Oxidation-Reduction of Sulfur

The form of sulfide present in the soil and sediment depends on pH, as shown below  

Under acidic conditions H2S is stable, and it will change to HS and to S as pH is increased  

FIGURE 11.4 Eh-pH diagram of distribution of sulfur species solution at 1 atm pressure and 25°C. (Modified from Krauskopf, 1979.) 

Sulfur exists in wetland soils in a variety of oxidation states between +6 and -2 and may be present in gaseons, solnble, and solid forms. Its most common oxidation states in wetlands are -2 (sulfhydrl, R-SH) and snlflde (HS ), 0 (elemental snlfnr), and +6 (sulfate) (Table 11.4). 

Oxidation State +6 +5 Example Sulfate Dithionate Formula soj- Sjor TABLE 11.5 Sulfur Cycling Processes 

---