This study is aimed at quantifying surface reaction controlled basaltic glass dissolution rates at far-from-equilibrium conditions. Towards this aim, steady-state basaltic glass dissolution rates were measured as a function of pH from 2 to 11 at temperatures from 6° to 50°C, and at near neutral conditions to 150°C. All rates were measured in open system titanium mixed flow reactors. Measured dissolution rates display a common pH variation; dissolution rates decrease dramatically with increasing pH at acid conditions, minimize at near neutral pH, and increase more slowly with increasing pH at basic conditions. The pH at which basaltic glass dissolution minimizes decreases with increasing temperature. Dissolution rates were interpreted within the context of a multioxide dissolution model. Constant temperature rates are shown to be consistent with their control by partially detached Si tetrehedra at the basaltic glass surface. Regression of far-from-equilibrium dissolution rates obtained in the present study and reported in the literature indicate that all data over the temperature and pH range 6° < T < 300°C and 1 < pH < 11 can be described within uncertainty using r+,geo = AA exp- (EA/RT)(a3H+/ aAl3+)1/3 where r+,geo signifies the geometric surface area normalized steady-state basaltic glass dissolution rate at far-from-equilibrium conditions, AA refers to a constant equal to 10-5.6 (mol of Si)/cm2/s, EA, designates a pH independent activation energy equal to 25.5 kJ/mol, R stands for the gas constant, T signifies temperature in K, and ai represents the activity of the subscripted aqueous species.
Matthildur B. Stefńsdóttir, Eydís S. Eiríksdóttir, Audur Andrésdóttir, Andri Stefńsson, Ingvi Gunnarsson, Björn Gudmundsson, Domenik Wolff-Boenisch, Jacques Schott, Stephan Kohler, and Oleg Pokrovsky assisted in many ways during the course of this work. Don Voight at Penn State University measured the surface area of the glasses. An early version of this paper benefited from critical reviews by two anonymous reviewers and the associate editor Enrique Merino. We are grateful for all of their help and assistance. The first draft of this manuscript was written at the 12th to 15th century castle at Castet d’Aleu located in the south of France while our families played in the garden and/or prepared lunch. We thank them for their understanding and patience. This research was supported by the Icelandic Science Foundation 962290098, the Research Fund of the University of Iceland, the Centre National de la Recherche Scientifique and the European Commission through the TMR program (contract HPRN-CT-2000-00058).