Mercury discharge into the environment, regardless of the source, has come under increased scrutiny from the U.S. Environmental Protection Agency (EPA). Mercury is classified as a persistent bioaccumulative toxin, and it is among the top 20 hazardous substances listed by the Agency for Toxic Substances and Disease Registry/Environmental Protection Agency.⁴ A 1996 EPA conference on mercury in the Midwest emphasized the need to eliminate mercury medical waste from entering the wastewater stream.⁵ Moreover, in 1997, the EPA issued a 1,700-page report to Congress stressing the need for closer scrutiny and regulation of mercury emissions.⁶ In 1998, the EPA and the American Hospital Association signed a memorandum of understanding to significantly cut hospital mercury wastes by 2005. This agreement included the total elimination of mercury-containing hospital wastes and a one-third reduction in other wastes.⁷

Mercury in dental amalgam exists in a fairly stable equilibrium, with only minute amounts released into the surrounding environment.^8–12 Despite this, amalgam should be recycled or processed properly and not be disposed of as garbage, medical infectious waste (for example, in "red bag" or biohazard containers) or in sharps containers. Furthermore, amalgam should not be rinsed down the drain into municipal sewer lines. Some communities incinerate municipal garbage, medical waste and sludge from wastewater treatment plants. The high temperatures used in incineration can alter the physical properties of amalgam to cause the release of mercury into the environment. Proper processing or disposal of dental amalgam waste prevents the release of additional mercury into the environment.

The chlorine disinfectant materials discharged the most mercury ions into solution.

Accordingly, amalgam excess from restorative procedures or amalgam scraps retrieved from dental unit suction traps can be processed for proper disposal, recycling or both. Some federal agency guidelines^13,14 and commercial recycling companies require that submitted amalgam that has been in contact with body fluids (for example, retrieved from dental unit suction traps) be disinfected before undergoing reclamation.¹⁵ The rationale for this disinfection is that materials retrieved from an oral evacuation system may be infectious and biologically hazardous. Some agencies that process recyclable medical materials also may require documentation that the submitted items are noninfectious.¹⁴

Sodium hypochlorite traditionally has been recommended as a disinfectant for amalgam retrieved from dental unit suction traps.² However, some investigations have demonstrated that oxidizing disinfectants—including bleach—mobilize soluble mercury from amalgam into the disinfectant solution.⁴ If the oxidizing disinfectant solution is not processed properly or is discarded into the public wastewater system, it adds to the environmental burden of mercury.

We conducted this study to evaluate the effect of representative disinfectants on mercury released from amalgam.

	MATERIALS AND METHODS

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We triturated a high-copper, spherical amalgam alloy in a dental amalgamator following the manufacturer’s recommendations. We stored the resulting dry amalgam alloy pellets in a sealed container for one month. We then pulverized the amalgam alloy pellets using a mill and put them through 900- and 710-micrometer standard testing sieves.

We prepared five samples for each disinfectant solution from each representative class (Table 1) using the following procedure. We placed 20 grams of the sized amalgam alloy particles into a new, sealable 50-milliliter plastic test tube, followed by 50 mL of a disinfectant solution. If required, we prepared the disinfectant solution following its manufacturer’s recommendations immediately before use. Then we sealed the test tube and agitated it for 30 seconds to disperse the amalgam alloy powder in the disinfection solution. Next, we left the mixture undisturbed for contact time recommended by the disinfectant’s manufacturer. Finally, we filtered the mixture through 0.2-µm filters and placed the supernatant in clean and dry test tubes, which then were sealed and submitted for testing.

	RESULTS

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The results of our study are given in Table 2 (page 918). We found that the chlorine disinfectant materials discharged the most mercury ions into solution, followed by the bromide, iodophor, peroxide/peracetic acid and phenolic disinfectants. The distilled water control discharged the same levels of mercury ions as the peroxide/peracetic acid disinfectant. Surprisingly, the quaternary ammonium compound disinfectant did not cause mercury ion release that was detectable above the 0.2 parts per billion threshold of the method used.

	DISCUSSION

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The most surprising finding was that the quaternary ammonium compound disinfectant did not mobilize mercury ions into solution.

We found that the chlorine disinfectants discharged the greatest amounts of mercury ion, followed by the bromide, iodophor, peroxide/peracetic acid and phenolic disinfectants. It is interesting to note that the peroxide/peracetic acid and triphenolic disinfectants discharged similar levels of mercury as the distilled water, which we used as a control. The most surprising finding of our evaluation was that the quaternary ammonium compound disinfectant did not mobilize mercury ions into solution. In fact, the initial test results were unexpected, so we considered them to be either a laboratory error or equipment malfunction, and we tested another group of samples. Repeated testing of the newly prepared samples combined with atomic absorption spectrophotometer recalibration produced the same results in both control and quaternary compound samples.

The rate of mercury released into a liquid environment depends on the bond strength of mercury in the amalgam; the presence, nature and stability of any surface oxide films; and the chemical transformations of mercury in the solution.¹⁶ Since we used the same amalgam in all of the tests, the binding of mercury in the different amalgam phases and the amalgam’s film-forming ability were the same. However, the properties and stability of the oxide surface pilms, which form a barrier to mercury dissolution, depend on the chemistry of the solution. Many chlorine-containing solutes destabilize metal oxide films, including the tin oxide of dental amalgam; this degrades the oxides’ ability to reduce the amount of mercury released. The observed high rate of discharge associated with the use of chlorine disinfectants can be attributed mainly to this effect. The comparative aggressiveness of the ions in the solutions with respect to oxide films may be largely responsible for the ranking in Table 2.

Another factor that may play a role in the release of mercury is the oxidation power of the environment within which the amalgam may exist. A higher oxidation power increases the electromotive potential of the metal, as well as the rate of conversion of elemental mercury-to-mercury ions. Although the mercury released from dental amalgam in synthetic saliva is insensitive to the potential in a wide range,¹⁶ the relationship may be different in other environments if the oxide properties are potential-dependent. Higher oxidation power also accelerates the transformation of the released elemental mercury-to-mercury ions.¹⁷ Since the solubility of elemental mercury in aqueous solutions is quite low, oxidation to ionic mercury may increase the mercury dissolution rate by lowering the solubility barrier.¹⁷

The relative oxidation potential of a solution can be determined by measuring the open-circuit potential of an inert electrode potential with respect to a reference electrode. In light of the surprising results we found in this study, we sought to compare the oxidative potentials of the quaternary ammonium compound and a representative chlorine disinfectant. Accordingly, we measured the reduction oxidation (redox) potentials of the quaternary ammonium compound and 5.25 percent sodium hypochlorite bleach, as well as the corrosion potential of the high-copper, spherical amalgam alloy in both solutions at 25 C using a temperature-controlled corrosion cell and a computer-controlled electronic potentiostat. We measured the stabilized potentials with respect to a standard saturated calomel electrode. The results presented in Table 3 show that both the redox potential and the corrosion potential of high-copper, spherical amalgam alloy were substantially lower in quaternary ammonium compound than in the bleach. It is possible that the low redox and corrosion potentials may have contributed to the low rate of mercury released by the quaternary ammonium compound by keeping the potential in a range beneficial for the formation of an oxide barrier and by reducing the rate of conversion of elemental to ionic mercury.

	CONCLUSIONS

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