Low-frequency electrical response to microbial induced sulfide precipitation

We investigated the sensitivity of low-frequency electrical measurements to microbe-induced metal sulfide precipitation. Three identical sand-packed monitoring columns were used; a geochemical column, an electrical column and a control column. In the first experiment, continuous upward flow of nutrients and metals in solution was established in each column. Cells of Desulfovibrio vulgaris (D. vulgaris) were injected into the center of the geochemical and electrical columns. Geochemical sampling and post-experiment destructive analysis showed that microbial induced sulfate reduction led to metal precipitation on bacteria cells, forming motile biominerals. Precipitation initially occurred in the injection zone, followed by chemotactic migration of D. vulgaris and ultimate accumulation around the nutrient source at the column base. Results from this experiment conducted with metals show (1) polarization anomalies, up to 14 mrad, develop at the bacteria injection and final accumulation areas, (2) the onset of polarization increase occurs concurrently with the onset of lactate consumption, (3) polarization profiles are similar to calculated profiles of the rate of lactate consumption, and (4) temporal changes in polarization and conduction correlate with a geometrical rearrangement of metal-coated bacterial cells. In a second experiment, the same biogeochemical conditions were established except that no metals were added to the flow solution. Polarization anomalies were absent when the experiment was replicated without metals in solution. We therefore attribute the polarization increase observed in the first experiment to a metal-fluid interfacial mechanism that develops as metal sulfides precipitate onto microbial cells and form biominerals. Temporal changes in polarization and conductivity reflect changes in (1) the amount of metal-fluid interfacial area, and (2) the amount of electronic conduction resulting from microbial growth, chemotactic movement and final coagulation. This polarization is correlated with the rate of microbial activity inferred from the lactate concentration gradient, probably via a common total metal surface area effect.

The Galactic metallicity gradient

We have previously published intermediate to hi,oh resolution spectroscopic observations of approximately 80 early B-type main-sequence stars situated in 19 Galactic open clusters/associations with Galactocentric distances distributed over 6 less than or equal to R-g less than or equal to 18 kpc. This current study collates and re-analyses these equivalent- width datasets using LTE and non-LTE model atmosphere techniques, in order to determine the stellar atmospheric parameters and abundance estimates for C, N, O, Mg, Al and Si. The latter should be representative of the present-day Galactic interstellar medium. Our extensive observational dataset permits the identification of sub-samples of stars with similar atmospheric parameters and of homogeneous subsets of lines. As such, this investigation represents the most extensive and systematic study of its kind to date. We conclude that the distribution of light elements (CI O, Mg & Si) in the Galactic disk can be represented by a linear, radial gradient of -0.07 +/- 0.01 dex kpc(-1) Our results for nitrogen and oxygen viz. (-0.09 +/- 0.01 dex kpc(-1) and -0.067 +/- 0.008 dex kpc(-1)) are in excellent agreement with that found from the study of HII regions. We have also examined our datasets for evidence of an abrupt discontinuity in the metallicity of the Galactic disk near a Galactocentric distance of 10 kpc (see Twarog et al. 1997). However, there is no evidence to suggest that our data would be better fitted with a two-zone model. Moreover, we observe a N/O gradient of -0.04 +/- 0.02 dex kpc(-1) which is consistent with that found for other spiral galaxies (Vila- Costas gr Edmunds 1993).

The effect of co-precipitation on cadmium(II) adsorption on hydrous aluminium(III) hydroxide in the presence of a range of chelates

The adsorption of cadmium(II) on freshly precipitated aluminium(III) hydroxide in the presence of a range of chelates has been investigated. By precipitating the metal, chelate and adsorbent together it is possible to change the pH variation of the metal-complex adsorption from anionic, ligand-like, binding to cationic binding. This is a general phenomenon and is explained by the formation of a ternary Al-O-Cd-L surface species. As a consequence of the preparation method, the pH edge is found to shift to lower pH values in the presence of the chelate which gives rise to an apparent increase in adsorption of Cd2+. This increase is, in general, most pronounced at [chelate] / [metal] > 1. Computer modelling shows that the observed trends result from the competition between Al-O-Cd-L and Al-L for the available aluminium( III) binding sites. The enhanced adsorption in the presence of phenylenediaminetetraacetate is anomalous since it is observed at a [ chelate] / [metal] approximate to 0.1 and cannot be interpreted by the simple competition model.

One-dimensional particle simulation of the filamentation instability: Electrostatic field driven by the magnetic pressure gradient force

Two counterpropagating cool and equally dense electron beams are modeled with particle-in-cell simulations. The electron beam filamentation instability is examined in one spatial dimension, which is an approximation for a quasiplanar filament boundary. It is confirmed that the force on the electrons imposed by the electrostatic field, which develops during the nonlinear stage of the instability, oscillates around a mean value that equals the magnetic pressure gradient force. The forces acting on the electrons due to the electrostatic and the magnetic field have a similar strength. The electrostatic field reduces the confining force close to the stable equilibrium of each filament and increases it farther away, limiting the peak density. The confining time-averaged total potential permits an overlap of current filaments with an opposite flow direction.

Performance evaluation of a zerovalent iron reactive barrier: Mineralogical characteristics

There is a limited amount of information about the effects of mineral precipitates and corrosion on the lifespan and long-term performance of in situ Fe° reactive barriers. The objectives of this paper are (1) to investigate mineral precipitates through an in situ permeable Fe° reactive barrier and (2) to examine the cementation and corrosion of Fe° filings in order to estimate the lifespan of this barrier. This field scale barrier (225' long x 2' wide x 31' deep) has been installed in order to remove uranium from contaminated groundwater at the Y-12 plant site, Oak Ridge, TN. According to XRD and SEM-EDX analysis of core samples recovered from the Fe° portion of the barrier, iron oxyhydroxides were found throughout, while aragonite, siderite, and FeS occurred predominantly in the shallow portion. Additionally, aragonite and FeS were present in up-gradient deeper zone where groundwater first enters the Fe° section of the barrier. After 15 months in the barrier, most of the Fe° filings in the core samples were loose, and a little corrosion of Fe° filings was observed in most of the barrier. However, larger amounts of corrosion (~10-150 µm thick corrosion rinds) occurred on cemented iron particles where groundwater first enters the barrier. Bicarbonate/ carbonate concentrations were high in this section of the barrier. Byproducts of this corrosion, iron oxyhydroxides, were the primary binding material in the cementation. Also, aragonite acted as a binding material to a lesser extent, while amorphous FeS occurred as coatings and infilings. Thin corrosion rinds (2-50 µm thick) were also found on the uncemented individual Fe° filings in the same area of the cementation. If corrosion continues, the estimated lifespan of Fe° filings in the more corroded sections is 5 to 10 years, while the Fe° filings in the rest of the barrier perhaps would last longer than 15 years. The mineral precipitates on the Fe° filing surfaces may hinder this corrosion but they may also decrease reactive surfaces. This research shows that precipitation will vary across a single reactive barrier and that greater corrosion and subsequent cementation of the filings may occur where groundwater first enters the Fe° section of the barrier.

A tip-high, Ca2+-interdependent, reactive oxygen species gradient is associated with polarized growth in Fucus serratus zygotes

We report the existence of a tip-high reactive oxygen species (ROS) gradient in growing Fucus serratus zygotes, using both 5-(and 6-) chloromethyl-2',7'-dichlorodihydrofluorescein and nitroblue tetrazolium staining to report ROS generation. Suppression of the ROS gradient inhibits polarized zygotic growth; conversely, exogenous ROS generation can redirect zygotic polarization following inhibition of endogenous ROS. Confocal imaging of fluo-4 dextran distributions suggests that the ROS gradient is interdependent on the tip-high [Ca2+](cyt) gradient which is known to be associated with polarized growth. Our data support a model in which localized production of ROS at the rhizoid tip stimulates formation of a localized tip-high [Ca2+](cyt) gradient. Such modulation of intracellular [Ca2+](cyt) signals by ROS is a common motif in many plant and algal systems and this study extends this mechanism to embryogenesis.