Alkalinity generation by Fe(III) reduction versus sulfate reduction in wetlands constructed for acid mine drainage treatment

Despite the widespread use of wetlands for acid mine drainage (AMD) treatment, alkalinity generating mechanisms in wetlands and their abiotic and biotic controls are poorly understood. While both dissimilatory sulfate reduction and Fe(III) reduction are alkalinity-generating mechanisms, only the former has been considered as important in wetlands constructed for AMD treatment. This study was conducted to determine the extent to which Fe(III) reduction occurs and the extent to which sulfate reduction versus Fe(III) reduction contributes to alkalinity generation in 5 wetlands constructed with different organic substrates (Sphagnum peat with limestone and fertilizer, Sphagnum peat, sawdust, straw/ manure, mushroom compost) that had been exposed to the same quality and quantity of AMD for 18–22 months. These substrates had Fe oxyhydroxide concentrations of 250–810 μmol Fe g−1 dry substrate. Flasks containing 100 g of wet substrate along with either 150 mL of wetland water or 130 mL of wetland water and 20 mL of 37 % formalin were incubated at 4 °C in January and 25 °C in May. On days 0, 2, 4, 8, 12 and 16, the slurry mixtures were analyzed for concentrations of H+, Fe2+ and SO42−. The bulk of the evidence indicates that for all except the mushroom compost wetland, especially at 25 °C, biologically-mediated Fe(II) reduction occurred and generated alkalinity. However, in none of the wetlands, regardless of incubation temperature, was there evidence to support net biological sulfate reduction or its attendant alkalinity generation. Sulfate reduction and concurrent Fe(III) oxyhydroxide accumulation may be important in the initial stages of wetland treatment of AMD, both contributing to effective Fe retention. However, as Fe(III) oxyhydroxides accumulate over time, Fe(III) reduction could lead not only to decreased Fe retention, but also to the potential net release of Fe from the wetland.