MECHANISM OF REDOX DISSOLUTION OF DIOXIDES OF TRANSURANIUM AND RARE-EARTH ELEMENTS .4. SORPTION OF PHOSPHORIC AND SELENIOUS ACIDS BY PLUTONIUM DIOXIDE

Sorption of phosphoric and selenious acids from nitric acid solutions (4 M HNO3) by plutonium dioxide is studied. The maximum of the phospha te or selenite ions sorbed by the dioxide is (6.1+/-0.4) x 10(16) mg(- 1) PuO2, or (1.0-1.1) x 10(-4) mol g(-1) PuO2. A change from the syste m free of the catalyzing anion to that with a fifth of the exchange-ca pable hydroxy groups replaced by the catalyzing anion affords a 3-10-f old increase in the rate of the reaction Ce(IV) + PuO2. Further change in the proportion of the PuO2 surface sites occupied by the catalyzin g anion from 20 to 100% causes a proportional increase of the rate. A model is proposed for interaction of the hydroxylated PuO2 surface wit h selenious and phosphoric acids in nitric acid solutions.