Antimonate Controls Manganese(II)-Induced Transformation of Birnessite at a Circumneutral pH

Abstract

Manganese (Mn) oxides, such as birnessite (δ-MnO2), are ubiquitous mineral phases in soils and sediments that can interact strongly with antimony (Sb). The reaction between birnessite and aqueous Mn­(II) can induce the formation of secondary Mn oxides. Here, we studied to what extent different loadings of antimonate (herein termed Sb­(V)) sorbed to birnessite determine the products formed during Mn­(II)-induced transformation (at pH 7.5) and corresponding changes in Sb behavior. In the presence of 10 mM Mn­(II)aq, low Sb­(V)aq (10 μmol L–1) triggered the transformation of birnessite to a feitknechtite (β-Mn­(III)­OOH) intermediary phase within 1 day, which further transformed into manganite (γ-Mn­(III)­OOH) over 30 days. Medium and high concentrations of Sb­(V)aq (200 and 600 μmol L–1, respectively) led to the formation of manganite, hausmannite (Mn­(II)­Mn­(III)2O4), and groutite (αMn­(III)­OOH). The reaction of Mn­(II) with birnessite enhanced Sb­(V)aq removal compared to Mn­(II)-free treatments. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that heterovalent substitution of Sb­(V) for Mn­(III) occurred within the secondary Mn oxides, which formed via the Mn­(II)-induced transformation of Sb­(V)-sorbed birnessite. Overall, Sb­(V) strongly influenced the products of the Mn­(II)-induced transformation of birnessite, which in turn attenuated Sb mobility via incorporation of Sb­(V) within the secondary Mn oxide phases.

Understanding the role of manganese oxides in retaining harmful metals: Insights into oxidation and adsorption mechanisms at microstructure level

“…(1) Research on Mn oxides' interactions with less abundant, toxic metals like thallium (Tl) [ 49 , 91 , 129 , 257 ] and antimony (Sb) [ 189 , [258] , [259] , [260] , [261] , [262] ] is limited and warrants further exploration.…”