Relative Low Concentration Research Articles

Concentration dependent transport of colloids in saturated porous media

A series of column experiments was undertaken to explore the influence of colloid input concentration (2, 1, 0.5, and 0.25 times a reference concentration), colloid size (negatively charged 3.2 and 1.0 μm carboxyl latex), and sand grain size (360, 240, and 150 μm quartz sands) on transport and deposition. A similar mass of stable mono-dispersed colloids was added to each column. For a given input concentration, decreasing the sand size and increasing the colloid size resulted in increased mass retention in the sand near the column inlet and lower relative concentrations in the effluent. For a given sand and colloid, increasing the input concentration produced less deposition and higher mass recovery in the effluent, especially for coarser sands and smaller colloids. Results of a time dependent attachment (blocking) and detachment model were not consistent with this behavior because the simulations predicted much less retention near the column inlet and a decreasing number of favorable attachment sites (mass of deposited colloids) with increasing input concentration in a given system (colloid and sand). A time dependent straining model (filling of straining sites) provided a better description of the effluent and deposition data, but still could not account for the observed concentration dependent mass recovery. Alternatively, the straining model was refined to include a liberation term that assumed that straining was hindered at higher concentrations (collision frequencies) due to repulsive colloid (aqueous phase)–colloid (strained) interactions. Simulations that included straining, liberation, attachment, and detachment significantly improved the description of the experimental data.

HRTEM analysis of the nanostructure of porous silicon

The nanometric structure of porous silicon makes this material to be very suitable for its use in many different fields, including optoelectronics and biological applications. In the present work, the structure of porous silicon was investigated in detail by means of cross-sectional high-resolution transmission electron microscopy and digital image processing, together with electron energy loss spectroscopy. The structure of the Si/porous silicon interface and that of the silicon nanocrystals that compose porous silicon have been analyzed in detail. A strong strain contrast in the Si/porous silicon interface caused by high stresses was observed. Accordingly, dislocation pairs are found to be a possible mechanism of lattice matching between porous silicon and the Si substrate. Finally, high relative concentration of oxygen in the porous silicon layer was observed, together with low relative electron concentration in the conduction band when compared to Si.

Effects of high pressure and microwave on pronase and α-chymotrypsin hydrolysis of β-lactoglobulin

The effects of hydrostatic high pressure, microwave irradiation, and conventional heating on hydrolysis of bovine β-lactoglobulin AB by pronase and α-chymotrypsin were studied. The high pressure treatments were performed between 100 and 300 MPa, the microwave irradiation was applied at different power conditions, pronase (30 W) and α-chymotrypsin (15 W). All enzymatic reactions were carried out during 10 and 20 min, at 40 °C. The initial rate of pronase and α-chymotrypsin hydrolysis of protein was increased by using either physical treatment during the enzymatic digestion. The microwave-assisted digestion by pronase was more effective than those performed under conventional heating. The only apparent microwave induced difference on protein digestion by α-chymotrypsin was in a lower relative concentration of peak eluting at 24.5 min, and the conversion of this fraction into final peaks eluting from 24.5 to 28.5 min (F1 fraction) after 20 min of incubation. Under high pressure, the β-lactoglobulin AB is completely hydrolyzed by pronase and α-chymotrypsin.

Effects of a phospholipid cosurfactant on external coalescence in water-in-oil-in-water double-emulsion globules

The activated form of N-glutaryl-phosphatidylethanolamine ( aNGPE) has been incorporated into water-in-oil-in-water (W 1/O/W 2) double-emulsion globules. Preparation of individual globules within a cylindrical capillary and video microscopy of their behavior enabled determination of the rate of coalescence of internal aqueous droplets (W 1) with the external aqueous phase (W 2), termed external coalescence. The presence of Tween 80 in the external phase at concentrations of 0.01 and 0.012 M effected such coalescence. The concentration of Span 80 in the oil phase exceeded that of aNGPE considerably, with the ratio of Span 80 to aNGPE molecules ranging from 60 to 1363. Despite the low relative concentration of aNGPE, external coalescence of the internal droplets was prolonged in globules containing this cosurfactant as compared to globules containing only Span 80. Increasing the amount of phospholipid in the oil phase did not significantly affect the rate of release, indicating that within our range of concentrations (22.0–91.3 μM) there was little variation in the amount of phospholipid adsorbing at the interface. These results indicate that, in addition to its suitable protein-binding functionality, aNGPE can prolong the release of internal droplets in a W 1/O/W 2 globule; its use as a cosurfactant shows potential benefits for drug-delivery applications.

Evaluating Charcoal Presence in Sediments and its Effect on Phenanthrene Sorption

Small amounts of charcoal in sediments can highly impact the sorption of organic compounds in single solute systems and at low relative concentrations. In the present study, the heterogeneous sorption behavior of charcoals is demonstrated through batch sorption experiments with phenanthrene. The utility of the black carbon content, as measured for soot materials, is evaluated as a measure of the nature of the charcoal material present in sediments. Black carbon content measurements improved predictions of sorptive properties through an additive model but were not sufficient in fully explaining the sorptive behavior observed for the sediments studied in the present work. This is also corroborated through the low and inconsistent measurements of black carbon content observed for laboratory-produced wood chars and natural charcoal samples. Since a quantitative means for prediction of sediment sorptive behavior is not readily available, a more detailed organic petrographical investigation of the charcoals present in sediment samples is pursued. This is a standard method that allows characterization of all (>1-2 μm) organic particles found in a sample. While petrographic methods provide a qualitative description of the sample, it is not possible to use the results to quantitatively predict the overall sorption properties based on this analysis alone. Thus, careful interpretation of qualitative (i.e. microscopy) and quantitative (i.e. black carbon fraction) results should lead us to the determination of a new quantitative property to be used as a predictive tool.

Novel Properties of DDAB: Matrix Effect and Interaction with Oleate

We report an investigation on the interaction of the positively charged surfactant DDAB (didodecyldimethylammonium bromide) with preformed vesicles from POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). Addition of 1.9 mM DDAB to preformed POPC vesicles of different concentrations results in mixed vesicles, owing to the avid uptake of DDAB by POPC. This leads to the so-called matrix effect: when preformed POPC liposomes with a narrow size distribution are present in the aqueous solution, there is a fast formation process of mixed vesicles and the final size distribution closely resembles that of the preformed POPC liposomes. The final mixed vesicle system is stable in size and size distribution. The effect is independent of the initial size of the POPC vesicles and requires low relative concentrations of POPC with respect to DDAB (up to 1:4). Aside this, we report about the time stability of DDAB vesicles as a function of the aqueous buffer ionic strength and we investigate the interaction between...

Variable alteration in heavy crude oils of west-central Saskatchewan, Canada

Heavy crude oils occurring in the Mississippian Bakken and Lower Cretaceous Mannville reservoirs in west-central Saskatchewan display variable geochemical composition in their light (C5–C8) and medium (C12–C30) molecular weight fractions. All oils show a depletion of normal alkanes, and to a lesser extent acyclic isoprenoids, which appears to increase progressively southward. As biodegradation seems to be the primary control of the n-alkane distributions, it appears that oils produced from deeper reservoirs in the south are more biodegraded than the oils in shallower Mannville reservoirs in the north. However, several other compositional characteristics, including gasoline range and aromatic fractions, are inconsistent with the trend in biodegradation defined by the relative n-alkane abundance. Oils from the shallower northern reservoirs are enriched in cycloalkanes, show lower relative concentrations of isoheptane, lower paraffin indices, much higher K1 ratios and lower relative concentrations of lower molecular weight naphthalenes. Moreover, some oils show a persistent presence of lower homologs of n-alkanes (C5–C8) and light aromatics such as benzene and toluene when higher (>C15) homologs are not present. Such compositional characteristics may indicate that some of the reservoirs containing biodegraded oil were subjected to a subsequent secondary charge of lighter, unbiodegraded crude or to different post-generative alteration mechanisms. This later charge of light hydrocarbons suggests the timing of most biodegradation to be pre-Eocene. Destructive microbial action resulting in preferential removal of n-alkanes, isoprenoids, alkylbiphenyls and methylated naphthalenes had little effect on higher molecular weight fractions. Therefore, despite their apparently very low gravities (average API of 14°), most of these oils suffered only light to moderate biodegradation. Although generally similar biomarker signatures indicate an overall genetic affinity and the same source rock for these oils, minor but systematic variations in the biomarker compositions likely reflect regional source organic facies variation or indicate a progressive change in oil maturity.

Dietary oxidized cholesterol decreases expression of hepatic microsomal triglyceride transfer protein in rats

The aim of this study was to compare the effects of dietary oxidized cholesterol and pure cholesterol on plasma and very low density lipoprotein (VLDL) lipids and on some parameters of VLDL assembly and secretion in rats fed two different dietary fats. Four groups of male growing Sprague-Dawley rats were fed diets containing pure or oxidized cholesterol (5 g/kg diet) with either coconut oil or salmon oil as dietary fat (100 g/kg diet) for 35 days. Rats fed oxidized cholesterol supplemented diets had significantly lower concentrations of triglycerides and cholesterol in plasma and VLDL than rats fed pure cholesterol supplemented diets irrespective of the type of fat. In addition, rats fed oxidized cholesterol supplemented diets had significantly lower relative concentrations of microsomal triglyceride transfer protein messenger ribonucleic acid (mRNA) than rats fed pure cholesterol supplemented diets. In contrast, hepatic lipid concentrations and the relative concentration of apolipoprotein B mRNA were not influenced by the dietary factors investigated. Parameters of hepatic lipogenesis (relative mRNA concentration of sterol regulatory element binding protein-1c and activity of glucose-6-phosphat dehydrogenase) were significantly reduced by feeding fish oil compared to coconut oil, but were not affected by the type of cholesterol. In conclusion, the data of this study suggest, that dietary oxidized cholesterol affects VLDL assembly and/or secretion by reducing the synthesis of MTP but not by impairing hepatic lipogenesis or synthesis of apolipoprotein B.

Haematological characteristics predicting susceptibility for ascites. 2. High haematocrit values in juvenile chickens

1. Male broilers of two different genetic stocks, a pure broiler sire line (A) and commercially available Ross broilers (B), were used to study the effect of differences in haematocrit values in 12-d-old chickens on the development of ascitic symptoms. Body weight gain (BWG), feed intake (FI) and feed conversion ratio (FRC) were measured from 2 to 5 weeks of age. Mortality was recorded from 2 to 6 weeks. The birds were kept at a low ambient temperature to stimulate the incidence of ascites. 2. From each stock 32 birds with the highest (High Ht) and 32 birds with the lowest (Low Ht) haematocrit values in venous blood were selected at 12 d of age for an examination on susceptibility for the ascites syndrome at 5 weeks. 3. At 2 weeks 448 birds per stock (including High Ht and Low Ht birds) were allotted to 32 floor pens (1 High Ht and 1 Low Ht in each pen). Venous blood samples were taken weekly in High Ht and Low Ht birds for haematocrit measurements and at week 5 also thyroid hormone (T3, T4) concentrations in plasma were measured. At 5 weeks all High Ht and Low Ht birds were examined post-mortem and arterial pressure index, heart weight, lung weight and liver weight (all relative to body weight) were recorded. High values for arterial pressure index, relative heart and liver weights and low values for relative lung weights and plasma thyroid hormone were considered to be indicators for ascites susceptibility. 4. Stock A, with the highest ascites mortality, had lower values for BWG and FCR than stock B. Throughout, a clear difference in Ht values between High Ht and Low Ht birds was maintained. No significant effects of 12-d haematocrit values were found on the incidence of ascitic symptoms until week 5. High Ht values tended to correlate with low relative lung weights and low plasma T3 concentrations in High Ht birds. 5. Within the groups of selected birds, there was an effect of stock on ascitic symptoms (arterial pressure index, relative heart weight and T3 values) at week 5. This is consistent with a higher ascites mortality in flock A than in flock B, over the whole period up to 6 weeks. Until 4 weeks both A and B were similar with respect to mean Ht values of High Ht and Low Ht groups. After 4 weeks Ht values were very high in the High Ht birds of stock A, possibly as a result of the susceptibility for ascites of stock A. There was a significant interaction between stock and Ht values on the ascites-related variable arterial pressure index. Only in stock A was high arterial pressure at 5 weeks related to high Ht values at d 12. 6. Overall it was concluded that a high Ht in blood of juvenile chickens has limited practical value in predicting incidence of ascites at 5 weeks of age.

Haematological characteristics predicting susceptibility for ascites. 2. High haematocrit values in juvenile chickens

1. Male broilers of two different genetic stocks, a pure broiler sire line (A) and commercially available Ross broilers (B), were used to study the effect of differences in haematocrit values in 12-d-old chickens on the development of ascitic symptoms. Body weight gain (BWG), feed intake (FI) and feed conversion ratio (FRC) were measured from 2 to 5 weeks of age. Mortality was recorded from 2 to 6 weeks. The birds were kept at a low ambient temperature to stimulate the incidence of ascites. 2. From each stock 32 birds with the highest (High Ht) and 32 birds with the lowest (Low Ht) haematocrit values in venous blood were selected at 12 d of age for an examination on susceptibility for the ascites syndrome at 5 weeks. 3. At 2 weeks 448 birds per stock (including High Ht and Low Ht birds) were allotted to 32 floor pens (1 High Ht and 1 Low Ht in each pen). Venous blood samples were taken weekly in High Ht and Low Ht birds for haematocrit measurements and at week 5 also thyroid hormone (T3, T4) concentrations in plasma were measured. At 5 weeks all High Ht and Low Ht birds were examined post-mortem and arterial pressure index, heart weight, lung weight and liver weight (all relative to body weight) were recorded. High values for arterial pressure index, relative heart and liver weights and low values for relative lung weights and plasma thyroid hormone were considered to be indicators for ascites susceptibility. 4. Stock A, with the highest ascites mortality, had lower values for BWG and FCR than stock B. Throughout, a clear difference in Ht values between High Ht and Low Ht birds was maintained. No significant effects of 12-d haematocrit values were found on the incidence of ascitic symptoms until week 5. High Ht values tended to correlate with low relative lung weights and low plasma T3 concentrations in High Ht birds. 5. Within the groups of selected birds, there was an effect of stock on ascitic symptoms (arterial pressure index, relative heart weight and T3 values) at week 5. This is consistent with a higher ascites mortality in flock A than in flock B, over the whole period up to 6 weeks. Until 4 weeks both A and B were similar with respect to mean Ht values of High Ht and Low Ht groups. After 4 weeks Ht values were very high in the High Ht birds of stock A, possibly as a result of the susceptibility for ascites of stock A. There was a significant interaction between stock and Ht values on the ascites-related variable arterial pressure index. Only in stock A was high arterial pressure at 5 weeks related to high Ht values at d 12. 6. Overall it was concluded that a high Ht in blood of juvenile chickens has limited practical value in predicting incidence of ascites at 5 weeks of age.

The effects of conjugated linoleic acids or an alpha-glucosidase inhibitor on tissue lipid concentrations and fatty acid composition of broiler chicks fed a low-protein diet

The effects of dietary conjugated linoleic acid (CLA) and an alpha-glucosidase inhibitor--Bay g 5421--on tissue lipid concentrations and fatty acid composition were investigated in male broiler chicks fed a low-protein diet. The trial comprised six isoenergetic broiler diets. One diet (high-protein diet) was used as a control diet and contained 230 g crude protein per kilogram. The other five diets were low-protein diets (180 g crude protein/kg) without additional supplement, or supplemented with 20 or 40 g/kg of a CLA-enriched oil, or supplemented with 50 or 100 mg of Bay g 5421/kg. Broilers fed the low-protein diets had higher concentrations of triglycerides and cholesterol in the liver than broilers fed the high-protein diet. Broilers fed low-protein diets supplemented with CLA had a higher cholesterol concentration in plasma, lower hepatic triglyceride concentrations, higher relative concentrations of saturated fatty acids (SFA), and lower relative concentrations of monounsaturated fatty acids (MUFA) in lipids of liver, thigh muscle, and adipose than broilers fed low-protein diets without CLA supplement. Broilers fed the low-protein diets supplemented with Bay g 5421 had lower concentrations of triglycerides and cholesterol in the liver and lower concentrations of triglycerides in plasma than broilers fed the low-protein diet without supplementation. Moreover, broilers fed diets with Bay g 5421 had higher relative concentrations of polyunsaturated fatty acids and lower relative concentrations of MUFA in lipids of liver, muscle, and adipose tissue. The results show that supplementation of the low-protein diets with CLA and Bay g 5421 affected tissue lipid concentrations and fatty acid composition in broilers, which are of practical relevance with regards to meat quality.

Release of bound aromatic hydrocarbons from late Archean and Mesoproterozoic kerogens via hydropyrolysis

Hydrogen-lean kerogens (atomic H/C<0.4) isolated from the 2.5-billion-year-old (Ga) Mt. McRae Shale, Hamersley Group, at Tom Price, Western Australia, were studied via hydropyrolysis, a continuous-flow technique that degrades organic matter in a stream of high-pressure hydrogen assisted by a dispersed Mo catalyst. The hydropyrolysates yielded predominantly phenanthrene and pyrene, and higher polyaromatic hydrocarbons and alkylated homologues were generated in low relative concentrations. Saturated hydrocarbons were not detected. The molecular and carbon isotopic compositions of the hydropyrolysates are very similar to aromatic hydrocarbons obtained by solvent extraction of the host rocks. Because molecular structures covalently attached to kerogen are unaffected by contamination, this indicates that both the bound and extractable aromatic fractions are syngenetic with the host rocks. Therefore, the results of the hydropyrolysis experiments provide compelling evidence for preserved bitumen of Archean age. The very high proportion of nonalkylated polyaromatic hydrocarbons in the hydropyrolysates is consistent with hydrothermal dehydrogenation of the kerogen, and a marked concentration difference of pyrene in rock extracts and hydropyrolysates might be explained by hydrothermal redistribution of the bitumen. The kerogen and bitumen composition is therefore consistent with models suggesting a hydrothermal origin for the giant iron ore deposits at Mt. Tom Price. Comparison of the Archean samples with hydropyrolysates from immature Mesoproterozoic kerogens from the Roper Group, McArthur Basin, Northern Territory, and with pyrolysis experiments on Proterozoic kerogens in the literature suggests that Precambrian kerogens are frequently highly aromatic and lipid-poor regardless of their degree of thermal preservation.

Surface modifications of carbon nanotube/polyacrylonitrile composite films by proton beams

Chemical and structural modifications induced by protons at the surface of polyacrylonitrile and carbon nanotube/polyacrylonitrile composite films are analysed by TEM, profilometry, IR and XPS. Changes induced in the film are related to the energy of incident particle and implanted dose. The irradiation of the composite films induces the emergence of the nanotubes outside of the polymer but with no increase of the surface roughness. Spectroscopic modifications in both films are globally similar, the most significant feature being a lower relative concentration of nitrogen with respect to carbon close to the surface.

Observations of 2,4,6-trichlorophenol degradation by ozone

The aqueous reactivity of 2,4,6-trichlorophenol (TCP) with ozone has been studied at laboratory-scale using a simple gas bubble/liquid contacting system. Degradation rate constants were measured directly and found to be 7.6 and 77.2 M −1 s −1 at pH 2 and 7.5, respectively. At pH 7.5, 10 min of ozonation (≡15 mM ozone consumption) achieved a 90% degradation of TCP, which corresponded to the release of approximately 2 mol Cl − per mol TCP. The presence of hydrogen peroxide in solution did not significantly increase the TCP degradation but increased the overall dechlorination to 2.7 mol Cl − per mol TCP. The presence of humic acid (HA) in solution was found to enhance the degradation rate of TCP at low relative HA concentrations (<0.6 g/g HA:TCP), but to reduce the rate at higher HA concentrations.

Messenger RNAs encoding the beta subunits of guinea pig (Cavia porcellus) luteinizing hormone (gpLH) and putative chorionic gonadotropin (gpCG) are transcribed from a single-copy gpLH/CGbeta gene.

Neither gene locus nor gene sequence characterizations have been reported for the beta subunits of guinea pig (gp) LH and putative gp chorionic gonadotropin (CG). Descriptions of this locus would allow comparison with functionally relevant molecular genetic features of other species' homologous loci including the single-copy equid LH/CGbeta gene and the primate LHbeta-CGbeta gene cluster locus. Contiguous cDNA and genomic DNA fragments spanning the entire mature coding sequence of gpLHbeta mRNA, gpCGbeta mRNA and a homologous gpLH/CGbeta gene were amplified using PCR methodologies. With the exception of one silent mutation, the two cDNA and the genomic sequences were identical where they overlapped. Comparison of guinea pig coding sequence with LHbeta, CGbeta and LH/CGbeta sequences of other vertebrate species revealed the following order of similarity expressed as per cent coding sequence identity: rhinoceros LHbeta (83.6%)>pig LHbeta (81.8%)>donkey LH/CGbeta=bovine LHbeta (81.5%)> horse LH/CGbeta (80.6%)>dog LHbeta (79.7%)>human LHbeta (78.2%)>rat LHbeta (77.9%)>human CGbeta (75.8%)>turkey LHbeta (52.7%); values that are generally consistent with recently postulated phylogenetic relationships. Like the consensus mammalian LHbeta gene, the 5'-flanking region of the gpLH/CGbeta gene contains a single TATA sequence 37 bp upstream of the translation start codon. The first in-frame stop codon occurred at codon position +122 which is consistent with the 121 amino acid residue length of the consensus mammalian mature LHbeta peptide. To estimate gene copy number, full-length gpLHbeta cDNA was radiolabeled and hybridized to Southern blots of guinea pig genomic DNA digested with a panel of six restriction endonucleases. The resulting simple hybridization pattern strongly suggested that there is a single-copy gpLH/CGbeta gene. Northern analysis of total pituitary RNA using the same probe indicated that gpLHbeta transcript size is indistinguishable from that of consensus mammalian pituitary LHbeta mRNAs ( approximately 750 nucleotides). Despite amplifying gpCGbeta from placental RNA, positive signal was not detected in Northern blot lanes containing guinea pig total RNA prepared from placentae collected at three gestational ages (17.3 days, 24.3 days and 68 days (term)). Other data suggest that inability to detect Northern blot signal could have been due to low relative tissue concentrations of gpCGbeta transcript and/or sampling at gestational time-points that missed peak periods of mRNA expression. We conclude that, with respect to gene copy number, coding sequence and pituitary mRNA size, the gpLH/CGbeta gene locus reflects the CTP-less consensus mammalian LHbeta condition. However, based on the capacity of this single-copy gene to express in both pituitary and placental tissues, gpLH/CGbeta also exhibits functional similarities with the single-copy equine LH/CGbeta locus.

Site-Selective Nitration of Tyrosine in Human Serum Albumin by Peroxynitrite

Peroxynitrite, which is formed in biological systems by the reaction of nitric oxide with superoxide anion, is a highly reactive molecule that can lead to cell injury or cell death. Reactions of peroxynitrite under physiological conditions include nitration of tyrosine-containing proteins or peptides, and we have been investigating the behavior of human serum albumin following exposure to peroxynitrite. Peroxynitrite, at relative concentrations ranging from 0.2 to 50 with respect to protein, was added to human serum albumin in buffer at pH 7.2. The resulting mixtures were dialyzed to remove small molecules, dried under vacuum, and then digested with trypsin. The digests were analyzed by high performance liquid chromatography with UV detection at 230 and 354 nm, the latter wavelength being selective for nitrotyrosine. At the higher relative concentrations of peroxynitrite, the 354-nm chromatograms contained a large number of peaks, including at least nine with molecular weights corresponding to nitration of nominal tryptic peptides. Following treatment with the lower relative concentrations of peroxynitrite, however, the 354-nm chromatograms were dominated by only two nitrated peptides; these were identified by comparison of LC retention times and collision-induced decomposition mass spectra as nitro-Y411TK413 and nitro-Y138LYEIAR144. Each of these tyrosines resides in a known reactive site within the protein, i.e., subdomains IIIA and IB, respectively.

Near-Infrared Two-Photon Excitation of Protoporphyrin IX: Photodynamics and Photoproduct Generation¶

The spectroscopy and photochemistry of protoporphyrin IX in ethanol and in Triton X-100 micelle solution have been examined using near-infrared two-photon excitation (TPE). TPE will allow photodynamic therapy with highly localized light dosage. We have determined that the photochemistry subsequent to TPE is very similar to that found for one-photon excitation. Moreover, the photoproducts observed possess very intense TPE fluorescence spectra, which allows their detection at low relative concentrations.

The Effect of Etchant Composition on Film Structure during Laser-Assisted Porous Si Growth

The structure of porous silicon films created by laser-assisted etching in aqueous fluoride solutions is shown to depend on the solution composition as well as the laser intensity. Simultaneous porous silicon growth and deposition of hexafluorosilicates has been observed for K+, Rb+ and Cs+ containing solutions, whereas little to no deposition occurs in Na+ and NH4+ containing solutions. Pore size decreases with higher laser intensity and high relative concentrations of HF to HF—2, while larger pores are formed at low laser intensity and high relative concentrations of HF—2 to HF.

Abiotic Behaviour of Organic Micropollutants in Soils and the Aquatic Environment. A Review: I. Partitioning

Recent research has confirmed the significance of organic carbon (OC) as the key sorbent for hydrophobic organic chemicals (HOC), as well as for many polar compounds. However, the triazine herbicides exhibit a variable affinity for soil organic matter (SOM) which is attributed to the extent of humification of the organic fraction. Charge transfer mechanisms are important for triazine sorption to OC and either proton or electron transfer may account for the reaction mechanism with humic acids. For many uron herbicides (e.g. chlorotoluron, metabromuron, chloroxuron, defenoxuron), sorption correlates with SOM. However, specific interactions between uron herbicides and a limited quantity of active constituents within SOM have also been proposed to explain deviations from sorption linearity at low herbicide relative concentration. Other studies indicate that isoproturon sorbs to organic colloids in solution and that a sorption threshold to SOM may be operative. Below the threshold, isoproturon appears to sorb predominantly to clays, indicating the presence of a limited number of ‘active’ sorptive sites within clay minerals. Research suggests that pesticide interactions with clay minerals may be influenced by near-surface clay geometry; the accessibility of the sorbing region of the sorbate to the active site of the clay; the identity of exchangeable cations on the clay and solution electrolytes. These recent studies indicate that interactions between micropollutants and soils and sediments often need to be evaluated on a compound-specific basis. This is especially the case for polar compounds which may partition to these environmental phases by diverse mechanisms.

Expansion of the Porous Solid Na2Zn3[Fe(CN)6]2·9H2O: Enhanced Ion-Exchange Capacity in Na2Zn3[Re6Se8(CN)6]2·24H2O

A technique for increasing the porosity in solid structures by replacing octahedral metal ions with hexanuclear cluster cores is extended with the expansion of the ion-exchange material Na2Zn3[Fe(CN)6]2·9H2O. At high relative cluster concentrations, the reaction between [Zn(H2O)6]2+ and Na4[Re6Se8(CN)6] produces Na2Zn3[Re6Se8(CN)6]2·24H2O (1), a compound exhibiting a porous three-dimensional framework isotypic with that of Na2Zn3[Fe(CN)6]2·9H2O. Its framework is characterized by hexagonal bipyramidal cages, each enclosing two Na+ ions and 24 water molecules in a volume of 1340 Å3more than triple the volume of the cages in the original structure. The expanded cavities and framework openings in compound 1 are shown to facilitate absorption of larger cationic complexes, specifically [M(H2O)6]n+ (M = Mg2+, Cr3+, Mn2+, Ni2+, Zn2+) and [Cr(en)3]3+, via exchange for the Na+ ions. Indeed, when the preparation of 1 is attempted using a slight excess of the zinc reactant, [Zn(H2O)6]2+ incorporates instead of Na+, leading to direct formation of the ion-exchanged compound [Zn(H2O)6]Zn3[Re6Se8(CN)6]2·18H2O (2). The crystal structure of 2 reveals the [Zn(H2O)6]2+ complexes to reside at the exact center of the hexagonal bipyramidal cages. With prolonged exposure to air, compound 1 and its ion-exchanged variants undergo a color change from orange to green, which is attributed to the one-electron oxidation of the [Re6Se8]2+ cluster cores in the solid framework. In a further parallel with ferrocyanide chemistry, the reaction between [Zn(H2O)6]2+ and Na4[Re6Se8(CN)6] at low relative cluster concentrations is found to yield [Zn(H2O)]2[Re6Se8CN)6]·13H2O (3), a phase exhibiting a two-dimensional framework structure isotypic with that of [Zn(H2O)]2[Fe(CN)6]·0.5H2O.