Calcium carbonate thermodynamics

Chemical scaling is another form of fouling that occurs in NF and RO plants. The thermodynamic solubility of salts such as calcium carbonate and calcium and barium sulfate imposes an upper boundary on the system recovery. Thus, it is essential to operate systems at recoveries lower than this critical value to avoid chemical scaling, unless the water chemistry is adjusted to prevent precipitation. It is possible to increase system recovery by either adjusting the pH or adding an antisealant, or both. 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

Fig. 6.2 Thermodynamic stability of calcium carbonates. Change in enthalpy is shown with polymorph.

Sverjensky DA, Shock EL, Helgeson HC (1997) Prediction of flie thermodynamic properties of aqueous metal complexes to 1000°C and 5 kb. Geochim Cosmochim Acta 61 1359-1412 Tarutani T, Clayton RN, Mayeda TK (1969) The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim Cosmochim Acta 33 987-996... 

The amount of calcium carbonate that will spontaneously dissolve in water if thermodynamic equilibrimn is attained is governed by the following reaction ... 

Away from the influence of the continents, the %CaC03 in the sediments is largely controlled by the processes that determine whether sinking detrital calcium carbonate survives the trip to the seafloor. Some of these processes are related to the thermodynamic controls on calciiun carbonate solubility and others are a consequence of the relative rates of particle sinking and dissolution. 

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... 

On the opposite end of the spectrum, thermodynamics cannot explain why some PIC can sink through undersaturated waters without dissolving to accumulate on the seafloor. This is a widespread phenomenon as evidenced by the spatial %CaCOj gradients seen in the surface sediments (Figure 15.5). If the saturation horizon dictated the survival of sinking and accumulating PIC, a sharp depth cutoff should exist below which calcium carbonate is absent from the surface sediment. The importance of this kinetic barrier to dissolution is also seen in the relatively high fraction of surfece-water PIC (20 to 25%) that accumulates in the sediments as compared to the low fraction of surfece-water POC (1%). 

It is of interest that two thermodynamically unstable forms of calcium carbonate, aragonite and vaterite. are found in living organisms as well as the more stable calcite. There appears to be no simple explanation for the distribution of the different forms in the various species. 

General Considerations. In order to facilitate the discussion of methods for calculating the saturation state of seawater with respect to calcium carbonate, initial consideration will be given to pure calcium carbonate phases. The method most frequently used expressing the saturation state of a solution with respect to solid phase is as the ratio (Q) of the ion activity (a) product to the thermodynamic solubility constant (K). For the calcium carbonate phase calcite, the expression for the saturation state is defined as (e.g., 13) ... 

EXAMPLE 22-4 The following thermodynamic data are given for the dissociation of calcium carbonate at 25°C AH = 178 kJ AG = 130 kJ AS° = 161 J/mole-deg. Estimate the temperature at which the dissociation pressure is 1 atm, ignoring the correction for heat capacity. 

The temperature dependence of equilibrium isotope exchange in the calcite-water system has been intensively studied since Urey (1947) first suggested that the paleotemperature of the ancient oceans could be estimated by the 0-isotope distribution between seawater and the calcium carbonate precipitated from it. Urey et al. (1951) argued that O-isotope equilibrium between seawater and CaC03 was likely and support for this idea has come from the close agreement between the CaC03-H20 isotopic fractionation observed in natural systems and those derived from both thermodynamic calculations and laboratory experiments (e.g. Epstein et al., 1951, 1953 Emiliani, 1955 O Neil et al., 1969 O Neil et al., 1975). 

Examples of such gaseous-solid reactions are comparatively common, and the fact that they are susceptible of simple thermodynamical treatment was discovered relatively early. Many solid compounds dissociate when the temperature is raised, giving off one or more gases. Examples of this are calcium carbonate, according to the equation... 

As the amount of CO2 stored increases, it becomes progressively more difficult to guarantee a physical barrier that prevents CO2 from returning to the atmosphere. Chemical conversion to a thermodynamically lower state would thus be desirable and is indeed possible. CO2 is the anhydrous form of carbonic acid and therefore can be used to displace weaker acids such as silicic acid. The formation of carbonates from silicates is well known as geological weathering. Thermodynamically, CO2 can be bound as a carbonate. In many instances, these carbonates dissolve in water, but some, such as magnesium and calcium carbonates, are remarkably stable as solids. Thus, mineral sequestration would provide a means of storing CO2. 

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