Studies on Leaching Behaviour of Sodium Borosilicate Glasses

Abstract

A combination of instrumental neutron activation analysis (INAA) and epithermal INAA (EINAA) methods has been developed for the simultaneous determination of multi-element concentrations. The elements measured includes Al, As, Ba, Ca, Ge, Co, Gr, Cs, Eu, Fe, Hf, I, K, La, Mg, Mn, Mo, Na, Nd, Ni, Pr, Rb, Sb, Sc, Si, Sm, Sn, Sr, Ta, Tb, Ti, Tm, U, V, W, Yb, Zn and Zr in glasses, and Al, Ge, Cs, Fe, La, Mn, Mo, Na, Nd, Ni, Pr, Sm, U and Win leachates. The precision and accuracy of the methods have been found to be within ±10% in most cases. Boron has been determined by an indirect INAA method applicable to samples containing > 1000 µg of B and by a spectrophotometric method suitable for 1 to 4 ppm B. A spectrophotometric method has modified for measuring Si (5-100 ppm) in leachates in the presence of F- ions. The suitability of sodium borosilicate glasses as host matrices for high-level waste has been evaluated by static leaching with distilled deionized water (DDW), synthetic granitic ground water (GGW) and synthetic Grande Ronde basaltic groundwater (BGW). The effects of leachant composition and pH, temperature (40° and 90°C), time (3-84 d), and surface area to volume ratio (0.010, 0.10 and 0.85 cm- 1 ) have been investigated. Results indicated a strong influence of the leachant composition through both its pH and nature as well as concentrations of ions present on the leach rate. In general, total mass loss was the highest in BGW primarily due to its high pH (>9). The presence of certain ions such as carbonates and sulfates in leachants increased its aggressiveness possibly through the formation of soluble complexes. The pH (4.5 - 7.5) of DDW and GGW influenced the precipitation of some elements such as La, Sm and U. Leach rates at 90° were 5-30 times higher than at 40°. Activation energies (32-63 kJ/mol) calculated for I-117 glass at 3 different SA/V ratios compares well with those reported in literature. The normalized elemental mass losses showed that the rate of leaching decreased with increasing SA/V ratio. The concentrations of La, Mn, Sm and U in leachate sampled at 90°C were lower compared to that after cooling to room temperature. Application of long-term dissolution models suggest that the primary mechanism of glass leaching is diffusion for up to 84 d.