dissolution ＆ precipitation thioarsenate

[chaoyu]

Please advise all the scholars in this forum, I am trying to generate thioarsenate by creating new reactant and new reactant (polysulfide) but after I do this no thioarsenate will be produced in the output, please let me know what is the reason for this

Code: [Select]

SOLUTION_MASTER_SPECIES
    S(0)          S2-2             0     64.12         
    As(s)         HAsSO3-2         0     155.943       
    S(00)         S3-2             0     96.12         
    S(r)             S               0     32.06
SOLUTION_SPECIES
H2AsO3- + S2-2 = HAsSO3-2 + HS-
    log_k     3.58
AsO3-3 + 2H+ = H2AsO3-
    log_k     29.2
HS- + S = S2-2 + H+
    log_k     -4.78
HS- = S + H+ + 2e-
    log_k     -4.57
H+ + HS- + S = H2S2
    log_k     10.36
HS- + 2S = S3-2 + H+
    log_k     2.92
HS- + 2S = HS3-
    log_k     10.75
H+ + HS- + 2S = H2S3
    log_k     15.06
HS- + S = HS2-
    log_k     5.25
H2S + H3AsO4 = H3AsSO3 + H2O
    log_k     11
AsO3-3+HS-+4H+=H3AsSO2+H2O
    log_k     47
AsO3-3+HS-+3H+=H2AsSO2-+H2O
    log_k     41.8
AsO3-3+HS-+2H+=HAsSO2-2+H2O
    log_k     27.9
AsO4-3+HS-+3H+=H2AsSO3-+H2O
    log_k     35.8
AsO4-3+HS-+2H+=HAsSO3-2+H2O
    log_k     25.6
AsO4-3+HS-+H+=AsSO3-3+H2O
    log_k     17.6
AsO4-3+2e-+2H+=AsO3-3+H2O
    log_k     2.17
SOLUTION_SPREAD
    -units    ug/l
 Temperature            pe    pH    As(3)    As(5)      S(00)      S(0) S(-2)
                                    ug/l     ug/l       ug/l      ug/l      
        14.1 -1.541881977 8.63 126.935   47.970 20929.408      8.64      
        14.4   2.086372895 8.53    0.000 120.924          0    47.712      
        14.2   10.48115309 8.36    0.000    3.651   9890.304 3317.664      
        12.3   2.990342718 8.16    0.000    1.398          0   141.408      
        12.4 -0.632757985 7.64    9.534   37.140   9903.552 7032.672      
        13.8   3.730690363 8.15    0.000    1.461   15455.68   6867.84      
        12.4 -2.182045643 8.44   17.955 298.290          0         0      
        14.8 -2.127175279 8.13 102.410   23.090          0         0      
        11.6 -2.106863404 8.42 194.780   44.340   4618.368 3301.248      
          16   10.89292045 8.49    0.000    0.636          0         0      
        14.3   3.332002665 8.34    1.443    3.156          0         0      
        13.4   1.052091751 8.05    1.591   25.252          0   594.624      
        15.1 -1.999247223 8.75 110.960 116.780    4130.24 3295.488      
        14.4    -2.5099486 8.21   67.590   36.770          0         0      
          15   -1.99819353 8.74           50.930          0      40.8      
        16.7 -1.807634672 8.38 215.590   96.320          0         0      
        13.8 -1.860960261 8.32 157.740   92.610   6120.576      7032      
        12.9 -2.181755166 8.63 152.690   49.860          0         0      
        12.5   0.222004539 7.75    0.034    7.600          0    94.752      
        12.8   7.973228224 8.34    0.000    1.005          0         0      
        13.8 -2.071436445 8.56 219.950   61.430    3827.52 7332.864      


SELECTED_OUTPUT 1
    -file                 selected_output_1.sel
    -totals               As(3)  As(3aqq)  As(5)  As(s)
    -molalities           AsO3-3  AsO4-3
    -activities           AsO3-3  AsO4-3  H2AsO3-  H2AsO4-
                          H2AsSO2-  H2AsSO3-
END


[dlparkhurst]

I am old and it has been years since I looked at the literature for polysulfides, but I think, at least in some papers, the polysulfides assume equilibrium with native sulfur.

However, you think there is an S(aq) species. One way to define the system is to assume there are three redox states of sulfur--S(6), S(0), and S(-2). You do not provide the entire As redox states, and I do not know what database you are using. I have taken AsO4-3 species from wateq4f.dat and used phreeqc.dat, which does not have As definitions, so the input file defines the entire As system.

I am using the definitions for S(6) and S(-2) from phreeqc.dat.

Still, I am not sure what you measured. With PHREEQC, you can only define concentrations for redox states defined in SOLUTION_MASTER_SPECIES. With the script below, you can define concentrations in SOLUTION for As(3), As(5), S(6), S(0), and S(-2).  I probably did not define the concentrations for S(0) and S(-2) correctly, but it depends on which species are included in your measurements.

Code: [Select]

SOLUTION_MASTER_SPECIES
As       H3AsO4         -1.0     74.9216         74.9216
As(+3)   AsO3-3         0.0      74.9216         
As(+5)   H3AsO4         -1.0     74.9216
S(0)          S 0     S   

SOLUTION_SPECIES
#As(5)
H3AsO4 = H3AsO4
    log_k           0.0
H3AsO4 = H2AsO4- + H+
    log_k -2.3
    delta_h -7.066 kJ
H3AsO4 = HAsO4-2 + 2H+
    log_k -9.46
    delta_h -3.846 kJ
H3AsO4 = AsO4-3 + 3H+
    log_k -21.11
    delta_h 14.354 kJ

#As(3)
AsO4-3 + 2e- + 2H+ = AsO3-3 + H2O
    log_k     2.17
AsO3-3 + 2H+ = H2AsO3-
    log_k     29.2

#S(0)
HS- = S + H+ + 2e-
    log_k     -4.57

#S(0)-S(-2)
HS- + S = S2-2 + H+
    log_k     -4.78
HS- + S = HS2-
    log_k     5.25
H+ + HS- + S = H2S2
    log_k     10.36
HS- + 2S = S3-2 + H+
    log_k     2.92
HS- + 2S = HS3-
    log_k     10.75
H+ + HS- + 2S = H2S3
    log_k     15.06


#As(3)-S(0)-S(-2)
H2AsO3- + S2-2 = HAsSO3-2 + HS-
    log_k     3.58
AsO3-3 + HS- + 4H+ = H3AsSO2 + H2O
    log_k     47
AsO3-3 + HS- + 3H+ = H2AsSO2- + H2O
    log_k     41.8
AsO3-3 + HS- + 2H+ = HAsSO2-2 + H2O
    log_k     27.9

#As(5)-S(-2)
H2S + H3AsO4 = H3AsSO3 + H2O
    log_k     11
AsO4-3 + HS- + 3H+ = H2AsSO3- + H2O
    log_k     35.8
AsO4-3 + HS- + 2H+ = HAsSO3-2 + H2O
    log_k     25.6
AsO4-3 + HS- +H+ = AsSO3-3 + H2O
    log_k     17.6

SOLUTION
-units ug/L
-temp 14.1
-pe   -1.54
-pH   8.63
As(3) 126.935
As(5) 47.97
S(0)  20929
S(-2) 8.64
END

It looks like you wanted to define the concentration of S3-2; I'm not sure which species that includes.  An alternative formulation would be to define S(0) [with S(aq) as the secondary master species], S(-1) [with S2-2 as the secondary master species], and S(-0.67) [with S3-2 as the secondary master species] as redox states in SOLUTION_MASTER_SPECIES. S, S2-2, and S3-2 would be defined with reactions that contain e-, H2, or O2 to make them secondary master species. You could then define total concentrations of S(0) (S), S(-1) (S2-2, HS2-, and H2S2) and S(-0.67) (S3-2, HS3-, and H2S3) in SOLUTION, in addition to S(6) and S(-2).