Pyrite dissolution at pH range

[Anvj0705]

Dear parkhurst,

I am trying to model my experimental results of pyrite dissolution (In N2 environment)in pH range 3.5 to 11.5 .
I put the parameters in the Palandri and Kharaka 2004 rate equation and run the program but I am getting negliable dissolution but in experiment it was around 1000 ppm Total Fe.

Code: [Select]

# Title Pyrite Dissolution

SOLUTION 1
temp 25
redox S(-2)/S(6)
units mol/kgw
ph 3.5
-water 1 #kg

PHASES 1
Fix_H+
H+ = H+
log_k 0

EQUILIBRIUM_PHASES 1
Fix_H+ -3.5 NaOH 10


KINETICS 1
Pyrite
 #-m 4.16E-3          # moles of solid per kg of water
 -m0 4.16E-3
 -parms 1   1    # total surface area per kg of water (m2/kgw) and the scaling factor
 -steps 108000 second in 100 steps        # define time steps
 INCREMENTAL_REACTIONS TRUE

RATES 1
pyrite
# from Palandri and Kharaka 2004
# experimental condition range T=20-40C, pH=1-4

-start
1 rem unit should be mol,kgw-1 and second-1
2 rem parm(1) is surface area in the unit of m2/kgw
3 rem calculation of surface area can be found in the note
4 rem M is current moles of minerals. M0 is the initial moles of minerals
5 rem parm(2) is a correction factor
10 rem acid solution parameters
11 a1=2.82E+02
12 E1=56900
13 n1=-0.500
14 n3=0.500
30 rem neutral solution parameters
31 a3=2.64E+05
32 E3=56900
33 n2=0.500
36 rem rate=0 if no minerals and undersaturated
40 SR_mineral=SR("pyrite")
41 if (M<0) then goto 200
42 if (M=0 and SR_mineral<1) then goto 200
43 if (M0<=0) then SA=PARM(1) else SA=PARM(1)*(M/M0)^0.67
50 if (SA<=0) then SA=1
60 R=8.31451
75 Rate1=a1*EXP(-E1/R/TK)*ACT("H+")^n1*ACT("Fe+3")^n3  #acid rate expression
80 Rate2=a3*EXP(-E3/R/TK)*ACT("O2")               #neutral rate expression
90 Rate=(Rate1+Rate2)*(1-Sr_mineral)*SA*parm(2)
100 moles= rate*Time
200 save moles
-end


SELECTED_OUTPUT 1
     -file pyrite_dissolution_1.txt
     -reset false

USER_PUNCH 1
-headings time SI(pyrite) pH Tot_Fe
-start
 10 punch TOTAL_TIME/3600 SI("Pyrite") LA("H+")*(-1) TOT("Fe")

USER_GRAPH 1
    -headings               time Fe S
    -axis_scale sy_axis     auto auto
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X -LA("H+")
20 GRAPH_Y TOT("Fe"),TOT("S")
  -end
    -active                 true

END

Palandri and Kharaka 2004, Rate equations are valid for small range pH 1-4. So Is there any other rate equation for alkaline pH range?

Thank you!!

[dlparkhurst]

Your rates are zero because both O2 and Fe+3 have zero concentrations in your solution. Here, I added atmospheric oxygen to a simplified calculation.

Code: [Select]

# Title Pyrite Dissolution
PHASES 1
Fix_H+
H+ = H+
log_k 0

RATES 1
pyrite
# from Palandri and Kharaka 2004
# experimental condition range T=20-40C, pH=1-4

-start
1 rem unit should be mol,kgw-1 and second-1
2 rem parm(1) is surface area in the unit of m2/kgw
3 rem calculation of surface area can be found in the note
4 rem M is current moles of minerals. M0 is the initial moles of minerals
5 rem parm(2) is a correction factor
10 rem acid solution parameters
11 a1=2.82E+02
12 E1=56900
13 n1=-0.500
14 n3=0.500
30 rem neutral solution parameters
31 a3=2.64E+05
32 E3=56900
33 n2=0.500
36 rem rate=0 if no minerals and undersaturated
40 SR_mineral=SR("pyrite")
41 if (M<0) then goto 200
42 if (M=0 and SR_mineral<1) then goto 200
43 if (M0<=0) then SA=PARM(1) else SA=PARM(1)*(M/M0)^0.67
50 if (SA<=0) then SA=1
60 R=8.31451
75 Rate1=a1*EXP(-E1/R/TK)*ACT("H+")^n1*ACT("Fe+3")^n3  #acid rate expression
80 Rate2=a3*EXP(-E3/R/TK)*ACT("O2")               #neutral rate expression
90 Rate=(Rate1+Rate2)*(1-Sr_mineral)*SA*parm(2)
100 moles= rate*Time
200 save moles
300 PRINT rate, time, rate1, rate2
-end
END
SOLUTION 1
temp 25
units mol/kgw
ph 3.5
O(0) 1 O2(g) -0.7
-water 1 #kg
END

USE solution 1

EQUILIBRIUM_PHASES 1
Fix_H+ -3.5 NaOH 10
KINETICS 1
Pyrite
 #-m 4.16E-3          # moles of solid per kg of water
 -m0 4.16E-3
 -parms 1   1    # total surface area per kg of water (m2/kgw) and the scaling factor
# -steps 108000 second in 100 steps        # define time steps
-steps 108000 #in 100 steps
INCREMENTAL_REACTIONS TRUE

USER_GRAPH 1
    -headings               time Fe S
    -axis_scale sy_axis     auto auto
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME
20 GRAPH_Y TOT("Fe"),TOT("S")
  -end
    -active                 true

END

[dlparkhurst]

Accidently left a PRINT statement in the RATES definition. But, it does illustrate how you can use PRINT statements to understand the rate calculation if needed. The printed values go to the output file.

[Anvj0705]

Thank you for your valuable comments.

But I worked in <10 ppm  oxygen environment and still I did not get the desired Iron concentration as I got in my experiment.

[dlparkhurst]

All I am saying is that you have two rates in the RATES definition, one depends on ACT("O2") and one depends on ACT("Fe+3). Using water without O2 and without Fe+3 as your initial condition, both rates will always be essentially zero.

So, you could set non-zero values for O2 and (or) Fe+3 in your initial condition. One option would be to start with analyzed concentrations near the beginning of your experiment. Then you can adjust parameters to match the remainder of your experiment.

Alternatively, you can add an additional Rate3 in your RATES definition that is non-zero in the absence of those two species, say a rate that depends only on H+.

You need something to get your rates started. After that you can change surface area, rate constants, and other parameters to match your rates.