Processes at the magnesium-bearing carbonates solution interface. II. Kinetics and mechanism of magnesite dissolution.

Steady-state dissolution rates of magnesite (MgCO3) were measured at 25 deg rees C as a function of pH (from 0.2 to 12), total dissolved carbonate conc entration (10(-5) < Sigma CO2 < 0.1 M), and ionic strength (0.002 < I < 0.5 M) using a mixed-flow reactor. Dissolution rates were found to be pH-indep endent at 0 < pH < 3, proportional to a(H+) at 3 < pH < 5, pH-independent a t 5 < pH < 8, and decreasing with increasing pH at pH > 8 and Sigma CO2 > 1 0(-3) M. In the acid pH region (3 less than or equal to pH less than or equ al to 5), the rates increase significantly with ionic strength. In the alka line pH region, carbonate and bicarbonate ions and ionic strength inhibit s ignificantly the dissolution rate even at far from equilibrium conditions. The surface complexation model developed by Pokrovsky et al. (1999a) was us ed 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 solution s, >MgOH2+; controls the dissolution kinetics. The following rate equation, consistent with transition state theory was used to describe magnesite dis solution kinetics over the full range of solution composition: R (mol/cm(2)/s)= [10(7.198) . {>CO3H degrees}(3.97) + 10(5.38) . {>MgOH2+}( 3.94)] . (1 - exp(-4A/RT)) where {>i} stands for surface species concentration (mol/m(2)), and A refer s to the chemical affinity of the overall reaction. This equation reflects the formation of two different precursor-activated complexes which contain four protonated >CO3H degrees 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, espec ially at close to equilibrium conditions, are consistent with those deduced from field measurements. Copyright (C) 1999 Elsevier Science Ltd.