Error in using SURFACE_MODIFY

[JW]

I would like to reduce the amount of surface complexation sites by using SURFACE_MODIFY.

As a preliminary, I tested it with a simple code as follows:
------------------------------
SURFACE_MASTER_SPECIES
      Surf_s      Surf_sOH

SURFACE_SPECIES
Surf_sOH = Surf_sOH
    log_k     0
H+ + Surf_sOH = Surf_sOH2+
    log_k     5.4
Surf_sOH = Surf_sO- + H+
    log_k     -6.7


SOLUTION   1                     
   pH   7
SURFACE   1                     
   -equilibrate   1                  
   -sites_units   density
   Surf_s   2.41   41    2000
END


SURFACE_MODIFY   1
   -component Surf_sOH
   -totals
      Surf_s   1000
   
RUN_CELLS
   -cells 1
   
END
--------------------------------------

Then, I've got the following error message.
--------------------------------------
ERROR: Analytical data entered twice for Surf_sOH.
ERROR: Program terminating due to input errors.
--------------------------------------

What is wrong with the code above?

Thank you in advance.

[Pak]

Is not sure that you can use Surface_Modify on a Phreeqc script that easy, never tried. I thought it was thought to be used in reactive transport simulations when you make a coupling by using iphreeqc or phreeqcRM.

[dlparkhurst]

Below is part of the listing from a DUMP of the surface. The component should be Surf_s, rather than Surf_sOH in your script.

However, note that 1000 moles would be a huge change in the number of surface sites (originally 0.32). Also, there is the problem of the O and H of the surface (see DUMP). By changing just the number of sites, you essentially are adding OH- to solution, which I think will be compensated by generating O2 and H+ with the following reaction:

2H2O -> O2 + 4H+ + 4e-

There is also a charge on the surface that would remain if just Surf_s is changed. This surface is relatively simple; most surfaces would have additional elements as well.

In short, I think you need to be very careful changing the number of sites. It's been a while, but I think when a surface is related to a phase, the surface is changed by adding or removing Surf_sOH, which means when you remove (or add) Surf_s, you must also modify O and H in the surface totals. For surface related to an equilibrium phase, I think you must define the SURFACE with Surf_sOH rather than Surf_s (I think that is how it knows to remove Surf_sOH, rather than just Surf_s. I don't think it would make a difference for SURFACE_MODIFY, but it wouldn't hurt.)

Code: [Select]

SURFACE_RAW                  1
  # SURFACE_MODIFY candidate identifiers #
  -type                      2
  -dl_type                   0
  -only_counter_ions         0
  -thickness                 1e-08
  -debye_lengths             0
  -DDL_viscosity             1
  -DDL_limit                 0.8
  -component                 Surf_s
    # SURFACE_MODIFY candidate identifiers #
    -formula_z               0
    -moles                   0
    -la                      -0.16069185197993
    -charge_balance          -4.1904572422299e-05
    -phase_proportion        0
    -Dw                      0
    -charge_name             Surf
    -master_element          Surf_s
    -totals
      H                      0.32809316295155
      O                      0.32813506752397
      Surf_s                 0.32813506752397


[Pak]

If I increase the amount of surface by X moles, I should remove this X from the amount of total O and H. And if I reduce the amount of surface by X, I should add this amount of X to the total O and H. If I want to keep the same amount on the system. Is that right?

thanks

[dlparkhurst]

Sounds backwards. Assuming SurfOH is uncharged, if you increase the surface, you should increase O and H; if you decrease the surface you should decrease O and H. You need to add and remove SurfOH.

I think you could use REACTION to add SurfOH. Not sure if this suits your application, but you could at least use it to check your logic.