Effects of solid-to-solution ratio on uranium(VI) adsorption and its implications

U(VI) adsorption onto goethite-coated sand was studied in batch experiments at a solid-to-solution ratio (SSR) ranging from 33.3 to 333 g/L. Batch kinetic experiments revealed that the presence of 10(-4) M phosphate increased both the initial rate and ultimate extent of U(VI) adsorption compared with phosphate-free systems. Our experimental U(VI) adsorption isotherms were independent of SSR in phosphatefree systems. However, the U(VI) adsorption isotherm became dependent on SSR in phosphate-containing systems (with a lower SSR resulting in stronger U(VI) adsorption). A surface complexation model (SCM) was used to conceptualize the interactions in systems containing U(VI), phosphate, and goethite contributing to this SSR effect. The SCM accounted for the effects of SSR on U(VI) adsorption reasonably well. This study implies that the extrapolation of batch-measured adsorption parameters of U(VI) (and potentially other radionuclides and metal(loid)s as well) to field conditions should be done with caution, especially in the presence of strongly interacting ligands.