Kinetics and mechanism of dolomite dissolution in neutral to alkaline solutions revisited

Steady-state dissolution rates of dolomite were measured at 25 degreesC in a mixed-flow reactor as a function of pH (from 5-12), ionic strength (0.002 < I < 0.1 M), total dissolved carbonate (10(-5) < Sigma CO2 < 0.1 M), Calc ium (10(-6)-0.003 M), magnesium (3.10(-7)-0.005 M), and inorganic (sulfate) and organic (acetate, ascorbate, formiate, tartrate, oxalate, citrate, and EDTA) ligands concentration. Dissolution rates were found to be pH-indepen dent at 6 less than or equal to 5 pH less than or equal to 8 and to decreas e with increasing pH at pH > 8 and Sigma CO2 > 10(-3) M. In the alkaline pH region, carbonate and bicarbonate ions significantly inhibit dissolution r ates at far from equilibrium conditions. Dissolved Ca was found to be a str ong inhibitor of dolomite dissolution at pH above 7, whereas dissolved Mg h as no effect on the dissolution rate. The surface complexation model develo ped by Pokrovsky, Schott, and Thomas (1999b) was used to correlate dolomite dissolution kinetics with its surface speciation. At the conditions of thi s study (5 < pH <less than or equal to> 12), dissolution is controlled by t he hydration of Mg surface sites and formation of > MgOH2+ species. This Ca and CO3-free surface precursor complex allows us to account for the inhibi ting effect of aqueous calcium and carbonate ions on dolomite dissolution. Based on these results and those of Pokrovsky, Schott, and Thomas (1999b), the following rate equation, consistent with transition state theory, was u sed to describe dolomite dissolution kinetics over the full range of soluti on composition: R = [k(CO3) . {CO3H degrees}(2.0) + k(Mg) . {> MgOH2+}(1.9)] . (1-exp(-1.9A /RT)) or, alternatively, at pH above 6 and I = 0.1 M, R = k(Mg)* . {K-CO3*.H-Ca*/K-CO3*.K-Ca* + K-Ca*.a(CO3)(2-) + a(CO3)(2-).a(C a2+)}(1.9) . [1-(Q/K-sp(0))(1.9)] where {>i} stands for surface species concentration (mol/m(2)), A refers to the chemical affinity of the overall reaction, k(CO3), k(Mg), k(Mg)*, K-CO 3*, K-Ca*, are constants, and (Q/K-sp(0)) stands for dolomite saturation in dex. This equation reflects the formation of two different precursor comple xes that contain two protonated > CO3H degrees and two hydrated > MgOH2+ gr oups in acid and in neutral and alkaline solutions, respectively. Crystalli zation of dolomite was found to occur in highly supersaturated solutions as confirmed by outlet solutions analysis and SEM observation of reacted grai ns. Very low dolomite crystallization rates (that is, similar to 10(-16) mo l/cm(2)/s) are consistent with those observed in natural conditions and pre dicted by the empirical model of Arvidson and Mackenzie (1997). Dolomite di ssolution rate is promoted by the addition of inorganic and organic ligands with the following effectiveness: sulfate approximate to formiate approxim ate to tartrate < acetate <less than> ascorbate less than or equal to oxala te < citrate much less than EDTA. The effect of these ligands can be modele d within the framework of the surface coordination theory.