Our client received a violation for discharging an effluent containing too much mercury. Because he was not aware of any process within his facility that used mercury-containing compounds, our assignment was to track down the source of the mercury and eliminate it, or treat the effluent in order to return to compliance. This also gave us a chance to look into the role of mercury in metal finishing in greater detail.

Mercuryóa.k.a. "quicksilver," a.k.a. "hydrargyrum (Hg)"óoccurs naturally in a free state, or mixed into its ores. The principal ore is cinnabar, which is mined in numerous countries, including Spain (Iberia), Austria, Canada, Mexico, China, Chile, and Japan. In the U.S., the states of California, Oregon, Nevada and Washington are our principal sources of cinnabar. Simply heating the ore beyond 500 C will distill off the mercury as a vapor that can then be condensed, to yield pseudo silver-like metal that is an elusive liquid at room temperature.

Mercury is very dense (13.5 grams per cc) and does not readily react with non oxidizing acids. It is a true metal, conducting electricity and existing in monovalent and divalent states. It readily forms solid solutions with numerous other metals, which are referred to as "amalgams." Chances are that the fillings in your teeth contain a mercury amalgam with silver, for example. Other amalgams are formed with copper, nickel and gold; but not iron, which is sometimes used for mercury containers.

Major uses for mercury and mercury compounds include dental fillings, batteries, feltmaking, thermometers, red pigments (vermilion), mercury vapor lamps, medicines, silent electrical (wall) switches, barometers and in semiconductor devices. A unique property of this liquid metal is its ease of vaporization. Simply exposing the metal to air, or blowing air through it, will vaporize the metal into a toxic gaseous state. Another unique property is the fact that mercury is a metal that has some solubility in water (about 20 ppb, at 20 C). If mercury metal is allowed to flow into a piping system, it may then reside for a long time, slowly dissolving into the water and releasing excessive concentrations into the discharge.

Common mercury compounds that have been used in metal finishing include mercuric chloride, sulfate, nitrate, cyanide, oxide and dichromate. Processes that include mercury-containing compound are zinc-mercury plating (we hope, no longer out there), and steel coloring solutions. Because mercury is present as a residual from one means of the production of sodium hydroxide, there is a possibility that excessive mercury may be present in any metal finishing process that contains "mercury cell" sodium hydroxide, including waste treatment.

The term "mad hatters in the story Alice In Wonderland was not based total in fiction. Felt production in the 1700s utilized mercury/mercury compounds. "Hatters" were well known to have the occupational hazard of going "mad," although the medical profession was not aware that mercury was the cause. Mercury guilders also suffered nervous trauma and eventual death, at early ages. The prime route of entry wasóand is todayóinhalation of the vapors. Mercury and some of its compounds can also be absorbed through the skin, and it can be ingested.

Acute symptoms of exposure include burning mouth/throat, thirst, shock, cardiac arrhythmia, nausea, vomiting, abdominal pain, bloody diarrhea, eye damage upon contact and a metallic cast to skin/eyes.

Chronic exposure symptoms include muscle weakness, sore mouth, anorexia, fever, nausea, abdominal pain, vomiting, diarrhea, headache, nervous disorders, kidney/liver dysfunction, hand tremors, insomnia, loss of memory, loosening of teeth and excessive salivation. There is some evidence that mercury/mercury compounds may be carcinogenic and teratogens (linked to birth defects). Mercury is a protoplasmic poison. After exposure, it circulates in the blood and is stored in the liver, kidneys, spleen and bone.

Skin contact with soluble mercury compounds can result in severe corrosive effects. Some mercury compounds create toxic mercury vapors.

Mercury and its compounds are regulated under OSHA, RCRA and SARA. OSHA PELs are: 0.05 mg/cubic meter for mercury vapor, 0.01 mg/cubic meter for alkyl organic mercurial compounds, and 0.10 mg/cubic meter for aryl organic and inorganic compounds. Under RCRA regulation, D-009 waste that contains more than 16 mg/Kg mercury must be sent for recycle. Waste that contains less than 16 mg/Kg must be treated to a TCLP level of 0.025 mg/L. D-009 wastewater must not contain more than 0.030 mg/L of mercury. While the metal finishing industry has no federal categorical standards on mercury in the wastewater discharges, mercury is typically regulated on a local level, at extremely low concentrationsó typically from 0.1 ppb to 0.5 ppb. Typical mercury compounds that may have been used in metal finishing processes in the past are:

Mercuric Nitrate

This mercury compound is a white, deliquescent powder. It is a powerful oxidizer that can form explosives with acetylene, ethanol, cyanide and sulfur. It decomposes upon heating, to yield elemental mercury and toxic oxides of nitrogen. Typical uses include bluing and coloring of steel, and as a sensitive analytical method for the determination of traces of chloride in acidic solutions by titration.

Mercuric Chloride

This is a white, crystalline material or powder that becomes volatile to a toxic vapor at 136 C, and decomposes to elemental mercury upon further heating. This compound is considered teratogenic. Mercuric chloride is included in some formulations for the coloring of ferrous alloys and in the old zinc-mercury alloy plating process.

Mercuric Oxide

This is a bright orange-red powder that was used for red paint pigments at one time. It is a powerful oxidizer that can readily explode upon mixture with reducing agents, phosphorus, sulfur, or magnesium. Mercuric oxide reacts violently with peroxide and ethanol. Heating mercuric oxide creates toxic mercury fumes. Past uses of this compound include a "mercury dip," to promote adhesion of plated deposits on tellurium-copper alloys and as an alternate source of mercury ions for zinc-mercury plating.

Zinc-mercury alloy plating was performed early in this century, in an attempt to obtain a more pleasing "white" zinc deposit. The bath contained zinc cyanide, sodium cyanide, sodium hydroxide, and mercuric chloride or oxide (0.25 g/L). This bath contained about 50,000 times the amount of mercury now allowed in the plant wastewater discharge. The anodes were zinc, containing 0.5-1 percent mercury. Needless to say, we doubt if this process is in use today. However, if you are aware that your facility may have used this process (or any mercury-containing process) in the past, and are having a difficult time with mercury violations, it may mean removing the associated plumbing that had handled the rinsewater from such an operation.

Because mercury is regulated to such low concentrations, chemical purity becomes a significant issue. Prime suspects for mercury contamination are sodium hydroxide, sulfuric acid, and color dyes used in anodizing (especially red ones). The vast majority of metal finishing processing solutions contain sodium hydroxide, so it becomes the prime suspect in a mercury hunt.

How does mercury get into sodium hydroxide? Sodium hydroxide is produced primarily in two ways: A diaphragm cell, and in the mercury cell. (See Fig.) The mercury cell consists of two plating compartments. In the first compartment, a graphite anode is used to "plate" sodium onto a mercury cathode out of a brine. The mercury-sodium amalgam is then used as an anode to plate sodium out of the mercury amalgam and into a water solution, which then becomes a sodium hydroxide solution. The sodium hydroxide solution can become as concentrated as 70 percent, using this method of production. Unfortunately, the sodium hydroxide is contaminated with as much as 1,000 ppb or more of mercury. Thus, mercury cell sodium hydroxide, has no place in a metal finishing shop.

The mercury you may find in sulfuric acid, dyes or other chemical products generally does not come from any production method. Its simply there as a chemical impurity.

On the Hunt

If you are on a mercury hunt, the first step is to contact each chemical supplier you use, and request a mercury analysis of the product or certification that the product is mercury-free. This may not necessarily allow you to eliminate all mercury sources.

You may still need to analyze various wastewater streams for mercury, in order to isolate specific sources. There is no "quick-and-dirty" test to detect a few ppb of mercury, so you will need a laboratory, equipped with a cold vapor atomic absorption spectrophotometer. One analysis typically costs $25, so choose your suspects wisely, concentrating on processes that contain sodium hydroxide and sulfuric acid.

The keys to compliance with mercury discharge limitations are detection and elimination, as opposed to treatment. If you find a chemical solution that contains a significant amount of mercury, contact the supplier for an alternate material.

Change suppliers if necessary. Other actions to eliminate mercury from your facility:

1. Eliminate the use of mercury-containing thermometers

2. Eliminate mercuric chrome from first aid kits

3. Check your quality control laboratory and eliminate any mercury-containing chemicals. If your plant performs acid copper plating, perhaps the laboratory uses a mercuric nitrate titration to determine the ppm chloride. Eliminate this procedure and substitute turbidimetric one.

4. Eliminate use of mercury vapor lighting. Broken fluorescent tubes can yield traces of mercury in the plant.

5. If you are plating onto electronic componentsóespecially semiconductors, solar cells, battery components, thin film transistors, infrared detectors and ultrasonic amplifiersórequest mercury-free certifications from your client. These devices may contain mercury-selenide or mercury telluride, which will then contaminate your processing baths.

Waste Treatment

Very low concentrations of mercury have been reduced to compliance level by treatment with organic sulfides, such as DTC (diethyl trithiocarbamates). Mercuric sulfide has an extremely low solubility (constant on the order of 1 x10 -54).

Not recommended, but practiced on occasion, was aeration of the waste stream, with and without reducing agents. The literature also contains references to reduction with sodium borohydride at pH 8-12. The reaction is carried out at room temperature, and takes 30 minutes retention. The amount of sodium borohydride required is 2-3 times the stoichiometric amount. The elemental mercury can then be gravity separated, or the water/mercury mixture can be filtered through a packing of zinc granules. The zinc-mercury amalgam is then sent out for recovery.

If You Must Handle Mercury

Spills of mercury or mercury-containing compounds or solutions must be cleaned up promptly and effectively. Wear nitrite rubber gloves and a self-contained breathing apparatus. Collect droplets of mercury with suction and place into a stoppered bottle. Fine droplets can be covered with an adsorbent. Workers who work with mercury-containing processes should undergo safety training specific to the hazards associated with the process and with mercury. Employees should take a daily shower and clothing change. Workers should have an annual (or more frequent) physical exam and blood/urine test for mercury.

Individuals who are pregnant, or who have kidney, liver, nervous system, sensitive skin, lung or stomach problems, should not work with or near mercury-containing processes. First aid is limited to flushing skin and eyes, upon contact, with large volumes of water. In all cases of exposure, a doctor must be consulted.

When All Else Fails

If you have made an extensive effort at eliminating mercury from your facility, and low-level violations of discharge regulations persist, you may be able to obtain an alternate limit on mercury in your discharge. In Chicago, for example, the following "relief" is available.

Discharge of mercury in concentrations exceeding 0.0005 mg/L are not allowed, except when:

1. The discharger does not use mercury, or the discharger uses mercury, and this use cannot be eliminated; or the discharger uses mercury only in chemical analyses or in laboratory or other equipment, and takes reasonable care to avoid contamination of wastewater; and

2. The discharge mercury concentration is less than 0.003 mg/L on a monthly average, 0.006 mg/L in a daily composite, and 0.015 mg/L in any grab sample;

3. The discharger is providing the best degree of treatment consistent with technological feasibility, economic reasonableness and sound engineering judgment. This may include no treatment for mercury; and

4. The discharger has an inspection program likely to reduce or to prevent an increase in the level of mercury discharges.

Another Source of Mercury (from P&SF, 2/93)

Apparently some platers are still using mercury amalgam ORP probes. These require the wastewater treatment operator to "strip" the old mercury layer off and dip the end of the probe into a puddle of mercury. The acid strip then becomes a source of mercury contamination when dumped into the treatment system. If the operator is "sloppy" and spills some mercury the result is further contamination. Metal finishers who use these probes must segregate the area where mercury is used and should send the stripping acid to a mercury reclaimer. The personnel should also be safety-trained in handling the mercury. The best solution to this problem is to find a non-mercury probe that works in your treatment system.

References

The following sources were used, to obtain information presented:

Sewage and Waste Control Ordinance, The Metropolitan Water Reclamation District of Greater Chicago.

Richard J. Lewis, Sr., Hazardous Chemicals Desk Reference, 2nd edition.

Allen G. Gray, Modem Electroplating, John Wiley, publisher.

Fred Lowenheim, Modern Electroplating, 3rd Ed., John Wiley, publisher.

Metal Finishing Guidebook and Directory, Metals and Plastics Publications, Inc.

E.R. Plunkett, M.D., Handbook of lndustrial Toxicology, Chemical Publishing Co. Inc.

W. John Williams, Handbook of Anion Determination, Butterworths, publisher.

Margaret-Ann Armour, Lois M. Browne, Gordon L. Weir, Hazardous Chemicals Information and Disposal Guide, University of Alberta.

N. Irving Sax, Richard J. Lewis Sr., Hawleys Condensed Chemical Dictionary, 11 th Ed.

Richard J. Lewis Sr., Saxs Dangerous Properties of Industrial Materials, 8th Ed., Van Nostrand Reinhold.

F. Albert Cotton, G. Wilkinson, Advanced Inorganic Chemistry, John Wiley, publisher.

Kenneth Cherry, Plating Waste Treatment, Ann Arbor Science, publisher.