NO2- redox

[Nicole5734]

Hi All,

I am a rookie and trying to simulate a redox reaction between NO2- and Fe2+. Experiments show that at pH 7 (buffered with HEPES), NO2- is stable in solution (added as NaNO2, 0.2 mM, solution is anoxic), and it would oxidize Fe(II) to Fe(III) if Fe(II) is added. When I used PHREEQC to make the solution, I found that even before I added Fe(II) to the solution (the solution only contained H2O and NaNO2, no O2),  NO2- was decomposed to NO3- and N2 (almost no NO2- left in the solution). I am wondering why is that. My code is as follows:

SOLUTION 1
   Temp   25
   pH      7   
   Na      0.2
   N(+3)   0.2

PHASES
        Fix_H+
        H+ = H+
        log_k  0.0
EQUILIBRIUM_PHASES 1      # to fix pH at 7
        Fix_H+   -7.0   NaOH    10.
      
SAVE solution 1
END


Then I used PhreePlot to construct an Eh-pH diagram for N (to see under what conditions NO2- would be stable). The diagram only shows NH4+, NH3, N2 and NO3-, the 4 species, no NO2-. I used llnl database and I checked that NO2- was in there. Why it dose not show on the Eh-pH diagram?

I am really confused why NO2- can not even exist in the solution, and wondering how PHREEQC does the calculation to decide that it can not exist (in reality NO2- is stable in solutions at neutral pH). Any comments and suggestions would be greatly appreciated. Thank you!

Nicole

[dlparkhurst]

Thermodynamics says the N2(aq) and NO3- tend to overshadow NO2-. However, the kinetics of N2(aq) reactions can be slow. In your system, you may want to assume that formation of N2(aq) does not occur. You can simulate this by decreasing the log K for the N2 definition. For example

Code: [Select]

SOLUTION_SPECIES
2 NO3- + 12 H+ + 10 e- = N2 + 6 H2O
-log_k 0 # 207.08

If you add this to your file, then N2(aq) will be negligible, and NO2- will be stable over a wider set of conditions. If you add Fe(2), it will be oxidized and NO2- will be reduced, probably to N(-3), but possibly to other intermediate valences of N are defined in your databasel. The nitrogen system is a little tricky in that you almost have to define which reactions you will allow.

In natural systems NO2- is usually relatively minor relative to NO3-, N2(aq), and NH4+. In addition, biological reactions tend to reduce NO3- to N2(aq), where it is relatively inert, whereas oxidation of N(-3) tends to skip N2 and got to nitrate.

[Nicole5734]

Thank you so much, dlparkhurst! It makes sense! I really appreciate it!
Based on the experiments, NO2- would be reduced to N2(g), NH4+ and N2O(g). So I am thinking, for simulation, I want to maybe suppress the formation of NO3-, but allow the formation of N2 and NH4+, so that NO2- can only be reduced not oxidized. If I want to do that, do I need to delete all the equations regarding NO3- and HNO3 in the database? Thank you very much!

[dlparkhurst]

I looked at the N system, and I think it may be easiest to start over. Here I have defined a new element "[N]", which is not related to N. [N]O2- is the new SOLUTION_MASTER_SPECIES. I have left a definition of [N]O3-, but disabled it by using a small log K. I also found that O2 and [N]2 were still more stable than [N]O2-, so I also disabled O2(aq).

Hopefully, I did the math right for rewriting the equations to [N]O2-. I was working from phreeqc.dat or Amm.dat. If want another database, you will have to use the appropriate log Ks and delta Hs. You will need to add [N]2(g) if gas will escape the solution, and you will need to add [N]2O(aq) and [N]2O(g) if you need them. However, I am afraid that like NO2-, N2O will not be thermodynamically stable relative to other aqueous species. If so, you may need to treat some of these reactions kinetically.

Code: [Select]

SOLUTION_MASTER_SPECIES
[N] [N]O2- 0 [N] 14.0067
[N](+5)     [N]O3- 0 [N]
[N](+3) [N]O2- 0 [N]
[N](0) [N]2 0 N
[N](-3) [N]H4+ 0 [N] 14.0067

SOLUTION_SPECIES
2 H2O = O2 + 4 H+ + 4 e-
#-log_k -86.08
      -log_k      -186.08
-delta_h 134.79 kcal
-dw 2.35e-9
-Vm  5.7889  6.3536  3.2528  -3.0417  -0.3943 # supcrt

[N]O2- = [N]O2-
-gamma 3.0 0
-dw 1.91e-9
-Vm  5.5864  5.8590  3.4472  -3.0212  1.1847 # supcrt

[N]O2- + H2O = [N]O3- + 2 H+ + 2 e-
# -log_k -28.570
      -log_k      -100
-delta_h 43.760 kcal
-gamma 3.0 0
-dw 1.9e-9  184  1.85  3.85
-Vm  6.32  6.78  0  -3.06  0.346  0  0.93  0  -0.012  1 # ref. 1

2[N]O2- + 8H+ + 6e- = [N]2 + 4H2O
      -log_k      149.94
      -delta_h    -224.61
-dw 1.96e-9
-Vm 7 # Pray et al., 1952, IEC 44. 1146

[N]O2-  + 8 H+ + 6 e- = [N]H4+ + 2 H2O
-log_k 90.507
-delta_h  -143.295 kcal
-gamma 2.5 0
-dw 1.98e-9  312  0.95  4.53
-Vm  4.837  2.345  5.522  -2.88 1.096  3  -1.456  75.0  7.17e-3  1 # ref. 1