Abstract:
Contaminant transport in ground water is an important environmental process that
affects a host of natural systems. Transport of contaminants is affected by many
processes, including aqueous complexation, adsorption, and redox reactions. One of the
most important components of an aquifer that controls transport is iron-oxide. In this
study, the effect of iron-oxides on an inorganic radionuclide and representative organic
contaminants were studied. Specifically, the role of iron-oxides in both the adsorption of
U(VI) on natural iron-rich sands and the reduction of carbon tetrachloride (CT) and
nitrobenzene (NB) by iron-oxide coated gold electrodes was investigated.
U(VI) was adsorbed to an iron-rich silica sand (containing appreciable amounts of
Al) over a range of experimental conditions. It was found that the iron-oxide components
of the heterogeneous sorbent were essential in determining the adsorption behavior of
U(VI). Four mathematical models were investigated in an attempt to elucidate the
advantages and disadvantages of applying the proposed models to adsorption data.
The effect of citrate on the adsorption of U(VI) by the iron-rich sand was also
studied, with citrate generally decreasing the affinity of the sorbent for U(VI). Removal
of highly reactive Fe and Al phases from the surface by citrate was investigated as a
possible explanation of the adsorption behavior of U(VI). Alteration of the sorbent
surface by citrate was found to play a determining role in decreasing the adsorption
capacity of the iron-rich sand for U(VI).
The rates of reduction of CT and NB by Fe(III)-oxide coated gold electrodes were
studied to gain insight into what controls reduction reactions on zero-valent metals.
Fe(III)-oxide films were deposited on gold electrodes, and Fe(II) sites were introduced
into the films by controlled electrochemical reduction of a small fraction of the Fe(III) in
the oxide. Mass transport kinetics were controlled through use of a well-defined flow-through
system approximating a wall-jet electrode configuration. The results from this
study support the view that the oxide film acts as a physical barrier, inhibiting direct
contact between the gold electrode and the contaminant organics, increasing the diffusion
path length through the film, and creating adsorption sites for the organic contaminants.
