Effect of humic acid on nickel(ii) sorption to Ca-montmorillonite by batch and EXAFS techniques study

The influence of humic acid (HA) on Ni(ii) sorption to Ca-montmorillonite was examined by using a combination of batch sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The sorption of Ni(ii) on HA-montmorillonite hybrids is strongly dependent on pH and temperature. At low pH, the sorption of Ni(ii) is mainly dominated by Ni-HA-montmorillonite and outer-sphere surface complexation. The EXAFS results indicate that the first coordination shell of Ni(ii) consists of ～6 O atoms at the interatomic distances of ～2.04 ? in an octahedral structure. At high pH, binary Ni-montmorillonite surface complexation is the dominant sorption mechanism. EXAFS analysis indicates the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5, while a Ni-Al layered double hydroxide (LDH) phase at the Ca-montmorillonite surface formed with pH 8.5. At pH 10.0, the dissolved HA-Ni(ii) complexation inhibits the precipitation of Ni hydroxide, and Ni-Al LDH phase forms. The rise of temperature increases the sorption capacity of Ni(ii), and promotes Ni-Al LDH phase formation and the growth of crystallites. The results are important to evaluate the physicochemical behavior of Ni(ii) in the natural environment.     

用EXAFS研究pH对Zn(Ⅱ)-TiO2体系吸附和微观构型的影响

应用延展X射线吸收精细结构(EXAFS)方法, 研究了不同pH对Zn(Ⅱ)在锐钛矿型TiO2表面吸附产物的微观构型的影响. 宏观的吸附-解吸实验表明, 随着pH值由5.8增大至6.8, 吸附等温线明显升高, Freundlich吸附常数由1.345 L/g增加到15.385 L/g; 而体系的不可逆性逐渐降低, 不可逆吸附系数(TⅡ)由0.43降低到0.23. 不同pH条件下吸附样品的EXAFS结果表明, Zn(Ⅱ)主要通过共用水合离子及TiO2表面的O原子结合到TiO2表面上, 第一配位层(Zn—O层)原子间距和配位数随着pH值增大逐渐降低, Zn(Ⅱ)在TiO2表面吸附形态从六配位向四配位转化;第二配位层(Zn—Ti层)分析结果表明, 存在2个典型的Zn—Ti原子间距, 即R1=0.319~0.334 nm(双齿方式结合的强吸附)和R2=0.366~0.378 nm(单齿方式结合的弱吸附), 随着pH值的升高, 强吸附位(CN1)逐渐减少而弱吸附位(CN2)逐渐增加, 其比值由2.12降低至0.89, 从而导致其在高pH值的条件下吸附量和可逆性明显增大. EXAFS结果从分子水平说明了该体系在不同pH值条件下表现出的可逆性差异是由于微观吸附状态不同所致.     

Solid-state 1H and 27Al NMR studies of amorphous aluminum hydroxides

Two kinds of amorphous aluminum hydroxides, a sample precipitated from admixing AlCl3 and NaOH aqueous solutions and the commercial product, were measured by 27Al and 1H solid-state NMR spectroscopy. Pentahedral and tetrahedral coordinations, as well as octahedral coordination of oxygen atoms for aluminum, are observed in 27Al magic angle spinning (MAS) spectra of both amorphous samples. In contrast, octahedral coordination is only observed in gibbsite, bayerite, and boehmite. According to 1H MAS-NMR spectra under conditions of high spinning rate (35 kHz) and high field (14.09 T), free waters and OH groups coupled with aluminum for amorphous samples are observed at approximately 5 and approximately 4.5 ppm, respectively, the latter peak being broader. This is consistent with the differential spectra between spin echo and transfer of populations in double resonance. We conclude that the subunits of AlO4, AlO5, and AlO6 in amorphous aluminum hydroxides are bound through hydrogen bonds with a wide distribution of bonding strength.     

Surface speciation of Cd(II) and Pb(II) on kaolinite by XAFS spectroscopy

Little spectroscopic evidence exists in the literature describing the surface complexation of cadmium (Cd) and lead (Pb) on kaolinite, the dominant clay mineral present in highly weathered soils of tropical and humid climates. X-ray absorption fine structure (XAFS) spectroscopy data at the Cd K and Pb L(III) edges were collected on Cd- and Pb-sorbed kaolinite samples and compared to a suite of reference materials including Pb and Cd sorbed on amorphous (am-)gibbsite. Cadmium formed dominantly (>75%) outer sphere complexes on kaolinite and a small fraction of CdOHCl complexes. In contrast Cd adsorbed as an inner sphere complex on gibbsite, suggesting that the Si tetrahedral sheet hindered Cd sorption to the Al octahedral sheet on kaolinite. Lead formed polymeric complexes, which bonded to kaolinite via edge sharing with surface Al octahedra. Two distinct Pb-Al edge-sharing distances on am-gibbsite, as opposed to one on kaolinite, suggested a similar steric hindrance effect for the surface complexation of polymeric Pb complexes on kaolinite. The results of this study show that the Si tetrahedral sheet limited the surface complexation of Cd and Pb on kaolinite, elevating kaolinite's permanent negative charge properties in retaining these heavy metals at its surface.     

Th uptake on montmorillonite: a powder and polarized extended X-ray absorption fine structure (EXAFS) study

The uptake process of Th(IV) onto montmorillonite was studied using powder and polarized-EXAFS (P-EXAFS) spectroscopy. Sorption samples were prepared in 0.1 M NaClO(4) solutions either undersaturated (pH 2 and 3, [Th](initial): 2.7x10(-6) to 4x10(-4) M) or supersatured (pH 5, [Th](initial): 4.3x10(-5) to 4x10(-4) M) with respect to amorphous ThO(2). Th loading varied between 1-157 micromol/g at pH 3 and 14-166 micromol/g at pH 5 and equaled 41 micromol/g at pH 2. At pH 5 and high surface loading the EXAFS spectrum resembled that of amorphous Th(OH)(4), suggesting the precipitation of a Th hydrous hydroxide. At low and intermediate surface coverage two O coordination shells at approximately 2.24 and approximately 2.48 A, and one Si shell at 3.81-3.88 A, were systematically observed regardless of pH. The formation of Th nucleation products and Th-Si solution complexes and the sorption of Th on a silica precipitate were excluded from the EXAFS spectra analysis and solution chemistry. In these conditions, Th was shown to bond the montmorillonite surface by sharing double corners with Si tetrahedra. This structural interpretation is consistent with surface coverage calculations which showed that the edge sites were saturated in the two highest concentrated samples (34 and 157 micromol/g) at pH 3.     

31P solid-state nuclear magnetic resonance study of the sorption of phosphate onto gibbsite and kaolinite

Sorption of phosphate onto gibbsite (gamma-Al(OH)3) and kaolinite has been studied by both macroscopic and 31P solid-state NMR measurements. Together these measurements indicate that phosphate is sorbed by a combination of surface complexation and surface precipitation with the relative amounts of these phases depending on pH and phosphate concentration. At low pH and high phosphate concentrations sorption is dominated by the presence of both amorphous and crystalline precipitate phases. The similarity between the single-pulse and CP/MAS NMR spectra suggests that the precipitate phases form a thin layer on the surface of the particles in close contact with protons from surface hydroxyl groups or coordinated water molecules. While the crystalline phase is only evident on samples below pH 7, amorphous AlPO4 was found at all pH and phosphate concentrations studied. As pH was increased the fraction of phosphate sorbed as an inner-sphere complex increased, becoming the dominant surface species by pH 8. Comparison of sorption and NMR results suggests that the inner-sphere complexes form by monodentate coordination to singly coordinated Al-OH sites on the edges of the gibbsite and kaolinite crystals. Outer-sphere phosphate complexes, which are readily desorbed, are also present at high pH.     

The structure of the homogeneous oxidation catalyst, Mn(II)(Br(-1))x, in supercritical water: an X-ray absorption fine-structure study

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure about Mn(II) and Br(-1) ions that exist as contact ion pairs in supercritical water. This work was performed to clarify why solutions of MnBr2 in supercritical water are known to effectively catalyze the aerobic oxidative synthesis of terephthalic acid from p-xylene as well as a number of other methylaromatic compounds. The Mn and Br K-edge spectra were collected at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne National Laboratory. The first-shell coordination structure about the Mn(II) ion changes from octahedral at ambient conditions to tetrahedral at supercritical conditions. Under supercritical conditions, the measured bond distances of Mn-OH2 and Mn-Br are 2.14 and 2.46 A, respectively. Direct contact ion pairs form with about 2 Br(-1) ions present in the first coordination shell of the Mn(II) ion. The structure of dissolved MnBr2, below 1.0 m, changes from essentially [Mn(II)(H2O)6]+2 to [Mn(II)(H2O)2(Br(-1))2] in supercritical water (scH2O). When an excess of Br(-1) ion is added, the bromide coordination number increases and the number of water molecules decreases. The results show that the initial MnBr2 catalyst in scH2O is tetrahedral with two Mn-Br contact ion pairs. The presence of the acetate anion deactivates the catalyst by formation of insoluble MnO.     

Effect of iron oxide coatings on zinc sorption mechanisms at the clay-mineral/water interface

Oxide surface coatings are ubiquitous in the environment, but their effect on the intrinsic metal uptake mechanism by the underlying mineral surface is poorly understood. In this study, the zinc (Zn) sorption complexes formed at the kaolinite, goethite, and goethite-coated kaolinite surfaces, were systematically studied as a function of pH, aging time, surface loading, and the extent of goethite coating, using extended X-ray absorption fine structure (EXAFS) spectroscopy. At pH 5.0, Zn partitioned to all sorbents by specific chemical binding to hydroxyl surface sites. At pH 7.0, the dominant sorption mechanism changed with reaction time. At the kaolinite surface, Zn was incorporated into a mixed metal Zn-Al layered double hydroxide (LDH). At the goethite surface, Zn initially formed a monodentate inner-sphere adsorption complex, with typical Zn-Fe distances of 3.18 A. However, with increasing reaction time, the major Zn sorption mechanism shifted to the formation of a zinc hydroxide surface precipitate, with characteristic Zn-Zn bond distances of 3.07 A. At the goethite-coated kaolinite surface, Zn initially bonded to FeOH groups of the goethite coating. With increasing aging time however, the inclusion of Zn into a mixed Zn-Al LDH took over as the dominant sorption mechanism. These results suggest that the formation of a precipitate phase at the kaolinite surface is thermodynamically favored over adsorption to the goethite coating. These findings show that the formation of Zn precipitates, similar in structure to brucite, at the pristine kaolinite, goethite, and goethite-coated kaolinite surfaces at near neutral pH and over extended reaction times is an important attenuation mechanism of metal contaminants in the environment.     

Ni(II) complexation to amorphous hydrous ferric oxide: an X-ray absorption spectroscopy study

Ni(II) sorption onto iron oxides and in particular hydrous ferric oxide (HFO) is among the important processes impacting its distribution, mobility, and bioavailability in environment. To develop mechanistic models for Ni, extended X-ray absorption fine structure (EXAFS) analysis has been conducted on Ni(II) sorbed to HFO. Coprecipitation revealed the formation of the metastable alpha-Ni(OH)(2) at a Ni(II) loading of 3.5 x 10(-3) molg(-1). On the other hand, Ni(II) formed inner-sphere mononuclear bidentate complexes along edges of FeO(6) octahedra when sorbed to HFO surfaces with Ni-O distances of 2.05-2.07 A and Ni-Fe distances of 3.07-3.11 A. This surface complex was observed by EXAFS study over 2.8 x 10(-3) to 10(-1) ionic strength, pH from 6 to 7, a Ni(II) loading of 8 x 10(-4) to 8.1 x 10(-3) molg(-1) HFO, and reaction times from 4 hours to 8 months. The short- and long-range structure analyses suggest that the presence of Ni(II) inhibited transformation of the amorphous iron oxide into a more crystalline form. However, Ni(2+) was not observed to substitute for Fe(3+) in the oxide structure. This study systematically addresses Ni(II) adsorption mechanisms to amorphous iron oxide. The experimentally defined surface complexes can be used to constrain surface complexation modeling for improved prediction of metal distribution at the iron oxide/aqueous interface.     

An EXAFS and TRLFS investigation on uranium(VI) sorption to pristine and leached albite surfaces

Uranium(VI) was sorbed to freshly ground and leached albite in batch and flow-through systems in the pH range 5.0-6.4. The uranium(VI) surface complexes were studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The EXAFS analysis of uranium(VI) sorbed on albite at pH 5.8 and 5 x 10(-6) M U(VI) showed one silicon atom at a USi distance of 3.09 A, which is indicative of the formation of an inner-sphere, mononuclear, bidentate uranium(VI) surface complex, Si(O)2UO2, on the silicate tetrahedra of albite. Two additional uranium(VI) sorption complexes were detected by TRLFS at higher initial aqueous U(VI) concentrations. However, the structure of these surface complexes could not be derived from EXAFS, since the measured EXAFS spectra represent the average of two surface complex structures. In order to simulate U(VI) sorption onto weathered feldspar surfaces, albite was leached with 0.01 M HClO4, resulting in surface material similar to amorphous silica gel. EXAFS showed that the equatorial oxygen shell of uranium(VI) sorbed on this material at pH 5.0 and 5.8 was split in two distances of 2.23 and 2.44 A. This indicates the formation of an inner-sphere surface complex.     

A multi-scale assessment of Pb(II) sorption on dolomite

Macroscopic sorption studies indicated that Pb sorption capacity was independent of pH over the pH range 5-7, while sorption as a function of reaction time up to two weeks for systems with no bulk precipitate phases showed continuous Pb uptake on dolomite. This could be due to diffusion of Pb into the micropores of dolomite as well as an increase in surface sites caused by particle size reduction during suspension mixing. Normalized XANES spectra for systems undersaturated with respect to Pb carbonate precipitates resembled the spectrum of Pb4(OH)4(4+), suggesting that Pb is mainly coordinated to dolomite as an inner-sphere surface complex. On the other hand, the XANES spectrum for 10(-3) M Pb at 1 atm CO2(g) in a 2 M Mg(NO3)2 background electrolyte solution resembled that of cerussite, while a sample at 5 x 10(-4) M Pb in equilibrium with air and 2 M Mg(NO3)2 resembled that of hydrocerussite. EXAFS analyses of sorption samples in chloride solutions showed that there were only first-shell contributions under 1 atm CO2(g), while higher shell contributions from Ca/Mg were seen at 10(-3.42) atm CO2(g). On the other hand, EXAFS samples prepared in nitrate solutions showed noticeable differences in speciation under different reaction conditions-from outer-sphere surface complexes at low Pb concentrations and pH, to inner-sphere surface complexes at moderate Pb concentrations and neutral pH, to the formation of Pb carbonate precipitates at the highest Pb loadings.     

Nature of metal binding sites in Cu(II) complexes with histidine and related N-coordinating ligands, as studied by EXAFS

Knowledge of the complexes formed by N-coordinating ligands and Cu(II) ions is of relevance in understanding the interactions of this ion with biomolecules. Within this framework, we investigated Cu(II) complexation with mono- and polydentate ligands, such as ammonia, ethylenediamine (en), and phthalocyanine (Pc). The obtained Cu-N coordination distances were 2.02 A for [Cu(NH(3))(4)](2+), 2.01 A for [Cu(en)(2)](2+), and 1.95 A for CuPc. The shorter bond distance found for CuPc is attributed to the macrocyclic effect. In addition to the structure of the first shell, information on higher coordination shells of the chelate ligands could be extracted by EXAFS, thus allowing discrimination among the different coordination modes. This was possible due to the geometry of the complexes, where the absorbing Cu atoms are coplanar with the four N atoms forming the first coordination shell of the complex. For this reason multiple scattering contributions become relevant, thus allowing determination of higher shells. This knowledge has been used to gain information about the structure of the 1:2 complexes formed by Cu(II) ions with the amino acids histidine and glycine, both showing a high affinity for Cu(II) ions. The in-solution structure of these complexes, particularly that with histidine, is not clear yet, probably due to the various possible coordination modes. In this case the square-planar arrangements glycine-histamine and histamine-histamine as well as tetrahedral coordination modes have been considered. The obtained first-shell Cu-N coordination distance for this complex is 1.99 A. The results of the higher shells EXAFS analysis point to the fact that the predominant coordination mode is the so-called histamine-histamine one in which both histidine molecules coordinate Cu(II) cations through N atoms from the amino group and from the imidazole ring.     

Adsorption and Precipitation of Aqueous Zn(II) on Alumina Powders

The products of aqueous Zn(II) sorption on high-surface-area alumina powders (Linde-A) have been studied using XAFS spectroscopy as a function of Zn(II) sorption density (Gamma=0.2 to 3.3 μmol/m(2)) at pH values of 7.0 to 8.2. Over equilibration times of 15-111 h, we find that at low sorption densities (Gamma=0.2-1.1 μmol/m(2)) Zn(II) forms predominantly inner-sphere bidentate surface complexes with AlO(6) polyhedra, whereas at higher sorption densities (Gamma=1.5 to 3.5 μmol/m(2)), we find evidence for the formation of a mixed-metal Zn(II)-Al(III) hydroxide coprecipitate with a hydrotalcite-type local structure. These conclusions are based on an analysis of first- and second-neighbor interatomic distances derived from EXAFS spectra collected under ambient conditions on wet samples. At low sorption densities the sorption mechanism involves a transformation from six-coordinated Zn-hexaaquo solution complexes (with an average Zn-O distance of 2.07 ?) to four-coordinated surface complexes (with an average Zn-O distance of 1.97 ?) as described by the reaction identical withAl(OH(a))(OH(b))+Zn (H(2)O)(6)(2+)--> identical withAl(OH(a)') (OH(b)')Zn(OH(c)')(OH(d)'+4H(2)O+zH(+), where identical withAl(OH(a))(OH(b)) represents edge-sharing sites of Al(O,OH,OH(2))(6) octahedra to which Zn(O,OH,OH(2))(4) bonds in a bidentate fashion. The proton release consistent with this reaction (z=a-a'+b-b'+4-c'-d'), and with bond valence analysis falls in the range of 0 to 2 H(+)/Zn(II) when hydrolysis of the adsorbed Zn(II) complex is neglected. This interpretation suggests that proton release is likely a strong function of the coordination chemistry of the surface hydroxyl groups. At higher sorption densities (1.5 to 3.5 μmol/m(2)), a high-amplitude, second-shell feature in the Fourier transform of the EXAFS spectra indicates the formation of a three-dimensional mixed-metal coprecipitate, with a hydrotalcite-like local structure. Nitrate anions presumably satisfy the positive layer charge of the Al(III)-Zn(II) hydroxide layers in which the Zn/Al ratio falls in the range of 1 : 1 to 2 : 1. Our results for the higher Gamma-value sorption samples suggest that Zn-hydrotalcite-like phases may be a significant sink for Zn(II) in natural or catalytic systems containing soluble alumina compounds. Copyright 2000 Academic Press.     

EXAFS studies on adsorption-desorption reversibility at manganese oxide-water interfaces. II. Reversible adsorption of zinc on delta-MnO2

Microscopic structures of Zn(II) adsorbed at delta-MnO(2)-water interfaces were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy. In a 0.1 M NaNO(3) solution of pH 5.50, hydrous Zn(II) was adsorbed onto the solid surface in the form of octahedral coordination. Adsorbed octahedral Zn(II) was located above and below the vacancy sites of delta-MnO(2). Each Zn was coordinated on one side to H(2)O molecules forming an H(2)O sheet and on the other side to oxygen atoms shared with layer MnO(6) octahedra forming a corner-sharing octahedral interlayer complex. The average Zn-O and Zn-Mn distances were 2.07+/-0.01 and 3.52+/-0.01 A, respectively. Macroscopic adsorption-desorption isotherms showed that, in contrast to that of the Zn-gamma-MnOOH system, adsorption of Zn(II) on delta-MnO(2) was highly reversible. EXAFS results indicated that the highly reversible adsorption was due to the weak adsorption mode of the corner-sharing linkage between the adsorbate and adsorbent polyhedra.     

X-ray absorption study of the solvation structure of Cu2+ in methanol and dimethyl sulfoxide

The solvation structure of Cu(2+) in methanol (MeOH) and dimethyl sulfoxide (DMSO) has been determined by studying both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions of the K-edge absorption spectra. The EXAFS technique has been found to provide a very accurate determination of the next-neighbor coordination distances, but it is inconclusive in the determination of the coordination numbers and polyhedral environment. Conversely, quantitative analysis of the XANES spectra unambiguously shows the presence of an average 5-fold coordination in both the MeOH and DMSO solution, ruling out the usually proposed octahedral Jahn-Teller distorted geometry. The EXAFS and XANES techniques provide coherent values of the Cu-O first-shell distances that are coincident in the two solvents. This investigation shows that the combined analysis of the EXAFS and XANES data allows a reliable determination of the structural properties of electrolyte solutions, which is very difficult to achieve with other experimental techniques.     

Heptametallic bowl-shaped complexes derived from conjugated Schiff-base macrocycles: synthesis, characterization, and X-ray crystal structures

The reaction of chelating conjugated macrocycles 1a-c with Zn(OAc)2 gives bowl-shaped heptanuclear Zn complexes featuring Zn in tetrahedral, octahedral, and square-pyramidal geometries. Crystallographic and NMR results indicate that vacant Zn coordination sites within the bowl may be accessed, suggesting that these coordination complexes may be used as mimics for Zn fingers and carbonic anhydrase.     

Sorption speciation of nickel(II) onto Ca-montmorillonite: batch, EXAFS techniques and modeling

The sorption speciation of Ni(II) on Ca-montmorillonite was evaluated using a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and modeling. The pH and temperature at the aqueous-montmorillonite interface affects both the extent of Ni(II) sorption as well as the local atomic structure of the adsorbed Ni(II) ions. At 0.001 mol L(-1) Ca(NO(3))(2) and low pH, the study reveals that the majority of Ni(II) is adsorbed in the interlayers of Ca-montmorillonite coordinated by six water molecules in an octahedron as an outer-sphere complex. At higher pH, inner-sphere surface complexes are formed. The Ni-Si/Al distances (R(Ni-Al) = 3.00 ?, R(Ni-Si1) = 3.10 ? and R(Ni-Si2) = 3.26 ?) determined by EXAFS confirm the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5 and 8.5. At pH 10.0, the Ni-Ni/Si distances (R(Ni-Ni) = 3.07 ? and R(Ni-Si) = 3.26 ?) indicates the formation of Ni-phyllosilicate precipitates. A rise in temperature promotes inner-sphere complexation, which in turn leads to an increase in Ni(II) sorption on Ca-montmorillonite. Sorption edges are fitted excellently by surface complexation model (SCM) with the aid of surface species determined from EXAFS spectroscopy.     

γ 辐照法制硫化镍纳米非晶及其晶化

以γ射线辐照硫代硫酸钠和硫酸镍的混合水溶液,所得沉淀物经乙醇多次清洗后６０℃下烘干。将所得粉末样品分别在氩气氛中１６０－５００℃焙烧４h,使用Ｘ射线衍射（ＸＲＤ ）及扩展Ｘ光吸收精细结构（ＥＸＡＦＳ）分析样品的结构。结果表明,１６０℃以下烘干所得样品为纳米非晶,其Ｎi－Ｓ壳层为四配位结构,Ｎi－Ｓ键长为０．２２１ｎｍ。经３００℃焙烧后,非晶默认为ＮiＳ微晶,Ｎi－Ｓ壳层变为八面体六配位（红镍矿型）结构,     

Nanogibbsite: synthesis and characterization

Nanogibbsite was synthesized using a supersaturated Al(OH)(3) solution which was prepared by titration of AlCl(3) with NaOH at pH 4.6. Excess chloride ions in the system were stripped off by dialyzing the Al(OH)(3) suspension against distilled water. The dialysis step is critical for initiation of gibbsite crystallization or the Al(OH)(3) suspension would remain amorphous. Chloride ions seem to mask the seeding sites and so retard the overall process of gibbsite formation. When subjected to heat treatment, gibbsite→alumina conversion occurred by two mechanisms. Nanogibbsite→α-alumina phase transition occurred forming χ- and κ-alumina polymorphs.     

A XANES and EXAFS Study of Hydration and Ion Pairing in Ambient Aqueous MnBr<Subscript>2</Subscript> Solutions

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to probe the first-shell coordination structure of Mn(II) in aqueous MnBr₂ solutions at ambient conditions from very dilute to the near saturation limit. The Mn K-edge EXAFS spectra for 0.05 and 0.2 m solutions showed that there was no Br(−I) in the first shell, and that the Mn(II) was fully hydrated with six water molecules in an octahedral arrangement. In contrast, for 6 m solution, the coordination number of water was reduced to about 5, and an average of about one bromine atom was present in the first shell as a contact ion pair. The 1s → 4p transition at 6545.5 eV confirmed the observation of Mn–Br contact ion pairs at high concentrations and the 1s → 3d transition at 6539.5 eV showed that the first shell coordination symmetry remained octahedral even in the presence of Mn–Br ion pairs.