Initial Dissolution Rate of the International Simple Glass as a Function of pH and Temperature Measured Using Microchannel Flow‐Through Test Method

Abstract

International simple glass (ISG) is a six-component alumino-borosilicate glass developed as a reference benchmark glass for six nations collaborating study on high-level nuclear waste glass dissolution/corrosion mechanism. In this study, aqueous dissolution tests were performed for the ISG using microchannel flow-through (MCFT) method to evaluate the initial dissolution rate of glass matrix, r 0 , precisely and systematically as a function of solution pH and temperature. The test results indicated that the r 0 shows a "V-shaped" pH dependence with a bottom at around pH4 at each temperature. Compared with Japanese reference glass of P0798, for which the r 0 showed a "U-shaped" pH dependence with a bottom at around pH6 in our previous study, the ISG shows the higher dissolution rate at basic pH, and lower dissolution rate at neutral to acidic pH. The results also indicated that the r 0 increases with temperature according to an Arrhenius law, and the apparent activation energy evaluated from Arrhenius relation is 62-77 [kJ/mol] at any pH from 3 to 10, which suggests the initial dissolution of ISG proceeds controlled by a surface-reaction mechanism in this pH range.

Determination of the Forward Dissolution Rate for International Simple Glass in Alkaline Solutions

“…The combination of the two data sets confirms the expected increase of the forward dissolution rate with pH. Table III and figure 3 do not show data obtained with our experimental setup in the pH range tested by Inagaki et al A test was however performed at pH 9 to see if the forward rate would be comparable with the ones found with the microchannel flow-through method [5]. In this test, the ISG did not show the expected behaviour: the dissolution rate, calculated from the dissolved Si concentration, did not decrease for increasing Si concentrations, as it should do following the first-order rate law [5,12].…”

“…The Si concentrations increased with decreasing flow rate, as expected, but not proportionally. Possibly, at this near-neutral pH, where incongruent dissolution with Si gel formation is favored, the resorption of dissolved Si on the altered glass surface has led to an underestimation of the matrix dissolution rate with respect to the values reported in [5] for pH values between 8 and 10. This would imply that the corresponding rates determined in our setup for the ISG at pH 9 are not forward rates, for which congruent Si dissolution is a prerequisite, especially when Si is used as dissolution indicator.…”

“…This discrepancy seems to increase at pH 14, but taking into account the large uncertainty encountered for SON68, the actual difference between the two glasses at this pH is difficult to estimate. The initial dissolution rate of the ISG was determined by Inagaki et al [5] at 25°C in KCl solutions with pH ranging from 3 to 10. As shown in figure 3, the results obtained in our study for the pH range 11.5-14 are in good agreement with their values determined for a lower pH range.…”

“…In our setup, with flow rates from 30 to 240 μL·min -1 (at pH 9), and a glass surface area of about 372 cm², the surface normalized flow rate is 0.08 to 0.65 μL·min -1 ·cm -2 . In the setup used in [5], with flow rates from 2 to 20 μL·min -1 , and a glass surface area of about 3 cm², the surface normalized flow rate is 0.67 to 6.7 μL·min -1 ·cm -2 . Because there is less water renewal per unit of glass surface area in our setup, resorption of Si is more likely.…”

“…Similar tests were performed with the ISG. The behaviour of the ISG was also investigated by different other authors [1,2,5,6], but mainly focussing on its dissolution at near neutral pH, depending on the type of radioactive waste repository chosen by the different countries involved in the collaboration.…”