H31-A10
Inhibition of Calcite Growth by Natural Organic Acids From the Florida Everglades at pH 8.5), P and 25), PC
Anthony Hoch, Michael Reddy and George Aiken, USGS, 3215), P Marine St., Boulder, CO 80303
Effectiveness of different organic acids as growth inhibitors
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.
Reduced Rated as f(Corganic acid)
Experimental Conditions
All experiments were run for 100 minutes under the following conditions:
0.40
A.
0.0
1.0
Organic acids used in experiments
2.0
3.0
4.0
5.0
Morphology of calcite crystals
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:
Sample
Mw
(daltons)
Mn
(daltons)
[COOH ]
(meq/g)
U3
F1
2BS
1747
1907
15), P19
1180
1238
1080
5), P.99
5), P.14
4.74
% Aromatic
functional
groups
19.0
18.2
15), P.4
Unreacted seed crystals (Baker Chemical
ACS grade CaCO3) show well-developed
rhombohedral morphology, with sharp,
straight edges.
Data for F1 Experiments
Data
Tirant added (mL)
5.0
0.0
0.2
0.5
1.0
2.0
5.0
4.0
3.0
2.0
2+
Ca
Since the experimental system replaces
Ca2+ and CO32- 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.
6.0
Calculation of Calcite Growth Rates
In this poster we present results showing effects of natural hydrophobic organic
acids from the Everglades on calcite crystal growth kinetics.
REDUCED RATE:
Crystal mass was increased by about 25), P% in
control experiments.
1.0
0.0
0
10 20
30 40
50
60 70
80 90 100
Time (min)
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.
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
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.
ABSOLUTE RATE:
R (mol/m2/min) = slope (l/min) * mtitrant (mol/l) /( massseed (g) * SAseed (m2/g))
Crystals grown in the presence of organic acids
such that R/Ro = 0.5), P 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 5), P0% of the
control experiment above (13%)
R/Ro = RWITH INHIBITOR/RCONTROL
C.
1
R2 = 0.7468
0.8
2BS
0.6
0.4
U3
0.2
F1
2
R = 0.7896
0.8
2BS
0.6
0.4
U3
0.2
F1
0
0
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), P, or 5), P0%
inhibition of growth rate.
% Carboxyl
functional
groups
14.4
11.6
15), P.4
B.
1
COA (mg/l)
Introduction
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).
0.60
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.
0.00
Experimental Data And Rate Calculation
It 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.
F1
U3
2BS
0.80
0.20
25), P C
pH
I (M)
[Ca]TOTAL (M) [CO3]TOTAL(M) PCO2 (atm)
T ( C)
0.0019
0.0019
10-3.5), P5), P
25), P 0.1 8.5), P00 0.005), P 4.44 0.04* 0.1
* saturation state varied slightly among experiments, due to Ca-DOC complexation
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.
Chemical properties of organic acids and effectiveness of crystal growth
inhibition
1.00
R/Ro = 1
indicates no
rate reduction
All of the organic acids studied were effective inhibitors of calcite growth at relatively
low concentrations ( 5), P mg/l) compared to concentrations observed in Everglades
surface waters (25), P to 5), P0 mg/l). Thus, although calcite supersaturation is observed in
Everglades waters, kinetic inhibition by natural organic acids probably prevents abiotic
precipitation from occurring.
14
15
16
17
18
19
aromatic groups (%)
20
10
12
1
R2 = 0.9882
o
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 CO32- titrants,
maintaining constant chemical conditions in
the reactor solutions.
Objective
Concentration of organic acids and crystal growth inhibition
o
Crystal growth experiments
reduced rate R/R
Calcite crystallization rates decreased at OA concentrations as low as 0.2 mg/l. Crystal growth was
almost entirely inhibited at the 5), P 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.
Discussion
reduced rate R/R
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), P mg/l.
Results
o
Highly reproducible calcite growth experiments were performed in a sealed reactor at constant pH,
temperature, supersaturation ( = 4.5), P), PCO2 (10-3.5), Patm), and ionic strength (0.1 M, KNO3).
Metastable supersaturated solutions were prepared by adding CaCl 2 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 CO32- 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.
Materials and Methods
reduced rate R/R
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 (CaCO 3).
R/Ro
Abstract
14
carboxyl groups (%)
16
2BS
0.8
2BS
Mn
Mw
0.6
0.4
0.2
U3
R2 = 0.9967
0
800
U3
F1
F1
1300
1800
molecular weight (daltons)
Summary and Implications for Natural Water Chemistry
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, naturallyoccurring 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.
References
Reynolds, R.C., 1979, Limnol. and Oceanog., 23(4), 5), P85), P-5), P97.
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.
Acknowledgements
Funding 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.
