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Kinetics of Calcite Precipitation in the Presence of Water-soluble Organic Ligands¹

ABSTRACT

The rate of precipitation of calcite is an important geochemical process which can influence the saturation state of natural waters with respect to pure calcite. The present study was established to determine the influence of water-soluble soil organic ligands on the rate of calcite precipitation. A reproducible seeded growth method was used to measure the rate constant for calcite precipitation at pH = 8.4 and 25°C, in the presence of varying amounts of total soluble organic carbon (C_TS) added as a water-soil extract (WSE) or Seward fulvic acid (FA). The rate equation which described the rate of calcite precipitation in the present experiments was: R = k_fs ²₂ ([Ca²⁺][CO^2-₃]-K_sp ^-2₂), where R = rate of calcite precipitation (mol L^–1s^–1), k_f = precipitation rate constant (L² mol^–1m^–2s^–1), s = surface area of calcite seeds (m²L^–1), ₂ = divalent ion activity coefficient, brackets represent concentrations (mol L^–1), and K_sp is the solubility of pure calcite at 25°C. Rate constants decreased to zero in the presence of 0.15 mM and 0.028 mM C_TS added as the WSE and FA, respectively. The organic C surface coverage on the calcite seeds corresponding to complete inhibition of calcite precipitation was 90 and 30 atoms C nm^–2 for the WSE and FA, respectively. The efficacy of inhibition of calcite precipitation in the presence of soluble soil organic matter explains why many natural waters, including soil solutions of Calciaquolls, are oversaturated with respect to pure calcite. Soluble polymeric carbon constituents which adsorb onto potential calcite seeds, render these surfaces inactive as sites for crystal growth.

¹ Contribution from the Minnesota Agric. Exp. Station, Dept. of Soil Science, University of Minnesota, St. Paul, MN 55108. Journal Series, Paper no. 14,777.

² Assistant and Associate Professor, respectively. Address for senior author: Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078.

Received for publication January 16, 1986.