The SOLUTION data block defines a sodium chloride solution that has 0.1 mmol/kgw ferrous iron (Fe_di) and is in equilibrium with atmospheric oxygen. Partial input data set for example 9 TITLE Example 9.-Kinetically controlled oxidation of ferrousĤ0 moles = (2.91e-9 + 1.33e12 * (ACT("OH-"))^2 * p_o2) * Fe_di * TIME A few of the transcriptions are shown in table 28, which gives the partial input file for this example. The master species for these elements are defined to be Fe_di+2 and Fe_tri+3, and all solution species, phases, exchange species, and surface species must be rewritten using these new elements and master species. Two new "elements" are defined in SOLUTION_MASTER_SPECIES-"Fe_di", which corresponds to Fe(2), and "Fe_tri", which corresponds to Fe(3). The calculation requires the uncoupling of equilibrium among the Fe(2) and Fe(3) species. kgw at pH = 7.0 through which air is bubbled the change in solution composition over time is calculated. This example models a reaction vessel with 10 mmol NaCl / kgw and 0.1 mmol FeCl The rate equation is highly non-linear in an unbuffered solution and must be integrated numerically. , the oxidation rate rapidly diminishes as pH decreases. Because the rate has quadratic dependence on the activity of OH However, FeĪnd it may also precipitate as iron oxyhydroxides, so that pH decreases during oxidation. The time for complete oxidation of ferrous iron is a matter of minutes in an aerated solution when pH is above 7.0. Is the total molality of ferrous iron in solution, and In water is given by (Singer and Stumm, 1970): This example illustrates the procedure for decoupling two valence states of an element (iron) and shows how PHREEQC can be used to calculate the kinetic oxidation of Fe Aqueous species that react kinetically must be defined essentially as separate elements with SOLUTION_MASTER_SPECIES. PHREEQC can also calculate kinetic reactions among aqueous species that are normally assumed to be in equilibrium, but this requires that the database be redefined. Kinetic reactions between solids and the aqueous phase can be calculated without any modification of the database. If the tolerance is not satisfied, then the integration over the time interval is automatically restarted with a smaller time interval. A check is performed to ensure that the difference between the 4th- and 5th-order estimates of the integrated rate over a time interval does not vary by more than a user-defined tolerance. Equilibrium is calculated for all solution-species, and for all exchange, equilibrium-phase, solid-solutions, surface assemblages and gas phases that have been defined. Equilibrium is calculated before a kinetic calculation is initiated and again when a kinetic reaction increment is added. The rate expressions are integrated with an embedded 4th- and 5th-order Runge-Kutta-Fehlberg algorithm. For transport calculations ( ADVECTION or TRANSPORT), kinetic reactions can be defined cell by cell by the number range following the KINETICS keyword ( KINETICS The rate expressions can be used in batch-reaction or transport calculations by using the KINETICS data block. Kinetic rate expressions can be defined in a completely general way in PHREEQC using Basic statements in the RATES data block. Kinetic Oxidation of Dissolved Ferrous Iron with Oxygen