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.

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

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 (ß-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.

Deciphering the photocatalytic degradation of polyaromatic hydrocarbons (PAHs) using hausmannite (Mn3O4) nanoparticles and their efficacy against bacterial biofilm.

Polyaromatic hydrocarbons (PAHs) are life-threatening organic pollutants that severely threaten ecosystems worldwide due to their poisonous qualities, cancer-causing properties, and mutation-causing qualities. Water and soil together form a critical component of the ecosystem that supports all life. Due to the pollutants that are being disposed of in them, their characteristics have changed, and their toxicity has increased. The goal of this study was to investigate the ability of hausmannite nanoparticles to degrade fluorene from soil and water. Using the chemical method, hausmannite nanoparticles were synthesized and further characterization was performed using UV-Vis, FTIR, DLS, XRD, and SEM-EDAX. Hausmannite significantly degraded fluorene using the batch adsorption method. The degradation was also confirmed by performing reactive kinetics using Freundlich's isotherm model and Langmuir's pseudo-second-order model of soil and water. In addition to the degradation efficacy, hausmannite was also proved to inhibit biofilm formation by Pseudomonas aeruginosa. The findings of the experiments confirmed the presence of hausmannite nanoparticles, as well as their physical properties, chemical properties, degradation properties, and parameters of the kinetic study. As a result, synthesized nanoparticles have been extensively utilized as a low-cost option for removing pollutants and microbial biofilm.

Plant mediated biosynthesis of Mn3O4 nanostructures and their biomedical applications.

Nanomaterials have drawn significant attention for their biomedical and pharmaceutical applications. In the present study, manganese tetra oxide (Mn3O4) nanoparticles were prepared greenly, and their physicochemical properties were studied. Taxus baccata acetone extract was used as a safely novel precursor for reducing and stabilizing nanoparticles. The synthesized nanoparticles were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Brunauer-Emmett-Teller (BET), and Barrett-Joyner-Halenda (BJH) and X-ray diffraction (XRD). The cytotoxicity of Mn3O4 (hausmannite) nanostructures was evaluated against murine macrophage cell line J774-A1 and U87 glioblastoma cancer cells for approximately 72 h. Spherical Mn3O4 nanoparticles with tetragonal spinel structures demonstrated minimal toxicity against normal body cells with CC50 around 876.38 µg mL-1. Moreover, Mn3O4 nanoparticles as well as the combination of antimoniate meglumine and Mn3O4 nanoparticles exhibited maximum mortality in Leishmania major. The synthesized nanominerals displayed a significant inhibitory effect against glioblastoma cancer cells at 100 µg mL-1. The selective cytotoxicity of Mn3O4 nanoparticles indicates that these biogenic agents can be employed simultaneously for diagnostic and therapeutic applications in medical applications.

Reductive transformation of birnessite and the mobility of co-associated antimony.

Manganese (Mn) oxide minerals, such as birnessite, are thought to play an important role in affecting the mobility and fate of antimony (Sb) in the environment. In this study, we investigate Sb partitioning and speciation during anoxic incubation of Sb(V)-coprecipitated birnessite in the presence and absence of Mn(II)aq at pH 5.5 and 7.5. Antimony K-edge XANES spectroscopy revealed that Sb(V) persisted as the only solid-phase Sb species for all experimental treatments. Manganese K-edge EXAFS and XRD results showed that, in the absence of Mn(II), the Sb(V)-bearing birnessite underwent no detectable mineralogical transformation during 7 days. In contrast, the addition of 10 mM Mn(II) at pH 7.5 induced relatively rapid (within 24 h) transformation of birnessite to manganite (~93%) and hausmannite (~7%). Importantly, no detectable Sb was measured in the aqueous phase for this treatment (compared with up to ∼90 µmol L-1 Sb in the corresponding Mn(II)-free treatment). At pH 5.5 , birnessite reacted with 10 mM Mn(II)aq displayed no detectable mineralogical transformation, yet had substantially increased Sb retention in the solid phase, relative to the corresponding Mn(II)-free treatment. These findings suggest that the Mn(II)-induced transformation and recrystallization of birnessite can exert an important control on the mobility of co-associated Sb.

Synthesis, phase transformation, and morphology of hausmannite Mn3O4 nanoparticles: photocatalytic and antibacterial investigations.

Nano structured Hausmannite (Mn3O4) has efficacious applications in numerous fields, such as catalytic, medical, biosensors, waste water remediation, energy storage devices etc. The potential application in wastewater treatment is due to its distinct structural features combined with fascinating physicochemical properties. Another area of interest is the oxidative properties imparted due to its reduction potential. Larger surface to volume ratio and high reactivity than the bulk form shows great progress as antimicrobial agent to control drug resistant microbial population. The distinct surface morphologies, crystalline forms, reaction conditions and synthetic methods exerts significant impact on the photo catalytic and bactericidal efficiency. Hence, the present paper focuses on a concise review of the multifarious study on synthetic methods of Mn3O4, growth mechanisms, structural forms, phase transformation and phase control, shape and dimensionality. The review also confers its applications towards photo catalytic and bactericidal studies.

Electrostatic Spray Deposition-Based Manganese Oxide Films-From Pseudocapacitive Charge Storage Materials to Three-Dimensional Microelectrode Integrands.

In this study, porous manganese oxide (MnOx) thin films were synthesized via electrostatic spray deposition (ESD) and evaluated as pseudocapacitive electrode materials in neutral aqueous media. Very interestingly, the gravimetric specific capacitance of the ESD-based electrodes underwent a marked enhancement upon electrochemical cycling, from 72 F∙g-1 to 225 F∙g-1, with a concomitant improvement in kinetics and conductivity. The change in capacitance and resistivity is attributed to a partial electrochemical phase transformation from the spinel-type hausmannite Mn3O4 to the conducting layered birnessite MnO2. Furthermore, the films were able to retain 88.4% of the maximal capacitance after 1000 cycles. Upon verifying the viability of the manganese oxide films for pseudocapacitive applications, the thin films were integrated onto carbon micro-pillars created via carbon microelectromechanical systems (C-MEMS) for examining their application as potential microelectrode candidates. In a symmetric two-electrode cell setup, the MnOx/C-MEMS microelectrodes were able to deliver specific capacitances as high as 0.055 F∙cm-2 and stack capacitances as high as 7.4 F·cm-3, with maximal stack energy and power densities of 0.51 mWh·cm-3 and 28.3 mW·cm-3, respectively. The excellent areal capacitance of the MnOx-MEs is attributed to the pseudocapacitive MnOx as well as the three-dimensional architectural framework provided by the carbon micro-pillars.