Environmental Engineering Reference
In-Depth Information
From Eqs. (
3.7
) and (
3.8
), the fluorescence of the ligand can be expressed in
terms of the protonation constant and the proton concentration as follows:
F
H
−
pH
−
F
F
−
F
L
−
pH
[
HL
]
L
=
K
′
HL
·[
H
]
(3.9)
=
Using the logarithm, Eq. (
3.9
) becomes
F
H
−
pH
−
F
F
−
F
L
−
pH
=
log
K
′
HL
−
pH
log
(3.10)
The plot of log(
F
H-pH
-
F
)/(
F
-
F
L-pH
) versus pH gives the values of log
K′
HL
from the intercept. For the occurrence of a further protonation at lower pH, two
protonation constants are operational that can be treated by another model (van
Den Berg
1984
).
3.5 Kinetics of M-Fulvic Acid Complexation
It has been shown that allochthonous fulvic acids are the main DOM compo-
nents studied in natural waters (Moran et al.
1991
; Malcolm
1990
; Ma et al.
2001
;
Mostofa et al.
2009a
). Therefore, it is vital to know how they form complexes with
metal ions. Changes in the full fluorescence spectral kinetic, i.e., in both Ex/Em
wavelengths of the fluorescence maxima occur in the EEM spectra of fulvic acid
or DOM during their complexation with trace elements (Wu et al.
2004a
,
c
). The
complexation of a fulvic acid (extracted from Cavan Bog, Canada) with several
metals (Cu
2
+
, Ni
2
+
, Co
2
+
, Cd
2
+
and Ca
2
+
) at pH 7 shows that the fulvic acid can
react rapidly with all metals studied (Wu et al.
2004c
). The result of pseudo-first
order kinetic plots demonstrates that fulvic acid has two major kinetically distin-
guishable binding sites, 'fast' and 'slow', having reaction half-lives of 1.3-3.9 and
34.7-69.3 s, respectively (Wu et al.
2004c
). The binding of copper to fulvic acid is
found to be fairly rapid, and the reaction is virtually at equilibrium after approxi-
mately 20-30 s (Lin et al.
1995
). Another study demonstrates that the three life-
times and emission wavelength maxima for three fluorophores in fulvic acid are
as follows: ~50 ps (392 nm), ~430 ps (465 nm), and 4.2 ns (512 nm) (Cook and
Langford
1995
). Kinetic changes of excitation-emission wavelengths of the fluo-
rescence maxima also suggest the presence of two major binding sites. For the
fast-reacting binding site, the rate constant and the site relative contribution are
in the order Cu
2
+
> Ni
2
+
> Co
2
+
> Cd
2
+
> Ca
2
+
, which agrees with the Irving-
Williams series, indicating affinity dependence of complexation kinetics (Wu et al.
2004c
). For the fast-reacting binding site in fulvic acid, proteins and other organic
ligands, the relative contribution of rate constant for bivalent metal complexes
follows the same order (Wu et al.
2002a
,
2004c
; Sidenius et al.
1999
; Irving and
Williams
1953
; Winzerling et al.
1992
). This result implies that metal ions react
