Figure 7
From: Activity of zero-valent sulfur in sulfidic natural waters
Free energy available per mole S0by disproportionation of S0to sulfide and sulfate. Based on data in Figures 5 and 6 and in Table 1 as well as [52]. ΔGR has been plotted against log(aH2S) because most of the ΔGR variation in this dataset is due to variation in H2S. Zero-valent sulfur is shown to be thermodynamically unstable in all the samples; that is, disproportionation would produce a decrease in free energy. The sample at upper right, lying very close to the equilibrium line, is the extremely sulfidic Mahoney Lake sample. It should be noted that ΔGR values depend on how reaction 19 is written; some previous workers [84] write the reaction to produce one mole of SO42−, rather than ¼ mole, and consequently obtain ΔGR values four times larger. This arbitrary choice affects the values on the vertical scale but not the pattern of the data. For reaction 19, ΔGR0 = +30.124 kJ/mol [71].