Environ. Sci. Technol. 1989,23,898-900
Acid Digestion for Sediments, Sludges, Soils, and Solid Wastes. A Proposed Alternative to EPA SW 846 Method 3050 David E. Klmbrough" and Janice R. Wakakuwa
California Department of Health Services, Division of Laboratories, Southern California Laboratory, 1449 W. Temple Street, Room 101, Los Angeles, California 90026-5698 EPA SW 846 Method 3050 (1) does not reproducibly recover either antimony or silver from soil or sludge samples. Both of these elements are regulated by Federal and California state law, necessitating that these elements be included in the routine scans for toxic metals. A method has been developed that will provide data for these elements that meets EPA mandated quality control requirements. The matrix is digested with a 4:l nitric acid-hydrochloric acid mixture and brought to near dryness. The digestate is washed in a solution of 1% nitric acid-1 % hydrochloric acid and filtered through filter paper that is washed with hot concentrated hydrochloric acid followed by hot deionized water. The filter paper and residue are refluxed in 5 mL of concentrated hydrochloric acid and filtered. The two filtrates are analyzed by either ICP or FAA.
Introduction At the present time there are no validated EPA digestion methods for the preparation of soils, sludges, sediments, and solid waste samples for the determination of antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, molybdenum, nickel, selenium, silver, thallium, vanadium, and zinc (these elements will hereafter be referred to as the "target elements") in a single digestion for determination by flame atomic absorption spectroscopy (FAA) or inductively coupled plasma-atomic emission spectroscopy (ICP). EPA Method 3050 in SW 846 (I) is satisfactory for most of the elements listed above but not for antimony or silver. The EPA removed silver and antimony from method 3050's list of validated elements in the third edition, a decision supported by research here and in other laboratories (2). The SW 846 establishes a 75-125% recovery for spikes or reference materials and reproducibility of less than 20% relative percent difference (RPD). Silver and antimony cannot be recovered within these limits by using method 3050. Since federal ( 3 , 4 )and state laws (5) list antimony and silver as toxic elements along with the other target elements, it is necessary to include them in the screening for toxic elements in solid matrices. This presents a problem; analysts must deal with solid matrices for antimony and silver but lack an officially recognized method. Multiple digestions, by methods that are not officially recognized, are time consuming, expensive, and legally ambiguous. In response to this problem a new method has been developed at the Southern California Laboratory (SCL) of the California Department of Health Services, hereafter referred to as the SCL method. The proposed method is capable of solubilizing both antimony and silver along with the other target elements by a single digestion procedure. The two methods were compared by this laboratory using three different matrices. Two samples of each matrix were digested by method 3050 and two samples were digested by the SCL method. Duplicate spikes, blanks, standard recoveries, and a reference material were also run by each method. Experimental Section Analytical Methods. (1)EPA S W 846 Method 3050 898
Environ. Sci. Technol., Vol. 23, No. 7, 1989
Table I. Standard Recovery' recovery, 70 SCL
analyte
3050
Ag
9.5 86 97 96 101 99 98 a7 97 98 97 87 94 96 93 99
As Ba Be Cd co Cr
cu
Mo Ni Pb Sb Se
T1 V Zn
98 102 103 102 99 105 94 94 96 92 95 88 91 96 103 95
ANOVA F 776 56 1.4 4.0 1.4 5.2 0.95 12 0.04 4.7 0.37 0.63 0.47 0.26 8.8 1.0
a All numbers are in percent recovery except F, which is dimensionless. Samples: 4 mL of 100 fig/mL multistandard: n = 3.
was used as designated in that manual which directs that 1.00-2.00 g of sample be digested first with 10 mL of 1:l nitric acid at 95 "C for 15 min, then 5 mL of concentrated nitric acid is added, and the resultant mixture is refluxed for 15 min. This step is repeated until the sample no longer changes in appearance. The digestate is then concentrated to 5 mL. The sample is treated with no more than 10 mL of 30% hydrogen peroxide. Finally, 5 mL of concentrated hydrochloric acid and 10 mL of deionized water are added and the sample refluxed for 15 min. The sample is then either filtered (Whatman 41 or equivalent) or centrifuged (2000-3000 rpm for 10 min), and the filtrate (or supernatant) is analyzed by either FAA or ICP. (2) The SCL method calls for 1.00-4.00 g of sample to be digested in a mixture of 20 mL of concentrated hydrochloric acid and 5 mL of concentrated nitric acid at ambient temperature. The sample and reaction mixture are slowly heated to 95 "C to prevent an overly vigorous reaction. The digestion is complete as evinced by the disappearance of NOz fumes (reddish brown) and the sample no longer changes in appearance. The sample is then evaporated to near dryness. The sample is washed with -40 mL of 1% nitric acid-1 70hydrochloric acid solution and filtered (Whatman 41 or equivalent). The filter paper is washed with no more than 5 mL of hot (95 "C) concentrated hydrochloric acid and then 20 mL of hot deionized water, with all of the filtrate being collected in one flask (this will be referred to as the primary filtrate). The filter paper and residue are placed back in the digestion vessel, 5 mL of concentrated hydrochloric acid is added, and the resultant mixture is refluxed at 95 "C until the filter paper disintegrates (approximately 10-15 min). The dissolved paper is then washed with deionized water and again filtered (this filtrate will be referred to as the secondary filtrate). A Perkin-Elmer 5500 ICP-AES was used to analyze samples following EPA SW-846 Method 6010. Comparison Procedure. This study consisted of the analyses of three different matrices by the 3050 and SCL
00 13-936X/89/0923-0898$01.50/0
0 1989 American Chemical Society
Table 11. Reference Material analyte
3050
Ag As Ba Be Cd co Cr cu Mo Ni Pb Sb Se Ti V Zn
13 30 131 7.4 41 42 90 58 23 64 47 12 26 23 38 68
concn, mg/kg SCL 'true value" 40 32 110 7.3 35 37 86 60 25 56 38 34 32 20 49 75
(X,+ X,)V/w= x,v/w = c
36 27 133" 7.1" 33 32 91" 55" 22 63" 44" 33 24 22 47" 68"
where XIis the concentration of element x in the primary filtrate, X2is the concentration of element x in the secondary filtrate, X , is the total concentration of element x = X I+ X2, V is the volume of volumetric flask used for both primary and secondary filtrates, w is the weight of sample taken, and C is the concentration of element x in the sample.
"These "true" values represent both the spiked quantity and the endogeneous quantities of each element quantitated as a total value.
Table 111. Sample Background Valuesasb
As Ba Cr Cu Ni Pb Zn
sample HJ
sample 4435
sample 4766
3050
3.23
3050
3.23
3050
3.23
