins and aromatic amino acids, and of all the compounds that can
initiate the reaction through self-generation of HO
•
via H
2
O
2
photo production.
