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Mercury
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Phase II Reports >> Technology Identification Subgroup Report
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Tech. Identification
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For more
information, contact David Eppstein by email at
deppstein@masco.harvard.edu,
or by calling 617-632-2860.
IV. SPECIES OF MERCURY IN WASTEWATER
The
Technology Identification Subgroup has learned that it is
important to understand the various species (forms) of mercury
that can exist in a wastewater stream and how they can react
with other stream contaminants because some mercury pretreatment
technologies can effectively remove only certain mercury
species. The various chemical species of mercury that can exist
in wastewater are: metallic, ionic, and organic. In addition,
these various species of mercury may bind to particulate matter
in the wastewater to form physical agglomerates containing
mercury.
Metallic
mercury is typically found in thermometers, manometers,
sphygmometers, fluorescent lamps and switching devices. This
form of mercury is a silver-colored liquid at room temperature
with a specific gravity of 13 (i.e., it is 13 times
heavier than water), and it is only slightly soluble in water.
Metallic mercury slowly vaporizes at room temperature and can
cause dangerous vapor concentrations in enclosed rooms. The
vapor form of metallic mercury is readily absorbed through the
lungs and is very toxic. Metallic mercury may be combined with
other metals to form amalgams (alloys).
Ionic
mercury exists when mercury atoms form covalent bonds with
halogens and other inorganic ligands (complex ions). Ionic
mercury can exist in two forms. With a single atom and an
overall +2 charge (Hg ++ ), the ionic mercury is in
the mercuric form. The mercurous form is diatomic with an
overall +2 charge (Hg 2 ++ ). The mercuric
form readily forms salts (e.g., mercuric chloride - HgCl 2 )
that are soluble in water. Mercuric chloride and Calomel (mercurous
chloride - Hg 2 Cl 2 ) are often used in
medical applications.
Organic
mercury (typified by methyl mercury) consists of mercury atoms
covalently bonded to organic groups. Often called organomercuric
compounds, these forms of mercury are quite soluble in water and
wastewater and are extremely toxic to aquatic life. These
compounds are readily absorbed by fish from their aqueous
environment and tend to become highly concentrated (bioaccumulated)
in the fish tissues. If fish having bioaccumulated organic
mercury are consumed, there can be major human health concerns.
In addition, inorganic mercury in the environment can be
converted by microbiological activity into methyl mercury
compounds that can be absorbed by fish.
The
various species of mercury can bind to the particulate matter
that may exist in ambient water or wastewater. Particulate-bound
mercury can move through the food chain through ingestion
(filter feeding organisms) or through re-conversion to dissolved
forms. Mercury-laden particulate matter can range in size from
tens of microns to sub-micron (colloidal). Typical EPA
methodology (Methods 200.7, 200.9, and 245.1) separate dissolved
from particulate mercury by filtration through a 0.45 micron (µm)
membrane filter.
As a
physical species of mercury (instead of the previous chemical
species), particulate mercury can often be a significant
fraction of total mercury in a wastewater stream. Moreover,
accumulations of metallic mercury or mercury-laden solids in
plumbing systems (at elbows, traps, and other points) can cause
chronic mercury contamination of the wastewater stream.
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