Experimental study of K-feldspar dissolution rates as a function of chemical affinity at 150°C and pH 9

Abstract

Steady state dissolution rates of a K-rich feldspar (K 0.81Na 0.15Ba 0.03Al 1.05Si 2.96O 8) were measured as a function of chemical affinity and aqueous Si and Al concentration in solutions containing 5 × 10 −3 m total K using a titanium mixed flow reactor at a temperature of 150°C and pH of 9.0. All dissolution experiments exhibited stoichiometric dissolution with respect to Al and Si. The concentration of aqueous silica and Al ranged from 1 × 10 −6 to 5 × 10 −4mol/kg and 4 × 10 −7 to 5 × 10 −4mol/kg, respectively, corresponding to K-feldspar chemical affinities ranging from ~90 to ~5 kJ/mol. Logarithms of measured dissolution rates are an inverse linear function of aqueous aluminum concentration, but independent of aqueous silica concentration at all chemical affinities greater than ~20 kJ/mol. These rates become increasingly controlled by chemical affinity as equilibrium is approached. This variation of steady state dissolution rates is consistent with their control by the decomposition of silica rich/aluminum deficient surface precursor complex. Taking account of transition state theory and the identity of reactions to form this precursor complex, an equation was derived to describe the steady state dissolution rates over the full range of chemical affinity. A simplified but less general version of this equation, which can be used to describe the steady state rates ( r) obtained in the present study can be expressed as r=k+ 1 a Al(OH) + −a H+ 1 3 (1 − exp (− A 3RT )) where k + stands for a rate constant equal to 1.7 × 10 −17 mol/cm 2/s, a H+ and aAl( OH) 4 − designate the activities of H + and Al(OH) 4 − , respectively, A refers to the chemical affinity of the overall reaction, R signifies the gas constant and T denotes the temperature in K. Corresponding experiments performed in a batch-type reactor illustrate the consistency between dissolution rates generated in open and closed systems.