Carbon dioxide calcium carbonate dissolution

Walter and co-workers (Walter and Burton, 1990 Walter et al., 1993 Ku et al., 1999) have made extensive efforts to demonstrate the importance of dissolution of calcium carbonate in shallow-water carbonate sediments. Up to — 50% carbonate dissolution can be driven by the sulfate reduction-sulfide oxidation process. In calcium carbonate-rich sediments there is often a lack of reactive iron to produce iron sulfide minerals. The sulfide that is produced by sulfate reduction can only be buried in dissolved form in pore waters, oxidized, or can diffuse out of the sediments. In most carbonate-rich sediments the oxidative process strongly dominates the fate of sulfide. Figure 6 (Walter et al., 1993) shows the strong relationship that generally occurs in the carbonate muds of Florida Bay between total carbon dioxide, excess dissolved calcium (calcium at a concentration above that predicted from salinity), and the amount of sulfate that has been reduced. It is noteworthy that the burrowed banks show much more extensive increase in calcium than the other mud banks. This is in good agreement with the observations of Aller and Rude (1988) that in Long Island Sound siliciclas-tic sediments an increased bioturbation leads to increased sulfide oxidation and carbonate dissolution. 

The effect of pH on the corrosion of zinc has already been mentioned (p. 4.170). In the range of pH values from 5 -5 to 12, zinc is quite stable, and since most natural waters come within this range little difficulty is encountered in respect of pH. The pH does, however, affect the scale-forming properties of hard water (see Section 2.3 for a discussion of the Langelier index). If the pH is below the value at which the water is in equilibrium with calcium carbonate, the calcium carbonate will tend to dissolve rather than form a scale. The same effect is produced in the presence of considerable amounts of carbon dioxide, which also favours the dissolution of calcium carbonate. In addition, it is important to note that small amounts of metallic impurities (particularly copper) in the water can cause quite severe corrosion, and as little as 0-05 p.p.m. of copper in a domestic water system can be a source of considerable trouble with galvanised tanks and pipes. 

FIGURE 44 Weathering. A weathered sandstone column. Calcite (composed of calcium carbonate) is dissolved by rain and groundwater (see Textbox 73). When stone in which calcite is a main component as, for example, sandstone, limestone, and marble, is in contact with water for long periods of time, it is weathered and partly or entirely dissolved. Pollutants such as sulfur dioxide are fundamental in accelerating the weathering and dissolution process. When sulfur dioxide, for example, dissolves in rainwater, it forms sulfuric acid, a strong acid that, at ambient temperatures, rapidly dissolves calcium carbonate. 

To convert calciiun carbonate to dolomite, some of the calcium must have been replaced by magnesiiun, requiring the partial dissolution of the carbonate. This process is promoted by contact with acidic pore water, such as occurs in organic-rich sediments because remineralization produces carbon dioxide. This is probably why dolomites are presently forming in detrital algal mats buried beneath sabkhat. The restricted extent of these modern dolomites reflects a kinetic hindrance to precipitation. Apparently dolomite precipitation in this setting is too slow to form substantial deposits when sea level is rapidly fluctuating. 

The fundamental reason for using lime-soda softening processes is to reduce the temporary hardness (carbonate hardness) content of the raw water in order to minimize risks of carbonate scaling in the user s cooling systems. Often some of the permanent hardness (noncarbonate hardness) is also removed, as is some silica. The principal temporary hardness salt is calcium bicarbonate, formed by dissolution of limestone (calcium carbonate) by water containing dissolved carbon dioxide. 

And finally, appendices provide a user s guide for the FREZCHEM model and tables of model parameters. Version 9.2 of this model includes the precipitation-dissolution of chloride, nitrate, sulfate, and bicarbonate-carbonate salts of calcium, magnesium, sodium, potassium, and ferrous iron. This version also contains strong acid chemistries (hydrochloric, nitric, and sulfuric), gas hydrate chemistries (carbon dioxide and methane), and tem-perature/pressure dependencies. Electronic copies of the FORTRAN code are available from the senior author (giles.marion dri.edu). 

A typical chemical reaction involved in weathering is the dissolution of calcium carbonate (limestone) by water containing dissolved carbon dioxide ... 

The balance between calcium carbonate production and dissolution is the major pH buffering mechanism of seawater over periods of time at least on the order of thousands of years ( ). The atmospheric carbon dioxide reservoir is less than 2 percent the size of the seawater reservoir ( ) and there is active exchange between these two reservoirs across the air-water interface. Consequently, the carbon dioxide content of the atmosphere and accumulation of calcium carbonate in the deep oceans are closely coupled. 

As a consequence of the realization that dissolution of magnesium- and calcium-containing sihcates controls carbon dioxide concentrations in the atmosphere over geologic time (e.g.. Equations (l)-(2)) and that such reactions may be important for subsurface carbon sequestration over human timescales, researchers have also become interested in the effect of dissolved inorganic carbon on dissolution. The indirect effect of CO2 species on dissolution of... 

Dissolution of feldspars is a logical source of dissolved silica, calcium, sodium, and potassium in groundwater. Similarly, the reaction of carbon dioxide-charged water with silicate minerals is a logical source of bicarbonate. Rogers (1987) examined these and other hypotheses using a mass-balance approach. In these calculations, chloride and sulfate were not considered, and the beginning concentrations were considered to be... 

Dissolution and hydration of lime or limestone in an acid medium containing carbon dioxide from the flue gas to form calcium and bicarbonate ions,... 

Stoichiometry and Solids Dissolution. One thousand cu ft of gas at standard conditions contains 3.16 g moles of sulfur dioxide when the sulfur dioxide content of the gas is 2500 ppm. At a liquid-to-gas ratio of 50 gal/standard 1000 cu ft, 16.7 mmoles of basic species/1. are required to react with this amount of sulfur dioxide. Very few of the simulated solutions in Tables III and IV attained this basicity except under extreme conditions of the variables, conditions unlikely to be controlled consistently in lime or limestone scrubbing. Consequently, under most conditions, additional basic species must enter the liquid phase in the scrubber to neutralize the dissolving gas. These species come from the dissolution of calcium carbonate or calcium sulfite in the scrubbing tower. The amount of solids dissolution required to achieve stoichiometry is reduced greatly by the presence of large amounts of magnesium in solution. 

The process is complex and involves simultaneous dissolution of calcium hydroxide and carbon dioxide, and crystallisation of calcium carbonate. Carbonation is generally carried out in a series of reactors under closely controlled pH, temperature and degree of supersaturation, to produce the required PCC morphology and particle size distribution (see section 31.2.3). Crystallisation can occur on the surface of the calcium hydroxide particles (producing scalenohedral crystals), in the aqueous phase (producing rhombohedral crystals) and at the gas-liquid interface. 

Figure 22.15 illustrates the dissolution of calcium carbonate by carbon dioxide. The apparent paradox is solved when we observe that the addition of a stoichiometric... 

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