Methods for Separation and Determination of Beryllium in Sediments and Natural Waters JOHN
R. MERRILL,1
MASATAKE HONDA,2
**and
JAMES R. ARNOLD2
Department of Chemistry, Princeton University, Princeton, N. In the course of studies on natural beryllium isotopes, methods were developed for spectrophotometric determination of beryllium in silicates at the parts-per-million level, for separation and purification of beryllium at this concentration from large sediment samples by solvent extraction and cation exchange, and for radiochemical purification of beryllium. The use of dispersed insoluble hydroxides on cation exchange columns permits rapid removal of beryllium, at concentrations below 10-10M, from large volumes of sea water. This method is of more general applicability. The nature of the reaction was also studied.
has two long-lived radio-
active isotopes, Be7, a gamma Beryllium emitter of half life 53 days, and Be10, a beta emitter of half life 2.5 X 106 years. These are among the few species which can be produced in large amounts by the bombardment of air by the cosmic radiation (2, 23). The discovery of these isotopes in nature (2, 8, 10) required techniques for separating beryllium chemically and radiochemically pure from various materials. Current research is mainly an effort to unravel the geochemical cycles of stable Be9 and
Be10
preparatory to an attempt to measure geological ages with Be10. This has raised difficult analytical problems whose solution may be of more general interest. The first requirement was a reliable microanalytical method for beryllium. The direct emission spectrographic technique is limited by a sensitivity of about 5 p.p.m. (average rocks contain about 3 p.p.m.) and inadequate precision. The only well proved method in the literature is the fluorimetric method using the reagent morin (25, 29). It was not adopted because the pure reagent was unavailable commercially when the work was begun, and because of the need for a special detection instrument. The methods adopted follow that of 1 Present address, Experimental Station, E. I. du Pont de Nemours & Co., Inc., Del. Wilmington, 2 Present address, School of Science and Engineering, University of California, La Jolla, Calif.
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ANALYTICAL CHEMISTRY
J.
Adam, Booth, and Strickland (1) in using spectrophotometry of the berylliumacetylacetone complex, Be-AA, but were modified in the separation process to include repeated solvent extraction of the acetylacetone complex, and the use of Be7 to measure chemical yield. Ion exchange methods were also developed based on fundamental studies which are described below. Pure beryllium must be recovered from large samples (up to 300 grams) of pelagic “red clays” for Be10 measurement. Solvent extraction and ion exchange methods were used, with the addition of stable Be carrier to measure chemical yield. Because the beryllium content of ocean water is about 5 X10-13 gram per ml. (19), special techniques were necessary. An insoluble oxide column (18) produced the initial concentration. The nature of this reaction was studied under several conditions. SPECTROPHOTOMETRIC ANALYSIS
Early experiments extending the pro-
cedure of Adam, Booth, and Strickland (1) with little modification gave erratic results. The size of sample and the ratio of possible interfering ions to
beryllium are far larger than contemplated in their work. Under these conditions beryllium was lost and at the same time impurities remained in the final solution. Experiments using Be7 tracer showed that Be was lost at several steps, and that some impurities were removed only gradually by the extraction cycle. When the solvent extraction cycles were increased to four (a fifth proved superfluous) all interfering ions were removed. The yield of beryllium was 60
to 75% under these conditions. The absence of impurities was verified by The peak using the absorption curve. at 295 mp is unique for beryllium (11); impurities encountered in practice give peaks at shorter wave length. It was now necessary to measure the chemical yield by introducing Be7 tracer into each analysis. The principle of the solvent extraction method is that the Be-AA complex is stable enough above pH 5 to be extracted into CCh even in the presence of excess (ethylenedinitrilo) tetraacetic acid
(EDTA). This is not true for any other known complex [less than 0.1% of U (VI) is extracted into the organic phase under the conditions described in “Second Extraction” below]. For BeAA, log Ki according to the authors’
measurements is 8.2 and log Ki 7.7, while for Be-EDTA, log K is 9.8 (see below). In the case of Al-AA, log Ki is 8.6, log Ki is 7.9, log K¡ is 5.8, while for Al-EDTA log K is 16.1. The low concentration of AA~ favors the EDTA complex in this case. The impurity which requires four extraction cycles for complete removal is not aluminum, according to our model experiments, but we have no other information about
it. Organic impurities absorbing in the ultraviolet must be minimized. The conditions for total wet destruction of organic substances are difficult to reach.
For example, the
use
of technical grade
EDTA in the first step resulted in
a
much increased absorption blank. The blank was also significantly different for different preparations of AA. PRELIMINARY SAMPLE TREATMENT
Pulverize the sample, dry several hours at 90° C., and weigh into a clean platinum dish. For materials containing a few parts per million of Be, use about 2 grams. Add carrier-free Be7 to monitor chemical yield. Run suitable blanks on reagents and tracer solution. Add slowly 6 grams of 48% HF solution, followed by 4 ml. of concentrated H2SO4. Heat the mixture with occasional swirling until SO3 fumes appear. After cooling add 2 grams of HF solution and 1 ml. of H2SO4. Heat the sample to dryness, with care to prevent spattering. Add 8 grams of KHSO4, and fuse at full Meker burner heat for about 3 minutes. Dissolve the cooled residue with a few drops of concentrated HC1 and 20 ml. of H20, and treat with several portions of boiling water until either dissolved or finely suspended. Separate the solution and suspended matter by centrifuging. Wash the precipitate with water and combine the supernatant liquids. Discard the final insoluble residue, probably consisting largely of CaSCL and fine Si02 [a negligible amount of Be7 and Be9 is discarded in the final residue (19)]. Partly neutralize the excess acid in the solution with saturated
Stockton shale (collected in Princeton, N. J.). This was pulverized and homogenized as a reference standard. The National Bureau of Standards rock
G-l and W-l (7) were also repeatedly analyzed. The results are given in Table I. Emission spectrographic analyses of G-l have given values of 2 and 4 p.p.m. (9). No other analyses of W-l are available. The procedure has also been used for samples
analyses of ocean sediments and sediment components in the range of 0.5 to 5.0 p.p.m. of Be (19). As little as 0.2 y of Be in a sample can be reliably7· measured. If 10 grams of a mineral sample (which can be treated without difficulty) are used, the limit of detection is 0.02 p.p.m. numerous
Figure 1. Absorption of beryllium-ocetylacetone complexes, and free acetylacetone
NaOH solution. The sample should be completely in solution at about pH 1
before proceeding.
EXTRACTION
The amount of EDTA required in the first solvent extraction depends on the amount of interfering cations (normally mostly Fe and Al) to be complexed. For the equivalent of about 2 grams of W-l rock the following amounts suffice. First Extraction. Just before use, dissolve 2 ml. of AA in 30 ml. of a 21% aqueous tetrasodium EDTA solution. Add enough (at least 20 ml.) of this solution to the sample solution to bring to pH 5 to 6 and adjust with HC1 if necessary. This nearly neutralizes the sample solution without causing any precipitation. Allow 15 minutes for the formation of the Be-AA complex (1). Extract the water phase (the volume of which can normally be kept to about 90 ml.) twice with 15-ml. portions of CC14. Slowly evaporate the combined CCI4 extract in the presence of about 8 ml. of concentrated HC1. Then add about 4 ml. of concentrated HNOa and boil almost to dryness (avoid charring). Add water. Second Extraction. Add 30 ml. of 10% disodium EDTA and adjust pH to 7 with NaOH. Add 5 ml. of 5% aqueous AA solution, and after 10 minutes extract with 15 ml. of CC14. Boil down the extract with HC1 and HNOa as above.
Third Extraction. Repeat the second extraction using only 20 ml. of 10% disodium EDTA. After boiling down with acid, add 5 ml. of H20 and boil again to remove color. Fourth Extraction. Add 1 ml. of 10% disodium EDTA and bring the pH to 7 with dilute NaOH. Add 0.5 ml. of 5% AA and check the pH. After 10 minutes extract the solution with 10 ml. of CHCla (used here because of easier back-extraction of free AA below). Drain off the extract,
discard the water phase, and wash the separatory funnel thoroughly. Return the CHCla to the funnel and backextract with two 20-ml. portions of 0.1M
NaOH to remove excess AA. Each back-extraction should last 90 seconds, while all other extractions should be continued for 2 minutes or more. Filter the CHCI3 through medium filter paper into a 10-ml. volumetric flask. Add fresh CHCla to bring the volume to 10 ml. ABSORBANCE MEASUREMENT
From the final solution reserve 2 ml. for Be7 counting. The absorbance measurement is made on a Beckman Model DU spectrophotometer, using a silica cell and a CHC13 blank. As a control the absorbance readings should be made at about ten points from 350 to 280 µ. The 350- and 340-m,u readings should be below about 0.01 absorbance unit, even when the peak absorbance is 1.5 units. The peak should be exactly at 295 mu. An experimentally determined blank (equivalent to 0.12 ± 0.03 7 of Be) must be subtracted from the 295- µ reading. The slope of the calibration curve has been experimentally determined as 2.93 7 Be per absorbance unit. Spectra of pure AA and Be and Al comThe plexes are shown in Figure 1. magnitudes of absorbance shown are arbitrary. If a very small amount of Be is present, it may be necessary to subtract an experimental blank to confirm the presence of a Be peak. ANALYTICAL DATA
The
procedure
was
Table
1.
Stockton Shale Sample, grams
7.19 7.31 7.23 7.72
1
Av 7.36 ± 0.21 p.p.m. .
using
For large scale separations a similar extraction method has been used, and more recently an ion exchange method. The initial step is the same for both.
Divide the sample into ca. 100-gram units. Disperse each unit in a small amount of H20 in a 1-liter heat-resistant polyethylene beaker. Add slowly 250 grams of 48% HF, then 80 ml. of concentrated H2SO4. Add about 10 mg. of BeO equivalent to carrier. Transfer to a 300-ml. Pt dish and heat slowly, expelling HF, SiF4, and H2S04. Finally the viscous solution solidifies. Extract this solid with about 300 ml. of H20 (the residue is organic matter and Al and Ca sulfates). If there is much unattacked original sample, further treatment with the KHS04 is needed, but this has not been necessary for our samples. Excess alkali metal ions should be avoided if the cation exchange process is to be used. Centrifuge and combine all final solutions from the sample. Add about a 20% excess of EDTA over the amount required to complex the Fe and Al present. Bring the volume to about 3 liters. Adjust the pH with NHa (aqueous) to about 6.4 (use the color of the Fe-EDTA complex as an indicator). When cool add 25 ml. of AA and stir until dissolved. Transfer to a separatory funnel and extract with three 250ml. portions of CeHe. Occasionally the organic layer must be centrifuged at this step. Combine the CeH6 fractions and wash with 500 ml. of H20 buffered
Replicate Analyses of Stand ard Sampl es G-l W-l
Be, p.p.m
2 3 10
tested
LARGE CORE SAMPLES
Sample, grams 5 3 5 5 7
Be, p.p.m.
3.0 3.3 3.7 3.4 3.0 3.3 ± 0.3 p.p.m.
Sample, grams 5 3 10 10 6 2
Be, p.p.m. 0 .76 0 .56 0 65 0 79 0. 69 .
0 .64
0.68 ± 0 .07 p.p.m.
VOL. 32, NO. 11, OCTOBER 1960
·
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at Table II.
Ion Exchange Methods
Separation from
Al, Al, Al, Al,
others others Fe, others others
Al, alkaline earths, U, others Ca, U, Cu
Form of Resin
Be
Eluent for Be
or
Al
Reference
Cation Exchange ca. 1M HC1 (Be) HR HR 0.05M Ca or Mg (Be) HR 0.4M oxalic acid (Al) NaR 3.5-4.0 (Al) EDTA, pH
(6, IS) (IS, U) This work
(21, SO)
=
NH
