Coagulation-flocculation of beech condensate: particles size distribution

Beech wood (Fagus sylvatica L.) condensate from a steaming operation was studied. The objective of our work was to study the precipitation of these wood extracts in presence of calcium ion after autoxidation at basic pH (8). The autoxidation was carried out at 250 rpm for 30 min, and flocculation was followed up for 30 min. An investigation with a laser sizer Mastersizer of Malvern has been done in order to study the influence of the agitation on the state of aggregation of the condensate. A negative correlation was observed between the mean size of particles and the agitation rate. Without stirring, flocculation rapidly occurred and the mean size of particles was high. Calcium-induced aggregation of the condensate was also found to be reversible toward agitation.     

Conformational properties of amphotericin B amide derivatives – impact on selective toxicity

Even though it is highly toxic, Amphotericin B (AmB), an amphipathic polyene macrolide antibiotic, is used in the treatment of severe systemic fungal infections as a life-saving drug. To examine the influence of conformational factors on selective toxicity of these compounds, we have investigated the conformational properties of five AmB amide derivatives. It was found that the extended conformation with torsional angles (,)=(290°,180° ) is a common minimum of the potential energy surfaces (PES) of unsubstituted AmB and its amide derivatives. The extended conformation of the studied compounds allows for the formation of an intermolecular hydrogen bond network between adjacent antibiotic molecules in the open channel configuration. Therefore, the extended conformation is expected to be the dominant conformer in an open AmB (or its amide derivatives) membrane channel. The derivative compounds for calculations were chosen according to their selective toxicity compared to AmB and they had a wide range of selective toxicity. Except for two AmB derivatives, the PES maps of the derivatives reveal that the molecules can coexist in more than one conformer. Taking into account the cumulative conclusions drawn from the earlier MD simulation studies of AmB membrane channel, the results of the potential energy surface maps, and the physical considerations of the molecular structures, we hypothesize a new model of structure-selective toxicity of AmB derivatives. In this proposed model the presence of the extended conformation as the only well defined global conformer for AmB derivatives is taken as the indicator of their higher selective toxicity. This model successfully explains our results. To further test our model, we also investigated an AmB derivative whose selective toxicity has not been experimentally measured before. Our prediction for the selective toxicity of this compound can be tested in experiments to validate or invalidate the proposed model.     

Oxygen reduction kinetics on graphite cathodes in sediment microbial fuel cells

Sediment microbial fuel cells (SMFCs) have been used as renewable power sources for sensors in fresh and ocean waters. Organic compounds at the anode drive anodic reactions, while oxygen drives cathodic reactions. An understanding of oxygen reduction kinetics and the factors that determine graphite cathode performance is needed to predict cathodic current and potential losses, and eventually to estimate the power production of SMFCs. Our goals were to (1) experimentally quantify the dependence of oxygen reduction kinetics on temperature, electrode potential, and dissolved oxygen concentration for the graphite cathodes of SMFCs and (2) develop a mechanistic model. To accomplish this, we monitored current on polarized cathodes in river and ocean SMFCs. We found that (1) after oxygen reduction is initiated, the current density is linearly dependent on polarization potential for both SMFC types; (2) current density magnitude increases linearly with temperature in river SMFCs but remains constant with temperature in ocean SMFCs; (3) the standard heterogeneous rate constant controls the current density temperature dependence; (4) river and ocean SMFC graphite cathodes have large potential losses, estimated by the model to be 470 mV and 614 mV, respectively; and (5) the electrochemical potential available at the cathode is the primary factor controlling reduction kinetic rates. The mechanistic model based on thermodynamic and electrochemical principles successfully fit and predicted the data. The data, experimental system, and model can be used in future studies to guide SMFC design and deployment, assess SMFC current production, test cathode material performance, and predict cathode contamination.     

Spectrophotometric and voltammetric characterization of a novel selective electroactive chemosensor for Mg2+

A novel azocalix[4]arene derivative, 5,11,17,23-tetrakis[(acetophenone)azo]-25,26,27,28-tetrahydroxycalix[4]arene (APC4), containing acetophenone azo groups at the upper rim was synthesized as a chemosensor. Its binding and sensing properties with alkali and alkaline earth metal ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) were investigated by UV-vis spectrophotometric and voltammetric techniques. The stoichiometric ratio and the association constant were determined spectrophotometrically as 1:1 and (1.94±0.31)×10⁵ L mol^?1 for the complex between Mg²⁺ and the chemosensor, respectively. Moreover, it was shown that the interaction between Mg²⁺ and the APC4 occurred by means of the phenol groups at the lower rim by voltammetric methods. The results of spectrophotometric and voltammetric experiments showed that the chromogenic chemosensor has high selectivity towards Mg²⁺ among the other used metal ions, especially the interfering Ca²⁺ ion.      

Co2+与BPPT络合反应动力学及动力学光谱法测定钴

Kinetics of complexation reaction of Co2+ with 2-benzoylpyridine-4-phenyl-3-thiosemicarbazone (BPPT)was spectrophotometrically examined at 421 nm. The ligand that is developed for a simple kinetic-spectrophotometric determination of Co2+ is based on 1:2 complex formation between Co2+ and BPPT. The complexation reaction was carried out in ethanol-water medium at 25 ℃. Kinetic and activation parameters of the complexation reaction were calculated, and the rate equation and the reaction mechanism were proposed. The calibration graph is linear in the concentration range of 0.10～2.91 mg·L-1 for the tangent method. The species that caused interference were investigated.     

A Cyclic Voltammetric Study of Some Porphyrazines

Summary.  Some porphyrazines with dimethylamino- or trimethylammoniumethylsulfanyl substituents were studied in dichloromethane or dimethylsulfoxide solution by cyclic voltammetric methods. The cyclic voltammogram of metal free octakis-(dimethyl-aminoethylsulfanyl)-porphyrazine is characterized by three one-electron reduction waves which show quasi-reversible behaviour at all sweep rates observed; the same holds for its cobalt(II) derivative. The quaternized octacationic derivative, however, exhibits four one-electron reduction waves which are reversible at all sweep rates. The reaction mechanisms and diffusion coefficients were investigated. Received October 17, 2000. Accepted (revised) December 11, 2000     

A tutorial proof that extreme points are basic feasible solutions

Given a set of m linear equations in n unknowns with the requirement that the solution space be nonnegative, a simple, heuristic proof is offered which shows that the extreme points of the set of feasible solutions are also basic feasible solutions. This proof can be used in many text treatments of Linear Programming which omit the proof on the grounds that it is too difficult to prove.     

Reaction of CF3 radicals with C2H6

The hydrogen transfer reaction between C₂H₆ and CF₃ radicals, generated by the photolysis of CF₃I, has been studied in the temperature range 298–617 K. The rate constant, based on the value of 10^13.36 cm³ mol^?1 s^?1 for the recombination of CF₃ radicals, is given by where k₂ is in cm³ mol^?1 s^?1 and E is in J mol^?1. These results are compared with those previously reported, and the following best value for k₂ is recommended:      

噻二嗪衍生物的合成及Pb2+在水/1,2-二氯乙烷界面转移的电化学性质

The transfer of Pb2+ facilitated by interfacial complexation with 5-(4-phenoxyphenyl)-6H-1,3,4-thiadiazin-2-amine(PPTA) at the polarized water/1,2-dicholoroethane(1,2-DCE) interface was investigated by cyclic voltammetry.We synthesized the thiadiazine derivative,PPTA,firstly.The transfer was performed at different metal concentrations and scan rates,and the obtained voltammetric transfer peaks toward Pb2+ ion over other divalent cations(Zn2+,Co2+,Ni2+,Cd2+,Hg2+,and Cu2+) were reversible.The dependence of the half-wave potentials of the Pb2+ ion on the concentration of PPTA in the organic phase indicates that the ion transfer is facilitated by the formation of 1:2(metal:ligand) complex in the organic phase with the association constant(lgβ2) of(17.1±0.2).     

Flow Partitioning in Fully Saturated Soil Aggregates

Microbes play an important role in facilitating organic matter decomposition in soils, which is a major component of the global carbon cycle. Microbial dynamics are intimately coupled to environmental transport processes, which control access to labile organic matter and other nutrients that are needed for the growth and maintenance of microorganisms. Transport of soluble nutrients in the soil system is arguably most strongly impacted by preferential flow pathways in the soil. Since the physical structure of soils can be characterized as being formed from constituent micro-aggregates which contain internal porosity, one pressing question is the partitioning of the flow among the “inter-aggregate” and “intra-aggregate” pores and how this may impact overall solute transport within heterogeneous soil structures. The answer to this question is particularly important in evaluating assumptions to be used in developing upscaled simulations based on highly resolved mechanistic models. In our synthetic model of soils, firstly we statistically generated a number of micro-aggregates containing internal pores. Then we constructed a group of diverse multi-aggregate structures with different packing ratios by stacking those micro-aggregates and varying the size and shape of inter-aggregate pore spacing between them. We then performed pore-scale flow simulations using computational fluid dynamics methods to determine the flow patterns in these aggregate-of-aggregates structures and computed the partitioning of the flow through intra- and inter-aggregate pores as a function of the spacing between the aggregates. The results of these numerical experiments demonstrate that soluble nutrients are largely transported via flows through inter-aggregate pores. Although this result is consistent with intuition, we have also been able to quantify the relative flow capacity of the two domains under various conditions. For example, in our simulations, the flow capacity through the aggregates (intra-aggregate flow) was less than 2 % of the total flow when the spacing between the aggregates was larger than $18\,\upmu \hbox {m}$ . Inter-aggregate pores continued to be the dominant flow pathways even at much smaller spacing; intra-aggregate flow was less than 10 % of the total flow when the inter- and intra-aggregate pore sizes were comparable. Although the results may not be exactly the same as those obtained from actual soil systems, such studies are making it possible to identify which model upscaling assumptions are realistic and what computational methods are required for detailed numerical investigation of hydrodynamics and microbial carbon cycling dynamics in soil systems.