Interstratification Patterns from the pH-Dependent Intercalation of a Tetracycline Antibiotic within Montmorillonite Layers

Little is known about the distribution of the intercalated molecules within the interstratified layers resulting from the pH-dependent interlayer adsorption of ionizable organic molecules, including antibiotics, within smectite-type clay minerals. Here we employed experimental and simulated X-ray diffraction (XRD) to characterize interstratification (or mixed layering) from the intercalation of oxytetracycline (OTC), a commonly used tetracycline antibiotic, within Na-montmorillonite layers at pHs 4, 5, 6, and 8. Our XRD data reveal that OTC is distributed nonrandomly in the interlayers such that Na- and OTC-saturated interlayers coexist. The profile of the full width at half-maximum intensity (fwhm), monitored as a function of increasing layer-to-layer distance (<i>d</i>₀₀₁), resulting from an increasing amount of intercalated OTC, reflects such mixed-layer crystals under the acidic pH conditions. A minimum in fwhm occurs at a d spacing of about 1.8 nm, which is to be the optimal <i>d</i>₀₀₁ for OTC-saturated layers, in agreement with molecular modeling results. Using the coordinates of the thermodynamically favorable configuration of the adsorptives in a model montmorillonite interlayer, we simulated XRD profiles to unravel the different patterns of interstratification from the experimental data. At both pHs 4 and 5, Na- and OTC-interlayers are randomly interstratified, whereas at pH 6, these layers are clustered, as characterized by a segregated interstratification pattern. The theoretical layer stacking sequences of the simulated XRD illustrate, as pH increases, the clustering of similar layer types with the exclusion of OTC intercalation from clay populations enriched in Na. At pH 8, both fwhm and <i>d</i>₀₀₁ indicate OTC adsorption primarily on external surface sites, not within interlayers. Our findings imply that, in addition to chemical speciation, a pH-dependent formation of montmorillonite crystallites with unexfoliated layers may be responsible both for the decreased OTC intercalation and for the increased binding on external sites, thus resulting in the different patterns of interstratification as a function of increasing pH

Fate of pathological prion (PrP^sc92–138) in soil and water: prion-clay nanoparticle molecular dynamics

<div><p>Pathogenic prion protein scrapie (PrP^sc) may contaminate soils for decades and remain in water in colloidal suspension, providing infection pathways for animals through the inhalation of ingested dust and soil particles, and drinking water. We used molecular dynamics simulations to understand the strong binding mechanism of this pathogenic peptide with clay mineral surfaces and compared our results to experimental works. We restricted our model to the moiety PrP(92–138), which is a portion of the whole PrP^sc molecule responsible for infectivity and modeled it using explicit solvating water molecules in contact with a pyrophyllite cleavage plane. Pyrophyllite is taken as a model for common soil clay, but it has no permanent structural charge. However, partial residual negative charges occur on the cleavage plane slab surface due to a slab charge unbalance. The charge is isotropic in 2D and it was balanced with K⁺ ions. After partially removing potassium ions, the peptide anchors to the clay surface via up to 10 hydrogen bonds, between protonated lysine or histidine residues and the oxygen atoms of the siloxane cavities. Our results provide insight to the mechanism responsible for the strong association between the PrP^sc peptide and clay nanoparticles and the associations present in contaminated soil and water which may lead to the infection of animals.</p></div

On–Off Mobilization of Contaminants in Soils during Redox Oscillations

Near-surface biogeochemical systems can oscillate between oxic and anoxic conditions. Under such periodic changes many redox-sensitive inorganic contaminants undergo speciation, mobility and toxicity changes. We investigated the changes to chromium (Cr), arsenic (As), selenium (Se), antimony (Sb) and uranium (U) mobility during a series of laboratory experiments where argillaceous substrates were subjected to successive cycles of oxidizing and reducing conditions. The <i>E</i>_H oscillated between −320 and +470 mV, induced via both abiotic and microbial forcings. Chemically induced cycles of oxidation and reduction were achieved via a combination of gas (N₂:CO₂ vs compressed air) and carbon (ethanol) addition, to stimulate the metabolism of a natively present microbial community. The contaminants were added either alone or as contaminant mixtures. Results show clear on–off switch mobility behavior for both major elements such as carbon (C), iron (Fe) and manganese (Mn) and for contaminants. Mn, Fe, and As were mobilized under anoxic conditions, whereas Sb, Se, and U were mobilized under oxic conditions. While As, Sb, and U were reversibly sorbed, Se and Cr were irreversibly sequestered via reductive precipitation. When present in aqueous solutions at high concentrations, Cr^VI prevented the reduction of Mn and Fe, and inhibited the mobilization of elements with lower <i>E</i>_H^o. To improve remediation strategies for multiple contaminants in redox-dynamic environments, we propose a mixed kinetic-equilibrium biogeochemical model that can be forced by oscillating boundary conditions and that uses literature rates and constants to capture the key processes responsible for the mobilization of contaminants in soils

Inhibition of U(VI) Reduction by Synthetic and Natural Pyrite

Reductive precipitation is an effective method of attenuating the mobility of uranium (U) in subsurface environments. The reduction of U­(VI) by synthetic and naturally occurring pyrite was investigated at pH 3.0–9.5. In contrast to thermodynamic calculations that were used to predict UO₂(s) precipitation, a mixed U­(IV) and U­(VI) product (e.g., U₃O₈/U₄O₉/U₃O₇) was only observed at pH 6.21–8.63 and 4.52–4.83 for synthetic and natural pyrite, respectively. Under acidic conditions, the reduction of UO₂²⁺ by surface-associated Fe²⁺ may not be favored because the mineral surface is nearly neutral or not negative enough. At high pH, the sorption of negatively charged U­(VI) species is not favored on the negatively charged mineral surface. Thus, the redox reaction is not favored. Trace elements generally contained within the natural pyrite structure can affect the reactivity of pyrite and lead to a different result between the natural and synthetic pyrite. Because UO₂(s) is extremely redox-sensitive toward U­(VI), the observed UO_2+x(s) phase reduction product indicates a surface reaction that is largely controlled by reaction kinetics and pyrite surface chemistry. These factors may explain why most laboratory experiments have observed incomplete U­(VI) reduction on Fe­(II)-bearing minerals

Nanocomposite Pyrite–Greigite Reactivity toward Se(IV)/Se(VI)

A nanopyrite/greigite composite was synthesized by reacting FeCl₃ and NaHS in a ratio of 1:2 (Wei et al. 1996). Following this procedure, the obtained solid phases consisted of 30–50 nm sized particles containing 28% of greigite (Fe²⁺Fe³⁺₂S₄) and 72% pyrite (FeS₂). Batch reactor experiments were performed with selenite or selenate by equilibrating suspensions containing the nanosized pyrite–greigite solid phase at different pH-values and with or without the addition of extra Fe²⁺. XANES-EXAFS spectroscopic techniques revealed, for the first time, the formation of ferroselite (FeSe₂) as the predominant reaction product, along with elemental Se. In the present experimental conditions, at pH 6 and in equilibrium with Se⁰, the solution is oversaturated with respect to ferrosilite. Furthermore, thermodynamic computations show that reaction kinetics likely played a significant role in our experimental system

Concentrations of aqueous Fe(II) in mg/L, of aqueous As(V) (black bars), As(III) (gray bars) and As(tot) (white bars) in mg/L, and of bacteria in million cells/mL

<p><b>Copyright information:</b></p><p>Taken from "Decoupling of arsenic and iron release from ferrihydrite suspension under reducing conditions: a biogeochemical model"</p><p>http://www.geochemicaltransactions.com/content/8/1/12</p><p>Geochemical Transactions 2007;8():12-12.</p><p>Published online 29 Nov 2007</p><p>PMCID:PMC2246110.</p><p></p> Each group of 3 bars includes data of experiments AD1, CP1, AD5, CP5 after 1 day, 28 days and 63 days of As-2LFh incubation. Error bars represent the standard deviation of duplicated measurements

Aqueous Fe (squares) and As (triangles) concentrations released during experiments AD1 (a), AD5 (b), CP1 (c), CP5 (d)

<p><b>Copyright information:</b></p><p>Taken from "Decoupling of arsenic and iron release from ferrihydrite suspension under reducing conditions: a biogeochemical model"</p><p>http://www.geochemicaltransactions.com/content/8/1/12</p><p>Geochemical Transactions 2007;8():12-12.</p><p>Published online 29 Nov 2007</p><p>PMCID:PMC2246110.</p><p></p> Empty symbols represent control experiments without bacteria. Proportions of released Fe and As are calculated by subtracting the concentration at the beginning of each experiment (Fig. 2) from the concentrations measured over time, and then dividing the differences by the initial solid content of Fe and As (Table 1), respectively. Error bars represent the standard deviation of duplicated measurements

Comparison of P-extractable As concentrations with the acid-leachable As content of the sediment collected with from (a) Chakdaha, India, and (b) Balia, Para, Bangladesh

<p><b>Copyright information:</b></p><p>Taken from "Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh"</p><p>http://www.geochemicaltransactions.com/content/9/1/1</p><p>Geochemical Transactions 2008;9():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2246114.</p><p></p> The two dashed lines in each panel correspond to a one-to-one correspondence and 10-fold higher HCl-extractable As concentrations, respectively. Also shown are dissolved As concentrations as a function of P-extractable As (c-d). Symbols identifying the profiles are the same as in Figure 2

Comparison of sediment Fe(II)/Fe ratios (a-b) and dissolved Fe (c-d) concentrations as a function of sulphate for needle-sampler samples collected in India (left) and Bangladesh (right)

<p><b>Copyright information:</b></p><p>Taken from "Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh"</p><p>http://www.geochemicaltransactions.com/content/9/1/1</p><p>Geochemical Transactions 2008;9():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2246114.</p><p></p> The symbols are color-coded in three ranges according to groundwater As concentrations. Dashed lines show the sulfate concentration in the Hooghly and Old Brahmaputra rivers

Histogram of EM conductivities around (a) Chakdaha, India and (b) Balia Para, Bangladesh

<p><b>Copyright information:</b></p><p>Taken from "Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh"</p><p>http://www.geochemicaltransactions.com/content/9/1/1</p><p>Geochemical Transactions 2008;9():1-1.</p><p>Published online 11 Jan 2008</p><p>PMCID:PMC2246114.</p><p></p> The location of the measurements is shown by black symbols on a contour map of EM conductivity, color-coded in the same ranges as in the histogram (c-d). Also shown are the locations of needle-sampler profiles using the same symbols of Fig. 2. (e-f) Lithological cross-sections for the two sites showing the distribution of clay/silt (black) and sand (white). (g-h) Contoured section of major cation concentrations for groundwater collected with the needle-sampler. Sections drawn with Ocean Data View [47]