Arsenic transport and transformation

[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: [Select]

##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

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.