ESM 282
INTRODUCTION
Mercury is present in the environment as a result of both natural and anthropogenic activities. A
persistent bioaccumulative toxic metal, the amount of mercury released into the environment has
increased significantly since pre-industrial times (see Figure 1). It‟s low boiling point allows
mercury to move freely through the environment and for the transformation from one chemical
compound to another (Ayres & Ayres, 147).
Most of the mercury present in the atmosphere is elemental mercury vapor, with only a small
fraction of it bound to airborne particles. This form may circulate in the atmosphere for up to a
year, resulting in the wide dispersal and transport of mercury up to thousands of miles away from
its source (EPA). In contrast, most mercury present in water, soil, sediment or plants and
animals is in the inorganic form of mercury salts and organic forms of mercury (e.g.,
methylmercury). Inorganic mercury is readily removed from the atmosphere by precipitation
and dry deposition (EPA). The methylation of mercury (i.e., the organic form) makes it
available for uptake by organisms such as bacteria, which are consumed by fish. Via this route,
mercury slowly makes it‟s way up the food chain, where it bioaccumulates in animal tissues.
Most of mercury‟s chemical compounds are in a form that is toxic to higher animals, resulting in
a variety of debilitating health effects.
Several studies have suggested that the relative contribution of mercury loadings to land and
water from atmospheric deposition can be substantial (EPA). However, there are few, if any,
permanent environmental sinks for mercury, resulting in indefinite environmental residence
times. Mercury is most persistent in sediments (~2.5*108 yr. residence time), followed by the
ocean and soils with residence times in the 1,000s of years. Mercury may be present in the
atmosphere for up to a year, although the average is only an ~11 day residence time. In addition,
mercuric species are subject to much faster atmospheric removal than elemental mercury (EPA).
Due to the properties of bioaccumulation and „biorecycling‟ of methyl mercury, environmentally
accumulated mercury will be available for many years (Ayres & Ayres, 147).
Figures 1a-1d: Pre-man versus 1978 flows of total mercury. (Ayres & Ayres, 150-151).
Atmosphere Terrestrial
80 70,000
70 60,000
60 50,000
kMT per ye ar
kMT per ye ar
50
40,000
40
30,000
30
20 20,000
10 10,000
0 0
Pre- 1978 Pre- 1978
man man
Ocean Sediment
41,900 330,000
41,800
kMT per ye ar (in millions)
328,000
41,700
kMT per ye ar
326,000
41,600
324,000
41,500
41,400 322,000
41,300 320,000
Pre- 1978 Pre- 1978
man man
ANTHROPOGENIC SOURCES
Mercury (Hg) has been mined for industrial uses for more than a century, and yet only a few
decades ago was it recognized to be a major environmental hazard. Today textbooks revere it as
a classic example of a toxic metal, which has the ability to bioaccumulate in the tissues of
animals of higher trophic levels. However, before the adverse environmental effects of mercury
were known, it was mined extensively due to its ability to conduct electricity, and the ability to
form amalgams with other precious metals (Ayres & Ayres, 147). The U.S.G.S. and the U.S.
Environmental Protection Agency (EPA) estimated that ~158 metric tons of mercury are emitted
annually into the atmosphere by anthropogenic sources in the United States. Of these 158 tons,
87% (137.4 tons) is from combustion point sources, 10% (15.8 tons) is from manufacturing point
sources, 2% (3.16 tons) is from area sources, and 1% (1.58 tons) is from miscellaneous sources
(EPA).
U.S. Anthropogenic Emissions
manufacturing
10%
area
2%
miscellaneous
1%
combustion
87%
Four specific source categories account for approximately 80% of the total anthropogenic
emissions—coal-fired utility boilers (33%), municipal waste combustion (19%),
commercial/industrial boilers (18%), and medical waste incinerators (10%) (EPA). These four
most significant sources represent high temperature waste combustion or fossil fuel processes.
For each of these operations, the mercury is present as a trace contaminant in the fuel or
feedstock. Because of its relatively low boiling point, mercury is volatized during high
temperature operations and discharged to the atmosphere as exhaust gas (EPA). Thus the main
environmental concern with anthropogenic mercury concerns emissions into the atmosphere.
Mercury Uses and Emissions Breakdown
Man first mined mercury in the form of the red colored mineral cinnabar for use as a pigment in
cosmetics and paints. A more profitable purpose for mercury arose in the nineteenth century
when it was used extensively to separate grains of gold and silver from riverbed gravel and
alluvial deposits (Ayres & Ayres, 160). Today the mining of silver and gold has replaced the use
of mercury as an amalgam with a process using cyanide. However, small mines that continue to
utilize mercury still exist (mostly in developing countries), as 1990 figures reported the
production of ~448 metric tons (Ayres & Ayres, 161).
The next major use of mercury (311 tons used in 1987) came during the first part of the twentieth
century, when its catalysts were used to manufacture acetic acid, acetaldehyde, and other
chemicals (Ayres & Ayres, 161). This use of mercury was again replaced, this time by ethylene.
The most consistent use of mercury, according to Ayres & Ayres, during the last century has
been in the manufacture of chlorine via the electrolytic process, hence the „mercury cell‟. Since
the 1940s chlorine production has steadily risen spurred by growing demands for elemental
chlorine as a germicide or bleach, polyvinyl chloride, chlorinated solvents, and many other uses,
all of which require mercury in the manufacturing processes. Tighter regulations and
bookkeeping over the latter half of the century have reduced the emissions of mercury per ton of
chlorine produced from 300mg/kg of product to 150mg/kg (Ayres & Ayres, 165). Mercury
emissions via the electrolytic process thus account for roughly 77 metric tons per year.
The largest single use of mercury has been in battery manufacturing, using approximately 250
tons in 1990 (EPA). Mercury has two main purposes for the production of batteries. The first
use is as a component in the zinc-mercury amalgam used as the anode in mercury oxide and
alkaline batteries and as a component in the cathode of mercury oxide batteries (EPA). The
second use serves to inhibit side reactions and corrosion of the battery casing material in carbon-
zinc and alkaline batteries (EPA). Before the 1980s, most batteries and some storage batteries
contained mercury in the form of mercuric oxide (HgO), zinc amalgam (Zn-Hg), mercuric
chloride (HgCl2), or mercurous chloride (Hg2Cl2) (EPA). Technological improvements made by
the battery industry within the last couple of decades are resulting in a phase out of mercury in
battery production. These improvements have shown to be successful, as the 1992 estimate of
mercury emitted from the production of batteries was 0.02 tons.
Approximately half of the batteries used in the United States end up in landfills or are dissipated
and/or incinerated, resulting in significant mercury emissions to the environment (see Figure 2).
The electrical industry also produces components using mercury such as florescent lamps, and
wiring devices and switches (fever thermometers and thermostats). These uses require roughly
141 tons of mercury annually (Jasinski), and emit less than 1 ton of Hg in the manufacturing
process. The emissions of mercury from these sources are more likely to occur when the product
is broken and/or discarded; hence the majority of these emissions end up in landfills and/or
incinerators as well.
Dental preparations (amalgams in fillings), and laboratory uses annually require roughly 57 tons
of mercury. Most of this mercury is captured in the use of the products manufactured; however
the end fate of these products is either incineration or landfilling, resulting in respective
emissions of 35 and 4 tons to the environment. Anti-fungal paints, measuring and control
instruments, and other miscellaneous uses comprise the remaining sources for mercury.
Together, these sources amount to approximately 16 metric tons of mercury lost to the
environment annually. The majority of losses occur via the manufacture of paints using mercury
as a paint additive. Current regulations however, have called for the elimination of mercury in
this process and emission levels are expected to reflect this decrease in future studies. A more
comprehensive and „outlined‟ diagram depicting all of these sources, uses, and emission figures
are found in Figure 2. (Note that more detail is depicted in the diagrams than is discussed.)
LEAKS TO ENVIRONMENT
Current emissions of mercury from manufacturing sources are generally low compared to
combustion sources (with the exception of chlor-alkali plants). The EPA identified that the
largest source of emissions is fossil fuel combustion (163 tons annually) by utility boilers
(particularly coal combustion). The other significant sources of mercury emissions are from
municipal waste combustors and medical waste incinerators. Because the vapor pressure of
mercury metal is strongly dependent on temperature, and it vaporizes readily under ambient
conditions, most of the mercury emissions are found in the atmosphere as mercury vapor.
Approximately 96 tons of mercury lost to the environment, mostly as mercury vapor, are from
area source manufacturing (Jasinski). The majority of losses, and these include losses to the
atmosphere (by incineration and/or dissipation) as well as to landfills occur post-production. As
stated earlier, these emissions are the result of products being discarded or broken. Intuitively
speaking, we can assume that if future consumption of mercury remains constant, emissions
from these manufacturing sources will remain constant as well. To combat emissions from fossil
fuel combustion, innovative ideas to increase fuel efficiency, as well as new technological
advancements, must be actualized by the utility industry, depending greatly on the nation‟s
future fuel energy needs. These leaks to the environment from each sector are shown in Figure
2.
Missing Mercury
As is demonstrated in Figure 2, the annual consumption of mercury in the US is greater than the
annual release. This results in a large quantity of „missing‟ mercury. The US Bureau of Mines
and the EPA (under the Toxic Release Inventory, or TRI) provide comparison data on mercury
consumption by the US chlor-alkali industry vs. mercury emissions in Table 1. The data
reported regarding „releases and transfers‟ for the production, consumption and major uses of
mercury indicate that inflows do not equal outflows for the mercury system. Apparently, a
significant portion of the mercury consumed by the chlor-alkali industry (which is all for
replacement uses) has mysteriously disappeared from these statistics. Thus, most of the mercury
being consumed is unaccounted for in the TRI (Ayres & Ayres, 167). This is a large discrepancy
that has only a handful of explanations.
One possibility is that the emissions reports are falsely generated (either intentionally or not),
due to a lack of information on the part of chlorine manufacturers. A second possibility lies in
the interpretation of the discrepancy between consumption and emission rates. It may be
assumed that the reported emissions rates refer only to air and process water, while most of the
mercury is probably accumulated in semi-solid or solidified sludges from process water
treatment. This sludge may be stored or buried on site or shipped to a landfill, and thus not
accounted for under the typical reporting framework. A third possibility lies in the fate of
mercury as a contaminant of the chlor-alkali products. Mercury contamination in caustic soda (a
Table 1: Mercury in chlorine production in the US (tonnes).
USBM Minerals EPA US Bureau of Mines
Yearbook
Year Consumption EPA OPPT* Releases Releases Transfers Total
1987 311 23.7
1988 455 15.7
1989 381 10.7 11.3 52.9 64.2
1990 247 11.0 11.0 58.5 69.5
1991 184 7.9 8.5 56.0 64.5
1992 209 6.3 4.5 8.7 13.2
* USEPA Office of Pollution Prevention and Toxics Releases
Source: Ayres & Ayres, 167.
product of chlor-alkali processing) has appeared as a significant contaminant; however, this
would only account for about 4.3 tonnes of missing mercury in the US (Ayres & Ayres, 168).
The most likely explanation for the missing mercury revolves around EPA mandated disposal
techniques. In 1992, the EPA banned the disposal of mercury-containing sludge produced at
chlor-alkali factories. In addition, at about this time the TRI was expanded to include reports for
„Source reduction and recycling‟. Thus, in 1992, 40 tonnes of mercury were released while 160
tonnes were recovered on site from chlor-alkali facilities (Ayres & Ayres, 168). This was
equivalent to 2.5 times the amount of mercury consumption reported for that year, suggesting
that the missing mercury had been accumulating in mercury-bearing waste sludges on site at the
chlorine plants (Ayres & Ayres, 168). These accumulations were not reported until 1992 when it
became illegal to continue their storage, prompting the need for recycling and thus reporting.
BIOGEOCHEMICAL FLOWS
Almost every part of the Earth‟s ecosystem contains some mercury. Due to its high vapor
pressure at normal atmospheric temperatures, mercury is constantly emitted from the earth
(although it rarely occurs in toxic concentrations) (Jasinski). Natural sources of significant
mercury emissions to the atmosphere may be classified as follows:
degassing from natural mineral deposits;
volcanic emissions;
photo-reduction of divalent mercury in natural waters;
biological demethylation of methyl or dimethyl mercury (Ayres & Ayres, 155).
In addition, the following sources serve as minor sources of mercury emissions:
geothermal sources (submarine volcanoes and faults and fractures in the earth‟s crust)
seismic activities (such as earthquakes);
erosion of mercury-bearing soils and rocks;
evaporation of mercury-containing water;
volatilization from soil;
animal excretions;
forest fires (Ayres & Ayres, 155).
All totaled, these sources account for the annual, global emission of approximately 317.0 kMT of
mercury into the atmosphere (Ayres & Ayres, 13), and are fairly comparable to anthropogenic
emissions. Approximately 2/3 of mercury emissions are thought to be attributable to natural
sources (oceanic and terrestrial), with the remaining 1/3 resulting from anthropogenic activities
(Ayres & Ayres, 155). Atmospheric additions of gaseous mercury have nearly doubled since the
nineteenth century (Ayres & Ayres, 156), with approximately 90 percent of these increases
attributed to anthropogenic activities. Natural water fluxes of mercury to water are not well
known, although a 1977 study estimated an annual, global discharge of mercury into the oceans
(due to natural weathering processes) of 3000 tonnes (Ayres & Ayres, 156).
Natural sources of mercury emissions result in the cycling of mercury through the biosphere,
with the concentration of natural mercury emissions at any one location generally found to be
quite low. However, anthropogenic sources often enhance these natural sources, resulting in
higher than normal localized levels (Jasinski). For example, the flooding of vegetated areas can
release a considerable quantity of soluble methylmercury into the water from the natural stock of
insoluble inorganic mercury (Baird, 361). This methylmercury may then enter the food chain,
resulting in elevated mercury levels in organisms.
Global sour ce s for air bor ne me rcur y.
Other
an th ro poge ni c
source s
Oceans
Worl d fo ssil fu el
co mbustion
Na tural te rrestri al
source s
The ratio of anthropogenic to natural sources of mercury is currently estimated at 6.5, with more
mercury being mined than resides in the topsoil (Ayres & Ayres, 156). In addition, the
mobilization rate of natural sources of mercury emissions into surface waters is very low
compared to anthropogenic sources (Ayres & Ayres, 157). The anthropogenic culprit here is
industrial water emissions. These emissions constitute a relatively small fraction of total
anthropogenic emissions, yet they may have a disproportional impact on biota (Ayres & Ayres,
157).
INTERACTIONS AND RESPONSES
The most significant sources of mercury emissions are industrial atmospheric emissions
(primarily chlor-alkali production), energy production and continental „degassing‟. Sources such
as natural geological processes and emissions from coal combustion and phosphate fertilizers
release significant amounts of mercury into the atmosphere, where it is often transferred to
waterways. Here, the biological transformation of inorganic to organic mercury (CH3Hg+) is
increased with the increased availability of organic matter. In this organic form, mercury is
available for uptake and it bioaccumulates in animal tissues, due to the high rate of mercury
uptake relative to elimination.
Bioaccumulation is more significant in aquatic systems than in terrestrial systems. Once
mercury enters the soil, it is strongly bound in the clays and other particles, resulting in a low
transfer to plants and herbivores. However, when organic mercury is uptaken, the effects may be
significant and irreversible. Detrimental health effects associated with mercury exposure include
the following:
deterioration of the nervous system (cellular damage);
impairment of hearing, speech, vision and gait (dulled senses);
involuntary muscle movements (affects motor skills);
corrosion of skin and mucous membranes;
difficulty chewing and swallowing.
Mercury present in fish is passed along to consumers of fish with increasingly greater toxicity, as
mercury is bioconcentrated and longer-lived organisms typically accumulate more.
Mercury does not appear to have any significant interactions with other metals, nor is there any
evidence pointing towards significant interactions between the natural and anthropogenic cycles.
A 1994 EPA report stated that natural sources of mercury emissions registered at approximately
1,000,000 Kg/year for both pre-industrial and current times (EPA). Thus, no increase was noted
in natural mercury fluxes as a result of increased anthropogenic emissions.
Substitutes for mercury are abundant and in strong demand. Public awareness regarding mercury
has increased in recent years due to several factors. One such factor lies in evidence stating that
mercury levels in the atmosphere of the industrial Northern Hemisphere are much higher and
rising than in the non-industrialized Southern Hemisphere (Ayres & Ayres, 152). This has led to
widespread concern regarding anthropogenic sources of mercury emissions, and a move towards
the reduction of mercury in consumer products. For example, mercury-amalgams are currently
being investigated for use in dental fillings. As was previously noted, the extensive use of
mercury in mining gold and silver has been phased out and replaced by the more efficient
cyanide process (Ayres & Ayres, 161). The use of mercury in batteries is being phased out as
well, due to concerns about landfill leakage of mercury into groundwater aquifers. These
batteries are now being replaced by rechargeable nickel-cadmium cell batteries. Lastly, mercury
once heavily used in fluorescent lamps is now being replaced by sodium vapor, which presents a
lesser toxicity hazard and is a more efficient light source.
Recycling of Mercury
Mercury consumption has rapidly declined since the late 1980s, with the most dramatic drop
noted in the case of electronic goods (especially in batteries and anti-fungal paint) (Ayres &
Ayres, 160). However, large quantities of these goods still remain, with primary disposal via
landfills. This is a potentially significant hazard as was previously noted (i.e., leakage if mercury
into ground water). A large number of mercury uses are potentially recyclable, including
industrial catalysts, mercury electrolytic cells (for chlorine production), electrical equipment and
instruments (Ayres & Ayres, 160). Unfortunately, much of the mercury present in these uses is
not recycled, but is instead dissipated into the environment.
Mercury recycling in the US is a relatively small-scale operation, with only five small chemical
companies that routinely process mercury-containing scrap (Ayres & Ayres, 178). Most „scrap‟
recycled includes fluorescent tubes, mercury vapor lamps, batteries, switches, electrical relays
and dental amalgam. However, current technologies do not allow for the recycling of mercurous
wastes from the chemical industry, a major producer. In addition, most mercury in consumer
products cannot be recycled economically, leading to the general discharge in landfills (all from
Ayres & Ayres, 178).
As was previously mentioned, mercury is extremely mobile due to its high vapor pressure and
low affinity for oxygen. Thus, landfill or sediment deposition is not a guarantee of long-term
immobilization. In addition, mercury‟s failure to oxidize rapidly in either the atmosphere or in
water points to the inevitable remobilization from virtually any use or waste. It seems quite
likely that all mercury that has been mined in the past will be, or has been, released into the
environment. This is a serious factor for consideration, as such losses are raising the
environmental background level of mercury.
Sources of mercury could be effectively controlled (or „plugged) via three primary routes:
product substitution, process modification, and materials separation. Product substitution, as was
previously mentioned, has already started to soar via the reduced use of and substitutes for
mercury. Process modification would be used primarily at chlor-alkali plants, removing mercury
with ~100% efficiency. Firms could also enjoy additional savings here as a result of reduced
recycling or disposal needs (~$4,590 per pound of mercury removed cost-effectiveness) (EPA).
Materials separation, the separation of low-volume materials containing high mercury
concentrations, could reduce mercury input to a combuster without removing energy content of
the waste stream (EPA). Such programs have been successfully implemented for household
batteries and at hospitals. All of these techniques will aid in the plugging of environmental leaks
for mercury.
CONCLUSION
Current information available for both natural and anthropogenic mercury cycles is seriously
flawed by a lack of reliable data on emission levels, trade, consumption and industrial uses of
mercury, especially outside of the US (Ayres & Ayres, 179). Estimates of flows for either cycle
vary significantly depending on the source. Thus, the statistical information presented in this
report is an approximation at best. While numerous sources of data exist for anthropogenic
flows of mercury, little to no data is available for natural mercury emissions beyond a composite
measure.
In addition, current measurement methods administered under the EPA‟s TRI fail to account for
large quantities of „missing‟ mercury. The inclusion of all sectors of the economy and a
modification requiring firms to account for all of their inputs and outputs of toxic materials (via a
demonstrated materials balance) would potentially eliminate this problem. It would also be
useful to gather emissions data for a number of source categories, including secondary mercury
production (recycling), commercial and industrial boilers, landfills, electric lamp breakage, and
iron and steel manufacturing (EPA). This would provide better estimates of mercury fluxes and
disposal techniques. Lastly, an accurate determination of the mercury flux from natural sources
would aid in determining the impact of US anthropogenic sources on the global mercury cycle,
as well as the impact of all mercury emissions in the US (EPA).
Anthropogenic activities have been identified as a significant source of mercury leaks to the
environment. These leaks may be significantly plugged in the future through government
regulations and innovative ideas for new technologies from the utility industry. Both of these
potential “plugs” have the capability for reducing the negative impacts associated with indefinite
mercury cycling by reducing overall atmospheric emissions. No significant interaction between
natural and anthropogenic mercury cycles was discovered.
The current analysis suggests that anthropogenic sources of mercury emissions are significant
but decreasing. EPA estimates for the future predict a significant decrease in mercury emissions
over the next 10 years as a result of new regulations. An increase in secondary production of
mercury is also expected as more recycling facilities commence operation to recover mercury
form discarded products and wastes. Lastly, mercury used in conjunction with manufacturing is
expected to decline, while chlorine production from mercury cell chlor-alkali plants is expected
to continue to account for most of the mercury use in the manufacturing sector (EPA).
REFERENCES
1) Ayres & Ayres. “Accounting for Resources”, Volumes I & II.
2) Baird, Colin, Environmental Chemistry (New York: W.H. Freeman and Company, 1995).
3) Environmental Protection Agency. “Mercury Study Report to Congress”, Volumes I & II.
Document # 452/R-97-004, 1997. Website: http://nsdi.epa.gov/ttn/uatw/112nmerc/mercury.html
4) Jasinski, Stephen M. “The Materials Flow of Mercury in the U.S.”, Volume 1.
U.S. Bureau of Mines Information Circular #9412, 1989. Website:
http://greenwood.cr.usgs.gov/pub/min-info-pubs/usbm-ic/ic-9412/
5) U.S.G.S. Website: http://minerals.usgs.gov/minerals/pubs/myb.html