DISSOLUTION KINETICS OF A LUNAR GLASS SIMULANT AT 25-DEGREES-C - THE EFFECT OF PH AND ORGANIC-ACIDS

The dissolution kinetics of a simulated lunar glass were examined at p H 3, 5, and 7. Additionally, the pH 7 experiments were conducted in th e presence of citric and oxalic acid at concentrations of 2 and 20 mM. The organic acids were buffered at pH 7 to examine the effect of each molecule in their dissociated form. At pH 3, 5, and 7, the dissolutio n of the synthetic lunar glass was observed to proceed via a two-stage process. The first stage involved the parabolic release of Ca, Mg, Al , and Fe, and the linear release of Si. Dissolution was incongruent, c reating a leached layer rich in Si and Ti which was verified by transm ission electron microscopy (TEM). During the second stage the release of Ca, Mg, Al, and Fe was linear. A coupled diffusion/surface dissolut ion model was proposed for dissolution of the simulated lunar glass at pH 3, 5, and 7. During the first stage the initial release of mobile cations (i.e., Ca, Mg, Al, Fe) was Limited by diffusion through the su rface leached layer of the glass (parabolic release), while Si release was controlled by the hydrolysis of the Si-O-Al bonds at the glass su rface (linear release). As dissolution continued, the mobile cations d iffused from greater depths within the glass surface. A steady-state w as then reached where the diffusion rate across the increased path len gths equalled the Si release rate from the surface. In the presence of the organic acids, the dissolution of the synthetic lunar glass proce eded by a one stage process. The release of Ca, Mg, Al, and Fe followe d a parabolic relationship, while the release of Si was linear. The re lative reactivity of the organic acids used in the experiments was cit rate > oxalate. A thinner leached layer rich in Si/Ti, as compared to the pH experiments, was observed using TEM. Rate data suggest that the chemisorption of the organic anion to surface silanol groups was resp onsible for enhanced dissolution in the presence of the organic acids. It is proposed that the increased rate of Si release is responsible f or the one stage parabolic release of mobile cations and the relativel y thin leached layer compared to experiments at pH 3 and 5.