Abstract

1
.
Inhibition of Calcite Growth by Natural Organic
Acids From the Florida Everglades at pH 8.5 and
25?C
H31-A10
Anthony Hoch, Michael Reddy and George Aiken,
USGS, 3215 Marine St., Boulder, CO 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 (W
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 CO32- 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
(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 Methods
Results
Discussion
Effectiveness of different organic acids as
growth inhibitors
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.
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 CO32- titrants,
maintaining constant chemical conditions in the
reactor solutions.
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.
R/Ro 1 indicates no rate reduction
Experimental Conditions All experiments were run
for 100 minutes under the following conditions
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
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.
Summary and Implications for Natural Water
Chemistry
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.

• 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.

Unreacted seed crystals (Baker Chemical ACS grade
CaCO3) show well-developed rhombohedral
morphology, with sharp, straight edges.
Experimental Data And Rate Calculation
Introduction 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. 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.
Data for F1 Experiments
Data 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.
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.
References Reynolds, 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.
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)
Calculation 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
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.