Colloid mobilization and transport in groundwater

Uranium(VI) adsorption to ferrihydrite: Application of a surface complexation model

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Publisher Summary It is noted that colloidal particles can be effectively transported through subsurface porous media under certain hydrogeochemical conditions. If present in large concentrations, mobile colloids can act as carriers for strongly sorbing contaminants and thereby provide an unretarded transport pathway for contaminants that are otherwise strongly retarded. This potential transport pathway can be considered in risk assessments of sites heavily contaminated with toxic chemicals, such as certain radionuclides, heavy metals, and hydrophobic organic compounds. The chapter reviews the current state of knowledge about the behavior of colloids in porous media and their role in contaminant transport. The chapter also identifies some important future research needs. Colloids are commonly defined as small particles or other entities with dimensions roughly between 1 nm and 1 μm. These size limits to dissolved molecules on one side and to larger suspended particles on the other side. Colloidal particles remain stable in suspension over long time periods, unless they coagulate to form larger aggregates or deposit onto surfaces of larger grains.

Mobile Subsurface Colloids and Their Role in Contaminant Transport

Surface catalysis of uranium(VI) reduction by iron(II)

Adsorption of uranyl species from bicarbonate solution onto hematite particles

The addition of humic acid to a hematite sol is shown to alter the uptake of uranium by the hematite particles. The magnitude of this alteration varies with the solution pH and the amount of humic acid added. Changes in uranium speciation, blocking of active surface sites, and extent of uranium-humic acid interactions are the dominant factors. Electrophoretic mobility data indicate that the surface charge of the particles becomes negative upon adsorption of humic acid, even at low humic acid concentration. The presence of uranium ions has little effect on this observation. Infrared spectroscopic analysis suggests that new bond formation may be involved in the uptake of humic acid or humic acid/uranium by hematite.

Effect of humic acid on uranium uptake by hematite particles

Colloidal particles of hydrated uranium oxide were formed by reducing a solution of sodium tricarbonatouranate(Vl) with sodium sulfide. These particles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and Fourier Transform infrared spectroscopy. Particle size was determined by photon correlation spectroscopy. Depending on conditions, the particles that formed consisted of either a mixed valency (U(IV) and U(VI)) uranium oxide or a complex uranium(VI) hydrous oxide. The mixed valency uranium oxide particles showed nearly spherical morphology, whereas the complex uranium(VI) hydrous oxide consisted of irregularly shaped particles. Electrokinetic measurements showed that both types of particles were negatively charged in near-neutral media.

Formation of uranium oxide sols in bicarbonate solutions

Adsorption of uranium(VI) species onto a magnetite sol was studied onto a magnetite sol was studied in the presence of bicarbonate ion in the pH range 7 to 9. The initial uranium uptake decreases as the pH increases, and an anionic hydroxy carbonate species is likely involved. At the highest pH and at uranium(VI) concentrations above 70 μmole · dm−3, a precipitate forms on the surface of the magnetite. A few studies were carried out on adsorption of uranium(VI) onto the same sol but in the presence of sodium chloride and in the pH range of 3 to 6. A small initial adsorption was noted at lower pH values, but this increased considerably when the pH reached 5.7. These adsorption results are roughly similar to those observed for adsorption of uranium(VI) onto a hematite sol when adsorption is compared on a unit surface area basis.

N.H Miller

Papers

Adsorption of uranyl species from bicarbonate solution onto hematite particles

Effect of humic acid on uranium uptake by hematite particles

Formation of uranium oxide sols in bicarbonate solutions

The adsorption of uranium(VI) onto a magnetite sol