Interactions of Dissolved Organic Matter with Mercury in the Florida Everglades
George Aiken
U.S. Geological Survey, Boulder, CO
Interactions of mercury (Hg) with dissolved organic matter (DOM) are hypothesized to
play important roles in controlling the reactivity, bioavailability and transport of Hg in
the Everglades. Little is known, however, about how Hg interacts with DOM or how
strong these interactions are. The questions being addressed by this research are: 1) By
what mechanisms and how strongly does Hg interact with DOM, and 2) What role do
these mechanisms play in controlling the effects that Hg has on organisms. This research
is driven by the hypothesis that the chemistry and structural characteristics of the DOM in
the Everglades strongly influence the processes that control Hg cycling and
bioavailability in the environment. A combined field/laboratory approach designed to
characterize DOM in the Everglades and to relate the structural characteristics of the
DOM to its reactivity with Hg is being used to test this hypothesis.
Most of the DOM in the Everglades originates from the degradation and leaching of
organic detritus derived from the algae, bacteria and macrophytes living within the
wetland environment. In addition, organic matter is also transported to the Water
Conservation Areas of the Everglades in the canals that drain the Everglades Agricultural
Area. Areas strongly influenced by the Everglades Agricultural Area had higher
dissolved organic carbon (DOC) concentrations, had a greater amount of hydrophobic
acids and hydrophobic neutrals, and the DOM was more aromatic than samples collected
from less impacted areas. Pore waters were found to contain greater DOC concentrations
than overlying surface waters, with the highest DOC concentrations in pore waters from
the more eutrophic study areas. DOC concentrations and the contributions of aromatic
organic molecules to the pool of molecules comprising the DOM were lower in those
areas with higher concentrations of methylmercury. The amount and nature of DOM in
the Everglades was found to be dependent on the dominant vegetation types,
hydroperiod, interactions of surface water with peat pore waters, and interactions with
canal water.
To better define the nature and strength of interactions between DOM and Hg, the
hydrophobic acid fraction (HPOA), hydrophilic acid fraction (HPIA), hydrophobic
neutral fraction (HPON), fulvic acid and humic acid were isolated from various surface
waters in the Florida Everglades. Using these isolates, Hg-DOM binding constants were
determined over a wide range of Hg(II) to DOM concentration ratios by means of a
modified equilibrium dialysis ligand exchange method. Very strong interactions (KDOM'
= 1023.20.5 L kg-1 at pH = 7.0 and I = 0.1), indicative of Hg-thiol bonds, were observed at
Hg(II)/DOM ratios below approximately 1 g Hg(II) per mg DOM. Hg(II)/DOM ratios
above approximately 10 g Hg(II) per mg DOM gave much lower KDOM' values (1010.70.5
L kg-1 at pH = 4.9 to 5.6 and I = 0.1), consistent with Hg(II) binding mainly to oxygen
functional groups. These results suggest that the binding of Hg(II) to DOM under natural
conditions (very low Hg(II)/DOM ratios ranging from 0.01 to 10 ng of Hg/mg of DOM)
is controlled by a small fraction of DOM molecules containing reactive thiol functional
groups. Similar strong Hg-DOM binding constants were obtained in studies of the
partitioning of Hg(II) between DOM and particulate organic matter (POM). In this work,
the HPOA isolate from F1, a northern, eutrophic site, was more effective at competing
with POM for Hg(II) than the sample from 2BS, a more pristine site.
Under most environmental conditions, therefore, it can be expected that only the
strongest DOM sites will interact with Hg(II). In the case of fully oxygenated Everglades
water (sulfide-free), the binding of Hg(II) by DOM should dominate Everglades’
dissolved inorganic mercury speciation. However, even for the case of strong binding
sites (KDOM' = 1023.20.5), Hg-sulfide complexes are predicted to dominate dissolved
inorganic Hg solution speciation in the presence of small concentrations (nanomolar) of
sulfide because of the strong sulfide affinity for Hg. Where measurable total sulfide
concentrations are present in the surface water and pore water of the Everglades,
therefore, Hg-sulfide complexes likely predominate.
A chemical equilibrium approach does not completely explain the behavior of Hg(II) in
the presence of DOM, however. For instance, chemical speciation models indicate that
pore waters in the Everglades, especially in the eutrophic areas, are supersaturated with
respect to cinnabar (mercuric sulfide, HgS); however, no cinnabar has been found in the
peat soils of the Everglades. Therefore, to better define the geochemical interactions
between DOC, Hg(II) and sulfide, experiments were designed using the organic matter
isolates and whole water samples to study interactions of DOC with Hg in cinnabar
dissolution and precipitation experiments. Cinnabar is a relatively insoluble solid (log Ksp
= -52.4) under most environmental conditions, but, in the presence of DOM, particularly
the humic fractions (HPOA, humic acid, and fulvic acid), a significant amount of Hg (up
to 1.7 M/mg C) was released from cinnabar over a period of seven days at pH 6.0. The
amount of Hg dissolved by various fractions of organic matter followed the order: humic
acid > HPOA  fulvic acid >> HPIA. The hydrophobic and hydrophilic neutral fractions
dissolved insignificant quantities of Hg from cinnabar. Model compounds such as
cysteine and thioglycolic acid dissolved small amounts of Hg from the cinnabar surface,
while other model compounds such as acetate, citrate, and EDTA dissolved no detectable
Hg. There was a positive correlation (R2 = 0.84) between the amount of Hg released and
the aromatic carbon content of the DOM.
Precipitation and aggregation of metacinnabar (black HgS) was inhibited in the presence
of low concentrations of humic fractions of DOM isolated from the Florida Everglades.
At low Hg concentrations (5x10-8 molar (M)), DOM prevented the precipitation of
metacinnabar. At moderate Hg concentrations (5x10-5 M), DOM inhibited the
aggregation of colloidal metacinnabar (Hg passed through a 0.1 m filter, but was
removed by centrifugation). At Hg concentrations greater than 5x10-4 M, mercury
formed solid metacinnabar particles that were removed from solution by a 0.1 m filter.
Organic matter rich in aromatic moieties was preferentially removed with the solid.
HPOA, humic and fulvic acids inhibited aggregation better than HPIA. Inhibition of
metacinnabar precipitation appears to be the result of strong DOM-Hg binding.
Prevention of aggregation of colloidal particles appears to be caused by adsorption of
DOM and electrostatic repulsion.
A general observation that can be drawn from all of our studies of Hg(II)-DOM
interactions is that the amount of dissolved Hg(II) present in a given system is greater in
the presence of reactive DOM. This is significant because many of the processes (both
abiotic and biotic) involved in Hg cycling in the Everglades are hypothesized to be
strongly dependent on the concentration of total dissolved Hg. Preliminary results of
ongoing wetland enclosure (mesocosm) experiments in the Everglades are consistent with
this hypothesis. In these experiments, mesocosms amended with the most reactive
organic matter from the Everglades, the HPOA fraction of the DOM obtained from site
F1, contained higher concentrations of total dissolved Hg than control enclosures
containing less reactive, native DOM. The DOM amended enclosures also were found to
have enhanced methylation (both biotic and abiotic pathways) of Hg(II) and enhanced
photo-oxidation of methylmercury relative to the controls. Future research will address
the mechanisms by which DOM influences these important reactions.
Aiken, George, U.S. Geological Survey, 3215 Marine Street, Boulder, CO, 80303
Phone: 303-541-3036, FAX: 303-447-2505, graiken@usgs.gov