| dc.description.abstract | The fate of actinides in the environment is of interest for a several
reasons. In oceanic surface waters actinides such as thorium and protactinium,
and in particular their ratio, are used as tracers of processes such as boundary
scavenging and paleocirculation. Thorium is also used to estimate residence
times and particle and colloid fluxes from the euphotic zone, which is useful in
global carbon budgets used to assess effects of global warming. Terrestrially,
contaminated areas in need of remediation, such as former nuclear weapons
production facilities, remain as repositories for no longer needed actinide
stockpiles or waste by-products such as plutonium. All three of these actinides:
thorium, protactinium, and plutonium are known to be particle-reactive but the
extent to which they sorb to immobile particles and mobile colloids can vary
with environmental conditions. Understanding controls on adsorption is
important in understanding uses and any limitations of these radioactive tracers
caused by colloids.
Often laboratory studies to understand actinide behavior are conducted
at concentrations (micro- to millimolar), which are orders of magnitude higher than they are found in the environment (femto- to picomolar). Colloids, a size
class of particles operationally defined as 1 nm to 1 µm in size, are ubiquitous in
aquatic systems. The effect colloids have on actinide particle association, i.e.
competitive or enhancing, can have a profound influence on the ultimate
behavior of the actinide.
The overall aim of this study is to assess sorption of thorium,
protactinium and plutonium onto silica particles as a proxy for inorganic
particles found in surface or ocean waters. In addition to the binary system of
actinide/silica, the ternary system actinide/ organic colloid/ silica were also
carried out to determine the affect of the organic colloid has on particle
association. In particular, extracellular polymeric substances (EPS) extracted
from laboratory grown bacteria and phytoplankton cultures were utilized as
they too are ubiquitous in aquatic systems and have shown to strongly complex
actinide ions, with EPS involved in oceanic scavenging of Th, as well as
immobilization/mobilization of Pu in contaminated areas on land. | en |
