Dent Mater8:359-361, November,1992

Environmental hazard evaluation of amalgam scrap P.L. Fan, S.-B. Chang, C. Siew

American Dental Association Health Foundation, Chicago, IL, USA

Abstract. Amalgam scrap was subjected to two different Environmental Protection Agency (EPA) extraction procedures to determine if it presents an environmental hazard. The results indicate that concentrations of mercury and silver in the extracts do not exceed the EPA's maximum allowable concentrations. It was concluded that amalgam scrap is not a hazardous solid waste. Proper handling of amalgam scrap disposal by recycling is, however, highly recommended.

INTRODUCTION Amalgam scrap is often saved in dental offices and sold to refiners for recycling of the silver and mercury (Council on Dental Materials, Instruments and Equipment, 1982; 1984; 1991). Proper handling of amalgam scrap during the recycling process does not cause environmental pollution from mercury. Mishandling by a refiner can result in mercury being released into the soil as evidenced in an incident in Connecticut (Strom, 1988). The disposal of amalgam scrap as a solid waste rather than as a recyclable material has raised questions about classifying amalgam scrap as hazardous waste. The criteria for Environmental Protection Agency (EPA) classification of solid waste materials as hazardous waste depend on their ignitability, corrosivity, reactivity and toxicity. The EPA's determination of toxicity is by the Extraction Procedure Toxicity Characteristic [EPTC] (Environmental Protection Agency, 1987a). EPTC defines the toxicity of a waste material by measuring the potential for the toxic constituents in the waste to leach out and contaminate ground water at levels which cause health or environmental concerns. It also establishes regulatory maximum allowable levels for a number of chemicals in the extracts when the solid waste is subjected to an established EPA extraction procedure (Environmental ProtectionAgency, 1987a; 1990). Inorganic species currently regulated by the EPA under EPTC are: arsenic, barium, cadmium, chromium, lead, mercury, selenium and silver. To determine ifa waste such as amalgam scrap should be regulated as a hazardous waste, the waste is subjected to the EPA established extraction procedure which is intended to simulate the leaching action that occurs in municipal landfills. If any of the extracted species exceeds the regulatory level, the waste is classified a hazardous waste and therefore requires special handling and disposal under EPA regulations. The dissolution of amalgam in aqueous solutions such as saline, saliva, diluted organic acids, etc. in the laboratory environment has been well documented in the dental literature (Takahashi et al.,1981; 1989; Chew et al., 1989; Okabe et al. 1987; Ferracane et al., 1987; Palaghias 1986a; 1986b; Konzons et al. 1982). The extent of amalgam dissolution was

found to be influenced by the types of amalgam studied, the conditions of the amalgam surfaces and the chemical nature of the solutions used in the studies. Although another study indicates that amalgam scrap would not be a hazardous waste (Roger, 1989), there is no published report on the evaluation ofamalgam scrap using the EPA extraction protocols. Before March 1990, the EPA's waste toxicity evaluation method was known as the Extraction Procedure Toxicity Test [EPTT] (Environmental Protection Agency, 1986; 1987b). In the EPTT procedure, a representative sample of the waste is extracted with an aqueous solution of acetic acid to be maintained at a pH of 5, and the extract is then analyzed to determine if the concentration of a particular element exceeds the EPTC value (Environmental Protection Agency 1986; 1987b). Frequent monitoring and adjustment ofthe pH ofthe solution during the 24 h extraction period as required by the EPTT affected the reproducibility of the test and also presented a hardship to the laboratory technician conducting it. In March 1990, the EPA revised and modified the EPTT and a new procedure identified as the Toxicity Characteristic Leaching Procedure [TCLP] was adopted (Environmental Protection Agency, 1990). With the TCLP, a preliminary evaluation step is included to determine the type of extraction fluid to be used. The new procedure eliminates the need to monitor the pH of the solution during the 18 h extraction period. The maximum allowable levels for the leachable inorganics as defined in the EPTC remain unchanged. This study utilized both the EPTT and the TCLP to determine if amalgam scrap should be classified as a hazardous waste, based on the maximum allowable levels for mercury and silver established by the EPA for EPTC. The concentrations of copper and tin were also measured even though they are not included in the EPA EPTC consideration.

MATERIALSAND METHODS All chemicals used were ACS grade. Amalgam scrap was collected from specimens used in conducting specification tests in the laboratory of the Council on Dental Materials, Instruments and Equipment of the American Dental Association. All scrap used was at least 30 d old and was stored dry in a closed container at room temperature. Twenty-four hours prior to the extraction, the scrap was reduced in size by hammering between a plastic plate and a steel plate to small pieces (estimated to be 0.5 mm or less in maximum dimension) to satisfy the particle size requirement of the testing method which specified that the particles pass through a 9.5 mm standard sieve or have a surface area of at least 3.1 cm2/g. Specimens that met these requirements were used for the

Dental Materials~November 1992 359

extraction procedure. Each specimen (100 g or more ) of the reduced size amalgam scrap was weighed and then placed in a 2L Nalgene polyethylene bottle. For EPTT, deionized water equal to 16 times the weight of the amalgam sample was added to the weighed sample in the extraction bottle. The solution pH was adjusted to 5.0+0.2 using 0.5 N acetic acid. No acetic acid was added if the pH was lower than 5.0. The specimens were then rotated on the extractor described below. The pH of the solution was frequently monitored and, if the pH rose above 5.2, it was adjusted to 5.0_+0.2 by the addition of acetic acid. This was done for at least the initial 6 h. If, at the end of the 24 h extraction period, the pH of a solution was not below 5.2, and, if the maximum amount of acid (4 mL per gram of solids) had not been added, the pH was again adjusted to 5.0+0.2 and the extraction was continued for an additional 4 h during which the pH was adjusted at 1 h intervals. After extraction, the liquid-solid ratio of the specimen was adjusted to 20:1 by adding an appropriate amount of deionized water. For TCLP (Environmental Protection Agency, 1990), a preliminary evaluation as defined in the procedure was done by adding 5.0 g of amalgam scrap to 96.5 mL of deionized water (pH = 6.23 _+0.11). After adding 3.5 mL of 1N HC1, the solution was heated to 50°C and held at that temperature for 10 min before it was allowed to cool to the ambient temperature. The pH of the solution was measured and found to be 2.52 _+ 0.14. Based on the result of this preliminary test, extraction fluid #1, as defined in the TCLP, was used in the extraction procedure. This extraction fluid was prepared by mixing 5.7 mL of glacial acetic acid, 64.3 mL of 1N sodium hydroxide and an appropriate amount of deionized water to make a final volume of 1 L. The pH of this extraction fluid was 5.0. Each amalgam sample (100 g or more) was weighed and then placed in a sample bottle. For each 100 g of amalgam sample, 2000 mL of extraction fluid were added to the bottle. Extraction was performed for 18 h on the rotating extractor described below. Extract solutions obtained by either the EPTT or the TCLP method were separated from the solid amalgam scrap by filtering through a 0.45 ~m, type HA, Millipore membrane (Millipore Corporation, Bedford, MA, USA). The filtrate was analyzed for silver, copper, tin, and mercury using the methods described below. Three specimens were tested along with two control blanks (i.e., without the inclusion of amalgam scrap in the extraction fluid). Mercury concentration was analyzed using a mercury vapor analyzer (PE AS-50, Perkin-Elmer Corp., Norwalk, CT, USA) with the cold-vapor technique. The concentrations of silver, copper, and tin in the extracts were determined by using a Perkin-Elmer flame atomic absorption spectrophotometer (AAS), model AAS-2386 (Perkin-Elmer Corp., Norwalk, CT, USA). The detection limits of these analytical methods were: mercury 0.005 mg/L, silver 0.1 mg/L, copper 0.2 mg/L, and tin 4 mg/L. The pH of the extraction liquid was monitored using an Orion 701A ion analyzer with Ross Model 8103 combination pH electrode (Orion Research Incorporated, Cambridge, MA, USA). The extraction procedure requires that the sample bottles be rotated at 30 rpm during the extraction period. This assembly was custom built to meet the EPA specifications (Environmental Protection Agency, 1986; 1990). The concentrations of mercury, silver, and copper from the 360 Fan et aL/Environmental evaluation of amalgam scrap

TABLE 1: CONCENTRATIONSOF MERCURY,SILVER, AND COPPER IN EXTRACTS USING EXTRACTIONPROCEDURETOXICITYTEST (EPI-I) AND TOXICITY CHARACTERISTICLEACHING PROCEDURE(TCLP) EPA Concentration, mg/L EPA Extraction Amalgam scrap specimens Control Maximum Protocol Metallic Allowable Element Mean+S.D. (n)* Range Mean+ (n) level, mg/L EPTT

Mercury 0.10_+0.02 (3) 0.07-0.12 Silver 0.3+0.2 (3) 0.1 -0.4 Copper 50 + 11 (3) 38- 60

Environmental hazard evaluation of amalgam scrap.

Amalgam scrap was subjected to two different Environmental Protection Agency (EPA) extraction procedures to determine if it presents an environmental ...
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