Highly enhanced oxidation of arsenite at the surface of birnessite in the presence of pyrophosphate and the underlying reaction mechanisms

Abstract

Manganese(IV) oxides, and more especially birnessite, rank among the most efficient metal oxides for As(III) oxidation and subsequent sorption, and thus for arsenic immobilization. Efficiency is limited however by the precipitation of low valence Mn (hydr)oxides at the birnessite surface that leads to its passivation. The present work investigates experimentally the influence of chelating agents on this oxidative process. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite was investigated using batch experiments and different arsenic concentrations at circum-neutral pH. In the absence of PP, Mn(II/III) species are continuously generated during As(III) oxidation and adsorbed to the mineral surface. Field emission-scanning electron microscopy, synchrotron-based X-ray diffraction and Fourier transform infrared spectroscopy indicate that manganite is formed, passivating birnessite surface and thus hampering the oxidative process. In the presence of PP, generated Mn(II/III) species form soluble complexes, thus inhibiting surface passivation and promoting As(III) conversion to As(V) from 60% in the absence of PP to 100% with 3 mM PP (0.5 mM initial As(III)). Enhancement of As(III) oxidation by Mn oxides strongly depends on the affinity of the chelating agent for Mn(III) and from the induced stability of Mn(III) complexes. Compared to PP, the positive influence of oxalate, for example, on the oxidative process is more limited. The present study thus provides new insights into the possible optimization of arsenic removal from water using Mn oxides, and on the possible environmental control of arsenic contamination by these ubiquitous non-toxic mineral species.

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

“…52 For example, exposure to Mn(III)-complexing PP has been shown to retard the activity of disordered Na-birnessite (δ-MnO 2 ) for Cr(III) to Cr(VI) conversion, 27 whereas similar exposure enhances the oxidation of isovalent As(III) to As(V) by hexagonal birnessite. 53 Contrary to this, adsorption of other Mn(III)-complexing ligands such as citrate 54 or phosphate 55,56 suppresses the oxidation As(III) by hexagonal birnessite, which has been attributed to passivation of adsorption sites by ligand complexes. Thus, multiple coupled factors, in addition to the crystallographic structure and composition, likely influence the oxidative response of the birnessite.…”

“…The different phases also have varying responses to ligand environments . For example, exposure to Mn­(III)-complexing PP has been shown to retard the activity of disordered Na-birnessite (δ-MnO 2 ) for Cr­(III) to Cr­(VI) conversion, whereas similar exposure enhances the oxidation of isovalent As­(III) to As­(V) by hexagonal birnessite . Contrary to this, adsorption of other Mn­(III)-complexing ligands such as citrate or phosphate , suppresses the oxidation As­(III) by hexagonal birnessite, which has been attributed to passivation of adsorption sites by ligand complexes.…”

Field and Laboratory Evidence for Manganese Redox Cycling Controlling Iron and Arsenic Retention in Household Sand Filters

“…25,26 We suggest that the flower-like nano morphology is similar to the birnessite structure reported in previous studies. 26,27 The identified mineral structures could originate from the filter materials or were newly formed during filtration. Further SEM imaging of the local raw sand materials should be included in future studies to better distinguish the origins of these precipitates.…”

Photocatalytic Generation of Reactive Oxygen Species on Fe and Mn Oxide Minerals: Mechanistic Pathways and Influence of Semiconducting Properties

“…14 In contrast, dissolved Mn III and the Mn III -containing birnessite family of minerals are a powerful oxidant, 15 which is due to their high standard reduction potential of the Mn III /Mn II redox couple, and can oxidize many heavy metals such as Cr III to Cr VI and As III to As V as well as many organic compounds. 16,17 Another striking difference pertains to the crystallographic arrangement.…”

“…Both aqueous Fe II and Fe II -containing green rust (GR) are potent reductants that can reduce nitrate, nitrite, chlorinated hydrocarbons (ethane, ethylene), and many heavy metals, including Cr VI to Cr III , Se VI to Se 0 , U VI to U IV , and Au III . In contrast, dissolved Mn III and the Mn III -containing birnessite family of minerals are a powerful oxidant, which is due to their high standard reduction potential of the Mn III /Mn II redox couple, and can oxidize many heavy metals such as Cr III to Cr VI and As III to As V as well as many organic compounds. , Another striking difference pertains to the crystallographic arrangement. Although both birnessite and GR have a flexible layered structure with typical 7–11 Å d 100 -spacing, in birnessite, the interlayer species consists of cations (Na I , K I , and others) with a single sheet of water molecules, while in GR, the interlayer species are anions, such as SO 4 2– , CO 3 2– , and Cl – , that compensate the excess positive charges of structural Fe III .…”