understanding surface in phreeqc

[tsnmaster]

Hello to everyone, i've got a  question related to keyword surface.
How  does this keyword consider changes in pH ?  i mean, i know that it would change the ionic strength but i'd like to know if i put the constant for a surface reaction asociated to equilibrium phases and then i change the pH, phreeqc will recognize the change or it depends only the amount of the phases that precipitate? I'm asking this because i know that the pH is related with the percentage of removal of metals as arsenic or chromium.

Thank you a lot

[dlparkhurst]

I think you are mixing up two different features of SURFACE.

First consider a SURFACE definition with a fixed number of moles of sites. As the pH changes, then the distribution of surface species changes. The log Ks for the surface species determine the distribution of species.

Second, SURFACE has the capability to adjust the number of sites as a mineral dissolves or precipitates. The distribution of species will still change as a function of pH, but there is the added change when the mineral dissolves and precipitates, changing the number of surface sites.

[tsnmaster]

ok, thank you a lot, i didn't understand the second one, so you mean that when i use SURFACE and i associate with EQUILIBRIUM PHASES, when one mol precipitates, change  the number of sites and it is capable of retain arsenic onto the surface?

Thank you

[dlparkhurst]

In the SURFACE definition, you make the number of sites of surface (let's use Surf_w for example) proportional to the number of moles of an EQUILIBRIUM_PHASE (say, goethite) (using the same user numbers for SURFACE and EQUILIBRIUM_PHASES). So, if there is no goethite, there are no surface sites. If goethite precipitates, then the number of surface sites is set proportionally.

If a surface is present (goethite is present) then it will equilibrate with the solution, sorbing As if the surface complexation reactions for As sorption on Surf_w (for example) are defined (SURFACE_SPECIES definitions).