Leaching flow rate, network dissolution

A distinguishing effect of flow rate is that saturation of the solution is never achieved under dynamic leaching conditions. Thus, network dissolution, a solubility limited process, is never allowed to halt. A steady-state condition is eventually established whereby leach rates of network formers equalize the rate of removal of species from the leachant due to the flowing solution. This can be illustrated by formulating a network dissolution model which incorporates the dependence of leaching of network formers on solubility limits and water flow rates. 

Waste form leach rates in a geologic repository will be affected by unknown water flow rates and by extensive cracking of the waste form monolith. An understanding of these effects is important in predicting the geochemical behavior of disposed radioactive waste forms over the full range of possible scenarios. The dependence of the waste form source term on the rate of renewal of aqueous solution is first established for the simple but important case of solubility-limited network dissolution control. 

In the present paper, flow rate effects on leaching are analyzed from a theoretical point of view and are rationalized in a consistent, generic leaching model incorporating the dependence of the mechanisms of selective leaching and network dissolution on solution feedback effects. 

Therefore, if leaching is network dissolution controlled, the higher the flow rate the sooner steady-state is achieved and the higher the leach rate, which accords with the experiments1 evidence. In particular, because 3 = A/V and - F/V,... 

Figure 2 shows the predicted, normalized cumulative mass losses based on the behaviors of silicon and sodium for three different values of the leachant renewal frequency. The physical parameters used refer to the leaching of PNL 76 68 borosilicate glass in deionized water at 90°C, (4) and reference is made to the geometric surface area, SA, of the sample. In particular, the curves corresponding to silicon and sodium tend to have the same, constant slope with increasing flow rate. In particular, the curves corresponding to = 1 day 1 practically coincide, indicative of network dissolution control. 

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