surface complexation

[amaury22]

I am new in PHREEQC.

i try to understant the values

Where i can see an explanation of how change, etc?
#name, site density, specific area/gram, mass
Oil_aCOOH 22.0 0.2 2.6 #Oil surface
Oil_bNH+ 75
Surf_cCaOH 4.9 0.2 0.2 #Calcite surface
Surf_dCO3H 4.9

There some formulas to caculate such values, sites, area/gram, mass?

How i change it?

[dlparkhurst]

Surface complexation is based on work on Hydrous Ferric Oxides as examined in the book by Dzombak and Morel. They have analyzed many reports to provide the characteristics of the HFO surface. They suggest values of 600 m^2/g for the surface area and 0.2 mol of weak sites and 0.005 mol of strong sites per mole of FeOOH. I think they use a gram formula weight of 89 for HFO.

Note that HFO is a transitory solid made in the lab with a given recipe and aged for a limited period of time. It probably does not correspond exactly to ferric oxyhydroxide in nature.

You appear to be working with completely different surfaces, in which case you must estimate the number of sites and the appropriate surface area for your system. Although the input uses two fields, the area/g and grams, the calculations only depend on the area, as calculated by the product of the two numbers. The area is important to the calculations for the charge density, the charge per area, which affect the potential term in the surface complexation equations. Large areas result in smaller charge density and smaller electrical potential effects; small areas may result in larger charge densities and larger electrical potential effects.

The number of sites can be defined directly as moles of sites, or as the number of sites per m^2, in which case the number of sites is equal to the product of the sites/m^2*m^2/g*g.

How you arrive at the sites and area is up to you. If you have empirical data, you could possibly fit these values. Otherwise, I refer you to the papers cited by Dzombak and Morel as to how these parameters were measured.

[amaury22]

Thank a lot.
I have been reading the paper of Dzombak and Morel.

Any case, i have some curiosity: when i simulated on PHREEQC there are database.

Are this parameters depending of such data?

This quantities depend on total water?
For example, if i take 0.5 kg of water or less

[dlparkhurst]

The data for Hfo in phreeqc.dat and pitzer.dat are from Dzombak and Morel, 1990. Input for SURFACE_MASTER_SPECIES and SURFACE_SPECIES may be in a database, may be included from a file (INCLUDE$), or part of the input file.

The data in SURFACE_MASTER_SPECIES and SURFACE_SPECIES do not depend on the solution composition.

The data in SURFACE do depend on the way you define your solution. SOLUTION generally generates a solution with 1 kg water, although -water in SOLUTION, MIX, or reactions may cause the mass of water to be other than 1 kg. SURFACE definitions for sites are ultimately moles (not moles per kg water) and area is m^2 (not m^2/kg water). So, you must determine the number of sites per kilogram water by the number of sites (and area) you define in SURFACE and the mass of water you define for your solution.