Sorption/desorption processes of uranium in clayey samples of the Bangombenatural reactor zone, Gabon

Experimental studies have been undertaken in order to provide new insights into the relative efficiency of the different mineral phases and sorption p rocesses for the control of U retention in the weathered zones surrounding the natural nuclear reactor at Bangombe (Oklo, Gabon). Clayey and Fe-oxihyd roxides rich samples from the oxidizing weathered zones located above the r eactor were examined. An experimental study of uranium adsorption/desorptio n processes in these samples was carried out using a uranium isotope exchan ge technique in order to estimate the proportion of uranium adsorbed on min eral surfaces. A sequential, extraction technique was used to identify the major U-containing minerals in the samples. In the U-rich iron crust rocks close to the reactor, the fraction of total uranium adsorbed at mineral sur faces is small. Extraction experiments reveal that a large part of uranium is associated to Fe-oxihydroxides, to minor P-rich phases, and presumably t o Mn-oxihydroxides. A possible mechanism for U retention is an incorporatio n into the structure of iron oxihydroxides and/or of ferric phosphates occu rring as surface precipitates on Fe-oxihydroxides. Traces of autunite-like mineral are also present in the zone. For the clayey samples in the weather ing profile, it may be inferred that several processes and minerals contrib ute significantly to U retention: adsorption processes occurring mainly at clay surfaces, association with traces of Mn-containing carbonates and iron oxihydroxides. A significant proportion of total U is adsorbed at mineral surfaces and is thereby easily accessible to weathering solutions. In a second part of this work, U-233 sorption data obtained on a Fe- and Mn -poor illitic Bangombe sample were modeled using a surface complexation mod eling approach. As a first approximation, it was assumed in modeling that u ranyl binding occurs at aluminol edge sites of the illite component. The bi nding constant required for modeling was firstly determined for the non-ele ctrostatic model (NEM) from experimental work on the U(VI)/hydrargilite (al pha-Al(OH)(3)) system. The fractional uptake of U(VI) on hydrargilite was m easured as a function of pH, for different ionic strength values (0.1 and 0 .01 M) under CO2- free conditions, and in solutions with a total concentrat ion of carbonate of 10(-3) M. The following surface complexation reaction: [GRAPHICS] with a p(C)K(1) value equal to -0.8 +/- 0.3 for the NEM (ionic strength: 0. 01 M) provided a reasonable description of the pH-dependent sorption of U(V I) on both a well-characterized hydrargilite and on a clayey sample of Bang ombe.