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

Abstract

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 3 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 μm, while at the same saturation value (Ω), when the DOC = 0.15 mM, the size of the crystals were <2 μm for the same period of time. A crystal growth experiment in the presence of different DOC concentrations and 2 m 2 L −1 of calcite crystals showed that precipitation was not detectable when 0.11 mmol m2 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 Ω values between 1 and 50 and a CO 2 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 Ω value of 9.0. Precipitation was completely inhibited in the Ω range of 1–24 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 Ω 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: R T = R CG + R HN, where R T is the total precipitation rate of calcite (mM s −1), R CG is the calcite precipitation rate due to crystal growth, and R HN is the precipitation rate due to heterogeneous nucleation. R CG for natural systems is related to the DOC of the suspension by the expression: R CG = sk CG([Ca +2] [CO 3 −2 − K SP) F(DOC) CG, where brackets represent activities, s is the surface area of the calcite crystals, k CG is the precipitation rate constant due to crystal growth, K sp is the solubility of pure calcite at 25°C, and F(DOC ) CG is the precipitation rate reduction for crystal growth with DOC. Experimentally we determined that R CG = 0 when DOC ≥0.05 mM. R HN was related to Ω with the following expression: R HN = k HN F( SA) ( log Ω - 2.5)F( DOC) HN , where k HN is the precipitation rate constant due to heterogeneous nucleation, F(SA) is a function of the surface area of the particles in suspension, 2.5 is the Ω value at which no further precipitation by nucleation was observed, and F(DOC) HN is the function representing the reduction in precipitation by heterogeneous nucleation due to inhibition of DOC. The above equation, developed for natural environments, predicts that the overall calcite precipitation rate is unaffected by the existing calcite surface area when the DOC in the system is ?0.05 mM.