Moly sorption

[kristengault]

Hi,
I'm trying to model Mo sorption to HFO with increasing pH. Based on the literature I was expecting HFO would pick up a lot up Mo at low pH. This doesn't appear to be the case for my simulation? I think it may be related to Mo sppn, which looks to be mostly H2MoO4 in my solution for the low pH runs. Sorption constants for H2MoO4 don't appear to exist in the database, so it doesn't look like it's being picked up - am I interpreting this correctly?

I couldn't find a constant for H2MoO4 sorption in the literature, but I expect it occurs? Is the lack of Mo sorption in my simulation below just a consequence of available data in the database? I was using minteqv4. The values there appear to be the D&M values (listed below my input file).

Thanks for your assistance!

SOLUTION 1 West PAG
    temp      25
    pH        3.5
    pe        4
    redox     pe
    units     mg/l
    density   1
    Ag        0.0017
    Alkalinity 1279
    Ca        758 charge
    Cd        0.0079
    Cu        0.3
    Fe        500
    K         51.3
    Mg        52.9
    Mo        15
    Na        31.2
    Ni        0.09
    S(6)      950
    Zn        0.51
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
SAVE solution 1
END
USE solution 1
SURFACE 1
    Hfo_sOH Ferrihydrite    equilibrium_phase 0.2    53300
    Hfo_wOH Ferrihydrite    equilibrium_phase 0.005
END
PHASES
fix_pH
    H+ = H+
    log_k     0
END
USE solution 1
EQUILIBRIUM_PHASES 2
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -3.6 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 3
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -3.8 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 4
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -4.0 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 5
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -4.2 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 6
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -4.2 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 7
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -4.4 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 8
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -4.6 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 9
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -4.8 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 10
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -5.0 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 11
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -5.5 NaOH      10
    USE surface 1
END
USE solution 1
EQUILIBRIUM_PHASES 12
    Ferrihydrite 0 0
    Gypsum    0 0
    O2(g)     -0.68 10
    fix_pH    -6.0 NaOH      10
    USE surface 1
END


Moly sorption constants in database:
Hfo_sOH + MoO4-2 + H+ = Hfo_sMoO4- + H2O
   log_k   9.5
   delta_h   0   kJ
      #                  Id:   8114800
      #        log K source:                                
      #      Delta H source:                                
      #T and ionic strength:   
Hfo_wOH + MoO4-2 + H+ = Hfo_wMoO4- + H2O
   log_k   9.5
   delta_h   0   kJ
      #                  Id:   8124800
      #        log K source:                                
      #      Delta H source:                                
      #T and ionic strength:   
Hfo_sOH + MoO4-2 = Hfo_sOHMoO4-2
   log_k   2.4
   delta_h   0   kJ
      #                  Id:   8114801
      #        log K source:                                
      #      Delta H source:                                
      #T and ionic strength:   
Hfo_wOH + MoO4-2 = Hfo_wOHMoO4-2
   log_k   2.4
   delta_h   0   kJ
      #                  Id:   8124801
      #        log K source:                                
      #      Delta H source:                                
      #T and ionic strength:   

[dlparkhurst]

Not sure where the Mo data in minteq.v4 comes from, maybe Dzombak and Morel, but I expect it was fit to a set of low pH experiments. Assuming the aqueous model was the same as minteq.v4 (maybe, maybe not), then the model should fit at low pH. Adding sorption of H2MoO4 would be inconsistent with the set of sorption constants used to fit the experimental data.

Mo does sorb at lower pH in your example. One reason for the limited amount of sorption is that you have a limited number of sites relative to Mo because you have a limited amount of Fe in your system. If you had more ferrihydrite, you would sorb more Mo.

Consider the results shown on this plot of the dissolved, sorbed, Hfo_s sites, Hfo_w sites.

USER_GRAPH 1
    -headings               pH Mo_dis Mo_sorbed Hfo_s Hfo_w
    -axis_titles            "pH" "Moles" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x -la("H+")
20 graph_y TOTMOL("Mo"), SURF("Mo","Hfo")
30 graph_y TOTMOL("Hfo_s"), TOTMOL("Hfo_w")
  -end

[sorsbys]

Hi all,

Check out Gustafsson (2003) and Gustafsson and Tiberg (2015) for updated moly thermo data.
https://www.kth.se/profile/gustafjp/page/publications-jp-gustafsson

Smedley and Kinniburgh (2017) is another good resource for tracking down available data.