Results 1 - 10 of 8284
Results 1 - 10 of 8284. Search took: 0.028 seconds
|Sort by: date | relevance|
[en] Enhanced reductive dehalogenation is an attractive treatment technology for in situ remediation of chlorinated solvent DNAPL source areas. Reductive dehalogenation is an acid-forming process with hydrochloric acid and also organic acids from fermentation of the electron donors typically building up in the source zone during remediation. This can lead to groundwater acidification thereby inhibiting the activity of dehalogenating microorganisms. Where the soils' natural buffering capacity is likely to be exceeded, the addition of an external source of alkalinity is needed to ensure sustained dehalogenation. To assist in the design of bioremediation systems, an abiotic geochemical model was developed to provide insight into the processes influencing the groundwater acidity as dehalogenation proceeds, and to predict the amount of bicarbonate required to maintain the pH at a suitable level for dehalogenating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative terminal electron-accepting processes, gas release and soil mineralogy. While calcite and iron oxides were shown to be the key minerals influencing the soil's buffering capacity, for the extensive dehalogenation likely to occur in a DNAPL source zone, significant bicarbonate addition may be necessary even in soils that are naturally well buffered. Results indicated that the bicarbonate requirement strongly depends on the electron donor used and availability of competing electron acceptors (e.g., sulfate, iron (III)). Based on understanding gained from this model, a simplified model was developed for calculating a preliminary design estimate of the bicarbonate addition required to control the pH for user-specified operating conditions.
[en] Acid atmospheric emissions within Europe and North America have decreased strongly since 1985 and most recent acidification studies have focused on the changes occurring within ecosystems as a result of this decreased deposition. This current study documents a soil acidification trend under ambient N deposition conditions over a 13 year period, suggesting that acidification continues to be a process of concern at this Calluna vulgaris dominated heathland with an acidic sandy soil. The annual manipulation of climatic conditions on this heathland simulated the predicted summer rainfall reduction (drought) and resulted in a long term retardation of the soil acidification trend. The pH of the soil solution significantly decreased over the course of the trial for both treatments, however, in the final 2 years the decline continued only in the Control treatment. This retardation is primarily associated with the reduction in rainfall leading to lower drainage rates, reduced loss of cations and therefore reduced lowering of the soil acid neutralizing capacity (ANC). However, a change in the underlying mechanisms also indicated that N transformations became less important in the Drought treatment. This change corresponded to an increase in groundcover of an air-pollution tolerant moss species and it is hypothesized that this increasing moss cover filtered an increasing quantity of deposited N, thus reducing the N available for transformation. A soil acidification lag time is expected to increase between the two treatments due to the cumulative disparity in cation retention and rates of proton formation. To the authors' knowledge, this is the first study in which such acidification trends have been demonstrated in a field-scale climate manipulation experiment. -- Highlights: ► A unique investigation of acidification on a field-scale climate manipulation trial. ► Soil acidification occurred over 13 years of ambient N deposition conditions. ► Repeated annual drought retards acidification by lower cation loss. ► Increasing cover of pollution tolerant mosses may reduce N input and proton formation. ► Soil acidification is still a concern on this N-saturated system.
[en] The current intensity through a nanoconstriction with relatively small atom-lead coupling is calculated in terms of the corresponding electrochemical potential which is assumed to be non-negative and referred to a reference electrode, given a certain electrolyte. In addition, a discussion in relation to off-resonant conduction is done including the derivation of an expression for the number of off-resonant states
[en] The basic physical principles underlying electron transfer between metal ions and other well-localised centres are summarized. Recent experimental progress is reviewed, with particular reference to electron transfer in the gas phase-between large molecules; in solution - results of volume of activation studies; and in the solid phase-transfer between paramagnetic centres. The potential value of neutron scattering spectroscopy, as a probe of low-energy electronic transitions in mixed-value materials, and as a method of measuring rate processes in the range 1010-1012 s-1, is discussed. (author). 86 refs.; 6 figs
[en] To clarify the vague points appeared in the literature, it was discussed that fractal analysis of electrode surfaces based on the concept of 'diffusion toward electrode surfaces' is only able to monitor surface roughness in scales larger than 10 nm. When inspecting fractality in atomic scale (and even up to 10 nm), electrochemical measurements are not reliable due to the presence of surface defects, which affect the electrochemical reaction. In other words, for fractal analysis of electrode surfaces, the diffusion layer width which acts as yardstick length, should be sufficiently large, incomparable to the scale of atomic inhomogeneities. To this aim, the experiment time should be sufficiently long or the diffusion coefficient should be sufficiently large
[en] The advantages of the use of ozone to clean water from cooling towers are highlighted
[nl]Waterbehandeling van het koelwater van koeltorens is noodzakelijk en wordt voomamelijk gedaan door toevoeging van chemicalien. Ozon als vervanger van de gebruikelijke chemicalien lijkt zeer geschikt om koeltorenwater te reinigen. Wordt aan de technische randvoorwaarden voldaan, dan is het een ideaal reinigingsmiddel dat met geringe operationele kosten de waterkwaliteit onderhoudt. Naast de reinigende eigenschappen, is het een groot voordeel dat de chemische belasting op het milieu en de daarbij komende locale lozingskosten sterk afnemen
[en] Highlights: • Very high-order spatial discretisations in electrochemical simulations are tested. • Asymmetric 4-to7-point approximations enable to use grids with less than 15 points. • Brute force resolution of the resulting problem is competitive in all cases studied. • Comparison between LU and QR decompositions and sparse matrix methods is performed. • Easy-to-implement, C++ example programs are provided. The use of very high order spatial discretisation in digital simulation of electrochemical experiments is assessed, considering up to asymmetric 8-point approximations for the derivatives. A wide range of conditions are examined, including several mechanisms and electrodes and potential-step and potential-sweep experiments. In all cases it is found that asymmetric multi-point approximations in combination with exponentially expanding grids provides very accurate results and with very reduced number of grid points (<15). Consequently, the direct (‘brute force’) resolution of the finite-difference equation system by standard matrix techniques becomes a competitive and more general alternative to specialised methods like the Thomas algorithm.
[en] Highlights: • The electrochemical synthesis of poly(o-methoxyaniline) (POMA) thin films was performed. • The concentration of monomer, and anodic charge influence the POMA film formation. • A relationship between the mass of the POMA, the anodic charge, and concentration of the OMA was established by EQCM studies. • The morphology of the films shows a compact first layer with spherical grains of different sizes distributed in this layer.
[en] The aim of this article is to provide insight on the use of a biological molecule-bacteriorhodopsin (bR) having all the basic properties necessary for the assembly of nanoscale electronic devices. Recent developments made during last decade supported by key references are reviewed in this contribution. Major emphasis on bR-based observations conducted in our laboratory has been elaborated. Important issues concerning structure, widely accepted photocycle of bR has been summarized. The possibility of nano-devices emanating from this biomolecule is briefly presented