Table 1.
Experimental conditions and resultsa
| Fractional abundances | |||||||||
| Exp. | pH | log ΣH2S | log ΣAs | As(III) | As(V) | S : As 1 : 1 |
S : As 2 : 1 |
S : As 3 : 1 |
S : As 4 : 1 |
| 1 | 7.24 | -7.51 | -4.82 | 0.998 | 0.002 | 0.000 | 0.000 | 0.000 | 0.000 |
| 2 | 7.30 | -5.49 | -4.87 | 0.988 | 0.005 | 0.001 | 0.005 | 0.002 | 0.000 |
| 3 | 7.20 | -4.96 | -4.87 | 0.917 | 0.015 | 0.012 | 0.032 | 0.024 | 0.000 |
| 4 | 7.01 | -4.28 | -4.93 | 0.102 | 0.011 | 0.005 | 0.072 | 0.811 | 0.000 |
| 5 | 7.20 | -3.85 | -4.90 | 0.007 | 0.001 | 0.001 | 0.027 | 0.962 | 0.003 |
| 6 | 7.12 | -3.51 | -4.91 | 0.009 | 0.002 | 0.007 | 0.139 | 0.838 | 0.005 |
| 7 | 7.04 | -3.49 | -4.91 | 0.011 | 0.003 | 0.004 | 0.094 | 0.865 | 0.023 |
| 8 | 7.17 | -2.97 | -4.85 | 0.000 | 0.004 | 0.000 | 0.063 | 0.847 | 0.086 |
| 9 | 7.16 | -3.50 | -5.63 | 0.000 | 0.004 | 0.004 | 0.078 | 0.902 | 0.012 |
| 10 | 7.00 | -3.00 | -5.55 | 0.000 | 0.012 | 0.006 | 0.125 | 0.805 | 0.053 |
| 11 | 6.79 | -5.96 | -4.85 | 0.994 | 0.010 | 0.000 | 0.000 | 0.000 | 0.000 |
| 12 | 7.24 | -5.47 | -4.88 | 0.973 | 0.016 | 0.002 | 0.004 | 0.000 | 0.000 |
| 13 | 7.05 | -4.96 | -4.87 | 0.960 | 0.005 | 0.014 | 0.021 | 0.000 | 0.000 |
| 14 | 7.20 | -4.66 | -4.91 | 0.896 | 0.011 | 0.038 | 0.054 | 0.001 | 0.000 |
| 15 | 7.30 | -4.48 | -4.94 | 0.748 | 0.030 | 0.026 | 0.089 | 0.095 | 0.000 |
| 16 | 9.13 | -5.73 | -4.88 | 0.942 | 0.033 | 0.012 | 0.005 | 0.009 | 0.000 |
| 17 | 10.02 | -4.99 | -4.94 | 0.840 | 0.000 | 0.114 | 0.045 | 0.003 | 0.000 |
| 18 | 9.83 | -4.48 | -4.89 | 0.650 | 0.132 | 0.127 | 0.185 | 0.009 | 0.000 |
| 19 | 9.80 | -3.80 | -4.89 | 0.290 | 0.035 | 0.024 | 0.198 | 0.454 | 0.000 |
| 20 | 10.90 | -3.57 | -4.86 | 0.054 | 0.034 | 0.041 | 0.251 | 0.620 | 0.000 |
| 21 | 9.51 | -4.54 | -4.96 | 0.800 | 0.000 | 0.124 | 0.063 | 0.005 | 0.000 |
| 22 | 9.67 | -4.24 | -4.87 | 0.710 | 0.000 | 0.200 | 0.090 | 0.000 | 0.000 |
| 23 | 9.20 | -3.96 | -4.94 | 0.470 | 0.050 | 0.071 | 0.221 | 0.184 | 0.000 |
aConcentrations in M. Concentrations of ΣH2S were calculated based on gravimetric mixing ratios of the stock NaHS and arsenite solutions, and the measured thioarsenite speciation; ΣH2S concentration in the stock NaHS solution was determined from the average of quadruplicate analyses using the methylene blue colorimetric method. Concentrations of ΣAs were determined by inductively coupled plasma-optical emission spectroscopy on unacidified samples. Precision of measurements: pH ~ 0.03; ΣH2S ~ 5%; ΣAs ~ 5%; species fractional abundance (F) ~5% for F > 0.1, ~10% for 0.1 > F > 0.01, ~20% for F < 0.01.