The Interactions Of Several Transition Metals With Hexagonal Birnessites

Manganese oxides are ubiquitous and very reactive minerals in geological environments. Because of their finite particle sizes, structural defects and high negative charges, Mn oxides have great effects on the fixation, migration and transformation of some elements (especially heavy metal(loids)) and organic pollutants in environments by reactions such as adsorption, isomorphous substitution and oxidation/reduction. Natural Mn oxides are usually enriched in various transition metals (TMs)(i.e. Co, Ni, Fe and V). Insertion of various TMs into Mn oxide matrixes follows different mechanisms which determine their geochemical behaviors. Incorporation of TM ions into the structures of Mn oxides results in certain changes in their substructures and physicochemical properties (e.g. morphologies, surface groups, adsorption, oxidation, catalytic oxidation and electrochemical properties). The environmental effects of Mn oxide minerals on the speciations and transformations of contaminants are either enhanced or reduced through these changes. It is a generally occurred phenomenon or process that Mn oxide minerals interact with various TMs in the environment. In present many reports are focused on the characteristics and mechanisms of adsorption of TM cations on birnessites, however it is yet unclear about the rules governing the isomorphous substitutions of TM cations for Mn in Mn oxide matrixes and as-induced changes in mineral substructures and physicochemical properties. In our studies, TM-containing hexagonal birnessites were obtained by doping TM cations (Co2+, Ni2+, Fe3+and V(V)) during birnessite crystallization or ion exchange reaction (Co2+). An array of techniques:powder X-ray diffraction (XRD), specific surface area (SSA), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure spectroscopy (XANES), extended X-ray absorption fine structure sprctroscopy (EXAFS) and atomic pair distribution function (PDF), and isothermal adsorption, oxidation and dissolution experiments were used to investigate the structures and properties of TM-containing birnessites and the crystal chemistry characteristics of TMs in birnessite structures, and to clarify their interaction mechanisms. The structure evolutions of birnessites with different Mn average oxidation states (AOSs) were also studied. The main results were listed as following:1. Co-doped birnessites with Co weight contents of0%(HB),3.46%(CoB5),6.23% (CoB10) and10.72%(CoB20) were synthesized by addition of Co2+during birnessite crystallization. Doping Co into the layers of birnessite had little effect on its crystal structure and micromorphology, but slightly reduced crystallinity, and decreased Mn AOS. The Co dopant existed mainly in the form of Co(Ⅲ)OOH in the birnessite structure, and the molar contants of hydroxyl groups on birnessite surfaces increased from12.79%of HB to13.05%,17.69%and17.79%for CoB5, CoB10and CoB20respectively. The maximum capacities of Pb2+adsorption on HB, CoB5, CoB10and CoB20were2538mmol/kg,2798mmol/kg,2932mmol/kg and3146mmol/kg, respectively. Part of the doped Co(Ⅲ) substituted for layer Mn (Ⅳ), resulting in the gain of negative charges of the layers and an increase in the contents of the hydroxyl groups on the mineral surfaces, mainly accounted for the improved Pb2+adsorption capacity. Owing to the higher standard redox potential of Co3+/Co2+than those of MnO2/Mn2+and Mn3+/Mn2+, apparent oxidation capacities of As(Ⅲ) by these doped birnessites were greatly enhanced, i.e.77.31%,78.07%,86.37%and91.00%for HB, CoB5, CoB10and CoB20respectively. The total As(Ⅲ) removal from solution was94.30%for CoB5and100%for both CoB10and CoB20, compared to92.03%for HB, by oxidation, adsorption and fixation, simultaneously.2. A series of birnessites with Co weight contents of1.16%(HC2),2.77%(HC5),5.90%(HC10) and7.88%(HC20) were obtained by ion exchange of Co2+at different initial concentrations with HB. Ion exchange of Co2+had little effect on birnessite crystal structure and micromorphology, but resulted in an increase in specific surface areas from19.26to33.35m2/g, and a decrease in both crystallinity and Mn AOS. It was due to that Mn(Ⅳ) in the layer structure was reduced to Mn(Ⅲ) during the oxidation process of Co2+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 Co2+/3+and Mn2+/above/below the vacant sites. This greatly attributed for the monotonous reduction in Pb2+adsorption capacity, from2538mmol/kg of HB to1500mmol/kg for HC20. The amounts of As(Ⅲ) oxidized by birnessites were enhanced after Co2+-ion exchange, but the apparent initial reaction rate was greatly decreased, i.e.0.0226,0.0175,0.0161,0.0123and0.0035min-1for HB, HC2, HC5, HC10and HC20respectively.3. Nanostructured birnessites containing different amounts of Ni were synthesized by addition of Ni2+to initial reactants. These Ni-rich birnessites had Ni contents as high as2.99%(Ni5) and6.08%(Ni10) in weight. EXAFS results showed that Ni5had23.7%of the total Ni (0.71wt.%) and Ni10had34.5%of the total Ni (2.10wt.%) in Mn octahedral layer with the remaining Ni located at vacancies and edge sites. The Ni-rich birnessites had weaker crystallinity and thermal stability, fewer layers stacked along the c axis,-1.5-2.7times larger surfaces areas compared to HB. Doping of Ni during birnessite crystallization enhanced the formation of vacancies in the layers; however, adsorption capacities for Pb2+and Zn2+by these Ni-rich birnessites were reduced, mainly because of the occupation of vacancies and edge sites by a large amount of Ni2+. These Ni-rich birnessites exhibited much higher oxidation capabilities and could completely oxidize As(III) in solution at rapider initial reaction rates than HB, owing to the increased Mn AOSs of the former. The results presented here demonstrated that, incorporation of Ni into the natural birnessites in ferromanganese nodules might be achieved both by direct coprecipitation of Ni with Mn to build the layers and migration over time from adsorbed Ni on the surface into the layers, and the latter may be the main mechanism.4. Fe-doped hexagonal birnessites with Fe weight contents of2.77%(Fe5) and5.56%(Fe10) were obtained by adding Fe3+into the initial reactants. Compared to the un-doped control Fe-doped birnessites had weaker crystallinity, i.e., less Mn layers stacking in the c direction, and larger surface areas. Combination of XPS, Mn K-edge XANES and EXAFS spectra demonstrated that Fe doping decreased the Mn AOS but had little effect on the basic layer structures and local Mn environments. Analysis of Fe K-edge EXAFS showed that81.2%and83.0%of the total Fe in the structures of Fe5and Fe10, respectively, were located in the interlayer spaces. Fe(Ⅲ) and Ni(Ⅱ) located in the birnessite layers exhibited in high-spin (HS) configurations whereas layer Mn(Ⅲ) and Co(Ⅲ) plausibly adopted low-spin (LS]-states. The effects of TM doping on the thickness of birnessite plate crystals along the c axis and the unit cell parameter b decreased in the order Fe>Ni>Co. Co and Fe incorporated into the birnessite layers by substitution for Mn(Ⅳ) while Ni substitutes for Mn(Ⅲ). Variations in K+contents could also reflect the spatial distributions of TMs in birnessite structure. Analysis of the variations in K+contents in doped birnessites and fitting of EXAFS data indicated that, most Fe (-81-83%) or Ni (-66-76%) incorporated into the birnessite structures existed in the interlayer regions while most Co (-71-80%) occurred in the Mn layers. The compatibility of these TM ions in the birnessite layers was decreased in the order Co>Ni>Fe. The smaller the difference between the coordination radius (CR) of Fe, Co or Ni and Mn(Ⅳ) or Mn(Ⅲ) that replaced, the more dopants were compatible within the Mn layers.5. Birnessites with V weight contents of1.33%(V2),2.83%(V5),5.16%(V8) and 10.26%(V10) were synthesized by doping pentavalent V (V(V)) during birnessite crystallization. With increasing V content, the crystallinity of V-doped birnessites significantly decreased, i.e., the numbers of [MnO6] layers stacked along the c axis decreased from14.5to～1and the crystal sizes in the a-b plane from137A to26A. As a consequence, the Mn-O bond lengths in [MnO6] and the edge-sharing Mn-Mn distances in birnessite layers slightly decreased although the basic layer structures and Mn average oxidation states changed negligibly, as revealed by Mn K-edge X-ray absorption structure (XAS) spectroscopic analysis. V K-edge XAS and differential pair distribution function (d-PDF) analyses showed that, V had a valence of+5and tetrahedronal symmetry, and was located on the surfaces of birnessite crystals as polynuclear oxyanionic clusters similar to [V6O16] clusters. V doping had greatly increased Pb2+adsorption capacities and affinities of birnessites, which could be mainly ascribed to the existence of V polynuclear clusters, and the decreased particle sizes and thus more edge sites.6. Powder X-ray diffraction (XRD) simulation and XAS techniques were used to investigate the structure evolutions of hexagonal birnessites with different Mn AOSs and the intrinsic structural factors determining their adsorption behaviors towards heavy metals. As the Mn AOSs of the synthetic hexagonal birnessites (from HB1to HB6) decreased from3.92to3.67, unit cell parameter b increased from2.838A to2.848A, and the Mn AOS had a negatively linear relationship with unit cell parameter b. The coherent scattering domains (CSD) in the a-b layer plane decreased from12.0nm to7.0nm, and～10.6-13.4layers stacking perpendicularly, on average. The contents of vacant octahedral sites decreased from18%to8%with decreasing Mn AOSs. Linear combination fit of XANES spectroscopy showed that these samples were predominantly composed of Mn4+with the amount of Mn2+/3+increasing from HB1to HB6. Analyses of Mn K-edge EXAFS spectroscopy using a full multiple scattering model demonstrated the similar crystal structure and Mn local environments for these hexagonal birnessites. However, apparent fractions of Mn sites occupied (focc) decreased from0.74for HB1to0.66for HB6, which can be attributed to the effects of decreasing vacancies and particle sizes. In addition, a trend of systematic elongation in the average bond-lengths for Mn-O and Mn-Mn shells was observed as the Mn AOS decreased.