The objective of this investigation was to carry out residual mercury (Hg) determinations and toxicity characteristic leaching procedure (TCLP) analysis of used amalgam capsules. All capsules tested retained Hg.  TCLP analysis of the triturated capsules showed Sybraloye and Contoure leached Hg at greater than the 0.2 mg/l Resource Conservation and Recovery Act (RCRA) limit.

This study provides knowledge of an additional, rarely acknowledged pathway of dental mercury released into our environment. Showing once again that dentists who are generally unaware of the hazards of mercury are unknowingly polluting the environment with used dental mercury amalgam capsules, which are discarded and later incinerated or land filled.

Dental Materials 18 (2002) 289±294

Residual mercury content and leaching of mercury and silver from used amalgam capsules

M.E. Stone*, E.D. Pederson, M.E. Cohen, J.C. Ragain, R.S. Karaway, R.A. Auxer, A.R. Saluta

The Naval Dental Research Institute, Building 1H, 310A B Street, Great Lakes, IL 60088-5259, USA

Introduction

Environmental aspects of mercury (Hg) have recently become an important issue for dentistry [1±6]. While environmental concerns continue to center on the heavy-metal content of dental-unit wastewater, issues involving solid- waste disposal have the potential to become equally significant. Two reports are available that describe the residual Hg content of disposable amalgam capsules [7,8] with the latter reporting values as high as 33.89 mg of Hg per capsule [8].

The policy that governs the disposal of solid waste in the United States is promulgated in the Resource Conservation and Recovery Act (RCRA) [9]. Through this act Congress provided the United States Environmental Protection Agency (USEPA) with the foundation to develop regulatory programs to manage solid waste, hazardous waste, medical waste, and underground storage tanks.

In 1965, Congress enacted the Solid Waste Disposal Act (SWDA), which provided the first federal statutory requirements intended to improve solid-waste disposal practices. The SWDA was modified in 1970 by the Resource Recovery Act, and modi- fied again in 1976 by RCRA. RCRA established a system for controlling hazardous waste from its point of generation to its final disposal (cradle-to-grave). RCRA has been amended many times since 1976, most notably by the Hazardous and Solid Waste Amendments (HSWA) of 1984. HSWA was fashioned principally in response to citizen concerns that existing methods of hazardous waste disposal were neither safe nor sufficient.

The Federal Facilities Compliance Act modified RCRA again in 1992. This act makes the federal government part of the regulated community, and it holds government facilities and managers to the same array of enforcement measures, including fines and penalties, as the rest of the nation. The latest legislative change to RCRA was the Land Disposal Program Flexibility Act of 1996, which provided regulatory flexibility for the land disposal of certain hazardous wastes.

To help determine if a waste is hazardous, the USEPA designed a laboratory analysis called Toxicity Characteris- tic Leaching Procedure (TCLP) [10,11], which determines the mobility of analytes in an acetic acid buffer solution. The concentration of regulated analytes in the extract deter- mines the toxicity characteristic of a sample, and therefore whether it is subject to disposal regulations under RCRA. The test was designed to predict whether landfill wastes might leach dangerous levels of chemicals into ground water. TCLP regulatory levels exist for 40 different toxic chemicals. The TCLP limits for Hg and Ag are 0.2 and 5.0 mg/l, respectively [10,11].

Facilities that generate less than 100 kg (220 lb) of hazardous waste per month or less than 1 kg (2.2 lb) per month of acutely hazardous waste are excused from most federal RCRA requirements and are classified as 'Conditionally Exempt Small Quantity Generators' (CESQGs) [9]. An essential caveat to this exemption is that some states prohibit the disposal of any hazardous waste into municipal solid-waste landfills, even from CESQGs. Facilities that produce solid waste are well warned to contact local, regional, and state solid-waste regulators to confirm regulatory requirements.

The present study was undertaken to assess residual Hg levels from currently available dental-amalgam capsules and to obtain information on the potential leaching of mercury (Hg) and silver (Ag) from these capsules. Leaching of these metals above regulatory levels could make the disposal of these capsules a vexing issue for the clinical dentist and for environmental regulators. It was hypothesized that certain capsule design features might provide opportunities for Hg and/or amalgam retention. These features included the presence of Hg-containing packets, mixing pestles, and internal grooves associated with the ultrasonically sealed capsules. In addition, it was reasonable to expect that residual Hg might be greater in double-spill than in single-spill amalgam capsules.

Discussion

While Hg contamination of dental-unit wastewater is one contributor to the environmental burden of clinical dentistry, the disposal of solid wastes is another major concern. This study examined both the residual Hg in used amalgam capsules and the leaching of Hg and Ag from used capsules. While it is known that used amalgam capsules contain resi- dual Hg [7,8], this is the first published study to demonstrate the leaching of metals from used capsules.

The residual Hg content of the 10 brands of capsules used in this study was less than the levels reported in two previous studies [7,8]. Barkmeier et al. [7] reported a mean Hg per capsule of 12.1 mg. Cheuk et al. [8] evaluated six different amalgams and found residual Hg to range from 2.75 mg/capsule for Valiant (Vivadent) to 15.55 mg/capsule for Dispersalloy (Dentsply/Caulk).

The mean residual Hg in this study ranged from 0.125 to 1.255 mg. The lower levels of residual Hg seen in this study may reflect the different digestion procedures used or the use of single-spill capsules in six of the 10 brands studied. The segregation of capsules into three groups appears to be a function of the capsule design. However, the definitive influence of capsule design on Hg retention could not be determined because of the overlapping design features of capsules used in the current study. More complicated capsule designs may tend to retain more amalgam and therefore more Hg. In the case of Dispersalloy, the pestle adds increased surface area where amalgam and Hg can adhere. In addition, forces generated in pestle containing-capsules during trituration may cause amalgam to adhere more tightly to insides of the capsules. As a group, the ultrasonically fused capsules retained the next highest amount of Hg. These capsules have a groove that tends to retain amalgam in an annular pattern (Fig. 1). The third group had the simplest capsule design and retained the least amount of Hg.

Fan et al. [14] and the United States Air Force Dental Investigation Service [15] have completed TCLP studies of amalgam scrap. However, to our knowledge, this is the first published report that applied this procedure to disposable amalgam capsules. The TCLP data from 10 batches of 25 capsules revealed two brands that exceeded the 0.2 mg/l RCRA limit for Hg (highlighted in Table 3). In a prior preliminary TCLP study [16] done with pooled 100-g samples across several different brands of amalgam capsules, Hg levels of TCLP extracts averaged 0.1291 mg/ l with one sample exceeding the RCRA limit for Hg (N ˆ 12, range ˆ 0.0369±0.6700, SD ˆ 0.1733), and Ag levels were non-detectable (, 0.0500 mg/l) except for one measurement of 0.0503 mg/l. It is of interest to note that the brands that retained the most Hg did not have the highest Hg levels in the TCLP extracts. This surprising and unantici- pated result is now the subject of further research.

In several cases, capsules that contained Hg packets did not mix due to failure of the packets to break open during trituration. This phenomenon was seen across many brands that used Hg packet technology and was the reason that only 20 capsules were used for the retained Hg analysis of the Optaloye II capsules (five Optaloye II capsules did not mix). Anecdotal information from several Navy dental clinics confirms this problem, which can be particularly severe in some lots of the same amalgam brand. Unruptured Hg packets can create a serious disposal dilemma as they contain a size- able amount of elemental Hg (Table 1).

Discarding capsules in municipal-solid-waste landfills is not ideal and could, in some cases, violate state solid-waste- discharge statutes, since some states regulate all generators of hazardous waste, even from CESQGs. Recovery of heavy metals through the recycling processes is environmentally more responsible than disposal in landfills where the potential exists for metals to leach into ground water. Reclaimed metals can be reused in the manufacturing of dental amalgam.

Incineration of used amalgam capsules must be avoided to prevent volatilization of Hg to the atmosphere. Deposi- tion of atmospheric Hg to land, lakes, rivers, and streams can be substantial [17] and lead to the bioaccumulation of organic Hg in fish and other aquatic organisms [17±26]. Nearly 100% of the Hg that concentrates in fish tissue is methylmercury [17,26]. High Hg levels in remote pristine lakes, where atmospheric deposition appears to be the key instrument of contamination, is further evidence of the importance of this pathway [17,24].

Conclusion

Capsule design features can influence retention levels of amalgam and Hg in used capsules. Retained Hg in used amalgam capsules can leach in amounts that make their disposal exigent and problematic. The leaching of Ag does not appear to be a subject for concern at current RCRA levels.