Environ. Sci. Technol. 2003, 37, 367-371
Toxicity Characteristic Leaching Procedure and Iron Treatment of Brass Foundry Waste DOUGLAS S. KENDALL* National Enforcement Investigations Center, U.S. Environmental Protection Agency, P.O. Box 25227, Building 53, Denver Federal Center, Denver, Colorado 80225
The Toxicity Characteristic Leaching Procedure (TCLP) is used by the United States Environmental Protection Agency to determine if wastes contain extractable components subject to hazardous waste regulations. This paper examines the limitations of the TCLP and the way it is used by studying a particular example. Waste casting sand from brass foundries to which iron metal has been added passes the TCLP test but when placed in a landfill for several years may start to leach lead, copper, and zinc. Results of TCLP tests of waste sand alone and with the additives iron metal, zinc metal, hydrous ferric oxide, and hematite are reported. Three processes were studied: reduction by metallic iron, sorption by hydrous ferric oxide, and precipitation of hydroxides. Lead, copper, and zinc behave differently with respect to these three processes, and their measurement allows some deductions as to what is occurring in a TCLP test or a landfill. Iron addition does not result in long-term stabilization of a waste placed in the ground. The chemistry of a laboratory extraction can be very different from the chemistry of a waste placed in the environment. Wastes that are treated to pass the TCLP test, but are not permanently stabilized, are a threat to the environment.
Introduction A leaching test, the Toxicity Characteristic Leaching Procedure (TCLP) (1), is used by the United States Environmental Protection Agency (EPA) to determine if a waste has the characteristic of toxicity and is therefore hazardous. If the extract from the leaching test contains more than the regulatory limit of arsenic, barium, cadmium, chromium, lead, mercury, selenium, silver, or any of a number of organic compounds, then the waste is considered toxic. The same leaching test is also used to determine if hazardous wastes, after treatment, can be placed in a hazardous waste landfill. The TCLP was designed for industrial wastes mixed with municipal waste. An acetate buffer is used to simulate leaching by organic acids from the decomposition of municipal wastes. The TCLP is expected to test a wide variety of wastes, and wastes passing the test are expected to be stable for many years when placed in a landfill. Waste generators use knowledge of the TCLP to modify their processes and waste treatment procedures so that their wastes qualify for land disposal or are not classified as hazardous. Sometimes an additive, such as lime, phosphate, * Author e-mail:
[email protected]; phone: (303)236-6089; fax: (303)236-5116. 10.1021/es020621n Not subject to U.S. Copyright. Publ. 2003 Am. Chem. Soc. Published on Web 12/06/2002
or iron, is physically mixed with a waste so that the mixture passes the TCLP test. When placed in a landfill, the additive may separate or degrade. If there is no permanent bond between the waste and the additive, it is more likely that the treatment may lose its effectiveness and that toxic substances will leach from the waste. Wastes that pass the test and are disposed of in landfills, yet are not stable over the long term, can release toxic substances into the groundwater (2). The example that was studied is a foundry in Texas that cast brass with a composition of 80-88% copper, 1.5-7% lead, 5-9% zinc, and 3-6% tin. Brass foundries use sand molds for their castings. The sand is recycled and becomes contaminated with lead, copper, and zinc. Waste sand should be managed correctly as the metals will leach in the TCLP test and in a landfill. To be able to dispose of their waste sand as nonhazardous, some foundries added about 10% by wt iron metal shavings or filings to their waste sand. This reduced the concentration of leachable lead to well under the TCLP regulatory limit of 5 mg/L (3). The waste sand of the foundry of this study was placed in a landfill but not mixed with municipal waste. After being in the landfill for 3-8 yr, waste sand was removed for testing as part of EPA enforcement efforts. Part of those efforts is this study, whose purpose was to understand iron treatment of waste foundry sand and its long-term stability. The effects of iron metal, hydrous ferric oxide (HFO), and other additives (dried HFO, hematite, and zinc) on TCLP tests of waste foundry sand were examined. Studies were also done of the sorption of lead, copper, and zinc by HFO and of the solubility of the hydroxides of lead, copper, and zinc under the conditions of the TCLP test. An additional objective was to consider whether using a prearranged leaching test can serve to determine which wastes are hazardous and which are suitable for placement in landfills. When there is a strong incentive for passing these tests, the result may not be permanently stabilized wastes and protected groundwaters. Perhaps another approach to evaluating wastes and their disposal is needed.
Experimental Section Sample Collection. The brass foundry and the landfill are located in Texas, southeast of Dallas. Waste foundry sand was obtained from the foundry. Waste foundry sand samples that had been treated with iron by the foundry were obtained from the municipal landfill where they had been buried. The material was segregated from the municipal waste and had been in the landfill for 3-8 yr, according to company records. Since the sampling, no additional waste sand has been added to the landfill. A drill rig was used to obtain sand from five borings in two different cells of the landfill. The cells were lined with clay, had a clay cap, and were about 7 m deep. The main purpose of the drilling was to obtain samples for TCLP testing for metals. Such samples are not preserved. Waste Characterization. Aqua Regia (three parts hydrochloric acid to one part nitric acid) was used to extract metals from the landfill samples and the waste foundry sand for the metals determination, which was done by inductively coupled argon plasma optical emission spectroscopy (ICP/OES). Quality control for the metal analyses included independent calibration verification standards, replicate analyses, known additions, and blanks. The waste sand sample from the foundry used for the TCLP tests was found to be predominantly quartz by X-ray diffraction. Casting sand may also contain clay and an organic resin (4). The effect of these minor constituents on metal leaching from the waste sand was not studied. The waste sand particles were sized by sieving, and 95% were found in the range of 0.043-0.43 mm. VOL. 37, NO. 2, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. TCLP Resultsa for Untreated Waste Foundry Sand and for Mixtures (by Weight) of Waste Sand with Various Additives Extracted with Fluid 1 sampleb
copper (mg/L)
lead (mg/L)
zinc (mg/L)
iron (mg/L)
final pH
waste foundry sand (n ) 3) waste foundry sand with 10% iron metal (n ) 4) waste foundry sand with 3.4% HFO (as Fe2O3) (n ) 4) waste foundry sand with 10% dried HFO (n ) 3) waste foundry sand with 10% Fe2O3 (hematite) (n ) 4) waste foundry sand with 10% zinc metal (n ) 4) waste from bore 2, 0-2.4 m (n ) 1) waste from bore 3, compositec (n ) 1) waste from bore 4, 0.6-1.5 m (n ) 1) waste from bore 4, 4.6-6.1 m (n ) 1) waste from bore 5, composited (n ) 1)
122 ((9) 0.4 ((0.2) 68 ((5) 75 ((6) 92 ((7) < 0.1 9.9 37 164 13 0.8
48 ((4) 0.4 ((0.2) 4.4 ((0.3) 9.0 ((0.7) 40 ((3)
