COMERN Project Description
16/02/16
PROJECT DESCRIPTION
1. Identification
Project Manager: Dr. Susannah Scott
Department of Chemistry, University of Ottawa,
Ottawa, Ont.
Research Title: Elucidation of photochemical rates and processes leading to the evasion of
mercury
Research Theme: Mercury sources and atmospheric chemistry
Theme Leader: Dr. Holger Hintlemann
Collaborators: Dr. Marc Amyot
Dr. David Lean
INRS – Eau, Québec, Qc
Department of Biology, University of Ottawa, Ottawa, Ont.
2. Project summary
This aim of this project is to understand and quantify light induced processes which lead to
evasion of mercury (Hg) from surface waters, soil and snow to the atmosphere. Such processes
are believed to be important in the global cycling of mercury, which results in its widespread
dispersion in the environment. The project has three subprojects, which create a link between
laboratory measurements and field studies:
(1) Fundamental photochemical reactions are being investigated. These involve the
broadband and single wavelength irradiation of mercuric solutions containing various
organic ligands and photosensitizers. Quantum yields, wavelength dependences and rate
constants for the photoreduction of Hg2+ are being measured under controlled conditions
in the laboratory.
COMERN Head Office: Université du Québec à Montréal, President-Kennedy Bldg – Suite PK-7150 C.P.8888,
Downtown STA (Qc) H3C 3P8. Phone: (514) 987-3601, Fax : (514) 987-3635
www.unites.uqam.ca/comern -- email: comern@uqam.ca
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COMERN Project Description
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(2) The distribution of oxidation states in naturally-occurring mercury found in atmospheric
and surface water is being investigated. This analysis is necessary in order to evaluate
whether observed rates of mercury evasion are consistent with proposed mechanisms of
photoreduction of the mercuric ion to elemental mercury. Also, measurements of
underwater spectral irradiance are being made in order to develop simple predictors for the
attenuation of the solar spectrum and to substantiate the ability of solar radiation to induce
photoreduction of various mercury-ligand complexes present in natural waters.
(3) Massive atmospheric Hg deposition events observed in Northern environments during the
spring may adversely affect aquatic systems upon thawing of the snow, eventually
creating a health hazard for native people in these regions. However, few studies have
probed the fate of the mercury in snow. We have recently shown that most of the newlydeposited Hg is released back to the atmosphere within 24-48 hr, most probably as a
result of photoreduction of the mercuric ion. These photoreactions are being investigated
in both snow and rain, via in situ irradiation of samples in quartz vessels. In order to
identify key mechanisms, we are evaluating the effect of organic radical inhibitors, as
well as probes allowing the identification of end-products.
3. Research objectives:
Part 1:
•
Measure absorption spectra for mercuric ion complexes with simple organic acids in order
to identify possible photochemical sensitivity.
•
Irradiate solutions of photochemically active complexes to evaluate quantum yields for
photoreduction as a function of wavelength.
•
Investigate the effect of photosensitizers, including other trace metal ions and conjugated
organic ligands.
•
Identify primary photoprocesses and photoinduced mechanisms for mercury reduction,
including the participated of reactive oxygen .
Part 2:
•
Develop methods to measure low levels of elemental mercury in water samples so that the
relative contribution of elemental mercury in precipitation can be determined.
•
Collect data on the attenuation of underwater spectral irradiance and relate the changes in
depth to such predictors as dissolved organic carbon (DOC), DOC fluorescence,
absorbance and the shape of the absorbance vs wavelength curve. Such information will
COMERN Head Office: Université du Québec à Montréal, President-Kennedy Bldg – Suite PK-7150 C.P.8888,
Downtown STA (Qc) H3C 3P8. Phone: (514) 987-3601, Fax : (514) 987-3635
www.unites.uqam.ca/comern -- email: comern@uqam.ca
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COMERN Project Description
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permit development of simple models to predict photochemical relationships (such as
photoreduction, photooxidation and photodegradation of MeHg) once action spectra have
been determined.
•
Conduct measurements of microbial oxidation and reduction in lakewaters in order to
delineate the relative contributions of microbial and photochemical processes.
Part 3:
•
Evaluate temporal fluctuations of Hg concentration in snowpacks, as a function of time
after deposition, solar radiation, temperature, ion concentrations.
•
Assess the impact of rain on Hg photoreduction in surface lake waters.
•
Identify key redox reaction mechanisms and action spectra in the field (High Arctic and
temperate areas; snow, rain and lake waters).
•
Link these data with laboratory results.
Details
Reduction of the mercuric ion in natural bodies of water and in upper soil layers leads to evasion
which adds to the atmospheric burden while reducing the amount of Hg available for methylation
or runoff into aquatic environments. Indeed, much of the mercuric ion which is wet-deposited
may be converted back to Hg0 and reenter the atmosphere within a few days. The reduction
appears to be largely photo-induced, although the nature of the primary photochemical processes
and the identities of the reducing agents are unknown. Similarly, photoreduction of mercuric
ions in clouds may alter the rate of wet deposition as Hg0 diffuses back into the gas phase. In
support of this, cloud water has been noted to be supersaturated with Hg0.
Humic substances are likely candidates for photoredox-sensitive ligands, although as the
concentrations of humic substances increase, the rate of photoreduction actually declines,
presumably due to reduced light penetration. Shorter wavelengths (< 440 nm) appear to be
primarily responsible for the photochemistry. Light attenuation in natural waters increases
exponentially with decreasing wavelength. The role of UV radiation, particularly at high
altitudes where its intensity is greatest, has not been extensively addressed.
In this project, we will measure the in situ photoreduction rate of aqueous Hg2+ in natural and
artificially-doped samples at specific wavelengths to determine quantum yields for Hg0
production. Diel patterns in Hg evasion will be related to the intensity and spectral composition
of solar radiation. With this information, along with reliable predictors of atmospheric and
underwater spectral irradiance, we will be able to model the rate of the photochemical loss
COMERN Head Office: Université du Québec à Montréal, President-Kennedy Bldg – Suite PK-7150 C.P.8888,
Downtown STA (Qc) H3C 3P8. Phone: (514) 987-3601, Fax : (514) 987-3635
www.unites.uqam.ca/comern -- email: comern@uqam.ca
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COMERN Project Description
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processes in clouds and surface waters.
The primary photoprocesses and the mechanisms of light-mediated reductions of aqueous and
particulate mercuric ions will be studied by flash photolysis. Irradiation of solutions of mercuric
ion in the presence of ligands such as formate and acetate, which are atmospherically relevant
since they are found in clouds, is expected to lead to ligand-induced photoreduction. The
involvement of catalytic cycles involving other metal ion complexes (such as those of trivalent
iron - Fe3+) whose photolyses generate reactive oxygen species (such as singlet oxygen and
superoxide) will be tested. A rigorous comparison of laboratory-derived photolytic mechanisms
and field-measured wavelengths, that are photochemically active, can then be undertaken. The
resemblance of photolytically-driven reduction processes of air-borne particulate divalent Hg and
divalent Hg in clouds to photoreduction in aquatic systems will be explored. This subproject will
provide a critical piece in quantifying fluxes of Hg across compartments (i.e. land-air and waterair) and add to our understanding how photoreductive processes mobilize ionic Hg by converting
it to more mobile species like Hg0.
COMERN Head Office: Université du Québec à Montréal, President-Kennedy Bldg – Suite PK-7150 C.P.8888,
Downtown STA (Qc) H3C 3P8. Phone: (514) 987-3601, Fax : (514) 987-3635
www.unites.uqam.ca/comern -- email: comern@uqam.ca
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