Sorption of U(VI) on goethite: effects of pH, ionic strength, phosphate, carbonate and fulvic acid.

U(VI) sorption on goethite was investigated as functions of pH, solid-to-liquid ratio (m/V), ionic strength and U(VI) concentration by a batch experimental method. Effects of phosphate, carbonate and fulvic acid (FA) on U(VI) sorption were examined. It was found that the sorption of U(VI) increases from 0% to 100% over the pH range of 2.5-4.5 and keeps constant in the high pH range. The sorption of U(VI) on goethite is insensitive to ionic strength. Different surface complexes in the framework of double-layer model were examined for fitting the sorption of U(VI) on goethite. A model with two mononuclear inner-sphere surface complexes, identical with triple bond SOUO(2)(+) and identical with triple bond SOUO(2)OH, was found capable of reproducing the pH sorption edges, the sorption isotherms and the sorption data with variable m/V in this study. The proposed model can also interpret the pH sorption edge collected at P(CO2)=10(-3.58) atm without considering any ternary surface complexes of carbonate. Moreover, it was found that the presence of phosphate at relatively high concentration (6 x 10(-4) mol/L) promotes U(VI) sorption. The presence of FA of 20 mg/L has little effect on the sorption of U(VI) on goethite.