Environ. Sci. Technol. 1990, 2 4 , 706-712
Truesdell, A. H.; Jones, B. F. J . Res. U S . Geol. Sum. 1974, 2, 233-248. Butler, J. N. Ionic Equilibrium; Addison Wesley: Don Mills, Ontario, 1964. Kielland, J. J. Am. Chem. SOC.1937, 59, 1675-1678. Helgeson, H. C.; Kirkham, D. H. Am. J . Sci. 1974, 274, 1199-1261. Nordstrom, D. K. Proceedings of the Hazardous Materials Management Conference/ West, Long Beach, Calif. Dec. 3-5, 1985. Bonner, A. J., Ed.; 1986; pp 453-457. Dubrovsky, N. M. Ph.D. Thesis, University of Waterloo, 1986. Dubrovsky, N. M.; Cherry, J. A.; Reardon, E. J.; Vivyurka, A. J. Can. Geotech. J . 1985,22, 110-128.
(42) Waser, J. Quantitative Chemistry; Benjamin: New York, 1966. (43) Whitfield, M. Ion Selective Electrodes for the Analysis of Natural Waters; Australian Marine Science Association: Sydney, Australia 1971; Handbook 2. (44) Nordstrom, D. K. Geochim. Cosmochim. Acta 1977, 41, 1835-1941. (45) Plummer, L. N.; Busenberg, E. Geochim. Cosmochim. Acta 1982, 46, 1011-1040. Received for review February 28,1989. Accepted December 27, 1989. Financial support was provided by the Natural Sciences and Engineering Research Council of Canada.
Colloidal Behavior of Actinides in an Oligotrophic Lake K. A. Orlandini,*~fW. R. Penrose,+,'B. R. Harvey,§M. B. Lovett,§and M. W. Findlay+,'
Environmental Research Division, Argonne National Laboratory, Argonne, Illinois 60439, and Ministry of Agriculture, Fisheries, and Food, Directorate of Fisheries Research, Lowestoft, Suffolk NR330HT, United Kingdom Understanding the speciation of low levels of actinides from fallout and from local sources in freshwater systems is important if we are to predict their distributions in the environment. Since these materials make excellent tracers for determining sedimentation rates and other environmental parameters, it is important to determine their physical and chemical properties in relatively pristine systems. Reported here are the results of actinide analyses in an artificial, oligotrophic lake in northwest Wales, United Kingdom, which is used as a source of cooling water for a nuclear power plant. The concentrations of the actinide elements plutonium, americium, thorium, and curium, and their distributions among different colloidal sizes were determined. Actinide concentrations in the dissolved fraction (10.45 pm) were as follows: 2397240Pu, 6.4-12.5 fCi/L; 241Am,2.5-18.2 fCi/L; 232Th,0.11-1.09 fCi/L; and W m , 0.3-1.4 fCi/L. The majority of the actinides in the lake were retained by hollow-fiber ultrafilters of 5-nm (nominal 1OOOOO MW) or 100-nm pore sizes; the actinides appeared to be bound reversibly to colloidal material of unknown composition. The two environmentally stable oxidation states of plutonium, IV and V, could be separated by ultrafiltration. These results indicate that submicron colloidal material can dominate the aqueous properties of actinides. Introduction
Numerous environmental studies in water and sediments have made use of trace metals to determine environmental impact, as well as act as indicators of geochemical parameters, e.g., sedimentation rates. Of particular use are actinide markers that can occur from fallout or from local sources. Some of the more useful of these markers include isotopes of plutonium, thorium, americium, and curium, which can be used as indicators of regional and locally sourced materials. The fate of many trace metal or organic pollutants in natural waters is controlled by their binding to particulate matter. The phase separation resulting from such binding, followed by sedimentation, is an important mechanism by
'Argonne National Laboratory. * Present address: Transducer Research, Inc., Naperville, IL
60540.
$Directorate of Fisheries Research. 706
Environ. Sci. Technol., Vol. 24, No. 5, 1990
which particle-reactive pollutants and trace metals can be removed from bulk water. The actinide elements have a strong affinity for particulates, but this affinity is modified by pH, inorganic ions, oxidation state, and the presence of colloidal organic matter. Depending on the circumstances, one or another of these parameters might predominate (1). For example, carbonate can form complexes with U(V1) that bind poorly to particulates. Actinides in the V oxidation state, such as Pu(V) and Np(V), are only weakly particle-reactive, and Am(III), Pu(IV), and Th(1V) are strongly reactive. Colloidal organic matter can strongly complex trivalent and tetravalent actinides at concentrations commonly encountered in the environment. Nelson et al. (2) demonstrated that concentrations of organic matter existing in many natural waters (1-20 mg/L) can compete with suspended particulates for the available actinides. The binding of actinides to particles (i.e., 10.45 pm) can be reduced by orders of magnitude by the presence of smaller colloidal organic matter. Other forms of colloidal material, such as clays and iron and manganese hydroxides, are also encountered in natural waters, but their roles are less well understood than that of organic colloids. Recent studies of natural colloidal systems by Santschi and others (3-5) have led to a renewed appreciation of the dynamic nature of these systems. Aside from the binding of ions to particulates and colloids, these materials seem to participate in equilibria involving aggregation and disaggregation as well. We have investigated the distribution of actinides among natural colloidal particles of various sizes. This study was done in an artificial oligotrophic lake located in northwest Wales, United Kingdom, which contains measurable traces of some important actinides. Methods
Study Site. Lake Trawsfynydd (Figure 1) is located in northwest Wales, United Kingdom. It is an artificial impoundment created in 1926 to supply a small hydroelectric station in the village of Maentwrog. Since 1965 the lake has been under the control of the Central Electricity Generating Board (CEGB). A Magnox-type twin 500-MW nuclear power reactor uses the lake as a source of cooling water to condense steam from the turbines (6). Spent fuel rods are cooled in a small pond that is scrubbed with ion-exchange resins. The washes from resin recycling,
0013-936X/90/0924-0706$02.50/0
0 1990 American Chemical Society
OUTFALL
-O
950 SCALE (m)
-
Figure 1. Map of Lake Trawsynydd, Gwynedd, Wales, United Kingdom, located 10 miles east and Inland of Porthmadog. Sampling locations A-C are described in the text.
together with active wastes from other sources, are drained in a controlled manner into the lake, such that radioactivity levels are maintained within acceptable limits. The outflow of the lake travels 10 mi to the sea. The lake supports a valuable sports fishery, which is not compromised by the trace levels of radioactivity. The lake is approximately 5.3 km2 in area, with an average depth of 5.2 m. The catchment area is 92 km2 and consists mainly of ancient granitic rocks (6). The region receives 1.76 m of rainfall per year, so that the residence time of water in the lake is short, -2 months. At full power, the volume of the lake is drawn through the reactor once a week. This can raise the water temperature as much as 10 "C above ambient. Maximum cooling efficiency is attained by channeling the hot water through a series of lagoons to the south end of the lake and drawing cold water through another dyked channel from the north end near the dam. Whitehouse stated that the lake does not stratify (6). That was our observation also, but in July 1986 we found a hole 20.5 m deep near the dam that contained anoxic water, apparently stabilized by a slight temperature difference (Figure 1, station C). Sampling. Samples were taken from the Bailey bridge (Figure 1,station A), from midlake (station B), and from the deepest sounding near the dam (station C). Midlake samples were taken from the CEGB boat by using a pump with a stainless steel and Teflon diaphragm, driven by compressed air. A 5-L polyvinyl chloride airtight chamber was included in the sampling line on the suction side to trap bubbles. This chamber was also used for direct measurements on the water with electrical probes or for removing samples for Winkler oxygen determination. A Palintest PTllO meter was used for pH measurement and a Palintest PT115 meter for conductivity (Wilkinson and Simpson, Ltd., Gateshead, Tyne and Wear, England). Samples were taken in 25-L polyethylene containers and transported immediately to the CEGB laboratory for fractionation. Samples were taken during the periods of April 28-May 1, July 21-24, and October 6-8,1986, and March 23-24, 1987.
Sample Fractionation. Samples were screened through 35-pm nylon mesh before all treatments. Portions were filtered through tared membrane filters (Millipore Corp., Type HAWP-293-25,0.45-pm pore size) to obtain dry weights of particulates. The remainder of each sample was filtered through nontared Millipore membranes before ultrafiltration. Hollow-fiber cartridges and apparatus were obtained from Amicon Corp., Danvers, MA. Cartridge types used were the 3000 molecular weight (MW) size (H10P3-20, nominal pore size 1.5 nm), 30000 MW size (H5P30-43, 2.5 nm), 100000 MW size (H5P100-43,5 nm), and 0.1 pm size (H5MP01-43). These were installed in a lightweight portable apparatus (Amicon Model DC-1OA) suitable for field use. Samples of screened and filtered water were circulated continuously through the cartridges by airdriven reciprocating pumps with Teflon and glass wetted parts (Asti, Cole-Parmer Corp). Back-pressure was often needed to produce a reasonable flow rate and was established by adjusting a needle valve on the output side of the cartridge. After a sample was reserved for actinide measurements,the filtrate was dialyzed through the 0.1-pm cartridge. A portion of this was reserved for analysis, and the remainder was split into three fractions for dialysis in the 1.5, 2.5-, and 5-nm cartridges. The product of each separation was a dialysate containing material smaller than the pore size of the hollowfiber filter and a concentrate containing the colloidal material. During the sampling period of March 1987, both dissolved organic carbon and actinides were measured in the concentrates. Since the original volumes were known, the analytical determinations on the concentrated processed volumes were used to determine the original concentrations. Actinide Analyses. Actinides were measured with techniques reported previously (7) and will only briefly be reviewed. For all radionuclide measurements, appropriate internal standards of 242Pu,230Th,and 243Amwere added to the samples. Samples on Millipore filters were ashed at 525 "C and dissolved in hot hydrochloric acid. Samples in solution were acidified with HC1, treated with 1mg/L iron(III), and precipitated with excess ammonia. The ferric hydroxide was recovered by filtration on Millipore filters. The filters were ashed in a muffle furnace and dissolved in 6 M HC1. After clarification by centrifugation and evaporation to dryness, samples were dissolved (8 M HN03) and applied to an AG-l-X8 anion-exchange column. The column effluents and 8 M HN03 washes were saved for americium and curium analysis. Thorium was eluted from the column with 12 M HC1, and plutonium with 0.1 M HCl/O.Ol M HF. The washes in 8 M HN03 were evaporated to dryness, dissolved in 9 M HC1, and washed through an AG-l-X8 column (9 M HC1) to remove and retain iron and uranium. The column washes, containing americium, curium, and rare earths, were evaporated, redissolved in 1M HCl, and then applied to an AG-50-X8 cation-exchange column. After the column was washed with 50-70 column volumes of 1 M HC1, americium and curium were then eluted with 6 M HC1. All samples were electroplated onto stainless steel planchets from an oxalic acid/HCl solution (8). CY activity was determined by high-resolution a spectrometry with silicon surface-barrier diodes. Separation of Pu(II1,IV) and Pu(V,VI) was carried out prior to analysis for some of the samples using either the bismuth phosphate (9) or the neodymium fluoride analytical procedure ( I O ) . Results obtained from either of Environ. Sci. Technol., Vol. 24, No. 5, 1990
707
Conductivity - I.LS c
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Figure 2. Water chemistry profiles at station C in the oligotrophic lake. From left to right: (A) July, 1986; (B) October 1986; (C) March 1987.
Table I. Chemical Properties of Lake Water Samples Taken at Site C (See Figure 1) parameter
surface
bottom
est error
20.5 0.5 19.2 11.7 1.7 37.5
0.5 0.1 0.1 0.2 0.1 0.5
6120-24186 water depth, m oxygen, mL/L temp, O C alkalinity, mg of CaC03/L reactive silica, mg of Si/L phosphorus, pg of P / L nitrate, pg of N/L
6.7 21.9 7.6 1.2 5.0 32
5.4 19.5 24.9 12
16.0 3.8 18.5 26.1 10
0.5 0.1 0.1 0.1
14.5 4.6 9.8 1.0
0.5 0.1 0.1 0.5
1
3/23/87 water depth, m oxygen, mL/L temp, "C phosphorus, pg of P / L
8.4 11.7 3.0
these methods were in good agreement (see Table 111). Other Analyses. Dissolved organic carbon was measured by P. Statham of the Department of Oceanography, University of Southampton, U.K., or in the United States, using a Sybron-Barnstead total organic carbon analyzer (Servomex Co., Norwood, MA). The instrument was calibrated with potassium hydrogen phthalate, 100 mg of carbon/L, as recommended by the manufacturer. A preparation of purified humic acids (25.5 mg/L) from a pond at Argonne National Laboratory (ANL) was used as a secondary calibration standard. Oxygen, alkalinity, reactive silica, phosphorus, and nitrate were measured in the analytical laboratories of the Directorate of Fisheries Research, by standard methods. Iron was measured by atomic absorption spectroscopy in an air/acetylene flame.
Results Water Chemistry. Analysis of water samples taken in April 1986 yielded concentrations of 5-6 mg/L sodium, 0.8 708
station
Envlron. Sci. Technol., Vol. 24, No. 5, 1990
239,240Pu
232Th 4128-29/86 0.91, 0.66 1.09, 0.56 0.89
A B, 2 m B, 9 m
12.0, 10.8 15.1, 9.8 10.7
A, 2 m C, 1 m C, 16 m
7.1, 5.7 6.1 3.3, 2.6
A C, 1 m C, 15 m
1016186 9.7 0.36 8.0, 11.7 0.30 5.9, 8.7, 7.8 0.28
A C, 1 m C, 13.5 m
9.9, 8.6 10.2, 9.3 9.3
1
1016186 water depth, m oxygen, mL/L temp, O C reactive silica, mg of Si/L nitrate, pg of N/L
Table 11. Concentrations of Actinides i n 0.45-pM Filtratesa 241Am
18.6, 17.7 1.4, 1.4 19.5, 13.2 1.4, 1.0 13.2 1.1
7/21/86 0.66, 0.43 6.5, 5.0 0.41 5.8 0.28*, 0.11* 2.5
3/23/87 0.54, 0.42 0.47, 0.46 0.53, 0.69
2"Cm
0.44, 0.33 0.40
0.40, 0.40
9.7 9.7 10.4
0.22 0.96 0.67
6.8, 6.4 6.6 7.6
0.53 0.63 0.76
OReplicate samples were taken when possible, and all the data are reported. All samples had propagated counting errors 4
100-450
5-100
2.5-5
Particle Size C l a s e
1.5-2.5
- nm
-
450
100-450 5-100
1.5-2.5
2.5-5
Particle Size Class
- nm
