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Example 4.-- Evaporation and Homogeneous Redox Reactions

Evaporation is accomplished by removing water from the chemical system. Water can be removed by two methods: (1) water can be specified as an irreversible reactant with a negative reaction coefficient in the REACTION keyword input, or (2) "H2O" can be specified as the alternative reaction in EQUILIBRIUM_PHASES keyword input, in which case, water is removed or added to the aqueous phase to attain a specified saturation index for a pure phase. This example uses the first method, the REACTION keyword data block is used to simulate concentration of rain water by approximately 20 fold by removing 95 percent of the water. The resulting solution contains only about 0.05 kg of water. In a subsequent simulation, the MIX keyword is used to generate a solution that has the same concentrations as the evaporated solution, but has a total of mass of water of approximately 1 kg.

The first simulation input data set (table 9) contains four keywords: (1) TITLE is used to specify a description of the simulation to be included in the output file, (2) SOLUTION is used to define the composition of rain water from central Oklahoma, (3) REACTION is used to specify the amount of water, in moles, to be removed from the aqueous phase, and (4) SAVE is used to store the result of the reaction calculation as solution number 2.

Table 9. Input data set for example 4

TITLE Example 4a.--Rain water evaporation
SOLUTION 1  Precipitation from Central Oklahoma
        units           mg/L
        pH              4.5   # estimated
        temp            25.0
        Ca              .384
        Mg              .043
        Na              .141
        K               .036
        Cl              .236
        C               .1      CO2(g)  -3.5
        S(6)            1.3
        N(-3)           .208
        N(5)            .237
        H2O     -1.0
        52.73 moles
SAVE solution 2
        -si      false
TITLE Example 4b.--Factor of 20 more solution
        2       20.
SAVE solution 3
All solutions defined by SOLUTION input are scaled to have exactly 1 kg (approximately 55.5 mol) of water. To concentrate the solution by 20 fold, it is necessary to remove approximately 52.8 mol of water (55.5 x 0.95).

The second simulation uses MIX to multiply by 20 the number of moles of all elements in the solution, including hydrogen and oxygen. This procedure effectively increases the total mass (or volume) of the aqueous phase, but maintains the same concentrations. The resulting solution is stored in solution 3 with the SAVE keyword. Solution 3 will have the same concentrations as solution 2 (from the previous simulation) but will have a mass of water of approximately 1 kg.

Selected results of the simulation are presented in table 10. The concentration factor of 20 is reasonable in terms of a water balance for the process of evapotranspiration in central Oklahoma (Parkhurst, Christenson, and Breit, 1993). However, the PHREEQC evaporation modeling assumes that evapotranspiration has no affect on the ion ratios. This assumption has not been verified and may not be correct. After evaporation, the simulated solution composition is still undersaturated with respect to calcite, dolomite, and gypsum. As expected, the mass of water decreases from 1 kg in rain water (solution 1) to approximately 0.05 kg in solution 2 after water was removed by the reaction. In general, the amount of water remaining after the reaction is approximate because water may be consumed or produced by homogeneous hydrolysis reactions, surface complexation reactions, and dissolution and precipitation of pure phases. The number of moles of chloride (mmol) was unaffected by the removal of water; however, the concentration of chloride (mmol/kg water) increased because the amount of water decreased. The mixing simulation increased the mass of water and the number of moles of chloride by a factor of 20. Thus, the number of moles of chloride increased, but the concentration is the same before (solution 2) and after the mixing simulation (solution 3) because of the increased mass of water.

Table 10. Selected results from example 4

An important point about homogeneous redox reactions is illustrated in the results of these simulations (table 10). Reaction calculations always produce redox equilibrium. The rain water analysis contained data for both ammonium and nitrate, but none for dissolved nitrogen. Although nitrate and ammonium should not coexist at thermodynamic equilibrium, the speciation calculation allows redox disequilibria and the concentrations of the nitrogen species are defined only by the input data. In the reaction (evaporation) step, redox equilibrium is attained for the aqueous phase, which caused ammonium to be oxidized and nitrate to be reduced, generating dissolved nitrogen. The equilibrium solution (solution 2) contains nitrate and dissolved nitrogen, but virtually no ammonium (table 10). This redox equilibration will occur in the reaction calculation because of the inherent redox disequilibrium in the definition of the rain water composition. Nitrogen redox reactions would have occurred even if the REACTION keyword had specified that no water was to be removed.

Table 9. Input data set for example 4
Table 10. Selected results from example 4

User's Guide to PHREEQC - 07 MAY 96
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