GENERAL IMPLICATIONS OF ALUMINUM SPECIATION-DEPENDENT KINETIC DISSOLUTION RATE LAW IN WATER-ROCK MODELING

Recent experimental and theoretical work has demonstrated that the dis solution rates of many aluminosilicate minerals are inversely proporti onal to the activity of aqueous Al3+. The consequences of these observ ations on the rates of natural geochemical processes have been calcula ted by the KIRMAT hydrochemical code. Comparisons are performed at the steady state limit of the pure advective transport through a homogene ous semi-infinite isothermal porous media at 25 and 150 degrees C. K-f eldspar, albite, and muscovite dissolution kinetics are studied over a broad range of initial pH (2-10) and aluminium concentration (from 1 x 10(-9) to 1 x 10(-3) molal) matching most the natural conditions. Re gardless of the mineral, the characteristic distance requires to reach equilibrium (l(eq)) is two and three orders of magnitude larger and l ower than predicted using the standard Transition State Theory (TST) l aw, respectively. The maximum decrease in muscovite and alkali-feldspa r dissolution rates due to aqueous aluminium at 25 degrees C is found at near to neutral pH, and at 150 degrees C it is found at basic pH. T he maximum dissolution rate increase at 25 degrees C at acid pH, but a t 150 degrees C it is found at basic pH. These results demonstrate tha t consideration of the effect of the aluminium speciation on aluminosi licate dissolution rates is required to improve the accuracy in water- rock interaction modelling. (C) 1998 Published by Elsevier Science B.V . All rights reserved.