Experiments suggest that dissolved organic ligands may primarily modify
mineral dissolution rates by three mechanisms: (1) metal-ligand (M-L)
complex formation in solution, which increases the degree of
undersaturation, (2) formation of surface M-L complexes that attack the
surface, and (3) formation of surface complexes which passivate or “protect”
the surface. Mechanisms (1) and (2) increase the dissolution rate and the
third decreases it compared with organic-free solutions. The types and
importance of these mechanisms may be assessed from plots of dissolution
rate versus degree of undersaturation.
To illustrate this technique, calcite, a common repository cementing and
vein-filling mineral, was dissolved at pH 7.8 and 22°C in
Na-Ca-HCO3-Cl solutions with low concentrations of three
organic ligands. Low citrate concentrations (50 μm) increased the
dissolution rate consistent with mechanism (1). Oxalate decreased the rate,
consistent with mechanism (3). Low phthalate concentration (< 50 μM)
decreased calcite dissolution rates; however, higher concentrations
increased the dissolution rates, which became faster than in inorganic
solutions. Thus, phthalate exhibits both mechanisms (2) and (3) at different
concentrations. In such cases linear extrapolations of dissolution rates
from high organic ligand concentrations may not be valid.