Defining manganese(II) removal processes in passive coal mine drainage treatment systems through laboratory incubation experiments

Oxic limestone beds are commonly used for the passive removal of Mn(II) from coal mine drainage (CMD). Aqueous Mn(II) is removed via oxidative precipitation of Mn(III/IV) oxides catalyzed by Mn(II)-oxidizing microbes and Mn oxide (MnO_x) surfaces. The relative importance of these two processes for Mn removal was examined in laboratory experiments conducted with sediments and CMD collected from eight Mn(II)-removal beds in Pennsylvania and Tennessee, USA. Sterile and non-sterile sediments were incubated in the presence/absence of air and presence/absence of fungicides to operationally define the relative contributions of Mn removal processes. Relatively fast rates of Mn removal were measured in four of the eight sediments where 63–99% of Mn removal was due to biological oxidation. In contrast, in the four sediments with slow rates of Mn(II) removal, 25–63% was due to biological oxidation. Laboratory rates of Mn(II) removal were correlated (R² = 0.62) to bacterial biomass concentration (measured by phospholipid fatty acids (PLFA)). Furthermore, laboratory rates of Mn(II) removal were correlated (R² = 0.87) to field-scale performance of the Mn(II)-removal beds. A practical recommendation from this study is to include MnO_x-coated limestone (and associated biomass) from an operating bed as “seed” material when constructing new Mn(II)-removal beds.