Geoscience Reference
In-Depth Information
from metal-ligand complexing experiments should improve the determination of stability
constants for complex formation.
Functional ligand group abundance associated with a given DOM source will be a pri-
mary control on metal-ligand associations, followed by the pH. Note that many functional
groups also exert some local control on solution pH, so the effects are related and probably
synergistic. For example, carboxyl groups may deprotonate, leaving the carboxylate anion
(COO
-
) to which a metal cation can bind, forming a complex and perhaps altering the
DOM's optical properties. In the well characterized IHSS humic substances, the ratio of
carboxyl groups to phenolic groups was found to be ~4:1 (Ritchie and Perdue,
2003
), so the
importance of carboxylic groups to metal-ligand fluorescence is clear. Carboxyls are most
abundant in terrestrial FA and least abundant in terrestrial HA, meaning that DOM fluores-
cence should be strongly affected by the presence of carboxyls because FAs (soluble at all
pHs) are the larger contribution to CDOM (Weishaar et al.,
2003
). Therefore, carboxylate
should be considered a major complexing ligand for most metals at low pH, and the effect
would be minimized at higher pHs where carboxylic acids are protonated.
Displacement of one metal by another can reduce the quenching effect. This was
observed by Willey (
1984
) and modeled with synchronous fluorescence on fulvic acids by
Cabaniss and Shuman (1987). Both studies showed an increase in fluorescence with Mg
2+
additions. Many other reports have investigated alkaline earth metal disruption of para-
magnetic metal complexation by DOM using fluorescence (Cabaniss and Shuman,
1988
;
Cabaniss,
1992
). The alkaline earth metals bind differently to DOM causing this effect,
which appears to result from conformational changes induced by the binding. Lu and Jaffe
(
2001
) and Wu et al. (
2004
) studied the effects of other species on metal-ligand complex-
ation, finding that the Cl
-
concentration also plays a role. At high pH, OH
-
becomes com-
petitive with DOM for metal binding (Cao et al.,
1995
). When these competitions occur,
the molecular conformation of DOM may change, as does the electrostatic environment
by raising pH. Functional group ionization also influences the ability of DOM to com-
plex metals, with an attendant effect on DOM fluorescence. The conformation changes
caused by incremental addition of the alkaline earth metals when river water mixes with
seawater also has implications for DOM biological and photochemical reactivity (see
Section 7.7
).
7.7 Effect of Salinity (Ionic Strength)
Changes to ionic strength appear to have a lesser effect on DOM fluorescence than pH and
metal-ligand interactions. However, several workers have observed bathychromic shifts
(blue shifts) and/or decreased fluorescence intensity of certain peaks in EEM spectra ana-
lyzed from samples across estuarine transitions where river water mixes with sea water
(e.g., Del Castillo et al.,
1999
; Kowalczuk et al.,
2003
; Alberts et al.,
2004
; Kowalczuk
et al.,
2009
), and during simulated mixing of freshwater and seawater (Boyd and Osburn,
2004
). Moreover, salinization can decrease SF intensity in the FA extracted from soils
(Cilenti et al.,
2005
; Provenzano et al.,
2008
). These observations imply that fluorescent
