Evaporation of seawater

Since the experimental studies of van t Hoff at the turn of the century, geochemists have sought a quantitative basis for describing the chemical evolution of seawater and other complex natural waters, including the minerals that precipitate from them, as they evaporate. The interest has stemmed in large part from a desire to understand the origins of ancient deposits of evaporite minerals, a goal that remains mostly unfulfilled (Hardie, 1991). 

In a series of papers, Harvie and Weare (1980), Harvie el al. (1980), and Eugster et al (1980) attacked this problem by presenting a virial method for computing activity coefficients in complex solutions (see Chapter 8) and applying it to construct a reaction model of seawater evaporation. Their calculations provided the first quantitative description of this process that accounted for all of the abundant components in seawater. 

To reproduce their results, we trace the reaction path taken by seawater at 25 °C as it evaporates to desiccation. Our calculations follow those of Harvie et al. (1980) and Eugster et al. (1980), except that we employ the more recent Harvie-Mpller-Weare activity model (Harvie et al., 1984), which accounts for bicarbonate. We include an HCO3 component in our calculations, assuming that the fluid as it evaporates remains in equilibrium with the CO2 in the atmosphere. 

In a first calculation, we specify that the fluid maintains equilibrium with whatever minerals precipitate. Minerals that form, therefore, can redissolve into the brine as evaporation proceeds. In react, we set the Harvie-Moilcr-Weare model and specify that our initial system contains seawater 

The dump command serves to eliminate the small mineral masses that precipitate when, at the onset of the calculation, the program brings seawater to its theoretical equilibrium state (see Chapter 6). The delxi and dxplot commands serve to set a small reaction step, assuring that the results are rendered in sufficient detail. 

6 The state (log Q/K) of React. 18.7 between gaylussite and aragonite at 0°C ( ) and 25°C ( ), showing which mineral is favored to form at the expense of the other. 

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The starting material for all industrial chlorine chemistry is sodium chloride, obtained primarily by evaporation of seawater. The chloride ion is highly stable and must be oxidized electrolytically to produce chlorine gas. This is carried out on an industrial scale using the chlor-alkali process, which is shown schematically in Figure 21-15. The electrochemistry involved in the chlor-alkali process is discussed in Section 19-. As with all electrolytic processes, the energy costs are very high, but the process is economically feasible because it generates three commercially valuable products H2 gas, aqueous NaOH, and CI2 gas. 

In a second example of a flow-through path, we model the evaporation of seawater (Fig. 2.6). The equilibrium system in this case is a unit mass of seawater. Water is titrated out of the system over the course of the path, concentrating the seawater and causing minerals to precipitate. The minerals sink to the sea floor as they... 

Table 24.3. Minerals formed during the simulated evaporation of seawater...

Fig. 24.7. Volumes of minerals precipitated during a reaction model simulating the evaporation of seawater as an equilibrium system at 25 °C, calculated using the Harvie-Mpller-Weare activity model. Abbreviations Ep = Epsomite, Hx = Hexahydrite.

Harvie, C.E., J. H. Weare, L. A. Hardie and H.P. Eugster, 1980, Evaporation of seawater, calculated mineral sequences. Science 208,498-500. 

The purpose of this exercise is to learn how to draw a probability ellipse from the mean values and covariance matrix, a topic to be further developed in Chapter 4. Na and Cl are incorporated into clouds during evaporation of seawater and are therefore strongly correlated. 

Sediments formed by the abiogenic precipitation of solutes from seawater are termed hydrogenous. Unequivocal examples of hydrogenous sediments are ones formed from the evaporation of seawater. The minerals deposited are collectively called evaporites and are the subject of Chapter 17. Others form with the assistance, to varying degrees, of marine microbes. For example, bacteria seem to play a role in the formation of Fe-Mn nodules and crusts. Some hydrogenous minerals, such as barite, celestite, glauconite, and francolite, are produced from the precipitation of elements... 

Evaporite A suite of minerals formed as a result of the evaporation of seawater. 

Sodium chloride is produced by solar evaporation of seawater or brine from underground salt deposits. It also is produced by mining rock salt. The commercial product contains small amounts of calcium and magnesium chlorides. 

Only about 10% of current world salt production comes from evaporation of seawater. Most salt is obtained by mining the vast deposits of halite, or rock salt, formed by evaporation of ancient... 

Seawater is the most abundant aqueous solution on Earth, and, as a consequence, has been the subject of countless studies (Millero 2001). The occurrence of marine evaporites has spurred much of the work on seawater solidification, which can occur by either evaporation or freezing. Early experimental work on the evaporation of seawater was done by Usiglio (1849). Whereas most evaporites on Earth probably involved mainly or only evaporation (particularly under hot, arid climatic conditions, 25-50 °C), cold-climate evaporation and freezing without evaporation are also common means by which... 

Manufacture from bitterns or mother liquors from the solar evaporation of seawater for salt. 

Special attention is being given to mineral extraction from bitterns formed by solar evaporation of seawater in arid waters or by seawater freezing in the Arctic areas. Such bitterns represent approximately 30-fold concentrated seawater with depleted concentrations of sodium, calcium, chloride and sulfate ions. 

It is generally agreed that most of the chloride in basinal brines has been derived from some combination of the subsurface dissolution of evaporites (e.g., Kharaka et al., 1985 Land, 1997) and the entrapment and/or infiltration of evaporated seawater (e.g.. Carpenter, 1978 Kharaka et al., 1987 Moldovanyi and Walter, 1992). Dissolution of halite produces waters dominated by sodium chloride. Evaporation of seawater produces waters having the general trends shown for ion-Br (Figure 5), Na-Cl (Figure 3) and Ca-Cl (Figure 4), but most formation waters have neither the cation (nor anion) composition of an... 

Although bromide and chloride are both monovalent anions of similar ionic radii (Br = 1.96 A, Cl = 1.81 A), Cl is strongly preferentially partitioned over Br into sodium, potassium, and magnesium halogen salts during precipitation (Hanor, 1987 Siemann and Schramm, 2000). During the initial evaporation of seawater, both bromide and chloride increase in concentration in the residual hypersaline waters, and the Br/Cl ratio of these waters does... 

Brines formed by subaerial evaporation of seawater should, in theory, have elevated Br/Cl ratios. Brines formed by the dissolution of halite should have low Br/TDS (Rittenhouse, 1967) and Br/Cl ratios (Carpenter, 1978 Kharaka et al., 1987). Brines representing these end-members and mixtures of these and/or meteoric and/or connate marine waters have been identihed in... 

Figure 9 Concentration trends of chloride versus bromide during the evaporation of seawater showing the initial precipitation point of evaporite mineral phases and possible mixing scenarios (after Matray, 1984). Also plotted are the groundwater data found in Table 2 for selected crystalline rock areas.

Pseudomorphs after halite are common throughout the McArthur Group. The halite appears to have formed by almost complete evaporation of seawater in shallow marine environments and probably represents ephemeral salt crusts. The general lack of association of halite and calcium sulfate minerals in these sediments probably resulted in part from the dissolution of previously deposited halite during surface flooding, but also indicates that evaporation did not always proceed beyond the calcium sulfate facies. 

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