Environmental Engineering Reference
In-Depth Information
waters (al Housari et al.
2010
; Brezonik and Fulkerson-Brekken
1998
; Arakaki
et al.
1999b
; Hoigné et al.
1989
; Schwarzenbach et al.
1993
; Nakatani et al.
2004
).
DOM and carbonate are the major scavengers or sinks of HO
•
in freshwaters
(White et al.
2003
; Nakatani et al.
2004
), but in seawater the bromide (Br
−
) ions
are actually the main scavengers (Song et al.
1996
; Nakatani et al.
2004
; Zafiriou
et al.
1987
). The steady-state concentration of HO
•
shows a large variability in
natural waters. Examples of concentration values reported in the literature are
(3.0-8.5)
×
10
-16
M in rivers (Brezonik and Fulkerson-Brekken
1998
; Arakaki
et al.
1999b
; Nakatani et al.
2004
), (9.41
±
0.12)
×
10
-17
M to (1.72
±
0.01)
×
10
-
16
M in estuarine waters (al Housari et al.
2010
), 12
×
10
-18
M in coastal sur-
face seawater and 1.1
×
10
-18
M in the open ocean (Mopper and Zhou
1990
). In
Antarctic waters the steady-state concentrations have been determined as 4.3
×
10
-
19
M in coastal waters and 2.6
×
10
-19
M in the open ocean (Qian et al.
2001
).
Elevated HO
•
concentration values (from 6.7
×
10
-15
to 4.0
×
10
-12
M) have been
described in surface stream waters contaminated with acidic mine drainage (AMD).
These waters have pH ~2.1-3.4, are highly rich of iron (6-1203 mg L
-1
) and have
a high concentration of NO
3
-
(5.9
×
10
-6
-5.8
×
10
-3
M) (Allen et al.
1996
). The
reported data suggest that the steady-state concentration of HO
•
can be very varia-
ble in different water systems. A major caveat that should be considered while com-
paring different studies is that the irradiation conditions are usually unequal, which
accounts for at least part of the variability. However, variations in the steady-state
HO
•
concentration have also been observed with waters of different origin under
the same irradiation conditions. The major factors that account for the variation of
the steady-state concentration of HO
•
in the aquatic environments are: (i) presence
of elevated concentrations of NO
2
-
and NO
3
-
ions; (ii) presence of the elevated
amounts of Fe
3
+
-containing complexes; (iii) occurrence of photo Fenton-type reac-
tions that take place between H
2
O
2
and reduced transition metal ions; (iv) amount
and nature of the dissolved organic matter (DOM).
The reciprocal of the consumption rate constant allows the assessment of
the life-time of HO
•
, which is (2.6-6.0)
×
10
-6
s in river, dew and cloud water
(Arakaki and Faust
1998
; Arakaki et al.
1999b
; Nakatani et al.
2004
) and several
times higher (3.0-66.0
×
10
-6
s) in remote polluted clouds, as estimated from a
modeling study (Jakob
1986
).
2.4 An HPLC Method for Measuring HO
•
in Irradiated
Natural Waters, Based on Benzene as Probe Molecule
This section reports a detailed description of a possible method that can be
adopted for the determination of HO
•
, based on benzene as a probe. The descrip-
tion is very detailed to enable the reader easily reproducing a similar experimental
set-up. Note that other probe molecules can also be used for HO
•
determination,
e.g. cumene (isopropylbenzene), pyridine and terephthalic acid (Mill et al.
1980
;
