Environmental Engineering Reference
In-Depth Information
standards decrease by up to 15 % at pH <4.5, there is little effect at pH 5-8, and
fluorescence is enhanced by up to 30 % at pH > 8. The peak B (tyrosine-like) is
more sensitive to pH changes than the other peaks (Hudson et al.
2007
; Reynolds
2003
).
Therefore, the fluorescence properties of FDOM are significantly affected by
pH, at a different extent for a variety of waters. The pH effect depends on several
factors such as the sources and chemical nature of DOM, the occurrence of differ-
ent functional groups, the presence of other organic substances and the contents
of trace elements. Four possible mechanisms are proposed from earlier studies
for the pH effect (Gosh and Schnitzer
1980
; Henderson et al.
2009
; Westerhoff
et al.
2001
; Laane
1982
; Patel-Sorrentino et al.
2002
; Myneni et al.
1999
): (i) the
alteration of the molecular orbitals of excitable electrons; (ii) physical changes
in the molecular shape caused by changes in charge density (for instance, humic
substances have a linear structure at high pH and coil when pH decreases); (iii)
competition between H
+
and metal ions to form complexes with the fluorescent
substances; and (iv) conformational changes in the molecules that can expose
or hide their fluorescent parts. These mechanisms for the pH effect are not well
understood. However, because changes in the fluorescence properties due to pH
variation from pH 2 to 12 are reversible, it is excluded that irreversible structural
changes may occur (Vodacek and Philpot
1987
; Patel-Sorrentino et al.
2002
).
Reversible pH-induced changes in the fluorescence properties may be caused
by two phenomena. First, the H
+
or OH
−
ions can alter the availability of elec-
trons to be excited in a specific functional group or fluorophore in a fluorescent
molecule, which can significantly modify the fluorescence intensity. Usually, the
intensity is increased by an enhancement of electron excitation and decreased by
an inhibition. For example, under neutral conditions the functional group (-CH
2
-
(NH
3
)-CH-COO
−
) bound to peak T-like flurophore in tryptophan can show a
resonance configuration that favors electron excitation and results into the highest
fluorescence intensity (Fig.
9
). The availability of electrons to be excited, and the
fluorescence intensity as a consequence, decreases both under acidic conditions
(-CH
2
-(NH
2
)-CH-COOH) and under basic ones (-CH
2
-(NH
2
)-CH-COO
−
). In
addition, the availability of non-bonding electrons (:NH-) in another functional
group of tryptophan (C
8
H
5
(NH)-) bound to the peak T
UV
-like flurophore would
be highest under neutral conditions, because there is no solvent effect on :NH-.
In contrast, the non-bonding electrons of :NH- can react either with H
+
or with
OH
−
. In both cases the reaction can significantly reduce the availability of non-
bonding electrons and, as a consequence, the fluorescence intensity in acidic and
in basic solutions.
The main functional groups bound to fulvic and humic acids are -COOH,
-COOCH
3
, -OH, -OCH
3
, -CH
=
O, -C
=
O, -NH
2
, -NH-, -CH
=
CH-COOH, -
OCH
3
, S-, O- or N-containing aromatic compounds, although their parent molecular
structures are unknown (Mostofa et al.
2009a
; Senesi
1990a
; Leenheer and Croué
2003
; Malcolm
1985
; Corin et al.
1996
; Peña-Méndez et al.
2005
; Seitzinger et al.
2005
; Zhang et al.
2005
). Such diverse functional groups have strong affinity for
complex formation with metal ions. Therefore, the pH effect on fulvic and humic
acids shows a different pattern compared to the tryptophan molecule.
+
