This keyword data block is used to define the amount and composition of each surface in a surface assemblage. The composition of a surface assemblage can be defined in two ways: (1) implicitly, by specifying that the surface assemblage is in equilibrium with a solution of fixed composition or (2) explicitly, by defining the amounts of the surfaces in their neutral form (for example, SurfbOH). A surface assemblage may have multiple surfaces and each surface may have multiple binding sites, which are identified by letters following an underscore.
Line 0a: SURFACE 1 Surface in equilibrium with solution 10 Line 1a: -equilibrate with solution 10 Line 2a: Surfa_w 1.0 1000. 0.33 Line 2b: Surfa_s 0.01 Line 2c: Surfb 0.5 1000. 0.33 Line 0b: SURFACE 3 Sites related to pure phase and kinetic reactant Line 1b: -equilibrate with solution 10 Line 3a: Surfc_wOH Fe(OH)3(a) equilibrium_phase 0.1 1e5 Line 3b: Surfc_sOH Fe(OH)3(a) equilibrium_phase 0.001 Line 3b: Surfd_sOH Al(OH)3(a) kinetic 0.001 2e4 Line 4: -no_edl Line 0c: SURFACE 5 Explicit calculation of diffuse layer composition Line 1c: -equilibrate with solution 10 Line 2d: Surfe_w 0.5 1000. 0.33 Line 5: -diffuse_layer 2e-8 Line 6: -only_counter_ions
number --Positive number to designate the following surface assemblage and its composition. A range of numbers may also be given in the form m-n , where m and n are positive integers, m is less than n , and the two numbers are separated by a hyphen without intervening spaces. Default is 1.
number --Solution number with which the surface assemblage is to be in equilibrium. Any alphabetic characters following the identifier and preceding an integer ("with solution" in line 1a) are ignored.
surface binding-site formula --Formula of surface species including stoichiometry of surface site and other elements connected with a pure phase or kinetic reactant. The formula must be charge balanced and is normally the OH-form of the surface binding site. If no elements other than the surface site are included in the formula, then the surface site must be uncharged. If elements are included in the formula, then these elements must be present in the pure phase or kinetic reactant.
name --Name of the pure phase or kinetic reactant that has this kind of surface site. If name is the name of a phase, the moles of the phase in the EQUILIBRIUM_PHASES data block with the same number as this surface number (3, in the example data block) will be used to determine the number of moles of surface sites (moles of phase times sites_per_mole equals moles of surface sites). If name is the rate name for a kinetic reactant, the moles of the reactant in the KINETICS data block with the same number as this surface number (3, in the example data block) will be used to determine the number of surface sites (moles of kinetic reactant times sites_per_mole equals moles of surface sites). Note the stoichiometry of the phase or reactant must contain sufficient amounts of the elements in the surface complexes defined in Line 3. In the example data block, there must be at least 0.101 mol of oxygen and hydrogen per mole of Fe(OH)3(a).
equilibrium_phase or kinetic_reactant--If equilibrium_phase is used, the name on the line is a phase defined in an EQUILIBRIUM_PHASES data block. If kinetic_reactant is used, the name on the line is the rate name for a kinetic reactant defined in a KINETICS data block. Default is equilibrium_phase. Optionally, e or k, only the first letter is checked.
-no_edl--Indicates that no electrostatic terms will be used in the mass-action equations for surface species and no charge-balance equations for the surfaces will be used. The identifiers -no_edl and -diffuse_layer are mutually exclusive and apply to all surfaces in the surface assemblage. Optionally, no_edl, -n[ o_edl], no_electrostatic, -n[ o_electrostatic].
-diffuse_layer--Indicates that the composition of the diffuse layer will be estimated, such that, the net surface charge plus the net charge in the diffuse layer will sum to zero. See notes following the example data block. The identifiers -diffuse_layer and -no_edl are mutually exclusive and apply to all surfaces in the surface assemblage. Optionally, diffuse_layer or -d[ iffuse_layer].
-only_counter_ions--Indicates that the surface charge will be counterbalanced in the diffuse layer with counter-ions only (the sign of charge of counter-ions is opposite to the surface charge). Charge balance by co-ion exclusion is neglected (co-ions have the same sign of charge as the surface). See notes following the example. The identifier -only_counter_ions only applies when the -diffuse_layer identifier is used and applies to all surfaces in the surface assemblage. Optionally, only_counter_ions or -o[ nly_counter_ions].
The default databases contain thermodynamic data for a surface named "Hfo" (Hydrous ferric oxide) that are derived from Dzombak and Morel (1990). Two sites are defined in the databases: a strong binding site, Hfo_s, and a weak binding site Hfo_w. Note that Dzombak and Morel (1990) used 0.2 mol weak sites and 0.005 mol strong sites per mol Fe, a surface area of 5.33e4 m 2 /mol Fe, and a gram-formula weight of 89 g Hfo/mol Fe; to be consistent with their model, the relative number of strong and weak sites should remain constant as the total number of sites varies.
The order of lines 1, 2, 3, 4, and 5 is not important. Lines 1 and, optionally, 4, 5, or 6 should occur only once within the keyword data block. Lines 2 and 3 may be repeated to define the amounts of all binding sites for all surfaces.
Lines 1a, 1b, and 1c require the program to make three calculations to determine the composition of each of the surface assemblages, termed "initial surface-composition calculations". Before any batch-reaction or transport calculations, three initial surface-composition calculations will be performed to determine the composition of the surface assemblages that would exist in equilibrium with the specified solution (solution 10 for all three surface assemblages in this example data block). The composition of the solution will not change during these calculations. In contrast, during a batch-reaction calculation, when a surface assemblage (defined as in example data block 1 or example data block 2 of this section) is placed in contact with a solution with which it is not in equilibrium, both the surface composition and the solution composition will adjust to reach a new equilibrium.
SURFACE 1 has two surfaces, Surfa and Surfb. Surfa has two binding sites, Surfa_w and Surfa_s; the surface area and mass for Surfa must be defined in the input data for at least one of the two binding sites. Surfb has only one kind of binding site and the area and mass must be defined as part of the input for the single binding site.
SURFACE 3 has one surface, Surfc, which has two binding sites, Surfc_w and Surfc_s. The number of binding sites for these two kinds of sites is determined by the amount of Fe(OH) 3 (a) in EQUILIBRIUM_PHASES 3, where 3 is the same number as the surface number. If m represents the moles of Fe(OH) 3 (a) in EQUILIBRIUM_PHASES 3, then the number of sites of Surfc_w is 0.1 m (mol) and of Surfc_s is 0.001 m (mol). The surface area for Surfc is defined relative to the moles of Fe(OH) 3 (a), such that the surface area is 100,000 m (m 2 ). During batch-reaction simulations the moles of Fe(OH) 3 (a) in EQUILIBRIUM_PHASES 3 may change, in which case the number of sites of Surfc will change as will the surface area associated with Surfc. Whenever Fe(OH)3(a) precipitates, the specified amounts of Surfc_wOH and Surfc_sOH are formed. These formulas are charge balanced and the OH groups are part of the formula for Fe(OH)3(a). The OH is not used in the initial surface-composition calculation, but is critical when amounts of Fe(OH)3(a) vary. Erroneous results will occur if the formula is not charge balanced; an error message will result if the elements in the surface complex (other than the surface site itself) are not contained in sufficient quantities in the equilibrium phase or kinetic formula.
The number of sites of Surfd in the example data block is determined by the amount of a kinetic reactant defined in KINETICS 3, where 3 is the same number as the surface number. Sites related to a kinetic reactant are exactly analogous to sites related to an equilibrium phase. The same restrictions apply--the formula must be charge balanced and the elements in the surface complex (other than the surface site itself) must be included in the formula of the reactant.
When -diffuse_layer is not used (default), to account for the charge that develops on the surface, an equal, but opposite, charge imbalance is attributed to the solution. Thus, charge imbalances accumulate in the solution and on the surface when surfaces and solutions are separated. This handling of charge imbalances for surfaces is physically incorrect. Consider the following, where a charge-balanced surface is brought together with a charge-balanced solution. Assume a positive charge develops at the surface. Now remove the surface from the solution. With the present formulation, a positive charge imbalance is associated with the surface, Z s , and a negative charge imbalance, Z soln , is associated with the solution. In reality, the charged surface plus the diffuse layer surrounding it would be electrically neutral and both should be removed when the surface is removed from solution. This would leave an electrically neutral solution. The default formulation is workable; its main defect is that the counter-ions that should be in the diffuse layer are retained in the solution. The model results are adequate, provided solutions and surfaces are not separated or the exact concentrations of aqueous counter-ions are not critical to the investigation.
The -diffuse_layer identifier is a switch that activates a different model to account for the accumulation of surface charge. When the -diffuse_layer identifier is used, the composition of the diffuse layer is calculated and an additional printout of the elemental composition of the diffuse layer is produced. The moles of each aqueous species in the diffuse layer are calculated according to the method of Borkovec and Westall (1983) and the assumption that the diffuse layer is a constant thickness (optional input with -diffuse_layer, default is 10 -8 m). The variation of thickness of the diffuse layer with ionic strength is ignored. The net charge in the diffuse layer exactly balances the net surface charge. Conceptually, the results of using this alternative approach are correct--charge imbalances on the surface are balanced in the diffuse layer and the solution remains charge balanced. Great uncertainties exist in the true composition of the diffuse layer and the thickness of the diffuse layer. The ion complexation in the bulk solution is assumed to apply in the diffuse layer, which is unlikely because of changes in the dielectric constant of water near the charged surface. The thickness of the diffuse layer is purely an assumption that allows the volume of water in the diffuse layer to remain small relative to the solution volume. It is possible, especially for solutions of low ionic strength, for the calculated concentration of an element to be negative in the diffuse layer. In these cases, the assumed thickness of the diffuse layer is too small or the entire diffuse-layer approach is inappropriate. However, the identifier -only_counter_ions offers an option to let only the counter-ions increase in concentration in the diffuse layer, and to leave the co-ions at the same concentration in the diffuse layer as in the bulk solution. The counter-ions have a higher concentration in the diffuse layer than without this option, because co-ion exclusion is neglected. The calculation of the diffuse-layer composition involves a computer intensive integration and an additional set of iterations. The -diffuse_layer identifier causes calculations to be 5 to 10 times slower than calculations with the default approach.
A third alternative for modeling surface-complexation reactions, in addition to the default and -diffuse_layer, is to ignore the surface potential entirely. The -no_edl identifier eliminates the potential term from mass-action expressions for surface species, eliminates any charge-balance equations for surfaces, and eliminates any charge-potential relationships. The charge on the surface is calculated and saved with the surface composition and an equal and opposite charge is stored with the aqueous phase. All of the cautions about separation of charge, mentioned in the previous paragraphs, apply to the calculation using -no_edl.
For transport calculations, it is much faster in terms of cpu time to use either the default (no explicit diffuse layer calculation) or -no_edl. However, -diffuse_layer can be used to test the sensitivity of the results to diffuse-layer effects. All solutions should be charge balanced for transport calculations.
surface binding-site formula --Formula of the surface binding site in its OH form, Surf_sOH and Surf_wOH in this example data block. It is important to include the OH in the formula or hydrogen and oxygen will be extracted from the solution during the reaction step, which will cause unexpected redox or pH reactions.
The difference between example data block 2 and example data block 1 is that no initial surface-composition calculation is performed in example data block 2; the initial states of the surfaces are defined to be in their OH form and not in equilibrium with any solution. Additional surfaces and binding sites can be defined by repeating lines 7 or 3. The -diffuse_layer, -only_counter_ions, or -no_edl identifier can also be included.
After a set of batch-reaction calculations has been simulated, it is possible to save the resulting surface composition with the SAVE keyword. If the new composition is not saved, the surface composition will remain the same as it was initially defined before the batch-reaction calculations. After it has been defined or saved, the surface composition may be used in subsequent simulations through the USE keyword. In ADVECTION and TRANSPORT simulations, the surface assemblages in the column are automatically updated after each shift.
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