Web Release Date: July 27,
Inhibition of Calcite Precipitation by
Natural Organic Material: Kinetics,
Mechanism, and Thermodynamics
and
Department of Environmental Sciences and Engineering, University of North Carolina, CB #7431, Chapel Hill, North Carolina 27599-7431, and U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303
Received for review March 9, 2005
Revised manuscript received July 19, 2005
Accepted July 20, 2005
Abstract:
The inhibition of calcite precipitation by natural organic
material (NOM) in solutions seeded with calcite was
investigated using a pH-stat system. Experiments were
carried out using three NOMs with different physical/chemical
properties. For each of the materials, inhibition was
found to be more effective at lower carbonate/calcium
ratios and lower pH values. The reduction in the precipitation
rate could be explained by a Langmuir adsorption model
using a conditional equilibrium constant. By identification of
the type of site on the NOM molecules that is involved
in the adsorption reaction, the "conditional" equilibrium
constants obtained at different solution compositions
converged to a single "nonconditional" value. The
thermodynamic data determined at 25 
C and 1 atm
suggest that the interaction between NOM molecules and
the calcite surface is chemisorptive in nature and that
adsorption is an endothermic reaction driven by the entropy
change. The greatest degree of inhibition was observed
for the NOM with the highest molecular weight and aromatic
carbon content. For a given type of NOM, the degree of
inhibition of calcite precipitation was dictated by the balance
between the enthalpy change and the entropy change of
the adsorption reaction.
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