The kinetics of proton promoted dissolution of K-montmorillonite was i
nvestigated at 298 +/- 1 K by batch experiments in the range 10(-1) gr
eater than or equal to [H+] greater than or equal to 10(-5) M using so
lutions of the constant KCI concentrations of 0.03 M, 0.10 M, and 1.0
M, respectively. In addition the concentration of adsorbed H+-ions ({H
+}, [mol/g]) was determined in the acidic range using both titration a
nd batch equilibration experiments. The dissolution rates R(Si) and R(
Al) [mol/(g . h)] were obtained from the observed increase with time o
f both dissolved Si(IV) and Al(III). R(Si) was found to be linearly de
pendent on {H+}: R(Si) = k .{H+}. The values of the first order rate c
onstant k [h(-1)] increase with increasing KCl concentration: 6.02 x 1
0(-4) +/- 2.4 x 10(-5) h(-1) (0.03 M KCl), 9.98 x 10(-4) +/- 7.1 x 10(
-5) h(-1) (0.10 M KCl), and 2.61 x 10(-3) +/- 3.9 x 10(-4) h(-1) (1.0
M KCl). Adsorption of H+-ions was interpreted in terms of the surface
complexation model. Proton uptake by the solid phase were formally att
ributed to H-K exchange and to protonation of edge surface aluminol gr
oups. The dissolution reaction does, however, not discriminate between
these two adsorption modes. At 0.10 M and 1.0 M KCl the observed rati
o Q = R(Si)/R(Al) was found to be close to the Si:Al ratio in the soli
d phase, suggesting congruent dissolution. In 0.03 M KCI and [H+] > 3.
2 x 10(-2) M, Q exceeded the Si:Al ratio of the solid. This deviation
from congruency at low KCl concentration was attributed to adsorption
of dissolved AI(III) by cation exchanger sites as reported by Charlet
et al. (1993b). Added Al(III) was found to inhibit the dissolution rea
ction at [H+] less than or equal to 10(-3) M. Observations by Charlet
et al. (1993b) suggest that this inhibition originates from adsorption
of Al(III) on crystal edge surface sites. Both the observed dissoluti
on stoichiometry and the inhibition by added Al(III) leads to the conc
lusion that dissolution occurs predominantely at the crystal edge surf
aces.