Supersaturated calcium carbonate solutions

The polymers produced can be used to control the crystal growth of calcium carbonate from supersaturated calcium carbonate solutions, in favourable cases control of form, size and nucleation density can be achieved. 

A carboxylate derivative of a fully aromatic, water-soluble, hyperbranched polyphenylene is considered as a unimolecular micelle due to its ability to complex and solubilize non-polar guest molecules [23]. The carboxylic acid derivative of hyperbranched polyphenylene polymer (HBP) (My,=5750-7077, Mn=3810-3910) consists of 40-60 phenyl units that branch outward from a central point forming a roughly spherical molecule with carboxylates on the outer surface. The free acid form of HBP was suspended in distilled water and dissolved by adding a minimum quantity of NaOH. The solution was adjusted to pH 6.2 with aqueous HCl. Calcium carbonate crystals were growth from supersaturated calcium hydrogencarbonate solution at room temperature. HBP gave... 

It has been proved that the basic polymers, which will be positively charged under acidic conditions, are also useful templates for the growth of calcium carbonate [243]. Hemispherical vaterite and needlelike aragonite can be selectively synthesized at the air/water interface by the mediation of poly(ethylene imines)(PEIs) dissolved in supersaturated calcium bicarbonate solution with different molecular weight of PEI blocks, suggesting that cationic polychains are also versatile templates for artificial material synthesis. 

Since the effluent from a softening unit is usually supersaturated with calcium carbonate at the usually high pH values, it is necessary to reduce the pH to a value that allows the solution to be exactiy saturated for the calcium-ion and carbonate-ion concentrations present. The relationship is... 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

The increased amount of carbonate ion present causes the supersaturation of calcium carbonate, which comes out of the solution. As the carbon dioxide is least soluble at the temperature of the hot metal surface, the calcium carbonate has its greatest supersaturation at the surface and therefore tends to deposit there. 

Also, calculate the holding time index. If the holding time is more than, say, 48 hours, expect potential problems with the dispersion and removal of suspended solids and sludges. Also expect potential problems with regard to the holding up of supersaturated minerals in solution, such as calcium carbonate. Again, some innovative solution may need to be found. 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Published reports discussing the influence of calcium carbonate solid surfaces on the phosphate ion concentration in natural water (2, 3) include 1)solubility product-based calculations 5) iTspontaneous precipitation experimehts from highly supersaturated solutions 7j 9) 3) adsorption of phosphate onto calcium carbonate (10, 11, 12, 13), These experiments have provided useful information, but several have not included activity coefficient or ion pair corrections in their analyses, seriously limiting the applicability of the results. 

Precipitation potential—The amount of calcium carbonate that will precipitate when the solution is left by itself from its supersaturating condition. 

Calcite speleothems are deposited in a cave when downward-percolating, carbonate-saturated grormdwaters, with a partial pressure of CO2 exceeding that of the cave atmosphere, enter a cave and become supersaturated with respect to calcium carbonate. If the degassing of CO2 from solution is slow and isotopic equilibrium is maintained between aqueous carbon phases and deposited carbonate, then two climatically-dependent variables (a fractionation effect and a water effect) will determine the O-isotopic composition of the speleothem calcite deposited in a cave. 

The disadvantage of this method is that it tends to form supersaturated solutions of calcium carbonate that make removal of the carbonate difficult. 

Kabasci, S., Althaus, W. and Weinspach, P.M. (1996). Batch precipitation of calcium carbonate from highly supersaturated solutions. Chemical Engineering Research and Design, 74, 765-772. 

Crystallization experiments were then carried out in supersaturated solutions of calcium carbonate. Under these conditions, rhombohedral calcite crystals were observed in the recognition sites of the calcite-imprinted polymer. Very few crystals... 

Crystallization of calciiun carbonate is highly dependent on precipitation conditions. In the classical method, much of the scientific investigation of calcium carbonate formation focused on the seed growth of anhydrous calcite crystals from solutions of low supersaturation [23-26]. A crystal can precipitate via epitaxy directly from the liquid solution so that the nuclei bear the same structure as the final crystal. 

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