Dissolution of HFO and Release of Arsenic

[DLipson]

Greetings,

I am trying to re-create Figure 5 of Welch et al (2000)* using the Phreeqc input file provided in their appendix, and after poring over the manual and conducting numerous trials I simply cannot get the input file to produce results even close to Figure 5.

Has anyone tried this? Does anyone have any insight whether I am doing something wrong or is there an issue with the journal article?

Any help would be greatly appreciated. Thank you in advance, -Dave Lipson

* Welch, A.H., D.B. Westjohn, D.R. Helsel, and R.B. Wanty. 2000. Arsenic in Ground Water of the United States: Occurrence and Geochemistry. Ground Water Vol. 38, No. 4, pp 589-604.

[dlparkhurst]

Add your input file to a post using the # button above the input text box and I will take a look.

[DLipson]

Thank you

Code: [Select]

TITLE d7_15
#Equilibrate HFO with 2 ug/L As-SAVE SURFACE
KNOBS
-pe 5
-diagonal
-iterations 400

SOLUTION 1
units ppm
TEMP 25.0
pH 7.
pe 10.
O(0) 1.20
Ca 1.20
Mg .80
Na 500.00
K 11.00
Cl 92.00 charge
S(6) 190.00
Alkalinity 840.00 as HCO3
Si 29.00
As 2 ppb

SURFACE 1
-equil solution 1
Hfo_w Fe(OH)3(a) 0.027 600 0.09

EQUILIBRIUM PHASES 1-2
#moles of Fe(0H)3 initially present
Fe(OH)3(a) 0.0 .0297
END

SOLUTION 0 Arsenate=0
units ppm
TEMP 25.0
pH 7.
pe 2.5
Ca 1.20
Mg .80
Na 500.00
K 11.00
Cl 92.00 charge
S(6) 190.00
Alkalinity 840.00 as HCO3
Si 29.00
USE SURFACE 1
EQUILIBRIUM_PHASES 1
Calcite 0.0 0.1
Siderite 0.0 0.0
Fe(OH)3(a) 0.0 0.0297
#moles of Fe(OH)3 initially present
REACTION 1
CH2O .00015
ADVECTION
-cells 1
-shifts 51
SELECTED_OUTPUT
-file test2.out
-totals As
END


[dlparkhurst]

I think sorption of CO3-2 and Si were not present in the database that was used for the calculations in the paper. The following disables those surface species, which results in a graph similar to the manuscript.

Code: [Select]

SURFACE_SPECIES
       Hfo_wOH + CO3-2 + H+ = Hfo_wCO3- + H2O
       #log_k 12.56
log_k -20
       Hfo_wOH + CO3-2 + 2 H+ = Hfo_wHCO3 + H2O
      # log_k 20.62
log_k -20

Hfo_wOH + H4SiO4 = Hfo_wH3SiO4 + H2O; log_k -20 #log_K 4.28
Hfo_wOH + H4SiO4 = Hfo_wH2SiO4- + H+ + H2O; log_k -20 #log_K -3.22
Hfo_wOH + H4SiO4 = Hfo_wHSiO4-2 + 2 H+ + H2O; log_k -20 #log_K -11.69
TITLE d7_15
#Equilibrate HFO with 2 ug/L As-SAVE SURFACE
KNOBS
-pe 5
-diagonal
-iterations 400

SOLUTION 1
units ppm
TEMP 25.0
pH 7.
pe 10.
O(0) 1.20
Ca 1.20
Mg .80
Na 500.00
K 11.00
Cl 92.00 charge
S(6) 190.00
Alkalinity 840.00 as HCO3
Si 29.00
As 2 ppb

SURFACE 1
-equil solution 1
Hfo_w Fe(OH)3(a) 0.027 600 0.09

EQUILIBRIUM PHASES 1-2
#moles of Fe(0H)3 initially present
Fe(OH)3(a) 0.0 .0297
END

SOLUTION 0 Arsenate=0
units ppm
TEMP 25.0
pH 7.
pe 2.5
Ca 1.20
Mg .80
Na 500.00
K 11.00
Cl 92.00 charge
S(6) 190.00
Alkalinity 840.00 as HCO3
Si 29.00
USE SURFACE 1
EQUILIBRIUM_PHASES 1
#Calcite 0.0 0.1
Siderite 0.0 0.0
Fe(OH)3(a) 0.0 0.0297
#moles of Fe(OH)3 initially present
REACTION 1
CH2O .00015
ADVECTION
-cells 1
-shifts 51
USER_GRAPH 1
    -headings               step As
    -axis_titles            "Step number" "As, ppb" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X STEP_NO
20 GRAPH_Y TOT("As")*GFW("As")*1e6
  -end
    -active                 true

END

[DLipson]

I understand the changes. Thank you very much!