Sulfate solution + H2S reaction

[Alberto]

Hello. I am trying to solve a problem without success. First some background:

A reaction used to precipitate sulfides from acidic sulfate solutions, using soluble sulfides (like Na2S, NaHS or H2S(aq)) is:

MSO4 + HS- = MS + SO4-2 + H+

However, thermodynamics predicts that if I add soluble sulfides to an acidic sulfate solution then elemental sulfur forms. If sulfur actually forms, this reaction will ruin the process by consuming sulfide+ sulfate instead of the dissolved metals. As far as I know, this does not happen, so I guess the reaction is slow below the boiling point of water.

In effect, PHRREQC   predicts a strong reaction between sulfate and sulfide (see attached pqi file showing the reaction between H2S and sulfate). In that file I tried to set the reaction velocity to zero, without success: the sulfate is completely consumed and turned into aqueous polysulfides.

Thank you in advance for your help.

[dlparkhurst]

First, any aqueous species containing the element "S" will react to equilibrium. The total amount of S in solution will be distributed among S aqueous species.

Second, the KINETIC reaction adds or removes elements from solution. The change in concentration in solution is the net change from the -formula definition. Thus, if you define

CH2O -1 CH4 0.5 CO2 0.5,

the net addition of C, H, and O is zero for each element, and the kinetic reaction will have no effect.

Third, I have never been a proponent of using polysulfide species. If you look in detail, it is often assumed that the polysulfide species are in equilibrium with native sulfur. I don't think that is necessarily the case. To me the polysulfide species are metastable species that are not stable in the long term. If you think elemental sulfur should form, you can add it as an equilibrium phase.

The only reason you are reacting aqueous sulfate and sulfide is because you have defined polysulfide species, either in the input file or the database file. If you use phreeqc.dat, then those reactions are not possible because the species are not defined, which I think in most cases is a better assumption. You say yourself that you do not expect the reactions that you are seeing.

Last, you have defined equilibrium with 1 atm H2(g) and 1atm H2S(g). Is this really what you want to do? You have defined zero moles of each present, so they will not have an effect until you have a lot of reaction progress.

[Alberto]

I did that because:

1) polysulfide species are expected to form when there are high quantities of aqueous sulfide and aqueous elemental sulfur. They exist, and ignoring them would likely be a mistake, specially when considering reactions like:

Pyrite + H2SO4 (aq, high concentration) = FeSO4(aq) + (1-x)H2S(aq) + xS°(aq)

Because pyrite is iron di-sulfide (i.e. the polysulfide S2-2) I tried to simulate that reaction, but I don't get the H2S. Then I tried with phyrrotite (FexS), that is well known to liberate H2S when adding any strong acid, and got the same result.

Something was wrong, then I removed the iron and tried with just sulfur compounds. Same problem.

2)I used H2S and H2 as EQUILIBRIUM_PHASES because I don't want the solution to become supersaturated with them at the later stages of reaction.

3)If I also include solid sulfur as an equilibrium phase then I got almost the same result, except that with solid sulfur instead of aqueous polysulfides.

My point is that I want to suppress sulfate reduction by sulfide, because that reaction is not observed in nature. I tried KINETICS because my guess is that while the reaction is thermodynamically possible, it is extremely slow at room temperature.

[dlparkhurst]

Sulfide will not reduce sulfate in the simulations unless there is an intermediate valence state that can form, namely the polysulfides. If you do not want sulfide to reduce sulfate, then remove the polysulfides. If you want equilibrium among all sulfur species, then your calculations indicate what thermodynamics has to say.

Alternatively, if you must have polysulfides, you must. (I don't think you need them.) You can set up a partial redox equilibrium system, which is not very easy in PHREEQC. For instance you could separate sulfide into its own element so that it would not react with sulfate except by a KINETIC process.

[Alberto]

You can set up a partial redox equilibrium system, which is not very easy in PHREEQC. For instance you could separate sulfide into its own element so that it would not react with sulfate except by a KINETIC process

Can you tell me how can I do that?

If I define sulfate and sulfide as different elements, how can I balance the reaction:

HS- + SO4-2 + 3H+ = 2S°(aq) + 2H2O   {here the "elements" sulfide-sulfur and sulfate-sulfur will be annihilated to make elemental sulfur, violating the mass balance}

I don´t want to completely eliminate that reaction, I just want to make it extremely slow using KINETICS (but I don't know how)

[dlparkhurst]

The following generates S0 slowly. You can adjust the rate constant to make it faster or slower. You are probably thinking about keeping all of the aqueous polysulfides, which this input file does not. But it does do what you asked for "I don´t want to completely eliminate that reaction, I just want to make it extremely slow using KINETICS".


SOLUTION_MASTER_SPECIES
    [S0aq]        [S0aq]           0     S               32
SOLUTION_SPECIES
   [S0aq] = [S0aq]
      log_k 0
SOLUTION 1
   pH   7
   S(6) 1
   S(-2) 1
   Na   1 charge
END
RATES
    S0_formation
-start
10 k = 100/(86400*365)
20 rate = k * TOT("S(6)") * TOT("S(-2)")
30 moles = rate * TIME
40 save moles
-end
END
USE solution 1
KINETICS 1
S0_formation
    -formula  S  -1 [S0aq]  1
    -tol      1e-08
-steps       3.15e8 in 10 steps # seconds
USER_GRAPH 1
    -headings               Years Sulfate Sulfide S0
    -axis_titles            "Years" "Moles per kilogram water" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x TOTAL_TIME / (86400 * 365)
20 graph_y TOT("S(6)"), TOT("S(-2)"), TOT("[S0aq]")
  -end
    -active                 true

[Alberto]

Thank you for the answer. This will make KINETICS work.

However, in the real world if you give a system enough time, it will approach equilibrium. For S°(aq) we have the following equilibrium:

HS- = [S0aq] + H+ + 2e-
    log_k     -5.41
    delta_h   65 kJ

(Of course this equation can be put also in terms of oxygen, but using e- the equation is simpler)

How can I make that the system approach equilibrium in the limit where time is very large?

[dlparkhurst]

SOLUTION_MASTER_SPECIES
    [S0aq]        [S0aq]           0     S               32
SOLUTION_SPECIES
   [S0aq] = [S0aq]
      log_k 0
SOLUTION 1
   pH   7
   temp 30
   S(6) 1
   S(-2) 1
   Na   1 charge
END
RATES
    S0_formation
-start
10 k = 1e12/(86400*365)
20 log_k = -5.41 - 65.0 * (298.15 - TK) / (LOG(10.0) * TK * 0.0083147 * 298.15)
30 log_equil = LA("[S0aq]") + LA("H+") + 2 * LA("e-") - LA("HS-") - log_k
40 rate = k * TOT("S(6)") * TOT("S(-2)") * 10^(1-log_equil)
50 moles = rate * TIME
60 save moles
-end
END
USE solution 1
INCREMENTAL_REACTIONS
KINETICS 1
S0_formation
    -formula  S  -1 [S0aq]  1
    -tol      1e-08
-steps       3.15e8 in 100 steps # seconds
USER_GRAPH 1
    -headings               Years Sulfate Sulfide S0
    -axis_titles            "Years" "Moles per kilogram water" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x TOTAL_TIME / (86400 * 365)
20 graph_y TOT("S(6)"), TOT("S(-2)"), TOT("[S0aq]")
  -end
    -active                 true

[ehsan]

hey dear
i have a problem with modeling of H2S in water in phreeqc
please help me to solve my problem.

[dlparkhurst]

OK.

Code: [Select]

SOLUTION
REACTION
H2S 1
0.001 mol
END