Insights into the stability assessment and reaction mechanisms of Mn-oxide-containing adsorbents for As(Ⅲ) removal in filter columns: Migration laws and stabilization mechanisms of Mn element

Abstract

This study focused on two Mn-oxide-containing adsorbents for As(Ⅲ) removal, namely granular iron-manganese composite oxide (GFMO) and granular iron-manganese-copper composite oxide (GFMCO). The comparative experiments results demonstrated that GFMCO exhibited superior performance in As(Ⅲ) removal and a more obvious Mn(II) release compared to GFMO. Furthermore, this study explored the approaches for the control of manganese release during As(Ⅲ) removal, identifying sodium hypochlorite (NaClO) oxidation followed by manganese sand filtration as the most effective method for capturing released Mn(Ⅱ) in water. Manganese sand columns effectively captured released Mn(Ⅱ) from effluent, while chlorine oxidation significantly improved manganese removal. The positive effect of copper on Mn(Ⅱ) removal by oxidants was also assessed. In addition, the solution pH significantly impacted manganese removal efficiency, with alkaline conditions being the most conducive. Moreover, the presence of sulfite notably accelerated manganese release. Characterization of the adsorption columns indicated that the manganese element undergoes release, migration, and speciation transformation within the filter systems, where redox reactions, adsorption processes, and autocatalytic oxidation processes were all involved. Not only NaClO oxidation but also autocatalytic oxidation with newly-formed Mn oxides contributed to the transformation Mn(Ⅱ) to Mn oxides, promoting the stabilization of Mn element in manganese sand filtration columns. This study provides valuable insights into the stability of Mn-oxide-containing adsorbents for As(Ⅲ) removal in the filter system and highlights on engineered approaches to control the transformation and migration of released manganese ions.