Surface chemistry and dissolution kinetics of divalent metal carbonates.

A surface complexation model (SCM) for divalent metal carbonates (Ca, Mg, Sr, Ba, Mn, Fe, Co, Ni, Zn, Cd, and Pb) is developed based on new electrophoretic measurements and correlation between aqueous and surface reactions stability constants. This SCM postulates the formation of the following surface species: >CO3H0, >CO3-, >CO3Me+, >MeOH0, >MeO-, >MeOH2+, >MeHCO30, and MeCO3- within the framework of a constant capacitance of the electric double layer. It can be used to describe the surface-controlled dissolution kinetics of divalent metal carbonates and allows determination of the order of dissolution reactions with respect to rate-controlling protonated carbonate surface groups in acid solutions (>CO3H0) and hydrated metal groups (>MeOH2+) in neutral to alkaline solutions. The reaction order with respect to protonated carbonate groups increases from 2 for MnCO3 and ZnCO3 to 4 for NiCO3, whereas for hydrated surface metals, it augments from 2 for ZnCO3 to approximately 4 for MnCO3 and NiCO3. The dissolution rates at 5 < or = pH < or = 8 increase in the order Ni < Mg < Co < Fe < Mn < Zn < Cd < Sr < or = Ca approximately = Ba approximately = Pb and correlate nicely with water exchange rates from the aqueous solution into the hydration sphere of the corresponding dissolved cations. Such a correlation allows the generation for all carbonates of a model describing their dissolution/precipitation kinetics, including the effect of various ligands, provided that rate constants and their activation volumes for water exchange around Me(II)-ligand dissolved complexes are available.