31.6 How hazardous is spilt mercury?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
31.6 How hazardous is spilt mercury?
First step - ensure any broken thermometer actually contained mercury, as
many only contain alcohol. Mercury has an appreciable vapour pressure at
ambient temperatures, thus if the mercury has split somewhere warm and with
limited air circulation, then vapour concentrations can accumulate. When
mercury drops any distance onto a surface, it splatters into hundreds of
minute globules, resulting in a large surface area. The major hazard is
the mercury vapour produced from the spill. Mercury usually ends up in carpet
or cracks in the surface, and so really is only a significant hazard to
children crawling around the floor. Do not over-react. If the location is
relatively cool and well-ventilated, there is little danger to adults. Remove
as much mercury as conveniently possible, and just remember when toddlers
come visiting that there is a slight potential hazard if the area is not
well-ventilated and is warm. Obviously, if you increase the ventilation, the
concentrations will decrease faster. The USA ACGIH TLV for mercury vapour is
0.05mg/m3, whilst the DFG ( Germany ) limit is 0.01mg/m3, and the vapour
pressure of mercury at 25C is 0.0018mm. At 25C, the equilibrium concentration
would be about 20mg/m3, which is 400 times the permitted TLV. It is unlikely
that this equilibrium would be reached in areas where there are significant
airflows, unless the mercury had been finely dispersed ( as in a blown
manometer, or dropped onto a very rough surface ).
Mercury vapour is rapidly oxidised to divalent ionic mercury by the tissues
of the body. Human volunteers exposed to tracer doses of elemental Hg
demonstrated first order kinetics for excretion with a half life of 60 days.
The lethal concentration for humans is apparently not known, but acute
mercurialism has resulted from exposures to concentrations within the range
1.2 - 8.5mg/m3. The human organism is able to absorb and excrete substantial
amounts of mercury, in some cases as high as 2 mg/day without exhibiting
any abnormal symptoms or physical signs [10].
The Dietary uptake for mercury was estimated to be :-
3 micrograms/day Adults
1 " " young children
1 " " infants.
and the adult uptake was estimated to comprise of
0.3 air via Hg(0),
0.1 water via Hg(2+),
3 food via Hg(CH3Hg+).
( EPA Mercury Criteria Document 1979 )
The CRC Handbook of Laboratory Safety [11] has a chapter on mercury hazards.
A good discussion of mercury ( and other metals ) is found in "Metals and
their Compounds in the Environment: Occurrence, Analysis and Biological
Relevance" [12].
The best method of removing spilt mercury is to use a vacuum with a flask
and pasteur pipette and chase the little globules around the floor while not
breathing :-). Seriously, a simple vacuum system, or even a pasteur pipette,
can remove most of the large globules. There are special commercial vacuum
cleaners, but never use a household one - as the expelled air will contain
mercury vapour, and the fine metal globules will contaminate the cleaner.
For nooks, crannies, and cracks - where the mercury is likely to remain
undisturbed, you can either apply flowers of sulfur ( fine elemental sulfur )
or zinc dust, with vigorous brushing to facilitate contact, and sweep up the
excess. If the mercury is going to be re-exposed ( by cleaning, foot traffic
etc., ), then the zinc dust may be preferred because of an apparently faster
reaction rate. However, if you have a light-coloured carpet, pouring yellow
or grey powder is not usually an option, and if the location is warm and not
well-ventilated near ground level, ensure that toddlers do not spend hours
every day playing there.
There have been several studies on the best methods to clean up spills,
including "Vaporisation of Mercury spillage" [13]. The abstract reports " A
report on an investigation of the problem in laboratories and industries of
mercury (Hg) vaporisation from small droplets in cracks and floors. The
efficacy of other fixing agents besides flowers of sulfur was metered.
The results show that the use of a sulfur, calcium oxide and water mixture
was the most successful mixture for fixing mercury droplets. A second
convenient technique is the use of an aerosol hair spray. A chelating soap
is available in some countries, and this would presumably be the method of
choice in dealing with spillages."
Another article includes methods based on amalgamating with zinc impregnated
in a metal sponge or scrubbing pad for picking up mercury [14], and another
investigates substances that can be used to remove spilled mercury - such as
iodised activated carbon, copper or zinc powders, molecular sieves of copper
or silver ions, and silica gel [15].
Dental amalgam is apparently a finely divided powder of a silver, tin,
and copper alloy that is mixed with the mercury. The setting time probably
is a function of the slow dissolution of the alloy in the mercury due to
the particle size of the powder used. The mass % of each individual metal
amalgam when mercury is saturated at 20C is Ag = 0.04, Cu = 0.0032, and
Sn = 0.62, but I've no idea if that is the ratio actually used. I presume
the ratio may be varied to obtain the desired physical properties, and that
there would be a theoretical excess of the alloy to ensure minimal free
mercury. The actual amount of mercury vapour from dental amalgam is low, but
directly measurable by sensitive mercury vapour analysers. The significance
of mercury vapour from dental amalgam to health has been very controversial,
however there are now practical alternatives in widespread use.
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