Use of orthophosphate complexing agents to investigate mechanisms limitingthe alteration kinetics of French SON 68 nuclear glass

This investigation was carried out to assess the protective properties of t he alteration film that develops during aqueous alteration of the French SO N 68 (R7T7-type) nuclear glass, notably by examining the behavior of some n etwork-forming cations in the presence of complexing anions. Glass alterati on was studied here in the presence of orthophosphate ions. Comparisons wer e established between two series of tests performed with a solution contain ing orthophosphate ions and control tests performed under the same conditio ns but without phosphates. The first series of experiments was performed un der initial rate conditions (i.e, in dilute media) to assess the effect of pH and phosphate concentration on the initial glass dissolution rate. Under these conditions, which ensure maximum chemical affinity of the glass diss olution reaction, phosphate adsorption occurs at the reaction interface onl y with acid pH values, at which the glass dissolution reaction is strongly inhibited. The elements that form complexes with the phosphates (Al, Fe, et c,) partially control glass dissolution in acidic media. Additional experim ents carried out under saturated conditions - notably with respect to Si - in a solution enriched with phosphates showed that rare earth and Ca phosph ates precipitated in the outer region of the alteration film, maintaining a glass dissolution rate significantly higher than in the control experiment . These observations have several implications. (1) Comparing the results o btained in the presence of phosphates and in the reference medium, the auth ors demonstrate deductively that glass dissolution is limited by the inner portion of the alteration film, i.e, the amorphous gel. (2) A kinetic law o f SON 68 glass dissolution cannot be based on silica alone; the results of these experiments contradict Grambow's model. (3) With regard to control of the glass dissolution kinetics by the protective properties of the gel, th is type of experiment shows that the relation between the chemical composit ion and the microstructure of the gel is an important aspect in modeling th e glass alteration kinetics, but that it is still poorly understood. (C) 20 00 Elsevier Science Ltd. All rights reserved.