Principal Transport Mechanism Research Articles

The cover thickness design of concrete structures subjected to chloride ingress from RBDO solution technique

Abstract: Diffusion is the principal transport mechanism of chloride ions into concrete pores. The chlorides trigger the reinforcements’ depassivation when its concentration at the concrete/reinforcement interface reaches the threshold level. Thus, the depassivation defines the initiation stage end and the propagation stage start. The structural safety reduces widely during the propagation stage because of the various deleterious mechanisms triggered by reinforcement’s corrosion. Therefore, the engineers should accurately predict and prevent the propagation stage start. The literature describes several models for evaluating the end of the initiation stage. However, few of them applies the Boundary Element Method (BEM) for this purpose, despite its known accuracy. Besides, enormous randomness affect the phenomenon. Thus, it is adequately handled solely in the probabilistic context. Optimisation techniques may be coupled in the problem modelling to propose adequate cover thickness values accounting for probabilities of failure. This study presents a Reliability-Based Design Optimisation (RBDO) approach for designing accurately the cover thickness of concrete structures subjected to chloride ingress. The BEM handles the diffusion modelling whereas the Monte Carlo simulation assesses the probabilities of failure. The RBDO is formulated in the context of Weighted Average Simulation Method (WASM), which requires only one assessment of the reliability analysis. It leads to a reliable and computationally efficient solution technique. The problem formulation and the implemented solution scheme are described herein. Moreover, one application is presented, in which the design results are interpreted properly.

Sediment provenance, sediment-dispersal systems, and major arc-magmatic events recorded in the Mexican foreland basin, North-Central and Northeastern Mexico

ABSTRACTThe Late Cretaceous-Paleogene Mexican foreland basin (MFB), defined herein, represents the southern continuation of the late Mesozoic Cordilleran foreland basin. Sandstone petrography, new detrital-zircon (DZ) U-Pb geochronology, and paleocurrent data indicate that much of the sedimentary fill of the basin was derived from an active Late Cretaceous-Paleogene magmatic arc, termed here the Mexican Cordilleran arc, on the western continental margin of Mexico. The oldest known strata of the proximal foreland basin in the Mesa Central consist of Cenomanian-Turonian turbidites. Sampled sandstones are compositional volcanic litharenites with abundant neovolcanic grains and a dominant, approximately syndepositional DZ age group ranging ~98–92 Ma that records a major magmatic event in the Mexican Cordilleran arc. Santonian-Campanian strata in the distal MFB consist of carbonate pelagites with abundant interbedded tuffs and tuffaceous sandstones. Represented by the Caracol and San Felipe formations deposited in the forebulge and back-bulge depozones, respectively, these strata form an arcuate outcrop belt ~700 km in length. DZ ages ranging ~85–74 Ma in the arc-derived tuffaceous strata record a second prominent magmatic event.Two principal transport mechanisms delivered volcanogenic sediment to the MFB from multiple, simultaneously active arc sources during Late Cretaceous time: (1) Cenomanian-Turonian east-directed transverse fluvial systems transported volcanic-lithic sand rich in young zircon grains; and (2) airborne ash clouds transported Santonian-Campanian zircon grains to the distal foreland basin in prevailing Late Cretaceous northwesterly winds. Axial transport of sediment derived from active arc sources, Proterozoic basement and derivative sedimentary rocks in northwestern Mexico, in addition to transverse transport from the thrust orogen itself, represents a younger sediment-routing system, modified by advance of the foreland fold-thrust belt, to the Maastrichtian-Paleogene foreland of northeastern Mexico.

Concentrations of Herbicides in Wetlands on Organic and Minimum-Tillage Farms.

Wetlands are abundant throughout the agricultural landscape of central Saskatchewan, Canada, and the biota present in these wetlands may be vulnerable to the toxic effects of pesticides used on nearby crops. We hypothesized that herbicide concentrations would be higher in wetlands on minimum-tillage farms than on organic (no herbicide use) farms, and that the principal transport mechanisms of runoff versus atmospheric deposition could be identified based on the concentrations in these two wetland types. To test these hypotheses, 29 herbicides were monitored for 5 yr in 16 wetlands on minimum-tillage farms and in seven wetlands on organic farms. Twenty herbicides were detected in wetlands on minimum-tillage farms versus 12 in wetlands on organic farms. Clopyralid, MCPA, 2,4-D, bromoxynil, dichlorprop, and dicamba were detected at a >50% frequency in wetlands on both minimum-tillage and organic farms. Concentrations of clopyralid were significantly higher in wetlands on minimum-tillage farms than in those on organic farms, whereas no significant difference was observed for any of the other five herbicides. Glyphosate, including its degradation product AMPA, was detected in >50% frequency only in wetlands on minimum-tillage farms where the mean concentration (1278 ng L) was higher than the concentration of other herbicides. Mass applied, vapor pressure, and water solubility were important determinants of herbicide concentrations and detections in wetlands. Herbicide concentrations in all but two samples were less than their respective Canadian guideline for protection of aquatic life, suggesting that, overall, individual herbicide concentrations in the wetlands were not toxic to biota.

Direct assessment of substrate binding to the Neurotransmitter:Sodium Symporter LeuT by solid state NMR.

The Neurotransmitter:Sodium Symporters (NSSs) represent an important class of proteins mediating sodium-dependent uptake of neurotransmitters from the extracellular space. The substrate binding stoichiometry of the bacterial NSS protein, LeuT, and thus the principal transport mechanism, has been heavily debated. Here we used solid state NMR to specifically characterize the bound leucine ligand and probe the number of binding sites in LeuT. We were able to produce high-quality NMR spectra of substrate bound to microcrystalline LeuT samples and identify one set of sodium-dependent substrate-specific chemical shifts. Furthermore, our data show that the binding site mutants F253A and L400S, which probe the major S1 binding site and the proposed S2 binding site, respectively, retain sodium-dependent substrate binding in the S1 site similar to the wild-type protein. We conclude that under our experimental conditions there is only one detectable leucine molecule bound to LeuT.

Chemistry in motion: tiny synthetic motors.

CONSPECTUS: Diffusion is the principal transport mechanism that controls the motion of solute molecules and other species in solution; however, the random walk process that underlies diffusion is slow and often nonspecific. Although diffusion is an essential mechanism for transport in the biological realm, biological systems have devised more efficient transport mechanisms using molecular motors. Most biological motors utilize some form of chemical energy derived from their surroundings to induce conformational changes in order to carry out specific functions. These small molecular motors operate in the presence of strong thermal fluctuations and in the regime of low Reynolds numbers, where viscous forces dominate inertial forces. Thus, their dynamical behavior is fundamentally different from that of macroscopic motors, and different mechanisms are responsible for the production of useful mechanical motion. There is no reason why our interest should be confined to the small motors that occur naturally in biological systems. Recently, micron and nanoscale motors that use chemical energy to produce directed motion by a number of different mechanisms have been made in the laboratory. These small synthetic motors also experience strong thermal fluctuations and operate in regimes where viscous forces dominate. Potentially, these motors could be directed to perform different transport tasks, analogous to those of biological motors, for both in vivo and in vitro applications. Although some synthetic motors execute conformational changes to effect motion, the majority do not, and, instead, they use other mechanisms to convert chemical energy into directed motion. In this Account, we describe how synthetic motors that operate by self-diffusiophoresis make use of a self-generated concentration gradient to drive motor motion. A description of propulsion by self-diffusiophoresis is presented for Janus particle motors comprising catalytic and noncatalytic faces. The properties of the dynamics of chemically powered motors are illustrated by presenting the results of particle-based simulations of sphere-dimer motors constructed from linked catalytic and noncatalytic spheres. The geometries of both Janus and sphere-dimer motors with asymmetric catalytic activity support the formation of concentration gradients around the motors. Because directed motion can occur only when the system is not in equilibrium, the nature of the environment and the role it plays in motor dynamics are described. Rotational Brownian motion also acts to limit directed motion, and it has especially strong effects for very small motors. We address the following question: how small can motors be and still exhibit effects due to propulsion, even if only to enhance diffusion? Synthetic motors have the potential to transform the manner in which chemical dynamical processes are carried out for a wide range of applications.

Separation of CO2 and N2 with a lithium-modified silicalite-1 zeolite membrane

Carbon dioxide capture and storage is gaining attention as a strategy to abate greenhouse gas emissions. However, currently available commercial processes to remove CO2 from flue gas streams, mainly from N2 are costly and energy intensive. A silicalite-1 membrane was prepared on a porous alumina support using hydrothermal synthesis. The resulting silicalite-1 membranes were modified using Li solutions in order to achieve better CO2 separation. SEM micrographs exhibited a thin, uniform and compact silicalite-1 film well adhered to the alumina support. Crystallinity of the membrane was confirmed by XRD. CO2 and N2 permeation experiments were carried out in a stainless steel module. CO2 single gas permeation was measured through a silicalite-1 membrane from room temperature to 400°C, having a 25psi pressure on the feed side. The resulting CO2 permeance measurements indicated molecular sieving as the principal transport mechanism for the unmodified silicalite-1 membrane, whereas surface diffusion is the principal transport mechanism for lithium-modified silicalite-1 membranes. The separation between CO2 and N2 takes place due to preferential CO2 adsorption and diffusion, which in turn hinders N2 permeation through the silicalite-1 pore network. A CO2/N2 separation factor up to 6 at 25psi and 400°C was obtained using the silicalite-1 modified membrane, whereas this was 1.46 using the unmodified silicalite-1 membrane.

Analysis of turbulent transport and mixing in transitional Rayleigh–Taylor unstable flow using direct numerical simulation data

Data from a 1152×760×1280 direct numerical simulation (DNS) [N. J. Mueschke and O. Schilling, “Investigation of Rayleigh–Taylor turbulence and mixing using direct numerical simulation with experimentally measured initial conditions. I. Comparison to experimental data,” Phys. Fluids 21, 014106 (2009)] of a transitional Rayleigh–Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipation and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh–Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. These results are useful for identifying the dynamically important terms requiring closure, and assessing the accuracy of the predictions of Reynolds-averaged Navier–Stokes and large-eddy simulation models of turbulent transport and mixing in transitional Rayleigh–Taylor instability-generated flow.

Gas hydrates in shallow deposits of the Amsterdam mud volcano, Anaximander Mountains, Northeastern Mediterranean Sea

We investigated gas hydrate in situ inventories as well as the composition and principal transport mechanisms of fluids expelled at the Amsterdam mud volcano (AMV; 2,025 m water depth) in the Eastern Mediterranean Sea. Pressure coring (the only technique preventing hydrates from decomposition during recovery) was used for the quantification of light hydrocarbons in near-surface deposits. The cores (up to 2.5 m in length) were retrieved with an autoclave piston corer, and served for analyses of gas quantities and compositions, and pore-water chemistry. For comparison, gravity cores from sites at the summit and beyond the AMV were analyzed. A prevalence of thermogenic light hydrocarbons was inferred from average C1/C2+ ratios <35 and δ13C-CH4 values of −50.6‰. Gas venting from the seafloor indicated methane oversaturation, and volumetric gas–sediment ratios of up to 17.0 in pressure cores taken from the center demonstrated hydrate presence at the time of sampling. Relative enrichments in ethane, propane, and iso-butane in gas released from pressure cores, and from an intact hydrate piece compared to venting gas suggest incipient crystallization of hydrate structure II (sII). Nonetheless, the co-existence of sI hydrate can not be excluded from our dataset. Hydrates fill up to 16.7% of pore volume within the sediment interval between the base of the sulfate zone and the maximum sampling depth at the summit. The concave-down shapes of pore-water concentration profiles recorded in the center indicate the influence of upward-directed advection of low-salinity fluids/fluidized mud. Furthermore, the SO42− and Ba2+ pore-water profiles in the central part of the AMV demonstrate that sulfate reduction driven by the anaerobic oxidation of methane is complete at depths between 30 cm and 70 cm below seafloor. Our results indicate that methane oversaturation, high hydrostatic pressure, and elevated pore-water activity caused by low salinity promote fixing of considerable proportions of light hydrocarbons in shallow hydrates even at the summit of the AMV, and possibly also of other MVs in the region. Depending on their crystallographic structure, however, hydrates will already decompose and release hydrocarbon masses if sediment temperatures exceed ca. 19.3°C and 21.0°C, respectively. Based on observations from other mud volcanoes, the common occurrence of such temperatures induced by heat flux from below into the immediate subsurface appears likely for the AMV.

Moisture sources for Central America: Identification of moisture sources using a Lagrangian analysis technique

We herein present an analysis of the sources of atmospheric moisture for Central America using a Lagrangian technique. The results of backward and forward moisture tracking analysis using the FLEXPART model has enabled the identification of the main sources of moisture that reach Central America, as well as an evaluation of their spatial evolution during their passage toward the region of interest. Data from the European Center for Medium‐Range Weather Forecasts (ECMWF) for a 5 year period (2000–2004) were used as input for the FLEXPART model. The applied method reproduces the variations in the location of the Intertropical Convergence Zone (ITCZ) over the study area very well. The primary source of moisture for Central America is identified over the Caribbean Sea, and a secondary source appears to exist near the equatorial Pacific region. The dominance of the Caribbean Sea region as a source of moisture for this region is clear, as is the importance of the Caribbean Low‐Level Jet (CLLJ) as the principal transport mechanism. These characteristics are confirmed by inspection of the moisture transport patterns and their seasonal behavior.

Induction of cells differentiation and ABC transporters expression by a myco-estrogen, zearalenone, in human choriocarcinoma cell line (BeWo)

The mycotoxin zearalenone, produced by Fusarium species, is a worldwide contaminant of concern in cereals and other plant products. Due to its estrogenic activity, zearalenone (ZEA) is known to have toxicological effect in animals on reproductive system and the placental transfer of ZEA was suggested by in vivo studies. Although passive diffusion is the principal transport mechanism across the placenta, several carrier-mediated transport protein such as ABC transporter (P-gp, MRP1, MRP2, BCRP) have been identified in the placenta. In this work, we have investigated the effect of ZEA on trophoblast differentiation and ABC transporter expression by using an in vitro model of transplacental barrier, the BeWo cell line. In the presence of 10 μM ZEA morphological (syncytium formation) and biochemical (hCG secretion) differentiation of BeWo cells were observed after a 48 h exposure. Results were compared to 17β-estradiol (E2) and an inducer of syncytialisation (forskolin). The influence of cell differentiation and ZEA exposure on expression profiles of major ABC transporters was investigated in BeWo cells: expression of mRNA MRP1, MRP2 and BCRP was induced after 24 h of ZEA exposure. Induction of P-gp, MRP1, and MRP2 protein was observed after 48 h of ZEA exposure. Similar results were obtained after forskolin exposure. Our study reported for the first time the implication of a food contaminant in biological effect and ABC transporter expression modulation in human choriocarcinoma cells.

Improved interpretation of in-diffusion measurements with confined swelling clays

Diffusion is considered the principal transport mechanism of radio-nuclides and other low-molecular-weight pollutants in compacted clays used as barriers at various disposal and storage sites, for example, at projected deep repositories for radioactive waste. Porous filters are routinely used to confine swelling clays in diffusion studies of radio-tracers. The presence of the filter gives rise to considerable mass-transfer limitations at the clay boundary that result in erroneous diffusion parameters. We have solved the problem of in-diffusion with due account for this phenomenon by means of Fourier transforms. By using literature data on the in-diffusion of traces of radioactive cesium in an argillaceous rock (Opalinus clay) and a compacted bentonite (FEBEX bentonite), we have demonstrated that taking into account the mass-transfer limitations considerably improves the quality of the theoretical fit of the time evolution of radio-tracer concentration in the reservoir. Besides that, we have shown that ignoring the mass-transfer limitations leads to a noticeable underestimation of both the effective diffusion coefficient and the specific sorption capacity of the clay.

Transdermal Delivery of Cytochrome C—A 12.4 kDa Protein—Across Intact Skin by Constant–Current Iontophoresis

To demonstrate the transdermal iontophoretic delivery of a small (12.4 kDa) protein across intact skin. The iontophoretic transport of Cytochrome c (Cyt c) across porcine ear skin in vitro was investigated and quantified by HPLC. The effect of protein concentration (0.35 and 0.7 mM), current density (0.15, 0.3 or 0.5 mA.cm(-2) applied for 8 h) and competing ions was evaluated. Co-iontophoresis of acetaminophen was employed to quantify the respective contributions of electromigration (EM) and electroosmosis (EO). The data confirmed the transdermal iontophoretic delivery of intact Cyt c. Electromigration was the principal transport mechanism, accounting for approximately 90% of delivery; correlation between EM flux and electrophoretic mobility was consistent with earlier results using small molecules. Modest EO inhibition was observed at 0.5 mA.cm(-2). Cumulative permeation at 0.3 and 0.5 mA.cm(-2) was significantly greater than that at 0.15 mA.cm(-2); fluxes using 0.35 and 0.7 mM Cyt c in the absence of competing ions (J ( tot ) = 182.8 +/- 56.8 and 265.2 +/- 149.1 microg.cm(-2).h(-1), respectively) were statistically equivalent. Formulation in PBS (pH 8.2) confirmed the impact of competing charge carriers; inclusion of approximately 170 mM Na(+) resulted in a 3.9-fold decrease in total flux. Significant amounts ( approximately 0.9 mg.cm(-2) over 8 h) of Cyt c were delivered non-invasively across intact skin by transdermal electrotransport.

Boson-Controlled Quantum Transport

We study the interplay of collective dynamics and damping in the presence of correlations and bosonic fluctuations within the framework of a newly proposed model, which captures the principal transport mechanisms that apply to a variety of physical systems. We establish close connections to the transport of lattice and spin polarons, or the dynamics of a particle coupled to a bath. We analyze the model by exactly calculating the optical conductivity, Drude weight, spectral functions, ground state dispersion and particle-boson correlation functions for a 1D infinite system.

Contributions of electromigration and electroosmosis to peptide iontophoresis across intact and impaired skin

d-(Arg)-Kyotorphin iontophoresis was investigated across intact and impaired skins in vitro. Iontophoretic flux increased from 68±12 to 538±116 nmol cm−2 h−1 when the peptide concentration in the anodal compartment was raised from 5 to 40 mM. Electromigration was the principal transport mechanism, accounting for ∼70% of total peptide delivery. Reducing the number of competing ions in the formulation significantly increased iontophoretic flux but did not affect convective solvent flow. The latter was independent of peptide concentration indicating that skin permselectivity was not modified by kyotorphin transport. Total iontophoretic flux was unaffected when the stratum corneum was removed by tape-stripping (146±34 versus 150±26 nmol cm−2 h−1). However, the contributions of the different transport mechanisms were significantly altered: (i) electromigration decreased, as more of the charge was carried by anions from the sub-dermal milieu; (ii) electroosmosis was absent; and (iii) passive permeation increased significantly. Transport rates across intact and impaired skin barriers were statistically indistinguishable when the donor electrolyte composition was modified; increased competition from anions was mitigated by the decreased Na+ levels in the formulation. Removal of Cl− ions from the receiver phase further increased peptide delivery, and also increased anodal electroosmosis.

Saturable Absorption of Glycerol in the Rat Intestine

The permeability of glycerol, a small hydrophilic solute, across the intestinal membrane would be low, if passive diffusion restricted to the paracellular route is the principal transport mechanism as generally assumed for this class of solutes. However, in the present study using a closed loop of rat small intestine in situ, we found that the absorption of glycerol was faster than that of urea, a probe solute widely assumed to permeate exclusively via the paracellular route. This finding is inconsistent with the paracellular permeation hypothesis, which predicts that the absorption of glycerol, which is larger than urea in terms of molecular size, could not be faster than that of urea. We also found that glycerol absorption was saturable. These findings suggest the involvement of carrier-mediated transport in intestinal glycerol absorption. Glycerol absorption in the colon was also saturable, suggesting the involvement of carrier-mediated transport, although it was much slower than that in the small intestine. Carrier-mediated glycerol transport might play an important role in absorbing glycerol liberated from dietary triglyceride. It would be interesting to further examine the possibility that a carrier-mediated glycerol transport system (or systems) might be involved in drug absorption and also that it might be utilized for oral drug delivery.

New and rare tropical and subtropical fishes from northern New Zealand

Five fish species (Platax teira, Abudefduf vaigiensis, Chromis flavomaculata, Coris dorsomacula, and Aluterus scriptus) are recorded from New Zealand for the first time, and five species (Forcipiger flavissimus, Chromis vanderbilti, Chrysiptera rapanui, Parma kermadecensis, and Thalassoma lutescens) are newly recorded from mainland New Zealand. Five other species that have been reported previously in the popular press, or by name only, are confirmed as members of the New Zealand fauna (Carcharhinus longimanus, Etelis carbunculus, Istiophorus platypterus, Acanthocybium solandri, and Aseraggodes bahamondei). New observations of 20 previously reported tropical and subtropical species are also presented. Many of the fishes were observed in autumn 1996, 1998, and 1999, indicating recruitment or immigration when sea surface temperature (SST) was higher than average. Major influxes of tropical and subtropical fishes apparently occur during warm summers, although some warm periods were not accompanied by influxes, and a few tropical and subtropical fishes arrived when SST was average or below average. The most likely upstream source of reef fish larvae is Norfolk Island, and the principal transport mechanism is probably the East Auckland Current. The discovery in north‐eastern New Zealand of single individuals of Parma kermadecensis and the yellow form of Chrysiptera rapanui, both previously regarded as Kermadec Islands endemics, provides evidence for occasional larval transport from the Kermadecs to north‐eastern New Zealand.

Conductivity of n-doped polyacetylene : dependence on doping level and temperature

In order to perform reliable and reproducible conductivity versus temperature measurements on n-doped (CH) x , we have designed and built a special electrochemical cell which allows the doping (or dedoping) process and the temperature variation to be made alternatively.Using an oriented film (obtained by the liquid crystal technique) and for the maximum concentration (16%) of K dopant, we have measured a room temperature conductivity of 10 4 S/cm. This conductivity decreases by less than 30% when T is decreased down to liquid helium temperature. We have also investigated, in the same cell, lower concentrations during doping and dedoping cycles. We present preliminary results showing a double network of curves σ = f(T) with y constant and σ = f(y) with T constant. At each doping level, we can correlate our results with the structural evolution of the system and its electronic properties (8) by measuring the open circuit voltage. Our principal aim is to study the possibility of a correlation between the variation σ(y) (i.e. the crystalline and electronic structure) and σ(T) (i.e. the principal transport mechanism).These results are discussed in the light of existing theoretical models such as: i) fluctuation induced tunneling, ii) variable range hopping, iii) transport by activation to a mobility edge, iv) weak localization in disordered metals. The last cannot be used to fit our data. The others can be used in specific doping level ranges (y 6%).We will also discuss differences in σ(T) between single phase systems and two-phase systems. We will study to what extent the evolution of σ(y > T) of our system can be described by modelling the conductivity of an heterogeneous system with two compounds having each different electronic properties and different intrinsic σ(T).

Polycyclic aromatic hydrocarbons in gulf of maine sediments: Distributions and mode of transport

Sediments from 19 stations in the Gulf of Maine were analyzed for 16 priority polycyclic aromatic hydrocarbon (PAH) compounds in 1983. Thirteen of the compounds were distributed widely with total concentrations ranging from 10–512 ppb (dry wt). These values are an order of magnitude lower than those observed in the coastal zone but higher than those on Georges Bank. PAH concentration was directly related to grain size, total organic carbon (TOC) and distance from source. PAHs appear to be accumulating in Wilkinson and Jordan Basins but not in Georges Basin. Observed PAH distributions support the conclusion that the principal transport mechanism is through the atmosphere with localized augmentation by sediment resuspension and transport from coastal embayments.

Transport processes in the oxidation of Ni studied using tracers in growing NiO scales

Experimental techniques have been developed for determining Ni63 and O18 tracer distributions in NiO scales ranging in thickness from ∼0.1 to 100 μ. These have been used to investigate Ni and O transport in scales on {100} Ni crystals and polycrystalline Ni in the temperature range 500–1300° C. NiO grown on {100} Ni crystals at 1000°C was uniform and compact and grew by the bulk diffusion of Ni in NiO by a vacancy mechanism. At temperatures below 800°C the principal transport mechanism was short-circuit diffusion of Ni in NiO. At all temperatures short-circuit diffusion of oxygen contributed to scale growth on polycrystalline Ni and was responsible for growth of the inner layer of duplex scales. The oxygen diffusion paths are believed to be micro-cracks induced by growth stresses.

Movement of Herbicides through Soil to Soybean Roots1

AbstractAbsorption of pesticides is one of the primary means by which pesticide residues enter the food chains of humans, domestic animals, and wildlife. Thus, a comparison was made between the theoretical and observed uptake of isopropyl m‐chlorocarbanilate (chlorpropham) and 2‐chloro‐4‐( ethylamino )‐6‐ (isopropylamino)‐s‐triazine (atrazine) from soil by soybean roots (Glycine max (L.)). The rate of absorption of both 14C‐herbicides as determined by liquid scintillation techniques was rapid initially and decreased as plant growth increased. These rate curves were approximately parallel throughout the growth range with the rate of absorption of atrazine being always greater than that of chlorpropham.The theoretical movement of the two herbicides to the root surface was calculated assuming certain boundary conditions for the system. Assuming the roots to be “perfect sinks,” the diffusion process accounted for the total quantity of herbicide absorbed except during the early hours following germination. As root growth increased, the roots became less of a sink for these two herbicides, and diffusion to a “perfect sink” could theoretically supply more than the amount absorbed. Assuming the rate of uptake to be proportional to the concentration at the root surface, a curvilinear rate constant was observed during the uptake period. The data indicated that the plant controlled the amount of herbicide accumulated as well as its principal transport mechanism to the root, probably through its influence on the permeability of the root or root saturation.