Uptake of uranium by carbonate crystallization from reduced and oxidized hydrothermal fluids

This work evaluated the immobilization of uranium (U) through incorporation into calcite under reduced and oxidized conditions. We investigated how much U could be entrapped by calcite crystallizing in chloride solutions in autoclaves at temperatures from 162 to 300 degrees C. The oxidation state of U was set by controlling oxygen fugacity via redox buffers. Uranium was introduced into calcite growth media as a solid oxide compound or U aliquot. We found the uptake of tetravalent U by calcite is higher than that of hexavalent U by up to four orders of magnitude. We estimate that crystallization of a few mg of calcite immobilizes all dissolved U when 1 kg of solution is saturated with UO2 under reduced hydrothermal conditions.

Automated summary

This study evaluated the immobilization of uranium in calcite under varied redox conditions, finding that tetravalent uranium was absorbed by calcite at a higher rate than hexavalent uranium by up to four orders of magnitude. It was estimated that a few milligrams of calcite crystallization under reduced hydrothermal conditions could immobilize all dissolved uranium in a UO2-saturated solution.