Impermeable clay barriers are frequently used to prevent contaminant migration into surrounding groundwater systems from various waste storage sites, such as tailings dams and radioactive waste. The standard clay used for these applications is montmorillonite - a swelling aluminosilicate with a very low hydraulic conductivity. We have identified that iron-rich montmorillonites (~20 wt% Fe), known as nontronites, also share this physical property regardless if the structural iron is reduced or oxidised. The chemical properties of nontronites have been an area of intense investigation since it has been shown that the structural iron in these clays is potentially redox-active. Here, the results of studies examining the uptake of U(VI) by two Australian nontronites and a montmorillonite over a large pH range (3 to 9) within environmentally relevant conditions (4 �M U(VI), 1 mM Ca, 0 or 1 mM Fe(II), �(CO2) = 0.0004 and �(O2) = 0.2 or 0.0) are presented. Very little difference in the uptake of U(VI) was observed between oxidised (�(O2) = 0.2) and chemically reduced (dithionite/citrate/bicarbonate) clays (�(O2) = 0.0). The addition of Fe(II) to either the oxidised or reduced clays (�(O2) = 0.0) also had little effect on the uptake of U(VI) < pH 6. However, U(VI) uptake was significantly enhanced (up to 40%) on all three clays above this pH. These results suggest that 'surface' Fe(II) rather than 'structural' Fe(II) in the clay minerals are redox-active towards U(VI). Uranium LIII-edge EXAFS spectroscopy analyses on selected samples showed that uranium neither precipitated as uraninite nor was structurally-incorporated in the clays or secondary Fe(III) oxyhydroxide minerals. This research indicates that 'redox-activated' clays may only be superior to traditional clay materials to reduce the subsurface migration of uranium within a restricted biogeochemical window.

Uranium and iron redox interactions at the surface of (iron-rich) Montmorillonites

248th National Meeting (2014)

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