PNNL-13501 - Pacific Northwest National Laboratory

Actinide Chemistry at the Aqueous-Mineral Interface by Atomic Force Microscopy
Study Control Number: PN00002/1409
David L. Blanchard Jr., Steven C. Marschman
The interaction of actinides with soil-mineral surfaces affects the migration of contaminants through the vadose zone and
aquifers. However, little direct, sub-micron and molecular level data on this interaction have been collected. Such data
could help validate models of actinide transport currently in use. This project investigated one aspect of that interaction,
the morphology of actinides deposited on a mineral from aqueous solution, using atomic force microscopy.
Project Description
The purpose of this project was to determine the
morphology of actinides deposited at mineral surfaces
using atomic force microscopy, and to use that to infer the
identity of the actinide species present and the mechanism
of their attachment to the surface (sorption, precipitation).
A neptunium(V) nitrate (NPO2NO3) solution was
synthesized and deposited on the surface of a single
crystal of a mineral, calcite, found in great abundance in
soils, particularly at the Hanford Site. Atomic force
microscopy was used to record the morphology of the
resulting neptunium deposits. This project advanced our
expertise in the area of the interaction of actinides with
surfaces.
Sorption/desorption and precipitation/dissolution
processes at mineral surfaces play a key role in
determining the rate of migration of contaminants through
soils and groundwater systems, and in the fixing of
contaminants in soil minerals. Much empirical work has
been performed on the macroscopic scale to determine
distribution coefficients between groundwaters (or
simulated groundwaters) and minerals. However, as
stated succinctly by Liang et al. (1996), any rigorous
attempt to understand and control the chemistry of the
aqueous-mineral interface must include an explicit model
of molecular-scale interactions at that interface.
Molecular scale models of sorption/desorption and
precipitation/dissolution have been hypothesized to
describe the results of the macroscopic experiments and
predict contaminant migration in real systems, with
varying success.
Atomic force microscopy has been used to study the
growth and dissolution of a number of minerals and to
study the interaction of solution species with mineral
surfaces. References for this large body of work may be
found in a number of reviews (Maurice and Lower 1998;
Maurice 1998; Hochella et al. 1998; Brady and Zachara
1996; Wicks et al. 1994). However, little molecular-level
data exist on the interaction of actinides with mineral
surfaces (Combes et al. 1992; Wersin et al. 1994). Three
other studies that focus on the interaction of uranium (U)
with mineral surfaces have recently appeared as
presentation abstracts only (Nitsche et al. 1999;
Wasserman et al. 1999; Morris et al. 1997).
Attention is now focused on subsurface radionuclide
contamination issues, but little knowledge exists on the
fundamental controlling processes for actinides.
Investigation of these processes at the molecular level is
key to understanding and developing accurate models of
subsurface contaminant migration.
Results and Accomplishments
A NpO2NO3 solution (10 mg neptunium per mL) was
prepared by purifying a solution containing multiple
anions (NO3 - , SO4 2- , PO4 3- , ClO3 - ), and neptunium in
multiple oxidation states (+IV, +V, and +VI).
Neptunium(V) crystalline deposits were formed by
evaporating a 10 µL drop of the neptunium solution on a
chip (10 mm x 5 mm x 2 mm) of freshly cleaved calcite.
Such a deposition may mimic processes that occur in the
vadose zone.
The neptunium deposits covered the calcite surface
extensively. Large crystals 50 µm to 200 µm across and
over 5 µm thick were observed, as well as dendritic
crystals 5 µm to 10 µm wide, hundreds of µm long, and
only 100 nm to 150 nm thick. Some ordering of both
large crystals and dendrites was observed in the calcite
substrate. The crystals covered most of the surface
exposed to the solution. A blank sample showed very few
crystals, indicating that the crystals are not simply
dissolved and recrystallized CaCO3.
A number of optical images (7) and atomic force
microscopy scans (57) were collected. Crystalline
deposits at the edge of the 10 µL drop coverage are shown
Nuclear Science and Engineering 359