Environmental Engineering Reference
In-Depth Information
According to the basic principle of fluorescence, the elements O, N, S, and P
as well as their related functional groups (C-O, C
=
C,
ϕ
-O, COOH, and C
=
O)
in fulvic acid can show fluorescence properties. Each of the functional groups in
fulvic acid is referred to as fluorophore. All fluorophores in a mother molecule
can exhibit fluorescence properties and any change in the molecule can have an
effect on the overall fluorescence properties (Senesi
1990a
). Fluorophores present
in allochthonous fulvic acid (or allochthonous humic acid or autochthonous fulvic
acid) either at peak C-region (Ex/Em
=
280-400/380-550 nm) or A-region (Ex/
Em
=
215-280/380-550 nm) in EEM spectra can be denoted as fluorochrome.
The molecular structure of fulvic acid is not yet known because of the complicated
chemical composition and relatively large molecular size. However, fulvic and
humic acids of vascular plant origin have allowed a partial identification of their
molecular structure as benzene-containing carboxyl, methoxylate and phenolic
groups, carboxyl, alcoholic OH, carbohydrate OH, -C
=
C-, hydroxycoumarin-like
structures, fluorophores containing Schiff-base derivatives, chromone, xanthone,
quinoline ones, as well as functional groups containing O, N, S, and P atoms.
Such functional groups include aromatic carbon (17-30 %) and aliphatic carbon
(47-63 %) (Leenheer and Croué
2003
; Malcolm
1985
; Senesi
1990b
; Steelink
2002
). All of the cited functional groups can be considered as major fluorophores
in fulvic and humic acids in natural waters. They can display two fluorescence
peaks: peak C at longer wavelength (or peak C-region) and peak A at shorter
ultraviolet (UV) wavelengths (or peak A-region) (Mostofa et al.
2009a
,
2005a
,
2010
,
2007a
; Senesi
1990a
; Coble et al.
1990
; Coble
1996
,
2007
; Mostofa KMG
et al., unpublished data; Komaki and Yabe
1982
; Schwede-Thomas et al.
2005
;
Nakajima
2006
). The electronic transition of the lowest energy that involves a
fluorophore in a molecule exhibits fluorescence (Ex/Em) with the highest intensity
at peak C and A-regions. When a fluorophore is degraded by photolytic processes,
another lowest energy fluorophore will subsequently produce the fluorescence
peak in the respective regions. Therefore, a particular peak (e.g., peak C, peak A,
peak T or T
UV
) of a fluorescent molecule is the outcome of the contribution of all
fluorophores present in the molecule itself.
The fluorescence properties of an organic molecule containing fluorophores depend
on several inner (or internal) and external (local physical conditions in the fluorophore's
microenvironment) factors associated with chemical structure (Mostofa et al.
2009a
;
Senesi
1990a
,
1990b
; Lakowicz
1999
; Tadrous
2000
; Wu et al.
2002
,
2004a
; Baker
2005
). The inner or internal factors are: (i) the probability of absorbing a photon; (ii) the
number of fluorophores or functional groups present in the molecule; and (iii) the
quantum yield that measures the probability of radiative decay from the excited state;
(iv) the extension of the
π
-electron system, which reduces the excitation energy and
shifts the emission wavelengths toward higher values; (v) heteroatom substitution on
aromatic compounds; (vi) electron withdrawing (meta-directing) functional groups in
aromatic compounds, which reduce the fluorescence intensity; (vii) electron-donating
(ortho-para directing) functional groups in aromatic compounds, which increase the
fluorescence efficiency; (viii) functional groups such as carbonyl, hydroxide, alkoxide
and amino ones, which shift fluorescence toward longer wavelengths; (ix) an increase
