EVAPORATING SEAWATER AND PRECIPITATING SALTS

The extended Debye-Hueckel/complexation theory works fine for calculating activity coefficients in dilute solutions and in concentrated brines without (much) SO4.
However, if seawater evaporates, the activities of the solutes in the brine and the salts that precipitate must be calculated with Pitzer equations. A sequence of calcite, sulfates and chlorides will then be calculated to precipitate.
Running the PHREEQC file sea_evap.txt shows the composition changes.
First, calcite precipitates, followed by gypsum, halite and various chlorides and sulfates. Gypsum transforms into anhydrite when the activity of H2O is reduced by the high salt concentrations.


Note the divergence of the solutes that together form a salt. The SO4 concentration is higher than the Ca concentration in seawater. Thus, if gypsum precipitates, SO4 increases while Ca decreases as explained in the gypsum example. The same reasoning explains the behavior of Na and Cl upon precipitation of NaCl. The composition of the various salts listed in the legend can be found in PITZER.DAT.
PHREEQC 2.16 can calculate the density and electric conductivity of the solution, see the density example.

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