This keyword data block is used to define a reaction and log K for each surface species, including surface master species. Normally, this data block is included in the database file and only additions and modifications are included in the input file. Surface species defined in Dzombak and Morel (1990) are defined in the default databases; the master species are Hfo_w and Hfo_s for the weak and strong binding sites of hydrous ferric oxide.
Association reaction for surface species. The defined species must be the first species to the right of the equal sign. The association reaction must precede all identifiers related to the surface species. Line 1a is a master-species identity reaction.
This example data block assumes that Surf_w and Surf_s are defined in a SURFACE_MASTER_SPECIES data block. Lines 1 and 2 may be repeated as necessary to define all of the surface reactions. An identity reaction is needed to define each master surface species, lines 1a and 1c in this example data block. The log K for the identity reaction must be 0.0, lines 2a and 2c in this example data block. This example data block assumes that Surf_w and Surf_s are defined in a SURFACE_MASTER_SPECIES data block.
An underscore plus one or more lowercase letters is used to define different binding sites for the same surface. In the example data block, association reactions for a strong and a weak binding site are defined for the surface named "Surf". Multiple surfaces may be defined simply by defining multiple master surface species (for example, Surfa, Surfb, and Surfc). Multiple binding sites can be defined for each surface by using an underscore followed by one or more lower case letters. Association reactions for each surface and binding site must be defined with SURFACE_SPECIES input.
Temperature dependence of log K can be defined with enthalpy of reaction (identifier delta_h) and the van't Hoff equation or with an analytical expression ( -analytical_expression). See SOLUTION_SPECIES or PHASES for examples.
The identifier -no_check can be used to disable checking charge and elemental balances (see SOLUTION_SPECIES). The use of -no_check is not recommended. If -no_check is used, then the -mole_balance identifier is needed to ensure the correct stoichiometry for the surface species. In PHREEQC version 1, the -no_check option was included to permit the stoichiometry of a species to be defined separately from the mass-action equation. Specifically, the sorption of uranium on iron oxides as described by Waite and others (1994) provides an example, where they use different coefficients in the mass-action equation than the mole-balance equations. However, activity of a surface species is defined as mole fraction of sites occupied by the species in PHREEQC version 2 which is inconsistent with activity that is defined as molality by Waite and others (1994) and PHREEQC version 1. It is noted that formulas with coefficients of only 1 in the mass-action-equation will give identical results for PHREEQC version 1 and 2. The -no_check and -mole_balance identifiers have been retained in version 2, but its use should be restricted to special sorption formulas.
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