| The effects of natural organic matter (NOM) and phosphates on the rate of heterogeneous calcite precipitation in calcite-seeded solutions were investigated using a pH-stat system under different solution conditions. The solution compositions were designed to maintain a constant degree of supersaturation with respect to calcite during the course of the precipitation experiments, but with different carbonate/calcium ratios and pH values.; In the absence of any precipitation inhibitors, the rate of calcite precipitation in the seeded solutions increased with increasing carbonate/calcium ratio or pH value for the same degree of supersaturation. This result contradicts the generally accepted empirical rate expression that the supersaturation state is the sole factor controlling calcite precipitation kinetics. By applying a surface complexation model, two surface species, >CO3 - and >CaCO3-, were found to be responsible for promoting calcite precipitation under conditions of supersaturation.; The inhibition of calcite precipitation by NOM, orthophosphate, and polyphosphate was found to be more effective at lower carbonate/calcium ratios and lower pH values. The reduction in the rate of calcite precipitation was found to conform to a Langmuir adsorption model using "conditional" equilibrium constants and total NOM or total orthophosphate/polyphosphate concentrations. Through detailed chemical speciation analyses, the "conditional" equilibrium constants obtained for different solution compositions was found to converge to a single "non-conditional" value for each of the inhibitors examined. Calcium-complexed NOM (NOM-Ca+), CaHPO40, CaP 2O72-, and CaP3O10 3-, were found to be the species responsible for adsorbing to the calcite surface, blocking the active crystal growth sites, and inhibiting calcite precipitation in solutions containing NOM, orthophosphate, pyrophosphate, and tripolyphosphate, respectively, suggesting that the complexation of anionic species with Ca2+ facilitates their adsorption on calcite and their ability to inhibit calcite precipitation.; The standard free energy change and standard enthalpy change of the adsorption reaction suggest that the adsorption of NOM molecules and orthophosphate on the calcite surface are endothermic reactions, chemisorptive in nature, and driven by entropy change, most likely resulting from the release of water molecules that accompanies the adsorption of the calcium complexes onto the calcite surface. |
