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CORRESPONDENCE Comment on “Acid Digestion for Sediments, Sludges, Soils, and Solid Wastes. A Proposed Alternative to EPA SW 846 Method 3050” SIR: Recently Kimbrough and Wakakuwa pointed out that the EPA SW 846 method 3050 fails to reproducibly recover Sb from soil or sludge samples (1). Despite these findings and the obvious implications, the EPA has approved a microwave oven digestion using nitric acid for the extraction of Sb from soils in preparation for analysis by flame atomic absorption (FAA), graphite furnace atomic absorption (GFAA), or inductively coupled plasma (ICP). The statement of work for this new EPA procedure (Document No. ILMO1.0) uses nitric acid in a sealed Teflon vessel with microwave heating for metal dissolution. Antimony is one of 22 metals listed in the scope and application. Thus, it is reasonable to assume that laboratories will be expected to routinely recover pollutant Sb from soil with this digestion. This correspondence offers further explanation of the failure of nitric acid to dissolve S b in the presence of silicates, based on our observations with microwave oven heated digestions. Although we are pleased that the EPA has accepted a closed vessel microwave digestion for the Contract Laboratory Program (CLP), we must caution users with regard to this specific digestion procedure when attempting to extract S b from potentially contaminated geological materials. Microwave heated acid dissolutions offer numerous advantages over conventional hot-plate digestions (2-7), but the use of only HNO, for the dissolution of S b from soils results in losses of this metal. For two geological materials, a standard soil from the Rocky Mountain Arsenal (RMA), and a National Institute of Standards and Technology (NIST) standard reference river sediment (SRM-2704), we found that a microwave-”0, digestion similar to the one in the EPA’s Statement of Work failed to recover S b for GFAA analysis (7). Thus, laboratories using a microwave-”0, digestion would report false negatives when assessing S b concentrations in soils. Antimony was not detected by GFAA in our microwave-HNO, digests of the NIST SRM-2704 reference material, the RMA soil, or the RMA soil spiked with approximately 40 pg of Sb(III)/g. Losses exceeding 70% occurred when 40 pg of Sb(III)/g was spiked into the RMA digest slurries (10 mL of HNO, and 0.5 g of soil) between two consecutive microwave dissolution heating programs. However, quantitative recoveries were obtained when Sb(II1) was spiked into 10 mL of HNO, (no soil) and then taken through the microwave-HNO, digestion procedure. Additionally, the Sb(II1) standard added to a RMA filtered digest after the microwave-”0, treatment was also quantitatively recovered. Thus, “loss” of S b (or failure to recover Sb) occurs when Sb(II1) is digested with HNO, in the presence of soil or sediment. These findings are consistent with other studies (8-10) reporting large losses of S b due to oxidation and subsequent adsorption. Using a radioactive tracer technique, two of the above studies (8,9) found lz2Sband 124Sbcarrier on glass surfaces of the digestion vessel or on filters when
it was not recovered in the extract. Bajo and Suter (8) suggested that when Sb(II1) is heated in the presence of only HNO, and HC104 the antimonic acid could be oxidized to Sb(V), perhaps existing as Sb205,and that this oxide was readily adsorbed by glass surfaces. More recently, Berry and Brett (11) have shown that when elemental S b is treated with concentrated HNOBa mixture of Sb203and Sb,O,(OH),(NO,), is formed. The latter decomposes to the trioxide when heated above 135 “C. As the trioxide, Sb is insoluble in water, dilute HNO,, or dilute H2S04,but soluble in HC1. From the trioxide the pentoxide can be formed in the presence of oxygen a t high pressures and temperatures (12). Consequently, we speculate that Sb205is formed during the microwaveHNO, digestion of soils and is removed by adsorption onto reactive silicate surfaces. The formation of adsorptive antimony oxides has been prevented for the digestion of organic matter by adding H2S04to a wet ashing mixture of HNO, and HC104 prior to digestion (8). Additionally, quantitative Sb recoveries from geological reference materials have been achieved when a HCl/HNO, acid mixture is used for digestion (1, 13). Using a 50/50 ratio of HC1/HN03, we were able to recover the majority of the S b present in the NIST SRM2704 with our microwave digestion procedure. The average recovery of 2.92 pg/g (15% RSD, n = 3) is 77% of the accepted value (3.79 pg/g) for this reference material. However, this acid dissolution mixture exceeded the pressure limits of all three of the Teflon vessels containing the NIST reference material, an indication that the microwave program used in the HNO, digestion is not suitable for digestions using both HNO, and HC1. Reducing the HC1 concentration in the digestion mixture to 20% did not eliminate overpressurization (two of the three vessels vented) and lowered the recovery of S b to approximately 60% (average 2.28 pg/g, 6.1% RSD), further indicating the need to use a less rigorous heating program with “OB/ HC1 acid mixtures. The use of HC1 during digestion shows promise for the recovery of Sb, most likely because in the presence of this acid Sb(V) forms the SbC&- anion, which does not adsorb on undigested siliceous materials. However, closed-vessel microwave digestions of soils with acid mixtures containing HC1 require further refinement, before routine application. Because of overpressurization, the high HC1 concentration required for good recoveries cannot be used with 100 psi PFA Teflon vessels without changing the microwave heating program that was used for the HNO, digestions. Currently available double-walled 200 psi vessels (14) may eliminate this problem. Other possible solutions include (1)allowing the HNO,/HCl and soil mixture to react for 10-15 min before capping, (2) using a two-step microwave digestion in which HC1 is added to a previously digested HNO, solution, (3) lowering the microwave power settings and/or decreasing the digestion time, or (4)using other acid mixtures that have been proven to recover S b in silicate materials (Le., HCl/HNO,/HF/boric acid) (15). In conclusion we feel, as did Kimbrough and Wakakuwa ( I ) , that Sb should not be included with pollutant metals that can be readily recovered from soil with a nitric acid digestion.
Not subject to US. Copyright. Published 1991 by the American Chemical Society
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Literature Cited (1) Kimbrough, D. E.; Wakakuwa, J. R. Environ. Sci. Technol. 1989, 23, 898-900. (2) Abu-Samra, A,; Morris, J. S.; Koirtyohann, S. R. Anal. Chem. 1975, 47,1475-1477. (3) Barret, P.; Davidowski, L. J., Jr.; Penaro, K. W.; Copeland, T. R. Anal. Chem. 1978,50, 1021-1023. (4) Kingston, H. M., Jassie, L. B., Eds. Introduction to Microwave Sample Preparation: Theory and Practice; American Chemical Society: Washington, DC, 1988. (5) Matthes, S. A,; Farrell, R. F.; Mackie, A. J. Journal of Technical Progress Report, U S . Bureau of Mines, 1983; No. 120. (6) Nadkarni, R. A. Anal. Chem. 1984, 56, 2233-2237. (7) Hewitt, A. D.; Reynolds, C. M. CRREL SR 90-19, Cold Regions Research and Engineering Laboratory, 72 Lyme Rd., Hanover, NH 03755, 1990. (8) Bajo, S.; Suter, U. Anal. Chem. 1982, 54, 49-51. (9) Gallorini, M.; Greenberg, R. R.; Gills, T. B. Anal. Chem. 1978,50, 1479-1481. (10) Merry, R. H.; Zarcinas, B. A. Analyst 1980,105, 558-563. (11) Berry, F. J.; Brett, M. E. Inorg. Chim. Acta 1984, 83, 167-169. (12) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Whiley-Interscience: New York, 1988; p 401. (13) Matthes, S. A. U.S. Department of the Interior, Bureau of Mines, Albany, OR, personal communication. (14) Tatro, M. E. Spectroscopy (Eugene,Oreg.) 1990,5, 17-20. (15) Matthes, S. A. Bureau of Mines Report of Investigations 8484, 1980.
Alan
D. Hewitt," James H. Cragin
Cold Regions Research and Engineering Laboratory Hanover, New Hampshire 03755-1290
SIR: The letter from Hewitt and Cragin points up the on-going dilemma of laboratories using EPA methods, which applies both to CLP laboratories and others that rely on the RCRA methods manual SW-846. Their review of the chemistry of antimony in acid digestions is consistent with the work done in our laboratory. However, we would like to add a few comments that may be important. Antimony will be "lost" in a digestion procedure whether or not silicates are present, if it is not already in
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As Hewitt and Cragin point out, the more HC1 that is present in the digestion mixture, the better the recoveries. However, mixtures of HCl and nitric acid produce an interesting phenomenon: a small amount of antimony is always trapped in the filter paper. The amount varies with the sample matrix, the total amount of antimony present, and the molecular form of the antimony, but it is rarely more than lo00 pg (2). At high concentrations this amount can be insignificant and ignored, but for samples where there is less than 5000 pg total antimony, this amount can be critical. For example, EPA SW 846 method 3050, which ends up being about 1:l HC1 to nitric acid, can accurately quantify a 2.0-g sample of soil contaminated with 10 000 pg/g antimony, but not one a t 100 pg/g. The same phenomenon can be seen with draft method 6020 and the aqua regia digestion used at our laboratory. To eliminate this phenomenon, the filter paper is washed with hot (95 "C) HC1 and then hot (95 "C) deionized water in our aqua regia method. For concentrations of greater than 20 000 pg/g, the filter paper and residue are redigested in 5 mL of concentrated HC1. These steps may be useful in microwave digestion procedures. Literature Cited
Funding for this work was provided by the Program Manager, Rocky Mountain Arsenal, through the U S . Army Toxic and Hazardous Materials Agency (USATHAMA) under contract DA-2522-IAR-1689, Durant Graves, Project Monitor. We especially thank Steve A. Matthes of the Bureau of Mines (Albany, O R ) , who provided helpful advice and suggestions throughout this study. We also thank Dr. Clarence Grant and Dr. Thomas Jenkins o f USACRREL, who reviewed the manuscript.
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a soluble form. For example, an aqueous solution of potassium antimony tartrate can be added to nitric acid at low concentrations and be recovered. However, if dry potassium antimony tartrate is digested with nitric acid, it will not be quantitatively solubilized, even in the absence of silicates ( I ) .
(1) Kimbrough, D. E.; Wakakuwa J. A study of the Linear Ranges of Several Acid Digestion Procedures. In Proceeding of the EPA's Sixth Annual Symposium on Solid Waste Testing and Quality Assurance;Washington DC, July 1990. (2) Kimbrough, D. E.; Wakakuwa, J. An Inter-Laboratory Study Comparing EPA SW 846 Method 3050 with a Proposed Alternative from the California Department of Health Services. In Proceeding of The EPA's Fifth Annual Symposium on Solid Waste Testing and Quality Assurance; Washington DC, July 1989. Reprinted in Waste Testing and Quality Assurance: Third Volume ASTM STP 1075, Tatsch, C. E., Ed.; American Society for Testing and Materials, Philadelphia, PA, 1991.
Janice R. Wakakuwa," David E. Kimbrough Department of Health Services Division of Laboratories Southern California Laboratory 1449 West Temple Street Los Angeles, California 90026-5698
0013-936X/9 1/0925-0986$02.50/0
0 1991 American Chemical Society
