Disposal of Mercury Waste

Mercuric chloride is one of the more poisonous compounds used in histotechnology. Its use is declining as a fixative as contamination of the environment becomes more of an issue. At one time the used fixatives were simple poured into the municipal waste disposal system and, presumably, ended up being discharged into lakes and rivers, and ultimately in the oceans. It was considered that the capacity of the oceans to absorb and dilute humanity's wastes was boundless. This is now known to be untrue, and mercury compounds are found in fish and other sea foods. These are not primarily from the histological laboratories, coming mostly from industrial uses, but every source, no matter how small, is a contributor to the problem.

There are two ways that waste mercury may be safely disposed of, encapsulation and recycling. Recycling is probably the more effective, but is not always possible as it requires that the wastes be sent to a chemical processing facility that handles such materials. In some countries, no such facilities exist and another method is required. That is encapsulation.

Encapsulation
As the name infers, encapsulation seeks to store waste mercury compounds in a way that stops it from contacting the environment. The compounds are sealed withing non-degrading materials, then stored in a place where they may not be disturbed for extended periods, such as a landfill. This may require local approval and permission from a local government agency. It is important that the disposal container be buried deeply and not be scavenged by recyclers or animals.

The most convenient materials to use for this form of disposal are heavy plastic or glass chemical jars, a heavy plastic pail or bucket, and used paraffin wax.

Please remember to wear gloves while doing this.

1. Keep a 500 mL or 1 litre jar in a convenient place, close by where the mercury fixatives are being used. Put some molten paraffin wax into the jar and coat all the glass surfaces with it. Allow to harden.
2. Discard all materials into this container, including filter paper, paper towels, washings from jars and tissue, etc. Leave the lid off and keep the jar in a warm place so that the water can evaporate and leave the solids behind. Periodically put some more molten paraffin wax into the jar to coat the contents and fill spaces.
3. When the jar is full, pour in some molten paraffin wax to ensure all spaces are filled and the contents cannot move. Screw the cap on the jar and coat the surface with several layers of molten paraffin wax, allowing each layer to harden before applying the next.
4. Obtain a heavy plastic pail which is large enough to completely contain the glass jar with about 50 mm space on each side. Place a layer of molten paraffin wax, about 50 mm thick, into the bottom and allow to almost harden. Place the jar containing the waste mercury compounds onto the wax base and add more paraffin wax, pouring it over the jar, raising the level in the pail by 3 inches or so. Allow to almost harden, and repeat until the jar is completely encased in paraffin wax. Finally, fill the pail with wax and attach the lid. The kind that is required to be hammered into place is the most appropriate.
5. If local regulations permit, this container may be buried in a landfill. Ensure it is completely and deeply buried and is not available to recyclers.

Recycling
Recycling is preferable to encapsulation if the facilities to do so are available. The procedure involves collection of the waste material, then chemically treating it to precipitate out mercury compounds. The whole is then filtered to collect the solids, which are sent to a facility dealing with heavy metal recovery. The filtrate is mercury free and may be disposed of into the regular waste water system. In order to ensure the filtrate is mercury free it is important that the procedure be followed as it is given, and that the specified amounts of waste treated and the amount of each chemical added not be changed.

Please remember to wear gloves while doing this.

1. Keep a 2 litre jar in a convenient place, close by where the mercury fixatives are being used. Place a mark on the outside where 900 mL reaches.
2. Discard all materials into this container, including filter paper, paper towels, washings from jars and tissue, etc. Keep the container closed when not being used so as to avoid spills and stop evaporation.
3. When the contents reach the 900 mL mark, check the pH and ensure it is above 8. Add 40 grams sodium carbonate and mix well. Leave overnight.
4. Filter.
5. Discard the filtrate. It should be mercury free.
6. Tip any globules of metallic mercury that are in the sediment into a jar and cap. Collect the rest of the sediment and store it until there is sufficient to ship to a hazardous waste treatment centre.

Further information
Wikipedia has an extensive article about mercury, including the following link to the USA Environmental Protection Agency recommendations about fish consumption.

EPA (US) Fish consumption guidelines

References
Hazardous Chemicals in the Histopathology Laboratory
Crookham, J. and Dapson, R.
Anatech Ltd

 

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