V(3) Doesn't Appear to Speciate

[awillou4]

Hello,

I'm attempting to model Vanadium sorption to HFO; however my V(3) concentrations remain fixed no matter how much V(2), V(4), and V(5) concentrations change due to surface complexion.  After searching through the database I've found a V(3) --> V(4) reaction:

VO+2 + e- + 2H+ = V+3 + H2O
   log_k   5.696
   delta_h   0   kJ
   -gamma   0   0
      #                  Id:   9019020
      #        log K source:   Bard85                       
      #      Delta H source:   MTQ3.11                       
      #T and ionic strength:            


as well as a V(4) --> V(5) reaction:


VO2+ + e- + 2H+ = VO+2 + H2O
   log_k   16.903
   delta_h   -122.7   kJ
   -gamma   0   0
      #                  Id:   9029030
      #        log K source:   Bard85                       
      #      Delta H source:   MTQ3.11                       
      #T and ionic strength:   

so I know the database has the two species mathematically connected.  Since minteq.v4 only provides a surface complexation reaction for V(5) (VO2+), perhaps the database needs a V(3) --> V(5) reaction added?  This doesn't seem necessary but I'm at a loss for why else V(3) is stagnant.  It may be worth noting that my two dominant species are V(3) & V(5).  The other species are generally 4+ orders of magnitude less prevalent.

Any insight would be much appreciated.  Thanks!

[dlparkhurst]

Please post a simple example. Also, note the database, or post it if it is not one distributed with PHREEQC.

[awillou4]

I've attached my PHREEQC database.  I had to split it into 2 parts so it would fit.  Simply paste part 2 to the end of part 1 if you'd like to use it.  I also experience the same V(3) issue when I use Minteq.v4.  You'll notice that V(OH)3 concentrations stay the same no matter the concentrations of other V species.  Let me know if you have any insight.

Here is a simple example of my input:

SURFACE 1
    Hfo_wOH    1.22023e-05    600       0.00543002
    Hfo_sOH    3.05057E-07
SURFACE 2
    Hfo_wOH    3.05057e-05    600       0.013575
    Hfo_sOH    7.62643E-07
SURFACE 3
    Hfo_wOH    6.10115e-05    600       0.0271501
    Hfo_sOH    1.52529E-06
SURFACE 4
    Hfo_wOH    9.15172e-05    600       0.0407251
    Hfo_sOH    2.28793E-06
SURFACE 5
    Hfo_wOH    0.000122023    600       0.0543002
    Hfo_sOH    3.05057E-06

SOLUTION 1
    temp      25
    pH        6.27
    pe        4
    redox     pe
    units     ppm
    density   1

    V         0.245
    -water    0.2 # kg


USE solution none

END


USE solution 1
USE surface 1
END


USE solution 1
USE surface 2
end


USE solution 1
USE surface 3
end


USE solution 1
USE surface 4
end


USE solution 1
USE surface 5
END

[dlparkhurst]

Why should V(3) concentrations change? You are not performing any redox reactions. You are simply removing some V(5) from the aqueous phase (to the surface). That should leave V(3) intact. The pe changes because the ratio of V(3) to V(5) in solution is changing.

[awillou4]

It may be that I'm simply ignorant of how one redox species responds to the removal of another.  I was under the impression that the removal of V(5) would create an imbalance, causing V(3) to decrease somewhat.  If this is not the case then I'll have to refresh my memory.  Thanks for your help.