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PubMed2005

Metal speciation in anoxic sediments: when sulfides can be construed as oxides.

Metal speciation in aquatic sediments is often characterized using wet chemical sequential extraction techniques. However, these methods are operationally defined and subject to artifacts, particularly when dealing with anoxic sediments, in which metal sulfide precipitates are likely to occur. Using X-ray absorption spectroscopy (XAS) and acid-volatile sulfide (AVS) analysis, we evaluated the effectiveness of one of the most widely used sequential extraction protocols, the Tessier method, at determining Zn and Pb speciation in anoxic wetland sediments. Sequential extraction results significantly underestimated the amount of Zn associated with sulfide phases as compared to the other two approaches. XAS analysis of ZnS amended sediments indicates that the most likely source of this conflict is an early dissolution of amorphous metal sulfide phases during the sequential extraction step corresponding to the extraction of iron and manganese oxides. The reagent mixture used in this step, hydroxylamine hydrochloride-HCl, is widely used in other sequential extraction protocols, including the BCR method, limiting their application to anoxic sediments. For this reason, current sequential extraction techniques should only be used on anoxic sediments with caution, and/or in conjunction with complementary approaches to assess metal speciation.

Chemistry Techniques, Analytical↗
PubMed2005

Variation in arsenic speciation and concentration in paddy rice related to dietary exposure.

Ingestion of drinking water is not the only elevated source of arsenic to the diet in the Bengal Delta. Even at background levels, the arsenic in rice contributes considerably to arsenic ingestion in subsistence rice diets. We set out to survey As speciation in different rice varieties from different parts of the globe to understand the contribution of rice to arsenic exposure. Pot experiments were utilized to ascertain whether growing rice on As contaminated soil affected speciation and whether genetic variation accounted for uptake and speciation. USA long grain rice had the highest mean arsenic level in the grain at 0.26 microg As g(-1) (n = 7), and the highest grain arsenic value of the survey at 0.40 microg As g(-1). The mean arsenic level of Bangladeshi rice was 0.13 microg As g(-1) (n = 15). The main As species detected in the rice extract were AsIII, DMAV, and AsV. In European, Bangladeshi, and Indian rice 64 +/- 1% (n = 7), 80 +/- 3% (n = 11), and 81 +/- 4% (n = 15), respectively, of the recovered arsenic was found to be inorganic. In contrast, DMAV was the predominant species in rice from the USA, with only 42 +/- 5% (n = 12) of the arsenic being inorganic. Pot experiments show that the proportions of DMAV in the grain are significantly dependent on rice cultivar (p = 0.026) and that plant nutrient status is effected by arsenic exposure.

Arsenicals↗
PubMed2005

Controls on arsenic speciation and solid-phase partitioning in the sediments of a two-basin lake.

Arsenic (As) regeneration from sediments of Spy Pond, a two-basin lake near Boston, MA, continues to result in seasonally elevated As levels despite the several decades that have elapsed since the pond's historical contamination by a pulse input of As. Solid-phase speciation and partitioning of As in the sediments appear to be primary determinants of both potential As regeneration rate and, conversely, the rate at which long-term burial of the pond's As burden will occur. Although iron (Fe) chemistry often controls As speciation in lakes, a higher rate of As regeneration in the south basin, accompanied by the absence of a correlation between regenerated As and Fe in this basin, suggest that additional factors operate to control As cycling in this lake. Solid-phase As speciation was remarkably similar between sediments of the two basins of this lake, about 40% of sedimentary As in upper sediments being in relatively labile (ionically bound and strongly adsorbed) fractions and a larger fraction of As being associated with recalcitrant minerals, likely sulfides. Extraction, X-ray fluorescence, and acid-volatile sulfide data collectively suggest that sedimentary As cycling in both basins is largely controlled by the formation of sulfide minerals. An accounting of the size of sulfur (S) pools supports this conclusion, showing that sufficient S exists in the north basin to control both Fe and As, while the south basin has lower S levels resulting in seasonal accumulation of Fe in the porewater.

Arsenic↗
PubMed2006

Speciation and quantitative mapping of metal species in microbial biofilms using scanning transmission X-ray microscopy.

A scanning transmission X-ray microscope illuminated with synchrotron light was used to investigate the speciation and spatial distributions of metals in a microbial biofilm cultivated from river water. Metal 2p absorption edge signals were used to provide metal speciation (through shapes of the absorption spectra) and quantitative spatial distributions of the metal species. This paper presents sample data and describes methods for extracting quantitative maps of metal species from image sequences recorded in the region of the metal 2p edges. Comparisons were made with biochemical characterization of the same region using images recorded at the C 1s and O 1s edges. The method is applied to detailed quantitative analysis of ferrous and ferric iron in a river biofilm, in concert with mapping Ni(II) and Mn(II) species in the same region. The distributions of the metal species are discussed in the context of the biofilm structure. These results demonstrate that soft X-ray STXM measurements at the metal 2p absorption edges can be used to speciate metals and to provide quantitative spatial distribution maps for metal species in environmental samples with 50 nm spatial resolution.

Biofilms↗
PubMed2004

Aqueous speciation studies of europium(III) phosphotungstate.

The incorporation of lanthanide ions into polyoxometalates may be a unique approach to generate new luminescent, magnetic, and catalytic functional materials. To realize these new applications of lanthanide polyoxometalates, it is imperative to understand the solution speciation chemistry and its impact on solid-state materials. In this study we find that the aqueous speciation of europium(III) and the trivacant polyoxometalate, PW9O34 9-, is a function of pH, countercation, and stoichiometry. For example, at low pH, the lacunary (PW11O39)7- predominates and the 1:1 Eu(PW11O39)4-, 2, forms. As the pH is increased, the 1:2 complex, Eu(PW11O39)2 11- species, 3, and (NH4)22[(Eu2PW10O38)4(W3O8(H2O)2(OH)4].44H2O, a Eu8 hydroxo/oxo cluster, 1, form. Countercations modulate this effect; large countercations, such as K+ and Cs+, promote the formation of species 3 and 1. Addition of Al(III) as a counterion results in low pH and formation of [Eu(H2O)3(alpha-2-P2W17O61)]2, 4, with Al(III) counterions bound to terminal W-O bonds. The four species observed in these speciation studies have been isolated, crystallized, and characterized by X-ray crystallography, solution multinuclear NMR spectroscopy, and other appropriate tech-niques. These species are 1, (NH4)22[(Eu2PW10O38)4(W3O8(H2O)2(OH)4].44H2O (P; a=20.2000(0), b=22.6951(6), c=25.3200(7) A; alpha=65.6760(10), beta=88.5240(10), gamma=86.0369(10) degrees; V=10550.0(5) A3; Z=2), 2, Al(H3O)[Eu(H2O)2PW11O34].20H2O (P, a=11.4280(23), b=11.5930(23), c=19.754(4) A; alpha=103.66(3), beta=95.29(3), gamma=102.31(3) degrees; V =2456.4(9) A3; Z=2), 3, Cs11Eu(PW11O34)2.28H2O (P; a=12.8663(14), b=19.8235(22), c=21.7060(23) A; alpha=114.57(0), beta=91.86(0), gamma=102.91(0) degrees ; V=4858.3(9) A3; Z=2), 4, Al2(H3O)8[Eu(H2O)3(alpha-2-P2W17O61)]2.29H2O (P; a=12.649(6), b=16.230(8), c=21.518(9) A; alpha=111.223(16), beta=94.182(18), gamma=107.581(17) degrees ; V=3842(3) A3; Z=1).

Crystallography, X-Ray↗
PubMed2004

Speciation of arsenic in different types of nuts by ion chromatography-inductively coupled plasma mass spectrometry.

In this work the quantitative determination and analytical speciation of arsenic were undertaken in different types of nuts, randomly purchased from local markets. The hardness of the whole nuts and high lipid content made the preparation of this material difficult for analysis. The lack of sample homogeneity caused irreproducible results. To improve the precision of analysis, arsenic was determined separately in nut oil and in the defatted sample. The lipids were extracted from the ground sample with the two portions of a mixture of chloroform and methanol (2:1). The defatted material was dried and ground again, yielding a fine powder. The nut oil was obtained by combining the two organic extracts and by evaporating the solvents. The two nut fractions were microwave digested, and total arsenic was determined by inductively coupled plasma mass spectrometry (ICP-MS). The results obtained for oils from different types of nuts showed element concentration in the range 2.9-16.9 ng g(-)(1). Lower levels of arsenic were found in defatted material (<0.1 ng g(-)(1) with the exception of Brazil nuts purchased with and without shells, 3.0 and 2.8 ng g(-)(1) respectively). For speciation analysis of arsenic in nut oils, elemental species were extracted from 2 g of oil with 12 mL of chloroform/methanol (2:1) and 8 mL of deionized water. The aqueous layer, containing polar arsenic species, was evaporated and the residue dissolved and analyzed by ion chromatography-ICP-MS. The anion exchange chromatography enabled separation of As(III), dimethylarsinic acid (DMAs(V)), monomethylarsonic acid (MMAs(V)), and As(V) within 8 min. Several types of nuts were analyzed, including walnuts, Brazil nuts, almonds, cashews, pine nuts, peanuts, pistachio nuts, and sunflower seeds. The recovery for the speciation procedure was in the range 72.7-90.6%. The primary species found in the oil extracts were As(III) and As(V). The arsenic concentration levels in these two species were 0.7-12.7 and 0.5-4.3 ng g(-)(1), respectively. The contribution of As in DMAs(V) ranged from 0.1 +/- 0.1 ng g(-)(1) in walnuts to 1.3 +/- 0.3 ng g(-)(1) in pine nuts. MMAs(V) was not detected in almonds, peanuts, pine nuts, sunflower seeds, or walnuts, and the highest concentration was found in pistachio nuts (0.5 +/- 0.2 ng g(-)(1)).

Arsenic↗
PubMed2005

Three-dimensional etching profiles and surface speciations (via attenuated total reflection-fourier transform infrared spectroscopy) of silicon nanowires in NH4F-buffered HF solutions: a double passivation model.

A systematic study of the etching behavior, in terms of three-dimensional profiles, of one-dimensional (1-D) silicon nanowires (SiNWs) in NH(4)F-buffered hydrofluoric acid (BHF) solutions of varying concentrations and pH values and the surface speciations of the resulting etched SiNW surfaces, as characterized by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, is reported. It was found that SiNWs are stable only in relatively narrow pH ranges of the BHF solutions. The results are rationalized in terms of a "double passivation" model. When SiNWs are etched in BHF solutions with pH values of 1-3, the surfaces are passivated with hydrogen (inner layer) giving rise to surface moieties such as Si-H(x) species (x = 1-3); at high HF concentrations, the H-terminated Si surfaces are covered with a hydrogen bonding network of HF and related molecules (oligomers, etc.), providing an outer-layer passivation. When SiNWs are etched in BHF solutions with pH values of 11-14 (by adding a strong base such as NaOH), the surfaces are oxygen-terminated with surface moieties such as Si-(O(-))(x)() species (x = 1-3); at high NH(4)F concentrations, the negatively charged Si surfaces are stabilized by NH(4)(+) ions via ionic bonding, again providing outer-layer passivation. In BHF solutions with pH values of 3-11, the surface speciation, consisting of Si-(OH)(x)(O(-))(y) (x + y = 1-3) species, is unstable and etched away rapidly. The surface speciations of SiNWs etched in various BHF solutions were explored via ATR-FTIR spectroscopy. It was found that, while etching SiNWs with HF-rich BHF solutions with pH < 4 gave rise to Si-H(x)() surface species, no surface Si-H(x) species were observed with SiNWs etched in BHF solutions with pH >/= 4 (HF/NH(4)F </= 1:1). In sharp contrast, etching of two-dimensional (2-D) Si wafers with either HF, NH(4)F, or BHF etchants produces Si-H(x) species on the surface. Finally, while HF is a much more efficient etchant than NH(4)F for 2-D Si surfaces, NH(4)F is found to be as efficient an etchant as HF for SiNWs (this work). We believe that these differences can be attributed to the nanometer size (corresponding to the roughness scale in 2-D wafers) of SiNWs on one hand and the lack of "passivation" by the hydrogen bonding network of HF and related molecules at pH >/= 4 on the other. These two factors, among others, contribute to the rapid hydrolysis of the surface Si-H(x)() species (and the etching of the SiNWs), particularly in BHF solutions with low HF/NH(4)F ratios and high pH values (pH >/= 4).

Journal Article↗
PubMed2001

Chemical speciation drives hydrothermal vent ecology.

The physiology and biochemistry of many taxa inhabiting deep-sea hydrothermal vents have been elucidated; however, the physicochemical factors controlling the distribution of these organisms at a given vent site remain an enigma after 20 years of research. The chemical speciation of particular elements has been suggested as key to controlling biological community structure in these extreme aquatic environments. Implementation of electrochemical technology has allowed us to make in situ measurements of chemical speciation at vents located at the East Pacific Rise (9 degrees 50' N) and on a scale relevant to the biology. Here we report that significant differences in oxygen, iron and sulphur speciation strongly correlate with the distribution of specific taxa in different microhabitats. In higher temperature (> 30 degrees C) microhabitats, the appreciable formation of soluble iron-sulphide molecular clusters markedly reduces the availability of free H2S/HS- to vent (micro)organisms, thus controlling the available habitat.

Animals↗
PubMed2005

Sexual behaviour: rapid speciation in an arthropod.

Theory predicts that sexual behaviour in animals can evolve rapidly, accelerating the rate of species formation. Here we estimate the rate of speciation in Laupala, a group of forest-dwelling Hawaiian crickets that is characterized primarily through differences in male courtship song. We find that Laupala has the highest rate of speciation so far recorded in arthropods, supporting the idea that divergence in courtship or sexual behaviour drives rapid speciation in animals.

Animal Communication↗
PubMed1987

Chromosomal rearrangements, speciation and the theoretical approach.

Many theoretical papers investigating the relationship between chromosomal change and speciation are found to have been based on erroneous data. For rather than considering those negatively heterotic, or at least potentially negatively heterotic rearrangements which can have a possible role in speciation, these papers have included substantial amounts of information on rearrangements which are not implicated in this process. Common forms of chromosomal repatterning such as heterochromatic addition and polymorphism are in this category. Their inclusion in theoretical studies, often cited as supporting or opposing a chromosomal involvement in speciation, invalidates these findings. A new approach is suggested.

Biological Evolution↗
PubMed2003

Speciation by host switch in brood parasitic indigobirds.

A growing body of empirical and theoretical work supports the plausibility of sympatric speciation, but there remain few examples in which all the essential components of the process are well understood. The African indigobirds Vidua spp. are host-specific brood parasites. Indigobird nestlings are reared along with host young, and mimic the mouth markings of their respective hosts. As adults, male indigobirds mimic host song, whereas females use these songs to choose both their mates and the nests they parasitize. These behavioural mechanisms promote the cohesion of indigobird populations associated with a given host species, and provide a mechanism for reproductive isolation after a new host is colonized. Here we show that all indigobird species are similar genetically, but are significantly differentiated in both mitochondrial haplotype and nuclear allele frequencies. These data support a model of recent sympatric speciation. In contrast to the cuckoo Cuculus canorus, in which only female lineages are faithful to specific hosts, host switches have led to speciation in indigobirds because both males and females imprint on their hosts.

Africa↗
PubMed2004

Evidence for ecology's role in speciation.

A principal challenge in testing the role of natural selection in speciation is to connect the build-up of reproductive isolation between populations to divergence of ecologically important traits. Demonstrations of 'parallel speciation', or assortative mating by selective environment, link ecology and isolation, but the phenotypic traits mediating isolation have not been confirmed. Here we show that the parallel build-up of mating incompatibilities between stickleback populations can be largely accounted for by assortative mating based on one trait, body size, which evolves predictably according to environment. In addition to documenting the influence of body size on reproductive isolation for stickleback populations spread across the Northern Hemisphere, we have confirmed its importance through a new experimental manipulation. Together, these results suggest that speciation may arise largely as a by-product of ecological differences and divergent selection on a small number of phenotypic traits.

Alaska↗
PubMed2005

Ecology: is speciation driven by species diversity?

Emerson and Kolm show that the proportion of species endemic to an island is positively related to its species richness and, assuming that endemism indexes speciation rate, they infer that greater species diversity accelerates diversification. Here we demonstrate that the same correlation between species richness and percentage endemism can arise even if within-island speciation is negligible, particularly when both endemism and species richness depend on attributes of islands (such as area) that influence the average age of resident populations. Island biogeography theory indicates that, where the average time to extinction is relatively long, diversity increases through colonization, irrespective of whether new species are formed; at the same time, islands on which populations persist for longer accumulate more endemic species as local populations differentiate and populations on neighbouring islands become extinct. We therefore suggest that species richness and endemism are correlated fortuitously owing to their mutual dependence on the life spans of populations on islands, which is unrelated to speciation itself.

Animals↗
PubMed1998

Speciation and absolute bioavailability: risk assessment of arsenic-contaminated sites in a residential suburb in Canberra.

Watson is a fully developed suburb of some 30 years in Canberra (the capital city of Australia). A plunge dip using arsenical pesticides for tick control was operated there between 1946 and 1960. Chemical investigations revealed that many soil samples obtained from the study area contained levels of arsenic exceeding the current health-based investigation levels of 100 mg kg-1 set by the National Healthy and Medical Research Council in Australia. For the speciation study, nine composite samples of surface and sub-surface soils and a composite samples of rocks were selected. ICP-MS analysis showed that arsenic levels in these samples ranged from 32 to 1597 mg kg-1. Chemical speciation of arsenic showed that the arsenite (trivalent) components were 0.32-56% in the soil and 44.8% in the rock composite samples. Using a rat model, the absolute bioavailability of these contaminated soils relative to As3+ or As5+ ranged from 1.02 to 9.87% and 0.26 to 2.98%, respectively. An attempt was made to develop a suitable leachate test as an index of bioavailability. However, the results indicated that there was no significant correlation between the bioavailability and leachates using neutral pH water or 1M HC1. Our results indicate that speciation is highly significant for the interpretation of bioavailability and risk assessment data; the bioavailability fractions of arsenic in soils from Watson are small and therefore the healthy impact upon the environment and humans due to this element is limited.

Animals↗
PubMed2000

Use of perchloric acid as a reaction medium for speciation of arsenic by hydride generation-atomic absorption spectrometry.

Simple and inexpensive methods for the speciation of arsenite, arsenate, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in environmental water samples were developed. In these methods a hydride generation-atomic absorption spectrometry (HG-AAS) technique was employed and perchloric acid (as a reaction medium), L-cysteine (as a pre-reducing agent for a certain contact time between its addition and analysis) and sodium tetrahydroborate(III) (NaBH4, as a reducing agent) were used. The use of L-cysteine greatly enhances the absorption signals of all four arsenic species at low acid concentration (0.001-0.04 M). The methods developed for the determination of total arsenic and total inorganic arsenic and speciation of the four arsenic species in environmental water samples are as follows. (i) DMA: 0.005 M acid and 0.04% NaBH4 in the absence of L-cysteine. DMA can also be speciated in the presence of L-cysteine as follows: 2 M acid, 2.5% L-cysteine after a contact time of approximately 5 min and 0.6% NaBH4. (ii) As(III): 5 M acid and 0.08% NaBH4 in the absence of L-cysteine. (iii) Total inorganic arsenic (As(III) + As(V)]: 8 M acid and 0.6% NaBH4 in the absence of L-cysteine. (iv) Total arsenic: 0.01 M acid, 5% L-cysteine after a contact time of 5 min and 2% NaBH4. (v) MMA: 8 M acid, 3% L-cysteine after a contact time of 50 min and 0.6% NaBH4. (vi) As(V): by difference. Detection limits and recoveries of added spikes for all analyses were found to be 0.5-1.7 ppb and 90-112% respectively.

Arsenic↗
PubMed1992

Use of radiochemical methods as tools for speciation purposes in environmental and biological sciences.

Chemical speciation for a few elements can be facilitated to a great extent by incorporating a suitable radioisotope into the system and measuring the radiation of the isolated species. This radiospiking can be applied to in vitro and in vivo labelled experiments. Radionuclides are, however, also present as an anthropogenic contaminant from various nuclear fission activities. The radiotracer should be added under such conditions that it behaves in exactly the same way as the isotopes it represents. It should possess an adequate radioactive half-life, and preferably be a gamma-emitter because of the ease of detection. Radiotracer labelling is now widely used to study speciation problems of many essential and toxic elements in body fluids and tissues. It can be used to trace the different locations where the element is metabolized and stored, and subsequently to detect the element in the isolated biocomponents. The determination of the location of a radiotracer in a cell by autoradiography proved to be impractical because of the lack of resolution. Radiochemistry is similarly very useful for investigating particular aspects of the speciation of heavy metals as they occur in the ecosystem, and to follow the fate and effects of fission nuclides in the environment as they are carried around by the water and air masses. However, in certain circumstances the behaviour of fission products appears to be different from that of their stable analogues. For the actinides they simply do not exist. Radiochemical methods are a major tool for identifying and quantifying the nuclides in the different species.

Animals↗
PubMed1995

Aluminium and silicon speciation in human serum by lon-exchange high-performance liquid chromatography-electrothermal atomic absorption spectrometry and gel electrophoresis.

Speciation of aluminium and silicon in serum was studied by a reliable and sensitive high-performance liquid chromatographic-electrothermal atomic absorption spectrometric (HPLC-ETAAS) hybrid method, based on the use of a polymeric anion-exchange column (Protein-Pak DEAE-5PW). This polymer-based column minimizes the risk of aluminium losses and of silicon contamination from the column during separation. The results obtained were compared with the results of previous studies carried out using different, complementary techniques including ultramicrofiltration, gel filtration and silica-based column for HPLC. In order to ascertain which protein(s) of serum actually bind(s) aluminium, gel electrophoresis was employed for the further separation of the column fractions obtained by HPLC and aluminium was determined in separate aliquots of the same fractions. From all the experiments, it appears that transferrin (Tf) is the only serum protein that binds aluminium and it contains about 90% of total serum aluminium. It was also confirmed that in the presence of desferrioxamine (DFO). aluminium is partly displaced from its complex with transferrin to a low molecular mass AL-DFO complex. Aluminum citrate seems to be the main low molecular mass aluminium species in serum, amounting to about (12 +/- 5% of the total aluminium in an aluminium-loaded serum sample. The proposed speciation procedure permits the simultaneous identification and determination of three aluminium species in metal-spiked serum (Al-Tf, Al-DFO and AI-citrate). The result for silicon suggest that it seems to be unspecifically adsorbed to several serum proteins and its speciation is not affected by the presence of DFO.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum↗
PubMed2001

Trace element speciation at cell membranes: aqueous, solid and lipid phase effects.

Biological interest in trace element speciation has tended to be polarised in very different ways according to the concerns of the investigators. Much of the ecological interest has centred on the effects of metal ions in the external environment with the "free metal ion model" dominating discussions of potential toxicity. By way of contrast biochemists have been much more concerned with the stereochemical and kinetic aspects of specificity in metal protein interactions in the cytoplasm. Separating these two sets of interests are the membrane biophysicists whose studies have concentrated on the channel concept. All three groups have tended to ignore speciation onto the solid phase. In the overall biological context, trace element speciation in the cell is more concerned with kinetics and the evolution of specificity of interaction between diverse ligands than with the conditions for equilibrium.

Animals↗