[Next] [Previous] [Up] [Top]



This keyword 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.


Line 1a: Surf_sOH = Surf_sOH
Line 2a       log_k     0.0
Line 1b: Surf_sOH + H+ = Surf_sOH2+
Line 2b:      log_k     6.3
Line 1c: Surf_wOH = Surf_wOH
Line 2c       log_k     0.0
Line 1d: Surf_wOH + H+ = Surf_wOH2+
Line 2d:      log_k     4.3
Line 1e: Surf_sOH + UO2+2 = (Surf_s2O2)UO2 + 2H+
Line 2e:      log_k     -2.57
Line 3:       -no_check 
Line 4:       -mole_balance (Surf_sO)2UO2



Keyword for the data block. No other data are input on the keyword line.

Line 1: Association reaction

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 the master-species identity reaction.

Line 2: log_k log K

log_k--identifier for log K at 25oC. Optionally, -log_k, logk, -l[og_k], or -l[ogk].

log K--Log K at 25oC for the reaction. Default 0.0. Log K for a master species is 0.0.

Line 3: -no_check

-no_check--indicates the equation defining the aqueous species should not be checked for charge and elemental balance. Optionally, no_check, or -n[o_check]. By default, all equations are checked. The only exceptions might be for bidentate surface sites. However, the identifier -mole_balance is needed to ensure that the proper number of atoms of each element and moles of surface sites are included in mole-balance equations.

Line 4: -mole_balance formula

-mole_balance--Indicates the stoichiometry of the species will be defined explicitly. Optionally, mole_balance, mass_balance, mb, -m[ole_balance], -mass_balance, -m[b].

formula--chemical formula defining the stoichiometry of the species. Normally, both the stoichiometry and mass-action expression for the species are determined from the chemical equation that defines the species. Rarely, it may be necessary to define the stoichiometry of the species separately from the mass-action equation. Sorption of uranium on iron oxides as described by Waite and others (1994) provides an example. They use different coefficients in the mass-action equation than the mole-balance equations. The chemical equation defining the species (Line 1e) is used to obtain the mass-action expression. By default, the formula for the species is derived from the sum of all the species in the equation excluding the defined surface species. The -mole_balance identifier is used to specify explicitly the stoichiometry of the surface species (Line 4).


Lines 1 through 4 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, 2a and 1c, 2c in this example. The log K for the identity reaction must be 0.0.

An underscore plus one or more lowercase letters is used to define different binding sites for the same surface. In the example, 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. 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.

Example problems

The keyword SURFACE_SPECIES is used in example problems 8 and 10. See the listing of the default database file in Attachment B for additional examples.

Related keywords


Example problems
Related keywords

User's Guide to PHREEQC - 07 MAY 96
[Next] [Previous] [Up] [Top]

Generated with CERN WebMaker