Redox Sequence Research Articles

Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions

Using a temperature-programmed online MS system, gas products are directly analyzed at low temperature to understand the reactivity of the surface intermediates on an Au/TiO2 catalyst that exhibits catalytic activity for CO oxidation. CO oxidation was performed over Au/TiO2 catalysts with Au particle size of 3nm, and a 7.3wt% loading, focusing on CO-TPD, steady state isothermal titration, high temperature TPR, and low temperature selective TPO experiments. With CO-TPD, we were able to observe directly, the presence of two different CO species adsorbed on the catalyst surface, as well as the respective desorption energies. Steady state isothermal titrations showed two transient processes with different redox sequences, both forming catalytic layers for intermediate storage and subsequent CO2 product formation. High temperature CO titration transients with 200°C pre-oxidation following either isothermal CO titration or high temperature TPR highlights the presence of two oxidative surface intermediate species with different kinetics, with the intermediates purposed as atomic oxygen at the Au NP perimeter and neighboring lattice sites. Finally, low temperature selective TPO beginning at −165°C showed that pre-reduction following oxidation up to −100°C is able to partially convert surface CO species into stable CO2 intermediates, as a higher activation energy was required to fully desorb the CO2 product from the catalyst surface.

Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands.

A series of tetradentate 2,2':6',2″:6″,2‴-quaterpyridine-type ligands related to ppq (ppq = 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) have been synthesized. One ligand replaces the 1,10-phenanthroline (phen) moiety of ppq with 2,2'-bipyridine and the other two ligands have a 3,3'-polymethylene subunit bridging the quinoline and pyridine. The structural result is that both the planarity and flexibility of the ligand are modified. Co(II) complexes are prepared and characterized by ultraviolet-visible light (UV-vis) and mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven H2-evolving activity of these Co complexes was evaluated under homogeneous aqueous conditions using [Ru(bpy)3](2+) as the photosensitizer, ascorbic acid as a sacrificial electron donor, and a blue light-emitting diode (LED) as the light source. At pH 4.5, all three complexes plus [Co(ppq)Cl2] showed the fastest rate, with the dimethylene-bridged system giving the highest turnover frequency (2125 h(-1)). Cyclic voltammograms showed a significant catalytic current for H2 production in both aqueous buffer and H2O/DMF medium. Combined experimental and theoretical study suggest a formal Co(II)-hydride species as a key intermediate that triggers H2 generation. Spin density analysis shows involvement of the tetradentate ligand in the redox sequence from the initial Co(II) state to the Co(II)-hydride intermediate. How the ligand scaffold influences the catalytic activity and stability of catalysts is discussed, in terms of the rigidity and differences in conjugation for this series of ligands.

Geochemistry of redox-sensitive trace elements in a shallow subterranean estuary

Submarine groundwater discharge (SGD) is an important component of chemical fluxes in the coastal ocean. The composition of SGD is influenced by biogeochemical reactions that take place within the subterranean estuary (STE), the subsurface mixing zone of fresh and saline groundwaters. The STE is characterized by redox gradients that affect the speciation and mobility of redox-sensitive elements (RSEs). We examined the distributions and behavior of the RSEs Mo, U, V, and Cr within the larger redox framework of a shallow STE and evaluated the source-sink function of the STE for these elements. We found that the advection of water through the STE and the apparent respiration of organic matter drives the formation of a “classic” redox sequence typically observed in diffusion-dominated fine-grained sediments. High concentrations of dissolved organic matter (up to 2.9mM) lead to extensive sulfide production (up to 1.8mM) within 3m of the surface. Both Mo and U are quantitatively removed as oxic surface waters mix into ferruginous and sulfidic zones. Molybdenum removal appears to occur where sulfide concentrations exceed ~11μM, a previously reported threshold for quantitative formation of highly particle-reactive thiomolybdate species. Uranium removal apparently occurs via reduction and formation of insoluble phases or sorption to sediments. It is not clear how readily sequestered metals may be returned to solution, but SGD may be an important sink in the marine budget for both Mo and U. In contrast, both V and Cr show non-conservative addition across the salinity mixing gradient. Increases in pH appear to promote dissolution of V from minerals within the shallow aquifer, and mobilization may also be associated with dissolved organic matter. Chromium enrichment is associated with higher dissolved organic matter and is likely due to the formation of soluble Cr-organic complexes. Fluxes of these elements were constrained using SGD volume fluxes, determined using radium isotopes as well as direct discharge measurements by Lee-type seepage meters, and concentrations in directly-sampled seepage (Mo: −0.21 to −7.7μmolm−2day−1; U: −0.02 to −0.6μmolm−2day−1; V: 0.05 to 2.0μmolm−2day−1; Cr: 0.12 to 4.4μmolm−2day−1).

Synthesis and electrochemical investigation of chromium(0) ferrocenyl-substituted carbene complexes

The series [(CO)5CrC(R)Fc], (1, R=OEt; 2, R=NHnBu) as well as [(OC)5CrC(R)Fc′(R)CCr(CO)5], (3, R=OEt; 4, R=NHnPr) of mono- and biscarbene chromium(0) complexes with Fc=FeII(C5H5)(C5H4) and Fc′=FeII(C5H4)2, were synthesized and characterized spectroscopically, electrochemically and computationally. Electrochemical studies on the new aminocarbene complexes revealed that 2 and 4 are oxidized (and reduced) at lower potential than the EtO derivatives 1 and 3 while the biscarbene complexes 3 and 4 provided evidence for interaction between different redox sites, including the Cr-centers. The Cr0/I couples are electrochemically reversible with the difference in formal potentials ΔE°′ for the consecutive oxidations of the chromium carbene entities being 151 and 105mV in 3 and 4, respectively. Computational and electrochemical results were mutually consistent in showing unambiguously that the Cr(0) center is oxidized before the ferrocenyl group in the carbene complexes 2, 3 and 4. Electrochemical experiments on 1 were also consistent with this redox sequence of events, but differently, our calculations suggest that for 1, the ferrocenyl group may be oxidized before Cr(0) oxidation. The ethoxycarbenes 1 and 3 also showed a carbene double bond reduction to an anion radical, −CrC, while the new aminocarbene 4 was the only derivative to show irreversible Cr(I) oxidation to Cr(II) within the potential window of the solvent.

Selective synthesis and redox sequence of a heterobimetallic nickel/copper complex of the noninnocent Siamese-twin porphyrin.

The Siamese-twin porphyrin (1H4) is a redox noninnocent pyrazole-expanded porphyrin with two equivalent dibasic {N4} binding sites. It is now shown that its selective monometalation can be achieved to give the nickel(II) complex 1H2Ni with the second {N4} site devoid of a metal ion. This intermediate is then cleanly converted to 1Ni2 and to the first heterobimetallic Siamese-twin porphyrin 1CuNi. Structural characterization of 1H2Ni shows that it has the same helical structure previously seen for 1Cu2, 1Ni2, and free base 1H6(2+). Titration experiments suggest that the metal-devoid pocket of 1H2Ni can accommodate two additional protons, giving [1H4Ni](2+). Both bimetallic complexes 1Ni2 and 1CuNi feature rich redox chemistry, similar to the recently reported 1Cu2, including two chemically reversible oxidations at moderate potentials between -0.3 and +0.5 V (vs Cp2Fe/Cp2Fe(+)). The locus of these oxidations, in singly oxidized [1Ni2](+) and [1CuNi](+) as well as twice oxidized [1CuNi](2+), has been experimentally derived from comparison of the electrochemical properties of the complete series of complexes 1Cu2, 1Ni2, and 1CuNi, and from electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS) (Ni and Cu K edges). All redox events are largely ligand-based, and in heterobimetallic 1CuNi, the first oxidation takes place within its Cu-subunit, while the second oxidation then occurs in its Ni-subunit. Adding pyridine to solutions of [1Ni2](+) and [1CuNi](2+) cleanly converts them to metal-oxidized redox isomers with axial EPR spectra typical for Ni(III) having significant dz(2)(1) character, reflecting close similarity with nickel complexes of common porphyrins. The possibility of selectively synthesizing heterobimetallic complexes 1MNi from a symmetric binucleating ligand scaffold, with the unusual situation of three distinct contiguous redox sites (M, Ni, and the porphyrin-like ligand), further expands the Siamese-twin porphyrin's potential to serve as an adjustable platform for multielectron redox processes in chemical catalysis and in electronic applications.

On the Formation Mechanism of Chromium Nitrides: An in situ Study

AbstractThree different synthetic routes to chromium nitrides were investigated using in situ neutron diffraction (SPODI, FRM‐II) and thermal analysis in ammonia: (i) The direct conversion of chromium powder in ammonia, (ii) the ammonolysis of chromium(III) chloride CrCl3, and (iii) the ammonolysis of chromium(III) sulfide Cr2S3. During the direct conversion of chromium first γ‐CrN was formed above 385 °C, above 567 °C additionally β‐Cr2N was detected. Finally, a complete conversion of γ‐CrN to β‐Cr2N was observed at 1080 °C. The ammonolysis of chromium(III) compounds (CrCl3 and Cr2S3) revealed a so far in 3d metal nitride synthesis unprecedented redox sequence: The compounds are reduced to chromium(II) chloride CrCl2 and chromium(II, III) sulfide Cr3S4, respectively, prior to the reoxidation of those intermediates to γ‐CrN. In contrast to the ammonolysis of 3d metal(II) halides, for chromium(III) chloride no temporary formation of ammoniates was observed. Since formation of γ‐CrN was detected only after annealing chromium(III) sulfide in flowing ammonia for 4 h to 5 h at maximum temperature, the formation of sufficient amounts of chromium(II, III) sulfide as the rate‐determining step for the nitride formation appears likely.

ChemInform Abstract: An Enzymatic Toolbox for Cascade Reactions: A Showcase for an in vivo Redox Sequence in Asymmetric Synthesis.

AbstractThe reaction cascade is realized by deployment of E.

An Enzymatic Toolbox for Cascade Reactions: A Showcase for an In Vivo Redox Sequence in Asymmetric Synthesis

Joined forces: Alcohol dehydrogenase, enoate reductase, and Baeyer–Villiger monooxygenase are combined in a cascade reaction by coexpression in E. coli to have a recombinant whole-cell biocatalyst. Such an artificial metabolic “mini”-pathway provides access to functionalized chiral compounds in high yields and optical purities as exemplified for kinetic resolutions, desymmetrizations, and regiodivergent biotransformations.

Activation of a σ-SnSn Bond at Copper, Followed by Double Addition to an Alkyne

Many synthetically useful copper-catalyzed transformations involve the activation of apolar or weakly polar σ-bonds (E-H and E-E' bonds, with E = C, B, Si, Sn, etc.). Yet, little is known so far about the associated elementary steps, and it is highly desirable to gain better knowledge regarding the way σ-bonds can be activated by copper to help further development in this area. To this end, we became interested in investigating the coordination and activation of apolar or weakly polar σ-bonds at copper using chelating assistance. Here we report investigations of gold and copper complexes deriving from the diphosphine-stannane [Ph2P(o-C6H4)Me2Sn-SnMe2(o-C6H4)PPh2] 1. The σ-SnSn bond of 1 readily undergoes oxidative addition at both gold and copper, giving bis(stannyl) Au(+) and Cu(+) complexes 2 and 3. Coordination of 1 to CuBr leads to the neutral complex 4 which features more σ-SnSn complex character. The ability of complex 3 to undergo insertion reactions with alkynes was then examined. With methyl propiolate, a clean reaction occurred, and the bis-stannylated alkene copper complex 5 was isolated. The structures of ligand 1 and complexes 2-5 have been unambiguously determined by multinuclear NMR spectroscopy and crystallography. These results substantiate the ability of copper to promote the addition of apolar σ-bonds to CC multiple bonds via a 2e redox sequence and draw thereby an unprecedented parallel with the group 10 metals.

铁炉渣施加对稻田甲烷产生、氧化与排放的影响

为了阐明铁炉渣施加对稻田甲烷产生、氧化与排放的影响,采用静态箱-气相色谱法对对照(CK)、2 Mg/hm²(Fe Ⅰ)、4 Mg/hm²(Fe Ⅱ)和8 Mg/hm²(Fe Ⅲ)铁炉渣施加后稻田甲烷产生、氧化与排放进行了测定与分析。研究结果表明:观测期内,CK、Fe Ⅰ、Fe Ⅱ和Fe Ⅲ样地甲烷产生量分别为0.06-8.87、0.12-8.28、0.15-7.84、0.17-7.82 mg·m^-2·h^-1,平均产生量分别为4.68、3.92、3.14、2.76 mg·m^-2·h^-1;甲烷氧化量分别是0.02-1.27、0.09-0.95、0.09-1.54、0.09-2.79 mg·m^-2·h^-1,平均氧化量为0.46、0.47、0.59、0.55 mg·m^-2·h^-1;甲烷排放分别是0.04-7.99、0.03-7.33、0.06-6.30、 0.08-5.12 mg·m^-2·h^-1,平均值分别为3.11、2.29、1.76、1.59 mg·m^-2·h^-1。铁炉渣的施加降低了甲烷产生量和排放通量,提高了甲烷氧化量。;Methane emission from biogenic sources is very important for the global CH₄ budget. Paddy fields have been identified as one of the major sources of anthropogenic CH₄ emissions to the atmosphere. Rice is the major cereal crop to feed more than half of the world's population, and the production of rice will increase with the expanding world population and may increase the associated CH₄ emissions and thereby accelerate global warming effects. Therefore, feasible soil management strategies need to be developed for reducing CH₄ emission from intensive rice farming while sustaining rice productivity.<br> Among the various factors, the content of soil oxidants (electron acceptors) and reductants (electron donors) play vital role in controlling CH₄ emissions from wetland rice agriculture. While electron acceptors can control CH₄ emission from paddy fields, it can be regenerated during the drained period. For example, Iron reduction is a dominant redox process within the redox sequence of anaerobic systems, and may suppress CH₄ production and its emission.<br> A field experiment was carried out in the Wufeng Agronomy Field of the Fujian Academy of Agricultural Sciences (26.1°N, 119.3°E), Southeast China during the rice growing season in April to July 2011. The soil at the experimental site was moist, poorly drained, clay loam, paddy field soil type. Bulk density of the soil before experimentation was 1.1 g cm^-3, and other chemical properties were: pH 6.5, organic carbon 18.1 g/kg, total N 1.2 g/kg, total P 1.1 g/kg.<br> In order to clarify the effect of iron slag amendment on paddy field methane production, oxidation and emission, the methane production, oxidation and emission of control (CK) and after 2 Mg/hm²(Fe Ⅰ), 4 Mg/hm²(Fe Ⅱ), and 8 Mg/hm²(Fe Ⅲ) iron slag application were determined by static chamber-gas chromatogram. The results showed: Methane production rates of CK, Fe Ⅰ, Fe Ⅱ and Fe Ⅲ plot were 0.06-8.87, 0.12-8.28, 0.15-7.84, 0.17-7.82 mg·m^-2· h^-1 during observation date and averaged values were 4.68, 3.92, 3.14, 2.76 mg·m^-2·h^-1,respectively. Methane oxidation rates were 0.02-1.27, 0.09-0.95, 0.09-1.54, 0.09-2.79 mg·m^-2·h^-1 and averaged values were 0.46, 0.47, 0.59, 0.55 mg·m^-2·h^-1, respectively. Methane emission fluxes were 0.04-7.99, 0.03-7.33, 0.06-6.30, 0.08-5.12 mg·m^-2·h^-1 and averaged values were 3.11, 2.29, 1.76, 1.59 mg·m^-2·h^-1, respectively. Iron slag suppressed methane production and emission, while it increased CH₄ oxidation rate.

Gold-Mediated Insertion of Oxygen into Silicon–Silicon Bond: An Original Au(I)/Au(III) Redox Sequence

The diphosphine–disilane iPr2(o-C6H4)SiMe2SiMe2(o-C6H4)PiPr2 reacts with AuCl(SMe2) via coordination of the two phosphines and oxidative addition of the σ-Si–Si bond. The ensuing bis(silyl) gold(III) complex has been unequivocally authenticated by NMR spectroscopy at −60 °C. Upon heating, it evolves cleanly to give a disiloxane gold(I) complex that has been fully characterized, including by X-ray diffraction analysis. Oxidation of the disilane proceeds via an original Au(I)/Au(III) redox sequence. According to 18O labeling experiments, both water and dioxygen are competent oxygen sources. Oxidative addition of the σ-Si–Si bond to form a bis(silyl) gold(III) complex seems to be a prerequisite for the disilane → disiloxane conversion to occur.

Studies on a Vinyl Ruthenium‐Modified Squaraine Dye: Multiple Visible/Near‐Infrared Absorbance Switching through Dye‐ and Substituent‐Based Redox Processes

The bis(vinyl ruthenium)-modified squaraine dye 1 was synthesized by treatment of [RuHCl(CO)(PiPr(3) )(2) ] with bis(ethynyl)-substituted squaraine 8. Spectroscopic and electrochemical measurements on 1 and its organic precursors 6-8 were performed to study the effect of the vinyl ruthenium "substituents," particularly with respect to (poly)electrochromism. Attachment of the vinyl ruthenium moieties endows metal-organic squaraine 1 with two additional oxidation waves and lowers the first two oxidation potentials by approximately 300 mV with respect to its organic precursors. Squaraines 6, 7, 8, and 1 strongly absorb at 648, 663, 656, or 709 nm. Although organic dyes 6, 7, and 8 fluoresce, no room-temperature emission is observed for 1. The radical cations and anions of 6, 7, 8, and 1 as well as the doubly oxidized dications have been studied by IR and UV/Vis/NIR spectroelectrochemistry, and the -/0/+/2+ redox sequences were found to be reversible in each case. Our results indicate that the 1(2-/-/0/+/2+) redox system constitutes a polyelectrochromic switch in which absorption in the visible or the near-infrared range is reversibly turned off or shifted deep into the NIR. They also show that radical cation 1(.+) is an intrinsically delocalized system with only little contribution from the outer vinyl ruthenium tags to the oxidation process. Dication 1(2+) constitutes a class-II mixed-valent system with two electronically different vinyl ruthenium moieties and has an open-shell singlet electronic ground-state structure. ESR and NMR spectra of chemically prepared 1(.+) and 1(2+) corroborate these results. It has also emerged that reduction involves an orbital that is strongly delocalized across the entire squaraine π system and strongly affects the peripheral vinyl ruthenium sites.

Statistical approaches and hydrochemical modelling of groundwater system in a small tropical island

The aquifer of Manukan Island of Borneo, Malaysia had been found to be affected by seawater intrusion associated with excessive groundwater exploitation. This research attempted to characterize the chemistry of an impacted zone in the island using factor analysis (FA), cluster analysis (CA) and a hydrochemical model package (PHREEQC). The factor scores were used to plot the spatial map and to group the relationships among the monitoring wells using CA. The results of FA analysis revealed that the three main processes associated with the seawater intrusion event are aquifer salinization, cation exchange process and redox sequences. Output from the PHREEQC simulation was used to support the findings from the multivariate analysis.

Molecular and catalytic properties of the aldehyde dehydrogenase of Gluconacetobacter diazotrophicus, a quinoheme protein containing pyrroloquinoline quinone, cytochrome b, and cytochrome c.

Several aldehyde dehydrogenase (ALDH) complexes have been purified from the membranes of acetic acid bacteria. The enzyme structures and the chemical nature of the prosthetic groups associated with these enzymes remain a matter of debate. We report here on the molecular and catalytic properties of the membrane-bound ALDH complex of the diazotrophic bacterium Gluconacetobacter diazotrophicus. The purified ALDH complex is a heterodimer comprising two subunits of 79.7 and 50 kDa, respectively. Reversed-phase high-pressure liquid chromatography (HPLC) and electron paramagnetic resonance spectroscopy led us to demonstrate, for the first time, the unequivocal presence of a pyrroloquinoline quinone prosthetic group associated with an ALDH complex from acetic acid bacteria. In addition, heme b was detected by UV-visible light (UV-Vis) spectroscopy and confirmed by reversed-phase HPLC. The smaller subunit bears three cytochromes c. Aliphatic aldehydes, but not formaldehyde, were suitable substrates. Using ferricyanide as an electron acceptor, the enzyme showed an optimum pH of 3.5 that shifted to pH 7.0 when phenazine methosulfate plus 2,6-dichlorophenolindophenol were the electron acceptors. Acetaldehyde did not reduce measurable levels of the cytochrome b and c centers; however, the dithionite-reduced hemes were conveniently oxidized by ubiquinone-1; this finding suggests that cytochrome b and the cytochromes c constitute an intramolecular redox sequence that delivers electrons to the membrane ubiquinone.

How to elucidate and control the redox sequence in vinylbenzoate and vinylpyridine bridged diruthenium complexes

Vinylbenzoate-bridged diruthenium complexes (RHC=CH)(CO)(P(i)Pr(3))(2)Ru(mu-4-OOCC(6)H(4)-CH=CH)RuCl(CO)(P(i)Pr(3))(2) (R = Ph, 3a or CF(3), 3b) and vinylpyridine-bridged (eta(6)-p-cymene)Cl(2)Ru(mu-NC(5)H(4)-4-CH=CH)RuCl(CO)(P(i)Pr(3))(2) (3c) have been prepared from their monoruthenium precursors and investigated with respect to the sequence of the individual redox steps and electron delocalization in their partially and fully oxidized states. Identification of the primary redox sites rests on the trends in redox potentials and the EPR, IR and Vis/NIR signatures of the oxidized radical cations and is correctly reproduced by quantum chemical investigations. Our results indicate that the trifluoropropenyl complex 3b has an inverse FMO level ordering (Ru1-bridge-Ru2 > terminal vinyl-Ru1 site) when compared to its styryl substituted counterpart 3a such that the primary oxidation site in these systems can be tuned by the choice of the terminal alkenyl ligand. It is further shown that the vinylbenzoate bridge is inferior to the vinylpyridine one with regard to charge and spin delocalization at the radical cation level. According to quantum chemical calculations, the doubly oxidized forms of these complexes have triplet diradical ground states and feature two interconnected oxidized vinyl ruthenium subunits.

The Synthesis of Triazadibenzo[cd,g]azulenes: How To Connect Four Different Rings in Just One Step?

Deprotonation at the methyl group of 6-methyl-4H-imid­azo[1,2-a]pyridines yields dimeric triazadibenzo[cd,g]azulenes in just one step. This reaction is interpreted as a cascade reaction involving an anionic electrocyclization reaction, a redox sequence and a subsequent dimerization process. The X-ray structural analysis revealed an anti arrangement of both tetracyclic subunits. By quantum chemical calculations the unexpected ring-closure reaction is interpreted in terms of electronic destabilization of the anionic intermediates 8 by configurational strain enabling the otherwise thermodynamically unfavorable cyclization process.

Mathematical modeling of biologically mediated redox processes of iron and arsenic release in groundwater

A one dimensional reactive transport model was developed in order to illustrate the biogeochemical behavior of arsenic and iron reduction and release to groundwater that accounts for the reaction coupling the major redox elements under reducing environment. Mass transport equation and the method of characteristics were used considering fundamental geochemical processes to simulate transport processes of different pollutants in mobile phase. The kinetic sub-model describes the heterotrophic metabolisms of several microorganisms. To model a complete redox sequence (aerobic or denitrifiers, Fe(III)-reduction, respiration bacteria of iron and arsenic compounds, and As(V) reduction) four functional bacterial groups (X1, X2, X3, and X4) were defined. Microbial growth was assumed to follow Monod type kinetics. The exchange between the different phases (mobile, bio, and matrix) was also considered in this approach. Results from a soil column experiment were used to verify the simulation results of the model. The model depicts the utilization of oxygen, nitrate, iron oxide and arsenic as electron acceptors for oxidation of organic carbon (OC) in a column. The OC as electron donor is one of the most important factors that affect the iron and arsenic reduction bacterial activity.

Multicomponent Molecular Layers that Exhibit Electrochemical Potential Flip on Au(111) by Use of Proton-coupled Electron-transfer Reactions

Abstract Interfacial ligand substitution of triruthenium(III) complexes confined on gold as a monolayer has proven to be useful to construct molecular heterostructures by quantitatively incorporating 4,4′-bipyridine-anchored diruthenium(III) complexes, whose multi-step redox sequence is reversibly tuned by proton-coupled electron-transfer reactions.

Monitoring and modelling of the solid-solution partitioning of metals and As in a river floodplain redox sequence

For a period of 2 a, pore water composition in a heavily contaminated river floodplain soil was monitored in situ. Pore water samples were collected 12 times over all seasons in a profile ranging from aerobic to sulphidic redox conditions, and As, Cd, Cr, Cu, Pb, Zn, Mn, Fe, Ca, Cl, SO4. DOC,IOC and pH were determined. The variability of pH, IOC, DOC and Ca was found to be rather small during the year and within the profile (rsd <0.04, 0.16. 0.24 and 0.22, respectively). The temporal variability of the metal and As concentrations was small, too, whereas changes with depth were distinct. Under sulphidic conditions, concentrations were below 1 mu g L-1 (Cd, Cu, Pb) or 10 mu g L-1 (Zn, As). The data set was compared with results from a geochemical model that was fully parameterised from literature data and included equilibrium speciation, sorption and mineral dissolution. The general pattern of the solid-solution partitioning of Cd, Cu, Zn and As in the profile was predicted well by mechanistic geochemical modelling on the basis of solid phase composition. Metals strongly bound to organic matter Such as Cd and Cu were predicted better than metals mainly present within a mineral. Detailed information regarding the presence of colloidal Fe and Mn in pore water might improve the prediction of the solid-solution partitioning of a number of metals. The study also indicates that the chemical behaviour of Pb is still not understood sufficiently. (c) 2008 Elsevier Ltd. All rights reserved.

Solid‐State Au/Hg Microelectrode for the Investigation of Fe and Mn Cycling in a Freshwater Wetland: Implications for Methane Production

AbstractThe solid‐state voltammetric gold‐amalgam microelectrode was used to measure multiple redox species (O, S, Fe and Mn) in situ at (sub)millimeter vertical resolution to elucidate rhizosphere processes in Jug Bay wetlands. In vegetated soil, a classic diagenetic redox sequence without any dissolved sulfide was observed in summer. However, the rhizosphere can be quite variable which is due to the introduction of O2 to the anoxic sediments by plants. In nonvegetated soil, the vertical concentration–depth profiles were relatively constant. The presence of Fe(II), Mn(II) and soluble Fe(III) in deeper sediments indicates the oxidation of Fe(II) as well as the nonreductive dissolution of Fe(III) and the reductive dissolution of Fe(III) and Mn(III, IV) solids. Mn(III, IV) and Fe(III) redox chemistry is important in organic matter mineralization mediated by bacteria and in suppressing methane formation. In addition, Mn(III, IV) also can oxidize Fe(II) to supply Fe(III) for bacterial Fe(III) reduction. Studying Fe and Mn cycling via voltammetric methods can give insights to methane production and loss as there is no methane sensor for sediment work at present.