Sulfur transformations related to revegetation of flue gas desulfurization sludge disposal sites

This study investigated factors controlling redox conditions in flue gas desulfurization (FGD) sludge and identified ways to minimize the production of phytotoxic reduced sulfur (S) species at FGD sludge disposal sites. The oxidation of reduced FGD sludge (Eh -385 mV) appears to be a two-step process mostly controlled by water content. Eighty percent of total sulfide (S^-2) in reduced sludge was oxidized within 20 h of exposure to air with constant water evaporation. When organic carbon (OC) was added to saturated oxidized sludge, the Eh dropped exponentially. Sulfate reduction began at an Eh of about -75 mV and reached a maximum at -265 to -320 mV. Water content, degree of mixing, concentration of OC, and temperature control the rate and extent of reduction of FGD sludge. This suggests that water saturation and OC inputs to revegetated disposal sites should be controlled, especially during warm temperatures, to prevent production of phytotoxic levels of S^-2.