CALCITE NUCLEATION AND PRECIPITATION KINETICS AS AFFECTED BY DISSOLVED ORGANIC-MATTER AT 25-DEGREES-C AND PH-GREATER-THAN-7.5

We examined the processes of calcite nucleation and crystal growth to determine the dominant mechanism for precipitation of calcite in the p resence of dissolved organic carbon (DOG). Our experiments, conducted at 25 degrees C and at a Ca:HCO3 molar ratio of 1:2, were performed wi th and without calcite seeds. Scanning electron micrograph observation s 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 mu m, whi le at the same saturation value (Omega), when the DOC = 0.15 mM, the s ize of the crystals were < 2 mu m for the same period of time. A cryst al growth experiment in the presence of different DOC concentrations a nd 2 m(2) L(-1) of calcite crystals showed that precipitation was not detectable when 0.11 mmol m(2) of carbon was coating the calcite cryst als. This surface coverage corresponds to a DOC in solution of 0.05 mM . The rate of calcite precipitation was measured at different concentr ations of DOC in quartz sand and quartz sand-Silver Hill illite suspen sions with calcite Omega values between 1 and 50 and a CO2 partial pre ssure of 35 Pa. The precipitation rate of calcite in the sand:solution suspensions decreased eightfold as the DOC increased from 0.02 to 0.1 5 mM at a Omega value of 9.0. Precipitation was completely inhibited i n the Omega range of 1-24 when the DOC was 0.3 mM or greater. In the s and-illite suspensions, a similar eightfold decrease in the precipitat ion rate of calcite was observed when the DOC increased from 0.28 to 2 .78 mM at a Omega value of 9.0. Differences in calcite precipitation r ates between sand and sand-clay systems are likely due to differences in the number of potential heterogeneous nucleation sites. Experimenta l data were described by the equation: R(T) = R(CG) + R(HN), where RT is the total precipitation rate of calcite (mM s(-1)), RCG is the calc ite precipitation rate due to crystal growth, and R(HN) is the precipi tation 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][CO3-2]-K-SP)f(DOC)(CG), where brackets represent activitie s, s is the surface area of the calcite crystals, k(CG) is the precipi tation rate constant due to crystal growth, K-SP is the solubility of pure calcite at 25 degrees C, and f(DOC)(CG) is the precipitation rate reduction for crystal growth with DOG. Experimentally we determined t hat R(CG) = 0 when DOC greater than or equal to 0.05 mM. R(HN) was rel ated to Omega with the following expression: R(HN) = k(HN)f(SA)(log Om ega-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 Omega value at which no fur ther precipitation by nucleation was observed, and f(DOC)(HN) is the f unction representing the reduction in precipitation by heterogeneous n ucleation due to inhibition of DOG. 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 i n the system is greater than or equal to 0.05 mM.