Hui Yin

Characterization of Co-doped birnessites and application for removal of lead and arsenite.

Nanostructured Co-doped birnessites were successfully synthesized, and their application for the removal of Pb(2+) and As(III) from aquatic systems was investigated. Powder X-ray diffraction, chemical analysis, nitrogen physical adsorption, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystal structure, chemical composition, micromorphologies and surface properties of the birnessites. Doping cobalt into the layer of birnessite had little effect on its crystal structure and micromorphology. Both chemical and XPS analyses showed that the manganese average oxidation state (Mn AOS) decreased after cobalt doping. The Co dopant existed mainly in the form of Co(III)OOH in the birnessite structure. Part of the doped Co(3+) substituted for Mn(4+), resulting in the gain of negative charge of the layer and an increase in the content of the hydroxyl group, which accounted for the improved Pb(2+) adsorption capacity. The maximum capacity of Pb(2+) adsorption on HB, CoB5, CoB10 and CoB20 was 2538 mmol kg(-1), 2798 mmol kg(-1), 2932 mmol kg(-1) and 3146 mmol kg(-1), respectively. The total As(III) removal from solution was 94.30% for CoB5 and 100% for both CoB10 and CoB20, compared to 92.03% for undoped HB, by oxidation, adsorption and fixation, simultaneously.

Co2+-exchange mechanism of birnessite and its application for the removal of Pb2+ and As(III).

Co-containing birnessites were obtained by ion exchange at different initial concentrations of Co(2+). Ion exchange of Co(2+) had little effect on birnessite crystal structure and micromorphology, but resulted in an increase in specific surface areas from 19.26 to 33.35 m(2)g(-1), and a decrease in both crystallinity and manganese average oxidation state. It was due to that Mn(IV) in the layer structure was reduced to Mn(III) during the oxidation process of Co(2+) to Co(III). The hydroxyl groups on the surface of Co-containing birnessites gradually decreased with an increase of Co/Mn molar ratio owing to the occupance of Co(III) into vacancies and the location of large amounts of Co(2+/3+) and Mn(2+/3+) above/below the vacant sites. This greatly accounted for the monotonous reduction in Pb(2+) adsorption capacity, from 2538 mmol kg(-1) for the unmodified birnessite to 1500 mmol kg(-1) for the Co(2+) ion-exchanged birnessite with a Co/Mn molar ratio of 0.16. The amount of As(III) oxidized by birnessite was enhanced after ion exchange, but the apparent initial reaction rate was greatly decreased. The present work demonstrates that Co(2+) ion exchange has great influence on the adsorption and oxidation behavior of inorganic toxic metal ions by birnessite in water environments.

Fe-doped cryptomelane synthesized by refluxing at atmosphere: Structure, properties and photocatalytic degradation of phenol

Fe-doped cryptomelanes were synthesized by refluxing at ambient pressure, followed by characterization with multiple techniques and test in photocatalytic degradation of phenol. The introduction of Fe(III) into the structure of cryptomelane results in a decrease in particle size and the contents of Mn and K(+), and an increase in the Mn average oxidation state (AOS), specific surface area and UV-vis light absorption ability. Mn and Fe K-edge extended X-ray absorption fine structure spectroscopy analysis indicates that some Fe(III) is incorporated into the framework of cryptomelane by replacing Mn(III) while the remaining Fe(3+) is adsorbed in the tunnel cavity. These Fe-doped cryptomelanes have significantly improved the photocatalytic degradation rate of phenol, with the sample of ∼3.04 wt.% Fe doping being the most reactive and achieving a degradation rate of 36% higher than that of the un-doped one. The enhanced reactivity can be ascribed to the increase in the coherent scattering domain size of the crystals, Mn AOS and light absorption, as well as the presence of sufficient K(+) in the tunnel. The results imply that metal doping is an effective way to improve the performance of cryptomelane in pollutants removal and has the potential for modification of Mn oxide materials.

Structure and properties of vanadium(V)-doped hexagonal turbostratic birnessite and its enhanced scavenging of Pb2+ from solutions

Vanadium(V)-doped hexagonal turbostratic birnessites were synthesized and characterized by multiple techniques and were used to remove Pb(2+) from aqueous solutions. With increasing V content, the V(V)-doped birnessites have significantly decreased crystallinity, i.e., the thickness of crystals in the c axis decreases from 9.8 nm to ∼0.7 nm, and the amount of vacancies slightly increases from 0.063 to 0.089. The specific surface areas of these samples increase after doping while the Mn average oxidation sates are almost constant. V has a valence of +5 and tetrahedral symmetry, and exists as oxyanions, including V₆O₁₆(2-), and VO4(3-) on birnessite edge sites by forming monodentate corning-sharing complexes. Pb LIII-edge extended X-ray absorption fine structure (EXAFS) spectra analysis shows that, at low V contents (V/Mn≤0.07) Pb(2+) mainly binds with birnessite on octahedral vacancy and especially edge sites whereas at higher V contents (V/Mn>0.07) more Pb(2+) associates with V oxyanions and form vanadinite [Pb₅(VO₄)₃Cl]-like precipitates. With increasing V(V) content, the Pb(2+) binding affinity on the V-doped birnessites significantly increases, ascribing to both the formation of the vanadinite precipitates and decreased particle sizes of birnessite. These results are useful to design environmentally benign materials for treatment of metal-polluted water.

Structure and properties of Co-doped cryptomelane and its enhanced removal of Pb2 + and Cr3 + from wastewater

Cryptomelane is a reactive Mn oxide and has been used in removal of heavy metal from wastewaters. Co-doped cryptomelane was synthesized by refluxing at ambient pressure and characterized by powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and extended X-ray absorption fine structure spectroscopy, and its performances for removal of Pb(2+) and Cr(3+) from aqueous solutions were investigated. Co doping has a negligible effect on the structure and morphology of cryptomelane but increases the specific surface area and Mn average oxidation state. Mn and Co K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that Co barely affects the atomic coordination environments of Mn, and distances of edge- and corner-sharing Co-Me (MeCo, Mn) pairs are shorter than those of the corresponding Mn-Me pairs, implying the replacement of framework Mn(III) by Co(III). These Co-doped cryptomelanes can quickly oxidize Cr(3+) to be HCrO4(-) and remove 45%-66% of the total Cr in the reaction systems by adsorption and fixation, and they have enhanced Pb(2+) adsorption capacities. Thus these materials are promising adsorbents for heavy metal remediation. The results demonstrate the design and modification of environmental friendly Mn oxide materials and can help us understand the interaction mechanisms of transition metals with Mn oxides.

Enhanced photocatalytic H 2 -production activity of C-dots modified g-C 3 N 4 /TiO 2 nanosheets composites

As a new carbon-based material, carbon dots (C-dots) have got widely preference because of its excellent electronic transfer capability. In this work, a novel ternary layered C-dots/g-C3N4/TiO2 nanosheets (CGT) composite photocatalysts were prepared by impregnation precipitation methods. The optimal ternary CGT composite samples revealed high photocatalytic hydrogen evolution rate in triethanolamine aqueous solutions, which exceeded the rate of the optimal g-C3N4/TiO2 composite sample by a factor of 5 times. The improved photocatalytic activity is owed to the positive effects of C-dots and layered heterojunction structure of TiO2 nanosheets and g-C3N4 sheets. C-dots in the CGT composites can serve as electron reservoirs to capture the photo-induced electrons. The well-defined layered heterojunction structure of CGT provides the intimate contact and the strong interaction of anatase TiO2 nanosheets and g-C3N4 sheets via face-to-face orientation, which restrains the recombination of photogenerated charge carriers, and thus enhances the photocatalytic H2-production activity. Electron paramagnetic resonance and transient photocurrent response proved the strong interaction and improved interfacial charge transfer of TiO2 nanosheets and g-C3N4 sheets, respectively. The mechanism of improving the photocatalytic H2-evolution activity was further confirmed by time-resolved fluorescence, electron paramagnetic resonance, transient photocurrent response and electrochemical impedance spectroscopy.

Mechanisms on the morphology variation of hematite crystals by Al substitution: The modification of Fe and O reticular densities.

Al substitution in hematite is ubiquitous in soils. With the increase of Al amount, the hematite morphology changes from rhombohedral crystals to disk-shaped ones, but the underlying mechanism is poorly understood. Herein, a series of Al-substituted hematite were synthesized and characterized by synchrotron X-ray diffraction (SXRD), field emission scanning electron microscopy (FESEM), high resolution electron transmission microscopy (HRTEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy, to investigate the effects of Al3+ substitution on the hematite structure and morphology. EXAFS and Rietveld structural refinement analyses find an increase in face-sharing (along c axis) Fe-Me (Me = Al, Fe) distances, edge-sharing (in a-b plane) Fe-Me (Me = Al, Fe) distances, and O-O average distances. Moreover, the face-sharing Fe-Me distances and O-O distances along c axis increase more significantly. This indicates a more apparent decrease in the reticular densities of Fe and O along the direction of c axis, which facilitates faster crystal growth along c axis and results in the evolution of morphology of Al-substituted hematite to disk-shaped crystals. The above results provide new insights into the morphology changes and environmental geochemistry behaviors of Al-contained hematite in soils, and are benefit for the control of crystal morphologies during its application as environmentally-friendly materials.

Self-assembly of birnessite nanoflowers by staged three-dimensional oriented attachment

Birnessite (layer-type Mn(III, IV) oxides with ordered sheet stacking) is the most common mineral species of manganese (Mn) oxides and has been demonstrated to be among the strongest sorbents and oxidants in surface environments. The morphology of birnessite is one of the key factors affecting its reactivity. Either biotic or abiotic birnessite samples usually consist of nanoflower-like crystals. However, the governing factors and mechanisms of morphological evolution of the nanoflower-shaped birnessite remain poorly understood. In this work, birnessite nanoflowers, as a natural birnessite analog, were synthesized and the intermediate products during birnessite crystallization were captured by instant freezing using liquid nitrogen. The processes and mechanisms of crystal growth of birnessite nanoflowers were investigated using a combination of high-resolution transmission electron microscopy (HRTEM), field-emission scanning electron microscopy (FESEM), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicate that primary hexagonal nanoflakes rapidly agglomerate to form nuclei-like substrates at the initial stages, and subsequently, these nanoflakes aggregate laterally and link serially on the substrates to form nanopetals through both rotation and edge-to-edge oriented attachment (OA) mechanism. This process is likely driven by hydrogen bonding between unsaturated O atoms at the edge planes of [MnO6] sheets. Meanwhile, the OA mechanism along the (001) plane is likely driven by Coulombic interactions and hydrogen bonding during the assembly process of the adjacent nanopetals. The morphological evolution occurred by the staged three-dimensional OA process that plays an essential role in the self-assembly of flower-like birnessite crystals. These findings provide further understanding of how nanoparticle assembly is directed to achieving desired shapes and sizes by fabricating nanomaterials through three-dimensional OA processes.

Contribution of Soil Active Components to the Control of Heavy Metal Speciation

Soil is the central organizer of the terrestrial ecosystem. Mineral, organic components, and microorganisms, which are major solid active components of the soil, profoundly affect the physical, chemical, and biological processes of soils including the behavior, transformation, and fate of various nutrients and pollutants (Violante et al. 2002). Heavy metal interaction with soil active components is recognized as being important in controlling heavy metal activities. Colloidal particles of soil organic matter (SOM), clay silicates, metal hydroxides, and microorganisms, which have large surface area and are often electrically charged, are considered as important adsorptive surfaces to bind heavy metals.

Coordination geometry of Zn2 + on hexagonal turbostratic birnessites with different Mn average oxidation states and its stability under acid dissolution

Hexagonal turbostratic birnessite, with the characteristics of high contents of vacancies, varying amounts of structural and adsorbed Mn3+, and small particle size, undergoes strong adsorption reactions with trace metal (TM) contaminants. While the interactions of TM, i.e., Zn2+, with birnessite are well understood, the effect of birnessite structural characteristics on the coordination and stability of Zn2+ on the mineral surfaces under proton attack is as yet unclear. In the present study, the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states (AOSs) on the coordination geometry of adsorbed Zn2+ and its stability under acidic conditions were investigated. With decreasing Mn AOS, birnessite exhibits smaller particle sizes and thus larger specific surface area, higher amounts of layer Mn3+ and thus longer distances for the first MnO and MnMn shells, but a low quantity of available vacancies and thus low adsorption capacity for Zn2+. Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn (IVZn) complexes on vacancies than octahedral (VIZn) complexes, and Zn2+ is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS. High Zn2+ loading favors the formation of VIZn complexes over IVZn complexes, and the release of Zn2+ is faster than at low loading. These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites, and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials.