Decomposition and Flux of Mercury Species from Water
Authors: Eva Santos and Anthony Carpi
John Jay College of Criminal Justice, 445 West 59th St, New York, NY, 10019
Introduction
Aim (cont.)
Results
Conclusions
Mercury has a complex biogeochemistry in which
different species of the metal participate in different
transport processes depending on the chemical
properties of the constituent in question. Several
environmental reactions influence this process by
driving the reduction or oxidation of mercury, and interconversion between mercury complexes. While the
reduction of HgCl2, HgO and HgS play important roles
on land surfaces, Hg+2 salts hydrolyze in water to form
HgClOH (in the case of mercuric chloride), Hg(OH)2,
Hg(OH)3-, or even Hg(OH)4-2 3. Mercury hydroxide
compounds exhibit different reduction/oxidation
potentials than other salts, and hydrolysis may affect
the behaviour of mercury on wet land surfaces as well
as in surface waters.
In addition, water appears to affect an increase in the
reduction and emission of mercury from soil even in the
dark – thus suggesting that there may be a physiochemical process at work as well. Therefore, this study
focuses on the kinetics of the formation and the
decomposition of easily reducible mercury complexes
in water, and the subsequent emission of elemental
mercury from these systems.
For both HgCl2- (fig. 3) and HgO-treated (fig. 4)
surfaces, mercury emissions showed an initial spike (
340min fig. 3, and 200 min fig. 4) which then slowly
declined, a behaviour typical for newly made samples.1
After drying, water was added to one of the surfaces
(red lines). A sudden peak in emissions was observed
from both HgCl2- and HgO-treated wet samples (3100
min fig. 3 and 2940 min for fig. 4) compared to the dry
samples. After a cycle of 24-hours in the dark (blue
shading in the figures), both samples were exposed to
light (and wet samples were still wet). Mercuric chloride
showed a slightly higher emission for the wet sample in
light compared to the dry sample. For mercuric oxide,
the wet sample showed considerably higher emissions
in light compared to dry HgO.
Others researchers have suggested that the peak
following water addition to soil is primarily due to the
redistribution of mercury in the soil matrix.3 However,
we have found evidence that water affects mercury
emissions even in the absence of a soil matrix. Hence,
our results suggest that there may be a chemical
process affecting mercury reduction and emission in
water both in dark and light. Mercuric oxide showed a
sustained increase in mercury emissions when wet,
and this difference carried over into the light samples.
Thus, it is likely that the addition of water causes the
formation of different ionic species that are more
susceptible to reduction. Mercuric chloride samples
showed an initial spike when wet which slowly
decreased over time. Further, the difference in
emissions between the wet and dry samples in light
was small. While water clearly affects the emission of
mercury from these samples, the exact mechanism of
this process is unclear and still under investigation.
HgCl2 dry vs wet
200
Flux (ng/hr/m2)
150
Figure 2 Mercury emissions from soil in response to simulated
precipitation events2
Methods
Acknowledgements
100
HgCl2 dry
HgCl2 wet
50
0
0
1000
2000
Drying period for
samples initially made
-50
Aim
It is known from previous studies that temperature and
radiation have the ability to increase mercury emission
from soil. From work in our lab and others, it has also
been suggested that the addition of water can lead to
significant increases in the emission of mercury from
soil (fig. 2). However, the mechanisms of how water
affects the emission of mercury from soil are unclear.
Several researchers have suggested that the addition
of water contributes to the physical redistribution of
mercury in the soil matrix, helping to bring divalent
mercury to the surface where it can be reduced to
elemental mercury by other factors including incident
light, and then emitted to the atmosphere.
The experimental procedure included treatment of two
Teflon surfaces with a dilute solution of HgCl2 and, in a
separate set of experiments HgO, in a dark cool area.
Upon drying, the two HgCl2 -treated Teflon surfaces
were placed in two separate chambers, where one
remained dry, while the other was treated with 248 ml
of ultra-pure Millipore (Milli-Q®) water to completely
saturate the sample. An identical procedure was used
for the two HgO-treated surfaces as well. Mercury
emissions were monitored continuously from both
surfaces. After addition of water both surfaces were
studied for 24-hours in the dark followed by a period
where both the wet and dry samples were exposed to
light. To measure mercury flux, a Tekran Mercury vapor
analyzer 2537 A unit was used in combination with a
dynamic chamber mercury flux (DFC). The air inside
the chamber was continuously flushed with a pump at a
known rate and the mercury concentration was
measured at the inlet and outlet of the chamber, the
difference providing a measure of the emission of
mercury from the surface. 3
4000
Water addition
to wet sample
24-hr dark
5000
6000
7000
Light exposure
24-hour period
Time (min)
Figure 3 Mercury emissions versus time from 2 replicate mercuric
chloride samples. At time = 3100 (10:30) min (highlighted in blue),
one sample was treated by adding 248 ml of water (red line) while
the other remained dry (blue line). At time = 4500 min full spectrum
light was shone on both samples.
Funding for this project was provided by the Program
for Research Initiative for Science Majors (PRISM) at
John Jay College of Criminal Justice. PRISM is funded
by the Title V, HSI-STEM and MSEIP programs within
the U.S. Department of Education; the PAESMEM
program through the National Science Foundation; and
New York State’s Graduate Research and Teaching
Initiative.
Works Cited
HgO wet vs HgO dry
200
150
100
Flux (ng/hr/m2)
Figure 1 The Mercury Cycle1
3000
Special thanks to Anthony Ho and Christina Hui.
HgO dry
HgO wet
50
0
0
1000
Drying period for
samples initially made
-50
2000
3000
4000
Water addition
to wet sample
24-hour dark
5000
6000
7000
Light exposure
24-hour period
Time (min)
Figure 4 Mercury emissions versus time from 2 replicate mercuric
oxide samples. At time = 2940(12:10) min (highlighted in blue), one
sample was treated by adding 248 ml of water (red line) while the
other remained dry (blue line). At time = 4500 min full spectrum light
was shone on both samples.
1. Ho, A. & Carpi, A. (2008). "Mechanisms of the
Reduction and Emission of Mercury in the
Environment," PRISM conference, New York, NY,
May 7, 2008
2. Hui, C., & Carpi, A. (2011). “Mercury emissions from
soil in response to simulated precipitation events,”
Poster Presentation at the 10th International
Conference on Mercury as a Global Pollutant,
Halifax, Nova Scotia, July 24-29.
3. Zhang, H. (2006). Photochemical Redox Reactions
of Mercury. Recent Developments in Mercury
Science, Structure and Bonding ,120: 37-79.
4. Gustin, M. & Stamenkovic, J. (2005). Effect of
watering and soil moisture on mercury emission
from soils. Biogeochemistry, 76: 215-232.