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Anal. Chem. 1980, 52, 825-828
Determination of Selenium in Environmental Materials by Neutron Activation and Inorganic Ion Exchange Daryl Knab' and Ernest S. Gladney Los Alamos Scientific Laboratory,
P.O.Box 1663, Los
Alamos, New Mexico 87545
A procedure for the determination of selenium in environmental samples is described. Neutron irradiated samples are dissolved in acid and 75Se Is adsorbed on an AI,O, inorganic ion-exchange column from 5 M HCI-1 M H,P04 solution. Tantalic acid is precipitated and filtered before loading on the column, which decreases interference from "'Hf, "qa, and =,Pa. The adsorbed 75Se is determined by Ge( Li) y spectrometry. Selenium concentrations determined by the procedure in NBS reference materials and USGS rock standards compare well with certified and other reported values.
Selenium is an essential micronutrient and also a toxic substance ( I ) , with a narrow tolerance range between necessary and toxic concentrations for many biological systems (2,3 ) . Because of the limited safe range, the need to add nutrient selenium to feed stock in parts of the country, and the number of industries which emit selenium bearing wastes, methods of accurately determining the selenium content in a variety of environmental and industrial matrices over a broad concentration range are necessary (4). Many methods of selenium determination are available (5-7), but none appear to be reliably accurate enough to satisfy the highly variable analytical requirements for environmental monitoring. A good compilation of current, 1970-1976, literature has been given by Poole e t al. (8). Classically, selenium has been determined gravimetrically, volumetrically (9, IO),and colorimetrically (11). Instrumentally, selenium can be measured by atomic absorption (12,13), gas chromatography ( 8 ) , fluorescence (14, 15), emission spectroscopy (16, 17), and neutron activation with y-ray spectroscopy (18-22). With the exception of y-ray spectroscopy, methods of selenium determination require sample decomposition and preparatory chemistry. Since the various methods for selenium determination are subject to numerous interferences, selenium is usually chemically isolated before measurement. Selenium is volatile under several conditions and considerable care is required to prevent volatility losses during sample dissolution, b u t dissolution with HN03-HF-HC104 has proven effective for most matrices (23,24). For inorganic samples analysis, oxidative fusions can be used for dissolution ( 2 5 ) , and organic samples can be decomposed by oxygen combustion methods (7, 8 ) . Assuming that sample dissolution is accomplished without loss, adequate selenium separations are still difficult because of lack of understanding of the complex chemical behavior of Se within the matrix and poor selenium oxidation state control (26). For instance, SeS, precipitations are not complete if Se(V1) is present. The same is true of precipitation of Se(0) with hydroxylamine and with the SeH2 outgassing method for flame atomic absorption. The reason is that the Se(V1) to Se(1V) reduction is deceptively slow under many conditions. Selenium with four functional oxidation states, each with very different chemical characteristics is amenable to separation by many techniques (27-29). In spite of, or because of, the many techniques available for separation and measurement,
reliably accurate and precise selenium determinations are difficult and expensive to attain. 'There is, therefore, a need for a short reliable separation procedure for selenium that gives recoveries in excess of 90% and is interference free for routine selenium determination in complex and variable matrices. A radiochemical separation procedure for neutron activated 76Seusing ion exchange on acidic A1203is described here. Selenium(1V) quantitively adsorbs on A1203from HC1-HBP04 solutions relatively free of interfering long lived activation products. T h e composition of the sample matrix does not adversely affect the selenium adsorption efficiency up to approximately 5 g of ash.
EXPERIMENTAL Apparatus. It is necessary to have access to a nuclear reactor equipped for neutron activation analysis. Separated samples are counted on four large, 60 to 80 cm3, Ge(Li) detectors (2.0 keV FWHM at 1332 keV) multiplexed into a 4096 channel pulse height analyzer. y-ray spectra are stored on computer compatible magnetic tapes for off line data reduction. Ion exchange columns are 0.7 cm i.d. X 19 cm tubes with a tapered tip and a 100 mL reservoir on top. Other equipment is standard commercially available laboratory apparatus. Reagents. Chromatographic grade, activated, acidic A1203(60 mesh) is available from Alfa corporation; it is used as received. Other reagents and chemicals are reagent grade and are utilized without additional purification. Procedure. Encapsulate 0.1 to 4 g of sample in polyethylene rabbits and irradiate for 7 h (convenient for our reactor schedule) at the Los Alamos Omega West Reactor in a thermal neutron flux of 1 X l O I 3 n/cm'/s. Allow the irradiated samples to decay at least 3 weeks before processing to reduce personnel exposure. Longer decay periods can be advantageous for reducing activity levels and interferences if sample turn-around time is not critical. Prepare standards by pipetting 50 pg of selenium as the oxide onto 47-mm Whatman 41 filter papers. Dry the filters, encapsulate, and irradiate the standards simultaneously with the samples. Chemically process the standards the same as the samples. Irradiated samples are divided into siliceous and nonsiliceous materials for dissolution. Transfer siliceous samples to Teflon beakers and add 500 pg selenium carrier, 10 mL HN03 and 10 mL HF for samples up to 1 g. Evaporate to about 1 mL; do not let the samples evaporate to dryness. Add 10 mL of "OB, 10 mL HF, and 2 mL of HCIOI. Evaporate to about 3 mL. Add 5 mL "OB, 1 mL HC1, 1 mL 0.1 M K2Crz07,1mg tantalum carrier, and 1mL saturated H3B03. Evaporate to HCIOI fumes and continue heating until the Cr3+ is oxidized to Cr20q', indicated by the color shift from green to bright orange. Upon cooling, the samples may be stored before further processing. For nonsiliceous matrices, transfer the irradiated sample to a glass beaker, add "OB to cover the sample, and add 500 pg Se carrier. Warm gently t o start the reaction and increase the heat as the reaction subsides; then cover with a watch glass. Continue refluxing until the brown fumes decrease, adding HNOBas needed. Let evaporate just to dryness. Add 10 mL " O B , 5 mL HC104, 0.25 mL HF, 1 mL 0.1 M K2Cr207,I mg tantalum carrier, and evaporate just t o HCIO, fumes. Add 1 mL saturated H3B03and fume until the Cr3+ is oxidized to Crz07'-. Upon cooling, the samples may be stored before further processing. Dissolve the residue from either dissolution procedure in 30 mL 6 M HCl. Cover and boil for 20 min to reduce the Se(V1)
0003-2700/80/0352-0825$01.00/0'C 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
to Se(1V). Remove from the heat, add 2 mL H3P04,and let cool. While the sample cools, prepare the M203ion-exchange column. Pack a small glass wool plug loosely into the tapered top of the ion exchange column. Pull the glass wool plug partway out of the column tip with a hooked wire, then check the column for adequate flow rate. Fill the tube with 5 M HC1-1 M H3P04and add 2.0 g Al,O,. Wash down the sides of the column until all of the A1203has been washed into the column proper. Let the solution drain to the top of the AZO3 bed. Wash the column with 5 mL 5 M HCl-1 M H3P04. Place a 9-cm Whatman GFA fiberglass filter on top of the column and filter the sample onto the column. Tantalic acid formed during the HNO-HC104 evaporation is retained by the filter. After the sample has drained to the top of the A1203,wash the beaker and filter with 5 mL 5 M HCl-1 M &PO.,. Wash the filter three times with 10 mL 5 M HC1-1 M H3P04. Wash the column twice with 10 mL 5 M HCl-1 M H3P04. Pull the small glass plug out of the tube, place it in a plastic scintillation vial, and rinse the A1203 into the vial. Count the separated samples on Ge(Li) detectors. The required counting time is a function of the samples being analyzed and is best determined experientally. The 75Se( t l l z = 120 d) y-rays at 265 and 280 keV are usually used to determine the Se content; for clean low level samples the 136-keV peak is more sensitive.
RESULTS AND DISCUSSION Selenium determinations by instrumental neutron activation analysis (INAA) preclude dissolution and separation losses, but have detection limits of about 1 ppm. Since that is inadequate for most environmental sample analyses, isolation of the 75Se from interfering activation products is necessary. For accurate chemically separated selenium determinations, yield corrections are preferred. However, no suitable radioactive tracer is available and gravimetric or reactivation carrier addition methods require obtaining and quantizing a pure product, which is very time consuming and expensive. Reactivation techniques can be used in this procedure, but yield determinations do not appear necessary. All of t h e matrices analyzed are adequately dissolved by the procedures described, coal and fly ash samples being the most difficult to dissolve. Some of the geological samples yield visible amounts of B a S 0 4 precipitate, but it does not carry appreciable selenium when filtered. Selenium carrier has been routinely added as a precaution against trace metal losses on containers and volatility losses during dissolution. The carrier addition is unnecessary since these losses are insignificant, using the prescribed conditions, and samples can be run carrier free. The A Z O 3columns have a limited capacity for selenium and the carrier should not exceed 1 mg. It is necessary to use care during dissolution to prevent baking or flaming the samples. Losses from 0 to 80% have been seen in samples t h a t were baked on high heat or which have flamed. These losses are not matrix related and carrier addition does not prevent the loss. It is necessary to use caution when dissolving organic material in HCIOI since the reaction can be violent. It is, therefore, recommended that most of the organic material be decomposed in H N 0 3 prior to adding HClO,. Geological and fly ash samples up to 0.5 g can be dissolved by fusion with 2 g NaOH 3 g NazOz in nickel crucibles with quantitative selenium retention. T h e resulting Se(V1) must be reduced to Se(IV) before loading on the column. Boiling the dissolved fusate in 3 t o 6 M HC1 causes SiOz to precipitate, which even with the filtering step frequently interferes with the column flow rates. T h e large acid volume required for dissolution (100-200 mL) of t h e fused sample also extends the time required for sample processing. Acidic A Z O 3has been found to be an effective and selective ion exchanger for Se out of HC1 media. Selenium(V1) and(1V) collect quantitatively on A1203from 1 M to 8 M HC1 solution from environmental matrices of up to 5 g sample ash; soil, bone, vegetation, and tissue samples work equally well. Phosphate added to the column preconditioning solution, for
+
decontamination requirements, precludes adsorption of the Se(VI), but does not affect Se(1V). It is necessary, therefore, to reduce any Se(V1) produced by the dissolution procedure to Se(1V). Selenium(V1) spontaneously reduces in HC1 solution, but the reaction rate is temperature and HC1 concentration dependent. I n 6 M HC1, 20 min a t boiling are required for complete reduction. At lower acid concentrations and/or temperatures, more time is required. Addition of H2O2, NH20H-HC1 or NaN02 does not appear to appreciably speed the reaction. Presumably Br- does, but this has not been pursued. Care is needed, if reducing agents are employed, to prevent reduction to Se metal, which is insoluble in HC1 and will filter out. Column dimension requirements are a function of flow characteristics, rather than the quantity of A1203needed. A 0.4-cm i.d. column containing 0.5 g Al,O, yields quantitative Se retention as does 1-cm i.d. column containing 6 g A1203. Our standard 0.7-cm i.d. column was selected for convenience. Two grams of Al2O3yields a settled bed about 3 cm high, which gives reliable retention and reasonable flow rates. Channeling losses are observed below 1 cm in the 0.7-cm i.d. column and beds exceeding 3 cm slow the flow rate excessively. Selenium adsorbs very strongly to A1203,so the column can be washed extensively to eliminate contaminants. Predominant interfering long-lived isotopes observed on the column are 32P,lSIHf, la2Ta,and 233Pa. Phosphorus-32 decays by high energy p emission, which produces bremsstralung radiation in the 75Se decay energy region causing a very high background. Protactinium and hafnium do not have decay y rays that interfere directly with 75Se, but also contribute to the background. Tantalum-182 has a decay peak at 264 keV which directly interferes with the preferred 75Sedecay peak at 265 keV. Phosphate adsorbs quantitatively on A1203from 1 M to 10 M HC1, but is reversible in the presence of excess phosphate. Preconditioning A1203 columns with 5 M HC1-1 M H3P04 prevents most of the 32Pfrom loading on the column and subsequent washes with the same solution yield decontamito the column nation factors of about lo5. Addition cf Pod3causes scandium, and probably other rare earth elements, to adsorb on the column. The 5 M HCl-1 M H3P04washes also elute the scandium, leaving about 3% on the column, but the high energy &Sc y rays do not normally interfere with the 75Se counting at that level. T h e ls2Ta264-keV peak interferes with the 265-keV peak of I5Se. Tantalum-182 also has a y-ray peak at 222 keV, which is interference free. T h e 264 counts can be calculated from the 264:222 ratio and can be subtracted from the 75Se265-keV peak. This correction is effective when the Ta:Se ratios are not too large, normally about 10, but the accuracy of the correction decreases inversely with the Ta:Se ratio. At a ratio of 100, found in Bandelier Tuff samples, the
