Equilibrium carbonate

Soft upland waters are aggressive to most metals, their behaviour depending to some extent on pH values as discussed on p. 2.53. They are inevitably unsaturated with respect to calcium carbonate and it is not usually practicable to modify the carbonate equilibrium to make them non-aggressive. 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Because 1 is several orders of magnitude larger than 2 or, we identify 1 as dominant. Notice, however, that the water equilibrium generates some hydroxide ions in the solution, so this equilibrium must be used to find the concentration of hydroxide ions. The third reaction involves a minor species, HCO3, as a reactant, so it cannot be the dominant equilibrium. However, just as the water equilibrium generates some hydroxide ions, the hydrogen carbonate equilibrium generates some carbonate anions, whose concentration must be determined. 

Fig. 5-1. Settling particles of biological origin carry carbon and alkalinity into the deep sea. Carbonate equilibrium reactions in the surface sea affect the atmospheric pressure of carbon dioxide.

The CO2 concentration in the earth s atmosphere is ultimately governed by the calcium carbonate equilibrium in the ocean (e.g., Berner et al. 1983). If the oceans are in equilibrium with calcite, which is usually the case, then to a reasonable approximation, the PCO2 of the atmosphere is defined by the equilibrium ... 

As a result of the observed limitations inherent in all indices (but the nevertheless high value placed on them as predictive tools), researchers have continued over the years to devise various charts, models, and computer programs as aids or replacements for them. These include the Kunz Model, Burroughs NTU Model, Captain BAISPEC Program, and Cooling Tower Carbonate Equilibrium (CTCE) Model. 

Solvent recovery G Activated carbon Equilibrium Steam stripping... 

Table 13.1 Temperature dependence of some important carbonate equilibrium constants. 

This shall be modeled with the following example. A fracture with an extension of 300 meters is given. Assume that, at the beginning of the simulation, this fracture is filled with groundwater that is in carbonate equilibrium. To simplify matters the following data shall be used pH 7.32... 

From the carbonate equilibrium expression derived previously, we know that... 

Finally, the inorganic carbon equilibrium is responsible for buffering seawater at a pH near 8 on timescales of centuries to millennia. 

The hydrological cycle involves pH variations. In fact, most natural waters have a pH between 4 and 9 that is controlled mainly by the carbon dioxide-carbonate equilibrium. For instance, as calculated in example 6.4, the pH of rainwater is around 5.7 because of its equilibrium with carbon dioxide, but as it touches the Earth and comes in contact with decomposing organic materials, it may acidify even further. On the other hand, if water comes in contact with alkaline environments, minerals or sediments — mainly carbonates (which upon dissolution in water generate strongly basic media), its pH will rise. 

The principal nonpolar-type adsorbent is activated carbon. Equilibrium data have been reported on hydrocarbon systems, various organic compounds in water, and mixtures of organic compounds (11,15,16,46,47). With some exceptions, the least polar component of a mixture is selectively adsorbed eg, paraffins are adsorbed selectively relative to olefins of the same carbon number, but dicyclic aromatics are adsorbed selectively relative to monocyclic aromatics of the same carbon number (see Carbon, activated carbon). 

F. E. Sulphide and carbonate equilibrium and the oxidation-reduction state of sulfur in the mineral regions of the Crimean mineral waters. Dokl. Akad. Nauk. SSSR 142(1) (1962). 

Positive values of AH show the endothermic nature of adsorption. Similar results have been reported for the adsorption of methylene blue by clay [19].The negative values of AG indicate the spontaneous nature of adsorption for mefriylene blue at 35, 40, 50 and 60 C. The positive values of AS suggest the increased randomness at the solid /solution interface during the adsorption of dye on coir pith carbon. Equilibrium data at different temperatures for the adsorption of dye on coir pith carbon do not follow the first order kinetic model but follows the second order kinetic model. The second order rate constants were in the range 0.357-0.879g/mg/min. 

Although the thermodynamic equilibrium of processes in seawater have been studied for many systems, only two systems of wide interest will be examined (i) the carbonate equilibrium in seawater and (ii) the solubility of Fe(III) in seawater. 

Assume Fca = Fco, and as a first order of approximation, ignore all carbonate equilibrium reactions in the solution (see text). In this case the steady-state diffusion-reaction equation for CO3 concentration, [COf ] (mol cm ), in the pore waters is... 

Figure 4.5. Aqueous carbonate equilibrium constant Pcoz- Water is equilibrated with the atmosphere pco2 10 atm), and the pH is adjusted with strong base or strong acid. Equations 7-9 with the constants (25°C) pATn = 1.5, pAT, = 6.3, pAT2 = 10.25, and pAT (hydration of CO2) = -2.8 have been used. The pure CO2 solution is characterized by the proton condition [H ] = [HCO ] + 2[C03 ] + [OH ] (see point P) and the equilibrium concentrations -log[H J = -log[HCO ] = 5.65 log[C02aq] = -log[H2C03 ] = 5.0 -log[H2C03] = 7.8 and -log[C03 ] = 8.5.

Moreover, these reactions lead to changes in inorganic carbonic equilibrium at the metallic interface... 

Carbonate precipitation controlled by photosynthetic CO2 assimilation is a major process in lakes and in the formation of travertines. However, in the marine environment carbonate precipitation is influenced by many processes and in only a few instances has been shown to be the direct result of photosynthetic CO2 assimilation. On the other hand, photosynthesis is a major factor in the natural carbonate cycle and controls the total carbonate equilibrium to the extent that a constant ratio between organic-C and carbonate-C is maintained in sediments even though little direct precipitational relation can be established between the two. 

The pH and carbonate species distribution for waters from different locations in the ocean (Table 4.4) are calculated by using data for At and DIG and the equilibrium constants. The equilibrium equations were solved with the computer program of Lewis and Wallace (1998) using the carbonate equilibrium constants fCj and K 2 of... 

Note that the approximation in the last step is only accurate in ocean waters with pH equal to or greater than 8.0. This is seen in Table 4.4, where COM and CO2 concentrations are evaluated in different water masses.) The above approximation indicates that addition of more DIC than Ac to the water results in a decrease in carbonate ion concentration (AAc -ADIC = ACOi ). Essentially more add, in the form of CO2 than base in the form of CO3 is added to the water during the solubilization of particulate matter. These trends are borne out in Table 4.4, where the actual carbonate species changes are calculated by using the complete carbonate equilibrium equations. 

Model geometry and parameters are compiled in Fig. 11.2. The strategy is to determine the concentrations of alkalinity and DIG in the surface ocean layer from the equations that describe the model d5mamics and the deep ocean values, and then to calculate the/coj in thermod5mamic equilibrium with these values at the mean surface water temperature and salinity. Garbonate plus borate alkalinity, Ac b. will be employed in our carbonate equilibrium calculations. (See Ghapter 6 for a discussion of the carbonate equilibrium calculations.)... 

Investigations of the state of carbonate equilibrium of water which is transiting the vadose zone have not been numerous. The equilibrium COj (carbon dioxide partial pressure) and state of saturation with respect to calcite was studied by Holland et al. (1964) in Indian Echo Cave, Pennsylvania, and Luray Caverns, Virginia, and by Thrailkill (1971) in Carlsbad Caverns, New Mexico. These investigations showed that seepage water entering the cave was in equilibrium with a f cO2 much higher than that of the normal atmosphere and was often supersaturated with respect to calcite. 

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