Rate Of Reoxidation Research Articles

Investigation of a chemically regenerative redox cathode polymer electrolyte fuel cell using a phosphomolybdovanadate polyoxoanion catholyte

Chemically regenerative redox cathode (CRRC) polymer electrolyte fuel cells (PEFCs), where the direct reduction of oxygen is replaced by an in-direct mechanism occurring outside of the cell, are attractive to study as they offer a solution to the cost and durability problems faced by conventional PEFCs. This study reports the first detailed characterization of a high performance complete CRRC PEFC system, where catholyte is circulated between the cathode side of the cell and an air-liquid oxidation reactor called the “regenerator”. The catholyte is an aqueous solution of phosphomolybdovanadate polyoxoanion and is assessed in terms of its performance within both a small single cell and corresponding regenerator over a range of redox states. Two methods for determining regeneration rate are proposed and explored. Expressing the regeneration rate as a “chemical” current is suggested as a useful means of measuring re-oxidation rate with respect to the cell. The analysis highlights the present limitations to the technology and provides an indication of the maximum power density achievable, which is highly competitive with conventional PEFC systems.

Effect of Re-Oxidation Rate of Additive Cation on Corrosion Rate of Stainless Steel in Boiling Nitric Acid Solution.

Fuel reprocessing plant is important for atomic energy cycle which is aimed to re-use uranium (U) and plutonium (Pu) from the spent nuclear fuels after barning-out in light water reactor. PUREX process, in which boiling nitric acid is used for dissolving fuel elements, is adopted in Japan. Many structural materials are used stainless steels protected by passive layers of the surface. At the beginning of developing the plant, many troubles of intergranular corrosion of stainless steels were experienced. According to the investigations of these problems, very low carbon type austenitic stainless steels were utilized for the materials.However, these stainless steels were corroded in some cases. For example, addition of hexavalent chromium (Cr (VI)) is heavily accelerated corrosion rate of these stainless steels in boiling nitric acid solutions. The corrosion potential of stainless steel in Cr (VI) added solution is much higher than that in pure nitric acid solution.The authors introduced that heptavalent vanadium (V (V)) was also accelerated corrosion rate of these stainless steels but the tendency of acceleration process was different from the case of containing Cr (VI). We analyzed the valence stats of Cr and V during corrosion test and showed that Cr (VI) reduced to Cr (III) with corrosion of stainless steel propagating. On the other hand, amount of V (V) was not changed during the corrosion test. It was concluded that V (IV) was easily re-oxidized in boiling nitric acid solution. Namely, V (V) is reduced to V (IV) on stainless steel surface by cathodic reaction of corrosion and then V (IV) is re-oxidized immediately to V (V) in the balk nitric acid solution.These data were obtained using non-radioactive solutions in the ordinary laboratory. The present work is aimed to compare the data of re-oxidation rate of actinides which are included in the actual solutions used in a reprocessing plant. We implemented a specially designed electrochemical test cell used for analyzing the valence states of actinide cations, i.e. neptunium (Np) and Pu, in a boiling nitric acid solution. The cell was set into glovebox in a hot laboratory which could be handled nuclear fuel materials. The cell was designed for minimizing the solution. The amounts of solution used are about 15 cm3 and sample size of working electrode is about 4 mm2. Electrochemical measurements and spectrum analyses were conducted with this apparatus. Analyzed cations were Cr (VI), V (V), Pu (VI) and Np (VI). Polarization curves showed that corrosion potentials of stainless steels changed to higher direction by addition of these cations and also corrosion current densities increased. The higher order of those is Cr (VI), Np (VI), V (V) and Pu (VI). Re-oxidation rates were obtained by the changes of absorption spectra with time using lower valence state ions added solutions, i.e. Cr (III), V (IV), Pu (IV) and Np (V). These species are re-oxidized to Cr (VI), V (V), Pu (VI) and Np (VI), respectively. Re-oxidation rates of them were very much different among them. Very fast re-oxidation rates were obtained by the V (IV) and Np (V) added solutions. A very low re-oxidation rate was obtained by the Cr (III) added solution. The Pu (IV) added solution was in the middle of them. The corrosion of stainless steel is controlled by corrosion potential of cation added solution at the initial stage and controlled by re-oxidation rate of cation as time passed. The element Np which indicates high corrosion potential and has fast re-oxidation rate influences too much the corrosion of stainless steel in boiling nitic acid solution among these elements in consideration.

Comparison of the Chemical Passivation of GaAs, In<sub>0.53</sub>Ga<sub>0.47</sub>As, and InSb with 1-Eicosanethiol

Self-assembled 1-eicosanethiolate layers were deposited on the oxide-free (100) crystal planes of GaAs, In0.53Ga0.47As, and InSb to protect the surfaces. The layer prevented re-oxidation in air for 30 min on GaAs but only 8 min on In0.53Ga0.47As based on the O 1s x-ray photoelectron spectroscopy state. The layer protected InSb from reoxidation for only 4 min based on the O Auger state. Well-ordered monolayers formed on GaAs and In0.53Ga0.47As based on transmission Fourier transform infrared (FTIR) spectroscopy. A partially ordered layer was formed on InSb based on attenuated total reflection FTIR. The increased reoxidation rate of InGaAs and InSb is due to the larger lattice parameter of these surfaces and their In content, which forms weaker bonds to S, Ga, and Sb compared to Ga bonding to As and S.

Polymer Electrolyte Fuel Cells Employing Heteropolyacids as Redox Mediators for Oxygen Reduction Reactions: Pt-Free Cathode Systems.

In this study, the heteropolyacids of H3+xPVxMO12-xO40 (x = 0, 2, and 3) were applied as redox mediators for the oxygen reduction reaction in polymer electrolyte fuel cells, of which the cathode is free from the usage of noble metals such as Pt/C. In this system, the electrochemical reduction of heteropolyacid over the carbon cathode and the subsequent reoxidation of the partially reduced heteropolyacid by exposure to the dissolved oxygen in the regenerator are important processes for continuous power generation. Thus, the redox properties of catholytes containing these heteropolyacids were investigated in detail. The substitution quantity of V in the heteropolyacid affected the onset reduction potential as well as the reduction current density, resulting in a difference in cell performance. The chemical composition of heteropolyacid also had a significant impact on the reoxidation property. Among the three compounds, H6PV3Mo9O40 was the most suitable redox mediator. Furthermore, the pH of the catholyte was found to be the crucial factor in determining the reoxidation rate of partially reduced heteropolyacid as well as cell performance.

Nitrite Reductase Activity in Engineered Azurin Variants.

Nitrite reductase (NiR) activity was examined in a series of dicopper P.a. azurin variants in which a surface binding copper site was added through site-directed mutagenesis. Four variants were synthesized with copper binding motifs inspired by the catalytic type 2 copper binding sites found in the native noncoupled dinuclear copper enzymes nitrite reductase and peptidylglycine α-hydroxylating monooxygenase. The four azurin variants, denoted Az-NiR, Az-NiR3His, Az-PHM, and Az-PHM3His, maintained the azurin electron transfer copper center, with the second designed copper site located over 13 Å away and consisting of mutations Asn10His,Gln14Asp,Asn16His-azurin, Asn10His,Gln14His,Asn16His-azurin, Gln8Met,Gln14His,Asn16His-azurin, and Gln8His,Gln14His,Asn16His-azurin, respectively. UV-visible absorption spectroscopy, EPR spectroscopy, and electrochemistry of the sites demonstrate copper binding as well as interaction with small exogenous ligands. The nitrite reduction activity of the variants was determined, including the catalytic Michaelis-Menten parameters. The variants showed activity (0.34-0.59 min(-1)) that was slower than that of native NiRs but comparable to that of other model systems. There were small variations in activity of the four variants that correlated with the number of histidines in the added copper site. Catalysis was found to be reversible, with nitrite produced from NO. Reactions starting with reduced azurin variants demonstrated that electrons from both copper centers were used to reduce nitrite, although steady-state catalysis required the T2 copper center and did not require the T1 center. Finally, experiments separating rates of enzyme reduction from rates of reoxidation by nitrite demonstrated that the reaction with nitrite was rate limiting during catalysis.

Oxidative dehydrogenation of isobutane to isobutene by pyrovanadates, M2V2O7, where M(II) = Mn, Co, Ni, Cu and Zn, and Co2VO4 and ZnV2O4: The effect of gold nanoparticles

The pyrovanadates M2V2O7, where MII=Mn, Co, Ni, Cu and Zn, have been investigated for the oxidative dehydrogenation of isobutane from 300 to 450°C, both with and without 5wt% gold nanoparticles. Reduction of the pyrovanadate compounds was generally observed, yielding products that contained V(IV) and/or V(III), and depended on both M(II) and the presence of gold nanoparticles. The most stable pyrovanadate was Ni2V2O7. Products identified by XRD were M2VO4, MVO3, MV2O4, or V2O3. The conversion of isobutane always increased with temperature, reaching 11% and 16% in the absence and presence of gold nanoparticles, respectively, at 450°C. Selectivities to isobutene were as high as 40–50%. Vanadium 2p3/2 XPS studies indicated that all catalysts exhibited surface V(V) and V(IV), while V(III) was present in those catalysts that generated V(III)-containing products. This suggests that catalytic activity depends on the rate of reoxidation of the lower oxidation states to V(V). Gold 4f7/2 XPS studies always indicated the presence of Au(0) and Au(I), and for some catalysts Au(III). The highest yields of isobutene correlate with the lowest Au(I) content. The reduction products Co2VIVO4 and ZnVIII2O4 were compared in their activities with Co2V2O7 and α-Zn2V2O7. The reduced phase Co2VO4 proved to be a good catalyst, comparable to the best 5wt% Au/M2V2O7 compositions.

Potential electron mediators to extract electron energies of RBC glycolysis for prolonged in vivo functional lifetime of hemoglobin vesicles.

Developing a functional blood substitute as an alternative to donated blood for clinical use is believed to relieve present and future blood shortages, and to reduce the risks of infection and blood type mismatching. Hemoglobin vesicle (HbV) encapsulates a purified and concentrated human-derived Hb solution in a phospholipid vesicle (liposome). The in vivo safety and efficacy of HbV as a transfusion alternative have been clarified. Auto-oxidation of ferrous Hb in HbV gradually increases the level of ferric methemoglobin (metHb) and impairs the oxygen transport capabilities. The extension of the functional half-life of HbV has recently been proposed using an electron mediator, methylene blue (MB), which acts as a shuttle between red blood cells (RBC) and HbV. MB transfers electron energies of NAD(P)H, produced by RBC glycolysis, to metHb in HbV. Work presented here focuses on screening of 15 potential electron mediators, with appropriate redox potential and water solubility, for electron transfer from RBC to HbV. The results are assessed with regard to the chemical properties of the candidates. The compounds examined in this study were dimethyl methylene blue (DMB), methylene green, azure A, azure B, azure C, toluidine blue (TDB), thionin acetate, phenazine methosulfate, brilliant cresyl blue, cresyl violet, gallocyanine, toluylene blue, indigo carmine, indigotetrasulfonate, and MB. Six candidates were found to be unsuitable because of their insufficient diffusion across membranes, or overly high or nonexistent reactivity with relevant biomolecules. However, 9 displayed favorable metHb reduction. Among the suitable candidates, phenothiazines DMB and TDB exhibited effectiveness like MB did. In comparison to MB, they showed faster reduction by electron-donating NAD(P)H, coupled with showing a lower rate of reoxidation in the presence of molecular oxygen. Ascertaining the best electron mediator can provide a pathway for extending the lifetime and efficiency of potential blood substitutes.

Metabolic and photosynthetic consequences of blocking starch biosynthesis in the green alga Chlamydomonas reinhardtii sta6 mutant.

Upon nutrient deprivation, microalgae partition photosynthate into starch and lipids at the expense of protein synthesis and growth. We investigated the role of starch biosynthesis with respect to photosynthetic growth and carbon partitioning in the Chlamydomonas reinhardtii starchless mutant, sta6, which lacks ADP-glucose pyrophosphorylase. This mutant is unable to convert glucose-1-phosphate to ADP-glucose, the precursor of starch biosynthesis. During nutrient-replete culturing, sta6 does not re-direct metabolism to make more proteins or lipids, and accumulates 20% less biomass. The underlying molecular basis for the decreased biomass phenotype was identified using LC-MS metabolomics studies and flux methods. Above a threshold light intensity, photosynthetic electron transport rates (water → CO2) decrease in sta6 due to attenuated rates of NADPH re-oxidation, without affecting photosystems I or II (no change in isolated photosynthetic electron transport). We observed large accumulations of carbon metabolites that are precursors for the biosynthesis of lipids, amino acids and sugars/starch, indicating system-wide consequences of slower NADPH re-oxidation. Attenuated carbon fixation resulted in imbalances in both redox and adenylate energy. The pool sizes of both pyridine and adenylate nucleotides in sta6 increased substantially to compensate for the slower rate of turnover. Mitochondrial respiration partially relieved the reductant stress; however, prolonged high-light exposure caused accelerated photoinhibition. Thus, starch biosynthesis in Chlamydomonas plays a critical role as a principal carbon sink influencing cellular energy balance however, disrupting starch biosynthesis does not redirect resources to other bioproducts (lipids or proteins) during nutrient-replete culturing, resulting in cells that are susceptible to photochemical damage caused by redox stress.

Effect of Oxidizing Slag on Cleanliness of IF Steel during Ladle Holding Process

AbstractThe effect of oxidizing slag on cleanliness of IF steel produced by ShouGang during ladle holding process was investigated by comparing the differences of slag composition, steel composition and inclusion morphology between two processes of different slag oxidability(Process‐A < Process‐B). The results showed that, the reoxidation of slag to steel was observed during holding process, and Process‐A with the lower slag oxidability was more helpful to improve the cleanliness of molten steel during holding process by comparing the T.O of the two processes. Due to the slag reoxidation during holding process, the large petal shaped and dendritic Al2O3 inclusions generated for Process‐A, and both the numbers of Al2O3 and Al2O3‐TixO inclusions increased significantly for Process‐B. In order to slow down reoxidation rate and improve the ability of slag to absorb inclusions, (FeO) content should be reduced and (CaO)/(Al2O3) should be increased in the case of liquid slag. In conclusion, whether prolonging ladle holding time should depend on (FeO) content, (CaO)/(Al2O3) etc. parameters of slag and initial T.O of molten steel.

Oxygen suppresses light-driven anodic current generation by a mixed phototrophic culture.

This paper describes the detrimental effect of photosynthetically evolved oxygen on anodic current generation in the presence of riboflavin upon illumination of a mixed phototrophic culture enriched from a freshwater pond at +0.6 V vs standard hydrogen electrode. In the presence of riboflavin, the phototrophic biomass in the anodic compartment produced an electrical current in response to light/dark cycles (12 h/12 h) over 12 months of operation, generating a maximum current density of 17.5 mA x m(-2) during the dark phase, whereas a much lower current of approximately 2 mA x m(-2) was generated during illumination. We found that the low current generation under light exposure was caused by high rates of reoxidation of reduced riboflavin by oxygen produced during photosynthesis. Quantification of biomass by fluorescence in situ hybridization images suggested that green algae were predominant in both the anode-based biofilm (55.1%) and the anolyte suspension (87.9%) with the remaining biovolume accounted for by bacteria. Genus-level sequencing analysis revealed that bacteria were dominated by cyanobacterium Leptolyngbia (∼35%), while the prevailing algae were Dictyosphaerium, Coelastrum, and Auxenochlorella. This study offers a key comprehension of mediator sensitivity to reoxidation by dissolved oxygen for improvement of microbial solar cell performance.

Kinetic Analysis of Lauric Acid Hydroxylation by Human Cytochrome P450 4A11

Cytochrome P450 (P450) 4A11 is the only functionally active subfamily 4A P450 in humans. P450 4A11 catalyzes mainly ω-hydroxylation of fatty acids in liver and kidney; this process is not a major degradative pathway, but at least one product, 20-hydroxyeicosatetraenoic acid, has important signaling properties. We studied catalysis by P450 4A11 and the issue of rate-limiting steps using lauric acid ω-hydroxylation, a prototypic substrate for this enzyme. Some individual reaction steps were studied using pre-steady-state kinetic approaches. Substrate and product binding and release were much faster than overall rates of catalysis. Reduction of ferric P450 4A11 (to ferrous) was rapid and not rate-limiting. Deuterium kinetic isotope effect (KIE) experiments yielded low but reproducible values (1.2–2) for 12-hydroxylation with 12-2H-substituted lauric acid. However, considerable “metabolic switching” to 11-hydroxylation was observed with [12-2H3]lauric acid. Analysis of switching results [Jones, J. P., et al. (1986) J. Am. Chem. Soc.108, 7074–7078] and the use of tritium KIE analysis with [12-3H]lauric acid [Northrop, D. B. (1987) Methods Enzymol.87, 607–625] both indicated a high intrinsic KIE (>10). Cytochrome b5 (b5) stimulated steady-state lauric acid ω-hydroxylation ∼2-fold; the apoprotein was ineffective, indicating that electron transfer is involved in the b5 enhancement. The rate of b5 reoxidation was increased in the presence of ferrous P450 mixed with O2. Collectively, the results indicate that both the transfer of an electron to the ferrous·O2 complex and C–H bond-breaking limit the rate of P450 4A11 ω-oxidation.

Variations in phytoplankton photo-physiology and productivity in a dynamic eutrophic ecosystem: a fast repetition rate fluorometer-based study

We investigated the photoacclimation state and photosynthetic rate of cells in the highly eutrophic upper Gulf of Thailand (UGOT), using fast repetition rate (FRR) fluorometry. Observations revealed differences in photosystem II (PSII) parameters: the maximum photochemical quantum efficiency of PSII (Fv/Fm), the functional absorption cross-section of PSII (σPSII) and the rate of reoxidation of the primary PSII electron acceptor (1/τQa), which were dependent on the hydrographic structure of the water column. FRR-derived parameters were used to estimate the chlorophyll a-normalized, photosynthetic rate (PB); the photosynthesis vs. irradiance (P–E) curves were derived from in situ PB profiles. The P–E parameters differed markedly between the upper stratified waters and the mixed water column; that is, the maximum photosynthetic rate |$P_{{\rm max}}^{\rm B} $| and the light saturation parameter (Ek) were higher in the upper stratified waters, while the initial slope of the P–E curve (α) was higher in the mixed waters. This indicates that cells acclimated to relatively bright light dominated the upper stratified waters, while cells acclimated to lower light dominated the mixed waters. A significant, positive relationship was found between the average PB in the euphotic zone and surface photosynthetically available radiation, from all profiles at both sampling sites, which suggests that phytoplankton photosynthesis in the UGOT was controlled primarily by irradiance. Furthermore, as stress from nutrient-limitation is unlikely in the UGOT, cells might realize their photosynthetic potential by means of photoacclimation, even under the different light regimes present in the UGOT.

Synthesis, Characterization, and Thermochemical Redox Performance of Hf4+, Zr4+, and Sc3+ Doped Ceria for Splitting CO2

We present results on the thermochemical redox performance and analytical characterization of Hf4+, Zr4+, and Sc3+ doped ceria solutions synthesized via a sol–gel technique, all of which have recently been shown to be promising for splitting CO2. Dopant concentrations ranging from 5 to 15 mol % have been investigated and thermally cycled at reduction temperatures of 1773 K and oxidation temperatures ranging from 873 to 1073 K by thermogravimetry. The degree of reduction of Hf and Zr doped materials is substantially higher than those of pure ceria and Sc doped ceria and increases with dopant concentration. Overall, 10 mol % Hf doped ceria results in the largest CO yields per mole of oxide (∼0.5 mass % versus 0.35 mass % for pure ceria) based on measured mass changes during oxidation. However, these yields were largely influenced by their rate of reoxidation, not necessarily thermodynamic limitations, as equilibrium was not achieved for either Hf or Zr doped samples after 45 min exposure to CO2 at all oxidation temperatures. Additionally, sample preparation and grain size strongly affected the oxidation rates and subsequent yields, resulting in slightly decreasing yields as the samples were cycled up to 10 times. X-ray diffraction, Raman, FT-IR, and UV/vis spectroscopy in combination with SEM-EDX have been applied to characterize the elemental, crystalline, and morphological attributes before and after redox reactions.

Characterization of photosynthetic electron transport chain in bioenergy crop Jerusalem artichoke (Helianthus tuberosus L.) under heat stress for sustainable cultivation

In order to explore the response of photosynthetic electron transport chain to heat stress, chlorophyll a fluorescence and modulated 820nm reflection transients were examined in the leaves of Jerusalem artichoke (Helianthus tuberosus L.). J step was elevated at 43°C, suggesting that electron transport beyond primary quinone electron acceptor of photosystem II (QA) was inhibited. I step was elevated at 45°C due to the lowered plastoquinol re-oxidation rate. Significant decrease in the QA reducing reaction centers per photosystem II (PSII) antenna chlorophyll and marked increase in relative variable fluorescence intensity at 300μs also occurred at 45°C, indicating that the electron transport chain from PSII donor side to QA was injured. Overall, the fragment between QA and plastoquinol is the most susceptible to heat in the electron transport chain before photosystem I (PSI). The maximal photochemical capacity of PSI was not affected by heat stress even at 48°C, and the interaction between PSII and PSI become unbalanced, when temperature rose to 45°C. At 45°C, prolonged PSI oxidation occurred in the expanding leaves because of the inhibited electron donation from PSII, however, PSI oxidation amplitude decreased in the expanded leaves. We infer that electron transport chain of PSI acceptor side is more sensitive to heat stress than the chain before PSI in the expanded leaves. Therefore, electrons in PSI reaction center could not be efficiently drained to the downstream at high temperature, and then PSI oxidation was blocked.

Visible light sensitized photocurrent generation from electrostatically assembled thin films of [Ru(bpy)3]2+ and the polyoxometalate γ*-[W18O54(SO4)2]4−: Optimizing performance in a low electrolyte medium

Under appropriate conditions, photocatalytic oxidation of benzyl alcohol by Dawson polyoxometalates, POM, can be sensitized by ruthenium(II) polypyridyl complexes under visible irradiation of electrostatic adducts formed between these materials. To realize the potential of such sensitization in a photoelectrochemical cell, thin film assembly of at an electrode interface of such materials is required. However, because they are electrostatically associated, the structural integrity of the Ru-POM complex can be compromised by the contacting electrolyte. Herein, thin layers of an electrostatic adduct formed between the polyoxotungstate γ*-[W18O54(SO4)2]4− and the ruthenium polypyridyl complex [Ru(bpy)3]2+ have been self-assembled on platinum electrodes for the first time, using alternate immersion layer-by-layer assembly. SEM and AFM imaging reveal that the adduct films exhibit a highly nanoporous structure with an average pore diameter of approximately 200nm; a film morphology that is clearly distinctive from films comprised of either parent ion; [(Hex)4N]4γ*-[W18O54(SO4)2] or [Ru(bpy)3](PF6)2 alone, and indicate that ion-pairing is what drives film porosity. Cyclic voltammetry of the adduct ([Ru–POM]) films was performed on Pt microelectrodes in the absence of, or presence of low levels of added supporting electrolyte to minimise any electrostatic disruption of the ionic film. Low concentrations of supporting electrolyte (1mM (Bu)4NBF4) decreased the resistance across the cell by an order of magnitude, compared with no electrolyte whilst maintaining the film integrity. Photoelectrochemical studies of [Ru–POM] films using visible light (λ>400nm) and benzyl alcohol as the sacrificial electron donor were performed in the absence of added electrolyte. Resonance Raman spectroscopy, along with the extinction of photocurrent on oxidation of the ruthenium center indicate that the benzyl alcohol oxidation is sensitized in the visible spectral region by the ruthenium center. Increasing the surface coverage by adding more layers resulted in a net decrease in photocurrent density due to ion-transport limitations. However, rates of electrochemical re-oxidation of the photochemically reduced films ([Ru–POM]red) increased exponentially with increasing applied overpotential up to the oxidation potential of the ruthenium center. The overall photocurrent was optimized at 59μAcm−2 in (1mM (Bu)4NBF4), which is almost an order of magnitude higher than currents generated under the same conditions (at 6.8μAcm−2) but in the absence of added electrolyte.

Re-oxidation Kinetics of Flash-Reduced Iron Particles in H2-H2O(g) Atmosphere Relevant to a Novel Flash Ironmaking Process

A novel flash ironmaking process based on hydrogen-containing reduction gases is under development at the University of Utah. The goal of this work was to study the possibility of the re-oxidation of iron particles in a H2-H2O gas mixture in the lower part of the flash reactor from the kinetic point of view. The last stage of hydrogen reduction of iron oxide, i.e., the reduction of wustite, is limited by equilibrium. As the reaction mixture cools down, the re-oxidation of iron could take place because of the decreasing equilibrium constant and the high reactivity of the freshly reduced fine iron particles. The effects of temperature and H2O partial pressure on the re-oxidation rate were examined in the temperature range of 823 K to 973 K (550 °C to 700 °C) and H2O contents of 40 to 100 pct. The nucleation and growth kinetics model was shown to best describe the re-oxidation kinetics. The partial pressure dependence with respect to water vapor was determined to be of first order, and the activation energy of re-oxidation reaction was 146 kJ/mol. A complete rate equation that adequately represents the experimental data was developed.

Inactivation of thepetEgene encoding plastocyanin causes different photosynthetic responses in cyanobacteriumSynechocystisPCC 6803 under light-dark photoperiod and continuous light conditions

Plastocyanin, encoded by the petE gene, can transfer electrons to photosystem I (PSI) and cytochrome c oxidase during photosynthetic and respiratory metabolism in cyanobacteria. We constructed a petE mutant of Synechocystis sp. strain PCC 6803 and investigated its phenotypic properties under different light conditions. When cultured under continuous light, inactivation of petE accelerated the plastoquinone pool reoxidation, slowed the reoxidation rate of the primary quinone-type acceptor, and decreased the connectivity factor between the individual photosystem II (PSII) photosynthetic units. Compared with the wild-type control, the petE mutant showed a decrease in its PSI/PSII fluorescence ratio and an increase in its dark respiration rate. When cultured under a light-dark photoperiod, the petE mutation caused an increase in the phycocyanin to chlorophyll ratio. Consequently, the mutant line was a darker blue than its wild-type counterpart. Moreover, the petE mutation increased the efficiency of light capture, nonphotochemical quenching, and linear electron transport activity, but decreased the functional absorption cross section of PSII. These results suggest that plastocyanin is involved in regulating the redox state of the photosynthetic electron transfer chain, and the petE mutation can induce interesting phenotypic properties that are specific to the light-dark photoperiod.

Influence of Chelating Agents on Biogenic Uraninite Reoxidation by Fe(III) (Hydr)oxides

Microbially mediated reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO(2(S)), has been proposed as a method for limiting uranium (U) migration. However, microbially reduced UO(2) may be susceptible to reoxidation by environmental factors, with Fe(III) (hydr)oxides playing a significant role. Little is known about the role that organic compounds such as Fe(III) chelators play in the stability of reduced U. Here, we investigate the impact of citrate, DFB, EDTA, and NTA on biogenic UO(2) reoxidation with ferrihydrite, goethite, and hematite. Experiments were conducted in anaerobic batch systems in PIPES buffer (10 mM, pH 7) with bicarbonate for approximately 80 days. Results showed EDTA accelerated UO(2) reoxidation the most at an initial rate of 9.5 μM day(-1) with ferrihydrite, 8.6 μM day(-1) with goethite, and 8.8 μM day(-1) with hematite. NTA accelerated UO(2) reoxidation with ferrihydrite at a rate of 4.8 μM day(-1); rates were less with goethite and hematite (0.66 and 0.71 μM day(-1), respectively). Citrate increased UO(2) reoxidation with ferrihydrite at a rate of 1.8 μM day(-1), but did not increase the extent of reaction with goethite or hematite, with no reoxidation in this case. In all cases, bicarbonate increased the rate and extent of UO(2) reoxidation with ferrihydrite in the presence and absence of chelators. The highest rate of UO(2) reoxidation occurred when the chelator promoted both UO(2) and Fe(III) (hydr)oxide dissolution as demonstrated with EDTA. When UO(2) dissolution did not occur, UO(2) reoxidation likely proceeded through an aqueous Fe(III) intermediate with lower reoxidation rates observed. Reaction modeling suggests that strong Fe(II) chelators promote reoxidation whereas strong Fe(III) chelators impede it. These results indicate that chelators found in U contaminated sites may play a significant role in mobilizing U, potentially affecting bioremediation efforts.

Nature of the active sites for the total oxidation of toluene by CuO[sbnd]CeO2/Al2O3

The binary metal oxide, CuOCeO2/γ-Al2O3, has been compared with the single oxide components CuO/γ-Al2O3 and CeO2/γ-Al2O3 for toluene total oxidation. The nature of the active sites was determined by means of several spectroscopic techniques, while the transient response technique TAP (Temporal Analysis of Products) was used to investigate the catalytic performance.The improved performance of the CuOCeO2/γ-Al2O3 catalyst compared to CuO/γ-Al2O3 is attributed to the formation of a Ce1−xCuxO2−x solid solution with a crystallite size of 6nm. Within this phase, oxidation of toluene occurs at Cu2+ sites and reduction of oxygen at Ce3+ sites. Similar to Wacker chemistry, two redox couples, Ce4+/Ce3+ and Cu2+/Cu1+, are operational. Apart from the solid solution, a copper oxide phase with a crystallite size of 100nm shows significantly lower catalytic activity.X-ray absorption near-edge structure (XANES) experiments at the copper and cerium edge indicate that Ce4+ is reduced at lower temperature than Cu2+. Upon re-oxidation with CO2 or H2O, Ce3+ is partly re-oxidized, while Cu0 is not. This explains an activity increase in the CuOCeO2/γ-Al2O3 in the presence of H2O or CO2. CuO/γ-Al2O3 shows loss of activity in the presence of H2O as site blocking is not compensated by an increase in the re-oxidation rate.

Drought-induced modifications of photosynthetic electron transport in intact leaves: Analysis and use of neural networks as a tool for a rapid non-invasive estimation

Water deficit is one of the most important environmental factors limiting sustainable crop yields and it requires a reliable tool for fast and precise quantification. In this work we use simultaneously recorded signals of photoinduced prompt fluorescence (PF) and delayed fluorescence (DF) as well as modulated reflection (MR) of light at 820nm for analysis of the changes in the photosynthetic activity in detached bean leaves during drying. Depending on the severity of the water deficit we identify different changes in the primary photosynthetic processes. When the relative water content (RWC) is decreased to 60% there is a parallel decrease in the ratio between the rate of excitation trapping in the Photosystem (PS) II reaction center and the rate of reoxidation of reduced PSII acceptors. A further decrease of RWC to 20% suppresses the electron transfer from the reduced plastoquinone pool to the PSI reaction center. At RWC below values 15%, the reoxidation of the photoreduced primary quinone acceptor of PSII, QA–, is inhibited and at less than 5%, the primary photochemical reactions in PSI and II are inactivated. Using the collected sets of PF, DF and MR signals, we construct and train an artificial neural network, capable of recognizing the RWC in a series of “unknown” samples with a correlation between calculated and gravimetrically determined RWC values of about R2≈0.98. Our results demonstrate that this is a reliable method for determination of RWC in detached leaves and after further development it could be used for quantifying of drought stress of crop plants in situ. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.