Calcite nucleation and precipitation kinetics as affected by dissolved organic matter at 25°C and pH > 7.5

We examined the processes of calcite nucleation and crystal growth to determine the dominant mechanism for precipitation of calcite in the presence of dissolved organic carbon (DOC). Our experiments, conducted at 25°C and at a Ca:HCO, molar ratio of 1:2, were performed with and without calcite seeds. Scanning electron micrograph observations showed that the particle size of calcite crystals in the presence of DOC = 0.02 mM and after 24 h reached values bigger than 100 pm, while at the same saturation value (n), when the DOC = 0.15 mM, the size of the crystals were <2 pm for the same period of time. A crystal growth experiment in the presence of different DOC concentrations and 2 m* L-’ of calcite crystals showed that precipitation was not detectable when 0.11 mmol m* of carbon was coating the calcite crystals. This surface coverage corresponds to a DOC in solution of 0.05 mM. The rate of calcite precipitation was measured at different concentrations of DOC in quartz sand and quartz sand-Silver Hill illite suspensions with calcite R values between 1 and 50 and a CO* partial pressure of 35 Pa. The precipitation rate of calcite in the sand:solution suspensions decreased eightfold as the DOC increased from 0.02 to 0.15 mM at a R value of 9.0. Precipitation was completely inhibited in the R range of l-24 when the DOC was 0.3 mM or greater. In the sand-illite suspensions, a similar eightfold decrease in the precipitation rate of calcite was observed when the DOC increased from 0.28 to 2.78 mM at a R value of 9.0. Differences in calcite precipitation rates between sand and sand-clay systems are likely due to differences in the number of potential heterogeneous nucleation sites. Experimental data were described by the equation: