Dissolution rates of talc as a function of solution composition, pH and temperature

Steady-state talc dissolution rates, at far-from-equilibrium conditions, were measured as a function of aqueous silica and magnesium activity, pH from 1 to 10.6, and temperature from 25 to 150 degrees C. All rates were measured in mixed flow reactors and exhibited stoichiometric or close to stoichiometric dissolution. All measured rates at pH > 2 obtained at a fixed ionic strength of 0.02 M can be described to within experimental uncertainty usingr(+) = S-BET (A(A) (a(H+)(2)/aMg(2+))(1/4) + A(B)) exp(-E-A/RT)where r+ signifies the BET surface area normalized forward talc steady-state dissolution rate, S-BET denotes the BET surface area of talc present in the reactor, A(A) and A(B) refer to pre-exponential factors equal to 5.0 x 10(-9) and 0.8 x 10(-9) mol/cm(2)/s, respectively, E-A designates an activation energy equal to 45 kJ mol(-1), R represents the gas constant, T denotes absolute temperature, and ai refers to the activity of the subscripted aqueous species. The first term of this rate expression is consistent with talc dissolution rates at acidic pH being controlled by the detachment of partially liberated silica tetrahedral formed at talc edge surfaces from the exchange of Mg2+ for two protons. Corresponding atomic force microscopic observations confirms that dissolution proceeds by the removal of T-O-T sheets from talc edges. At pH <=, 2, the Mg2+ for proton exchange is so extensive that talc T-O-T sheets break apart leading to increased surface area and accelerated rates, whereas rates appear to be pH independent at pH >= 7. (C) 2007 Published by Elsevier Ltd.