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# Example 7.-- Gas-Phase Calculations

This example demonstrates the capabilities of PHREEQC to model the evolution of gas compositions in equilibrium with an aqueous phase under the conditions of either fixed pressure or fixed volume of the gas phase. For a fixed-pressure gas phase, when the sum of the partial pressures of the component gases exceeds the specified pressure of the gas phase, a gas bubble forms. Once the bubble forms, the volume and composition of the gas bubble vary with extent of reactions. For a fixed-volume gas phase, the aqueous solution is in contact with a head space of fixed volume. The gas phase always exists in this head space and the pressure and composition of the gas phase vary with extent of reactions.

Gas-liquid reactions can be modeled in three ways with PHREEQC: (1) a gas can react to maintain a fixed partial pressure using EQUILIBRIUM_PHASES data block, (2) a fixed-pressure, multicomponent gas phase can be modeled using the GAS_PHASE data block with the -fixed-pressure identifier (default), or (3) a fixed-volume, multicomponent gas phase can be modeled using the GAS_PHASE data block with the -fixed-volume identifier. Conceptually, an infinite gas reservoir is assumed for the fixed-partial-pressure approach, as in the atmosphere or sometimes in the unsaturated zone. The partial pressure of the gas component in the reservoir does not change regardless of the extent of reactions. If the gas reservoir is finite and the pressure on the gas phase is constant, as in gas bubbles in estuarine and lake sediments, then a fixed-pressure gas phase is appropriate. If the gas reservoir is finite and the volume that the gas phase fills is constant, as in an experiment with a fixed head-space, then a fixed-volume gas phase is appropriate.

In this example, the GAS_PHASE data block is used to model the decomposition of organic matter in pure water under fixed-pressure and fixed-volume conditions, with the assumption that carbon, nitrogen, hydrogen, and oxygen are released in the stoichiometry CH 2 O(NH 3 ) .07 by the decomposition reaction. With no other electron acceptors available in pure water, the pertinent microbiological decomposition reaction is methanogenesis. The carbon and nitrogen released by organic decomposition are assumed to react to redox and gas-solution equilibrium. Aqueous carbon species are defined in SOLUTION_MASTER_SPECIES or SOLUTION_SPECIES of the default databases for two valence states, carbon(+4) and carbon(-4) (methane); no intermediate valence states of carbon are defined. Aqueous nitrogen may occur in the +5, +3, 0, and -3 valence states. The gas components considered are carbon dioxide (CO 2 ), methane (CH 4 ), nitrogen (N 2 ), and ammonia (NH 3 ).

Figure 7. --Composition of the gas phase during decomposition of organic matter with a composition of CH 2 O(NH 3 ) 0.07 in pure water under conditions of fixed volume and fixed pressure for the gas phase. [Partial pressure of ammonia gas is less than 10 -7 atmospheres throughout (not shown).]

In the first simulation, the initial water is defined to be a ground water in equilibrium with calcite at a partial pressure of carbon dioxide of 10 -1.5 . Pure water is defined with the SOLUTION data block by using defaults for all values (pH = 7, pe = 4, temperature = 25 o C); calcite and carbon dioxide are defined with EQUILIBRIUM_PHASES; and SAVE is used to save the equilibrated solution (table 26). SELECTED_OUTPUT is used to define a file ( ex7.sel ) to which specified data are written for each calculation. All default printing to the file is suspended with the identifier -reset false. The additional identifiers cause specific data items to be written to the selected-output file for each calculation in each simulation: -simulation, the simulation number; -state, the type of calculation (initial solution, reaction, transport, and others); -reaction, the amount of the reaction added for each calculation (as defined in the REACTION data block); -si, the saturation index of each mineral or log partial pressure of each gas that is specified; and -gas, the moles in the gas phase of each gas component that is specified.

In the second simulation, the organic decomposition reaction with a carbon to nitrogen ratio of approximately 15:1 is added irreversibly to the solution in increments ranging from 1 to 1000 mmol ( REACTION keyword). A gas phase, which initially has no moles of gas components present, is allowed to form if the sum of the partial pressures exceeds 1.1 atm; only CO 2 , CH 4 , N 2 , and NH 3 are allowed in the gas phase ( GAS_PHASE data block). In the third simulation, the same initial solution and reaction are used as in the second simulation. The gas phase initially has no moles of gas components present, but is defined to have a fixed volume of 22.5 L, which is the volume of the fixed-pressure gas phase after reaction of 1.0 mol of organic matter. After 1.0 mol of reaction, both the fixed-pressure and fixed-volume gas phases will have the same pressure, volume, and composition; at all other reaction increments the pressure, volume, and composition will differ between the two gas phases.

Table 26. --Input data set for example 7

```TITLE Example 7.--Organic decomposition with fixed-pressure and
fixed-volume gas phases
SOLUTION 1
EQUILIBRIUM_PHASES 1
Calcite
CO2(g)  -1.5
SAVE solution 1
SELECTED_OUTPUT
-reset false
-file ex7.sel
-simulation     true
-state          true
-reaction       true
-si CO2(g) CH4(g) N2(g) NH3(g)
-gas CO2(g) CH4(g) N2(g) NH3(g)
END
#  Simulation 2: Decomposition of organic matter, CH2O(NH3).07,
#  at fixed pressure of 1.1 atm
USE solution 1
GAS_PHASE 1 Fixed-pressure gas phase
-fixed_pressure
-pressure       1.1
CO2(g)          0.0
CH4(g)          0.0
N2(g)           0.0
NH3(g)          0.0
REACTION 1
CH2O(NH3)0.07     1.0
1. 2. 3. 4. 8. 16. 32 64. 125. 250. 500. 1000. mmol
END
#  Simulation 3: Decomposition of organic matter, CH2O(NH3).07,
#  at fixed volume of 22.5 L
USE solution 1
USE reaction 1
GAS_PHASE 1 Fixed volume gas phase
-fixed_volume
-volume         22.5
CO2(g)          0.0
CH4(g)          0.0
N2(g)           0.0
NH3(g)          0.0
END```

For the fixed-pressure gas phase, a bubble forms when nearly 3 mmol of reaction have been added (fig. 7). Initially the gas is more than 90 percent CH 4 and less than 10 percent CO 2 , with only minor amounts of N 2 and NH 3 (NH 3 partial pressures were less than 10 -7 atm throughout the batch-reaction calculation and are not plotted). The volume of gas produced ranges from less than 1 mL at 3 mmol of reaction to 22.5 L after 1 mol of reaction. After 1 mol of reaction is added, nearly all of the carbon and nitrogen is in the gas phase.

For the fixed-volume gas phase, the gas phase exists for all amounts of organic decomposition (fig. 7). Initially the gas is primarily CO 2 with significant amounts of CH 4 and a small amount of N 2 . As the gas composition evolves, CO 2 and CH 4 partial pressures become nearly equal. The N 2 partial pressure is always an order of magnitude smaller than CO 2 and CH 4 ; NH 3 partial pressures are always small (not shown). The partial pressures of the fixed-volume gas phases are smaller than the fixed pressure gas phase up to 1.0 mol of reaction. If the reaction continued beyond 1.0 mol, the pressure of the fixed-volume gas phase would continue to increase and would be greater than the pressure in the fixed-pressure gas phase, which remains constant. Conversely, the volume of the fixed-pressure gas phase is less than the volume of the fixed-volume gas phase until 1.0 mol of reaction. If the reaction continued beyond 1.0 mol, the volume of the fixed-pressure gas phase would exceed the volume of the fixed-volume gas phase.

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