H2S oxidation

[hkkv]

Hi,

I am trying to model the reduction of H2S from groundwater by oxidation.
The formula and the rate are given:

HS- + 2O2 = SO4-2 + H+

0.003325*S(-2)^1.34 * O2(aq)^0.56

what am i doing wrong? and how should i enter the formula in the KINETICS block?

Code: [Select]

TITLE Hydrogen Sulfide Oxidation
PHASES
Fix_H+
       H+ = H+
       log_k  0
SOLUTION 1 groundwater
units mg/l
water 1
temp 25
pH 7.5 charge # pH=5.763
Sg 1 # dissolved H2S


EQUILIBRIUM_PHASES 1
Fix_H+   -7.5   NaOH   10 #2.475e-05 mol NaOH required to fix pH to 7.5 
#Fix_H+   -7   NaOH   10
#Fix_H+   -8   NaOH   10
SAVE SOLUTION 2
END

USE SOLUTION 2
EQUILIBRIUM_PHASES 2
   CO2(g)   -3.398     # 0.04%, log(0.0004)=-3.398
   Oxg(g)   -0.678     # 21%,   log(0.21)=-0.678     # O2(g) decoupled in phreeqc.dat only
   Ntg(g)   -0.102     # 79%,   log(0.79)=-0.102     # N2(g) decoupled in phreeqc.dat only     
SAVE  SOLUTION  3
END

USE SOLUTION 3
RATES
H2S_Oxidation                     
-start   #r = k * [S(-2)]^m * [O2]^n             
10 rate = parm(1) * TOT("Sg")^parm(2) * MOL("Oxg")^parm(3)
20 moles = rate * time           
30 save moles                   
-end

KINETICS
H2S_Oxidation
 #HS- + 2O2(g) = SO4-2 + H+.
 
-formula 
-parms 0.003325  1.34  0.5     # k  m  n
-steps  2592000  in  30 steps   # 30 days in seconds

INCREMENTAL_REACTIONS true


USER_GRAPH 1
-headings
-chart_title "Hydrogen sulfide removal"
-axis_titles "Time[day]" "sulfites [mg/ L]"   
-connect_simulations true
-start
10 graph_x TOTAL_TIME/86400 
20 graph_y  KIN("H2S_Oxidation")
-end
END


[dlparkhurst]

I think you need to add O2 to your -formula. That at least will get you started.

Code: [Select]

-formula O2 +1


[MichaelZ20]

In this case SO4 will not increase.
May be it will be better to set O2(g) instead of Oxg(g) in EQUILIBRIUM_PHASES, O2 instead of Oxg in RATES, and to use
-formula Sg -1   O2 -2  SO4 1 ?

[dlparkhurst]

Here is a revision of your script. Nothing wrong with the way you started, but this is a bit simpler. The main point to look at is the -formula. You are converting unreactive Sg to reactive S and unreactive Oxg to reactive O2.

Code: [Select]

SOLUTION 2 groundwater
units mg/l
water 1
temp 25
pH 7.5 # pH=5.763
Sg 1 # dissolved H2S
Na    1 charge
END

USE SOLUTION 2
EQUILIBRIUM_PHASES 2
   CO2(g)   -3.398     # 0.04%, log(0.0004)=-3.398
   Oxg(g)   -0.678     # 21%,   log(0.21)=-0.678     # O2(g) decoupled in phreeqc.dat only
   Ntg(g)   -0.102     # 79%,   log(0.79)=-0.102     # N2(g) decoupled in phreeqc.dat only     
SAVE  SOLUTION  3
END

USE SOLUTION 3
RATES
H2S_Oxidation                     
-start   #r = k * [S(-2)]^m * [O2]^n             
10 rate = parm(1) * TOT("Sg")^parm(2) * MOL("Oxg")^parm(3)
20 moles = rate * time           
30 save moles                   
-end

KINETICS
H2S_Oxidation
 #HS- + 2O2(g) = SO4-2 + H+.
 
-formula  Sg -1 H2S +1 Oxg -1 O2 +2
-parms 0.003325  1.34  0.5     # k  m  n
-steps  2592000  in  30 steps   # 30 days in seconds

INCREMENTAL_REACTIONS true


USER_GRAPH 1
-headings
-chart_title "Hydrogen sulfide removal"
-axis_titles "Time[day]" "sulfites [mg/ L]"   
-connect_simulations true
-start
10 graph_x TOTAL_TIME/86400
20 graph_y  KIN("H2S_Oxidation")
-end
END


[MichaelZ20]

I think that the formula should be
-formula  HSg -1  HS +1  Oxg -1  O2 +2 ,
since following the considered equation (HS- + 2O2(g) = SO4-2 + H+), just HS- (not H2S) is oxidized.

[dlparkhurst]

Thanks Michael, you are right that Sg represents just the sulfur, not the hydrogen. I correct my formula as follows:

Code: [Select]

-formula  Sg -1 S +1 Oxg -1 O2 +2

Using HSg and HS is not wrong, but PHREEQC only considers the net reaction, so H is removed with HSg, but added with HS, with net zero change in the H concentration.