The role of iron-oxides in uranium(VI) adsorption and the kinetics of contaminant organic reduction.

摘要

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.