Leaching of uranium and thorium from monazite: I. Initial leaching

Details Leaching of uranium and thorium from monazite: I. Initial leaching Leaching of uranium and thorium from monazite: I. Initial leaching

Jul 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.1867e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 54, Issue 7, pp.1867-1877

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Scientific paper

Interaction of three natural monazite specimens with a bicarbonate-carbonate solution was investigated for times up to 6.8 years. Dissolution was observed to be incongruent with respect to 238 U and 232 Th as well as their radiogenic daughters 234 U, 230 Th, and 228 Th. Leaching was divided into a very rapid initial stage lasting a few hours and a slower process active for the remaining time. The initial stage was modeled as the sum of a contribution from a mechanico-chemically damaged portion of the specimen, which did not exhibit isotopic selectivity in leach properties, and a contribution from the selective removal of recoil daughter products from their recoil tracks in the surface of the otherwise undamaged bulk mineral. The latter effect is greater for short-lived 228 Th compared to long-lived 234 U. A correlation between the magnitude of the effect and the half-life of the radiogenic nuclide suggests an upper limit of ~ 10 6 years for the timescale of natural track annealing. After the initial dissolution stage, insoluble precipitates of the intermediate product in the Th chain, 228 Ra, provide a supplementary source of 228 Th by radioactive decay. Contributions to these precipitates come from dissolution-released and recoil-released 228 Ra. This source is manifest as large apparent release rates of 228 Th which begin after several weeks of leaching. Preannealing of a specimen at 800°C depresses the elemental Th leach rate but enhances the amount of 228 Th / 232 Th fractionation. This enhancement is associated with rejection of Ra from the mineral during annealing of -recoil damage.

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