Experiments suggest that dissolved organic ligands may primarily modifymineral dissolution rates by three mechanisms: (1) metal-ligand (M-L)complex formation in solution, which increases the degree ofundersaturation, (2) formation of surface M-L complexes that attack thesurface, and (3) formation of surface complexes which passivate or “protect”the surface. Mechanisms (1) and (2) increase the dissolution rate and thethird decreases it compared with organic-free solutions. The types andimportance of these mechanisms may be assessed from plots of dissolutionrate versus degree of undersaturation.

To illustrate this technique, calcite, a common repository cementing andvein-filling mineral, was dissolved at pH 7.8 and 22°C in Na-Ca-HCO3-Cl solutions with low concentrations of threeorganic ligands. Low citrate concentrations (50 μm) increased thedissolution rate consistent with mechanism (1). Oxalate decreased the rate,consistent with mechanism (3). Low phthalate concentration (< 50 μM)decreased calcite dissolution rates; however, higher concentrationsincreased the dissolution rates, which became faster than in inorganicsolutions. Thus, phthalate exhibits both mechanisms (2) and (3) at differentconcentrations. In such cases linear extrapolations of dissolution ratesfrom high organic ligand concentrations may not be valid.