AbstractAbstractCalcium 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), PCO2 (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 Ca 2+ 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 (COA) 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.

Materials and MethodsMaterials and Methods

Inhibition of Calcite Growth by Natural Organic Acids From the Florida Everglades at pH 8.5 and 25C

Experimental Data And Rate CalculationExperimental Data And Rate Calculation

Summary and Implications for Natural Water ChemistrySummary and Implications for Natural Water Chemistry

Calculation of Calcite Growth RatesCalculation of Calcite Growth Rates

ABSOLUTE RATE: R (mol/m2/min) = slope (l/min) * mtitrant (mol/l) /( massseed (g) * SAseed (m2/g))

REDUCED RATE: R/Ro = RWITH INHIBITOR/RCONTROL

IntroductionIntroductionIt is well known that the inorganic mineral phase, calcite (CaCO3), is commonly supersaturated in natural waters, with no observed precipitation (Reynolds, 1979). The reason for this phenomenon is that crystal growth is greatly reduced by various naturally-occurring kinetic inhibitors, e.g. magnesium ion, phosphate ion and dissolved organic carbon.

In the Everglades calcite precipitation associated with periphyton (shallow algal mats) has a pronounced effect on pH, pO2, pCO2, calcium concentration and bicarbonate concentration. Calcite precipitation may also influence the bioavailability of phosphorous and may be responsible for low natural concentrations of phosphorous in Everglades waters (Browder et al., 1994).

Calcite precipitates in the presence of algae because of the localized environment of high pH and elevated calcium and bicarbonate concentration. It has been proposed that presence or absence of calcite in the periphyton is related to hydrology and effects of dilution on supersaturation (Gleason and Stone, 1994), with no consideration of inhibitory kinetic effects.

In this poster we present results showing effects of natural hydrophobic organic acids from the Everglades on calcite crystal growth kinetics.

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

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Anthony Hoch, Michael Reddy and George Aiken, USGS, 3215 Marine St., Boulder, CO 80303Anthony Hoch, Michael Reddy and George Aiken, USGS, 3215 Marine St., Boulder, CO 80303

H31-A10

ResultsResults DiscussionDiscussion

Crystal growth experiments

We used a constant composition reactor, in which, calcite seed crystals were added to metastable, supersaturated solutions containing the organic acids. Crystallization began immediately upon seeding, and was accompanied by a drop in pH. The pH drop triggered addition of Ca2+ and CO3

2- titrants, maintaining constant chemical conditions in the reactor solutions.

Effectiveness of different organic acids as growth inhibitors

Morphology of calcite crystals

Scanning electron microscope images were obtained for unreacted seed crystals, seed crystal grown for 100 minutes in the absence of organic acids and seed crystals grown for 100 minutes with organic acid, such that R/Ro = 0.5, or 50% inhibition of growth rate.

Concentration of organic acids and crystal growth inhibition

All of the organic acids studied were effective inhibitors of calcite growth at relatively low concentrations ( 5 mg/l) compared to concentrations observed in Everglades surface waters (25 to 50 mg/l). Thus, although calcite supersaturation is observed in Everglades waters, kinetic inhibition by natural organic acids probably prevents abiotic precipitation from occurring.

Chemical properties of organic acids and effectiveness of crystal growth inhibition

Below, we have plotted chemical characteristics of the organic acids versus reduced rate in the presence of 1.0 mg/l inhibitor. Rate reduction correlates most strongly with molecular weight and aromaticity. Work is in progress to determine more precisely what properties of the organic acids are most responsible for crystal growth inhibition.

Organic acids used in experiments

Hydrophobic organic acids (humic acid + fulvic acid), isolated using XAD resins from surface waters in Water Conservation Areas 2A and 2B, were used in experiments and had the following characteristics:

AcknowledgementsAcknowledgementsFunding for A. Hoch was provided by the USGS through the National Research Council research associateship program.Logistical support for Everglades sample collection was provided by the South Florida Water Management District, Larry Fink and Peter Rawlich.

Experimental Conditions

All experiments were run for 100 minutes under the following conditions:

Data

Since the experimental system replaces Ca2+ and CO3

2- stoichiometrically as calcite precipitation progresses, the quantity of titrant added is proportional to the quantity of calcite precipitated and crystal growth rates may be calculated from the slopes shown to the right. Note that higher concentrations of organic acids result in more shallow slopes.

R2 = 0.7468

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aromatic groups (%)

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2BS

U3

F1

R2 = 0.7896

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carboxyl groups (%)

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R2 = 0.9967

R2 = 0.9882

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U3 U3

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C.T (C) pH

25C I (M) [Ca]TOTAL (M) [CO3]TOTAL(M) PCO2 (atm)250.1 8.5000.005 4.440.04* 0.1 0.0019 0.0019 10-3.55

* saturation state varied slightly among experiments, due to Ca-DOC complexation

Sample Mw

(daltons)Mn

(daltons)[COOH ](meq/g)

% Aromaticfunctional

groups

% Carboxylfunctional

groupsU3 1747 1180 5.99 19.0 14.4F1 1907 1238 5.14 18.2 11.6

2BS 1519 1080 4.74 15.4 15.4

Of the three organic acids studied, the sample from the F1 site was the strongest inhibitor, followed by U3 and 2BS. Note that the F1 inhibitors slowed the growth reaction to about 60% of the control rate at 0.2 mg/l organic acid concentration, which is very low compared to natural Everglades waters.

Data for F1 Experiments

Unreacted seed crystals (Baker Chemical ACS grade CaCO3) show well-developed rhombohedral morphology, with sharp, straight edges.

Control experiments with no organic acids yielded morphologies characterized by continuous planes of new crystal growth, with smooth edges and step features on the face perimeters.

Crystal mass was increased by about 25% in control experiments.

Crystals grown in the presence of organic acids such that R/Ro = 0.5 exhibit planes of new growth that are not continuous, because "poisoning" of growth sites by adsorbed organic acids has prevented surface-nucleated growth spirals from coalescing.

Crystal mass increased by about 50% of the control experiment above (13%)

• Hydrophobic organic acids cause dramatic inhibitory effects on calcite growth kinetics. Acids collected from localities only a few miles apart have measurably different inhibiting abilities related to their chemical properties.

•This work illustrates that kinetic effects induced by spatially-variable, naturally-occurring growth inhibitors should be considered when attempting to understand chemical processes associated with periphyton and the inorganic deposition of calcitic muds in the Everglades.

•The significant effect of organic acids on calcite growth kinetics suggests that organic/inorganic interactions should be considered as an important parameter when modeling interactions of minerals with natural waters.

Reduced Rated as f(Corganic acid)

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R/Ro = 1 indicates no

rate reduction

ReferencesReferencesReynolds, R.C., 1979, Limnol. and Oceanog., 23(4), 585-597.Browder, J.A., P.J. Gleason and D.R. Swift, 1994, In Everglades: The Ecosystem and its Restoration (S.M. Davis and J.C. Ogden, eds.) St. Lucie Press.Gleason, P.J. and P. Stone, 1994, In Everglades: The Ecosystem and its Restoration (S.M. Davis and J.C. Ogden, eds.) St. Lucie Press.