Abstract

1
.
Calcite Growth-Rate Inhibition by Fulvic Acid and
Magnesium -- Potential Reduction of Calcite
Formation Rate in Marine Calcifying Organisms ,
M.M. Reddy, US Geological Survey, Denver, CO
Abstract Crystallization rates are measured in
the presence and absence of a natural hydrophobic
organic acid (a humic acid/fulvic acid isolated
from the Florida Everglades, FA, at a solution
concentration of 0.5 mg/L), magnesium ion (at a
solution concentration of 10-4 M), and in the
presence of both FA and magnesium ion. Organic
matter adsorbed from the air onto the seed
crystals has no influence on the measured calcite
crystal-growth rates.   FA and magnesium ion
alone reduced calcite crystal-growth rates by 47
and 38 , respectively, compared to control
experiments containing no added growth-rate
inhibitor. Growth-rate experiments in the
presence of both FA and magnesium ion reduced the
calcite growth rate to 5 of the control rate a
calcite growth-rate reduction almost ten times
greater than either individual inhibitor.
Magnesium ion inhibits calcite growth rates by
substitution of magnesium ion for calcium ion at
the calcite crystal-growth site. In contrast,
polycarboxylate acid anions, such as natural
hydrophobic organic acids inhibit calcite growth
rates by binding multiple carboxylate groups on
the calcite surface.   In combination, FA and
magnesium ion interactions in solution and/or on
the growing calcite surface dramatically decrease
calcite growth rates. Thus, natural hydrophobic
organic acids, a large reactive global carbon
reservoir, can reduce calcification rates.
Moreover, trace metal incorporation during
carbonate mineral growth is strongly growth-rate
dependent. Trace metal concentrations in marine
carbonates are used for reconstructing past ocean
chemistry and paleoclimate for example, the
magnesium-to-calcium ratio of calcifying marine
organisms is a proxy for past sea surface
temperatures.   These results suggest that FA
and magnesium ion concentrations at
biocalcification sites in marine calcifying
organisms mediate calcite crystallization rates.
Thus, FA and magnesium ion influence metal-ion
partitioning and must be considered when using
trace-metal records as paleoclimate proxies.
Materials and Methods
Results
Discussion
Effectiveness of F1, magnesium and both together
as growth inhibitors
Crystal growth experiments A constant composition
reactor, employing calcite seed crystals added
to metastable, supersaturated solution, is used
for all experiments. Crystallization accompanied
by a drop in the supersaturated solution pH
begins immediately upon seeding. The solution pH
drop triggers addition of calcium and carbonate
titrants and maintains constant chemical
conditions and calcite supersaturation in the
reactor solutions.
Hydrophobic organic acid and magnesium influences
on calcite growth rates Everglades hydrophobic
organic acids inhibit calcite growth rates at
relatively low concentrations compared to
dissolved organic carbon concentrations observed
in Everglades surface waters. Calcite
supersaturation in Everglades waters, maintained
by kinetic inhibition by natural organic acids
probably prevents abiotic precipitation from
occurring. Magnesium (10-4M) reduced calcite
crystal-growth rates (by 38 ) compared to
control experiments. Remarkably, the calcite
growth-rate in the presence of both FA and
magnesium ion is reduced to 5 of the control
rate a calcite growth-rate reduction almost ten
times greater than either individual inhibitor.
Organic matter adsorbed from air onto the seed
crystals has no influence on the measured calcite
crystal-growth rates.   Hydrophobic organic
acids cause dramatic inhibitory effects on
calcite growth kinetics. Everglades hydrophobic
organic acids and magnesium ion interactions in
solution and/or on the growing calcite surface
dramatically decrease calcite growth rates.
Mechanistic details of the synergistic growth
rate reduction caused by magnesium and
hydrophobic organic acids requires additional
investigation. Magnesium inhibits calcite growth
rates by substitution for calcium ion at growth
site (Reddy and Hoch, 2000 and references
therein). In contrast, polycarboxylate acid
anions, such as Everglades' hydrophobic organic
acids inhibit calcite growth rates by binding
multiple carboxylate groups on the calcite
surface and perhaps pinning step advancement
(Reddy and Hoch, 2000).
Magnesium (10-4M) reduced calcite crystal-growth
rates (by 38 ) compared to control experiments.
The F1 sample, the strongest inhibitor isolated
from locations in Everglades Water Conservation
Area 2A, slowed the calcite growth reaction to a
value of about 47 of the control rate at 0.5
mg/l. Everglades surface waters dissolved organic
carbon concentrations at site F1 are 25 to 50 mg
C/L. Calcite growth-rate in the presence of both
FA and magnesium ion is reduced to 5 of the
control rate.
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 (consisting of soluble humic acid
and fulvic acid), isolated using XAD resins from
surface waters in Everglades Water Conservation
Areas 2A, had the following characteristics
Morphology of calcite crystals Scanning
electron microscope images 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 (R/Ro
0.5) are below
Summary
Objective To study and quantify calcite (CaCO3)
crystal growth kinetic effects of a hydrophobic
organic acid isolated from site F1 in the Florida
Everglades in the presence and absence of added
magnesium ion.
Natural hydrophobic organic acids, a large
reactive global carbon reservoir, reduce
calcification rates. Trace metal incorporation
during biocalcification is growth-rate dependent
mediating trace metal concentrations in marine
carbonates recording past ocean chemistry and
paleoclimate. The Mg/Ca in carbonate minerals of
calcifying marine organisms is a proxy for past
sea surface temperatures. Hydrophobic organic
acids and magnesium concentrations at
biocalcification sites in calcifying organisms
regulate calcite crystallization rates and the
extent of trace metal incorporation in the
calcium carbonate. Hydrophobic organic acids and
magnesium concentrations influence metal-ion
partitioning and must be considered when using
trace-metal records as paleoclimate proxies.
Unreacted seed crystals (Baker Chemical ACS grade
CaCO3) show well-developed rhombohedral
morphology, with sharp, straight edges.
Experimental Data And Rate Calculation
Introduction Calcite (the stable calcium
carbonate polymorph at ambient temperature and
pressure) is frequently supersaturated in natural
waters, with no observed precipitation (see for
example, Reynolds, 1979). Stable solution
supersaturation occurs because crystal growth
rates are greatly reduced by naturally-occurring
kinetic inhibitors, such as magnesium ion,
phosphate ion and dissolved organic carbon
present in sufficient solution concentrations
(Reddy and Hoch, 2000, and references
therein). In the Florida Everglades, calcite
precipitation associated with periphyton (shallow
algal mats) impacts surface water pH, pO2, pCO2,
and calcium and carbonate concentrations. Algae
precipitate calcite from adjacent surface water
because of a localized environment of elevated
(with respect to surface water composition) pH
and calcite supersaturation. Gleason and Stone
(1994) propose that presence or absence of
calcite in the periphyton is related to hydrology
and effects of dilution on supersaturation,
without consideration of inhibitory kinetic
effects of dissolved constituents. Results
presented here demonstrate calcite crystal growth
kinetic inhibition due to a natural hydrophobic
organic acid from a site in the Everglades in the
presence and absence of magnesium ion.
Data The constant composition experiment
replaces calcium and carbonate ions
stoichiometrically as calcite precipitates.
Therefore, the quantity of titrant added is
proportional to the quantity of calcite
precipitated and crystal growth rates are
calculated from the titrant added versus time
plots. Magnesium, F1 organic acid, and both
together result in more shallow titrant added
versus time 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 Reddy, M.M. and Hoch, A.R.,2000, In
Advances in Crystal Growth Inhibition
Technologies, Amjad, Z., ed., Kluwer
Academic/Plenum Publishers, New York,
pp.107-120. Reynolds, R.C., 1979, Limnol. and
Oceanog., 23(4), 585-597. 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 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 Logistical support for
Everglades sample collection was provided by the
South Florida Water Management District.
Assistance in laboratory work and data reduction
by Tony Hoch is acknowledged . Contact
mmreddy_at_usgs.gov