ADSORPTION OF GADOLINIUM ON THE SURFACE OF GOETHITE IN THE SYSTEM NACL-H₂O-AR-CO₂ AT 25°C

The sorption behavior of Gd³⁺ onto goethite has been studied as a function of acidity, total ionic strength, and concentrations of metal and carbonate in the system. Gadolinium is a neutron-absorber and an analogue of the transuranium elements. Goethite is calculated to be a reaction product of corrosion of nuclear waste packages. Batch experiments have shown a significant effect of pH and total concentration of gadolinium on the sorption of this metal. In the pH range from 4.5 to 6.0 the uptake of Gd³⁺ by goethite changes from 5 to 95 % in Ar-media At pH=5.5, the partition coefficient of Gd between goethite and 0.1mNaCl solution decreases approximately 0.5 log units when concentration of gadolinium increases from 5 to 100 ppm It was established that the precipitation of gadolinium hydroxides in the goethite-free system starts at pH>8. This suggests that in pure Ar media, surface complexation, rather than precipitation of Gd-bearing phases, takes place. The adsorption edge for Gd onto goethite is independent of ionic strength over the range from 0.01 to 1.0 investigated, implying an inner-sphere surface complexation mechanism.

The influence of CO₂ on gadolinium adsorption was studied in 0.1mNaCl-(0.001-0.04) mCO₂ solutions. The experiments show considerable uptake of Gd³⁺ in CO₂ -bearing solutions. Nevertheless, control experiments (i.e., without goethite) indicated the precipitation of a gadolinium carbonate-bearing phase. The goethite surface facilitates this precipitation. The result obtained will be used to develop a comprehensive surface-complexation model for the sorption of Gd³⁺ onto goethite, relevant to evaluation of the mobility of radionuclides from nuclear waste in the subsurface.

^XPermanent address: Institute of Experimental Mineralogy RAS, Chernogolovka Moscow District, 142432, Russia