Intermediate Oxidation States Research Articles

Red-Ox Reactions in Ionic Liquids and Their Impact on Electrodeposition of Metals and Alloys

In the absence of water, there are fewer restrictions on the nature of ionic metal species that can exist in an electrolyte. Ions with high valencies can comproportionate with low valent species to form stable intermediate oxidation states. In the case of alloy electrodeposition, the initial oxidation states of the metals in the system may change spontaneously if a red-ox reaction between different ions is possible. Without the passivating effects of oxygen or water, the surface of the anode remains chemically active and can be attacked by ions with more positive reduction potentials. H+ ions, if present, can participate in the oxidation of some species, forming hydrogen gas that leaves the system. Examples of red-ox reactions in selected ionic liquid electrolytes and some proposed methods of taking these processes into account are discussed.

The effect of pressure on sulphur speciation in mid- to deep-crustal arc magmas and implications for the formation of porphyry copper deposits

Piston cylinder experiments are used to investigate the effect of oxygen fugacity (ƒO2) on sulphur speciation and phase relations in arc magmas at 0.5–1.5 GPa and 840–950 °C. The experimental starting composition is a synthetic trachyandesite containing 6.0 wt% H2O, 2880 ppm S, 1500 ppm Cl and 3800 ppm C. Redox conditions ranging from 1.7 log units below the Ni–NiO buffer (NNO − 1.7) to NNO + 4.7 were imposed by solid-state buffers: Co–CoO, Ni–NiO, Re–ReO2 and haematite–magnetite. All experiments are saturated with a COH fluid. Experiments produced crystal-bearing trachydacitic melts (SiO2 from 60 to 69 wt%) for which major and volatile element concentrations were measured. Experimental results demonstrate a powerful effect of oxidation state on phase relations. For example, plagioclase was stable above NNO, but absent at more reduced conditions. Suppression of plagioclase stability produces higher Al2O3 and CaO melts. The solid sulphur-bearing phases and sulphur speciation in the melt are strong functions of ƒO2, as expected, but also of pressure. At 0.5 GPa, the anhydrite stability field is intersected at NNO ≥ +2, but at 1.0 and 1.5 GPa, experiments at the same ƒO2 produce sulphides and the stability field of sulphate moves towards higher ƒO2 by ~1 log unit at 1.0 GPa and ~1.5 log units at 1.5 GPa. As a result, models that appeal to high oxidation state as an important control on the mobility of Cu (and other chalcophiles) during crustal differentiation must also consider the enhanced stability of sulphide in deep- to mid-crustal cumulates even for relatively oxidized (NNO + 2) magmas. Experimental glasses reproduce the commonly observed minimum in sulphur solubility between the S2− and S6+ stability fields. The solubility minimum is not related to the Fe content (Fe2+/Fe3+ or total) of the melt. Instead, we propose this minimum results from an unidentified, but relatively insoluble, S-species of intermediate oxidation state.

Growth kinetics and complex characterization of PECVD SiOxNy dielectric films

This paper is devoted to the study of patterns of dielectric film growth depending on the parameters of the plasma chemical deposition process. The study has revealed the influence of the basic reagents’ content on the changes in surface morphology, breakdown voltage and stoichiometry (transition to the intermediate oxidation states of silicon) of dielectric films. Furthermore, an exponential pattern of the change in the films’ growth rate has been registered. When increasing the N/Si ratio, a nonlinear dependence of the change in the dielectric films’ band gap has been recorded. When increasing the SiH4/N2O ratio, a shift of the peak positions of the interband interactions relative to the band gap boundaries has been detected. The dataset on breakdown voltage and band structure suggests a certain optimum of barrier properties of SiOxNy dielectric films at the basic reagents’ ratio close to 0.3–0.4.

Sulfur dynamics during long-term ecosystem development

Long-term soil and ecosystem development involves predictable changes in nitrogen (N) and phosphorus (P) availability and limitation, but far less is known about comparable changes in sulfur (S) despite its importance as an essential plant macronutrient and component of soil organic matter. We used a combination of elemental analysis, X-ray absorption spectroscopy, hydrolytic enzyme assays, and stable S isotope ratios to examine S in soil and leaf tissue along the 120,000-year Franz Josef chronosequence, New Zealand. Total soil S concentrations increased during the early stages of pedogenesis and then declined as soils aged. There was little variation in soil N:S ratios along the chronosequence other than in the youngest (5 year old) soil, although the carbon (C):S ratio increased markedly in the oldest soils and the P:S ratio decreased continuously along the chronosequence. Foliar S concentrations and N:S ratios varied widely among common plant species but did not change consistently with increasing soil age, although foliar P:S declined for several species in the older stages of the chronosequence. The chemical nature of soil organic S extracted from mineral and organic horizons and determined by S K-edge X-ray absorption near-edge fine-structure (XANES) spectroscopy was dominated by C-bonded S distributed approximately equally in highly-reduced and intermediate oxidation states, although ester-bonded S was also abundant throughout the chronosequence. Soil sulfatase activity expressed on a soil C basis was highest in young soils, indicating low S availability in the early stage of pedogenesis. Enzymatic C:S and N:S ratios varied little during ecosystem development, although the enzymatic P:S ratio increased continuously along the chronosequence. Stable S isotope ratios (δ34S) increased along the chronosequence, particularly in the early stages of pedogenesis, reflecting a shift in S inputs from primary mineral S to oceanic sulfate in atmospheric deposition. Overall, this first comprehensive assessment of S along a long-term soil chronosequence suggests that S availability is low in the earliest stage of pedogenesis, but then remains stable throughout the progressive and retrogressive phases of ecosystem development, despite pronounced shifts in the chemistry and dynamics of other nutrients.

The effect of oxide shell thickness on the structural, electronic, and optical properties of Si-SiO2 core-shell nano-crystals: A (time dependent)density functional theory study

Due to their tunable properties, silicon nano-crystals (NC) are currently being investigated. Quantum confinement can generally be employed for size-dependent band-gap tuning at dimensions smaller than the Bohr radius (∼5 nm for silicon). At the nano-meter scale, however, increased surface-to-volume ratio makes the surface effects dominant. Specifically, in Si-SiO2 core-shell semiconductor NCs the interfacial transition layer causes peculiar electronic and optical properties, because of the co-existence of intermediate oxidation states of silicon (Sin+, n = 0–4). Due to the presence of the many factors involved, a comprehensive understanding of the optical properties of these NCs has not yet been achieved. In this work, Si-SiO2 NCs with a diameter of 1.1 nm and covered by amorphous oxide shells with thicknesses between 2.5 and 4.75 Å are comprehensively studied, employing density functional theory calculations. It is shown that with increased oxide shell thickness, the low-energy part of the optical transition spectrum of the NC is red shifted and attenuated. Moreover, the absorption coefficient is increased in the high-energy part of the spectrum which corresponds to SiO2 transitions. Structural examinations indicate a larger compressive stress on the central silicon cluster with a thicker oxide shell. Examination of the local density of states reveals the migration of frontier molecular orbitals from the oxide shell into the silicon core with the increase of silica shell thickness. The optical and electrical properties are explained through the analysis of the density of states and the spatial distribution of silicon sub-oxide species.

Effect of Alkyl Group on MxOy(-) + ROH (M = Mo, W; R = Me, Et) Reaction Rates.

A systematic comparison of MxOy(-) + ROH (M = Mo vs W; R = Me vs Et) reaction rate coefficients and product distributions combined with results of calculations on weakly bound MxOy(-)·ROH complexes suggest that the overall reaction mechanism has three distinct steps, consistent with recently reported results on analogous MxOy(-) + H2O reactivity studies. MxOy(-) + ROH → MxOy+1(-) + RH oxidation reactions are observed for the least oxidized clusters, and MxOy(-) + ROH → MxOyROH(-) addition reactions are observed for clusters in intermediate oxidation states, as observed previously in MxOy(-) + H2O reactions. The first step is weakly bound complex formation, the rate of which is governed by the relative stability of the MxOy(-)·ROH charge-dipole complexes and the Lewis acid-base complexes. Calculations predict that MoxOy(-) clusters form more stable Lewis acid-base complexes than WxOy(-), and the stability of EtOH complexes is enhanced relative to MeOH. Consistent with this result, MoxOy(-) + ROH rate coefficients are higher than analogous WxOy(-) clusters. Rate coefficients range from 2.7 × 10(-13) cm(3) s(-1) for W3O8(-) + MeOH to 3.4 × 10(-11) cm(3) s(-1) for Mo2O4(-) + EtOH. Second, a covalently bound complex is formed, and anion photoelectron spectra of the several MxOyROH(-) addition products observed are consistent with hydroxyl-alkoxy structures that are formed readily from the Lewis acid-base complexes. Calculations indicate that addition products are trapped intermediates in the MxOy(-) + ROH → MxOy+1(-) + RH reaction, and the third step is rearrangement of the hydroxyl group to a metal hydride group to facilitate RH release. Trapped intermediates are more prevalent in MoxOy(-) reaction product distributions, indicating that the rate of this step is higher for WxOy+1RH(-) than for MoxOy+1RH(-). This result is consistent with previous computational studies on analogous MxOy(-) + H2O reactions predicting that barriers along the pathway in the rearrangement step are higher for MoxOy(-) reactions than for WxOy(-).

Unexpected Modifications of Cysteines in VDAC3: Indication that VDAC3 may Signal the Mitochondrial Intermembrane Redox State

The accumulation in mitochondria of oxidative agents must be signaled to the cell. Neverthless the organelle is the source of redox species. The accumulation in mitochondria of oxidative agents must be signaled to the cell and an organelle heavily loaded with ROS evidenced. We addressed specific structural questions related to the function of VoltageDependentAnion-selectiveChannel isoform 3 (VDAC3) cysteines. We show that VDAC3 (1–3) may be relevant for signaling the redox potential existing in the mitochondrial intermembrane space. We found that VDAC3 can be progressively modified by an accumulation of ROS, resulting in the oxidation at different extents of the exposed cysteine residues. We discovered that each single VDAC3 molecule in the membrane can contain a differently oxidated set of cysteine residues, thus giving rise to what we call “redox isomers” (4). A disulfide bridge was evidenced by mass spectrometry (5). A recent paper indicated a putative disulfide bridge that we did not find by MassSpec (6), neither the authirs detected intermediate oxidation states. Since this complex oxidation pattern is a consequence of the ROS level in the IMS, VDAC3 monitores the redox homeostasis. In our opinion this work represents a pathbreaking finding in the field of mitochondrial redox sensing.Acknowledgements MIUR-PRIN 2010-2011 n. 2010CSJX4F[1] Messina A. et al, 2012, BiochimBiophysActa 1818, 1466-1476[2] Checchetto V. et al, 2014, Cell Physiol. Biochem. 34, 842-53[3] Messina A. et al, 2014, Mol. Biosyst. 10, 2134-45[4] Reina S. et al 2015, SUBMITTED[5] Saletti R. et al, 2015, SUBMITTED[6] Okazaki M. et al, 2015, BiochimBiophysActa, in press

Time-Resolved XAS Investigation of the Local Environment and Evolution of Oxidation States of a Fischer–Tropsch Ru–Cs/C Catalyst

Here we present a time-resolved in situ X-ray absorption spectroscopy investigation of the local chemical environment and the electronic structure of the active metal (Ru) and the promoter (Cs) present in catalysts supported over high-surface-area graphite. These catalysts were submitted to a reduction treatment under hydrogen gas and then to alternate atmospheres of Fischer–Tropsch reactants, CO/He, and H2/He atmospheres, respectively. This study reveals that Ru reduction is a complex process taking place through a two-step mechanism via an intermediate oxidation state and occurs in parallel to the Cs partial reduction. It also demonstrates a close association between Cs and Ru atoms as first neighbors. The evidence that the Ru–Cs particle morphology reverses (from flat 2D to a 3D shape with low coordination numbers) when the H2 atmosphere is changed with CO proves the strong interaction of CO with the surface of the Ru–Cs nanoparticles, a point confirmed by microcalorimetry of CO adsorption. To our know...

Reduction specifics of rare-earth orthovanadates (REE = La, Nd, Sm, Dy, Ho, Er, Tm, Yb, and Lu)

The reduction specifics of REE orthovanadates LnVO4 (Ln = La, Nd, Sm, Dy, Ho, Er, Tm, Yb, and Lu) have been studied using the temperature-programmed reduction (TPR) method. Hydrogen and carbon monoxide were chosen as reducing agents. The reduction temperature is found to depend both on the REE and the reducing agent. REE orthovanadates are reduced in the range 1033–1153 K not forming phases that contain vanadium in intermediate oxidation states. In CO, the reduction temperature is found to be higher than in H2 for all orthovanadates. TPR data have been used to calculate the activation energies of reduction of REE orthovanadates using the Kissinger equation. The effective activation energies of reduction depend on the REE and the reducing agent and are in the range 41–147 kJ/mol.

Highly active water oxidation on nanostructured biomimetic calcium manganese oxide catalysts

Nanostructured biomimetic birnessite CaxMnOy oxides with an intermediate Mn oxidation state and proper structural distortion show improved catalytic activity for the WOR.

Investigating the Distribution of Chemical Forms of Sulfur in Prostate Cancer Tissue Using X-ray Absorption Spectroscopy.

The use of synchrotron radiation may shed more light on the study of prostate cancer, one of the leading diseases among men. In the presented study the microbeam setup at the PSI Swiss Light Source combined with fluorescence detected X-ray absorption spectroscopy (XAS) was applied to determine two-dimensional (2D) imaging of distributions of various chemical sulfur forms in prostate cancer tissue sections, since sulfur is considered important and essential in cancer progression. The research focused on prostate tissues obtained during routine prostatectomies on patients suffering from prostate cancer.Our previous studies using μ-XAS point measurements on prostate cancer cell lines showed the differences in fractions of various forms of sulfur between cancerous and non-cancerous cells. Therefore, in this experiment the chosen areas of prostate cancer tissues were scanned to get the full picture of the chemical composition of tissue, which is highly heterogeneous. The incident X-ray beams of energies tuned to spectroscopic features of the near-edge region of sulfur K-edge absorption spectra were used to provide contrast between chemical species presented in the tissue. Next, the relative content of the three main sulfur forms, found in biological systems, was calculated and the results are presented in a form of 2D color maps. These maps are correlated with the microscopic histological image of the scanned area.The main findings show that sulfur occurs in prostate tissue mainly in reduced form. The oxidized form of sulfur is present mostly in prostatic stroma, while sulfur in intermediate oxidation state is present in trace amount.

On the mechanism of luminescence from porous silicon nanostructures

A strong nonlinear increase in the photoluminescence intensity under laser excitation at room temperature is found for porous silicon obtained by anodic oxidation. It is shown that the maximum photoluminescence intensity corresponds to samples of anodically oxidized porous silicon in the intermediate oxidation state. Laser excitation is found to increase the intensity of vibrational modes in the О3–SiH, Si–O–Si, and Si–O–H configurations with respect to the Si–Hn mode intensity in IR absorption spectra. It is experimentally confirmed that the oxide structure on the surface of silicon crystallites and the structure of the Si/SiO2 interface determine to a great extent the photoluminescence characteristics.

Age, igneous petrogenesis, and tectonic setting of the Bilihe gold deposit, China, and implications for regional metallogeny

The Bilihe gold deposit, along the northern margin of the North China Craton (NCC), is hosted in Permian intrusions that were emplaced into Bainaimiao Group greenschist- to amphibolite-facies Mesoproterozoic basement rocks, the early Paleozoic Bainaimiao Arc, and late Paleozoic sedimentary and volcanic cover rock sequences. The Middle Ordovician to Early Silurian calc-alkaline arc comprises volcanic sequences of basalt, andesite, and felsic lavas, and an intrusive complex ranging from gabbro through granite. The deposit is genetically related to ilmenite-series and highly fractionated quartz diorite porphyry host rocks. The most fractionated variety of the quartz diorite porphyry is granitic, which contains dendritic quartz phenocrysts and occurs at the top part of the intrusions. Gold mostly occurs as Au grains (>990 fineness) in trails in dendritic quartz, and the gold trails typically occur along certain crystallographic elements of dendritic quartz. The gold is believed to have crystallized directly from magma at temperatures of at least 750–800°C. The Fe2O3/(Fe2O3+FeO) ratio of the quartz diorite ranges from 0.21 to 0.29, and there is significantly more ilmenite than magnetite, suggesting that the magma was moderately reduced. There are also post-ore syenogranite porphyries in the deposit area. Zircon U-Pb LA-ICP-MS dating in this study shows that two samples of the volcanic wallrock have ages of 274±3Ma and 270±4Ma, two samples of gold-forming quartz diorite porphyries have ages of 261±2Ma and 259±3Ma, with 440–400Ma and 1344–1316Ma cores, and one syenogranite porphyry sample is dated at 253±3Ma.The mineralized quartz diorite porphyries exhibit a high-K calc-alkaline composition, high Mg# (46–51), and a high content of compatible elements (V: 114–164ppm; Cr: 80–93ppm; Ni: 37–42ppm) and Rb, Th, and U, as well as depletions in Ba, Sr, and HFSE (e.g., Ti, Nb, and Ta), suggesting melt from a slab fluid-metasomatized enriched mantle. The LREE enrichment, the intermediate oxidation state, and the presence of the Silurian and Mesoproterozoic zircon cores indicate that crustal materials also played a significant role in porphyry formation. In addition, the rocks have slightly negative εNd(t) (−2.5–0.1), but variable (87Sr/86Sr)i (0.70328–0.70573). Based on the above features, we propose that quartz diorite porphyries were produced by partial melting of the metasomatized mantle coupled with the assimilation of crustal materials of the Bainaimiao Group, Late Ordovician-Silurian subduction-related plutons, and Late Carboniferous-Early Permian marine sedimentary rocks. The assimilation of rocks of the Bainaimiao Group, which have an elevated gold content of 26.3ppb, is speculated to have played important roles in the formation of a rarely documented magmatic gold deposit. In contrast, syenogranite porphyries are characterized by high contents of K2O+Na2O (10.3–10.6wt%), Al2O3 (16.3–16.5wt%), K, Hf, and Zr; low CaO (0.6–1.2wt%), Mg# values (17–21), compatible elements (V: 16–21ppm; Cr: 4.9–7.9ppm; Ni: 2.8–4.5ppm), and Eu, Sr, and HFSE; and positive εNd(t) (1.4–1.5) with low (87Sr/86Sr)i (<0.70431), indicating that the porphyries were formed by partial melting of the subduction-modified lower crust with a minor input of asthenospheric materials.It is concluded that the quartz diorite porphyries formed during the southward subduction of the Paleo-Asian ocean underneath the northern margin of the NCC. This confirms that the northern margin was not a passive continental margin, but an active margin with subduction and development of a continental arc. In addition to Bilihe, there are at least four more gold deposits (Hadamiao, Changshanhao, Saiyinwusu, and Chaihulanzi) that formed at the northern margin of the NCC from Middle Permian to Early Triassic. Such a timing indicates that they may also be ultimately related to the tectonic regime imposed by southward subduction of the Paleo-Asian ocean. Based on the new understanding of the tectonic setting, with significant known mineral occurrences along the length of the margin, it is inferred that the >1000-km-long northern margin of NCC may have a large potential for subduction-related mineralization.

Spatial distributions of sulphur species and sulphate-reducing bacteria provide insights into sulphur redox cycling and biodegradation hot-spots in a hydrocarbon-contaminated aquifer

Dissimilatory sulphate reduction (DSR) has been proven to be one of the most relevant redox reactions in the biodegradation of contaminants in groundwater. However, the possible role of sulphur species of intermediate oxidation state, as well as the role of potential re-oxidative sulphur cycling in biodegradation particularly at the groundwater table are still poorly understood. Here we used a combination of stable isotope measurements of SO42−, H2S, and S0 as well as geochemical profiling of sulphur intermediates with special emphasis on SO32−, S2O32−, and S0 to unravel possible sulphur cycling in the biodegradation of aromatics in a hydrocarbon-contaminated porous aquifer. By linking these results to the quantification of total bacterial rRNA genes and respiratory genes of sulphate reducers, as well as pyrotag sequencing of bacterial communities over depth, light is shed on possible key-organisms involved.Our results substantiate the role of DSR in biodegradation of hydrocarbons (mainly toluene) in the highly active plume fringes above and beneath the plume core. In both zones the concentration of sulphur intermediates (S0, SO32− and S2O32−) was almost twice that of other sampling-depths, indicating intense sulphur redox cycling. The dual isotopic fingerprint of oxygen and sulphur in dissolved sulphate suggested a re-oxidation of reduced sulphur compounds to sulphate especially at the upper fringe zone. An isotopic shift in δ34S of S0 of nearly +4‰ compared to the δ34S values of H2S from the same depth linked to a high abundance (∼10%) of sequence reads related to Sulphuricurvum spp. (Epsilonproteobacteria) in the same depth were indicative of intensive oxidation of S0 to sulphate in this zone. At the lower plume fringe S0 constituted the main inorganic sulphur species, possibly formed by abiotic re-oxidation of H2S with Fe(III)oxides subsequent to sulphate reduction. These results provide first insights into intense sulphur redox cycling in a hydrocarbon contaminant plume, which widens the perspective of redox processes and microbial interactions ongoing in contaminated aquifers.

Chromium (VI)-induced transformation is enhanced by Zn deficiency in BALB/c 3T3 cells.

Hexavalent chromium [Cr(VI)] is a carcinogenic heavy metal that is reduced to intermediate oxidation states, such as Cr(V) and Cr(IV), in the process of forming stable Cr(III) forms; it is these intermediate forms that are thought to be responsible for much of the DNA damage and mutations that are induced by Cr(VI). Metallothionein (MT), a heavy metal-binding protein, is induced by zinc and other heavy metals and protects cells from the toxic effects of these metals by sequestering them. MT cannot bind Cr, but by scavenging reactive oxygen species through its cysteine residues, it may act as a protective factor against Cr(VI)-induced DNA lesions by reducing Cr(VI) directly to Cr(III), thereby avoiding the creation of the toxic intermediates. Here, we showed that Zn deficiency decreased MT expression in BALB/3T3 clone A31-1-1 cells and caused them to become highly susceptible to Cr(VI)-induced transformation. To obtain Zn-deficient cultures, cells were cultured in medium supplemented with 10% Chelex(®)-100 chelating resin-treated FBS. The increase in susceptibility to transformation was abolished by culturing the cells with supplemental Zn (50 µM). Previously, we reported that Cr(VI) inhibits MT transcription by preventing the zinc-dependent formation of a complex of metal response element-binding transcription factor-1 (MTF-1) and the co-activator p300. Our results suggest that the carcinogenicity of Cr(VI) is enhanced by MTF-1 dysfunction.

Temperature-Dependent Chemoselective Hydrosilylation of Carbon Dioxide to Formaldehyde or Methanol Oxidation State

Tethered Ru–S complexes were found to catalyze the hydrosilylation of carbon dioxide with a variety of simple monohydrosilanes selectively to bis(silyl)acetals or silylated methanol, respectively. The chemoselective conversion to either the formaldehyde or methanol oxidation state was solely dependent on the reaction temperature. This is the first example of a catalyst system that gives access to both of the intermediate oxidation states in a controlled way.

In situ oxidation kinetics of magnetite nanoparticles by total scattering

Iron oxide nanoparticles (NPs) show different structures as a function of oxidation state. In particular, magnetite (Fe3O4) NPs are easily oxidized in air at moderate temperatures, eventually yielding maghemite (Fe2O3). Oxidation proceeds via the creation of iron vacancies. While the vacancies may be created with a random distribution throughout the octahedral Fe sites, they eventually order over a specific subset of these sites, lowering the symmetry from F-centered (magnetite) to P-centered (cubic maghemite). By ex situ X-ray Total Scattering studies of magnetite-maghemite NPs in different oxidation states[1] we have recently studied, by the DFA method[2], the correlation between particle diameter, stoichiometry and lattice parameter in polydisperse NP samples unraveling also the size dependence of lattice parameter and composition. Moreover, we have shown indirect evidence of the formation of a polycrystalline surface layer of maghemite on a magnetite core in the intermediate oxidation states. Motivated by the excellent ex-situ results, we have also performed in-situ studies where magnetite NPs were oxidised in air at moderate temperatures (50-200 C). We present here an in-situ study performed at the X04SA-Materials Science beamline of the Swiss Light Source synchrotron[3]. Total Scattering X-ray diffraction patterns were collected every few minutes, while the oxidation was completed within several hours. The mechanism of NPs oxidation - whereas a surface oxidised layer is formed by outwards diffusion of Fe, then the vacancies so created order themselves giving rise to the maghemite-magnetite phase transition, will be examined in great detail. We will discuss, on robust statistical basis, the calculation of kinetic and diffusion constants, the temperature effect on the lattice constant and on the thickness of the surface oxidised layer; the different possible structural models for the cubic-maghemite NPs. We thank for support Fondazione Cariplo (2009-0289).

Sulfur accumulation in Melanudands during development by upbuilding pedogenesis since 14–15 cal. ka

To understand the contribution of upbuilding pedogenesis to the accumulation of sulfur (S) in Melanudands, we investigated the vertical distribution of S species using K-edge X-ray absorption near-edge structure (XANES) spectroscopy of S species and vertical variation of δ34S values of total S in two Japanese Melanudands developed above tephra Nt-S, dated at 14–15cal.ka. These soils have been developed by upbuilding pedogenesis, in which episodic accumulations of volcanic ash and eolian dust transported from the Asian continent are added incrementally to the soil surface while topdown pedogenesis occurs concomitantly forming soil horizons. The oxidation states of S compounds determined by XANES analysis varied widely in the soil profiles. The predominant S species was highly oxidized S in ester sulfates and inorganic sulfates (+6 oxidation state). Proportions of S with reduced and intermediate oxidation states increased episodically to as much as 66%. Ester sulfate-S was found in the soil horizons with estimated ages less than 10calka. A close correlation was found between ester sulfate-S concentrations and Al-associated organic S concentrations (p<0.05); indeed, the concentrations themselves were roughly similar. The δ34S values of total S increased with depth in the two soil profiles. These results suggest that episodic deposits of volcanic ash and eolian dust trapped detritus and humus beneath them during the buildup of the soil surface. The organic S compounds in this material might be decayed as topdown pedogenic processes in the soils. Production of ester sulfate-S may be related to the warm and humid climate of the Holocene, which may have accelerated microbial activity responsible for transformation of reduced to highly oxidized S. Ester sulfate-S might have become Al-associated forms and accumulated in the soils due to their high pedogenic mineral contents.

The electrodeposition of Zn-Ti alloys from ZnCl2-urea deep eutectic solvent

The electrochemical behavior of Ti(IV) and the electrodeposition of Zn-Ti alloys were investigated in a ZnCl2-urea (1:3 molar ratio) deep eutectic solvent containing 0.27 mol L−1 TiCl4. The electrochemical reduction of Ti(IV) to Ti was complicated by the formation of intermediate oxidation states of Ti(III) and Ti(II), as well as the precipitation of TiCl3. It was possible to prepare Zn-Ti alloys containing 5.8–16.7 at.% Ti. The composition and surface morphology of Zn-Ti alloys depended on deposition potential and temperature. The deposits could be indexed to a disordered hexagonal close-packed structure similar to pure Zn and were completely chloride-free. The current efficiency for the deposition of Zn-Ti alloys varied from 38.4 to 67.9 %.

An optical sensor for pesticide determination based on the autoindicating optical properties of peroxidase

During the enzymatic reaction of the heme-protein Horseradish peroxidase (HRP) with hydrogen peroxide there are changes in the molecular absorption spectra of HRP and its different oxidation states which can be used for quantitative determination of the substrate. One of these intermediate oxidation states is the HRPII, with iron as an oxyferryl. This compound is assumed to be responsible for the organophosphate pesticide degradation in the Fenton reaction. In this work, the enzymatic HRP-H₂O₂ reaction has been studied, based on the effect of different pesticides on the mechanism reaction; these modifications have been used for the quantitative determination of pesticides. A mathematical model has been developed relating to the analytical signal with the pesticide concentration. Three organophosphate pesticides (diazinon, trichlorfon and tetrachlorvinphos) and one sulfamide (dichlofluanid) have been used to demonstrate the viability of the methodology and the accomplishment fulfillment of the model. Tetrachlorvinphos was chosen as the pesticide model to develop the optical sensor film for continuous pesticide determination, consisting of HRP immobilized in a polyacrylamide gel. The sensor can be used for at least 15 days and responds linearly to tetrachlorvinphos concentrations in the range from 4.0 × 10(-7) to 4.0 × 10(-6)mol L(-1). The main advantage of the methodology is its reversibility in contrast to the irreversible Fenton reaction. The HRP-H2O2 methodology has been used to measure the pesticides in a waste water sample spiked with tetrachlorvinphos.