Inhibition of Oxyanions on Redox-driven Transformation of Layered Manganese Oxides

Abstract

Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn­(II)-driven reductive transformation of δ-MnO2, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6–8 and a wide range of Mn­(II)/MnO2 molar ratios. Without the oxyanions, δ-MnO2 transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn­(II)/MnO2 ratios (0.09 and 0.13) and into δ-MnOOH and Mn3O4 with minor poorly crystallized α- and γ-MnOOH at high Mn­(II)/MnO2 ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn­(II,III)-oxyanion ternary complexes on vacancies of δ-MnO2, adversely interfering with electron transfer, Mn­(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.

Impacts of environmental levels of hydrogen peroxide and oxyanions on the redox activity of MnO2 particles

“…can be generated at high Mn(II)/MnO2 ratios (0.5 and 1). 60 In our XRD patterns, broad bands (hump) have appeared between 12° and 22° and between 25° and 35° in the samples reacted with Mn(II) (Mn(II)/MnO2 molar ratio = 0.3) or H2O2 (H2O2/MnO2 molar ratio = 0.6), suggesting changes in the structure or ordering of the birnessite mineral sheets (Fig. 4).…”

“…It was previously reported that unlike nitrate, silicate or phosphate may interact with MnO2 through hydrogen bonding or formation of outer-sphere complexes with surface hydroxyl groups of MnO2. [59][60][61][62] On the other hand, previous studies reported that the presence of dissolved silicate may decrease the oxidation rate constants of MnO2 toward chlorinated compounds. 63,64 They have attributed the decrease in MnO2 reactivity to the surface-bound silicate but none of them have provided adsorption data of silicate onto MnO2.…”

Synchronous Moderate Oxidation and Adsorption on the Surface of γ-MnO2 for Efficient Iodide Removal from Water

“…Dissolved silicate tends to adsorb onto colloidal MnO 2 by hydrogen bonds and cover the MnO 2 surface; further particle aggregation and growth are inhibited accordingly . Additionally, phosphate and silicate form oxygen anion ternary complex with Mn­(II,III) at the vacancy of MnO 2 and interfere with the electron transfer and structure arrangement . This effect inhibits the adsorption of iodine accordingly.…”

“…43 Additionally, phosphate and silicate form oxygen anion ternary complex with Mn(II,III) at the vacancy of MnO 2 and interfere with the electron transfer and structure arrangement. 44 This effect inhibits the adsorption of iodine accordingly. These anions compete for and occupy the active sites on the γ-MnO 2 surfaces, and iodine removal is inhibited by these different mechanisms.…”

In Situ Regulation of MnO2 Structural Characteristics by Oxyanions to Boost Permanganate Autocatalysis for Phenol Removal

“…9 Additionally, by studying Raman spectra and transformation behavior of MnO 2 nanoparticles in the presence of various oxyanions, a generalized effect of oxyanions on the structural alteration in MnO 2 has been proposed, including adjustment in the size, average Mn valence, oxygen species, and preferentially exposed planes of MnO 2 . 13,18,19 These structural characteristics are crucial for determining the oxidative and catalytic capacity of MnO 2 . 15,20 Therefore, oxyanions may affect the degradation efficiency of Mn(VII) by controlling the oxidative and catalytic capacity of MnO 2 .…”