Re: Arsenite Sorption Model

["David L Parkhurst" <dlpark@xxxxxxxx>]

By using the same chemical symbol, As, you need a reaction that connects
As(3) to As(5).  This is the reaction from wateq4f.dat.

#        H3AsO3 = H3AsO3
#        log_k   0.
      H3AsO4 + 2H+ +2e- = H3AsO3 + H2O
      logk 18.897
      delta_H -30.015 kcal

However, the consequence of doing this is that As(3) and As(5) will always
react to redox equilibrium. If you want As(3) independent of As(5), it is
possible to disconnect the redox states, but As(3) would have to be renamed
and all the reactions redefined. The following example separates As(3) from
As(5) and As(5) will not react to As(3) or vice versa. If you want minerals
or other reactions, you would have to rewrite them to use "Arsenite" as in
the following example.

One final possibility, probably much more than you want, is that, given the
separation defined below, it would be possible to write a KINETIC reaction
that would transfer Arsenite to As(5) according to some rate expression.

David

SOLUTION_MASTER_SPECIES
Arsenite   H3ArseniteO3         0.0     74.9216         74.9216

SOLUTION_SPECIES
#Arsenite3  master species
        H3ArseniteO3 = H3ArseniteO3
        log_k           0

#H2ArseniteO3-             478
        H3ArseniteO3 = H2ArseniteO3- + H+
        log_k           -9.228
        delta_h 6.56 kcal

#HArseniteO3-2             479
        H3ArseniteO3 = HArseniteO3-2 + 2H+
        log_k           -21.33
        delta_h 14.2 kcal

#ArseniteO3-3              480
        H3ArseniteO3 = ArseniteO3-3 + 3H+
        log_k           -34.744
        delta_h 20.25 kcal

#H4ArseniteO3+             481
        H3ArseniteO3 + H+ = H4ArseniteO3+
        log_k           -0.305

SURFACE_SPECIES
#   Arsenite
        Hfo_wOH + H3ArseniteO3 = Hfo_wH2ArseniteO3 + H2O
        log_k   5.41


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled