Synthesis of manganese spinel nanoparticles at room temperature by coprecipitation

Effects of cobalt doping on the reactivity of hausmannite for As(III) oxidation and As(V) adsorption

Hausmannite is a common low valence Mn oxide mineral, with a distorted spinel structure, in surficial sediments. Although natural Mn oxides often contain various impurities of transitional metals (TMs), few studies have addressed the effect and related mechanism of TM doping on the reactivity of hausmannite with metal pollutants. Here, the reactivity of cobalt (Co) doped hausmannite with aqueous As(III) and As(V) was studied. Co doping decreased the point of zero charge of hausmannite and its adsorption capacity for As(V). Despite a reduction of the initial As(III) oxidation rate, Co-doped hausmannite could effectively oxidize As(III) to As(V), followed by the adsorption and fixation of a large amount of As(V) on the mineral surface. Arsenic K-edge EXAFS analysis of the samples after As(V) adsorption and As(III) oxidation revealed that only As(V) was adsorbed on the mineral surface, with an average As-Mn distance of 3.25-3.30 Å, indicating the formation of bidentate binuclear complexes. These results provide new insights into the interaction mechanism between TMs and low valence Mn oxides and their effect on the geochemical behaviors of metal pollutants.

Hausmannite as potential As(V) filter. Macroscopic and spectroscopic study of As(V) adsorption and desorption by citric acid

Arsenic (As) is a toxic element that leads the list of human health threats and is one of the priority contaminants in soil and water. In order to remove As(V) and/or reduce its mobility, filters and amendments with high affinity for As(V) adsorption are used in drinking water treatment or directly applied to the soil, thereby promoting its immobilization. Hausmannite and hematite were compared by in-situ Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) spectroscopy and batch experiments for evaluating As(V) adsorption and sequential desorption by citrate. The pH and contact time were used as variables. Hausmanite adsorbed more As(V) than hematite. As(V) was adsorbed on the mineral surface of simultaneously inner- and outer-sphere species. Inner-sphere bidentate complex form preferentially at high pH, early adsorption time and low surface loading, while the monodentate species should be responsible to increase total As(V) adsorption at low pH, later adsorption kinetics and higher As(V) surface loading. Citrate was effective in causing As(V) desorption at higher citric acid concentrations and higher pH values. After a long time of incubation, the neogenesis of a manganite by hausmnannite oxidation was observed. Concomitantly, less As(V) was desorbed by citrate desorption, even in the presence of high citric acid concentrations. Hausmannite was an efficient mineral for As(V) removal and immobilization.

Fast Microwave-Assisted Hydrothermal Synthesis of Pure Layered δ-MnO₂ for Multivalent Ion Intercalation

This work reports on the synthesis of layered manganese oxides (δ-MnO₂) and their possible application as cathode intercalation materials in Al-ion and Zn-ion batteries. By using a one-pot microwave-assisted synthesis route in 1.6 M KOH (Mn_VII:Mn_II = 0.33), a pure layered δ-MnO₂birnessite phase without any hausmannite traces was obtained after only a 14 h reaction time period at 110 °C. Attempts to enhance crystallinity level of as-prepared birnessite through increasing of reaction time up to 96 h in 1.6 M KOH failed and led to decreases in crystallinity and the emergence of an additional hausmannite phase. The influence of Mn_II:OH⁻ ratio (1:2 to 1:10) on phase crystallinity and hausmannite phase formation for 96 h reaction time was investigated as well. By increasing alkalinity of the reaction mixture up to 2.5 M KOH, a slight increase in crystallinity of birnessite phase was achieved, but hausmannite formation couldn’t be inhibited as hoped. The as-prepared layered δ-MnO₂ powder material was spray-coated on a carbon paper and tested in laboratory cells with Al or Zn as active materials. The Al-ion tests were carried out in EMIMCl/AlCl₃ while the Zn-Ion experiments were performed in water containing choline acetate (ChAcO) or a ZnSO₄ solution. Best performance in terms of capacity was yielded in the Zn-ion cell (200 mWh g⁻¹ for 20 cycles) compared to about 3 mAh g⁻¹ for the Al-ion cell. The poor activity of the latter system was attributed to low dissociation rate of tetrachloroaluminate ions (AlCl₄⁻) in the EMIMCl/AlCl₃ mixture into positive Al complexes which are needed for charge compensation of the oxide-based cathode during the discharge step.