B44A-01
Prions, Radionuclides and Clays: Impact of clay interlayer "acidity" on toxic compound speciation
The physical and chemical processes that are the basis of contaminant retardation in clay rich medium, such as soil or nuclear waste repository, have been studied at the molecular level by a combination of molecular dynamics (MD), electron paramagnetic spectroscopy (EPR) and neutron diffraction with isotopic substitution (NDIS). The speciation of contaminants such as Sm, a radionuclide analogue, and Cu, bound to Prion protein (PrP), has been studied upon adsorption in clay interlayers. We used as molecular probe the P5-Cu(II) complex, where the P5 pentapeptide(92-96 PrP residues) represents one of the five Cu(II) binding site present in PrP, the key protein involved in diseases known as transmissible spongiform encephalopathies. In both cases, the pH of the interlayer has been inferred from the metal ion coordination, here used as a molecular reporter. In circum neutral pH waters, samarium is present as Sm(OH)3° species and should not be adsorbed in clay interlayer by "cation exchange" unless its hydrolysis is altered. Samarium NDIS results indicate that whether the number of oxygen nearest neighbours varies only from 8.5 to 7, as Sm penetrates the interlayer, the number of hydrogen nearest neighbours drops from 12 to 6. The high affinity of clay for Sm shows that a change in Sm hydrolysis occurs in the clay interlayer, but is directly followed by the formation of a surface complex with montmorillonite siloxane plane functional groups which prevents the determination of a "local pH". Conversely, has been found to be a much more sensitive interlayer water pH probe. and this peptide domain is involved in the misfolding of the protein,a transconformation which may lead to the pathogenic PrPSc form. We have therefore studied by EPR spectroscopy the adsorption of Cu(II)-P5 complexes on montmorillonite, and found the clay to have a large and selective adsorption capacity for the various [Cu(P5)H-n](2-n)+ complexes where n is the number of deprotonated amido function. The speciation of the Cu-ligand complex was found to be different, in bulk water (Hureau et al., 2006) and in clay suspensions, where n increases. This new speciation of the copper metal ions, used as a molecular probe, allows to "measure" the pH of interlayer water which is shown to be significantly lower than in bulk water pH. Molecular models for PrP attachment to the clay basal plane and Sm location within the clay interlayer were obtained by MD computations. Implications on PrP pathogenicity, following carcase burial and particle ingestions, and on radionuclide mobility, following nuclear waste burial in clay rich repository sites, will be discussed.
B44A-02
Short-term d13C changes in cultivated soils from Mexico
The soils of the Mexican Volcanic Belt are part of ecosystems subjected to strong human impact during the last six centuries. One measurable characteristic of the soil is the stable carbon isotopic relation of the soil organic matter (SOM) or d13C. The d13C SOM parameter is a genetic characteristic of soil reflecting the relative proportion of C3 and C4 that comes from colonizing plants having different photosynthetic C pathway and is used as a high-spatial resolution tool to infer paleoenvironmental changes.The d13C mean signatures of C3 and C4 plants are -27 and -13 %o, respectively. This work focuses on short-term changes in d13C on soils subjected to controlled agricultural practices during 2002-2005 in two sites of Mexico with similar annual precipitation and temperature. The tepetate was broken up 20y ago and ameliorated with fertilizers and organic matter. In both sites three experimental treatments consisting of traditional soil management and two variations of this one were evaluated. Traditional treatment implies low fertilizer and any chemical input, sowing annual crops during the rainy season and, in general, using low energy input. The crops planted were: legumes C3, oat C3, and a mixture of maizeC4 and beanC3, and wheatC3. The Improved and Organic treatments, had higher input of N and P as chemical fertilizers, and of organic manure (manure or compost), respectively. Soil samples were collected from the plow layer in Tlaxcala and in Michoacán, before C4 maize was planted. An Andisol from a pine-oak (C3 species) forest close to the Atecuaro site was also sampled up to 40 cm. This soil was considered a reference site not recently influenced by human activity. To analyze the d13C ratios of the SOM carbonate free samples, a routine combustion method and mass spectrometry (Finnigan MAT250) were used. In both agricultural sites a general excess of C3 species over C4 was evidenced through a mass balance equation derived from experimental d13C values (generally less than -20%o). The common feature for the soils under the Traditional and Traditional improved treatments was a C3 enrichment of the superficial SOM component compared to the underlying layers as a consequence of the dominance of the cultivated C3 species. A similar but more accentuated negative shift is also observed in the SOM from the forest soil (non-cultivated soil d13C -25.2), so the interpretation is uncertain. In the Traditional Organic treatment a clear and perceptible increment of d13C in the SOM carbon signature was observed. This was attributed mainly to the fact that cows manure may contain a lot of C4 coming from feedstuff rich in corn grain that is provided to the animals during grass shortage periods . However, the maize crop introduced in the rotation during the 3rd year had no major effect on the tepetates carbon isotopic signature. The stable isotopic carbon data corresponding to a short period (4 years) of observation in uniformly managed soil ecosystems showed that d13C changed due to the quality of the residues (relative abundance of C3/C4 species) incorporated to the SOC, but this memory is susceptible to undergo changes in the short term and could be rapidly reversed as a consequence of crop management.
B44A-03
Stability of Commercial Small-Sized Cerium Oxide in the Presence of Biological Material: Dilucidating Relationships between Reactivity and Toxicity of Nanomaterials
Cerium is the most abundant lanthanide and generally the only one to undergo redox reactions at the Earth's surface. Although rarely studied in natural environments, the redox chemistry of cerium may regulate metal toxicity. Unlike Ce(III) or other lanthanide ions, Ce(IV) has shown a remarkably efficacy to hydrolyze DNA. While Ce(IV) has been recognized as an important candidate to occupy peptidases catalytic centers, Ce(III) is virtually inactive for peptide hydrolysis. The selectivity of Ce as Ce(IV) relates to the specific coordination of water molecules and their orientation. Ce(IV) may bind selectivity to biomolecules to instigate conformation changes or cleavage of complexes, which affect metabolic pathways pivotal to growth and survival. For instance, Ce(IV) promotes the selective cleavage of RNA-type substrates, cyclic monophosphates, peptides, or monocleotides such as AMP, leading to mixtures of nucleosides and nucleobases. Association constants for Ce(IV)-DNA complexes are reported to be higher in magnitude for single stranded than double stranded DNA, while cleavage rates for either complexes are comparable. Complexation of Ce(IV) with mitoxantrone results in the intercalation of such complex into DNA, enabling mitoxantrone to bind effectively with DNA, along with concomitant conformational changes in the DNA double helix and inhibition of DNA synthesis. To the authors' knowledge, however, little information is available on the reactivity as it relates to toxicity of Ce-bearing nanoparticles widely used in nanotechnological applications. Here, we study molecular interactions between small-sized CeO2 and biomolecules(e.g., DNA, RNA, proteins) using carbon and cerium spectroscopy. Suspension stability as determined by aggregation kinetics was studied by Dynamic Light Scattering (DSL) and UV. In addition, acidophiles and fungi cultures were analyzed by nephelometry to estimate population density and growth rate values. Results show a progressive increase in the transformation of biomolecules (as % carbon) with decreasing CeO2 particle diameter (13 < d < 84 Å), which substantiates an intimate relation between CeO2 unit cell expansion and reactivity towards organics susceptible to undergo redox transformations. As shown by C and Ce spectroscopy, organic polymers that form because of oxidation are distributed next to the mineral surface and its occurrence is coupled to Ce reduction-oxidation. As evidenced by DSL and UV experiments conducted for the pH 2 to 8 range, the aggregation behavior of nanoCeO2 is susceptible to pH variations imposed because the presence of biological moieties itself over solid concentration.
B44A-04
Dissolution Kinetics of Arsenopyrite in the Presence of Iron(III)-Sequestering Biogenic Ligands at pH 5.
Arsenopyrite is one of the most important natural sources of Arsenic on Earth. Arsenopyrite is relatively insoluble in pure water. That is not the case if it is exposed to environmental conditions. Notably, arsenopyrite surfaces exposed to biological activity undergo changes in lattice energy, surface morphology, particle size, and other properties, typical of mineral dissolution. Iron biogeochemical cycling is pivotal to electron transferring in nature. Therein, the need to further scrutinize on the mechanism of arsenopyrite dissolution induced by biological activity. In oxic environments, such as highly weathered soils or surficial seawater, microorganisms and higher plants produce biogenic ligands such as siderophores to mobilize Fe that otherwise would be unavailable. Siderophores ligands facilitate the dissolution of natural particles that represent a primary reservoir of iron. In this paper we study the stability of arsenopyrite in the presence of desferrioxamine (DFO-B), a common siderophore ligand, at pH 5. Arsenopyrite specimens from mines from Panasqueira, Portugal, were used for this study. Batch dissolution experiments of arsenopyrite (1 g L-1) in the presence of DFO-B ([DFO-B]0 ƒ¬ 200 ƒÝM) were conducted for 7 days. The initial pH was adjusted to 5. Samples were stirred at 150 rpm. Sieving was conducted to homogeneized the particle size 0.149-0.1mm before conducting the dissolution experiments. Corresponding experiments in the absence of DFO-B for the purpose of comparison were also conducted. Analyses for soluble metals were conducted by AA and ICP-AES. Surface characterization was conducted by XRD and SEM-EDX. Incrustations of Pb(0) were detected in the arsenopyrite samples used for this study. In the presence of DFO-B, releases of Fe, As, and Pb showed positive trends with time. A shallower dependency was observed for release of Fe, As, and Pb in the presence of water only under similar experimental conditions. Detected concentrations of Fe, As, and Pb, after 100 h of reaction time in the presence of DFO-B were 0.3, 0.26, and 0.13 ƒÝM, respectively. Concentrations of Fe, As, and Pb, in the presence of water only were ca. 0.06, 0.13, and 0.01 ƒÝM, correspondingly. Hence, the effectiveness of DFO-B for releasing Pb was almost three times higher than that for releasing Fe (further details on Pb dissolution as affected by DFO-B are provided in Cornejo et al.). These results cannot be accounted for by size¡Xto-charge considerations prevailing in metal complexation by DFO-B only. Elemental sample enrichment as evidenced by SEM-EDX supporte the idea the Fe-S subunit bond energy is limiting for Fe release, while likely, the mechanism(s) of dissolution for Pb is independent and occurs concurrently to than for Fe and As.
B44A-05
Characterization of Ozone-driven and Photo-assisted Oxidation of Indoor Organic Pollutants
Understanding the oxidation mechanisms of volatile and semivolatile organic compounds (VOCs and SVOCs) is critical to assess their environmental fate, and to design efficient indoor air cleaning technologies. Ozone-driven oxidation of gas phase and surface-bound indoor chemicals generates potentially harmful partially oxidized byproducts and ultrafine particulate matter. The composition of chemically complex secondary aerosol particles formed during ozonation has been only partially elucidated. Air cleaning technologies relying on photocatalytic oxidation of VOCs have the potential to improve indoor air quality in buildings in a cost-effective way. Current research on this area focuses on the development of novel materials that can improve the VOC conversion efficiency and mineralization yields over an extended catalyst lifetime.
B44A-06
The Use of Small-Particle Sized TiO2 Supported on Clays as Photocatalytic Materials: A Low- Cost Alternative Technology for the Degradation of Air Pollutants
Assisted photocatalysis by TiO2 is an advanced oxidation process that has been employed for air and water remediation. Clays are natural porous materials bearing high surface areas and interlayer spacing that allows entrapment of small-sized particles. Pillared clays exchanged with small-sized TiO2 can constitute materials with interesting photocatalytic properties because high surface area values and large contents of mesospores, which enables analyte trapping. Furthermore, intercalation at the clay interlayer enables TiO2 to become more resistant to aggregation when in solution. Just recently it has been reported that clays can lead to increases in the photocatalytic activity of TiO2 when the mesopores size is adequate to host organic solutes and ensure their effective interaction with the TiO2 particles. In this paper we study the photocatalytic properties of small-sized TiO2 supported on the following clay samples: Montmorillonite [SWy-2, Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10 ] from Crook Country, Wyoming, USA; Hectorite [SHCa-1, Na0.4Mg2.7Li0.3Si4O10(OH)2 ] from San Bernardino. Country, California, USA; Kaolinite [KGa-1b, Al2Si2O5(OH)4 ] from Washington Country, Georgia, USA. Deposition of TiO2 on the clay surface was conducted by using a sol-gel synthetic method. Anatase TiO2 particles transformation at the clay interlayer was achieved by thermic treatment at 180 „aC. Material characterization was conducted using FTIR microspectroscopy, Scanning Electron Microscopy (SEM), and XRD analysis. The organic compound used as probe was ethanol
B44A-07
Decay of the Singlet Excited States of Nitro-Polycyclic Aromatic Hydrocarbons and its Relevance in the Photochemical Degradation
Nitro-polycyclic aromatic hydrocarbons (NPAHs) are important toxic pollutants formed directly during fuel combustion and indirectly by atmospheric nitration of polyaromatic hydrocarbons. Since photochemical reactions determine the atmospheric ambient stability of these compounds, understanding the dynamics of their singlet excited states is of great importance. Electronically excited NPAHs evolve through two parallel pathways: Intersystem crossing leading to the formation of the first triplet state and the photoinduced dissociation of nitrogen (II) oxide. In this contribution we present the first time-resolved emission measurements of the singlet excited states which are the precursors in these primary photo-processes. We analyzed 1-nitronaphthalene, 9- nitroanthracene, 1-nitropyrene, 6-nitrochrysene and 3-nitrofluoranthene in solution samples. Except for 1- nitronapthalene, where a single exponential is observed, for the rest of the compounds the emission shows double exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule. The n* nature of the S1 and T1 states together with the presence of upper n* triplet states nearly isoenergetic to S1 accounts for the ultrafast triplet formation rates. This appears to be a common feature in all NPAHs.
B44A-08
Stability of Commercial Lead in the Presence of Iron(III)-Sequestering Biogenic Ligands: Implications for Metal Mobility and Bioavailability in Natural Iron-Rich Environments
Mexico is a leader in the production of lead. Exposure of soils contaminated with lead ore waste, ingestion of contaminated waters, or inhalation of suspended particles, among others are examples of common routes of exposure. Solubility often becomes limiting for Pb bioavailability, therein the need to further characterize natural processes that contribute to its mobility in the environment. In oxic environments, such as highly weathered soils or surficial seawater, microorganisms and higher plants produce biogenic ligands such as siderophores to mobilize Fe that otherwise would be unavailable. Siderophores ligands facilitate the dissolution of natural particles that represent a primary reservoir of iron. On the other hand, lead sorbs strongly to minerals, particularly to those bearing high contents of iron. Siderophore ligands have been identified to influence the adsorption behavior of Pb(II) on iron mineral surfaces. Yet, little is known on how siderophore ligands may affect the stability of Pb(0). In this paper we study the stability of Pb(0) in the presence of desferrioxamine (DFO-B), a common siderophore ligand. Batch dissolution experiments of Pb(0) (1 g L-1) in the presence of DFO-B ([DFO-B]0 ƒ¬ 200M) at pH 5 were conducted for 7 days. The adsorption behavior of DFO-B was also characterized. Corresponding experiments in the absence of DFO-B for the purpose of comparison were also conducted. Analyses for soluble Pb were conducted by AA and ICP-AES. Solid characterization was conducted by XRD and SEM-EDX. Analyses for soluble Pb reveal concentrations of up to ca. 40 ppm. White small-sized particles were identified after reaction, regardless of the presence of DFO-B. The suspension chemical composition influenced colloidal stability. In the presence of DFO-B, forming solids showed small aspect ratio ( < 2m), while remained suspended in supernatant solutions. In the absence of DFO-B, solids formed showed varied in size. The were found to remain associated with the Pb(0). Flocculation was observed to occur notingly if in the absence of DFO- B. Intermolecular forces at the mineral-water interface are sought to account for these observatons. As revealed by XRD and SEM-EDX, solid composition is attributed to various lead oxide and hydroxides. The presence of DFO-B influence the solid composition. Thermodynamic considerations lead the authors to suggest that sideophore ligands could act as templates for preferential nucleation of secondary phases, while effectively abating the reaction activation energy of formation.