Surface Complexation Leaching Question

[JRen]

I have a question regarding setting up a surface complexation leaching model.

I want to set up a surface that is then contacted by a liquid. I want this surface to contain a significant amount of arsenic at the beginning of the simulation.

Hence, I want the surface to consist of mostly Hfo_wOH, but also some Hfo_wHAsO4-. I want the arsenic to be present on the surface initially and then leach into the liquid. However, I have been unable to define the initial surface with this arsenic content. I always get numerous errors including the following:

ERROR: Analytical data entered twice for Hfo_wOH.

Is there a way to define the surface composition which includes trace contaminants before the simulation?

My input file is as follows:
SURFACE 1
    Hfo_wHAsO4-    6.83e-06    1.27      20
    Hfo_wOCd+    1.39e-07
    Hfo_wOH    0.000352
    Hfo_wOHSeO3-2    1.39e-06

SOLUTION 2, pH 4.5
    temp      25
    pH        7.5
    pe        -1
    redox     pe
    units     mol/l
    density   1
    -water    0.2 # kg

PHASES
Scorotide
    FeAsO4:2H2O = AsO4-3 + Fe+3 + 2H2O
    log_k     -20.249
Ca4(OH)2(AsO4)2:4H2O
    Ca4(OH)2(AsO4)2:4H2O + 8H+ = 4Ca+2 + 6H2O + 2H3AsO4
    log_k     40.2
Ca5(AsO4)3(OH)
    Ca5(AsO4)3(OH) + 10H+ = 5Ca+2 + H2O + 3H3AsO4
    log_k     37.8
Ca3(AsO4)2:4H2O
    Ca3(AsO4)2:4H2O + 6H+ = 3Ca+2 + 4H2O + 2H3AsO4
    log_k     20.22
CaHAsO4:H2O
    CaHAsO4:H2O + 2H+ = Ca+2 + H2O + H3AsO4
    log_k     4.22
Ferrarisite
    Ca5(HAsO4)2(AsO4)2:9H2O + 10H+ = 5Ca+2 + 9H2O + 4H3AsO4
    log_k     27.75
Guerinite
    Ca5(HAsO4)2(AsO4)2:9H2O + 10H+ = 5Ca+2 + 9H2O + 4H3AsO4
    log_k     28.55
CaSeO3:2H2O
    CaSeO3:2H2O = Ca+2 + 2H2O + SeO3-2
    log_k     -7.76
CdSeO3
    CdSeO3 = Cd+2 + SeO3-2
    log_k     -8.9
Fe2(SeO3)3
    Fe2(SeO3)3 = 2Fe+3 + 3SeO3-2
    log_k     -30.7
Fe2(SeO3)3:6H2O
    Fe2(SeO3)3:6H2O = 2Fe+3 + 6H2O + 3SeO3-2
    log_k     -41.58
MgSeO3:6H2O
    MgSeO3:6H2O = 6H2O + Mg+2 + SeO3-2
    log_k     -5.36
MgSeO3
    MgSeO3 = Mg+2 + SeO3-2
    log_k     -8.99
Na2SeO3
    Na2SeO3 = 2Na+ + SeO3-2
    log_k     -3.51
Fix_H+
    H+ = H+
    log_k     0
SURFACE_SPECIES
Hfo_wOH + Mg+2 = Hfo_wOMg+ + H+
    log_k     -15
Ca+2 + Hfo_wOH = Hfo_wOCa+ + H+
    log_k     -15
HAsO4-2 + Hfo_wOH2+ = Hfo_wHAsO4- + H2O
    log_k     4
Cd+2 + Hfo_wOH = Hfo_wOCd+ + H+
    log_k     -2.91
Hfo_wOH + Na+ = Hfo_wONa + H+
    log_k     -9.1
Hfo_wOH + SeO3-2 = Hfo_wOHSeO3-2
    log_k     5.17
H+ + HSeO3- + Hfo_wOH = Hfo_wHSeO3 + H2O
    log_k     12.69
H2AsO4- + Hfo_wOH2+ = Hfo_wH2AsO4 + H2O
    log_k     2
Cl- + Hfo_wHAsO4- = Hfo_wCl + HAsO4-2
    log_k     6
SOLUTION_SPECIES
H3AsO4 = H3AsO4
    log_k     0
H3AsO4 = H2AsO4- + H+
    log_k     -2.243
H3AsO4 = HAsO4-2 + 2H+
    log_k     -9.001
H3AsO4 = AsO4-3 + 3H+
    log_k     -20.597
Ca+2 + H2AsO4- = CaH2AsO4+
    log_k     1.3
Ca+2 + HAsO4-2 = CaHAsO4
    log_k     2.66
AsO4-3 + Ca+2 = CaAsO4-
    log_k     4.36
H2AsO4- + Mg+2 = MgH2AsO4+
    log_k     1.52
HAsO4-2 + Mg+2 = MgHAsO4
    log_k     2.86
AsO4-3 + Mg+2 = MgAsO4-
    log_k     6.34
H2SeO3 = HSeO3- + H+
    log_k     -2.57
H2SeO3 = H2SeO3
    log_k     0
H2SeO3 = SeO3-2 + 2H+
    log_k     -11.24

[dlparkhurst]

You can only define surface site names, like Hfo_w or Hfo_wOH, in a SURFACE definition; no other elements.

You can define the surface to be in equilibrium with a solution. So if you set the pH and adjust the amounts of As, Cd, and Se in the SOLUTION, you could get a surface with the amounts of As, Cd, and Se that you want.

This input gets you within an order of magnitude for each element. The competition for sites makes it a little tricky.

SOLUTION
pH    7
-units mmol/kgw
As   0.0000001
Cd    0.0001
Se   0.00001
END
USER_PRINT
10 PRINT SURF("As", "Hfo_w")
SURFACE 1
Hfo_wOH 0.000352 1.27 20
-eq solution 1
END
USER_PRINT
10 PRINT "Surface sites, moles: ", TOT("Hfo_w")
20 PRINT "As sorbed, moles:     ", SURF("As", "Hfo")
30 PRINT "Cd sorbed, moles:     ", SURF("Cd", "Hfo")
40 PRINT "Se sorbed, moles:     ", SURF("Se", "Hfo")
USE solution 1
USE surface 1
END