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why are the element is not balance before and after the reaction?
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Topic: why are the element is not balance before and after the reaction? (Read 3061 times)
xbk
Contributor
Posts: 2
why are the element is not balance before and after the reaction?
«
on:
April 18, 2016, 10:41:01 AM »
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
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dlparkhurst
Top Contributor
Posts: 3665
Re: why are the element is not balance before and after the reaction?
«
Reply #1 on:
April 18, 2016, 11:05:11 PM »
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.
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xbk
Contributor
Posts: 2
Re: why are the element is not balance before and after the reaction?
«
Reply #2 on:
April 19, 2016, 09:44:00 PM »
Thank you very much for your valuable advice
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why are the element is not balance before and after the reaction?