The mechanism, rates and consequences of basaltic glass dissolution: I. Anexperimental study of the dissolution rates of basaltic glass as a function of aqueous Al, Si and oxalic acid concentration at 25 degrees C and pH=3 and 11.

Steady state basaltic glass dissolution rates were measured as a function o f aqueous aluminum, silica, and oxalic acid concentration at 25 degreesC an d pH 3 and 11. All rates were measured in mixed flow reactors, performed in solutions that were strongly undersaturated with respect to hydrous basalt ic glass, and exhibited stoichiometric Si versus Al release. Rates are inde pendent of aqueous silica activity, but decrease with increasing aqueous al uminum activity at both acidic and basic conditions. Increasing oxalic acid concentration increased basaltic glass dissolution rates at pH 3, but had little affect at pH 11. All measured rates can be described within experime ntal uncertainty using r = k(a(H+)(3)/a(Al+3))(0.35) where r signifies the surface area normalized basaltic glass steady state d issolution rate, k refers to a rate constant equal to 10(-11.65) (mol of Si )/cm(2)/s, and a(i) represents the activity of the subscripted aqueous spec ies. The observation that all rates obtained in the present study can be de scribed by a single regression equation supports strongly the likelihood th at basaltic glass dissolution is controlled by a single mechanism at both a cidic and basic pH and in both the presence and absence of organic acids. T aking account of the dissolution mechanisms of similarly structured and com positioned minerals, and previously published studies of basaltic glass dis solution behavior, basaltic glass dissolution likely proceeds via 1) the re latively rapid and essentially complete removal of univalent and divalent c ations from the near surface; 2) aluminum releasing exchange reactions betw een three aqueous H+ and Al in the basaltic glass structure; followed by 3) the relatively slow detachment of partially liberated silica. The breaking of Al-O bonds does not destroy the glass framework; it only partially libe rates the silica tetrahedral chains by removing adjoining Al atoms. Basalti c glass dissolution rates are proportional to the concentration of partiall y detached framework Si tetrahedra near the surface, which is linked throug h the law of mass action for the Al/proton exchange reaction to aqueous alu minum activity. Copyright (C) 2001 Elsevier Science Ltd.