Environmental Factors Controlling the Electronic Properties and Oxidative Activities of Birnessite Minerals

Abstract

Recent studies of birnessite-type lattices show that they have an unusually low-lying band structure with an electron affinity value of 6.0 eV, which is the highest among all common oxides and sulfides in an aqueous solution. This study reports on the underlying mechanism through which environmental factors, such as organic and inorganic ligands, exert a strong influence on the band structure and oxidative reactivity of various synthetic and natural birnessite phases. Specifically, the study evaluated the effect of (i) disorder/crystallinity, (ii) common aqueous cations, and (iii) Mn(II) and Mn(III) complexing anionic ligands on Cr(III) to Cr(VI) oxidation activity, Mn dissolution, and the band structure of birnessite mineral with the aim of finding correlations between them. The results show that the high oxidative activity of various birnessites is coupled with their high electron affinity, which, in turn, is modulated by various aqueous anions and cations through their dynamic influence on the dissolution-controlled structural Mn(III)/Mn(II) content. Inorganic cations of different ionic radii and hydration strengths control the interlayer spacing of the basal sheets, while inorganic and organic aqueous anions control the extent of Mn(II) and Mn(III) complexation. Their combined effects are seen to govern the efficiency of Mn dissolution from the crystal, which modulates the birnessite activity and band structure.