AbstractAbstract

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 MethodsMaterials and Methods

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

Experimental Data And Rate CalculationExperimental Data And Rate Calculation

SummarySummary

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

IntroductionIntroductionCalcite (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.

ObjectiveObjectiveTo 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.

ResultsResults DiscussionDiscussion

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.

Effectiveness of F1, magnesium and both together as growth inhibitors

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:

Hydrophobic organic acid and magnesium influences on calcite growth rates

Everglade’s 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. Everglade’s 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).

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:

AcknowledgementsAcknowledgementsLogistical 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@usgs.gov

Experimental Conditions

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

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.

.

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

Sample Mw

(daltons) Mn

(daltons) [COOH ] (meq/g)

% Aromatic functional

groups

% Carboxyl functional

groups U3 1747 1180 5.99 19.0 14.4 F1 1907 1238 5.14 18.2 11.6

2BS 1519 1080 4.74 15.4 15.4

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.

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 13%.

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.

R/Ro = 1 indicates no

rate reduction

ReferencesReferencesReddy, 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.

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Control

0.5 mg C/L F1

10^-4 M Mg

F1 + Mg

time, minutes

Ca2+

titra

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