John W. Morse

The chemistry of orthophosphate uptake from seawater on to calcite and aragonite

Abstract The chemistry of orthophosphate uptake from synthetic seawater onto the surfaces of synthetic calcite, aragonite and low-magnesium biogenic calcite has been studied, in order to elucidate the kinetics of the process (generally believed to be the major control of dissolved reactive phosphate in carbonate-rich marine sediments). Our results differ from those obtained by others, who have studied orthophosphate uptake in low ionic strength solutions and at much higher supersaturations relative to apatite. In both ‘free drift’ and chemostat experiments, Mg and F have only a minor effect on the reaction rate. Even at constant solution composition the rate of orthophosphate uptake was found to decrease by 10 6 over a two week period. The data from the ‘free drift’ experiments can be fitted to the Elovich equation. This indicates that the kinetics observed for this reaction can be explained by an exponential decrease in available surface reaction sites and/or a linear increase in the activation energy associated with chemisorption as the reaction proceeds.

The solubility of calcite and aragonite in seawater of 35%. salinity at 25°C and atmospheric pressure

The solubilities of synthetic, natural and biogenic aragonite and calcite, in natural seawater of 35%. salinity at 25°C and 1 atm pressure, were measured using a closed system technique. Equilibration times ranged up to several months. The apparent solubility constant determined for calcite of 4.39(±0.20) × 10−7 mol2 kg−2 is in good agreement with other recent solubility measurements and is constant after 5 days equilibration. When we measured aragonite solubility we observed that it decreased with increasing time of equilibration. The value of 6.65(±0.12) × 10−7 mol2 kg−2, determined for equilibration times in excess of 2 months, is significantly less than that found in other recent measurements, which employed equilibration times of only a few hours to days. No statistically significant difference was found among the synthetic, natural and biogenic material. Solid to solution ratio, contamination of aragonite with up to 10 wt% calcite and recycling of the aragonite made no statistically significant difference in solubility when long equilibration times were used. Measured apparent solubility constants of aragonite and calcite are respectively 22( ± 3)% and 20( ± 2)% less than apparent solubility constants calculated from thermodynamic equilibrium constants and seawater total activity coefficients. These large differences in measured and calculated apparent solubility constants may be the result of the formation of surface layers of lower solubility than the bulk solid.

The interaction of manganese(II) with the surface of calcite in dilute solutions and seawater

The interaction of Mn2+ with the surface of calcite in aqueous solutions is complex. In dilute solutions the Mn2+ is rapidly absorbed, MnCO33 nucleates on the calcite surface and then grows by a first order reaction with respect to the initial Mn2+ concentration. At higher ionic strengths in NaCl solutions, the rate of these processes is slower, but the same general pattern persists. In solutions containing Mg2+, at the concentration of seawater and in seawater, the nucleation phase of the uptake process does not appear to occur. The long-term uptake rate of Mn2+ on the surface of calcite in seawater is first order with respect to the dissolved Mn2+ concentration. The rate constant is over three orders of magnitude smaller than that found in dilute Mg2+-free solutions. A probable explanation for the slower growth rate in seawater is that MnCO3 is not nucleated on the calcite surface due to the presence of high Mg2+ concentrations. The Mg2+, through site competition, prevents enough Mn2+ from being adsorbed to reach a critical concentration for MnCO3 nucleation. This behavior is similar to that found for orthophosphate with calcite surfaces in dilute solutions and seawater. It indicates that rhodochrosite cannot nucleate in carbonate-rich recent sediments unless the Mg2+ concentration is lowered below that of seawater. Measurements of the solubility of rhodochrosite in seawater gave results from an undersaturation approach to equilibrium in excellent agreement with those found in previous studies in dilute solutions. When equilibrium was approached from supersaturation, approximately fifty times more calcium was precipitated than Mn2+. The measured solubility was over twice that determined from undersaturation. It is possible that a Mn—calcite containing 25 to 30 mol% MnCO3 formed on the rhodochrosite from the supersaturated solutions. Consequently, it is doubtful that pure rhodochrosite controls the concentration of Mn2+ even in calcium carbonate-poor marine environments.

A simple technique for surface area determination

An inexpensive apparatus for BET (Brunauer, Emmett and Teller) surface area measurement using krypton with a lower limit of 0.001 m2 has been developed. Operation is made simple by the use of an MKS Baratron pressure gauge and a constant-volume system.

The carbonate system in the western Mediterranean sea

Abstract Measurements of the pH and total alkalinity have been made on waters collected in the western Mediterranean Sea. These results have been used to examine the elements of the carbonate system, HCO3−, CO32−, CO2, ΣCO2, PCO2, and specific alkalinity. The saturation of Mediterranean Gibraltar is supersaturated with respect to calcium carbonate. Our results for the saturation state (Ω) for Mediterranean waters are in good agreement with the results of Alekin (Geochemistry, 206, 239–242, 1972) and those calculated from the GEOSECS (Geochemical Ocean Sections) test station in the eastern Mediterranean. The saturation state of calcite and aragonite in deep Mediterranean waters in higher than that of deep North Atlantic waters.

Magnesium interaction with the surface of calcite in seawater.

Magnesian calcite overgrowth containing 4 (� 2) mole percent magnesium carbonate forms on calcite exposed to natural seawater near the ocean surface. This magnesian calcite is approximately 30 percent less soluble in seawater than pure calcite. The formation of the magnesian calcite of reduced solubility may have a major influence on calcite accumulation in deep sea sediments.

A literature review of the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on OTEC heat exchangers

Abstract An investigation has been made of available data on the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on Ocean Thermal Energy Conversion (OTEC) heat exchangers. Pertinent oceanographic data is lacking at or near potential OTEC sites for the calculation of the degree of saturation of seawater with respect to calcium carbonate. Consequently, only “extrapolated” saturation values can be used. These indicate that near surface seawater is probably supersaturated, with respect to the calcium carbonate phases calcite and aragonite, at all potential OTEC sites. The deep seawater that would be brought to the surface at the potential Atlantic Ocean sites is also likely to be supersaturated with respect to calcium carbonate. The deep seawater at the potential Pacific Ocean sites may be slightly undersaturated. The fact that OTEC heat exchangers will be operating in seawater, which is supersaturated with respect to calcium carbonate, means that if nucleation of calcite or aragonite occurs on the heat exchanger surfaces, significant growth rates of calcium carbonate scale may be expected. The potential for calcium carbonate nucleation is highest at cathodic metal surface locations, which are produced as the result of aluminum corrosion in seawater. Consequently, corrosion and scale formation may be closely related. What the possible effects of biofouling may be on this process are not known.