Calcium carbonate saturation levels

Dilute seawater is undersaturated in carbonates, which reduces the likelihood of forming protective ctdcareous films on a metal surface. In deep ocean waters, the calcareous deposits are not spontaneously formed in an ambient environment and are often not precipitated under cathodic protection conditions [6]. In the cold waters of the deep ocean environmental zones, the precipitation and/or dissolution of the calcareous deposits is mainly controlled by the calcium carbonate saturation level, II [25]. 

The Distribution of Calcium Carbonate Saturation States and Their Relation to Sediment Marker Levels... 

Ryznar stability index. The Ryznar stability index (RSI) attempts to correlate an empirical database of scale thickness observed in municipal water systems to the water chemistry. Like the LSI, the RSI has its basis in the concept of saturation level. Ryznar attempted to quantify the relationship between calcium carbonate saturation state and scale formation. The Ryznar index takes the form... 

Calcium—In general, calcium (as CaCOs) below 800 ppm should not result in calcium sulfate scale. In arid climates, however, the critical level may be much lower. For calcium carbonate scaling tendencies, calculate the Langelier Saturation Index or the Ryznar Stability Index. 

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. 

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 CleanOX process is limited in saturated matrices possessing very low permeabilities and high calcium carbonate levels, as well as in groundwater with high total organic carbon content. In addition, the presence of iron-metabolizing bacteria can reduce the effectiveness of the iron catalyst, resulting in operations and maintenance problems for the system. 

System pH is also a critical factor that affects the saturation level. For calcium carbonate, a higher pH decreases the saturation level (more scaling), but for silicates, the reverse is true, so many cooling systems operating at high pH (say, >9.5) can tolerate silica levels of up to 200 ppm SiC>2 without significant risk of silicate scaling. 

Calcium carbonate is accumulating in deep ocean sediments, in which the overlying water is undersaturated with respect to both aragonite and calcite, and sediment marker levels closely correspond to unique saturation states. This indicates that dissolution kinetics play an important role in determining the relation between seawater chemistry and calcium carbonate accumulation in deep ocean basins. It is, therefore, necessary to have knowledge of the dissolution kinetics of calcium carbonate in seawater if the accumulation of calcium carbonate is to be understood. 

Figure 7. The depth distribution of the Ro and calcite saturation levels, the foraminiferal lysocline and the calcium carbonate compensation depth in the Western and Eastern Atlantic Ocean (after Ref. 40)...

Selective extractions, chemical equilibria calculations, and crystallization measurements presented here imply that the hydrous iron oxides, even in the carbonate dominated Genesee River, play a major part in inorganic phosphorus transport by sediments in the fluvial system. Saturation levels of inorganic phosphate and calcium carbonate minerals in the Genesee River... 

As soon as the calcite saturation level is reached, biogeochemical carbonate crystals accumulate, mixed with remains of ostracods sediments prograding towards the lake centre form foresets. Foresets can include turbidite beds a few centimetres thick due to turbid underflow. Calcium carbonate precipitates only when Characeae are present. Along the talus, and with depth, the water cools down increasing calcite dissolution. 

Figure 3 Bathymetric profiles of calcium carbonate (calcite) saturation for hydrographic stations in the Atlantic and Pacific Oceans (data from Takahashi etai 1980). Carbonate saturation here is expressed as ACOa ", defined as the difference between the in situ carbonate ion concentration and the saturation carbonate ion concentration at each depth ACOa " = [C03 ]seawater - [COa Jsaturation)-The saturation horizon corresponds to the transition from waters oversaturated to waters undersaturated with respect to calcite (A 003 = 0). This level is deeper in the Atlantic than in the Pacific because Pacific waters are COa-enriched and [C03 ]-depleted as a result of thermohaline circulation patterns and their longer isolation from the surface. The Atlantic data are from GEOSECS Station 59 (30°12 S, 39°18 W) Pacific data come from GEOSECS Station 235 (16°45 N,161°23 W).

The saturation level (SL) of water in a mineral phase is a good indicator of the potential for scaling due to that specific sealant. Saturation level is a ratio between the ion activity product (lAP) and the thermodynamic solubility product of a specific compound in that water. For example, when calcium carbonate (CaCOj) is the sealant SL is defined as... 

Indices based upon ion-association models provide a common denominator for comparing results between systems. For example, calcite saturation level calculated using free calcium and carbonate... 

The saturation levels for common cooling-water scales were calculated, including calcium carbonate, calcium sulfate, amorphous silica, and magnesium hydroxide. Brudte saturation levels were included because of the potential for magnesium silicate formation as a result of the adsorption of silica upon precipitating magnesium hydroxide. Three categories of systems were encountered [24] ... 

In this example, water is said to be saturated with calcium carbonate when it will neither dissolve nor precipitate calcium carbonate scale. This equilibrium condition is based upon an imdisturbed water at constant temperature which is allowed to remain imdisturbed for an infinite period of time. Water is said to be undersaturated if it can still dissolve calcium carbonate. Supersaturated water wiU precipitate calcium carbonate if allowed to rest. If water is undersaturated with respect to calcium carbonate, the SL value will be less than 1.0. When water is at equilibrium, SL will be 1.0 by definition. Water which is supersaturated with calcium carbonate will have a saturation level greater than 1.0. As the saturation level increases beyond 1.0, the driving force for calcium carbonate crystal formation or crystal growth increases. 

It has been shown that water supersaturated with calcium carbonate often develops tubercular deposits which do not inhibit corrosion on mild steel. This behavior is typically associated with water of low buffer capacity. Puckorius also warned against using saturation-level-derived indices as the basis for predicting corrosion problems in cooling systems. 

X will be a small value when either calcium is high and carbonate low, or carbonate is high and calcium low. It will increase to a maximum when equal parts of calcium and carbonate are present. As a result, these calculations will provide vastly different values for waters with the same saturation level. Although the original momentary excess index was applied only to calcium carbonate scale, the index can be extended to other scale-forming species. In the case of sulfate, momentary excess is calculated by solving forX in the relationship... 

The saturation indices discussed previously can be calculated based upon total analytical values for all possible reactants. Ions in water, however, do not tend to exist totally as free ions. Calcium, for example, may be paired with sulfate, bicarbonate, carbonate, phosphate, and other species. Bound ions are not readily available for scale formation. This binding, or reduced availability of the reactants, decreases the effective ion-activity product for a saturation-level calculation. Early indices such as the LSI are based upon total analytical values rather than free species primarily because of the intense calculation requirements for determining the distribution of species in a water. Speciation of a water requires numerous computer iterations for the following ... 

Indices based upon ion association models provide a common denominator for comparing results between systems. For example, calcite saturation level calculated using free calcium and carbonate concentrations has been used successfully as the basis for developing models which describe the minimum effective scale inhibitor dosage that will maintain clean heat-transfer surfaces. The following cases illustrate some practical usage of the ion association model. 

Category 3. These systems demonstrated an extraordinarily high scale potential for at least calcium carbonate and brucite. These systems operated with a recirculating water chemistry similar to that of a softener rather than of a cooling system. The Category 3 water chemistry was above the maximum saturation level for calcium carbonate where traditional inhibitors such as phosphonates are able to inhibit scale formation. 

---