CURRENT RESEARCH Submicron Particle Size and Charge Characteristics of 2399240Pu in Natural Waters James J. Alberts*, Morris A. Wahlgren, Donald M. Nelson, and Paul J. Jehn Radiological and Environmental Research Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, 111. 60439
The submicron particle size distribution and charge characteristics of atmospheric fallout 239,240Pu are determined by ultrafiltration and ion-exchange resin techniques at extremely low activities (-1 femtocurie/L) in rain, snow, and surface waters of the northcentral and southeastern United States. Results indicate that fallout 239,240Pu exhibits different size and charge characteristics as well as total activities in waters of differing pH and anionic composition. These observations are of importance in attempting to predict the geochemical behavior of plutonium on a wide geographical scale. The concern over possible environmental contamination by radionuclides has been enhanced by proposals of major increases in the use of nuclear power to ease the demands on petroleum-generated energy. As the supply of uranium is exhausted, plutonium usage will be required for continued nuclear power generation. Therefore, the fate of the isotopes of plutonium in the biogeochemical cycles of the environment is of particular concern, especially since the bioavailability of this element may be affected by its chemical state and size spectrum. Although the chemistry of plutonium is one of the best understood of all elements at the laboratory scale, little is known of the solution chemistry of this element at environmental levels, which are approximately 10 orders of magnitude less than usually observed in laboratory studies. For several years, Argonne National Laboratory has been studying the distribution of plutonium in the waters, sediments, and biota of the Great Lakes in an effort to derive a predictive capability for this element at concentrations M) which are realistic in an environmental framework ( I ) . One phase of this program has been determination of the distribution in the colloidal and subcolloidal size ranges, and the ionic charge of plutonium which has been in the environment for several years. This report presents some of the findings from this study and compares the submicron particle and total charge distribution pattern of 239,240Pupresent in precipitation and surface waters from diverse areas of the eastern United States as a result of atmospheric fallout from nuclear testing. Methods Water samples from Lake Michigan were taken at a station approximately 13 km southwest of Grand Haven, Mich. (water depth = 67 m), from the University of Michigan research vessel, R/V Mysis. Water samples from 3 m were taken using a submersible pump, while those from 60 m depth were taken by repeated lowerings of a 30-L Niskin sampling bottle. Two hundred liters of water were pressure filtered through 3.0-wm and then through 0.45-1m membrane filters (Millipore Corp.). Two 50-L samples of filtered water were immediately spiked with 236Pu(1.2 dpm) as an internal standard, and 100 mL of concentrated HC1 which lowered the pH to approximately 2.
A third 50-L filtered water sample was taken for ultrafiltration through a Bio-Fiber 80 Miniplant (Bio-Rad Lab.) which has an approximate size retention of particles greater than 30 000 molecular weight (MW) or about 30 8, diameter. The water which passed through the fiber bundle was collected and spiked as above. The fiber bundle was then backflushed with deionized water which was spiked and retained. The final 50 L of filtered water were passed (1 L/min) through successive ion-exchange resin beds containing Y2 lb of anion resin (Dowex 1A X 8,100-200 mesh, chloride form) and l/2 lb of cation resin (Dowex 50W X 8, 100-200 mesh, hydrogen form). The water which passed through these resins was discarded. The fractionation scheme outlined in Figure 1 has also been accomplished by passing water samples through the ultrafiltration fibers prior to ion exchange. Both methods give comparable results. The scheme used here was chosen because of its flexibility aboard a ship. Snow samples were collected by placing large polyethylene sheets on the ground after the beginning of a snow storm. The snow was then transferred to large (200 L) covered plastic vats and allowed to melt at room temperature (approximately two days), but with gas exchange to the atmosphere. The melt water was then passed through the same fractionation scheme as the Lake Michigan water samples. The pH of the sample was determined with a Beckman pH meter with combination glass electrode.
Raw H20
J
Deck Screen
80 p + 25
u
.1
M e m b r a n e Filter
M e m b r a n e Filter
0.45
IJ
1
Hollow Fiber U l t r a f i l t r a t i o n
30.000 M W Retention
. 1 Ultrafiltered H20
A n i o n Resin Dowex 1
- X8
4
Cation Resin Dowex 50 W
- X8
J . Residual
Figure 1. Flow diagram of fractionation scheme employed for 239*240Pu particle size and charge distribution studies of natural water sam-
ples Volume 11, Number 7, July 1977
673
Rain water samples were taken by placing two large boards (1.2 x 2.4 m) covered with polyethylene so that the water
falling on them during a rain storm would be funneled and collected in a large plastic vat. The pH of the solutions was determined, and the water collected in this manner was then fractionated as above. Snow and rain samples were collected on the Argonne site. Water samples from Banks Lake, a small black water lake in south Georgia whose limnological parameters have been previously described ( Z ) , and from Okeefenokee swamp in Georgia, were obtained by J. E. Schindler, University of Georgia. Water was pumped into large carboys from a depth of 1 m. The samples were immediately air freighted to this laboratory, where they were fractionated within four days of collection. Observations on Lake Michigan waters showed that the distribution of plutonium activity did not change in this period of time in samples stored a t a pH of approximately 8. The pH of the samples from Georgia was 4-5, and it is assumed that the observed distributions were not altered significantly during transit and storage. The various water samples were analyzed for 239,240Pu by the method of Golchert and Sedlet ( 3 ) .The resins were dried a t 110 "C, ashed a t 500 "C, dissolved in concentrated nitric acid, spiked with 23fiPu(1.2 dpm), evaporated to dryness, dissolved in concentrated HBr to ensure isotopic exchange, evaporated to dryness again, taken up in nitric acid, and then analyzed in the same manner as the water samples. The activity of 2393240Puwas determined by comparing the integrated count under that peak with that in the 2 3 f i P internal ~ standard peak.
Discussion The results of this investigation are summarized in Table in I ( & I counting error). The concentrations of 239,240Pu
filtered surface and deep waters from Lake Michigan exhibit the same lower surface values relative to deep waters as previously reported in unfiltered samples taken during a summer season ( 4 ) . Water samples from lakes in the southeastern United States and of precipitation samples from the Argonne area have significantly higher 239,240Pu activities than filtered water in Lake Michigan. The decrease in 239,240Pu concentration in Banks Lake surface waters between May and December may indicate a seasonal trend similar to that observed in Lake Michigan, but more analyses will have to be undertaken to substantiate these values. One possible explanation for the increased 239.240Pu activity values observed in lake samples taken from places other than Lake Michigan may be found in the pH differences observed in these samples. Lake Michigan surface waters during August and September are supersaturated with calcium carbonate (5, 6) and hence have a p H of approximately 8. All samples which are higher in 239,240Pu activity have pH values 3-4 units more acidic than Lake Michigan. It may be postulated that the presence of carbonate, which forms a strong complex with plutonium (7), is the major factor controlling the level of filterable 239,240Pu in waters of high pH. This hypothesis is supported by the fact that surface waters of a southeastern pond from South Carolina, with pH values approximately the same as Lake Michigan, have 2393240Pu activities less than or equal to those of Lake Michigan, despite the fact that the sediments have approximately five times the 2y9,240Pu activitylg of Lake Michigan sediment (Alberts and Corey, unpublished data). Examination of the data in Table I reveals striking differences in the distribution of 239,240Pu in the submicron particie fractions of these water samples. With the exception of the September 1974 3-m sample, Lake Michigan water does not activity in have a significant fraction of its filterable 239,240Pu
Table 1. Activity Values of 2399240Pu (fCi/L) in Various PhysicochemicallyDefined Components of Natural Watersa 30 000 M W
pn
Of
water
Lake Michigan Sept. 1974
Filtered water
ultrafiltered water
f 0.03
Ultrafilter backwash
Anion exchangeable
Cation exchangeable
Ultrafilterable noncharged
f 0.02
0.42
f 0.06
