Basalt glass dissolution

Daux, V., Guy, C., Advocat, T., Crovisier, J.-L., Stille, P. 1997. Kinetic aspects of basaltic glass dissolution at 90°C role of aqueous silicon and aluminium. Chemical Geology, 142, 109-126. 

Oelkers, E. H. Gislason, S. R. 2001. The mechanism, rates and consequences of basaltic glass dissolution I. An experimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25 °C and pH = 3 and 11. Geochimica et Cosmochimica Acta, 65, 3671-3681. 

The main features of basalt glass dissolution are illustrated on Figure 1. For all elements one can observe after an ephemeral period of fast dissolution a linear sleady-state dissolution period. Looking at the stoichiometry of cation release with respect to silica, we can recognize two different groups of cations ... 

Figure 1. (a) Examples of basalt glass dissolution at pH 2.5 plotting silica release in solution us ft function of time and temperature, (b) Rx of Na and Mg to Si at 50"C versus solution pH and time,... 

Crovisier, J. L., Fritz, B., Grambow, B., Eberhart, J. P. 1985. Dissolution of basaltic glass in seawater experiments and thermodynamic modelling. In Werme, L. O. (ed) Scientific Basis for Nuclear Waste Management IX. Material Research Society Symposium Proceedings, 50, 273-280. 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

Here k is the rate constant, and Kj is the equihbrium constant for an exchange reaction between protons and the metal M, at the surface. Oelkers argues that when the term Kfa +la Y is small, significant M, remains in the surface leached layer, and the rate equation simplifies in that the denominator becomes unity. For such a case, the logarithm of the far-from-equilibrium rate becomes linearly related to the logarithm of the activity of the aqueous species M, and is dependent only upon pH and activity of M,. Oelkers (2001b) has used this simplified rate equation to describe dissolution of basalt glass... 

Figure 2,5 (a) An Arrhenius plot of log k versus I/TXK) for the dissolution rates of various silicate rocks and minerals. The data points and curves for rhyolite, basalt glass, and diabase are from Apps (1983), as is the curve labeled silicates, which Apps computed from the results of Wood and Walther (1983). Curves for the S1O2 polymorphs are based on Rimstidt and Barnes (1980). Modified from Langmuir and Mahoney (1985). Reprinted from the National Well Water Assoc. Used by permission, (b) An Arrhenius plot of log k versus 1 /T(K) for the precipitation of quartz and amorphous silica based on Rimstidt and Barnes (1980). Reprinted from Geochim. Cosmochim. Acta, 44, J.D. Rimstidt and H.L. Barnes, The kinetics of silica water reactions, 1683-99, 1980, with permission from Elsevier Science Ltd, The Boulevard. Langford Lane. Kidlington OXS 1GB, U.K. 

This chapter is organized under three topical areas. First, by analyzing the dissolution and speciation data of a basalt glass and various oxides we show that the surface characteristics and the dissolution behavior of complex oxides can be modeled from the properties of their constituent oxide components. Then we examine the main features of the steady-state dissolution of multiple oxides and their implications for the application of the surface coordination theory. Finally, we discuss the problem of the nature of the active dissolution sites by analyzing recent data on the dissolution of strained minerals. 

Figure 2. Plot of the dissolution rate of basalt glass versus solution pH at 50, 100, 150, and 200°C.

Schott (Chapter 12, this volume) applies the speciation approach to the dissolution of basalt glass. As Schott points out, such glasses have little or no structural ordering and thus interpretation may be rendered more simple than in crystalline mixed oxides (e.g., olivine and feldspars). The dissolution behavior of... 

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