PROCESSES AT THE MAGNESIUM-BEARING CARBONATES/SOLUTION INTERFACE. II. KINETICS AND MECHANISM OF MAGNESITE DISSOLUTION. - REACTION RATES OF PROCESSES IN NATURAL WATERS

Abstract

Steady-state dissolution rates of magnesite (MgCO3) were measured at 25°C as a function of pH (from 0.2 to 12), total dissolved carbonate concentration (10-5 < ΣCO2 < 0.1 M), and ionic strength (0.002 < I < 0.5 M) using a mixed-flow reactor. Dissolution rates were found to be pH-independent at 0 < pH < 3, proportional to aH+ at 3 < pH < 5, pH-independent at 5 < pH < 8, and decreasing with increasing pH at pH > 8 and ΣCO2 > 10-3 M. In the acid pH region (3 =< pH =< 5), the rates increase significantly with ionic strength. In the alkaline pH region, carbonate and bicarbonate ions and ionic strength inhibit significantly the dissolution rate even at far from equilibrium conditions. The surface complexation model developed by Pokrovsky et al. (1999a) was used to correlate magnesite dissolution kinetics with its surface speciation. Dissolution rates in the acid pH region are controlled by the protonation of >CO3- surface complexes. In neutral and carbonate-rich alkaline solutions, >MgOH2+ controls the dissolution kinetics. The following rate equation, consistent with transition state theory was used to describe magnesite dissolution kinetics over the full range of solution composition: R (mol/cm2/s)=[107.198.{>CO3H°}3.97+105.38.{>MgOH2+}3.94 ].(1-exp(-4A/RT)) where {>i} stands for surface species concentration (mol/m2), and A refers to the chemical affinity of the overall reaction. This equation reflects the formation of two different precursor-activated complexes which contain four protonated >CO3H° species in acid solutions and four protonated (hydrated) >MgOH2+ groups in neutral and alkaline solutions. The very low magnesite dissolution/precipitation rates predicted by this equation, especially at close to equilibrium conditions, are consistent with those deduced from field measurements.