Environmental Engineering Reference
In-Depth Information
spectra of EPS show that the fluorescence intensity of the peak T
UV
-region is
higher than the peak T-region, which is a similar behavior as tryptophan (Fig.
1
e)
(Mostofa et al.
2009a
; Zhang et al.
2010
). The fluorescence intensity of EPS and
its complexation with Hg
2
+
significantly varies with pH, and is highest under neu-
tral conditions (Fig.
2
; Zhang et al. (
2010
)).
3 Complexation Theorem
The complexation theorem depends on the linear or non-linear relationship
between formed complex and fluorescence quenching (Ryan and Weber
1982a
;
Cabaniss and Shuman
1988
; Hays et al.
2003
,
2004
; Ryan and Ventry
1990
; Cook
and Langford
1995
). The nonlinearity depends on several factors (Hays et al.
2004
; Cook and Langford
1995
), which can be distinguished as (i) Sample type,
pretreatment, and age of the DOM; (ii) Overlap of fluorophores that typify chemi-
cally different complexation sites; (iii) The wavelengths (or scanning method) at
which the fluorescence measurements are acquired; (iv) The varied physicochemi-
cal microenvironments of the fulvic acid structure as characterized by either a sin-
gle fluorophore or fluorophore set; (v) Cooperational and molecular configuration
changes due to metal loading or ionization state; and (vi) Shielding of binding
sites or fluorescence (i.e., controlling photophysical effects). In addition (vii) the
fluorescence intensity of fulvic and humic acids either at peak C-region or peak
A-region might be the result of various types of fluorophores (functional groups)
associated with those substances (Malcolm
1985
; Senesi
1990
; Steelink
2002
).
Sequential decomposition of the fluorophores bound at peak C- and A-regions in
fulvic and humic acids by photoinduced degradation decreases the fluorescence
intensity at the cited peaks during the transport from rivers to lakes and oceans
(Mostofa et al.
2005a
;
b
,
2007
,
2010
; Amador et al.
1989
; Malcolm
1990
; Allard et
al.
1994
; Moran et al.
2000
; Xie et al.
2004
; Li and Crittenden
2009
; Minakata et
al.
2009
). Such changes may affect the nature and contents of fluorophores bound
to fulvic and humic acids in various types of natural waters. Such changes are
responsible for the variation of the binding capacity of metal ions, thereby exerting
a significantly effect on the cited linear or non-linear relationship. The complexa-
tion theorem can therefore be expressed in two ways: (1) Theory of metal-ligand
complexation based on linearity; and (2) Theory of modified Multisite Stern-
Volmer (MSV) equation based on nonlinearity.
3.1 Theory of the M-DOM Complexation Based on Linearity
The fluorescence quenching of an organic ligand (e.g. fulvic acid, termed as L) by
complexation with a metal ion (e.g. Cu
2
+
, termed as M) assuming a simple 1:1
metal ligand coordination ratio (Fig.
3
) can be expressed in terms of the individ-
ual fluorescence of each species, as depicted by Ryan and Weber (
1982a
,
b
). The
