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<p>Methylmercury is a bioaccumulative neurotoxin that severely endangers human health.
Humans are exposed to methylmercury through consumption of contaminated aquatic fish.
To date, effective strategies for preventing and remediating methylmercury contamination
have remained elusive, mainly due to the lack of knowledge in regard to how methylmercury
is generated and degraded in the aquatic environment. The goal of this dissertation
was to study the mechanisms of two transformation processes that govern the fate of
methylmercury in natural settings: microbial mercury methylation and methylmercury
photodegradation. The role of mercury speciation (influenced by environmental conditions)
in determining the reactivity of mercury in these biological and photochemical reactions
was the focus of this research.</p><p>Methylmercury production in the aquatic environment
is primarily mediated by anaerobic bacteria in surface sediments, particularly sulfate
reducing bacteria (SRB). The efficiency of this process is dependent on the activity
of the methylating bacteria and the availability of inorganic divalent mercury (Hg(II)).
In sediment pore waters, Hg(II) associates with sulfides and dissolved organic matter
(DOM) to form a continuum of chemical species that include dissolved molecules, polynuclear
clusters, amorphous nanoparticles and after long term aging, bulk-scale crystalline
particles. The methylation potential of these mercury species were examined using
both pure cultures of SRB and sediment slurry microcosms. The results of these experiments
indicated that the activity of SRB was largely determined by the supply of sulfate
and labile carbon, which significantly influenced the net methylmercury production
in sediment slurries. The availability of mercury for methylation decreased during
aging. Dissolved Hg-sulfide (added as Hg(NO3)2 and Na2S) resulted in the highest methylmercury
production. Although the methylation potential of humic-coated HgS nanoparticles decreased
with an increase in the age of nanoparticle stock solutions, nano-HgS was substantially
more available for microbial methylation relative to microparticulate HgS, possibly
due to the smaller size, larger specific surface area and more disordered structure
of the nanoparticles. Moreover, the methylation of mercury derived from nanoparticles
cannot be explained by equilibrium speciation of mercury in the aqueous phase (<0.2
<em>f</em>Ým, the currently-accepted approach for assessing mercury bioavailability
for methylation). Instead, the methylation potential of mercury sulfides appeared
to correlate with the extent of dissolution and their reactivity in thiol ligand exchange.
Additionally, partitioning of mercury to a diverse group of bulk-scale mineral particles
and colloids (especially FeS) may be an important process controlling the mercury
speciation and subsequent methylmercury production in natural sediments.</p><p>In
surface waters, sunlight degradation is believed to be the predominant pathway for
the decomposition of methylmercury. The mechanism of this process was investigated
in a series of photodegradation experiments under natural sunlight and UV-A radiation,
and in the presence of DOM and selective quenchers for photo-generated reactive intermediates.
The results suggested that singlet oxygen generated from photosensitization of DOM
drove the photodecomposition of methylmercury. The rate of methylmercury degradation
depended on the type of methylmercury (CH3Hg+) binding ligand present in the water.
CH3Hg -thiol (e.g., glutathione, mercaptoacetate, DOM) complexes were significantly
more reactive in photodegradation compared to other methylmercury complexes (CH3HgCl
or CH3HgOH), which may be because thiol-binding can effectively decrease the activation
energy and thus enhance the reactivity of methylmercury molecules toward the Hg-C
bond breaking process. These findings challenge the long-accepted view that water
chemistry characteristics do not affect the kinetics of methylmercury sunlight degradation,
and help explain recent field observation that methylmercury photodegradation occurred
rapidly in freshwater lakes (where CH3Hg-DOM dominate methylmercury speciation) but
relatively slowly in sea water (where CH3Hg-Cl control methylmercury speciation).</p><p>Overall,
this dissertation has demonstrated that chemical speciation of inorganic mercury and
methylmercury determines their availability for microbial methylation and sunlight
degradation, respectively. The abundance of these available mercury species is influenced
by a variety of environmental parameters (e.g., DOM). This dissertation work contributes
mechanistic knowledge toward understanding the occurrence of methylmercury in the
aquatic environment. This information will ultimately help construct quantitative
models for accurately predicting and assessing the risks of mercury contamination.</p>
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