Influence of Sample Pretreatment on P Speciation in Sediments Evaluated with Sequential Fractionation and P <i>K</i>-edge XANES Spectroscopy

Sequential phosphorus (P) fractionation procedures are one of the most widely used wet chemical methods for characterizing P pools in soils and sediments, but have also been criticized repeatedly for their lack of accuracy to measure chemically specified phosphate fractions. In the recent investigation, sediments from two different sample locations with the same pretreatments were analyzed with sequential P fractionation. To verify traditional assignments of P fractionation results, P K-edge X-ray absorption near edge structure (XANES) spectroscopy was applied on the sediments and especially on the residues after the sequential extraction steps. Results of both methods indicated that the influence of sample pretreatment on the distribution of P pools was much lower compared to the effects of different sample origins. Kettle hole sediments were dominated by moderately labile iron (Fe) and aluminum (Al) associated P species, whereas Bodden sediments contained more stable calcium (Ca)–P species. Sample pretreatment of sediments can be similar to traditional soil sample pretreatment without causing fundamental changes in P speciation. The P K-edge XANES spectroscopy confirmed most assumptions of sequential P fractionation.</p

Watching Iron Nanoparticles Rust: An <i>in Situ</i> X‑ray Absorption Spectroscopic Study

Iron nanoparticles and iron oxide nanoparticles are among the most commonly studied nanomaterials because of their applications in fields ranging from catalysis to ferrofluids. However, many synthetic methods give iron nanoparticles with large size distributions, and it is difficult to follow the kinetics of iron nanoparticle oxidation reactions and the relative speciation of iron oxidation states in real time. Herein, we introduce a simple approach of controlling the sizes of Fe@Fe_<i>x</i>O_<i>y</i> nanoparticles and a novel method for following Fe@Fe_<i>x</i>O_<i>y</i> nanoparticle oxidation <i>in situ</i> in liquid solutions by Fe K- and L-edge X-ray absorption near-edge structure (XANES) spectroscopy. XANES results show that these Fe@Fe_<i>x</i>O_<i>y</i> nanoparticles have similar XANES spectra before exposure to air. <i>In situ</i> XANES measurements allow for quantitative oxidation kinetics of different nanoparticle sizes to be followed; results show that the rate of Fe(0) oxidation increases with a decrease in average nanoparticle size. However, the rate of Fe core size depletion was found to be ca. 0.02 nm/min for all the nanoparticle systems studied. This suggests similar oxidation mechanisms are at work for all the particle sizes studied. This work shows that <i>in situ</i> liquid cell XANES can be used to follow oxidation state and coordination environment changes in Fe NP dispersions

A critical review and evaluation of some P-research methods

Important tools for the investigation of phosphorus (P) in soil are the acid digestion in aqua regia (AR) for total P, the sequential P fractionation (SF), 31P nuclear magnetic resonance (31P-NMR) spectroscopy and P K-edge X-ray absorption near-edge structure (P K-edge XANES) spectroscopy. The objective of the present study was to critically compare the outcomes of these methods in order to improve the data evaluation and interpretation. For this purpose, we analyzed total P (Pt) and the P speciation of three groups of soil samples (extremely P-rich, P-poor acidic, P-poor to P-rich alkaline). The recovery of the extracted P between the different methods depended on the soil characteristics, resulting in recommendations for improving the P extraction. The 31P-NMR spectra were calibrated to a monoester standard which provided a better consistency of the peak position in comparison to the shifting of the orthophosphate peak to 6.0 ppm. The SF and 31P-NMR agreed in the inorganic P (Pi) and Po proportions, especially in the alkaline fraction. The correlation between Po from P K-edge XANES and diesters from 31P-NMR indicated that P K-edge XANES detected rather diesters than monoesters in soils. Correlations between SF and P K-edge XANES results confirmed the presence of specific Pi species in the sequentially extracted fractions. However, the SF is still relatively unspecific for interpreting Po species from a certain fraction. A more specific description of the Po composition in fractions from the SF requires investigation of these fractions by 31P-NMR and P K-edge XANES.</p

In Situ X‑ray Absorption Spectroscopic Study of Fe@Fe_<i>x</i>O_<i>y</i>/Pd and Fe@Fe_<i>x</i>O_<i>y</i>/Cu Nanoparticle Catalysts Prepared by Galvanic Exchange Reactions

Fe@Fe_<i>x</i>O_<i>y</i> core–shell nanoparticles have been previously shown to be a versatile support for catalytic metals such as Pd and Cu. However, the resulting structure, metal speciation, and performance of such catalysts in catalytic reactions are still poorly understood. Herein, we synthesize Fe@Fe_<i>x</i>O_<i>y</i>-supported Pd and Cu nanoparticles by controlling the molar ratios of Fe@Fe_<i>x</i>O_<i>y</i> nanoparticles to Pd²⁺ or Cu²⁺ species. Scanning transmission electron microscopy analyses show that Pd or Cu NPs are deposited on the exterior shell of the Fe@Fe_<i>x</i>O_<i>y</i> nanoparticles. In situ X-ray absorption near-edge structure (XANES) spectra were used to follow the formation processes of Fe@Fe_<i>x</i>O_<i>y</i>/Pd and Fe@Fe_<i>x</i>O_<i>y</i>/Cu nanoparticles and the performance of Fe@Fe_<i>x</i>O_<i>y</i>/Pd nanoparticles for Suzuki–Miyaura cross-coupling reactions. The results show that different molar ratios of Fe@Fe_<i>x</i>O_<i>y</i> nanoparticles to Pd²⁺ or Cu²⁺ lead to different morphologies of the resulting supported-NP structures. In situ XANES results show that Fe@Fe_<i>x</i>O_<i>y</i> nanoparticles can effectively fully reduce Pd or Cu salts over the course of ∼20 min to give small Pd or larger Cu nanoparticles on the surface and can also rereduce oxidized Pd in Suzuki–Miyaura cross-coupling reactions

Enhanced Fluorescence in Tetraylnitrilomethylidyne–Hexaphenyl Derivative-Functionalized Periodic Mesoporous Organosilicas for Sensitive Detection of Copper(II)

Highly fluorescent and copper­(II)-responsive periodic mesoporous organosilicas (TH-PMOs) were successfully obtained by using a (tetrayl­nitrilo­methylidyne–hexaphenyl (TH))-derived tetrasiloxane (TH-Si₄) as the organosilica precursor. The TH unit was embedded within the framework of PMOs by four silyl groups without forming associated species, and high fluorescence quantum yields were achieved even for the PMOs prepared from 100% organosilane precursor. The optical studies indicated that the intramolecular rotation of TH was restricted by the framework of TH-PMOs, resulting in the decline of the nonradiative decay process and enhanced monomeric fluorescence emission. The unique structure of the TH groups not only assured their aggregation-induced enhanced emission (AIEE) characteristics but also provided potential coordinating sites for metal ions. Therefore, the enhanced fluorescence of PMOs showed a highly selective response to copper ions in aqueous solution with the detection sensitivity up to the 10^–8 M level. Moreover, the diffusion process of Cu²⁺ and the competitive effect between Cu²⁺ and Fe²⁺ on TH-PMOs were measured by STXM; the results reveal that the specific binding between TH and Cu²⁺ brings about a relatively high adsorption capacity of the hybrid material toward Cu²⁺

Carbon Nanofilaments Functionalized with Iron Oxide Nanoparticles for in-Depth Hydrogen Sulfide Adsorption

The purification of hydrogen prior of its use in various applications, such as fuel cells, is of paramount importance. Although there are many commercial ways to obtain hydrogen sulfide, the need to reach very low concentration values, at the ppm or even at the ppb level, is the main motivation behind this work. This work examines the production and utilization of a new, low H₂S breakthrough and high capacity adsorbent, made of iron nanoparticles embedded in carbon nanofilaments. It is produced by a 2-step functionalization methodology: acid pretreatment and iron wet impregnation. This novel adsorbent was characterized by scanning transmission electron microscope, X-ray absorption near edge structure, Brunauer Emmet and Teller calculations, and thermogravimetric analysis, and the adsorption efficiency was measured for different iron-loadings, temperatures, and H₂S breakthrough values. Operating conditions and metal-loading that allow a decrease of H₂S concentration from 500 ppm to below 1.5 ppm are reported. It has also been found that acid treatment influences metal dispersion and, due to the nanometric nature of adsorbents, the process is not controlled by mass diffusion phenomena

Characterization of Mercury Binding onto a Novel Brominated Biomass Ash Sorbent by X‑ray Absorption Spectroscopy

Recent laboratory and field-scale experiments demonstrated the potential for brominated industrial solid waste from biomass combustion (Br-Ash) to be an efficient, cost-effective alternative to activated carbon for capturing mercury from coal-fired power plants. To develop this attractive alternative technology to a commercially sustainable level, a better understanding of mercury capture mechanisms by Br-Ash is required. For this purpose, X-ray absorption fine-structure (XAFS) spectra of Br-Ash were collected at the Hg LIII-edge, Br K-edge and S K-edge, and analyzed to determine the local bonding environment of mercury atoms. The coordination environment of Hg was compared with that on a commercial brominated activated carbon. Our results indicate that the mercury was captured by chemisorption on both the commercial and biomass ash sorbents; however, the mercury binding environment was different for each sorbent. Mercury was found to bind to the reduced sulfur by the commercial brominated activated carbon, in contrast to mercury binding with carbon and bromine on the brominated biomass ash. Based on the results obtained, a mechanism of Hg capture involving oxidation of elemental Hg followed by binding of the oxidized mercury on the surface of the sorbent near Br was proposed for the brominated biomass ash

Molecular Mechanisms of Chromium(III) Immobilization by Organo–Ferrihydrite Co-precipitates: The Significant Roles of Ferrihydrite and Carboxyl

The interaction mechanisms of heavy metals with organo–Fe hydroxides co-precipitates (OFC) remain unclear due to the structural complexity of the OFC. In this study, batch experiments were conducted to investigate the immobilization mechanisms of Cr­(III) by the OFC, which was prepared by co-precipitating Fe3+ with rice/rape straw-derived dissolved organic carbon, through sorption and co-precipitation using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy and scanning transmission X-ray microscopy (STXM). At an Fe/C molar ratio ≥ 0.3, both the sorption and co-precipitation immobilized the majority of Cr­(III), but the co-precipitation desorbed less Cr­(III) than the sorption regardless of DOC loadings and sources. In contrast, Cr­(III) immobilization was significantly reduced at an Fe/C molar ratio of 0.1 for both reactions. Linear combination fitting of Cr K-edge XANES spectra revealed the predominance of ferrihydrite-bound Cr­(III), but enhanced organic Cr­(III) occurred with increased organic carbon (OC) loading for both the sorption and co-precipitation. STXM coupled with multi-edge XANES analysis confirmed the primary association of Cr­(III) with ferrihydrite and directly probed carboxyl as the binding site for Cr­(III) retention on the OC constituents of the OFC. These results provided new molecular-level insights into the Cr­(III) retention mechanisms on the OFC, particularly for the interactions of Cr­(III) and OC constituents of the OFC, which could benefit the management of Cr-contaminated soils with straw returning

Water-Soluble Copper(I) Hydroxide Catalyst and Its Formation in Ligand-Free Suzuki–Miyaura Cross-Coupling Reactions

Copper has emerged as an alternative metal for metal-catalyzed cross-coupling reactions due to its low cost, readily availability, and low toxicity. This article reports a water-soluble active molecular catalyst and its formation in the ligand-free Suzuki–Miyaura (SM) cross-coupling reactions with copper iodide as the precatalyst in aqueous solutions. The SM coupling is homogeneous in nature, and the molecular catalyst is Cu­(OH) in its singlet electronic state as identified by experimental and computational UV–vis absorption spectroscopy. The Cu­(OH) catalyst is generated through the leaching of oval-shaped Cu2O nanoparticles, which are characterized with X-ray Auger electron spectroscopy, X-ray absorption spectroscopy, and transmission electron microscopy. The soluble Cu­(OH) species is stable for at least 4 weeks under ambient conditions

Further Understanding of the Electronic Interactions between N719 Sensitizer and Anatase TiO₂ Films: A Combined X-ray Absorption and X-ray Photoelectron Spectroscopic Study

In this study, the electronic properties of N719 adsorbed onto anatase were comparably investigated by using X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) techniques. Sensitized TiO2 films made from two different nanocrystalline anatase powders were investigated: a commercial one (Solaronix) and our synthetic variety produced through aqueous synthesis. This was done to investigate how our aqueous-produced nanocrystalline anatase substrates compared with commercial products and to observe whether both nanocrystalline anatase anodes behaved in a similar manner in terms of their bonding and electronic interactions. Surface coordination changes to Ti−O groups previously reported via Ti K-edge extended X-ray absorption fine structure (EXAFS) data [using transmission or fluorescence yield (FY)] between the pure TiO2 and the adsorbed state were not observed in our measurements via the Ti L or K X-ray absorption near-edge structure (XANES) (nor EXAFS) data for both substrates via a surface-sensitive detection technique (total electron yield, TEY). This is likely due to the probing depth of TEY mode (5−10 nm), in which the coordination changes that occur to the surface groups, which should in turn affect the XANES spectrum, are not observed at Ti K- or L-edge XANES spectrum. The C and N K-edge XANES spectra of the N719 adsorbed onto two TiO2 films were for the first time evaluated in this work. From the C K-edge XANES data, the spectral changes revealed that additional electronic states occur between dye molecules and TiO2 surface. The C K-edge XANES spectra allowed us to propose that electronic interactions do not only occur through the covalent bonding of the anchoring groups but also through the aromatic electron density of the bipyridine groups and the d states found in TiO2. This was further confirmed via XPS analysis by monitoring the N bipyridine groups before and after sensitization. XPS used in combination with XAS (in TEY mode) provided complementary information owing to its higher surface sensitivity. The Ti 2p and O 1s XPS spectra showed that adsorption of the dye on TiO2 leads to a change of the surface dipole and/or a change in the Fermi level position in the band gap, which shifts all the core levels of TiO2. These are not equal for both TiO2 substrates in spite of them being nanocrystallnine anatase. This effect was found to be greater for the N719−aqueous TiO2 system than the respective Solaronix one. For the N 1s and S 2p XPS, the shift toward higher energy indicated that there exists an additional H-bonding interaction of the NCS ligand of the dye molecule with the TiO2 surface groups (OH/H2O)