Inhibition of Calcite Growth by Natural Organic Acids from the Everglades at pH 8.5 and 25^oC

Anthony Hoch, Michael Reddy, George Aiken

USGS, 3215 Marine Street, Boulder, Colorado 80303

ABSTRACT

Calcium carbonate mineral crystallization plays an integral role in the water chemistry of terrestrial ecosystems. Natural organic acids (OA) reduce or inhibit crystal growth. This study examines the kinetic effects of OAs collected from the Florida Everglades on the growth of calcite (CaCO₃).

Highly reproducible calcite growth experiments were performed in a sealed reactor at constant pH, temperature, supersaturation ( = 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.

Organic acids were added to HCO₃^- solutions prior to preparation of supersaturated solutions. OA isolates used were non-volatile hydrophobic acids (primarily fulvic acid) from 3 water samples collected in a North-South transect across the Everglades. OA from the northern site had higher molecular weight and was more aromatic in character than that from the south. All OA samples had similar acid characteristics. Experimental concentrations (C_OA) of OA ranged from 0 to 5 mg/l.

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

OBJECTIVE

To study and quantify kinetic effects of hydrophobic organic acids isolated from the Florida Everglades on the important inorganic process of calcite (CaCO₃) crystal growth.