RADIONUCLIDE SORPTION TO ROCKS AND MINERALS - EFFECTS OF PH AND INORGANIC ANIONS .2. SORPTION AND SPECIATION OF SELENIUM

Sorption of Se-75 to rocks (metamorphosed chert and shale, and ganodio rite) and minerals (hematite, magnetite and calcite) was investigated in 0.001 N, 0.01 N and 0.1 N solutions of Na2SO4, NaCl, Na2CO3 and NaH CO3. The effect of pH and co-existing anions on the sorption of this r adionuclide is discussed in terms of surface complex formation. Variat ion in the speciation of Se-75 during the sorption experiment was exam ined. A low pH solution produced a protonated surface on the sorbent w hich attracted the selenium anions. Of the co-existing anions (SO42-, Cl-, CO32- and HCO3-) which compete with selenium anions for sorption sites, SO42- was the most effective competitor. Oxidation of radioacti ve Se (first added as selenite) to selenate was promoted in the presen ce of sorbents. There was a positive correlation between the ratio of selenate in solution and the ratio of Se sorbed. One explanation for t his correlation is that the oxidation reaction of selenite to selenate is caused in part by electron transfer in the surface complex formed between the sorbent and selenite, resulting in the release of selenate into solution. Greater selenite sorption therefore leads to a larger amount of selenate in the solution. Because Se is one of the most mobi le radionuclides contained in the final form of radioactive wastes, we compared its affinity for geological materials with other nuclides. N uclide affinity for the solid phase, with the exception of calcite, wa s in the order Co greater than or equal to Cs greater than or equal to Mn greater than or equal to Sr greater than or equal to Se in slightl y acidic to basic solutions (pH 5.5 to 10.1) and Cs > Se greater than or equal to Co greater than or equal to Mn congruent to Sr in acidic s olutions (pH 4.5 to 4.8). Radionuclide sorption to calcite differed fr om that to other geological samples, the affinity for which decreased in the order Mn greater than or equal to Co greater than or equal to S r greater than or equal to Se greater than or equal to Cs (pH 6.5 to 1 0.2).