Mineralogy and characterization of arsenic, iron, and lead in a mine waste-derived fertilizer.

The solid-state speciation of arsenic (As), iron (Fe), and lead (Pb) was studied in the mine waste-derived fertilizer Ironite using X-ray absorption spectroscopy, Mössbauer spectroscopy, and aging studies. Arsenic was primarily associated with ferrihydrite (60-70%), with the remainder found in arsenopyrite (30-40%). Lead was observed almost exclusively as anglesite (PbSO4), with <1% observed as galena (PbS). The identification of As in oxidized Fe oxides and Pb as PbSO4 is in disagreement with the dominant reduced phases previously reported and suggests As and Pb contained within the mine waste-derived product are more bioavailable than previously considered. Aging studies in solution result in Ironite granules separating into two distinct fractions, an orange oxide precipitate and a crystalline fraction with a metallic luster. The orange oxide fraction contained As adsorbed/precipitated with ferrihydrite that is released into solution when allowed to equilibrate with water. The fraction with a metallic luster contained pyrite and arsenopyrite. A complete breakdown of arsenopyrite was observed in Ironite aged for 1 month in buffered deionized water. The observations from this study indicate As and Pb exist as oxidized phases that likely develop from the beneficiation and processing of mine tailings for commercial sale. The potential release of As and Pb has important implications for water quality standards and human health. Of particular concern is the quantity of As released from mine waste-derived products due to the new As regulation applied in 2006, limiting As levels to 10 microg L(-1) in drinking water.

Hexahydro-1,3,5-trinitro-1,3,5-triazine transformation by biologically reduced ferrihydrite: evolution of Fe mineralogy, surface area, and reaction rates.

Microbial respiration of Fe(III) oxides has been shown to produce reduced Fe phases that are capable of transforming a variety of oxidized contaminants. Little data, however, are available on how these Fe phases evolve over time and how this evolution may affect their ability to reduce contaminants. Here,the evolution and reactivity of biologically reduced ferrihydrite were monitored over a period of 14 months. Solids were collected from a culture of Geobacter metallireducens (GS-15) thatwas incubated with ferrihydrite (as the electron acceptor) for 0, 7, 10, 20, 75, and 400 days. Mineralogical composition and surface area of the biologically reduced solids were characterized using Mössbauer spectroscopy, X-ray diffraction, and BET with N2 adsorption. By day 10, ferrihydrite began to transform, and a nanoparticle magnetite/maghemite phase, as well as two ferrous phases, was observed. One of the ferrous phases was identified as siderite, whereas the other could not be positively identified. Likely candidates, however, include Fe(OH)2(s) or an adsorbed Fe(II) species. Over the next few months, ferrihydrite was completely reduced and evolved into a mixture containing about 70% magnetite/maghemite, 19% siderite, and 11% of the second Fe(II) phase. The effect of incubation time on the reactivity of the biologically reduced solids was evaluated by measuring the kinetics of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) transformation. The only products observed were the three reduced nitroso products. Rate coefficients (k) for RDX transformation were dramatically influenced by incubation time with half-lives of about 1 month observed in the presence of solids incubated for 10 and 20 days, 3 months with solids incubated for 75 days, and negligible removal with solids incubated for 400 days. The loss of reactivity was not directly correlated to any one mineralogical variable but may be due to particle size or surface chemistry changes in the reactive Fe phase or to cell die-off and the accumulation of cell lysis products after consumption of the electron acceptor. The dramatic effect of incubation time on the rate of RDX removal highlights a potential limitation of studying complex systems, as we have here, in batch reactors and suggests that incubation time is an important variable to consider when measuring and comparing rates of contaminant reduction.

Inhibition of bacterial perchlorate reduction by zero-valent iron.

Perchlorate was reduced by a mixed bacterial culture over a pH range of 7.0-8.9. Similar rates of perchlorate reduction were observed between pH 7.0 and 8.5, whereas significantly slower reduction occurred at pH 8.9. Addition of iron metal, Fe(0), to the mixed bacterial culture resulted in slower rates of perchlorate reduction. Negligible perchlorate reduction was observed under abiotic conditions with Fe(0) alone in a reduced anaerobic medium. The inhibition of perchlorate reduction observed in the presence of Fe(0) is in contrast to previous studies that have shown faster rates of contaminant reduction when bacteria and Fe(0) were combined compared to bacteria alone. The addition of Fe(0) resulted in a rise in pH, as well as precipitation of Fe minerals that appeared to encapsulate the bacterial cells. In experiments where pH was kept constant, the addition of Fe(0) still resulted in slower rates of perchlorate reduction suggesting that encapsulation of bacteria by Fe precipitates contributed to the inhibition of the bacterial activity independent of the effect of pH on bacteria. These results provide the first evidence linking accumulation of iron precipitates at the cell surface to inhibition of environmental contaminant degradation. Fe(0) was not a suitable amendment to stimulate perchlorate-degrading bacteria and the bacterial inhibition caused by precipitation of reduced Fe species may be important in other combined anaerobic bacterial-Fe(0) systems. Furthermore, the inhibition of bacterial activity by iron precipitation may have significant implications for the design of in situ bioremediation technologies for treatment of perchlorate plumes.

Spectroscopic evidence for Fe(II)-Fe(III) electron transfer at the iron oxide-water interface.

Using the isotope specificity of 57Fe Mössbauer spectroscopy, we report spectroscopic observations of Fe(II) reacted with oxide surfaces under conditions typical of natural environments (i.e., wet, anoxic, circumneutral pH, and about 1% Fe(II)). Mössbauer spectra of Fe(II) adsorbed to rutile (TiO2) and aluminum oxide (Al2O3) show only Fe(II) species, whereas spectra of Fe(II) reacted with goethite (alpha-FeOOH), hematite (alpha-Fe2O3), and ferrihydrite (Fe5HO8) demonstrate electron transfer between the adsorbed Fe(II) and the underlying iron(III) oxide. Electron-transfer induces growth of an Fe(III) layer on the oxide surface that is similar to the bulk oxide. The resulting oxide is capable of reducing nitrobenzene (as expected based on previous studies), but interestingly, the oxide is only reactive when aqueous Fe(II) is present. This finding suggests a novel pathway for the biogeochemical cycling of Fe and also raises important questions regarding the mechanism of contaminant reduction by Fe(II) in the presence of oxide surfaces.

Different patterns of synaptic transmission revealed between hippocampal CA3 stratum oriens and stratum lucidum interneurons and their pyramidal cell targets.

Stratum lucidum (SL) interneurons likely mediate feedforward inhibition between the dentate gyrus mossy fibers and CA3 pyramidal cells, while stratum oriens (SO) interneurons likely provide both feedforward and feedback inhibition within the CA3 commissural/associational network. Using dual whole-cell patch-clamp recordings between interneurons and CA3 pyramidal cells, we have examined SL and SO interneurons and their synapses within organotypic hippocampal slice cultures. Biocytin staining revealed different morphologies between these interneuron groups, both being very similar to those found previously in acute slices. The kinetics of IPSCs were similar between the two groups, but the reliability of synaptic transmission of SL interneuron (SL-INT) IPSCs was significantly lower than the virtually 100% reliability (non-existent failure rates) of SO-INT IPSCs. The SL-INT IPSCs also had a lower quantal content than the SO-INT IPSCs. In addition, SL-INTs were less likely than SO-INTs to innervate or to be innervated by nearby CA3 pyramidal cells. Paired-pulse stimulation at 100 ms interstimulus intervals produced similar paired-pulse depression in both interneuron synapses, despite the significantly higher failure rate of IPSCs produced by the SL-INTs compared with SO-INTs. CV analysis supported the hypothesis that paired-pulse depression was presynaptic. During repetitive, high frequency stimulation (>10 Hz for 500 ms) the two different synapses exhibited distinctly different forms of short-term plasticity: all SL interneurons displayed significant short-term facilitation (mean 113% facilitation, n=4), while, by contrast, SO interneuron synapses displayed either short-term depression (mean 42% depression, n=5 of 8) or no net facilitation or depression (n=3 of 8). These results indicate that the synaptic properties of interneurons can be quite different for interneurons in different hippocampal circuits.

Excitatory synapses from CA3 pyramidal cells onto neighboring pyramidal cells differ from those onto inhibitory interneurons.

The glutamatergic pyramidal cell (PYR) to pyramidal cell synapse was compared to the PYR to inhibitory interneuron (INT) synapse in area CA3 of rat hippocampal roller-tube cultures. Paired-pulses and tetanic stimulations of a presynaptic PYR were conducted utilizing dual whole-cell patch-clamp recordings of either two PYRs or of a PYR and visually identified stratum oriens INT. Differences in synaptic characteristics were observed, depending on the postsynaptic target cell. Across cell pairs the variation of EPSC amplitudes was much larger for postsynaptic PYRs than for INTs. EPSCs recorded from INTs had faster rise times and shorter decays than those recorded in PYRs. There were also differences in the short-term plasticity of these synapses. Dual PYR:PYR recordings during paired-pulse stimulation at 100 ms interstimulus intervals demonstrated no modulation of EPSC amplitudes, while PYR:INT synapses showed paired-pulse depression. During trains of action potentials, the PYR:PYR EPSCs followed the presynaptic action potential train reliably, with little depression of EPSCs, while PYR:INT EPSCs demonstrated failures of transmission or profound depression after the initial EPSC. These results indicate multiple differences at both the pre- and postsynaptic level in the characteristics of pyramidal cell synapses that depend on the postsynaptic target's identity as either PYR or INT.

Adaptive changes in the somatotopic properties of individual thalamic neurons immediately following microlesions in connected regions of the nucleus cuneatus.

We examined the ability of thalamic neurons in the ventrobasal complex to show adaptive changes in receptive field properties following the loss of projections from the nucleus cuneatus. Thalamic responses to air jet stimulation were tested at multiple peripheral sites before and after making discrete microlesions in topographically-matched regions of the nucleus cuneatus. Prior to making a lesion, crosscorrelation analysis and orthodromic microstimulation were used to confirm the source of cuneate neurons projecting to the thalamic recording site. A total of 69 thalamic neurons were recorded from 29 rats. Following placement of a microlesion (100-200 microns diameter) in the nucleus cuneatus, 34 thalamic neurons did not show significant changes in stimulus-induced responses, possibly because the lesion was too small or because critical sites in the receptive field were not tested. The remaining 35 neurons were affected by cuneate microlesions, but the change in responsiveness varied according to stimulation site. When the most responsive site in the receptive field was examined, 24 neurons exhibited significant decreases and three neurons showed significant increases in responsiveness. Cuneate microlesions produced decreases at moderately responsive sites, but the reduction in response magnitude was smaller than at the most responsive site. When responses near the receptive field boundary were examined, 11 neurons displayed significant increases and only four neurons showed significant decreases. For two neurons without well-defined receptive field boundaries, cuneate microlesions caused new excitatory responses to emerge from sites that had formerly caused a slight inhibition of spontaneous activity. In all cases of increased responsiveness, the changes appeared on only one side of a neuron's receptive field. This asymmetry may account for the fact that the probability of detecting receptive field expansion increased from 27% (six of 22 experiments) to 71% (five of seven experiments) when the number of stimulation sites located throughout the receptive field was increased. These results indicate that the receptive field structure of individual neurons shows adaptive properties immediately after loss of the predominant ascending inputs.

A comparative analysis of coordinated neuronal activity in the thalamic ventrobasal complex of rats and cats.

There are substantial differences in the incidence of inhibitory neurons in the ventrobasal complex of rat and cat thalamus. This marked dissimilarity in neuronal composition suggests that there should be corresponding differences in the orchestration of neural activity in these regions during cutaneous stimulation. To explore this possibility, we conducted a cross-correlation analysis of neuronal activity in the ventroposterolateral (VPL) nucleus of anesthetized rats and cats. Pairs of neurons representing hairy skin were recorded simultaneously with one or two electrodes during air jet stimulation of multiple sites throughout the receptive fields. Cross-correlation histograms indicated that correlated activity among adjacent neurons occurred in three distinct patterns. In one pattern, classified as narrow-unimodal, the discharge of one neuron preceded a discharge in the partner neuron over a narrow interval of time (< 5 ms). Narrow-bimodal patterns were characterized by responses in which the temporal order of discharges from the two neurons was variable, but the interspike intervals were always < 5 ms. In wide-unimodal patterns, the discharge of one neuron was correlated with subsequent discharges in the partner neuron over a wide interval of time (> 5 ms). In rat VPL, two-thirds of the 58 neuron pairs showing correlated responses were characterized by narrow-unimodal responses and nearly one-third of the neuron pairs displayed narrow-bimodal patterns. Only one pair of rat VPL neurons were characterized by a wide-unimodal pattern of coordination. By comparison, half of the 61 adjacent neuron pairs with coordinated responses in cat VPL were characterized by narrow-unimodal patterns. Slightly more than one-third of the correlated neuron pairs had narrow-bimodal patterns, while the remainder (13%) were classified as wide-unimodal responses. Pairs of neurons separated by 340-405 microns discharged synchronously in a pattern that was similar to the temporal relationship expressed in the narrow-bimodal patterns found among adjacent neurons. In both species, the wide-unimodal patterns had the strongest coordinated responses as measured by the correlation coefficient. Although inhibitory relationships did not appear in correlation histograms that had been corrected for stimulus coordination, cross-correlation analysis of the raw spike trains revealed brief (10-40 ms) periods of inhibition that were associated with cat VPL neurons exhibiting wide-unimodal coordination patterns. In rat VPL, most inhibition involved longer (30-60 ms) periods of inhibitory oscillations appearing amidst a much larger rhythmic pattern. These results suggest that correlation patterns transpiring over narrow (< 5 ms) time intervals represent the coordination of activity among neighboring thalamocortical relay neurons.(ABSTRACT TRUNCATED AT 400 WORDS)