Impact of uranyl - calcium - carbonato complexes on uranium(VI) adsorption to synthetic and natural sediments

Adsorption on soil and sediment solids may decrease aqueous uranium concentrations and limit its propensity for migrationinnaturalandcontaminatedsettings.Uraniumadsorption will be controlled in large part by its aqueous speciation, with a particular dependence on the presence of dissolved calcium and carbonate. Here we quantify the impact of uranyl speciation on adsorption to both goethite and sediments from the Hanford Clastic Dike and Oak Ridge Melton Branch Ridgetop formations. Hanford sediments were preconditioned with sodium acetate and acetic acid to remove carbonate grains, and Ca and carbonate were reintroduced at defined levels to provide a range of aqueous uranyl species. U(VI) adsorption is directly linked to UO ₂²⁺ speciation, with the extent of retention decreasing with formation of ternary uranyl - calcium - carbonato species. Adsorption isotherms under the conditions studied are linear, and K_d values decrease from 48 to 17 L kg^-1 for goethite, from 64 to 29 L kg ^-1 for Hanford sediments, and from 95 to 51 L kg^-1 for Melton Branch sediments as the Ca concentration increases from 0 to 1 mM at pH 7. Our observations reveal that, in carbonate bearing waters, neutral to slightly acidic pH values (∼5) and limited dissolved calcium are optimal for uranium adsorption.