Seawater Freezing

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 

To experimentally validate the Gitterman model, we prepared an artificial seawater sample that had the composition of seawater s liquid partially frozen down to — 23 °C (Marion et al. 1999). To this sample we added an excess of mirabilite crystals to ensure an adequate sulfate source. The sample was then placed in a — 26 °C temperature-controlled bath and allowed to equilibrate with periodic sampling and analyses over a 12-week period. The precipitation of hydrohalite between — 23 °C and — 26 °C (Fig. 3.16) led to an initial decrease in the sodium molality. Magnesium, on the other hand, was conserved in the solution phase, as ice formed and hydrohalite precipi- 

In our assessment of the more stable freezing pathway, we must distinguish between more stable and natural. Natural crystallization processes 

An aspect of seawater freezing that will play a role in our discussion of a snowball Earth (Sect. 5.1.3) is the quantity of water that remains unfrozen at subzero temperatures. For validation of this facet of the model, see Fig. 3.17. For a seawater system starting with 1.0kg water at 0°C, ice starts 

Although there are many similarities in evaporating and freezing of seawater (Fig. 5.4), these processes generally produce different suites of minerals. 

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Fig. 3.16. a Ringer-Nelson-Thompson and b Gitterman pathways for seawater freezing. Symbols represent experimental data lines are model calculations. Reprinted from Marion and Farren (1999) with permission... 

Fig. 3.17. Amount of unfrozen water during seawater freezing. Experimental measurements are from Richardson (1976). Reprinted from Marion and Grant (1994) with permission...

Fig. 5.2. Salt precipitation during seawater freezing to the eutectic temperatures along the Ringer-Nelson-Thompson and Gitterman pathways Reprinted from Marion et al. (1999) with permission...

Program listings and FORTRAN codes for various version of the FREZCHEM model (versions 5.2 to 9.2) are available from the senior author (giles.marion dri.edu http //frezchem.dri.edu). In this appendix, we first describe the data input then we examine four output files that deal with (1) seawater freezing, (2) strong acids, (3) gas hydrates, and (4) a pressure application. But before discussing these examples, there are some limitations that the user needs to be aware of. 

Table A.2 is model output for seawater freezing at 253.15 K. Beneath the title, the output includes temperature, ionic strength, density of the solution (p), osmotic coefficient amount of unfrozen water, amount of ice, and pressure on the system. Beneath this line are the solution and gaseous species in the system. The seven columns include species identification, initial concentration, final (equilibrium) concentration, activity coefficient, activity, moles in the solution phase, and mass balance. The mass balance column only contains those components for which a mass balance is maintained. The number of these components minus 1 is generally the number of independent components in the system (in this case, 8 — 1 = 7). The mass balances (col. 7) should equal the initial concentrations (col. 2). This mass balance comparison is a good check on the computational accuracy.

Herut B, Starinsky A, Katz A, Bein A (1990) The role of seawater freezing in the formation of subsurface brines. Geochim Cosmochim Acta 54 13-21 Hoffman PF, Kaufman AJ, Halverson GP, Schrag DP (1998) A Neoprotero-zoic snowball Earth. Science 281 1342-1346 Hoffman PF, Schrag DP (2000) Snowball Earth. Sci Am, January 2000, pp 68-75... 

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. 

Herut B., Starinski A., Katz A., and Bein A. (1990) The role of seawater freezing in the formation of subsurface brines. Geochim. Cosmochim. Acta 54, 13-21. 

Yaqing W., Xiaobai C., Guanglan M., Shaoqing W., and Zhenyan W. (2000) On changing trends of dD during seawater freezing and evaporation. Cold Reg. Sci. Technol. 31, 27-31. 

Desalination by freezing has also been under development for a number of years, but it has not yet become commercially feasible. This method is based on the fact that when an aqueous solution (in this case, seawater) freezes, the solid that separates from solution is almost pure water. Thus, ice crystals from frozen seawater at desalination plants could be rinsed off and thawed to provide usable water. The main advantage of freezing is its low energy consumption, as compared with distillation. The heat of vaporization of water is 40.79 kJ/mol, whereas that of fusion is only 6.01 kJ/mol. Some scientists have even suggested that a partial solution to the water shortage in Cafifomia... 

Fish blood has an osmotic pressure equal to that of seawater. If seawater freezes at — 2.3°C, what is the osmotic pressure of the blood at 25°C To solve this problem, what as-surt5)tions must be made ... 

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