ba-1975-0147.ch012

12 Collection and Analysis of Radionuclides in Seawater

Downloaded by PEPPERDINE UNIV on September 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0147.ch012

W. B. S I L K E R Radiological Sciences Department, Battelle, Pacific Northwest Laboratories, Richland, Wash. 99352

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Be and fission products from seawater are collected and their concentrations quantified by analysis. The basic sampling system consists of a unit which filters the water to remove particulate material and then directs the sample flow through a bed of aluminum oxide which retains a determinable amount of the various radionuclides. Fractional radionuclide adsorption wasfirstevaluated in the laboratory under simulated field conditions and then verified by experiments at sea. Analysis of the samples is done by anticoincidence-shielded multidimensional gamma ray spectrometry, which provides a sensitive means of radionuclide detection.

adioactive material i n the world's oceans has provided many useful tracers for investigations of various aspects of oceanography, geo chemistry, and other fields of study. These radionuclides include those present i n the decay chains of the natural uranium and thorium series, fission products and plutonium isotopes resulting from nuclear weapons testing, and the cosmogenic radionuclides. One of the cosmogenic radio nuclides that has proved most useful is Be, which is a spallation product resulting from the reaction of cosmic rays with atoms of oxygen and nitrogen. The techniques used for sample collection and analysis are varied and are dictated by the concentration of the particular radionuclide and the instrumental sensitivity available for its measurement. For those ra dionuclides that are present i n sufficiently high concentration, a 10-100 1. sample of water is collected, and the material is isolated and concen trated by co-precipitation and radiochemical separation. Several in situ 7

139 Gibb; Analytical Methods in Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

140

ANALYTICAL METHODS IN OCEANOGRAPHY

concentration methods have been developed i n which the material of interest is collected on a matrix that is exposed to the water. Folsom ( I ) uses ferrocyanides of cobalt or copper which are specific for cesium collec tion to measure the concentration of C s . The amount of water contacted is determined by measuring the quantity of inert cesium, a conservative element, retained i n the collector. Jute fiber or sponges impregnated with hydrous ferric oxide were used by L a i (2) to extract silicon and S i from tens of tons of seawater. In yet another application, Moore and Reid (3) use acrylic fibers impregnated with manganese oxides to extract radium isotopes from several thousand liters of seawater. 137


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Radionuclide Collection In our particular case, we are interested in measuring the concentra tions of Be and the gamma emitting fission products in seawater which are present at levels of a few tenths to hundredths of a dpm/1. To obtain a sufficiently large sample, characteristically 40001. of water are processed, and the large-volume water sampler (4) (Figure 1) is used. The bottom section of this unit contains eight parallel filters which remove particulate material greater than 0.3 μ i n diameter. Sample flow is then directed through a 0.64-cm thick bed of neutral alumina which retains a determinable fraction of the radionuclides of interest. The alkali metals and alkaline earths which are present i n rather high concentrations in the ocean are not adsorbed and pass through the alumina bed. The effi ciency with which the aluminum oxide retained various radionuclides was evaluated, both i n the laboratory and i n the Pacific Ocean. The radionuclides present i n the seawater served as tracers for this process. The ocean experiment thus eliminated any inconsistencies resulting from differences i n either the chemical or physical state between the prepared solution and the natural states of the various radionuclides. 7

Laboratory investigations were conducted with a small system that provided prefilters and a 1 c m X 0.63-cm deep bed of aluminum oxide adsorbent. A sample of seawater containing one or more radionuclides of interest was passed through this system at a flow rate of 50 m l / m i n , which is equivalent to the operational shipboard flow rate of 37.85 l . / m i n for the large ocean sampling system. Collection efficiencies, determined by gamma counting, were evaluated both by comparison of the spike concentrations i n the influent and effluent streams and by assay of the amounts retained by the bed with respect to the total throughput. The results from both of these methods were i n agreement. The retention efficiencies for the various radionuclides measured by this method are included i n Table I. Sorption of the radionuclides was constant at flow rates below 50 m l / m i n / c m but decreased b y 10% at 100 m l / m i n / c m 2

2

Gibb; Analytical Methods in Oceanography Advances in Chemistry; American Chemical Society: Washington, DC, 1975.

2


12.

siLKER

Radionuclides in Seawater

Figure 1.

141

The Battelle large volume water sampler

and 27% at 150 m l / m i n / c m . T h e retention efficiencies were maintained after the equivalent of approximately 6000 1. passed through the ocean system. This is i n excess of the 4000 1. normally required to measure adequately the concentration of ocean radionuclides. Using the facilities aboard the R / V Yaquina, a research vessel oper ated b y Oregon State University, the collection efficiencies of the radio nuclides existent i n the ocean were determined in situ. A large reaction vessel on the ship allowed the trace elements and radionuclides to be separated from about 600 1. of ocean water b y precipitation-scavenging reactions while a concurrent water sample was passing through the ocean sampling system. This experiment was repeated at four stations along the Oregon and Washington coasts, which were within the influence of 2


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the Columbia River plume yet contained 32% salinity. The Columbia River proximity provided adequate concentrations of radioactive tracers for the experiment. After the water was collected i n the reaction vessel, 288 p p m ferrous sulfate was added. Addition of a stoichiometric quantity (100 p p m ) of potassium permanganate thus resulted i n formation of hydrous ferric and manganese oxides. Scavenging efficiencies for this precipitation reac tion have been reported previously (5,6). The resulting suspension was agitated for 10-20 min and allowed to settle for 12-24 hr, after which the supernatant liquid was siphoned. The precipitate was removed and Table I.

Percent Uptake of Soluble Radionuclides from Seawater by Alumina 0

Radionuclide

Laboratory

51 M

60 w

124 I44

2U

B i

Ru

Sb Ce

(226

2S4

R a

66 68 0.5 44 10 44 59 17 3 62 4 40

60 43 25 1.5 14 45 47 18 2.5 58 2.5 56

'Be «Sc Cr Mn Co Zn ""Zr-Nb

ioe

Ocean Test

)

X h

± ± ± ± ± =fc ± ±

14 35 0.5 44 6 45 13 2 rfc 2 ± 29 =fc 6 ± 7

Those materials that pass on 0.3μ membrane filter are assumed to be soluble. Flow = 50 ml/min/cm . β

2

eventually isolated by filtration, dried, and transferred into a suitable counting container. The filters and alumina, which were removed from the ocean sampler, were also dried and transferred into counting con tainers. A l l of these samples were analyzed w i t h a multidimensional gamma-ray spectrometer (5), and the amounts of the various radioactive species were determined from the resultant spectra. B e d efficiencies calculated according to the following equation are also included i n Table I : Efficiency =

d/m Wfr) d/m (ppt) — d/m

(filter)

m

( 1 )

The quantities of radioisotopes used i n the laboratory tests were so large that the error caused by counting statistics was less than 3 % . Large errors attached to the values from the ocean test arise i n part from the counting statistics from the low concentrations of radionuclides i n -


12.

srjLKER

Radionuclides in Seawater

volved and i n part from the division of a small number by the difference of two relatively large numbers. Good agreement was obtained between the two tests for all isotopes except M n and C r . It is felt that these dis crepancies resulted from differences in the chemical or physical form of the isotopes in the two systems, which emphasizes the need for in situ evaluation under the conditions from which field samples are expected to be obtained. These comparisons also show that for cases where the natural state of the radionuclides can be duplicated, laboratory evalua tion of retention efficiencies are satisfactory. Currently, we monitor the retention efficiencies i n a different manner. Laboratory tests demonstrated that adsorption of the different radio active species by aluminum oxide obeyed the Freundlich isotherm. Successive beds of the same thickness removed the same fraction of the amount of radionuclide passing the preceding bed as was retained by the upstream bed. In other words, if a bed of a given thickness removed 50% of a particular isotope, a second bed of the same thickness removed 50% of the residual or 25% of the amount initially present. Thus, by using two 0.64-cm beds in series and by measuring the concentrations of radionuclides on each bed, the collection efficiencies for all radio nuclides can be calculated easily. The precision of the sampling method was determined from the analysis of four surface water samples collected at a station i n the North Equatorial Atlantic Ocean. Water was simultaneously pumped through two sampling units which were reloaded, and the process was repeated. The analytical results obtained from these four replicate samples, to gether with the retention efficiencies for the various radionuclides are given i n Table II. The range of the replicate measurements agrees reasonably well with that which would be expected from statistical con siderations. The poor precisions recorded for C e and R a arise in part from poor counting statistics and also from the fact that only 2.5% of the R a is retained by the aluminum oxide bed. As previously mentioned, aluminum oxide is used as our primary collector, but other media have been used for special situations. For example, when the Hanford reactors were operating, quantities of hexavalent C r were discharged to the Columbia River and subsequently to the ocean, and we were interested in studying the dispersion rate of the Columbia River plume. Alumina d i d not collect the dichromate ion efficiently. B y using alumina saturated with stannous chloride, the chromium was reduced o a contact to the trivalent state, and this was very efficiently retained on the bed. W e also found that by saturating alumina with barium sulfate, we could collect radium isotopes, pre sumably by a replacement reaction with the barium on the matrix. For 5 4


143

51

144

226

226

5 1


144


Table II. Sample

me


1 2 3 4 Average Precision (%)

Measured Radionuclide Concentrations

ba-1975-0147.ch012

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