Dissolution Kinetics of a Simple Analogue Nuclear Waste Glass as a Function of Ph, Time and Temperature
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DISSOLUTION KINETICS OF A SIMPLE ANALOGUE NUCLEAR WASTE GLASS AS A FUNCTION OF pH, TIME AND TEMPERATURE. Kevin G. Knauss, William L. Bourcier, Kevin D. McKeegan, Celia I. Merzbacher, Son N. Nguyen, Frederick J. Ryerson, David K. Smith, Homer C. Weed and Leon Newton Lawrence Livermore National Laboratory, Livermore, CA 94550 ABSTRACT We have measured the dissolution rate of a simple five-component borosilicate glass (Na20, CaO, A120 3, B20 3, Si0 2) using a flow-through system. The experiments were designed to measure the dissolution rate constant over the interval pH 1 through pH 13 at 3 temperatures (250, 500 and 70 0C). Dilute buffers were used to maintain a constant pH. Analyses of solutions and solid surfaces provided information that is used to develop a kinetic model for glass dissolution. Under all conditions we eventually observed linear dissolution kinetics. Instrongly acidic solutions (pH 1 to pH 3) all components but Si were released in their stoichiometric proportions and a thick, Si-rich gel was formed. In mildly acidic to neutral solutions the gel was thinner and was both Si- and Al-rich, while the other components were released to solution in stoichiometric proportions. In mildly to strongly alkaline solutions all components were released to solution in stoichiometric proportions. By varying the flow rate at each pH we demonstrated a lack of transport control of the dissolution rate. The dissolution rates were found to be lowest at near-neutral pH and to increase at both low and high pH. A rate equation based on transition-state theory (TST) was used to calculate dissolution rate constants and reaction order with respect to pH over two pH intervals at each temperature. At 250C between pH 1 and pH 7 based on the Si release rate the log rate constant for glass dissolution (g glass/m2od) was -0.77 and the order with respect to pH was -0.48. Between pH 7 and pH 13 the log rate constant for glass dissolution was -8.1 and the order with respect to pH was +0.51. The measured simple glass dissolution rate constants compare very well with constants estimated by fitting the same TST equation to experimental results obtained for SRL-165 glass and to dissolution rate estimates made for synthetic basaltic glasses. INTRODUCTION Many studies have investigated the effect of solution composition, especially pH, on the rate at which both natural [1] and synthetic [2,3] glasses dissolve. However, most of these studies were conducted under conditions inwhich it is difficult to determine whether or not dissolution is stoichiometric and difficult to derive either the rate constant for dissolution over any pH interval or the order for the rate equation with respect to H+activity. Many studies failed to control pH throughout the runs and consequently pH varied as reaction proceeded [2,3], although the importance of eliminating this problem inorder to obtain data useful inconstructing rate equations was realized early on [4]. In addition these studies tended to focus on the relative effects of pH change on the rates and no a
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