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Energy & Fuels 2007, 21, 728-734
Development of a Miniaturized Technique for Measuring the Leachability of Toxic Trace Elements from Coal-Biomass Co-combustion Ash Residues A. George,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW7 2BY, UK ReceiVed September 28, 2006. ReVised Manuscript ReceiVed January 29, 2007
The toxicity characteristic leaching procedure (TCLP) is a standardized test method for evaluating worstcase scenario leaching of metals from landfill sites. The method stipulates a minimum requisite sample mass of 100 g and may not be used if this sample size is unavailable. A new “microleaching” test was designed, and a sensitivity analysis was conducted on attenuated sample masses, ranging from 10 to 150 mg, using the TCLP and a modified leaching apparatus. Tests were carried out to determine the precision and accuracy of the procedure, at the reduced mass range, using two TCLP certified reference materials (CRMs). A Polish coal ash, produced at conditions studied in co-combustion work was also subjected to the microleaching test. ICP-MS analyses were conducted on the resulting leachates to determine the concentrations of certified elements, Ag, As, Ba, Cd, Cr, Hg, Pb, and Se. Tests for each sample mass and each material were repeated six times. The metrics used to validate the new method were that the relative standard deviation (RSD) of the six experiment means should be below 20% and that, in this instance, there should be a (10% difference between the certified and empirically determined mean value. Given these constraints, it was found that results were repeatable for sample masses above 75 mg and concurred with certified values for sample masses above 100 mg for the three samples studied. It was concluded that the microleaching test could be used for sample masses of above 100 mg without relinquishing the theoretical boundaries set by the TCLP.
1.0. Introduction More than 300 million tons per annum of fly ash and bottom ash are produced globally from coal combustion for power generation.1,2 Coal is set to maintain a dominant position in fulfilling global energy demand, forecast to increase from 4790 million tons in 2002 to 7030 million tons in 2030.3 Therefore, the volume of ash produced is forecast to increase significantly. Approximately 40% of the ash residue produced is used in other industrial and construction processes such as cement manufacture, while the unused proportion is disposed of in landfill sites, holding ponds, lagoons, and slag heaps.4 A significant quantity of heavy metals and other toxic elements contained in coal ash are also, by default, disposed of in this manner. Older landfill sites, where measures were not taken to control leachate and landfill runoff, are liable to release trace metals into the environment over long periods of time. Remediation techniques to harness leachate by lining and capping these older landfill sites are often not undertaken, due to economic considerations and lack of regulatory incentives. Contemporary landfill sites do incorporate methods for toxic substance containment. However, all landfill sites will eventually fail,5 due to linings becoming pierced or the degradation of the lining material, in cases of clay and dual layer lining. In these cases, * Corresponding author. E-mail:
[email protected]. (1) 2005 Coal Combustion Product (CCP) Production and Use SurVey; ACAA (American Coal Ash Association): Aurora, CO, 2005. (2) Smith, I. Land Uses of Coal Fly Ash - Benefits and Barriers; IEA Clean Coal Centre: London, 2005; p 3. (3) 2004 IEA World Energy Outlook; International Energy Agency: Paris, 2005; Chapter 5, p 70. (4) Ugurlu A. Leaching characteristics of fly ash. EnViron. Geol. 2004, 46, 890.
it is very difficult to ascertain the location of, and to make a repair of, the fault. Trace elements leached in sufficient quantities and concentrations can enter the environment via ground and surface waters and ultimately find their way into the food chain. Cases where anthropogenic sources of trace elements have caused damage to human health are well documented.6-8 As such, it is important to quantify the potential for trace elements leaching in landfill sites and thus assess the propensity of toxic emissions to breach regulatory limits. In these instances, while tests can quantify the degree of leaching of toxic elements from a particular site, it is also meaningful to quantify the mobility of these trace elements in the environment.9 A variety of leaching tests are available for the characterization of waste leaching; however, tests are often inappropriately applied. To this end, calls have been made for the standardiza(5) Lee, G. F.; Jones-Lee, A. Deficiencies in Subtitle D Landfill Liner Failure and Groundwater Pollution Monitoring. Presented at the NWQMC National Conference Monitoring: Critical Foundations to Protect Our Waters; US Environmental Protection Agency: Washington, D.C., 1998. (6) Kudo, Y.; Fujikawa, S.; Miyahara, J.; Zheng, H.; Takigami, M.; Sugahara; Muramatsu, T. Lessons from Minamata mercury pollution, Japan - after a continuous 22 years of observation. Water Sci. Technol. 1998, 38 (7), 187. (7) Fryzek, J. P., Ph.D.; Mumma, M. T. M. S.; McLaughlin, J. K., Ph.D.; Henderson, B. E., M.D.; Blot, W. J. Cancer Mortality in Relation to Environmental Chromium Exposure. J. Occup. EnViron. Med. 2001, 43 (7), 635. (8) Chowdhury, U. K.; Biswas, B. K.; Chowdhury, T. R.; Samanta, G.; Mandal, B. K.; Basu, G. C.; Chanda, C. R.; Lodh, D.; Saha, K. C.; Mukherjee, S. K.; Roy, S.; Kabir, S.; Quamruzzaman, Q; Chakraborti, D. Groundwater Arsenic Contamination in Bangladesh and West Bengal, India. EnViron. Health Perspect. 2000, 108 (5), 393. (9) Slocombe, S. D. Environmental monitoring for protected areas: Review and prospect. EnViron. Monit. Assess. 1992, 21 (1), 49.
10.1021/ef0604846 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007
Leachability of Toxic Trace Elements Table 1. Parameters Subjected to Ruggedness Testing in US EPA TCLP parameter
extent parameter tested
liquid/solid ratio extraction time headspace buffer acidity acid washed filters filter type bottle type
19:1 versus 21:1 16 versus 18 h 20% versus 60% 190 versus 210 mequiv yes versus No 0.7 versus 0.45 µm borosilicate versus flint glass
tion and harmonization of leaching tests where there has thus far been none.10 In the case of landfills, a test has been developed by the US EPA, known as the toxicity leaching characteristic procedure (TCLP).11,12 Although this test has its limitations with regard to assessing real time leaching behavior, it is appropriate for use in a landfill worst-case scenario and is a commonly used test for this purpose.13,14 Results obtained from the TCLP may be directly related to emission limits without the requirement for further trace element mobility studies. The TCLP stipulates a minimum sample size of 100 g. Leaching tests carried out in the field are usually not susceptible to constraints of sample availability. This is not the case in laboratory-based situations where available sample masses are often very small, rendering sample analysis difficult to achieve. In this study, the TCLP method was tested using sample masses in the 10-150 mg range. The test was modified to deal with smaller masses, without invalidating the key parameters governing leaching, as defined by the initial TCLP design. 2.0. Experimental Procedure 2.1. Procedural Overview. It was deemed that the TCLP would be valid for smaller sample masses if, at the attenuated sample mass, for a given certified reference material (CRM), two events were fulfilled and one assumption made. The first event was that results obtained are repeatable. This would both imply method consistency and intersample homogeneitysan important parameter in conducting leaching tests. Second, that results concur with certified values stated for the given reference materials. Hence, experiments were designed to test for two parameters, i.e., the precision and accuracy of the experiments. Given positive outcomes with respect to CRM agreement, the assumption would need to be made that samples to be tested had the same properties of homogeneity as the CRMs. This assumption is reasonable as both the CRMs and the experimental samples were of the same nature, namely combustion ashes. To further test this assumption with regard to precision, a Polish coal bottom ash underwent the same leaching tests as the CRMs and its homogeneity was tested. This ash was produced via the same combustion method as the ashes for which the microleaching test was being designed. Additionally, the US EPA TCLP method was subjected to ruggedness tests to assess the sensitivity of the protocol to slight perturbations in the parameters. These parameters are shown in Table 1. These tests were repeated in the microleaching procedure (10) Van Der Sloot, H. A. End of black box approach? A step towards more sustainable landfills. Waste Manage. 2005, 25, 461. (11) Blackburn, W. B.; Show, I.; Taylor, D. R.; Marsden, P. J. CollaboratiVe Study of the Toxicity Characteristics Leaching Procedure (TCLP); report no. EPA/600/4-87/045, EPA: Washington, DC, 1986; EPA contract 68-03-1958. (12) Newcomer, L. R.; Blackburn, W. B.; Kimmell, T. A. Performance of the Toxicity Characteristic Leaching Procedure; Wilson Laboratories, S-Cubed, EPA: Washington, DC, 1986. (13) Cohen, B.; Lewis, A. E.; Petersen, J.; Von Blottnitz, H.; Drews, S. C.; Mahote, S. I. The TCLP and its applicability for the characterisation of worst case leaching of wastes from mining and metallurgical operations. AdV. EnViron. Res. 1999, 3 (2), 152. (14) Kimmell, T. A.; Williams, L. R.; Sorini, S. S. The RCRA Toxicity Characteristic Leaching Procedure (TCLP); A Concept for a New Method. Federal Facil. EnViron. J. 2001, 12 (3), 7.
Energy & Fuels, Vol. 21, No. 2, 2007 729 Table 2. Certified Reference Material Metals Concentrations in Solid and Leachate element
reference value in leachate (ppm)
arsenic barium cadmium chromium lead mercury selenium silver
89.53 0.44 146.9 12.7 154.6
