Determination of chromium valence over the range Cr(0)–Cr(VI) by electron energy loss spectroscopy

Chromium is a redox active 3d transition metal with a wide range of valences (−2 to +6) that control the geochemistry and toxicity of the element. Therefore, techniques that measure Cr valence are important bio/geochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report an electron energy loss spectroscopy (EELS) technique along with an extensive suite of affined reference spectra that together, unlike other methods, can determine Cr valence (or at least constrain the possible valences) at high-spatial resolution (tens-of-nanometer scale) across a wide valence range, Cr(0)–Cr(VI). Fine structure of Cr-L_2,3 edges was parametrized by measurement of the chemical shift of the L₃ edge and the ratio of integrated intensity under the L₃ and L₂ edges. These two parameterizations were correlated to Cr valence and also the dⁿ orbital configuration which has a large influence on L-edge fine structure. We demonstrate that it is not possible to unambiguously determine Cr valence from only one fine-structure parameterization which is the method employed to determine metal valence by nearly all previous EELS studies. Rather, multiple fine-structure parameterizations must be used together if the full range of possible Cr valences is considered. However even with two parameterizations, there are limitations. For example, distinguishing Cr(IV) from Cr(III) is problematic and it may be difficult to distinguish low-spin Cr(II) from Cr(III). Nevertheless, when Cr is known to be divalent, low- and high-spin dⁿ orbital configurations can be readily distinguished.     

Modeling the reversible,diffusive sink effect in response to transient contaminant sources

A physically based diffusion model is used to evaluate the sink effect of diffusion-controlled indoor materials and to predict the transient contaminant concentration in indoor air in response to several time-varying contaminant sources. For simplicity, it is assumed the predominant indoor material is a homogeneous slab, initially free of contaminant, and the air within the room is well mixed. The model enables transient volatile organic compound (VOC) concentrations to be predicted based on the material/air partition coefficient (K) and the material-phase diffusion coefficient (D) of the sink. Model predictions are made for three scenarios, each mimicking a realistic situation in a building. Styrene, phenol, and naphthalene are used as representative VOCs. A styrene butadiene rubber (SBR) backed carpet, vinyl flooring (VF), and a polyurethane foam (PUF) carpet cushion are considered as typical indoor sinks. In scenarios involving a sinusoidal VOC input and a double exponential decaying input, the model predicts the sink has a modest impact for SBR/styrene, but the effect increases for VF/phenol and PUF/naphthalene. In contrast, for an episodic chemical spill, SBR is predicted to reduce the peak styrene concentration considerably. A parametric study reveals for systems involving a large equilibrium constant (K), the kinetic constant (D) will govern the shape of the resulting gasphase concentration profile. On the other hand, for systems with a relaxed mass transfer resistance, K will dominate the profile.     

Increased sediment oxygen uptake caused by oxygenation-induced hypolimnetic mixing

Hypolimnetic oxygenation systems (HOx) are increasingly used in lakes and reservoirs to elevate dissolved oxygen (O₂) while preserving stratification, thereby decreasing concentrations of reduced chemical species in the hypolimnion. By maintaining an oxic zone in the upper sediment, HOx suppress fluxes of reduced soluble species from the sediment into the overlying water. However, diminished HOx performance has been observed due to HOx-induced increases in sediment O₂ uptake. Based on a series of in situ O₂ microprofile and current velocity measurements, this study evaluates the vertical O₂ distribution at the sediment-water interface as a function of HOx operation. These data were used to determine how sediment O₂ uptake rate (JO₂) and sediment oxic-zone depth (z_max) were affected by applied oxygen-gas flow rate, changes in near-sediment mixing and O₂ concentration, and proximity to the HOx. The vertical sediment-water O₂ distribution was found to be strongly influenced by oxygenation on a reservoir-wide basis. Elevated JO₂ and an oxic sediment zone were maintained during continuous HOx operation, with z_max increasing linearly with HOx flow rate. In contrast, JO₂ decreased to zero and the sediment became anoxic as the vertical O₂ distribution at the sediment-water interface collapsed during periods when the HOx was turned off and near-sediment mixing and O₂ concentrations decreased. JO₂ and z_max throughout the reservoir were found to be largely governed by HOx-induced mixing rather than O₂ levels in the water column. By quantifying how JO₂ and z_max vary in response to HOx operations, this work (1) characterizes how hypolimnetic oxygenation affects sediment O₂ dynamics, (2) contributes to the optimization of water quality and management of HOx-equipped lakes and reservoirs, and (3) enhances understanding of the effect of mixing and O₂ concentrations in other systems.     

Solving the problem at the source: Controlling Mn release at the sediment-water interface via hypolimnetic oxygenation

One of the primary goals of hypolimnetic oxygenation systems (HOx) from a drinking water perspective is to suppress sediment-water fluxes of reduced chemical species (e.g., manganese and iron) by replenishing dissolved oxygen (O₂) in the hypolimnion. Manganese (Mn) in particular is becoming a serious problem for water treatment on a global scale. While it has been established that HOx can increase sediment O₂ uptake rates and subsequently enhance the sediment oxic zone via elevated near-sediment O₂ and mixing, the influence of HOx on sediment-water fluxes of chemical species with more complicated redox kinetics like Mn has not been comprehensively evaluated.This study was based on Mn and O₂ data collected primarily in-situ to characterize both the sediment and water column in a drinking-water-supply reservoir equipped with an HOx. While diffusive Mn flux out of the sediment was enhanced by HOx operation due to an increased concentration driving force across the sediment-water interface, oxygenation maintained elevated near-sediment and porewater O₂ levels that facilitated biogeochemical cycling and subsequent retention of released Mn within the benthic region. Results show that soluble Mn levels in the lower hypolimnion increased substantially when the HOx was turned off for as little as ∼48 h and the upper sediment became anoxic. Turning off the HOx for longer periods (i.e., several weeks) significantly impaired water quality due to sediment Mn release. Continual oxygenation maintained an oxic benthic region sufficient to prevent Mn release to the overlying source water.     

Use of in-stream reservoirs to reduce bacterial contamination of rural watersheds

An investigation into bacterial water quality problems was conducted on an interconnected stream and irrigation system within the Oldman River Basin of southern Alberta, Canada. Levels of indicator bacteria, including fecal coliforms, generic Escherichia coli and fecal streptococci, were repeatedly measured in streams and irrigation return canals of this river basin during the summer of 2001. Bacterial-loading segments of the irrigation/stream system were identified through a comparison of indicator bacteria levels in pairs of upstream and downstream sites. Mann-Whitney U-tests indicated that reservoirs significantly reduced bacterial counts. A temporal comparison of E. coli counts and river discharges suggested that these indicator bacteria do not originate from within in-stream sediments. Site-specific as well as cumulative inputs from a variety of non-point sources are likely to be responsible for the high downstream levels of indicator bacteria in this water system. The use of management practices such as in-stream reservoirs may significantly reduce contamination, and increase the quality of limited rural water supplies to allow their reuse and safe discharge into downstream water sources. The identification of bacteria-loading river/canal segments could also be used to prioritize restoration projects.     

Smooth and conductive DNA-templated Cu₂O nanowires: growth morphology, spectroscopic and electrical characterization

DNA strands have been used as templates for the self-assembly of smooth and conductive cuprous oxide (Cu?O) nanowires of diameter 12-23 nm and whose length is determined by the template (16 μm for λ-DNA). A combination of spectroscopic, diffraction and probe microscopy techniques showed that these nanowires comprise single crystallites of Cu?O bound to the DNA molecules which fused together over time in a process analogous to Ostwald ripening, but driven by the free energy of interaction with the template as well as the surface tension. Electrical characterization of the nanowires by a non-contact method, scanned conductance microscopy and by contact mode conductive AFM showed the wires are electrically conductive. The conductivity estimated from the AFM cross section and the zero-bias conductance in conductive AFM experiments was 2.2-3.3 S cm?1. These Cu?O nanowires are amongst the thinnest reported and show evidence of strong quantum confinement in electronic spectra.     

Polymer/clay nanocomposites as VOC barrier materials and coatings

The transport properties of select volatile organic compounds were measured in polyurethane/clay nanocomposite barrier membranes as a function of clay content. The nanocomposites were fabricated by two different processing methods involving stirring and sonication of the clay particles. The concentration of Cloisite^® 30B in the nanocomposite was varied from 0 to 50 wt%. Characterization of membrane transport properties was achieved via a gravimetric sorption method. Material-phase diffusivity coefficients (D) decreased with increasing Cloisite^® concentration, while changes in the material/VOC partition coefficients (K) depended on the molecular interactions of the VOCs with the membrane material.     

4,4‐Dimethyl‐ and Diastereomeric 4‐Hydroxy‐4‐methyl‐ (2S)‐Glutamate Analogues Display Distinct Pharmacological Profiles at Ionotropic Glutamate Receptors and Excitatory Amino Acid Transporters

Subtype‐selective ligands are of great interest to the scientific community, as they provide a tool for investigating the function of one receptor or transporter subtype when functioning in its native environment. Several 4‐substituted (S)‐glutamate (Glu) analogues were synthesized, and altogether this approach has provided important insight into the structure–activity relationships (SAR) for ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs), as well as the excitatory amino acid transporters (EAATs). In this work, three 4,4‐disubstituted Glu analogues 1 – 3 , which are hybrid structures of important 4‐substituted Glu analogues 4 – 8 , were investigated at iGluRs and EAATs. Collectively, their pharmacological profiles add new and valuable information to the SAR for the iGluRs and EAAT1–3.     

The prediction of crack growth in bonded joints under cyclic-fatigue loading I. Experimental studies

The performance of adhesively-bonded joints under monotonic and cyclic-fatigue loading has been investigated using a fracture-mechanics approach. The joints consisted of an epoxy film adhesive which was employed to bond aluminium-alloy substrates. The effects of undertaking cyclic-fatigue tests in (a) a ‘dry’ environment of 55% relative humidity at 23°C, and (b) a ‘wet’ environment of immersion in distilled water at 28°C were investigated. In particular, the influence of employing different surface pretreatments for the aluminium-alloy substrates was examined. In addition, single-lap joints were tested under cyclic fatigue loading in the two test environments, and a back-face strain technique has been used which revealed that crack propagation, rather than crack initiation, occupied the dominant proportion of the fatigue lifetime of the single-lap joints. In Part II, the data obtained in the present Part I paper will be employed to predict theoretically the lifetime of the adhesively-bonded single-lap joint specimens.