Surface complexation model_Effect of dosage on uranium removal

[Francis Adu]

Dear All,

I would be glad if anyone could share his/her thought on my model for me. I am modeling the removal of uranium by iron, silicon, and aluminum hydroxides. The main objective is to study the effect of varying the adsorbent dosage on the removal at a fixed pH of 8. In the model attached; I am only varying the dosage from 0.05-0.2g. However, when I change the mass, there is no appreciable change in the removal as I am recording also 100% removal at all the different dosages. I would be glad if anyone could check my codes or suggest any reason why significant differences are not recorded whenever there is a change in the dosage. I have attached the codes below for your reference;


PHASES 1

    fix_pH
    H+ = H+
    log_k     0


SURFACE_MASTER_SPECIES 1
       Surf_al  Surf_alOH
       Surf_si  Surf_siOH
       Surf_fe  Surf_feOH
     
SURFACE_SPECIES 1
Surf_alOH = Surf_alOH
log_k 0.0
-no_check
Surf_alOH + H+ = Surf_alOH2+
log_k  12.3

Surf_alOH = Surf_alO- + H+
log_k  -13.16

Surf_alOH + UO2+2 = Surf_alOUO2+ + H+
log_k   1.4

Surf_alOH + 2UO2+2 + CO3-2 + 3H2O = Surf_alO(UO2)2CO3(OH)3-2 + 4H+
 log_k     0.99

Surf_siOH = Surf_siOH
 log_k    0.0

Surf_siOH + H+ = Surf_siOH2+
 log_k    -0.95

Surf_siOH = Surf_siO- + H+
 log_k     -6.90

Surf_siOH + UO2+2 = Surf_siOUO2+ + H+
log_k      0.99

Surf_siOH + UO2OH+ = Surf_siOUO2OH + H+
 log_k     1.0

Surf_siOH + UO2(CO3)3-4 = Surf_siOHUO2(CO3)3-4
 log_k     8.0

Surf_siOH + UO2(OH)3- = Surf_siOHUO2(OH)3-
 log_k     6.90

 Surf_feOH = Surf_feOH
  log_k 0.0

Surf_feOH +H+ = Surf_feOH2+
  log_k 7.47

Surf_feOH = Surf_feO- + H+
  log_k -9.51

Surf_feOH + UO2+2 = Surf_feOUO2+ + H+
 log_k  5.20

Surf_feOH + UO2+2 + CO3-2 = Surf_feOHUO2CO3
  log_k 15.95


SURFACE 1
Surf_siOH  2.773e-04 500  0.05
Surf_alOH  9.243e-06
Surf_feOH  8.596e-04

SOLUTION 1
    temp      20
    pH        8
    pe        4
    redox     pe
    units     mmol/l
    density   1
    C(4)      1
    Cl        10
    Na        11
    U         250 ug/L
    -water    0.1 # kg
END

Thank you

[dlparkhurst]

I'm not sure how you were adjusting the surface parameters. If you changed only the number of grams, then the number of sorption sites would not vary; only the surface area would change. The surface area would most affect the potential factor, which is related to the amount of work needed to bring a charged ion from solution to a surface site.

Regardless of the surface area, the number of sites is sufficient to remove almost all of the uranium. Surf_fe is the most important surface site for sorbing the uranium.

Another way to define the number of surface sites is by use of the -sites DENSITY option, which allows the number of sites to depend on the number of grams that is defined. The input is sites per nanometer squared. I have adjusted the SURFACE definition to use -site DENSITY to produce the same number of sites in your definition for 0.05 g. I repeated the calculation with fewer and fewer grams, which corresponded to fewer and fewer sites. The amount of U sorbed is plotted against the total number of Surf sites. It requires about 1e-4 fewer sites before uranium is not completely sorbed. You seemed to expect a variation as the number of sites increased from 0.05 g. You need to look at the log Ks of the sorption reactions if you want sorption to be less complete.

Code: [Select]

PHASES 1

    fix_pH
    H+ = H+
    log_k     0


SURFACE_MASTER_SPECIES 1
       Surf_al  Surf_alOH
       Surf_si  Surf_siOH
       Surf_fe  Surf_feOH
     
SURFACE_SPECIES 1
Surf_alOH = Surf_alOH
log_k 0.0
-no_check
Surf_alOH + H+ = Surf_alOH2+
log_k  12.3

Surf_alOH = Surf_alO- + H+
log_k  -13.16

Surf_alOH + UO2+2 = Surf_alOUO2+ + H+
log_k   1.4

Surf_alOH + 2UO2+2 + CO3-2 + 3H2O = Surf_alO(UO2)2CO3(OH)3-2 + 4H+
log_k     0.99

Surf_siOH = Surf_siOH
log_k    0.0

Surf_siOH + H+ = Surf_siOH2+
log_k    -0.95

Surf_siOH = Surf_siO- + H+
log_k     -6.90

Surf_siOH + UO2+2 = Surf_siOUO2+ + H+
log_k      0.99

Surf_siOH + UO2OH+ = Surf_siOUO2OH + H+
log_k     1.0

Surf_siOH + UO2(CO3)3-4 = Surf_siOHUO2(CO3)3-4
log_k     8.0

Surf_siOH + UO2(OH)3- = Surf_siOHUO2(OH)3-
log_k     6.90

Surf_feOH = Surf_feOH
  log_k 0.0

Surf_feOH +H+ = Surf_feOH2+
  log_k 7.47

Surf_feOH = Surf_feO- + H+
log_k -9.51
 
Surf_feOH + UO2+2 = Surf_feOUO2+ + H+
log_k  5.20

Surf_feOH + UO2+2 + CO3-2 = Surf_feOHUO2CO3
log_k 15.95

SOLUTION 1
    temp      20
    pH        8
    pe        4
    redox     pe
    units     mmol/l
    density   1
    C(4)      1
    Cl        10
    Na        11
    U         250 ug/L
    -water    0.1 # kg
END
#SURFACE 1
#Surf_siOH  2.773e-04 500  0.05
#Surf_alOH  9.243e-06
#Surf_feOH  8.596e-04

     
END
USER_GRAPH 1
    -axis_titles            "Surf sites, moles" "U sorbed, moles" ""
    -axis_scale x_axis      auto auto auto auto log
    -axis_scale y_axis      auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTMOL("Surf_al") + TOTMOL("Surf_fe") + TOTMOL("Surf_si")
20 GRAPH_Y SURF("U","Surf")
  -end
    -active                 true
USE solution 1
SURFACE 1
    -sites DENSITY
    Surf_siOH  6.6803 500  0.05
    Surf_alOH  0.22267
    Surf_feOH    20.708   
END
USE solution 1
SURFACE 1
    -sites DENSITY
    Surf_siOH  6.6803 500  0.0005
    Surf_alOH  0.22267
    Surf_feOH    20.708   
END
USE solution 1
SURFACE 1
    -sites DENSITY
    Surf_siOH  6.6803 500  0.000005
    Surf_alOH  0.22267
    Surf_feOH    20.708   
END
USE solution 1
SURFACE 1
    -sites DENSITY
    Surf_siOH  6.6803 500  0.0000005
    Surf_alOH  0.22267
    Surf_feOH    20.708   
END
USE solution 1
SURFACE 1
    -sites DENSITY
    Surf_siOH  6.6803 500  0.00000005
    Surf_alOH  0.22267
    Surf_feOH    20.708   
END
USE solution 1
SURFACE 1
    -sites DENSITY
    Surf_siOH  6.6803 500  0.000000005
    Surf_alOH  0.22267
    Surf_feOH    20.708   
END



[Francis Adu]

Dear Parkhurst,

Thank you very much for your time in answering my question. The explanation has explicitly clarified a lot of things for me.