Crystal Growth of Calcium Carbonate in Seawater. The Effect of Temperature and of the Presence of Inhibitors

Abstract

The spontaneous precipitation of calcium carbonate from supersaturated seawater solutions has been investigated in the temperature range 25–80°C and under conditions of sustained supersaturation at pH = 8.5. The supersaturation values with respect to calcite varied from 10 to 100 and in all cases the phase precipitated at the initial stages was vaterite, which afterwards transformed into aragonite. This has been established by Scanning Electron Microscopy and powder X-Ray Diffraction analysis of the precipitates. The precipitation was largely homogeneous, as suggested by an average value of surface energy equal to 41.6 mJ/m², which was calculated for 25, 35 and 50°C. The induction times measured increased significantly with decreasing supersaturation and the rates of precipitation showed a parabolic dependence on supersaturation, pointing to a surface diffusion mechanism. An activation energy of 34.2 kJ/mol was calculated. At 80°C and at supersaturation values lower than 6 the precipitation of magnesium hydroxide made it impossible to obtain measurements. Amorphous magnesium hydroxide has been detected by SEM in the precipitates obtained. In the absence of magnesium ions from the synthetic seawater, the solutions were stable at 80°C at supersaturations lower than 17. The precipitates obtained in this case were analyzed by SEM and XRD and small vaterite and aragonite crystals transforming into calcite were detected. The dominance of calcite was due to the absence of magnesium, which favors the formation of aragonite. The inhibition of the calcium carbonate crystal growth in seawater has also been investigated at 80°C with the use of a polymer of polyacrylic acid with molecular weight equal to 2000. Its strong inhibitory effect (concentrations in the seawater solutions lower than 0.5 ppm) has been attributed to adsorption on active crystal growth sites.