THE ROLE OF PHOSPHATE ADSORPTION ON URANIUM CO-ADSORPTION TO GOETHITE-COATED SAND

The adsorption of U(VI) by Fe(III)-oxides such as goethite can dominate the partitioning of U(VI) and control uranium mobility in oxic environments. Common complexing ligands such as carbonate, phosphate, and dissolved natural organic matter have profound and sometimes unexpected impacts on U(VI) adsorption. We studied U(VI) adsoption on goethite-coated sand (to mimic natural Fe-coated subsurface materials) as a function of pH in systems closed to the atmosphere, in the presence and absence of added phosphate. In the high pH range, the adsorption of U(VI) decreased in the presence of phosphate and the majority of phosphate was in the dissolved form. This decrease in U(VI) adsorption was attributed to the formation of dissolved complexes between uranyl and phosphate. In the low pH range, however, batch experiments showed that the presence of phosphate increased both the rate and extent of U(VI) adsorption. At low pH, phosphate was strongly bound by the goethite surface and the increased adsorption of U(VI) was attributed to the formation of a ternary surface complex involving both uranyl and phosphate. In addition, U(VI) adsorption experiments at different solid to solution ratios (SSR) indicated that U(VI) adsorption isotherms were linear and independent of SSR in the absence of phosphate, whereas U(VI) adsorption isotherms were non-linear and changed with SSR in the presence of phosphate. A surface complexation model (SCM) that incorporates the interactions involving uranyl and phosphate can successfully predict U(VI) adsorption both in the presence and absence of phosphate under a range of conditions and explain the effects of SSR on U(VI) adsorption isotherms.