Environmental Engineering Reference
In-Depth Information
Fig. 9
Relative intensities of
the HFEPR spectra of [HA-
Mg
2
+
] sample (
solid lines
,
open symbols
) and metal-
free HA (
dashed lines
,
solid
symbols
) at different pH. The
intensities are measured at
g
=
2.0041 {for [HA-Mg
2
+
]
(
square
) and the metal-
free HA (
(filled square
)}, at
g
=
2.0054 for [HA-Mg
2
+
]
(
circle
) and at
g
=
2.0057 for
metal-free HA (
).
Squares
correspond to the low-pH
radical forms and circles
correspond to the high-pH
radical forms. Data source:
Christoforidis et al. (
2010
)
●
pH 7 and in donation of electrons from carboxylic functional groups to metal
ions. Changes of pH (either acidic or basic) can result in protonation and depro-
tonation that ultimately increase the excitation energy of the functional groups,
thereby decreasing the fluorescence intensity and the electron donation from the
functional group to the metal ions. The changes of the fluorescence intensity of
EPS and its complexation with Hg(II) as a function of pH are depicted in Fig.
2
.
Similarly, donation of unpaired electrons from the -NH group in the aromatic ring
[C
8
H
5
(NH)-] of the tryptophan structure is highest under neutral condition (pH 7).
Donation of electrons is significantly reduced with changes of pH (either acidic or
basic), following either protonation of the unpaired electrons by H
+
in acidic con-
ditions or deprotonation by OH
-
in basic solution.
Overall, the pH effect therefore plays a very significant role in metal-DOM
complexation in natural waters.
5.4 Effects of Ions (Cations and Anions) and of Ionic
Strength (
I
)
The M-DOM complexation is significantly affected by the occurrence of ions (cat-
ions ca.
Na
+
, Ca
2
+
and Mg
2
+
and anions ca.
CL
−
, CO
3
2
−
, OH
−
) and by the ionic
strength (I) (Cabaniss
1992
; Sonke and Salters
2006
; Christoforidis et al.
2010
;
Iskrenova-Tchoukova et al.
2010
; Smith and Martell
1987
; Cabaniss and Shuman
1988
; Fu et al.
2007
; Cao et al.
2004
; Takahashi et al.
1997
; van Loon et al.
2004
;
Lu and Jaffe
2001
; Glaus et al.
2000
; Pinheiro et al.
2000
). A computational
