This example demonstrates the irreversible reaction capabilities of PHREEQC in modeling the oxidation of pyrite. Oxygen (O 2 ) and NaCl are added irreversibly to pure water in five varying amounts (0.0, 1.0, 5.0, 10.0, and 50.0 mmol); the relative proportion of O 2 to NaCl in the irreversible reaction is 1.0 to 0.5. Pyrite, calcite, and goethite are allowed to dissolve to equilibrium and carbon dioxide partial pressure is maintained at 10 -3.5 (atmospheric partial pressure). In addition, gypsum is allowed to precipitate if it becomes supersaturated
Table 19. --Input data set for example 5
TITLE Example 5.--Add oxygen, equilibrate with pyrite, calcite, and goethite.
SOLUTION 1 PURE WATER
pH 7.0
temp 25.0
EQUILIBRIUM_PHASES 1
Pyrite 0.0
Goethite 0.0
Calcite 0.0
CO2(g) -3.5
Gypsum 0.0 0.0
REACTION 1
O2 1.0
NaCl 0.5
0.0 0.001 0.005 0.01 0.05
SELECTED_OUTPUT
-file ex5.sel
-total Cl
-si Gypsum
-equilibrium_phases pyrite goethite calcite CO2(g) gypsum
END
Pure water is defined with SOLUTION input (table 19), and the pure-phase assemblage is defined with EQUILIBRIUM_PHASES input. By default, 10 mol of pyrite, goethite, calcite, and carbon dioxide are present in the pure-phase assemblage; gypsum is defined to have 0.0 mol in the pure-phase assemblage. Gypsum can only precipitate if it becomes supersaturated; it can not dissolve because no moles are initially present. The REACTION data block defines the irreversible reaction that is to be modeled. In this example, oxygen ("O2") will be added with a relative coefficient of 1.0 and NaCl will be added with a relative coefficient of 0.5. The steps of the reaction are defined to be 0.0, 0.001, 0.005, 0.01, and 0.05 mol. The reactants can be defined by a chemical formula, as in this case (O 2 ) or by a phase name that has been defined with PHASES input.Thus, the phase name "O2(g)" or "Halite" from the default database file, could have been used in place of "O2" or "NaCl" to achieve the same result. The number of moles of the element oxygen added to the aqueous phase in each reaction step is equal to the stoichiometric coefficient of oxygen in the formula "O2" (2.0) times the relative coefficient (1.0) times the moles of reaction defined by the reaction step (0.0, 0.001, 0.005, 0.01, or 0.05); the number of moles of chloride added at each step is the stoichiometric coefficient of chlorine in the formula "NaCl" (1.0) times the relative coefficient (0.5) times the moles in the reaction step. SELECTED_OUTPUT is used to write the total concentration of chloride, the saturation index of gypsum, and the total amounts and mole transfers of pyrite, goethite, calcite, carbon dioxide, and gypsum to the file ex5.sel after each equilibrium calculation.
The results for example 5 are summarized in table 20. When no oxygen and sodium chloride are added to the system, a small amount of calcite and carbon dioxide dissolves, and trace amounts of pyrite and goethite react; the pH is 8.28, the pe is low (-4.94) because of equilibrium with pyrite, and gypsum is six orders of magnitude undersaturated (saturation index -6.13). As oxygen and sodium chloride are added, pyrite oxidizes and goethite, being relatively insoluble, precipitates. This reaction generates sulfuric acid, decreases the pH, slightly increases the pe, and causes calcite to dissolve and carbon dioxide to be released. At some point between 10 and 50 mmol of oxygen added, gypsum reaches saturation and begins to precipitate. When 50 mmol of oxygen and 25 mmol of sodium chloride have been added, a total of 9.00 mmol of gypsum has precipitated.