Dissolution of CO2 in Water

The simple models of cation exchange presented here have not yet considered as a competing cation, even though it is always present in clay-water systems. cations are produced by water dissociation or, more importantly (because carbonic acid is more acidic than water), by the dissociation of carbonic acid formed from the dissolution of CO2 in water ... 

The dissolution of CO2 in water and the dissociation of carbonic acid can be expressed by the following equations ... 

Different from most gas hydrate formers, CO2 is not hydrophobic. At 273 K, the solubility of CO2 in water varies between 0.08 and 1.46 mol/L with the partial pressure of CO2 between 0.1 and 3.4 MPa [14]. The dissolution of CO2 in water produces species including dissolved unhydrated CO2, hydrated CO2, H2CO3, HCO3-, and CO. ... 

Figure 13.4 shows the overall scheme of these reactions, without the acid H2CO3 being shown. In this formulation, the vertical arrow represents the dissolution of CO2 in water. This is seen as a physical equilibrium (not achemical one) and is described by Henry s law. The two horizontal arrows represent the first and the second ionizations of carbonic acid (= dissolved CO2 + H2O), which are seen as chemical reactions. Ions have almost no vapor pressure they do not exist in the gas phase (except at flame temperatures). The carbon-containing species can leave the liquid only by forming dissolved CO2, which has a vapor pressure. 

Addition or removal of CO2 in an aqueous environment is regulated by life processes, primarily by respiration and photosynthesis. Respiration increases the amount of CO2, causing a drop in pH and CO3 concentration and therefore a higher solubility of CaCOs, thus promoting carbonate dissolution. Photosynthesis decreases the amount of CO2 in waters causing an increase in pH and COl" concentration (and therefore a lower solubility of CaC03) and thus promotes carbonate precipitation. 

Solute depression of the melting point can also occur due to the dissolution of gases in water. Ordinarily the solubility of ordinary gases is so small that this depression is not easily measurable. However, some gases, such as CO2, have a far higher solubility in water and solute depression can be easily measured with a thermistor. 

The presence of CO2 influences the pH of an aqueous phase, due to formation mid dissociation of carbonic acid. The resulting pH is equal to values of about 2.9 at 200 bar and 40°C (9). The dissolution of metals in water determines die extent of leaching and extraction. To study the pH... 

Besides the self-dissociation of water one should take into account the presence of traces of ionic impurities, such as the HCOs ions coming from the dissolution of CO2 in the water used to prepare the solution. Because it is very difficult to make a precise estimation of the contribution of these impurities at high temperature and pressure, it is convenient to measure the conductivity of the water used to prepare the solution under the same conditions as the solution measurements. The solvent conductivity is discounted from the specific conductivity of the solution in order to obtain the real conductivity of the electrolyte. 

The investigated spray process is based on the dissolution of supercritical carbon dioxide (SCCO2) into the liquid which shall be atomized. Therefore, CO2 is one of the central substances within this project. The carbon dioxide is provided by YARA with a purity of 99.9% (v/v). The critical temperature of CO2 is 304.13 K the critical pressure amounts 7375 MPa. The solubility of CO2 in water is in the order of 2-5 wt.% at 313 K and a pressure range of 2-15 MPa [5]. The solubility of CO2 in polymers is in the range of 10 wt.% at 353 K and 14 MPa [6]. 

When the above reaction is reversed and CO2 is lost from the water, calcium carbonate deposits are formed. The concentration of CO2 in water determines ihe extent of dissolution of calcium carbonate. The carbon dioxide that water may gain by... 

The relevant equations to describe the dissolution of calcium carbonate in rainwater are the solubility product expression for CaCOs and two equations describing the ionization of CO2 in water (that is, carbonic acid). The three equations are as follows ... 

Data on the solubihty of magnesium hydroxide in water are not all in agreement, but the solubihty is extremely low. The extent of Mg(OH)2 solubihty is 10 mg/L, which is about 1/100 the solubihty of Ca(OH)2. In concentrated solutions of NH Cl and NH CO, the solubihty of Mg(OH)2 is markedly increased, but in no instance does its solubihty equal that of MgCO in water heavily permeated with CO2. Dolomitic hydrates are slightly less soluble than high calcium hydrates, but much nearer the latter in value than Mg(OH)2, because the presence of MgO and Mg(OH)2 does not impede the dissolution of its Ca(OH)2 constituent. 

An increase in seawater alkalinity (for example, by the dissolution of CaCOa) would decrease pC02 in seawater and decrease the Revelle factor (10), Thus CaCOa dissolution would provide a a strong negative feedback in response to an increased level of CO2 in the atmosphere and ocean. However, the surface water of temperate and tropical oceans is supersaturated with respect to CaCOa by several fold. It is not likely that the dissolution of CaCOa would provide a negative feedback to the air-sea CO2 transfer process in the near future. 

The formation and dissolution of CaCOa in the ocean plays a significant role in all of these effects (34)- CaCOa is produced by marine organisms at a rate several times the supply rate of CaCOa to the sea from rivers. Thus, for the loss of CaCOa to sediments to match the supply from rivers, most of the CaCOa formed must be redissolved. The balance is maintained through changes in the [COa] content of the deep sea. A lowering of the CO2 concentration of the atmosphere and ocean, for example by increased new production, raises the [COa] ion content of sea water. This in turn creates a mismatch between CaCOa burial and CaCOa supply. CaCOa accumulates faster than it is supplied to the sea. This burial of excess CaCOa in marine sediments draws down the [COa] - concentration of sea water toward the value required for balance between CaCOa loss and gain. In this way, the ocean compensates for organic removal. As a consequence of this compensation process, the CO2 content of the atmosphere would rise back toward its initial value. 

The terrestrial weathering of organic matter derived from shales and soils results in the oxidation of carbon, which generates CO2. Dissolution of this CO2 in water produces carbonic acid. This weak acid serves to enhance chemical weathering reactions... 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

Experimental investigations of carbon dioxide mineral trapping have been started, but there is little understanding of the processes involved on the molecular level, due to the complex nature of the brine and the physical conditions present in the brine aquifers. One such complexity issue is the dissolution of CO2 from the gaseous phase into aqueous solution. This process is thermodynamically unfavorable with a ArG° value of 2.00 kcal/mol, in pure water at STP [3]. This becomes even more thermodynamically unfavorable as salts are introduced into the solution. Figure 17.1 shows how the solubility of CO2 in aqueous solution is also dependent upon the salt concentration and salt composition, even from simple salt solutions. According to Fig. 17.1, there is no correlation with size and the ability for the solution to uptake the C02. 

In Figure 2, the interfacial tension of coffee oil with a high content of volatile flavours against CC>2 is depicted. Mixtures like this are of particular interest for high pressure spray extraction. At increasing density of the fluid CO2 -phase, interfacial tension is decreased by dissolution of CO2 at the interface. In this case, presence of surface active material in the liquid phase, e.g. proteins, rather seem to be of subordinate importance. With respect to foam formation these surfactants neither show their known stabilising effect as long as no polar phase such as water is added. 

At high pH values some anionic ligands like EDTA4- and NTA3- compete with hydroxyl and carbonate ions (usually present in water due to the dissolution of CO2 from the air) for the metal ions existing in solution. On the other hand, at low pH it is the metals and the protons that compete for the ligand. 

The amount of dissolved CO2 in water can be obtained from Henry s law, but its dissolution behavior is more complex than that of O2 because C02 reacts with water to form carbonic acid. This is a weak acid with two dissociation constants that determine the aqueous concentrations of protons, carbonate, and... 

Instead of using volatile organic solvents, it would be beneficial, for compatibility with fuel cells and the environment, to use just water as the solvent. The solubility of AB in water is high, 25 wt.% [14]. Aqueous AB solutions are also relatively stable if kept under an inert gas atmosphere [10, 111]. In air the solution possesses a much lower stability which is believed to result from an increase in acidity in the solution due to the dissolution of CO2 [111]. The stability of the aqueous AB solution is obviously kinetically controlled since the decomposition is thermodynamically favored due to the formation of strong B—O bonds according to ... 

Clearly, there are real differences in ki between experiments. The highest value of ki is estimated from the data of Weyl (9 )y who directed a jet of CO2-saturated water (pH = 3.9) onto the surface of calcite. Weyl found that the rate of solution varied with the jet velocity. His rates imply that kj varies from 0.11 to 0.23 when velocity of the jet increases from 18 to 35 m sec The smallest value of k (.0073) is derived from the data of Tomlnaga et al. (10). These authors rotated a disk of marble in HCl solutions (0.1750 - 0.5317N) at 485 rpm. Rate of dissolution was followed by the volume of CO2 evolved. After an initial period for saturation of the acid with CO2, rate of gas evolved becomes linear in the cumulative amount of CO2 produced. Because the acid concentration decreased as calcite dissolved, we extrapolated the observed linear relation in CO2 production back to the initial condition to estimate Initial rates under known acid concentrations. Correction to pH via the Davies equation leads to the rates shown for these authors in Figure 6. 

Figure 2.7. Gibbs function, system enthalpy, and system entropy variations with the extent of reaction for the dissolution of gaseous CO2 in water C02(g) = C02(a

Equilibrium properties of the C02/sea-water system have been well researched from an experimental standpoint. In particular, the clathrate hydrate forming conditions T < 285K andP>4MPa) are well established. Several experiments have been performed under conditions mimicking the direct injection process and have attempted to study the dissolution rate of CO2 in seawater. Under direct injection conditions, the injected CO2 is in the form of a liquid droplet and a thin spherical shell of CO2 clathrate hydrate of structure I is observed to form around the CO2 drop, separating it from the sea water. The process of hydrate formation has many similarities with that of crystallization, i.e., it can be divided into a nucleation phase and a growth phase. For CO2 clathrates, the nucleation phase involves the formation of a... 

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