X-ray Absorption Near-edge Spectroscopy Measurements Research Articles

Long-and short-range structure of SnO2 nanoparticles: Synthesis and photo(electro)catalytic activity

We report herein a detailed study on the influence of hydrothermal treatment temperature on both long- and short-range structures of SnO2 nanoparticles (NPs) applied as photocatalysts for the discoloration of organic pollutants and as photoanodes for water splitting. Synchrotron X-ray diffraction and X-ray absorption near-edge spectroscopy measurements confirmed the enhancement of the structural order of SnO2 NPs as a function of hydrothermal temperature. Fourier transform infrared spectroscopy revealed that the hydrothermal treatment increased the amount of hydroxyl groups on the SnO2 NPs surface. Regarding the photocatalytic activity, the NPs were able to promote the discoloration of different dyes that can act as potential organic pollutants. The photoelectrocatalytic performance of the samples depended on the hydrothermal treatment temperature, with the degree of crystallinity and surface hydroxyl groups playing a significant role in their performance as photoanodes. In particular, the NPs treated at a higher temperature presented a better degree of crystallinity, in addition to many hydroxyls on their surface, leading to increased mobility of the photogenerated charge carriers and improving the interaction between the molecules degraded and the material surface. The results demonstrated that the hydroxyls adsorbed on the SnO2 surface favor the formation of hydroxyl radicals, a species that indirectly participate in the photocatalytic oxidation of rhodamine B dye. The photoelectrocatalytic tests showed that the NPs treated at 200 °C increased oxygen evolution reaction performance.

The effect of Ge concentration on the Ni diffusion across AuGe metal interface of GaAs/AlGaAs heterostructure

X-ray absorption near edge spectroscopy (XANES) has been employed to investigate the effect of Ge concentration on the diffusion of Ni in AuGe/Ni/AuGe stack, which is an Ohmic contact to the GaAs/AlGaAs heterostructure. Four different AuGe weight ratios are chosen- one eutectic (AuGe 75:25) and three offeutectic (AuGe 88:12, 95:05 and 100:0). The AuGe/Ni/AuGe stacks are annealed at 100˚C and 300˚C temperatures for 2hrs to facilitate the diffusion of Ni in to AuGe matrix. XANES measurements were carried out at 45˚ incidence angle and total reflection geometry. Metallic Ni, NiO and Ni3Ge were used as reference samples for XANES analysis. The detailed XANES analysis confirmed that the high concentration of Ge in AuGe sandwich layers restricts the diffusion of Ni in to AuGe matrix. A possible mechanism behind the Ni diffusion into AuGe matrix is propounded based on the XANES result.

Analysis of Cimetidine Crystal Polymorphs by X-ray Absorption Near-Edge Spectroscopy.

Sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) measurements were performed to characterize the crystal polymorphs of the active pharmaceutical ingredients (APIs) containing sulfur atoms. Cimetidine (CIM) was used as a model API. Each crystal form of CIM has its own XANES spectrum, so we can discriminate the crystal form by its spectrum. The analysis of the crystal structure of CIM revealed that the difference in the shape of XANES spectra was ascribable to the difference in the C-S-C bond angle of CIM molecules and the intermolecular hydrogen bonds, such as C-H···S and N-H···S, and S-S interaction. It was found that the peak shape of the XANES spectrum is gentle when the C-S-C bond angle is large, while the peak shape can be steep when the C-S-C bond angle is small. Furthermore, it was found that the peak energy values varied depending on the hydrogen bonds and S-S interaction. By linear combination fitting using XANES spectra, it was possible to quantify the ratio of CIM form A crystal in mixed powders of form A and monohydrate crystals. These results indicate that XANES measurements can be a useful technique to evaluate the crystal polymorphism of APIs containing S atom in pharmaceutical formulation.

X-ray Absorption Near-Edge Spectroscopy Analysis of Indomethacin in Crystalline Forms and in Amorphous Solid Dispersions.

Chlorine K-edge X-ray absorption near-edge spectroscopy (XANES) measurements were performed to characterize the crystal polymorphs of identical active pharmaceutical ingredients (APIs) containing chloride atoms and their amorphous solid dispersions (ASDs). Indomethacin (IMC), of which three crystal forms (α, β, and γ) have been reported, was used as a model API. The shape of XANES spectra was unique to each IMC crystal. The analysis of the crystal structures of IMC revealed that chlorine atoms of the IMCα form had unique intermolecular interactions and halogen bonds with oxygen atoms, while those of the IMCγ form do not have any notable interactions. This result showed that XANES measurements can detect weak interatomic interactions. The shapes of the ASD spectra were clearly different from those of the crystals, suggesting that the environment around the Cl atom of IMC was different from that of the crystals. A thermal stress test was then performed to study the transformation from the amorphous form to the crystalline form of IMC in the ASD. The powder X-ray diffraction (PXRD) patterns indicated that amorphous IMC transformed into crystals during the thermal stress test. In accordance with the PXRD results, the XANES spectra also transformed from ASD to crystalline form. These results indicate that the IMC transformation could be monitored by XANES measurement. Our findings led us to conclude that XANES measurement is a novel approach for the evaluation of crystal polymorphs of APIs and the crystalline state of APIs in ASDs.

A Combined Study of Tc Redox Speciation in Complex Aqueous Systems: Wet-Chemistry, Tc K-/L3-Edge X-ray Absorption Fine Structure, and Ab Initio Calculations

The combination of wet-chemistry experiments (measurements of pH, Eh, and [Tc]) and advanced spectroscopic techniques (K- and L3-edge X-ray absorption fine structure spectroscopy) confirms the formation of a very stable Tc(V)-gluconate complex under anoxic conditions. In the presence of gluconate and an excess of Sn(II) (at pe + pH ≈ 2), technetium forms a very stable Tc(IV)-gluconate complex significantly enhancing the solubility defined by TcO2(s) in hyperalkaline gluconate-free systems. A new setup for "tender" X-ray spectroscopy (spectral range, ∼2-5 keV) in transmission or total fluorescence yield detection mode based on a He flow cell has been developed at the INE Beamline for radionuclide science (KIT light source). This setup allows handling of radioactive specimens with total activities up to one million times the exemption limit. For the first time, Tc L3-edge measurements (∼2.677 keV) of Tc species in liquid (aqueous) media are reported, clearly outperforming conventional K-edge spectroscopy as a tool to differentiate Tc oxidation states and coordination environments. The coupling of L3-edge X-ray absorption near-edge spectroscopy measurements and relativistic multireference ab initio methods opens new perspectives in the definition of chemical and thermodynamic models for systems of relevance in the context of nuclear waste disposal, environmental, and pharmaceutical applications.

Geochemical changes of Mn in contaminated agricultural soils nearby historical mine tailings: Insights from XAS, XRD and, SEP

Manganese is ubiquitous in nature and essential for some metabolic reactions, but also it is present in the environment as a product of anthropogenic activities. Despite evidence linking high concentrations of Mn to several diseases, it is not commonly considered during environmental risk assessment of soils. Changes in chemical forms, oxidation state, and speciation of Mn in impacted agricultural soils by historical mine tailings in central Sonora in northwestern Mexico were studied using X-ray Absorption Near Edge Spectroscopy (XANES), X-ray Diffraction (XRD), and Sequential Extraction Procedures (SEP) techniques. Mn concentrations ranged from 1093 to 2919 mg·kg−1 in these soils, which exceeded the local geochemical background by up to six-fold. According to SEP, the tendency found in agricultural soils was reducible (42–66%) > soluble and exchangeable (16–42%) > residual (5–22%) > oxidizable (4–24%). In contrast, the tendency in sulfide-rich tailings samples was soluble and exchangeable (47–49%) > residual (35–41%) > oxidizable (5–11%) > reducible (5–7%), and in oxide-rich materials was residual (21–79%) > soluble and exchangeable (1–30%) > reducible (1–45%) > oxidizable (2–15%). The considerable percentage of the soluble forms indicated the potential of mobilization and availability to exposed organisms. No Mn-bearing mineral phases were identified with XRD, which suggested that manganese was mostly found in low-crystalline forms. XANES measurements indicated that the oxidation state of Mn in agricultural soil samples varied from Mn2+ through Mn4+. Conversely, the main oxidation state in mine tailings samples was Mn2+. Substantial changes were detected in Mn XANES spectra in the whole sample and residues left from SEP in agricultural soils, which confirmed the presence of all chemical forms (e.g., soluble and exchangeable, reducible, oxidizable, residual). A well-defined structure spectrum was obtained in soils once low crystallinity species and/or organic compounds were removed, which was similar to that acquired for mine tailings, suggesting mobility from tailings deposit to agricultural soils. Substantial contribution of compounds with mineral structures like birnessite (((Na, Ca, K)0.6(Mn4+, Mn3+)2O4·1.5H2O) and minor contributions of hausmannite (Mn3O4), spessartine (Mn3Al2(SiO4)3) and szmikite (MnSO4) were identified by Linear Combination Fitting (LCF) in soils, whereas the significant contribution of structures like hausmannite and spessartine, and minor contributions of rhodochrosite were found in tailings material. An LCF analysis in residues from SEP also suggests that the chemical species involved in the Mn mobilization were scachite and szmikite. In general, the results obtained indicate that leaching processes favored the presence of chemical species of Mn with a considerable potential of mobility and toxicity in agricultural soils interacting with historical mine tailings in central Sonora.

Study of reactively sputtered nickel nitride thin films

Nickel nitride (Ni–N) thin film samples were deposited using reactive magnetron sputtering process utilizing the different partial flow of N2 (RN2). They were characterized using x-ray reflectivity (XRR), x-ray diffraction (XRD) and x-ray absorption near-edge spectroscopy (XANES) taken at N K-edge and Ni L-edges. From XRR measurements, we find that the deposition rate and the density of Ni–N films decrease due to successively progression in RN2, signifying that Ni–N alloys and compounds are forming both at Ni target surface and also within the thin film samples. The crystal structure obtained from XRD measurements suggest an evolution of different Ni–N compounds given by: Ni, Ni(N), Ni4N, Ni3N, and Ni2N with a gradual rise in RN2. XANES measurements further confirm these phases, in agreement with XRD results. Polarized neutron reflectivity measurements were performed to probe the magnetization, and it was found Ni–N thin films become non-magnetic even when N incorporation increases beyond few at.%. Overall growth behavior of Ni–N samples has been compared with that of rather well-known Fe–N and Co–N systems, yielding similarities and differences among them.

The magneto-elastic and optical properties of multiferroic GaFeO3-δ

Temperature-dependent Raman spectroscopy, X-ray absorption near-edge spectroscopy (XANES), optical absorption spectroscopy (OAS) and ac-susceptibility measurements have been carried out on GaFeO3 samples to understand the effect of sintering temperature (TS) on optical and magneto-elastic properties. The temperature-dependent ac-susceptibility measurements suggest the systematic scaling in the values of ferri-magnetic transition temperature (TC) with TS. The observed variation has been understood in terms of enhancement in the Goodenough-Kanamori-Anderson coupling. Instead of expected anharmonic behavior in Raman spectra around TC, several vibrational modes of rigid FeO6 units were found to show anomalous behavior, which suggests the significant magneto-elastic coupling contributions to phonon frequencies. The strength of magneto-elastic coupling has been quantified by using the phonon renormalization function (Δw) and is found to scale systematically with TS. Further, optical absorption study demonstrates the increase in intensity and red shift in charge-neutral d-d transitions, which infers the increase in oxygen non-stoichiometry with TS. The oxygen non-stoichiometry has been further quantified by the XANES measurements. The present investigations on GaFeO3 samples reveal that TC can be tuned by varying the oxygen stoichiometry and can be very beneficial for spintronic devices applications.

Iron Nanoparticles Encapsulated in S,N-Codoped Carbon: Sulfur Doping Enriches Surface Electron Density and Enhances Electrocatalytic Activity toward Oxygen Reduction.

Development of highly efficient nonprecious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acidic media is challenging but of great significance. Herein, an effective ORR catalyst based on Fe nanoparticles encapsulated by S,N-codoped few-layer defective carbon (Fe@S,N-DC) was synthesized via a microwave-assisted strategy. The obtained Fe@S,N-DC nanocomposite showed a remarkable electrocatalytic activity toward ORR in acidic media, with a half-wave potential (E1/2) of +0.785 V versus reversible hydrogen electrode, which was 80 mV more positive than that of the sulfur-free counterpart (Fe@N-DC). Furthermore, due to the protection by the S,N-codoped carbon shell, the Fe@S,N-DC nanocomposite displayed apparent stability with only a 13 mV negative shift of E1/2 after 10,000 cycles and excellent tolerance to methanol. X-ray absorption near-edge spectroscopy measurements confirmed the formation of multiple defective sites on the S,N-codoped carbon surface and strong interfacial electron transfer from the Fe core to the outer carbon surface, as compared to the sulfur-free counterpart. The enriched electron density on the defective carbon surface of Fe@S,N-DC, induced by the interfacial electron transfer, facilitated the reduction of O2 to OOH*, leading to enhanced ORR performance. These results shed light on the significance of S doping in Fe-N-C catalysts in the design of high-performance NPM catalysts for ORR in acidic media.

Examination of Natural and Standard Fe<sub>3</sub>O<sub>4</sub> Powders Using X-Ray Absorption Near-Edge Spectroscopy (XANES)

Investigation of Fe K-edge X-Ray Absorption Near Edge Spectroscopy (XANES) spectra of Fe3O4 (FeO.Fe2O3) from natural source compared with the Fe3O4 standard is presented. The natural Fe3O4 powder was prepared from ironstone of Tanah Laut, Kalimantan Selatan by co-precipitation method. XANES measurements in transmission mode were performed at the Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand. XRD phase analysis confirms that the synthesized Fe3O4 powder is a single phase, but it cannot determine the proportion of Fe2O3 and FeO in the structure. TEM measurement confirms that the particle size of natural Fe3O4 about 10 nm. Qualitative analysis of the pre-edge XANES data revealed that the absorbing atom in the XAS measurement is Fe3+. Meanwhile, the absorption edge (E0) values of natural and standard Fe3O4 powders were 7126.44 eV and 7125.02 eV, respectively. The proportion was then acquired using XANES data analysis through Linear Combination Fitting (LCF). It was found that the natural Fe3O4 sample consisted of 98 wt. % Fe2O3 and 2 wt.% FeO, while the standard Fe3O4 powder consisted of 96 wt. % Fe2O3 and 4 wt. % FeO. The mechanism of the absorption in both samples is also described and compared.

High-Pressure Synthesis of Lu2NiIrO6 with Ferrimagnetism and Large Coercivity.

Double-perovskite Lu2NiIrO6 was synthesized at high pressure (6 GPa) and high temperature (1300 °C). Synchrotron powder X-ray diffraction indicates that its structure is a monoclinic double perovskite (space group P21/ n) with a small, 11% Ni/Ir antisite disorder. X-ray absorption near-edge spectroscopy measurements established Ni2+ and Ir4+ formal oxidation states. Magnetic studies indicate a ferrimagnetic transition at 207 K. The low-temperature magnetization curve of Lu2NiIrO6 features broad hysteresis with a coercive field as high as 48 kOe. These results encourage the search for hard magnets in the class of 3d/5d double-perovskite oxides.

Fe K-Edge X-ray absorption near-edge spectroscopy (XANES) and X-ray diffraction (XRD) analyses of LiFePO4 and its base materials

XANES analysis has been performed with the aim of knowing the Fe oxidation state in a synthesized LiFePO4 and its base materials. XANES measurements were performed at SLRI on energy around Fe K-edge. An XRD analysis has also been performed with the aim of knowing the phase composition, lattice parameters and crystallite size of the LiFePO4 as well as the base materials. From the XRD analysis, it was found that the dominating phase in the iron sand sample was Fe3O4 and the only phase found after calcination was LiFePO4. The latter phase exhibited crystallite size of 100 nm and lattice parameters a = 10.169916 Å, b = 5.919674 Å, c = 4.627893 Å. Qualitative analysis of XANES data revealed that the oxidation number of Fe in the sample before calcination was greater than that after calcination and Fe in the natural iron sand, indicated by the E0 values of 7129.2 eV, 7120.6 eV and 7124.4 eV respectively.

X-ray absorption spectroscopy of Mn doped ZnO thin films prepared by rf sputtering technique

A set of r.f. sputter deposited ZnO thin films prepared with different Mn doping concentrations have been characterised by Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Spectroscopy (XANES) measurements at Zn, Mn and O K edges and at Mn L2,3 edges apart from long range structural characterisation by Grazing Incident X-ray Diffraction (GIXRD) technique. Magnetic measurements show room temperature ferromagnetism in samples with lower Mn doping which is however, gets destroyed at higher Mn doping concentration. The results of the magnetic measurements have been explained using the local structure information obtained from EXAFS and XANES measurements.

Measurement of labile arsenic speciation in water and soil using diffusive gradients in thin films (DGT) and X-ray absorption near edge spectroscopy (XANES)

Environmental context Both inorganic and organic arsenic species are toxic to the environment when labile. The Diffusive Gradients in Thin Films technique, equipped with ferrihydrite binding gel enables measurement of labile arsenic species from water and soil solutions. This study indicated that labile arsenic species are quantitatively adsorbed to the gel, and that they are stable for up to 2 weeks following deployment. Abstract Arsenic speciation was determined in the solution extracted from a ferrihydrite binding gel layer in a DGT unit (FB-DGT) deployed in water using coupled high-performance liquid chromatography and hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS). However, the extent of change in arsenic speciation during the extraction process is unknown. By identifying the arsenic species in the FB-DGT gel directly, using X-ray absorption near edge spectroscopy (XANES) fitting with model arsenic compounds, we obtain a better understanding of the ability of FB-DGT to measure labile arsenic species in solution. The results presented herein confirm that FB-DGT accumulated labile inorganic and methylated arsenic species. Arsenic species bound to the FB-DGT gel were stable for up to 2 weeks following deployment. However, caution should be applied when interpreting the proportion of As species measured by HPLC-HG-AFS in solution extracted from FB-DGT because the distribution of arsenic species in extracted solutions was found to be modified by the extraction process. Some (~20%) of arsenate was converted into arsenite, and a significant amount (~25%) of dimethylarsinic acid (DMAsV) was converted into monomethylarsonic acid (MMAsV) or arsenate (AsV). Only inorganic arsenite (iAsIII) was stable during the extraction process. These findings suggested that, based on the XANES measurement, although As species were quantitatively accumulated in the FB-DGT gel, the FB-DGT technique was still unsuitable for studying As speciation. This is because changes in arsenic speciation were observed to occur during gel extraction, prior to HPLC-HG-AFS measurement, and because the XANES technique is available for routine measurements.

Local electronic structure of ZnO nanorods grown by radio frequency magnetron sputtering

We report the structural properties and local electronic structure of vertically aligned ZnO nanorods fabricated on silicon substrate without external catalyst by radio frequency magnetron sputtering. X-ray diffraction and morphological studies reveal the hexagonal wurtzite crystal structure with preferential orientation along the (002) crystallographic plane and vertical alignment of the ZnO nanorods. Angle dependent X-ray absorption near-edge spectroscopy measurements of the vertically aligned ZnO nanorods at O K and Zn L-edges divulge that the tip and side wall surfaces of the nanorods are terminated by the oxygen and zinc ions respectively.

Synchrotron XRF and XANES investigation of uranium speciation and element distribution in fluid inclusions from unconformity‐related uranium deposits

AbstractFluid inclusions from two quartz samples of the McArthur River and Rabbit Lake unconformity‐related uranium deposits (Athabasca Basin, Canada) were analysed by synchrotron X‐ray fluorescence (SXRF) and X‐ray absorption near‐edge spectroscopy (XANES) to shed light on (i) the detailed chemistry of the fluids having transported the uranium and (ii) the speciation of uranium in these fluids. The analysed samples contain variable proportions of NaCl‐rich or CaCl2‐rich (25–35 wt% salts) fluid inclusions that homogenise into the liquid phase between 120 and 200°C. For unknown reason, all of the CaCl2‐rich fluid inclusions decrepitate under X‐ray beam after a few seconds, precluding any SXRF or XANES measurement. SXRF on 12 homogenised NaCl‐rich fluid inclusions from the Rabbit Lake sample shows that the fluid inclusions contain appreciable amounts of Br, Fe, Sr, transition metals (Mn, Ni, Cu, Zn), Pb, U and rare earth elements (REE) (La, Ce), with concentrations being relatively homogeneous among fluid inclusions. Within the complex McArthur River sample (numerous fluid inclusions + randomly distributed solids), statistical analyses of hyperspectral SXRF images were performed using the non‐negative matrix approximation (NNMA) method. This strategy allows distinguishing the three domains contributing to the overall signal: (i) the quartz matrix, which notably contains significant amounts of Cr, Cu and Pb; (ii) the fluid inclusions characterised by high amounts of Br, Fe, Sr and transition metals; and (iii) La‐Ce ± Fe solids. Part of the U and REE are spatially associated with distinct optically invisible solids within the quartz matrix. XANES on four McArthur River sample fluid inclusions at room temperature and at 150°C (fluid inclusion trapping temperature) as well as in solid and liquid U(IV) and U(VI) standards, respectively, shows that the uranium has remained in the form of U(VI) from trapping to present in the fluid inclusion.

Efficient charge carriers induced by extra outer-shell electrons in iron-pnictides: a comparison between Ni- and Co-doped CaFeAsF

A comprehensive study of the difference between CaFe1−xNixAsF and CaFe1−xCoxAsF systems has been carried out by measuring the efficient charge carrier concentration, the valence states and the superconducting phase diagram. It is found that at the same doping level, Ni doping introduces nearly twice the number of charge carriers as Co doping. However, x-ray absorption near-edge spectroscopy measurements reveal that the valence state of Fe in both systems is close to 2, indicating that there is no valence mismatch. We suggest that the charge carriers in CaFe1−xMxAsF (M=transition metal elements) are not induced by valence mismatch but come from the difference in the number of outer-shell electrons. We also suggest that with Ni and Co doping, the systems change from a multi-band material in the underdoped regions to a single-band state in the overdoped regions.

Defect Chemistry and Thermomechanical Properties of Ce0.8PrxTb0.2 − xO2 − δ

The oxygen nonstoichiometry of was measured as a function of at temperatures between 600 and by coulometric titration and thermogravimetry. A nonideal solution model, allowing for a linear δ dependence of the partial molar enthalpy of reduction in the dopants, could successfully reproduce the experimentally determined oxygen nonstoichiometry. X-ray absorption near-edge spectroscopy measurements were performed at the Ce/Pr/Tb L3 and L2 edges. The valence state of each dopant was affected by the presence of the co-dopant. The redox properties strongly depended on the lattice strain energy and the mean metal–oxygen bond strength. The thermal and chemical expansion coefficients were determined by dilatometry. The strongly nonlinear behavior of the thermal expansion coefficient originated from the chemical strain due to increasing oxygen nonstoichiometry with increasing temperature.

Local structure of Mn dopants in CuAlS2and CuGaS2

The local structure around Mn atoms in Mn-doped CuAlS2 and CuGaS2 chalcopyrites is investigated using the X-ray absorption fine structure method. Extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near-edge spectroscopy measurements at the Mn K-edge indicate that Mn substitutes on the cationic atoms sites. In the CuAlS2 structure Mn is located at both Cu and Al sites, with a preference for the Cu sites for higher Mn concentrations. Discrimination between Mn at two cationic sites is impossible for CuGaS2:Mn using EXAFS due to almost identical photoelectron scattering amplitudes and phases for Cu and Ga atoms. The nearest-neighbour Mn–S distance for all studied compositions is close to 2.41 Å which is a typical value for the Mn–S tetrahedral bond length. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products

The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II) (aq) in pH 6–8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 × 10 −9 atm O 2). The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Fe/Tc(IV) form was observed when 11 μmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II) (aq) = 0.05–0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. Heterogeneous reduction predominated after approximately 0.01 mmol/L of Fe(II) was oxidized. The heterogeneous reaction was more rapid, and was catalyzed by Fe(II) that adsorbed to the Fe/Tc(IV) redox product. Wet chemical and Fe–X-ray absorption near edge spectroscopy measurements (XANES) showed that Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox products after reaction termination. 57Fe-Mössbauer, extended X-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements revealed that the Fe/Tc(IV) solid phase was poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with an octahedral Tc(IV) dimer bound in bidentate edge-sharing mode to octahedral Fe(III) associated with surface or vacancy sites in ferrihydrite. The precipitate maintained Tc(IV) aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O 2· nH 2O (s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O 2· nH 2O (s) as a result of its intraparticle/intragrain residence. Precipitates of this nature may form in anoxic sediments or groundwaters, and the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.