Applications and Case Studies > Groundwater flow studies
Arsenic transport and transformation
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Zhaoyang:
Dear All,
I am modeling arsenic transport and transformation in groundwater with PHREEQC. Even though transport is not included at this stage, I always get a negative FeS in my model. I was wondering if you know how to solve this issue. In addition, I don’t know how to define reaction R6 in PHREEQC. Could you please advise how to do that? Thanks for your help in advance.
--- Code: ---##Reaction expressions
## 2CH2O+SO4-2 = H2S + 2HCO3- (R1) ### CH2O - Doc; HCO3- - Bicarbonate
## Fe(OH)2.436(AsO4)0.188 + 0.125HS- + 1.311H+ = 0.188AsO4-3 + Fe+2 + 1.936H2O + 0.125SO4-2 (R2) ### Fe(OH)2.436(AsO4)0.188 - Afeas; AsO4-3 - Asfive; Fe+2 - Fe_di
## Fe(OH)2.436(AsO4)0.188 + 1.311H+ + 0.125CH2O= 0.188AsO4-3 + Fe+2 + 1.936H2O + 0.125HCO3- (R3)
## HS- + Fe+2 = FeS + H+ (R4) ### FeS - Fes
## Fes + 2.25O2 + 1.5H2O = SO4-2 + FeOOH + 2H+ (R5) ### O2 - Do; FeOOH - Feooh
## SurfAs + HCO3- = SurfHCO3- + As (R6) ### As - Aszero
## Fe(OH)2.436 + As = As bearing Crystalline iron(hydr)oxide (R7) ### Fe(OH)2.436 - Bion; As bearing Crystalline iron(hydr)oxide - Casber
SOLUTION_MASTER_SPECIES
Doc Doc 0 Doc 12
Bicarbonate Bicarbonate- 0 Bicarbonate 61
Afeas Afeas 0 Afeas 123.4
Asfive Asfive-3 0 Asfive 75
Fes Fes 0 Fes 88
Do Do 0 Do 32
Feooh Feooh 0 Feooh 89
Aszero Aszero 0 Aszero 75
Bion Bion 0 Bion 97.3
Casber Casber 0 Casber 123.4
Salt Salt 0 Salt 58.5
Fe_di Fe_di+2 0 Fe_di 56
SOLUTION_SPECIES
Doc = Doc
log_k 0
Bicarbonate- = Bicarbonate-
log_k 0
Afeas = Afeas
log_k 0
Asfive-3 = Asfive-3
log_k 0
Fes = Fes
log_k 0
Do = Do
log_k 0
Feooh = Feooh
log_k 0
Aszero = Aszero
log_k 0
Bion = Bion
log_k 0
Casber = Casber
log_k 0
Salt = Salt
log_k 0
Fe_di+2 = Fe_di+2
log_k 0.0
SOLUTION 0
units mg/L
Salt 0.001
pH 7.33
Do 3.30
Doc 5.0
Aszero 4
S(6) 232
Salt 11400
Bicarbonate 220
Afeas 100
Bion 100
Casber 100
END
RATES
Sulfate_reduction
-start
1 k1= 1e-4
10 rate = -k1*TOT("Doc") * TOT("S(6)")
20 moles = rate * TIME
30 SAVE moles
-end
As_release_1
-start
1 k2= 6e-7
10 rate = -k2*TOT("H") * TOT("S(-2)")
20 moles = rate * TIME
30 SAVE moles
-end
As_release_2
-start
1 k3= 6e-7
10 rate = -k3*TOT("H") * TOT("Doc")
20 moles = rate * TIME
30 SAVE moles
-end
Iron_sulfide_formation
-start
1 k4= 8e-2
10 rate = -k4*TOT("S(-2)") * TOT("Fe_di")
20 moles = rate * TIME
30 SAVE moles
-end
Iron_sulfide_oxidze
-start
1 k5= 8e-6
10 rate = -k5*TOT("Do")
20 moles = rate * TIME
30 SAVE moles
-end
As_Crystalline
-start
1 k6= 4.6e-6
10 rate = -k6*TOT("Aszero") * TOT("Fe_di")
20 moles = rate * TIME
30 SAVE moles
-end
SOLUTION 1-40
units mg/L
Salt 0.001
pH 7.33
Do 3.30
Doc 5.0
Aszero 4
S(6) 232
Salt 11400
Bicarbonate 220
Afeas 100
Bion 100
Casber 100
KINETICS 1-40
Sulfate_reduction
-formula Doc 2 SO4 1 HS -1 Bicarbonate -2 H -1
-m 0
As_release_1
-formula Afeas 1 HS 0.125 H 1.311 Asfive -0.188 Fe_di -1 H2O -1.936 SO4 -0.125
-m 0
As_release_2
-formula Afeas 1 H 1.311 Doc 0.125 Asfive -0.188 Fe_di -1 H2O -1.936 Bicarbonate -0.125
-m 0
Iron_sulfide_formation
-formula HS 1 Fe_di 1 Fes -1 H -1
-m 0
Iron_sulfide_oxidze
-formula Fes 1 Do 2.25 H2O 1.5 SO4 -1 Feooh -1 H -2
-m 0
As_Crystalline
-formula Bion 1 Aszero 1 Casber -1
-m 0
END
--- End code ---
Best regards,
Zhaoyang
dlparkhurst:
That's too complicated for me. I typically use (1) pure equilibrium, where a standard database is used and reactions occur as CH2O or O2 are kinetically added to the solution, or (2) redox decoupling as in example 9.
Make sure the definitions in SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES represent aqueous species, not mineral phases. For instance Feooh looks a lot like the solid phase Goethite, but not like aqueous species Fe+3, FeOH+2, Fe(OH)2+, etc. Similarly, you have no further speciation definitions for some of your SOLUTION_MASTER_SPECIES, for example As(5) has species AsO4-3, HAsO4-2, H2AsO4-; CO2, CO3-2.
Sorry, I don't know where to begin making your simulation work.
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