Surface species in a inverse mass model

[joserevueltas94]

Hello, I am trying to account for HPO4-2 retention in soils as a surface reaction in a inverse mass model, however, I am not sure how to consider in "PHASES" this reaction. I would be grateful for any help you can give me. This is my code.

SURFACE_MASTER_SPECIES
  Surf    SurfOH
SURFACE_SPECIES
  SurfOH = SurfOH
          log_k   0.0
  SurfOH  + H+ = SurfOH2+
          log_k   7.29
  SurfOH2+  + 0.5HPO4-2 = Surf(HPO4)0.5 + H2O
          log_k   7.29

PHASES

CH2O
      CH2O + H2O = CO2 + 4H+ + 4e-

Cysteine
      C3H7NO2S + 8H2O = 3CO2 + NH4+ + SO4-2 + 18e- + 19H+

Ca3(PO4)2(Amorfo)
      Ca3(PO4)2 = 3Ca+2 + 2PO4-3
      log_k -25.2

NO2_1
    NH4+ + 2H2O = NO2- + 8H+ + 6e-

NO3_1
     NO3- + 10 H+ + 8 e-  = NH4+ + 3 H2O

CaCO3:6H20
     CaCO3:6H2O = Ca+2 + CO3-2 + 6H2O
     log_k -7.461

OCP #Octacalcium phosphate
    Ca8H2(PO4)6:5H2O = 8Ca+2 + 2H+ + 6PO4-3 + 5H2O
    log_k -96.6


END




SOLUTION 1   80 cm depth - 20/12/2019   
   units mg/l   
pH   8.20   
Na   253.81   
N(-3)   0.29   as NH4
K   24.17   
Ca   62.49   
Mg   26.72   
Alkalinity   597.90   as HCO3
Cl   166.77   
N(3)   0   as NO2
N(5)   11.41   as NO3
P   11.06   
S(6)   130.93   
      
      
SOLUTION 2   Wastewater 16/01/2020   
   units mg/l   
pH   8.14   
Na   189.42   
N(-3)   25.85   as NH4
K   30.71   
Ca   39.47   
Mg   26.29   
Alkalinity   496.36   as HCO3
Cl   156.11   
N(3)   32.82   as NO2
N(5)   4.32   as NO3
P   4.74   
S(6)   87.33   
      
      
SOLUTION 3   Gravity sample - 16/01/2020   
   units mg/l   
pH   7.88   
Na   243.60   
N(-3)   0.00   as NH4
K   20.87   
Ca   61.86   
Mg   24.02   
Alkalinity   478.57   as HCO3
Cl   180.59   
N(3)   2.35   as NO2
N(5)   41.50   as NO3
P   11.16   
S(6)   134.16   


END

INVERSE_MODELING
        -solutions 1 2 3
        -uncertainty 0.05
        -phases
               Calcite
               #CaCO3:6H20
               Ca3(PO4)2(Amorfo) 
               #OCP
               H2O(g) preci
               KX  preci
               CaX2
               NaX 
               #Cysteine diss
               NH4X  preci
               MgX2
               N2(g) preci
               CH2O  diss
               NO2_1 diss
               #NO3_1 prec
               O2(g)
               Surf(HPO4)0.5
        -range
        -balances
                P
                S(6)
                Mg
                Cl
        -force_solutions true
        #-minimal
       
END

[dlparkhurst]

Looks like you need a small source of P. I would probably write it off to uncertainty in when and where you sampled. I don't think I would use a surface as the source/sink of P, just because it is unclear what the stoichiometry of that reaction is unclear (what cation is involved?). Your apatite is as good a phase as any to account for a change in P, but you don't need too much.

I'd like to see a trend from multiple analyses before I'd draw too many conclusions from inverse modeling. From the data you have, the only thing that seems convincing is that nitrite is oxidized to nitrate. The rest is just a jumble of ion exchange reactions. Wastewater is certainly not a constant composition, so I would guess there is uncertainty with time that could account for most of the differences in cations and other concentrations.

So, if you have considered the trends that you think are consistent, the reactions that seem plausible, and the reasonable sources and sinks for the elements, I think that is what you achieve with inverse modeling. I don't think you will get any definitive result by more effort.