Environ. Sc/. Technol. 1984, 18, 130-132
Peterle, T. J. Nature (London) 1969,224, 620. Sodergren, A.; Svensson, B.; Ulfstrand, S. Environ. Pollut. 1972,3,25-37. Swain, W. R. J. Great Lakes Res. 1978, 4 , 398-407. Murphy, T. J.; Rzeszutko, C. P. J. Great Lakes Res. 1977, 3,305-312. Strachan, W. M. J.; Huneault, H. J. Great Lakes Res. 1979, 5, 61-68. Smith, J. H. H.; Bomberger, D. C., Jr.; Haynes, D. L. Chemosphere 1981, 10, 281-290. Wolff, C. J. M.; van der Heijde, H. B. Chemosphere 1982, 11, 103-117. Allen, H. E.; Halley, M. A. "Assessment of Airborne Inorganic Contaminants in the Great Lakes"; International Joint Commission: Windsor, 1980; Appendix B to the
Science Advisory Board Report, pp 1-160. (16) Strachan, W. M. J.; Huneault, H.; Schertzer, W. M.; Elder, F. C. In "Hydrocarbons and Halogenated Hydrocarbons in the Aquatic Environment"; Afghan, B. K.; Mackay, D., Eds.; Plenum Press: New York, 1980; pp 387-396. (17) Inland Waters Directorate "Analytical Methods Manual (1979)"; Inland Waters Directorate: Department of the Environment, Ottawa, 1979.
Received for review March 28, 1983. Accepted September 14, 1983. The development of the sampler and acquisition of three prototypes was funded by the Department of Environment, Environmental Contaminants Contract Fund, under Grant 9904-EMS-IWD.
Rapid Method for the Digestion of Sewage and Sludge for Metal Analyses Jeppe S. Nielsen" and Steve E. Hrudey
Department of Civil Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G7
A steam digestion method for metal determinations in sewage and sludge is described and compared to methods using nitric acid digestion and high-speed homogenization. The steam digestion method employs a domestic pressure cooker, provides quantitative metal recoveries, is not prone to metal contamination, and requires less reagents and supervision than the methods with which it was compared.
Introduction Measurement of total metal concentrations in sewage and sludge generally involves initial sample digestion to destroy the organic matrix and to solubilize the metals. However, there are disadvantages associated with the digestion methods commonly used. Dry-ashingin a muffle furnace at high temperatures (550 "C), followed by dissolution in dilute acid, gives incomplete recovery of volatile metals such as cadmium and lead (2-3). On the other hand, low-temperature ashing methods have been reported to give incomplete metals recovery (4). Wet digestion methods using nitric ("OB), hydrochloric (HCl), sulfuric (H2S04),hydrofluoric (HF), and perchloric (HC104)acids and hydrogen peroxide (H202), singly or in combination, can be hazardous and generally require considerable sample handling ( I ) . Digestion at atmospheric pressure (open digestion) with HN03 is one of the methods recommended for wastewaters by "Standard Methods" (5) and the US. Environmental Protection Agency (6). However, open HN03 digestion has been found to give low results for some metals in sludges (3,4). Mixtures of HNO:, with HC1 or H202have generally given better results (2,4, 7). As well, low recoveries of lead have been found with the use of HN03/H2S04mixtures (2,8) because of the formation of insoluble lead sulfate. Generally, the most efficient metal recoveries can be obtained by acid digestion in high-pressure vessels ( I ) . However, these methods are expensive and can be limited by the small weight of sample that can be contained in the pressure vessel (1). Recently, a digestion method utilizing maceration and ultrasonic energy generated by a high-speed homogenizer has been described (2,9,10). This method yielded results *Present address: Department of Civil Engineering,New South Wales Institute of Technology, New South Wales 2007, Australia. 130
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for a wide range of metals which were not significantly different from those obtained by using high pressure or open digestion with HN03/H202and HN03/H2S04mixtures. The major advantage of high-speed homogenization was that it only required 5 min per sample. In this paper, we present a rapid and simple sludge digestion method in which acidified samples are steam digested under low pressure. The results from this method are compared with those when open acid digestion and high-speed homogenization are used. Sewage and sludge samples from Edmonton as well as a U.S. Environmental Protection Agency reference municipal sludge sample were used. The methods were compared for cadmium, chromium, copper, nickel, and zinc.
Experimental Section Apparatus. Steam digestion was carried out in an aluminum Model 411 domestic pressure cooker of 11-L capacity (Presto, Scarborough, Ontario). Homogenization was performed with a Polytron Model PT 10-35 homogenizer (Brinkmann Instruments Ltd., Rexdale, Ontario) by using a specially constructed titanium probe of the same design as the stainless steel Polytron PT 10 ST probe. Metals were determined by flameless atomic absorption spectroscopy by using a Model 5000 spectrophotometer (Perkin-Elmer, Norwalk, CT) with deuterium background correction, an HGA 2200 graphite furnace, and an AS-1 autosampler with 20-pL sample injection volumes. Reagents. High-purity deionized water from a R015Q2 Laboratory Water system (Millipore Corp., Bedford, MA) and Aristar nitric acid, "OB (British Drug Houses), were used throughout. Certified atomic absorption standard solutions (Cd metal, C U ( N O ~ Ni(N03)2, )~, ZnO in water; Fisher Scientific Co.) were in "OB, and KzCrZO7 used to prepare the atomic absorption calibration standards. Samples. Grab samples of raw sewage, activated sludge, mixed liquor, and primary sludge from the City of Edmonton Gold Bar Wastewater Treatment Plant and a reference quality control sample of dried municipal digested sludge from the U.S.Environmental Protection Agency (sample no. 2792, WP976) were used. The raw sewage was undiluted whereas the mixed liquor and primary sludge samples were diluted 1:lO (v/v) with high purity water prior to digestion. The reference sludge was
0013-936X/84/0918-0130$01.50/0
0 1984 American Chemical Society
Operating Conditions for the Atomic Absorption Spectrophotometer and Furnace metal cu Cr conditions Cd
Table I.
357.9
228.8 wavelength, nm 0.7(L)a slit width, nm integration time, s 5 85 drying temp, C 40 drying time, s 300 ashing temp, C 35 ashing time, s 2100 atomization temp, C atomization time, s 5 a ( L ) designates low slit height option.
0.7(L) 10 85 40 1100 35 2700 10
O
324.7 0.7( L) 5 85 40 2700 5
Ni
Zn
232.0 0.2( L) 5 85 40 800 35 2700 5
307.6 0.7( L) 5 85 40 300 35 2400 5
Table 11. Digestion Results Obtained by the Steam, Open HNO,, and Homogenization Methods with Raw Sewage, Mixed Liquor and Primary Sludge digestion method, mg/La sample metal steam open HNO, homogenization raw sewage Cd 0.0031 * 0.0004 0.0025 i 0.0002 0.0016 * 0.0003 0.172 f 0.003 0.243 * 0.008 Cr 0.245 i 0.009 0.081 i 0.021 0.069 i 0.010 cu 0.090 f 0.011 0.062 k 0.006 0.031 f 0.001 Ni 0.052 i 0.003 0.292 0.01 0.30 i 0.03 Zn 0.31 i 0.01 mixed liquor Cd 0.0150 i. 0.0001 0.0205 i 0.0030 0.0183 i 0.0010 Cr 2.00 i 0.01 2.00 i 0.01 1.42 i 0.14 cu 0.375 2 0.025 0.433 i 0.194 0.167 i 0.029 Ni 0.217 f 0.028 0.213 i 0.029 0.195 i 0.001 Zn 1.4 i 0.2 2.3 i 0.2 1.6 i 0.03 0,099 i 0.001 primary sludge Cd 0.116 i 0.007 0.129 i 0.007 Cr 12.7 f 0.4 10.5 i 0.3 10.4 0.17 cu 4.72 i 0.03 4.62 2 0.15 4.18 i 0.11 Ni 1.50 i 0.03 1.20 i 0.01 1.40 i 0.03 Zn 12.0 f 0.1 10.4 f 0.1 10.4 f 0.1 aConcentration presented as mean i 1 standard deviation based on triplicate injections of each of three samples. +_
Table 111. Blank Values Obtained with the Steam, Open HNO,, and Homogenization Methods concentration in blank, mg/L metal Cd Cr cu Ni Zn
steam 0.0000
open HNO, homogenization 0.0003 0.0000
0.002 0.000
0.005 0.000
0.008
0.000
0.004
0.004
0.00
0.03
0.01
0.000
prepared by adding 20.00 mL of 1% (v/v) HNO, to 0.2 g weights of dried sludge. Metal contamination was avoided by washing all laboratory glassware and polyethylene sample bottles in liquid detergent, soaking in 2 M HC1 for several hours, and rinsing thoroughly with tap and high-purity water. Digestion Methods. Sample for steam digestion were acidified (1%, v/v) with concentrated "OB, and then 20-mL "a were transferred to 50-mL Pyrex test tubes. The test tubes were covered with aluminum foil, placed upright in beakers in the pressure cooker, and steam di-
gested for 1 h at 210 kPa (2 atm). The digested supernatant was decanted and analyzed. The open acid digestion method was performed according to APHA (5)and USEPA (6)as follows. Sample volumes of 20.00mL were transferred to 100-mL Pyrex beakers, acidified with 3.0 mL of concentrated "OB, and gently boiled to near dryness on a domestic electric hotplate. Further 3.0-mL volumes of concentrated HN03 were added to the samples, and the beakers were covered with watch glasses and gently boiled with refluxing until the solution was light colored and clear. Four 3.0-mL additions of acid were required for each sample, and the samples were boiled to near dryness after the last addition of concentrated acid. About 20 mL of dilute HN03 ( l % , v/v) was then added to each sample and warmed. The samples were decanted into 50-mL volumetric flasks, the walls of the beakers were washed twice with 1% "OB, and the rinsings were added to the volumetric flasks. The samples were cooled, made up to mark with 1% HNO,, and analyzed. Samples to be digested by high-speed homogenization were also acidified ( l % , v/v) with concentrated "0,. Samples (20 mL) were then transferred to Pyrex 50-mL
Table IV. Metal Concentrations Determined in the U.S.EPA Reference Municipal Sludge after Steam Digestion reference concentration, mg/kg mean rangeb
a
no. outside
reference metal measured concentration,a mg/kg range Cd 16.2, 17.9, 24.1 20.77 2.49-39.1 0 Cr 157, 145,152 204.5 115-294 0 cu 847, 905, 853 1095.3 831-1360 0 Ni 196, 209, 209 198.3 164-233 0 Zn 1165,1149, 1 2 5 1 1323.1 1190-1450 2 Each value is the mean of three injections t o the graphite furnace. bEach range is mean f ( t o . 9 5X standard deviation). Environ. Sci. Technol., Vol. 18, No. 2, 1984
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test tubes and homogenized for 5 min at near maximum speed (setting 8). The supernatant was decanted and analyzed. Each type of sample was digested in triplicate by each method. Blank high-purity water samples were concurrently treated and digested in the same manner as the sewage and sludge samples. Metal Analyses. The operating conditions for the flameless atomic absorption metal determination are listed in Table I. Pyrolytically coated graphite tubes were used, and triplicate injections to the furnace were made for all the samples. Where necessary, samples were diluted so that their absorbance values fell on the linear part of the respective calibration curves. Blanks and standards were analyzed after about every third sample to correct for instrumental drift and graphite tube deterioration in the furnace. The absence of matrix interferences was checked by the method of standard additions by using raw sewage and mixed liquor samples. Metal recoveries from these samples were not significantly different (p C 0.05) from those using standard metal solutions prepared with 1% (v/v) HN03
Results and Discussion Table I1 shows total metal concentrations determined in raw sewage, mixed liquor, and primary sludge samples from the City of Edmonton Wastewater Treatment Plant which were digested by the steam, open HN03, and homogenization methods. Metal recoveries from samples which had been steam digested were equivalent or better than those obtained with either of the other digestion methods for all but one of the 45 cases. Only zinc recovery from mixed liquor was significantly (p C 0.05) lower for steam digestion than for open HN03 digestion. Furthermore, the precision (i.e., standard deviation) of the steam digestion method was generally as good as for either of the other two digestion methods. The results of the blanks which were digested and analyzed together with the samples are shown in Table 111. Blank values for the steam digestion method were negligible. In contrast, the cadmium, chromium, nickel, and zinc blank values obtained with the other digestion methods were 5-20% of the corresponding metal concentrations determined in raw sewage (Table 11). The steam digestion method was further evaluated by analyzing, in triplicate, a sample of reference municipal sludge from the U.S. Environmental Protection Agency by this method. The results are shown in Table IV together with the total metal concentration range supplied for the
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reference sludge. All our experimental results were within the reference ranges except for two of the three zinc results.
Summary A steam digestion method using a domestic pressure cooker has been demonstrated to give quantitative metal recoveries from raw sewage, mixed liquor, primary sludge, and reference sludge samples. The steam digestion method was also less prone to contamination than the open HN03 digestion and high-speed homogenization methods with which it was compared. From an operational point of view, the steam digestion method required less reagents and considerably less manipulation than the open HNOBdigestion method and enabled more samples to be digested simultaneously. Steam digestion was more convenient than homogenization because it allowed simultaneous digestion of up to 36 samples, and it avoided the irritating noise generated by the homogenizer. Registry No. Cd, 7440-43-9;Cr, 7440-47-3;Cu, 7440-50-8;Ni, 7440-02-0; Zn, 7440-66-6; water, 7732-18-5.
Literature Cited (1) Ritter, C. J. Am. Lab. (Fairfield, Conn.) 1982, 14 (€9,72. (2) Carrondo, M. J. T.;Perry, R.; Lester, J. N. Anal. Chim. Acta
1979,106, 309. (3) Jennies, S. W.; Katz, S. A.; Mount, T. Am. Lab. (Fairfield, Conn.) 1980, 12 (a), 18. (4) Delfino, J. J.; Enderson, R. E. Water Sewage Works 1978, 125, R-32. (5) American Public Health Association “Standard Methods for the Examination of Water and Wastewater”, 15th ed.; American Public Health Association: Washington, DC, 1980; Pt. 302. (6) U.S. Environmental Protection Agency “Methods for Chemical Analysis of Water and Wastes”; U.S. Environmental Protection Agency: Cincinnati, OH, 1976; No. EPA-625/6-74-003a, p 81. (7) Andelman, H.; Jenniss, S. W.; Katz, S. A. Am. Lab. (Fairfield, Conn.) 1981, 13 ( E ) , 31. (8) Agemian, H.; Chau, A. S. Y. Anal. Chim. Acta 1975,80,61. (9) Carrondo, M. J. T.; Perry, R.; Lester, J. N. Sci. Total Environ. 1979, 12, 1. (10) Stoveland, S.; Astruc, M.; Perry, R.; Lester, J. N. Sci. Total Environ. 1978, 9, 263.
Received for review April 18, 1983. Accepted August 11, 1983. This study was supported by a research grant from the Municipal Engineering Branch, Standards and Approvals Division of Alberta Environment, by a research contract from the Water and Sanitation Department of the City of Edmonton, and by the Natural Sciences and Engineering Research Council of Canada.
