Steel slag-limestone reactor for Mn removal with tolerance to Fe: Experiments and mechanisms

Mn generally coexists with Fe in mine drainage and groundwater under reducing conditions. The major limitation in Mn removal by passive treatment has been the inhibition by Fe2+. In this study, an efficient and simple passive treatment system for Mn was developed and verified for tolerance to Fe. The highest Mn removal efficiency and tolerance to Fe2+ were exhibited by steel slag mixed with limestone, which was compared with steel slag and steel slag mixed with Mn-coated gravel. Moreover, additional experiments with CaCO3, NaHCO3, Na2CO3, and steel slag were performed to determine the Mn removal mechanism. Herein, precipitation of Mn as carbonates near the surface of limestone was found to be the major mechanism for the Fe2+ tolerance. Scanning electron microscopy with energy dispersive spectroscopy and X-ray photoelectron spectroscopy analyses and geochemical modeling also supported Mn removal as carbonates by the reaction with limestone. This removal reaction is not inhibited by Fe2+. Furthermore, the Mn carbonates would be easily oxidized to Mn(III,IV) (hydr)oxides. Therefore, steel slag mixed with limestone can be a promising substrate for concurrent Mn and Fe removal in various environments. [Display omitted] •Fe coexisting with Mn inhibits removal of Mn in steel slag reactor.•Steel slag mixed with limestone exhibited the highest tolerance to Fe.•Formation of Mn carbonates near the limestone surface resulted in the tolerance to Fe.•Slag mixed with limestone is a promising substrate to remove Mn in the presence of Fe.