INHIBITION OF CALCITE GROWTH BY NATURAL ORGANIC MATERIAL FROM THE FLORIDA EVERGLADES AT pH = 8.5 and 25^OC

BY A.R. Hoch, M.M. Reddy, and G.R. Aiken

U.S. GEOLOGICAL SURVEY

The crystallization of calcium carbonate minerals plays an integral role in the water chemistry of terrestrial ecosystems. Natural organic material (NOM) has been shown to reduce or inhibit crystal growth. The purpose of this study is to quantify and understand the kinetic effects of NOM collected from the Florida Everglades on the crystal growth of calcite and other common carbonate minerals such as aragonite and siderite.

Highly reproducible calcite growth experiments were performed in a sealed reactor at constant pH, temperature, supersaturation (Omega = 4.5), P_CO2 (10^-3.5atm), and ionic strength (0.1 M, KNO₃). Metastable supersaturated solutions were prepared by adding CaCl₂ solutions dropwise to NaHCO₃ solutions, then adjusting pH with KOH. Metastability was verified in all experiments for at least 60 min. by monitoring pH. Crystal growth began immediately upon addition of well-characterized calcite seed crystals. Calcite growth was achieved using a constant composition reactor. Calcium and CO₃^2- ions were replenished stoichiometrically in response to the pH decrease accompanying calcite formation. Crystallization rates were monitored continuously by recording rates of Ca²⁺ addition.

NOM was added to HCO₃^- solutions prior to preparation of supersaturated solutions. NOM was in the form of non-volatile hydrophobic acids (primarily fulvic acid) from 3 water samples collected in a North-South transect across the Everglades. NOM from the northern site had a higher molecular weight and was more aromatic in character than that from the south. All NOM samples had similar acid characteristics. Experimental concentrations of NOM ranged from 0.0 to 5.0 mg/l.

Calcite crystallization rates decreases at NOM concentrations as low as 0.2 mg/l. Crystal growth was almost entirely inhibited at the 5 mg/l level using NOM from the northern site. NOM with higher molecular weight and aromaticity was more effective as a growth inhibitor than NOM with lower molecular weight and aromaticity. SEM imaging revealed new growth steps on calcite seed surfaces with no secondary nucleation. Ca-NOM complexation in solution cannot account for decreased growth rates. We attribute calcite growth inhibition to the blockage of surface growth sites by NOM.

The above abstract has been submitted to AGU for the 1998 spring meeting, in Boston, MA, May 26-29, 1998.