δ-MnO2 Surface Research Articles

MnO2 as efficient and robust catalyst for halogen-free synthesis of cyclic carbonate from CO2 and epoxides

Cyclic carbonate is an important fine chemical/intermediate and their synthesis from CO2 and epoxides is economical and green pathway, however, it is still a challenge to develop a efficient and robust catalyst for halogen-free synthesis of cyclic carbonate. Herein, we successfully prepared large-scale MnO2 catalysts by microfluidics method, and the MnO2 especially δ-MnO2 showed >99.9 % propylene carbonate yield under 393 K, 3 MPa CO2 and DMF solvent. Moreover, the initial TOF value over δ-MnO2 on the basis of total catalyst amount was 5.80 h−1, which was the maximum among all the reported heterogeneous metallic oxide catalysts for halogen-free synthesis of propylene carbonate from CO2 and epoxides, and the superior catalytic performance can be ascribed to the abundant moderate acidic–basic active sites on the δ-MnO2 surface and the synergistic effect of δ-MnO2 and DMF. Furthermore, the δ-MnO2 can be reused at least 4 times and exhibited wide substrate applicability.

Understanding interaction mechanism between δ-MnO2 and Li2O2 in nonaqueous lithium-oxygen batteries

Focusing on the δ-MnO2 acting as catalysts in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes of the cathode in nonaqueous lithium-oxygen battery, the adsorption models of key intermediates LixOy on the δ-MnO2 surface and the interface model of δ-MnO2 and Li2O2 crystals are established based on density functional theory (DFT) calculations. In terms to the energy change, the δ-MnO2 shows appropriate adsorption energy to the Li2O2 molecule and crystal, benefiting to the proceeding of ORR and OER processes. As for the electron distribution, the δ-MnO2 is an indirect band gap semi-conductor, the semi-conductive characteristic is preserved after the LixOy adsorption, going against the initial Li2O2 nucleation. During the OER process, the three-phase interface of δ-MnO2/Li2O2/O2 proves to be electronic conductive, guaranteeing the capability of combination between electrons and lithium ions. To further optimize the electronic conductivity of δ-MnO2, the δ-MnO2 doped with Li element, which proves to be a stable structure, is proposed to enhance the catalytic activity.

HCHO oxidation over the δ-MnO2 catalyst: Enhancing oxidative activities of surface lattice oxygen and surface adsorbed oxygen by weakening Mn-O bond

The usages of HCHO in construction and decoration materials lead to serious indoor air pollution, removing the indoor HCHO to improve air quality is urgently required. In this work, we developed a new procedure to synthesis ultra-thin nanosheet δ-MnO2 without using traditional templates. The Mn-O bond was much weaker in the ultra-thin nanosheet δ-MnO2 than those in the β- and γ-MnO2. As a consequence, surface oxygen lattice of the δ-MnO2 was the most removable, generating the most oxygen vacancy on the surface. The oxidative ability of the lattice oxygen was the most enhanced, active surface adsorbed oxygen species were the most present on the δ-MnO2 surface. These contributed to an unexpected performance that 100 ppm HCHO was fully oxidized at 35–45 °C depending on the relative humidity between 10 and 50% at high space velocity of 300 L/(gh). When compared with HCHO oxidation performance of MnO2 in literatures, the current ultra-thin nanosheet δ-MnO2 lists the most active HCHO oxidation catalyst as regards to the lowest activation energy, the lowest temperature of full HCHO conversion and the highest rate of HCHO oxidation, giving the most promising potential application for practical indoor HCHO elimination.

Effect of manganese oxides on arsenic speciation and mobilization in different arsenic-adsorbed iron-minerals under microbially-reducing conditions

The oxidation and immobilization of arsenic (As) by manganese oxides have been shown to reduce As toxicity and bioavailability under abiotic conditions. In this study, we investigate the impact of manganese oxide (δ-MnO2) on the fate of different Fe-minerals-adsorbed As in the presence of As(V)-reducing bacteria Bacillus sp. JQ. Results showed that in the absence of δ-MnO2, As release in goethite was much higher than in ferrihydrite and hematite during microbial reduction. Adding 3.1 mM Mn reduced As release by 0.3%, 46.3%, and 6.7% in the ferrihydrite, goethite, and hematite groups, respectively. However, aqueous As was dominated by As(III) in the end, because the oxidation effect of δ-MnO2 was limited and short-lived. Additionally, the fraction of solid-phase As(V) increased by 9.8% in ferrihydrite, 39.4% in goethite, and 7.4% in hematite in the high-Mn treatments, indicating that δ-MnO2 had the most significant oxidation and immobilization effect on goethite-adsorbed As. This was achieved because goethite particles were evenly distributed on δ-MnO2 surface, which supported As(III) oxidation by δ-MnO2; while ferrihydrite strongly aggregated, which hindered the oxidation of As(III). Our study shows that As-oxidation and immobilization by manganese oxides cannot easily be assessed without considering the mineral composition and microbial conditions of soils.

Study on the Adsorption Mechanism of Cobalt and Nickel in Manganese Sulfate by δ-MnO2.

Manganese has excellent performance in removing metal ions from aqueous solutions, but there are few studies on the adsorption and removal of heavy metal impurities in metal salt solutions. In this paper, the adsorption of cobalt and nickel ions in MnSO4 solution by δ-MnO2 prepared from two different manganese sources was studied. The optimum adsorption conditions were as follows: When the concentration of Mn2+ was 20 g/L, δ-MnO2 addition was 10 g/L, Co2+ concentration was 80 mg/L, Ni2+ concentration was 80 mg/L, reaction time was 60 min, reaction temperature was 80 °C, and pH value was 7, the adsorption rate of Co2+ and Ni2+ reached more than 80%. The manganese dioxide adsorbed by heavy metals was analyzed and detected. The results showed that MnOOH appeared in the phases of both kinds of δ-MnO2, and their morphologies were dense rod-like structures with different lengths and flake-like structures of fine particles. Co and Ni were distributed on the surface and gap of MnO2 particles, and the atomic percentage of Co was slightly higher than that of Ni. The new vibration peaks appeared near wave numbers of 2668.32, 1401.00, and 2052.19 cm–1, which were caused by the complexation of cations such as Co2 + and Ni2 + with hydroxyl groups. Some cobalt and nickel appeared on the surface of δ-MnO2, and the surface oxygen increased after adsorption. The above characterization revealed that the adsorption of cobalt and nickel in manganese sulfate by δ-MnO2 was realized by the reaction of its surface hydroxyl with metal ions (M) to form ≡SOMOH.

Oxygen Limitation Accelerates Regeneration of Active Sites on a MnO2 Surface: Promoting Transformation of Organic Matter and Carbon Preservation.

Birnessite (δ-MnO2) is a layered manganese oxide widely present in the environment and actively participates in the transformation of natural organic matter (NOM) in biogeochemical processes. However, the effect of oxygen on the dynamic interface processes of NOM and δ-MnO2 remains unclear. This study systematically investigated the interactions between δ-MnO2 and fulvic acid (FA) under both aerobic and anaerobic conditions. FA was transformed by δ-MnO2 via direct electron transfer and the generated reactive oxygen species (ROS). During the 32-day reaction, 79.8% of total organic carbon (TOC) in solution was removed under anaerobic conditions, unexpectedly higher than that under aerobic conditions (69.8%), suggesting that oxygen limitation was more conducive to the oxidative transformation of FA by δ-MnO2. The oxygen vacancies (OV) on the surface of δ-MnO2 were more exposed under anaerobic conditions, thus promoting the adsorption and transformation of FA as well as regeneration of the active sites. Additionally, the reaction of FA with δ-MnO2 weakened the strongly bonded lattice oxygen (Olatt), and the released Olatt was an important source of ROS. Interestingly, a part of organic carbon (OC) was preserved by forming MnCO3, which might be a novel mechanism for carbon preservation. These findings contribute to an improved understanding of the dynamic interface processes between MnO2 and NOM and provide new insights into the effects of oxygen limitation on the cycling and preservation of OC.

Coprecipitation mechanisms of Zn by birnessite formation and its mineralogy under neutral pH conditions

Birnessite (δ-Mn(IV)O2) is a great manganese (Mn) adsorbent for dissolved divalent metals. In this study, we investigated the coprecipitation mechanism of δ-MnO2 in the presence of Zn(II) and an oxidizing agent (sodium hypochlorite) under two neutral pH values (6.0 and 7.5). The mineralogical characteristics and Zn–Mn mixed products were compared with simple surface complexation by adsorption modeling and structural analysis. Batch coprecipitation experiments at different Zn/Mn molar ratios showed a Langmuir-type isotherm at pH 6.0, which was similar to the result of adsorption experiments at pH 6.0 and 7.5. X-ray diffraction and X-ray absorption fine structure analysis revealed triple-corner-sharing inner-sphere complexation on the vacant sites was the dominant Zn sorption mechanism on δ-MnO2 under these experimental conditions. A coprecipitation experiment at pH 6.0 produced some hetaerolite (ZnMn(III)2O4) and manganite (γ-Mn(III)OOH), but only at low Zn/Mn molar ratios (< 1). These secondary precipitates disappeared because of crystal dissolution at higher Zn/Mn molar ratios because they were thermodynamically unstable. Woodruffite (ZnMn(IV)3O7•2H2O) was produced in the coprecipitation experiment at pH 7.5 with a high Zn/Mn molar ratio of 5. This resulted in a Brunauer–Emmett–Teller (BET)-type sorption isotherm, in which formation was explained by transformation of the crystalline structure of δ-MnO2 to a tunnel structure. Our experiments demonstrate that abiotic coprecipitation reactions can induce Zn–Mn compound formation on the δ-MnO2 surface, and that the pH is an important controlling factor for the crystalline structures and thermodynamic stabilities.

Nature of High- and Low-Affinity Metal Surface Sites on Birnessite Nanosheets

Birnessite nanosheets (δ-MnO2) are key reactive nanoparticles that regulate metal cycling in terrestrial and marine settings, yet there is no molecular explanation for the sorption selectivity of metals which controls their enrichment. This fundamental question was addressed by optimizing the structure of Ni, Cu, Zn, and Pb surface complexes on δ-MnO2 and by calculating the Gibbs free-energy change (ΔG) of the sorption reactions with density functional theory. The sorption selectivity follows the order Pb > Cu > Ni > Zn in good agreement with experimental data. Cu, Ni, and Zn bind preferentially to layer edges at low surface coverage forming double-edge-sharing (DES) complexes, whereas Pb binds extensively with high selectivity over the three transition metals to both layer edges (DES bonding) and to basal planes forming triple-corner-sharing (TCS) complexes. Pb has a similar affinity for the DES and TCS sites at pH 5 and a higher affinity for the TCS sites at circumneutral pH. The Pb DES and TCS complexes are both dehydrated at the δ-MnO2–water interface and feature a trigonal pyramidal geometry with three surface O atoms. The high stability of the two new Pb complexes arises from the hybridization between the Pb 6s/6p and O 2p states, forming a strong covalent Pb–O/OH bond at the δ-MnO2 surface. The quantum chemical results provide a mechanistic and energetics insight into the metal uptake on δ-MnO2 that extends what extended X-ray absorption fine structure (EXAFS) spectroscopy alone can provide.

Insight into influence mechanisms of pyrite and vernadite on the degradation performance of 2,2′,5-trichlorodiphenyl in a pyrophosphate-chelated Fenton-like reaction

Pyrophosphate (PP)-chelated Fenton-like reaction (PP-Fe2+/H2O2, PCFR) has been proven as a promising remediation means for polychlorinated biphenyl-polluted soils. Pyrite (FeS2) and vernadite (δ-MnO2) are typically hot-spot soil redox minerals, but whether both of them can promote the degradation performance of PCFR has been still ambiguous. Therefore, this study insightfully investigated the influence degree and corresponding mechanisms of FeS2 and δ-MnO2 on the degradation of 2,2′,5-trichlorobiphenyl (PCB18) in a PCFR. Results demonstrated that FeS2 slightly increased the degradation efficiency and kinetic rate constant of PCB18 by 1.2% and 17.0%, respectively, but δ-MnO2 significantly decreased them by 51.9% and 93.1%, respectively, in the PCFR. The slight promoting effect of FeS2 was attributed to the homogeneous Fenton-like reaction catalyzed by the released Fe ions from FeS2, rather than the acceleration of aqueous Fe(III/II) cycle through the direct oxidation of FeS2 because the formal potential of PP-complexed Fe3+/Fe2+ pair was lower than that of SO42−/FeS2 pair. As for δ-MnO2, the aggravation of H2O2 disproportionation by δ-MnO2 and the generation of amorphous ferric (hydr)oxide from the adsorbed PP-complexed Fe3+ on the surface of δ-MnO2 played a dominant role in decreasing the degradation performance of PCB18 in the PCFR. Meanwhile, the beneficial effects of Fenton-like reactions catalyzed by surface Mn and released Mn2+ from δ-MnO2 were markedly suppressed by both detrimental pathways. Results of this study revealed that the highly redox activity of FeS2 and δ-MnO2 was unable to efficiently enhance the remediation performance of PCFR.

Photo-stimulated anoxic reduction of birnessite (δ-MnO2) by citrate and its fine structural responses: Insights on a proton-promoted photoelectron transfer process

The photo-enhanced dissolution of semiconducting birnessite by organic reductants prevalently occurs in the euphotic zones of natural aquatic systems. However, the underlying photoelectrons transfer mechanisms and kinetic factors are not fully understood. In this study, the anoxic photoreduction of δ-MnO2 (a phase analogue for natural birnessite) by citrate under simulated solar irradiation at pH 4.0, 5.5 and 7.0 was investigated, and the structural evolution of δ-MnO2 during the reaction was examined. The photoreduction of δ-MnO2 was observed at all three pHs. ~86.0% and ~32.7% Mn2+ was released into the solution at pH 4.0 and 5.5, respectively; while negligible amount of dissolved Mn2+ was produced at pH 7.0. Mn average oxidation state (AOS) in both the bulk and surface of δ-MnO2 lowered significantly with decreasing pH (3.39/2.56, 3.57/3.07 and 3.78/3.19 in bulk/surface at pH 4.0, 5.5 and 7.0, respectively). The increase in the photoreduction rate and extent of δ-MnO2 at lower pHs indicated the photoelectron transfer was pH-dependent. From the amount of reduced Mn(III/II) in structure and Mn2+ in solution, the average photoelectron transfer amount per mol Mn was calculated as 1.81, 0.95 and 0.22 at pH 4.0, 5.5 and 7.0, respectively. The retention of Mn(II) (~13% in content) in the structure of reacted δ-MnO2 was only observed at pH 4.0 rather than higher pHs. The photoreduction of Mn(IV) involved two successive steps of single-electron transfer to produce Mn(III) and further Mn(II), and the above results suggested the second-step photoelectron transfer was more effective under acidic conditions. The potential difference between the conduction band (CB) and MnO2/Mn2+ half-reaction was enlarged as pH decreased, which conduced to the optimal effectiveness of photoelectron transfer and thorough reduction of Mn(IV/III) under lower pH conditions. The pH-dependent Mn photoreduction occurred throughout the structure of δ-MnO2, whereas for the dark reaction, the influence of pH on the reduction extent only took effect on the surface. These observations implied the dark-reaction electron transfer predominantly occurred at the surface sites. By contrast, according to the semiconductor energy band theory, the excited CB photoelectrons were shared by all atoms in the lattice of δ-MnO2, and each Mn atom had equal chance to be reduced by photoelectrons. Furthermore, the accumulation of photo-reduced and large-size Mn(III/II) in the layer caused the spatial expansion of the in-plane crystallization, and could possibly alter the layer symmetry or even promote the long-term structural transformation towards tunneled Mn oxides. This work provides insights into the photocatalytic geochemical performance of the naturally-abundant birnessite, which is influenced by varied pH conditions on Earth's surface.

Role of Reduced Sulfur in the Transformation of Cd(II) Immobilized by δ-MnO2.

Mn oxides are the major sinks for Cd(II) in the aquatic environment. At the redox interface, reduced sulfur might affect the fate of sorbed Cd(II) by either reducing Mn oxides or forming strong complexes with Cd(II). Here, we investigated the fate of Cd(II) immobilized on δ-MnO2 affected by reduced sulfur (S2- and cysteine). A low concentration of S2- led to Cd(II) migration from vacant sites to edge sites, while a high concentration of S2- largely converted Cd(II) adsorbed on the surface of δ-MnO2 to CdS. At low pH, the cysteine addition led to the release of Cd(II) initially adsorbed at the δ-MnO2 vacant sites into the solution and caused the migration of a small portion of Cd(II) to the δ-MnO2 edge sites. At high pH, a high concentration of cysteine led to the detachment of Cd(II) from δ-MnO2, Cd(II) readsorption by Mn(III)-bearing minerals, and Cd-cysteine formation. Changes of Cd(II) speciation were caused by δ-MnO2 dissolution induced by reduced sulfur, the competition of generated Mn(II/III) for the adsorption sites, and the precipitation of Cd(II) with reduced sulfur. This study indicates that reduced sulfur is a critical factor controlling the fate of Cd(II) immobilized on Mn oxides in the aquatic environment.

Quantitative investigation of ZnO nanoparticle dissolution in the presence of δ-MnO2.

The widespread use of zinc oxide nanoparticles (ZnO NPs), the second most produced nanomaterial, inevitably leads to their release into the environment. In this study, dissolution and transformation of ZnO NPs in the presence of δ-MnO2, an abundant and ubiquitous manganese (Mn) oxide mineral, was investigated via a suite of techniques covering bulk to molecular scales. Dissolution kinetics indicated that the presence of δ-MnO2 significantly affected ZnO NP dissolution rate/trend and equilibrium Zn2+ concentration, which were found to be mainly dependent on the concentration and mass ratio of ZnO NPs and δ-MnO2. Approximately 300mg ZnO NPs per g δ-MnO2 was expected for ZnO NP uptake at pH7.0 via ZnO NP dissolution and surface Zn2+ adsorption. X-ray diffraction (XRD), ζ potential, high-resolution transmission electron microscopy (HR-TEM), and Zn K-edge X-ray absorption spectroscopy (XAS) results revealed that when the mole content of ZnO NPs was less than the total adsorption sites of δ-MnO2 surface, ZnO NPs were completely dissolved and adsorbed on δ-MnO2 surface in the form of inner-sphere complexes. A fraction of ZnO NPs persisted when the mole ratio of ZnO to δ-MnO2 further increased. These results suggest that the transformation and fate of ZnO NPs is affected by environment-relevant minerals such as Mn oxides due to their huge capacity of fixing dissolved metal cations at the surface or interlayer structure.

Transformation of m-aminophenol by birnessite (δ-MnO2) mediated oxidative processes: Reaction kinetics, pathways and toxicity assessment

The m-aminophenol (m-AP) is a widely used industrial chemical, which enters water, soils, and sediments with waste emissions. A common soil metal oxide, birnessite (δ-MnO2), was found to mediate the transformation of m-AP with fast rates under acidic conditions. Because of the highly complexity of the m-AP transformation, mechanism-based models were taken to fit the transformation kinetic process of m-AP. The results indicated that the transformation of m-AP with δ-MnO2 could be described by precursor complex formation rate-limiting model. The oxidative transformation of m-AP on the surface of δ-MnO2 was highly dependent on reactant concentrations, pH, temperature, and other co-solutes. The UV-VIS absorbance and mass spectra analysis indicated that the pathway leading to m-AP transformation may be the polymerization through the coupling reaction. The m-AP radicals were likely to be coupled by the covalent bonding between unsubstituted C2, C4 or C6 atoms in the m-AP aromatic rings to form oligomers as revealed by the results of activation energy and mass spectra. Furthermore, the toxicity assessment of the transformation productions indicated that the toxicity of m-AP to the E. coli K-12 could be reduced by MnO2 mediated transformation. The results are helpful for understanding the environmental behavior and potential risk of m-AP in natural environment.

Impact of dissolved O2 on phenol oxidation by δ-MnO2.

Although redox reactions of organic contaminants with manganese oxides have been extensively studied, the role of dissolved O2 in these processes has largely been overlooked. In this study, the oxidative degradation of phenol by δ-MnO2 was investigated under both oxic and anoxic conditions. Dissolved O2 inhibited phenol degradation due to its promoting role in the reoxidation and precipitation of reduced Mn(ii) to Mn(iii) on the δ-MnO2 surface, resulting in partial transformation of δ-MnO2 to "c-disordered" H+-birnessite at pH 5.5 and feitknechtite, manganite, and hausmannite at pH 7.0 and 8.5. The reformed Mn(iii) phases could reduce phenol oxidation by blocking reactive sites of δ-MnO2. In addition, dissolved O2 caused a higher degree of particle agglomeration and a more severe specific surface area decrease, and hence lower reactivity of δ-MnO2. These findings revealed that after reductive dissolution by phenol and reoxidation by dissolved O2 throughout continuous redox cycling, δ-MnO2 became less reactive rather than being regenerated. These results can provide new insights into the understanding of the oxidation of organic contaminants by manganese oxides in the natural environment.

Transformation of oxytetracycline by redox-active Fe(III)- and Mn(IV)-containing minerals: Processes and mechanisms

Abiotic mechanisms of oxytetracycline degradation by redox-active minerals, Fe(III)-saturated montmorillonite (Fe-SWy) and birnessite (δ-MnO2), were studied to better understand the environmental behavior of tetracycline antibiotics in aqueous systems. Kinetics of dissipation (adsorption, oxidation and formation of transformation products (TPs)), was investigated up to 7 days, and reaction mechanisms were elucidated based on identification of TPs by liquid chromatography mass spectrometry. Oxytetracycline was completely removed from solution by both minerals, however kinetics, TPs and mechanisms were distinct for each mineral. Oxytetracycline oxidation by δ-MnO2 occurred within minutes; 54 identified TPs were detected only in solution, most of them exhibited decreasing levels with time. In contrast, oxytetracycline was completely adsorbed by Fe-SWy, its degradation was slower, only 29 TPs were identified, among them 13 were surface-bound, and most of the TPs accumulated in the system with time. Oxytetracycline transformation by δ-MnO2 involved radicals, as was proven by electrochemical degradation. Reductive dissolution was observed for both minerals. X-ray photoelectron spectroscopy demonstrated accumulation of Fe(II) on Fe-SWy surface, whereas Mn(II) was primarily released from δ-MnO2 surface. Highly oxidized carbon species (i.e., newly formed TPs) were observed on the surface of both minerals interacting with oxytetracycline. This study demonstrates the impact of structure and reactivity of redox-active minerals on removal and decomposition of tetracycline antibiotics in aqueous systems.

Application of EXAFS with a bent crystal analyzer to study the pH-dependent microstructure of Eu(III) onto birnessite

Herein, Eu(III) retention mechanism and microstructure on δ-MnO2 as a function of solution pH were studied using both the conditional macroscopic method and molecular-level high resolution EXAFS technique with a bent crystal analyzer. The results suggested that the sorption Eu(III) onto δ-MnO2 was strongly dependent on pH but not on ionic strength, implying the formation of inner-sphere surface complexation for Eu(III) on δ-MnO2 surface. Molecular-level results from EXAFS methods showed that Eu was surrounded by ∼8O atoms in first coordination shell at REu-O ≈ 2.42 Å, and second shell of Mn atoms at REu-Mn ≈ 3.91 Å was found in four sorption samples from different solution pH. All this results reflected the formation of a bidentate surface complex with Eu(III) bonding by corner sharing to MnO6-octahedron on δ-MnO2 surface. Both the macroscopic findings and microstructure of Eu(III) at the δ-MnO2-water interface enhance our understanding for the reaction dynamics and states of Eu(III) and related radionuclides in natural environment.

The effects of iron(II) on the kinetics of arsenic oxidation and sorption on manganese oxides

In this study, As(III) oxidation kinetics by a poorly-crystalline phyllomanganate (δ-MnO2) in the presence and absence of dissolved Fe(II) was investigated using stirred-flow and batch experiments. Chemically synthetic δ-MnO2 was reacted with four influent solutions, containing the same As(III) concentration but different Fe(II) concentrations, at pH 6. The results show an initial rapid As(III) oxidation by δ-MnO2, which is followed by an appreciably slow reaction after 8h. In the presence of Fe(II), As(III) oxidation is inhibited due to the competitive oxidation of Fe(II) as well as the formation of Fe(III)-(hydr)oxides on the δ-MnO2 surface. However, the sorption of As(III), As(V) and Mn(II) are increased, for the newly formed Fe(III)-(hydr)oxides provide additional sorption sites. This study suggests that the competitive oxidation of Fe(II) and consequently the precipitation of Fe(III) compounds on the δ-MnO2 surface play an important role in As(III) oxidation and As sequestration. Understanding these processes would be helpful in developing in situ strategies for remediation of As-contaminated waters and soils.

Hydrothermal synthesis and characterization of surface-modified δ-MnO2 with high Fenton-like catalytic activity

Surface-modified δ-MnO2 with a hierarchical structure was synthesized via a hydrothermal redox reaction between toluene and KMnO4. Toluene is oxidized to produce benzoic acid which rapidly adsorbs onto the surface of δ-MnO2. The δ-MnO2 sample prepared with toluene exhibited a much higher catalytic activity for the Fenton-like oxidation of methylene blue in aqueous solution than that prepared without toluene. The enhanced catalytic activity might be attributed to the surface-bound persistent free radicals produced from the electron transfer from the adsorbed C6H5–COO− to Mn(IV).

Birnessite (δ-MnO2) mediated degradation of organoarsenic feed additive p-arsanilic acid.

p-Arsanilic acid (p-ASA), is a widely used animal feed additive in many developing countries, and is often introduced to agricultural soils with animal wastes. A common soil metal oxide, birnessite (δ-MnO2), was found to mediate its degradation with fast rates under acidic conditions. Experimental results indicate that adsorption and degradation of p-ASA on the surface of δ-MnO2 were highly pH dependent, and the overall kinetics for p-ASA degradation and formation of precursor complex could be described by a retarded first-order rate model. For the reaction occurring between pH 4.0 and 6.2, the initial rate equation was determined to be rinit=2.36×10(-5)[ASA]0.8[MnO2]0.9[H+]0.7. p-ASA first forms a surface precursor complex on δ-MnO2 during degradation, followed by formation of p-ASA radicals through single-electron transfer to δ-MnO2. The p-ASA radicals subsequently undergo cleavage of arsenite group (which is further oxidized to arsenate) or radical-radical self-coupling. Instead of full mineralization (with respect to arsenic only), about one-fifth of the p-ASA "couples" to form an arsenic-bearing azo compound that binds strongly on δ-MnO2. The fast transformation of p-ASA to arsenite and arsenate mediated by δ-MnO2 significantly increases the risk of soil arsenic pollution and deserves significant attention in the animal farming zones still using this feed additive.

Chemical processes for the extreme enrichment of tellurium into marine ferromanganese oxides

Tellurium, an element of growing economic importance, is extremely enriched in marine ferromanganese oxides. We investigated the mechanism of this enrichment using a combination of spectroscopic analysis and adsorption/coprecipitation experiments. X-ray Absorption Near-Edge Structure (XANES) analysis showed that in adsorption/coprecipitation systems, Te(IV) was oxidized on δ-MnO2 and not oxidized on ferrihydrite. Extended X-ray Absorption Fine Structure (EXAFS) analysis showed that both Te(IV) and Te(VI) were adsorbed on the surface of δ-MnO2 and ferrihydrite via formation of inner-sphere complexes. In addition, Te(VI) can be structurally incorporated into the linkage of Fe octahedra through a coprecipitation process because of its molecular geometry that is similar to the Fe octahedron. The largest distribution coefficient obtained in the adsorption/coprecipitation experiments was for the Te(VI)/ferrihydrite coprecipitation system, and it was comparable to those calculated from the distribution between natural ferromanganese oxides and seawater. Our XAFS and micro-focused X-ray fluorescence (μ-XRF) mapping of natural ferromanganese oxides showed that Te was structurally incorporated as Te(VI) in Fe (oxyhydr)oxide phases. We conclude that the main process for the enrichment of Te in ferromanganese oxides is structural incorporation of Te(VI) into Fe (oxyhydr)oxide phases through coprecipitation.This mechanism can explain the unique degree of enrichment of Te compared with other oxyanions, which are mainly enriched via adsorption on the surface of the solid structures. In particular, the great contrast in the distributions of Te and Se is caused by their oxidized species: (i) the similar geometry of the Te(VI) molecule to Fe octahedron, and (ii) quite soluble nature of Se(VI). Coexisting Mn oxide phases may promote structural incorporation of Te(VI) by oxidation of Te(IV), although the surface oxidation itself may not work as the critical enrichment process as in the case of some cations. This enrichment mechanism also means that ferromanganese oxides mainly scavenge dominant Te(VI) species from seawater and do not affect its species distribution in seawater, as described in a previous model. The variation in Te abundances and the correlation of Te concentration with the growth rate of natural ferromanganese oxides are consistent with the coprecipitation mechanism.