Migration and Transformation Behavior of Mn2+ and NH4+-N in Electrolytic Manganese Residue at Different Leaching pH Environments: Release Kinetic Model, Physical Phase Changes, and Formation of Manganese Oxide

Mn2+, NH4+-N, and heavy metals in electrolytic manganese residue (EMR) have severely polluted the environment. This study investigated the migration and transformation behaviors of Mn2+ and NH4+-N in EMR at different leaching environments. The results showed that the release of Mn2+ and NH4+-N was a secondary kinetic process. Physical phase analysis indicated that Mn2+ and NH4+-N lost in leaching were mainly derived from (NH4)2Mg(SO4)2·6H2O, (NH4)2SO4, and MnSO4·H2O from EMR, and Mn lost from EMR during leaching was mainly exchangeable and in a soluble state, followed by a carbonate bond state. NH4+-N was specifically retained in EMR by electrostatic and ion exchange interactions, and a part of NH4+-N in EMR can be converted to NO3–-N on the surface of manganese oxide formed during the alkaline leaching environment. The accumulated maximum release of Mn2+ and NH4+-N under an alkaline leaching environment was significantly reduced compared to an acidic environment, since Mn2+ mainly reacted with OH– to generate Mn(OH)2, MnOOH, and MnO2. In addition, releasing of heavy metal ions from EMR was a continual process. This study provides theoretical support for the harmless treatment and resource utilization of EMR.