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July 23, 2003

The Effect of Phosphate on Lanthanide Sorption by an Oxide Mineral: X-ray Absorption and Magnetic Studies

S.-j. Yoon¹, P.A. Helmke¹, W.F. Bleam¹, and J.E. Amonette^2
1Department of Soil Science, University of Wisconsin, Madison, WI; ²Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, WA

The feasibility of using phosphate to enhance radionuclide immobilization on mineral surfaces was examined using trivalent lanthanide ions as chemical analogues of actinide radionuclides. Phosphate dramatically increased the amount of lanthanide sorbed on aluminum oxide surfaces at pH 5, the product apparently being ultra-fine particles of LnPO₄.

High-level radioactive wastes containing actinide elements have been introduced to the environment with the advent of nuclear weapons and nuclear energy. Sorption of the actinides by soil materials decreases their mobility and bioavailability. Understanding their sorption mechanism is important for forecasting their behavior in soils and designing environmental remediation procedures.

Sorption studies involving radionuclides, such as ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Am, are relatively rare because the U.S. Nuclear Regulatory Commission grants few licenses for the possession and handling of these elements. Furthermore, it is difficult to control experimental conditions because their oxidation states are unstable. Other actinide ions (²³²Th⁴⁺, ²³⁷Np⁵⁺, and ²³⁸U⁶⁺) and non-radioactive lanthanide ions (Ln³⁺) have served as chemical analogues for those unstable actinide ions in various environmental studies. The utility of these analogues, verified by numerous studies for solution chemistry, may also extend to mineral surface chemistry.

In this study, trivalent lanthanide ions (Eu³⁺, Gd³⁺, and Dy³⁺) serve as chemical analogues of trivalent actinides to explore the sorption chemistry of actinide ions (Ac³⁺) on mineral surfaces as affected by phosphate, a ubiquitous oxyanion found on all oxide mineral surfaces in nature. The in situ immobilization of actinides by phosphate may be feasible considering both actinide and lanthanide phosphates are highly insoluble and stable in geological formations.

Orthophosphate has proven effective for immobilizing lead (Pb²⁺) as insoluble lead phosphate precipitates on phosphate minerals. Our study asked whether adsorbed phosphate anions at oxide mineral surfaces react with lanthanide ions, immobilizing lanthanide as phosphate surface precipitates analogous to the reaction observed with Pb2+.

The amount of lanthanide ions sorbed on boehmite (γ-AlOOH) surfaces, estimated by neutron activation analysis, dramatically increased in the presence of phosphate at pH 5 (Figure 1). The structure of the sorbed lanthanide was determined by x-ray absorption spectroscopy (beamlines X23B and X10C at the National Synchrotron Light Source, Brookhaven National Laboratory), as well as by magnetic susceptibility measurements (Institute for Rock Magnetism at University of Minnesota and Kansas State University) and electron paramagnetic resonance (EPR) spectroscopy (EMSL, Pacific Northwest National Laboratory).

The extended x-ray absorption fine structure (EXAFS) shows that the sorbed lanthanide ions react with phosphate to form lanthanide phosphate precipitates of indeterminate size on the oxide surface (Figure 2). The superparamagnetic behavior of dysprosium and gadolinium in the surface precipitates indicates the lanthanide phosphate precipitates are uniform, ultra-fine particles (radii less than 10 Å assuming spherical particles) distributed on the oxide surface. Even in the absence of phosphate, sorbed Gd³⁺ ions apparently form precipitates on boehmite surfaces rather than being adsorbed as isolated ions as evidenced in EPR spectra (not shown).

To the extent that Ln³⁺ ions serve as suitable chemical analogues for Ac³⁺ ions, the results of this study predict the ubiquitous phosphate, adsorbed to all mineral surfaces in the environment, would enhance the sorption of Ac³⁺ ions leading to the mineral-surface precipitation of ultra-fine AcPO₄ particles.

BEAMLINE
X10C, X23B

FUNDING
U.S. Department of Agriculture
Hatch Program
U.S. Department of Energy through the William R. Wiley Environmental Molecular Sciences Laboratory

PUBLICATION
Yoon, S., Helmke, P.A., Amonette, J.E., and Bleam, W.F. X-ray Absorption and Magnetic Studies of Trivalent Lanthanide Ions Sorbed on Pristine and Phosphate-Modified Boehmite Surfaces. Langmuir 18, 10128-10136 (2002).

FOR MORE INFORMATION
Soh-joung Yoon
Environmental Sciences Department
Brookhaven National Laboratory, Upton, NY
Email: syoon@bnl.gov