SURFACE Code Block - units of input

[sorsbys]

Hello,

I am using the Dzombak & Morel DDL model in the SURFACE code block, which requires moles of sites as input.

The units in the rest of the code block (m^2/g and g) are in terms of grams, leading me to believe that the molar sites input refers to moles in one gram of soil material.

However, PHREEQC example 14 indicates that the input is in moles/liter of solution. Furthermore, the concentration of the complexed species is reported in molality units (mol/kgw). This is similar to Visual MINTEQ's approach, which uses an input of grams of sites per liter of solution.

My question is, does PHREEQC handle the molar input as moles/gram, or moles/kgw?

There are three orders of magnitude between the two, and my results (molybdenum in an acidic aquifer) show an unrealistically low sorbed concentration if it's moles/gram, but reasonable levels if it's moles/kgw. I did update PHREEQC's MINTEQv4 database to include the newer data from Gustafsson (2003), so I don't think the database is the issue (it was, prior to updating).

Thanks!

[dlparkhurst]

Manual's description of input:

Line 2: surface binding site, ( sites or site density ) , specific_area_per_gram, grams, [ Dw coefficient ]

surface binding site --Name of a surface binding site.

sites --Total number of sites for this binding site, in moles; applies when -sites_units is absolute .

site density --Site density for this binding site, in sites per square nanometer; applies when -sites_units is density .

specific_area_per_gram --Specific area of surface, in m 2 /g (square meter per gram). Default is 600 m 2 /g.

grams --Mass of solid for calculation of surface area, g (gram); surface area is grams times specific_area_per_gram . Default is 0 g.

-------------------

The surface area (in m^2) is the product of specific area times grams, in all cases.

The number of sites is in moles when -sites_units absolute. The number of sites per kgw or L depends on the mass of water or volume of solution. Usually, but not necessarily, the mass of water in the solution is near 1, and the number of moles and mol/kgw are nearly the same numerically.

If the -site_units are "density", then a value is given for sites per nanometer squared, and the total number of sites is the surface area times the site density times the appropriate conversion factor to convert the surface area from m^2 to nanometer^2 and the number of sites to moles of sites. The result of the calculation is the number of moles of sites; again not mol/kgw, mol/L, or mol/g.

[sorsbys]

Thank you!