Supersaturated calcium carbonate

Figure 11. An SEM picture of crystals, grown at the air/water interface of a supersaturated calcium carbonate...

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. 

Gypsum, carbonate, incompatible waters or supersaturation Calcium carbonate, incompatible waters... 

Aragonite. Calcium carbonate is a common deposit in shallow tropical waters as a constituent of muds, or in the upper part of coral reefs where it precipitates from carbon dioxide-rich waters supersaturated with carbonate from intense biological photosynthesis and solar heating. Deposits of ooHtic aragonite, CaCO, extending over 250,000 km in water less than 5 m deep ate mined for industrial purposes in the Bahamas for export to the United States (19). 

Although they may be completely soluble in the lower temperature bulk water, these compounds (eg, calcium carbonate, calcium phosphate, and magnesium siUcate) supersaturate in the higher temperature water adjacent to the heat-transfer surface and precipitate on the surface. 

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... 

Schierholtz and Stevens (1975), Noor and Mersmann (1993) and Chen etal. (1996) determined nucleation rates by integrating the total crystal number formed over a period and related it to an estimate of supersaturation in the precipitation of calcium carbonate, barium carbonate and barium sulphate respectively. 

The reactor has been successfully used in the case of forced precipitation of copper and calcium oxalates (Jongen etal., 1996 Vacassy etal., 1998 Donnet etal., 1999), calcium carbonate (Vacassy etal., 1998) and mixed yttrium-barium oxalates (Jongen etal., 1999). This process is also well adapted for studying the effects of the mixing conditions on the chemical selectivity in precipitation (Donnet etal., 2000). When using forced precipitation, the mixing step is of key importance (Schenk etal., 2001), since it affects the initial supersaturation level and hence the nucleation kinetics. A typical micromixer is shown in Figure 8.35. 

With insufficient carbon dioxide of type 3 (and none of type 4) the water will be supersaturated with calcium carbonate and a slight increase in pH (at the local cathodes) will tend to cause its precipitation. If the deposit is continuous and adherent the metal surface may become isolated from the water and hence protected from corrosion. If type 4 carbon dioxide is present there can be no deposition of calcium carbonate and old deposits will be dissolved there cannot therefore be any protection by calcium carbonate scale. 

Very hard waters are usually not very aggressive provided that they are supersaturated with calcium carbonate. Underground waters with a low pH value and high carbon dioxide content are, however, aggressive unless corrective treatment is applied. 

Saturation Index an index which shows if a water of given composition and pH is at equilibrium, supersaturated or unsaturated with respect to calcium carbonate (or to magnesium hydroxide). 

The most common source is the supersaturation and subsequent scaling of minerals originating in the MU water. Insoluble calcium carbonate in the form of calcite (CaC03) resulting from the thermal decomposition of soluble calcium bicarbonate [Ca(HC03)2] is a classic example. Calcium carbonate quickly forms a white, friable deposit. In addition, the hydrolysis of excess bicarbonate increases... 

Oceanic surface water is everywhere supersaturated with respect to the two solid calcium carbonate species calcite and aragonite. Nevertheless carbonate precipitation is exclusively controlled by biological processes, specifically... 

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

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... 

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... 

Figure 8-10 shows the first 200 years of evolution of the concentrations at the same depths as plotted in Figure 8-9. The concentrations of both total carbon and calcium at a 500-centimeter depth decrease at first and then increase. This decrease occurs because I used starting values equal to seawater values. The waters were initially supersaturated and started out by precipitating calcium carbonate. This initial precipitation was overwhelmed at the shallower depths by the rapid addition of carbon as a result of respiration.

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). 

If f) is greater than 1, the water mass is supersaturated and calcium carbonate will spontaneously precipitate until the ion concentrations decrease to saturation levels. When Cl is less than 1, the water mass is undersaturated. If calcium carbonate is present, it will spontaneously dissolve imtil the ion product rises to the appropriate saturation value. Although calcium is a biointermediate element, it is present at such high concentrations that PIC formation and dissolution causes its concentration to vary by less... 

The saturation state of seawater can be used to predict whether detrital calcite and aragonite are thermodynamically favored to survive the trip to the seafloor and accumulate in surfece sediments. Any PIC or sedimentary calcium carbonate exposed to undersaturated waters should spontaneously dissolve. Conversely, PIC and sedimentary calcium carbonate in contact with saturated or supersaturated waters will not spontaneously dissolve. Typical vertical trends in the degree of saturation of seawater with respect to calcite and aragonite are shown in Figure 15.11 for two sites, one... 

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

Because warm surface seawater is usually supersaturated with respect to calcium carbonate, abiogenic precipitation of calcite and aragonite does occur, at least when supersaturations are very high. These conditions are limited to shallow water where temperatures can get sufficiently high, namely coastal tropical seas. 

Oolite A hydrogenous precipitate commonly found in carbonate sediments of continental shelves located in the tropics. They are composed primarily of calcium carbonate and are thought to be an abiogenic precipitate formed from warm seawater supersaturated with respect to calcite and aragonite. 

For example, normal urine is supersaturated with calcium oxalate. To prevent formation of renal calculi (stones)719 an inhibitory glycoprotein is present and slows the formation and growth of crystals.720 Under some disease conditions calcium carbonate stones may form in pancreatic ducts. A17 kDa lectinlike glycoprotein called lithostatine has been proposed to inhibit stone formation by binding to certain planes on CaC03 microcrystals just as antifreeze proteins (Box 4-D) inhibit ice formation.721 However, this proposed function for lithostatine is doubtful.722 723 Pathological deposits of crystalline calcium pyrophosphate and basic calcium phosphates are sometimes present in joints,724 even in Neanderthal skeletons.725... 

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. 

In the. first run, acid feed rate was gradually reduced without immediate evidence of scale formation and then was cut off entirely. Heat transfer coefficients started to drop immediately and at the end of the week the tubes were coated with calcium carbonate scale. In this first run, the pH entering the evaporator was 8.3 and leaving it was 8.4. Langelier, Caldwell, and Lawrence have measured the equilibrium pH above which a sea water concentrate is supersaturated with respect to calcium carbonate and magnesium hydroxide (4). Under the conditions of this test, the following conditions were encountered ... 

---