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(8) Chaney, R. L.; Mielke, H. W.; Sterrett, S. B. Environ. Geochem. Health 1989, 11, 105. (9) Davis, A.; Ruby, M. V.; Bergstrom, P. Enuiron. Sei. Technol. 1992, 26 (3), 461. (10) Davis, A.; Ruby, M. V.; Bergstrom, P. Hazardous Materials Control Symposium Proceedings; Hazardous Materials Control Institute: Greenbelt, MD, 1991; pp 564-569. (11) Barltrop, D.; Meek, F. Arch. Enuiron. Health 1979,34,280. (12) Jeffcoat, R. A. Distribution of Lead in Rats After Repeated Exposure t o Lead Compounds in Feed Prepared by Research Triangle Institute, Research Triangle Park, NC, for National Institute of Environmental Health Sciences: Research Triangle Park, NC, 1991. (13) Freeman, G. B.; Johnson, J. D.; Liao, S. C.; Feder, P. I.; Killinger, J. M.; Chaney, R. L.; Bergstrom, P. D. Chem. Speciation Bioavailability 1991, 3 (3/4), 121. (14) Allison, J. D.; Brown, D. S.; Novo-Gradac, K. J. 1991. MZNTEQA2, A Geochemical assessment model for enuironmental systems: Version 3.0 User's Manual. EPA/ 600/3-9/021; U S . Environmental Protection Agency: Athens, GA, 1991. (15) Bovington, C. H.; Jones, A. L. Faraday Soc. Trans. 1970, 66, 2088. (16) Nordstrom, D. K. In Acid Sulphate Weathering; Soil Science Society of America: Madison, WI, 1982. (17) Blowes, D. W.; Reardou, E. J.; Jambor, J. L.; Cherry, J. A. Geochim. Cosmochim. Acta 1991,55, 965. (18) Bezwoda, W.; Charlton, R.; Bothwell, T.; Torrance, J.; Mayet, F. J . Lab. Clin. Med,. 1978, 92, 108. (19) Hunt, J. N.; Spurrell, W. R. J . Physiol. 1951, 113, 157. (20) Malagaleda, J. G.; Longstreth, G. F.; Summerskill, W. H. J.; Go, V. L. W. Gastroenterology 1976, 70, 203.
(21) Nriagu, J. 0. Geochim. Cosmochim. Acta 1974, 38, 887. (22) Berner, R. A. A m . J . Sci. 1978, 278, 1235. (23) Kornicker, W. A.; Presta, P. A.; Paige, C. R.; Johnson, 0. M.; Hileman, 0. E., Jr.; Snodgrass, W. J. Geochim. Cos. mochim. Acta 1991,55, 3531. (24) Tortora, G. J. Principles of H u m a n Anatomy, 2nd ed,; Harper and Row: New York, 1980; p 605. (25) Arnold, J. G.; Dubois, A. Dig. Dis. Sci. 1983, 28, 73;. (26) Luce, R. W.; Bartlett, R. W.; Parks, G. A. Geochim. Cosmochim. Acta 1972, 36, 35. (27) Duggan, M. J.; Inskip, M. J.; Rundle, S. A.; Moorcroft, J. S. Sci. Total Enuiron. 1985, 44, 65. (28) LaVelle, J. M.; Poppenga, R. H.; Thacker, B. J.; Gieay, J. P.; Weis, C.; Orthoudt, R.; Vandervoort, C. Chem. c'jeci. ation Bioauailability 1991, 3(3/4), 105. (29) Handbook of Chemistry and Physics, 56th ed.; Weti.-&R. C., Ed.; CRC Press: Cleveland, OH, 1975. (30) Streitwieser, A.; Heathcock, C. H. Introduction to Organic Chemistry, 3rd ed.; Macmillan Publishing Co.: New York, 1989; p 1197. (31) Giordano, T. H. Geochim. Cosmochim. Acta 1989,56,359. (32) Sparks D. L. Kinetics of Soil Chemical Processes, 1st ed.; Academic Press: San Diego, 1989; p 210. (33) Garrels, R. M. In Research in Geochemistry, 1st ed.; Abelson, P. H., Ed.; Wiley: New York, 1959; pp 25-37. (34) Aagaard, P.; Helgeson, H. C. Am. J . Sci. 1982, 28%. 237. (35) Holdren, G. R.; Berner, R. A. Geochim. Cosmochim. Acta 1979,43, 1161. Received for review November 19, 1991. Revised manuscript received February 25, 1992. Accepted February 27, 19!d.
COMMUNICATIONS Evidence for Thorium Isotopic Disequilibria due to Organic Complexation in Natural Waters Jeffrey S. Gaffney," Nancy A. Marley, and Kent A. Orlandlni Environmental Research Division, Building 203, Argonne National Laboratory, Argonne, Illinois 60439
Introduction Thorium and uranium parent-daughter relationships have been used classically to investigate several problems of environmental significance including the modeling of radionuclide transport and scavenging processes on geological time scales (I+), mixing and circulation of surface waters and groundwaters (&9), and dating of groundwaters and other geochemical materials (10, 11). They have also been used as analogues for a broad range of metals which exhibit similar adsorption properties (12). The principal assumptions in these methods are as follows: (1) the different isotopes are in chemical equilibrium in the water column, (2) observed deviations from equilibrium are due to differing source terms, and (3) the deviations can be used to date the geochemical process in question as the system returns to equilibrium. Recently, attention has been given to the existance of a third phase in the transport of radionuclides through adsorption to colloids (13-16). Isotopic disequilibria has been demonstrated for the uranium isotopes between colloidal (>1.5 nm) and solution phases (15). In addition, 1248
Envlron. Sci. Technol., Vol. 26, No. 6, 1992
it has been shown that thorium and uranium call form stable complexes with humic and fulvic acids 111 the aqueous phase and this interaction plays an important role in solubilization of these actinides (17-20). The n-obilization of thorium and uranium isotopes from rare earth deposits in Brazil is thought to be due to complexation of these species with humic substances (4,21,22). This interaction is currently neglected in geochemical modeling of transport processes. We report here observati>ns of thorium isotopic disequilibria in natural waters that appear to be due to preferential complexation of this actinidp with organic complexing agents (humic and fulvic acds) in macromolecule fractions within the water. Experimental Procedures Samples of water were obtained from Volo Bog, a small glaciated bog located in an Illinois Nature Preserve northwest of Chicago. The water is of low nutrient cc;n@nt with a pH of 5.5. Hollow-fiber ultrafiltration of the gamples was performed by using Amicon filters after the samples were prefiltered with 0.45-pm membrane filters.
~013-936X~92/0926-1248$03.00/0 0 1992 American Chemlcal Sociev
Gble I. Dissolved Organic Carbon (DOC), Trace Metal, Bpd Radionuclide Concentrations in Colloidal Samples from Volo Bog Size Fractions units
DOC thorium-228 thorium-232 thorium-230 radium-226 uranium-238 calcium magnesium iron manganese
2
1
1.6 1.45 0.37 0.74 7.10 0 2.0 2.2 120 0
4.7 0 0 0
4.44 0 0 0
120 25
fractionn 3 4 2.9 8.17 13.33 7.76 5.33 17.58 2.3 2.9 95 7
12.8 31.06 2.58 11.85 0 44.99 20.3 6.6 65 1
5 7.2 44.42 4.15 5.54 27.53 14.06 55.3 33.2 0 6
"(1)0.45-0.1 pm, (2) 0.1 prn-lOOK, (3) 100-30K, (4) 30-3K, (5)
t s K molecular weight. loo
z
E
1
D3cc
l
0 4 5 4 - 3 ly 0 1%-
i03K
i00K-30K
30K-3K
0.45 pm), Filtrate (
