Processes > Oxidation and reduction equilibria

why are the element is not balance before and after the reaction?

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xbk:
Hi!
I am trying to model the reaction between pyrite and nitrate,with various products such as NO2, N2, NH4.only to find that total N after the reaction not equal to total N bofore the reaction.I am wondering  which reason can lead to this result, and how to deal with this problem.
this is my input files below:

SOLUTION_MASTER_SPECIES
       Nitrite       Nitrite-         0     Nitrite         46
SOLUTION_SPECIES
Nitrite- = Nitrite-
    log_k     0
SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mg/kgw
    density   1
    C(4)      363.095
    N(5)      291.765
    Na        236.67
    O(0)      6.5
    P         99.36
    -water    0.45 # kg

RATES
    pyrite
-start
 10  NO3 = MOL("NO3-")
 20 if (NO3 <= 0) then goto 200
 40 K= 2.6/14/1000*0.63*2.415161
 50 Kb=1*0.004
 60 rate = K*NO3/(Kb+NO3)
 70 moles = rate*TIME/2.415161
200 SAVE moles
-end
    denitrosification
-start
 10  Nitrite = MOL("Nitrite-")
 20 if (Nitrite <= 0) then goto 200
 30 NO3=MOL("NO3-")
 40  K=0.008
 50 Kno3 = 0.004
 60 Kn = 0.1
 70 rate= K*Nitrite/(Nitrite+Kn)*Kno3/(Kno3+NO3)
 80 moles = rate*time/15
200 SAVE moles
-end


KINETICS 1
pyrite
    -formula  FeS2  0.59711 N02-  -1.339761 N2  -0.5377 Nitrite-  1.339761 Ntg  0.5377
    -m        10
    -m0       10
    -tol      1e-08
denitrosification
    -formula  FeS2  4 N02-  15 N2  -7.5 Nitrite-  -15 Ntg  7.5
    -m        1
    -m0       1
    -tol      1e-08
-steps       7 in 7 steps # seconds
-step_divide 1
-runge_kutta 3
-bad_step_max 500
-cvode true
-cvode_steps 100
-cvode_order 2


USER_GRAPH 1
    -headings               step Nitrate Nitrite  SO4-2 NH4+ CO2(aq)  o2(aq)  N2  Fe2+
    -axis_titles            "time" "Molality" ""
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  t
  -start
10 graph_x TOTAL_TIME
20 graph_y   mol("NO3-")    mol("Nitrite-")    mol("SO4-2")  mol("AmmH+") mol("CO2") mol("o2")  mol("Ntg")  mol("Tie+2")
  -end
    -active                 true


END

dlparkhurst:

First, let's do the reaction the easy way by simply adding FeS2 to solution. Amm.dat is used, and the rate of pyrite oxidation is defined by RATES. Electron acceptors are chosen by thermodynamics, so that NO3- is reduced to N2, with minimal NO2- formation. The input file is attached as N-equilibrium.pqi.

The hard way is to define kinetic reactions for each redox reaction. I have implemented the first part, where pyrite is oxidized and NO2- is created (attached N-disequilibrium). To complete the process, you need to additional rates. One option is to write a rate that converts NO2- ("[N3]O2- in the file) to N2O (another new "element") and another that converts N2O to N2 (Ntg) based on some rate expressions that should ultimately produce equilibrium among the various redox states.

Note that PHREEQC will add the net change in moles for each element in -formula in KINETICS.

xbk:
Thank you very much for your valuable advice

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