Lake evaporate solute concentration

The coupling between the hydrological forces and chemical processes —H->C—is responsible for transport of solutes, and this coupling is also to some extent reflected in the water-flow-rate dependence of some mineral dissolution reactions. The reverse coupling between chemical forces and hydrological processes—C- H—is seen in such phenomena as chemical density stratification in lakes, evaporative mixing caused by solute concentration increase in a surface water layer, and chemical density-driven water currents. 

This system is represented by a closed basin, made of impermeable rocks and filled in the past by a saline alkaline lake. Water in this case could not permeate downwards but only evaporate, so the deposit develops horizontally, instead of vertically as in the previous occurrence. Here pH and salinity in the fluids tend to increase, giving rise to brines, c.g., basic, alkali-rich solutions. Concentric zones of authigenic minerals are so formed, from an outer and upper ring of little altered glass and clay minerals, to zeolites, analcime and a finally alkali-feldspars. A good example for this type of occurrence is Lake Tecopa, California, where the zeolitic ring is constituted by phillipsitc, clinoptilolite and erionite, followed by the central feldspar zone [36]. 

The evaporation of water from a saturated solution leaves a solution in which the ion concentrations exceed the solubility limit. To return to equilibrium, the salt must precipitate from the solution. Evaporation is used to mine sodium chloride and other salts from the highly salty waters of inland seas such as Great Salt Lake in Utah and Israel s Dead Sea. 

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation. 

In most commercial processes, borax is obtained from lake brines, tincal and colemanite. The primary salt constituents of brine are sodium chloride, sodium sulfate, sodium carbonate and potassium chloride. The percent composition of borax as Na2B40 in brine is generally in the range 1.5 to 1.6%. Borax is separated from these salts by various physical and chemical processes. The brine solution (mixed with mother liquor) is subject to evaporation and crystahzation for the continuous removal of NaCl, Na2C03 and Na2S04, respectively. The hot liquor consists of concentrated solution of potassium salts and borate components of the brine. The insoluble solid particles are filtered out and the liquor is cooled rapidly in continuous vacuum crystallizers under controlled conditions of temperatures and concentrations to crystallize KCl. Cystallization of borax along with KCl from the concentrated liquor must not occur at this stage. KCl is separated from the hquor by filtration. Bicarbonate then is added to the liquor to prevent any formation of sodium... 

The resulting Soln. C is a predominantly NaCl solution similar to terrestrial seawater (Soln. D, Table 5.3). Had we chosen a concentration factor of 600-fold, the agreement would have been even better. In any case, the concentration factor is arbitrary. The point is that simple processes, starting with a dilute Fe-Mg-HC03-rich solution formed by reaction of water with ultra-mafic and mafic rocks, evaporation, and carbonate precipitation, converted the solution into an Earth-like seawater NaCl brine. The Na/Mg ratio of solution C is 9.9, while terrestrial seawater has a Na/Mg ratio of 8.8 (Soln. 5.3D). Note also the similar pH values (8.03 and 8.05, Table 5.3). This solution did not (cannot) evolve into an alkali soda-lake composition as some have hypothesized or assumed for Mars (e.g., Kempe and Kazmierczak 1997 Morse and Marion 1999) because the mass of hypothesized soluble iron and magnesium and the low solubility of their respective carbonate minerals are sufficient to precipitate most of the initial soluble bicarbonate/carbonate ions. 

These processes occur by precipitation through evaporative concentration of a solute in the aqueous medium until its dissolution capacity is exceeded. Then, a solid is formed and deposited either as a sediment or on a nearby surface. These products are called evaporites. A typical example is the deposition and formation of calcium carbonate stalactites and stalagmites. Evaporation is a major process in arid areas and it influences the chemistry of surface waters. That is why in saline lakes, inland seas, or even in estuaries, evaporites of NaCl or NaCl/KCl and deposits of CaS04 and CaC03 are formed. Here, CaS04 generally precipitates first, and then NaCl. 

Von Damm and Edmond (1984) utilized the lakes of the Ethiopian and northern Kenya rift zones to examine reverse weathering (the formation of authigenic clay minerals), because here evaporative concentration had not proceeded to the extent that salt precipitation interfered with a mass balance approach. They found that —60% of an alkalinity deficit could be accounted for by processes other than carbonate precipitation, and concluded that solute magnesium was lost as rapidly to clay as solute calcium was to carbonate. This situation, particularly in volcanic terrain, was also initially recognized at saline Lake Abert, Oregon, by Jones and VanDenburgh (1966). 

Figure 13 TEQUIL model (Moller et aL, 1997) plots illustrating quantities of salts precipitated (in moles) as a function of fixed increment of solution evaporated for four bodies of brine in the Bonneville desert (initial solution chemistry and model output after Kohler, 2002). Note that relative abundances reflect the concentration of the starting fluid, which is appreciably greater for the West Pond and Shallow Brine Aquifer solutions than for the Great Salt Lake or Reynolds North crustal pore fluids. Of most significance is the ratio of sulfate and chloride salts. In this regard, note the similarities between the Great Salt Lake and West Pond brines. In contrast, the greater association of sulfate with calcium in the Reynolds...

The dilute inflow to the East African basins acquires most of its alkalinity by the rapid hydrolysis of volcanic glass and lavas, producing high initial Na, SiOi, and HCO concentrations (Jones et al., 1977). Waters in the region are therefore nearly exclusively of the Na-COa or Na-COa-Cl type. Most of the other solutes are lost to carbonate or silicate precipitation (Jones et al., 1977 Beadle, 1981 Renaut and Tiercelin, 1994). Sulfate in East African waters is often removed from solution during evaporative concentration, probably due to reduction, especially in lake-marginal wetlands (Deocampo and Ashley, 1999). 

Hode of Preparation.—Jets of steam (Jvmerolles or suf-Jioni) charged with boracic acid, HBOg, which issue from the earth in Tuscany, are conducted into lagoni (little lakes), in which the acid condenses. The solution thus formed, after being concentrated by heat, is neutralized with sodium carbonate, and, on evaporation, yields crystals of borax. 

Large lithium reserves are available in South America. A resource of special interest is the dried up salt lake Atacama, 2500 m above sea level in northern Chile. The main component is halite, rock salt, NaCl. In cavities a concentrated salt solution is present, in which the lithium content is as high as 0.15%. This solution is transported to nearby Antofagasta. In a chemical factory there the lithium carbonate is prepared from the chloride. This carbonate is an important export product. Lithium-containing brines are also available in Nevada in the USA. The brines are pumped from the ground through a series of open dams. Through solar evaporation over 12 to 18 months the brine increases its lithium concentration to about 0.6%. Soda is added and lithium carbonate precipitates. 

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