Isabelle Le Hécho

Selenium speciation analysis at trace level in soils.

This paper describes the development of an analytical methodology to determine speciation of selenium present in soils at trace level (μg kg(-1)). The methodology was based on parallel single extractions and high performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC-ICPMS). Two complementary chromatographic separations were used to confirm Se species identity. Different extractants, selected on the basis of sequential extraction schemes, were compared. Ultrapure water, 0.1 molL(-1) phosphate buffer (KH(2)PO(4)/K(2)HPO(4)) at pH 7 and 0.1 molL(-1) sodium hydroxide extractants were finally chosen owing to their efficiency in extracting Se and compatibility with Se species stability. These extractants allow also assessing respectively water-soluble Se (i.e. the most mobile Se fraction), exchangeable Se (i.e. sorbed onto soil component surface) and Se bound to soil organic matter. This methodology gives thus information on Se mobility related to its distribution in soil with preservation of original Se speciation. Detection limits range from 3 to 29ng(Se)L(-1) and from 0.1 to 10 μg(Se)kg(-1), allowing determination of Se species concentrations in extracts from soils containing native Se at trace level. The methodology was applied to three soils with total Se concentrations between 210 and 1560 μg(Se)kg(-1).

Distribution and speciation of ambient selenium in contrasted soils, from mineral to organic rich.

Selenium adsorption onto oxy-hydroxides mainly controls its mobility in volcanic soils, red earths and soils poor in organic matter (OM) while the influence of OM was emphasized in podzol and peat soils. This work aims at deciphering how those solid phases influence ambient Se mobility and speciation under less contrasted conditions in 26 soils spanning extensive ranges of OM (1-32%), Fe/Al oxy-hydroxides (0.3-6.1%) contents and pH (4.0-8.3). The soil collection included agriculture, meadow and forest soils to assess the influence of OM quality as well. Trace concentrations of six ambient Se species (Se(IV), Se(VI) and 4 organo-Se compounds) were analyzed by HPLC-ICP-MS in three extractants (ultrapure water, phosphate and sodium hydroxide) targeting Se associated to different soil phases. The Kd values determined from ultrapure water extraction were higher than those reported in commonly used short-term experiments after Se-spiking. Correlations of ambient Se content and distribution with soil parameters explained this difference by an involvement of slow processes in Se retention in soils. The 26 Kd values determined here for a wide variety of soils thus represent a relevant database for long-term prediction of Se mobility. For soils containing less than 20% OM, ambient Se solubility is primarily controlled by its adsorption onto crystalline oxy-hydroxides. However, OM plays an important role in Se mobility by forming organo-mineral associations that may protect adsorbed Se from leaching and/or create anoxic zones (aggregates) where Se is immobilized after its reduction. Although for the first time, inorganic Se(IV), Se(VI) and organo-Se compounds were simultaneously investigated in a large soil collection, high Se proportions remain unidentified in each soil extract, most probably due to Se incorporation and/or binding to colloidal-sized OM. Variations of environmental factors regulating the extent of OM-mineral associations/aggregation may thus lead to changes in Se mobility and bio-availability.

Single walled carbon nanotube length determination by asymmetrical-flow field-flow fractionation hyphenated to multi-angle laser-light scattering

Asymmetrical flow field-flow fractionation (AFlFFF) hyphenated to multi-angle laser-light scattering (MALS) was evaluated in order to determine single walled carbon nanotube (SWCNT) length distribution. Fractionation conditions were investigated by examining mobile phase ionic strength and pH, channel components and cross-flow rate. Ammonium nitrate-based mobile phase with 10(-5)molL(-1) ionic strength and pH 6 allows the highest sample recovery (89±3%) to be obtained and the lowest loss of the longest SWCNT. A cross-flow rate of 0.9mLmin(-1) leads to avoid any significant membrane-sample interaction. Length was evaluated from gyration radius measured by MALS by comparing SWCNT to prolate ellipsoid. In order to validate the fractionation and the length determination obtained by AFlFFF-MALS, different SWCNT aliquots were collected after fractionation and measured by dynamic light scattering (DLS). AFlFFF is confirmed to operate in normal mode over 100-2000nm length. MALS length determination after fractionation is found to be accurate with 5% RSD. Additionally, a shape analysis was performed by combining gyration and hydrodynamic radii.

Nanoparticle characterization by cyclical electrical field-flow fractionation.

In this work, the analytical potential of cyclical electrical field flow fractionation (CyElFFF) for nanomaterial and colloidal particle characterization has been experimentally demonstrated. Different operating parameters were investigated in order to evaluate their effect on the mechanisms of retention and fractionation power of CyElFFF. The voltage and frequency of the oscillating electrical field appeared to be the most influential parameters controlling the separation mode. Mobile phase flow rate was also found to be a key parameter controlling the fractionation efficiency. This work allowed the definition of operating conditions such that a reliable CyElFFF analysis could be performed on different nanoparticles on the basis of the direct comparison of their theoretical and experimental behavior. The results show that this technique in optimized conditions is a powerful tool for electrophoretic mobility based separation and characterization of various nanoparticles.

Tributyltin and triphenyltin uptake by lettuce.

This paper provides quantitative information on the transfer of TBT (tributyltin) and TPhT (triphenyltin) from sludged soil to cultivated lettuce. The effect of their initial concentrations in the soil (varying from 20 to 50 microg(Sn)kg(-1) for each triorganotin), sludge amount (between 1% and 9%), and cultivation duration (32-54 days) was evaluated by means of experimental designs. The impact of the cultivation temperature at 13 degrees C and 19 degrees C on organotin fate in the soil/plant system was also considered. The final concentration of a given organotin in the plant roots was found to depend directly on its initial concentration in the soil. A total of (85+/-15)% of initial TBT in the soil was still present at the end of the experiments, regardless of the cultivation duration. Consequently, TBT appeared to be taken up by lettuce continually. A total of (75+/-5)% of TPhT was found to be degraded in the soil at 54 days. So, this compound could have been taken up by the plant at the beginning of the cultivation. Sludge amount seemed to have a negative effect on TPhT concentration in a plant at 32 days. This could be due to the quantitative TPhT sorption onto the sludge, observed just after spiking. Organotin plant uptake appeared to be more important at 19 degrees C than at 13 degrees C. TBT and TPhT were mainly accumulated in the roots, and up to 2% and 10% of TPhT and TBT, respectively, were translocated to the shoots. Despite TPhT degradation, products in large amounts were present in the soil and were not significantly taken up by the plant. They possibly remained immobilized on solid phases of the sludged soil.

Field study of time-dependent selenium partitioning in soils using isotopically enriched stable selenite tracer

A better understanding of selenium fate in soils at both short and long time scales is mandatory to consolidate risk assessment models relevant for managing both contamination and soil fertilization issues. The purpose of this study was thus to investigate Se retention processes and their kinetics by monitoring time-dependent distribution/speciation changes of both ambient and freshly added Se, in the form of stable enriched selenite-77, over a 2-years field experiment. This study clearly illustrates the complex reactivity of selenium in soil considering three methodologically defined fractions (i.e. soluble, exchangeable, organic). Time-dependent redistribution of Se-77 within solid-phases having different reactivity could be described as a combination of chemical and diffusion controlled processes leading to its stronger retention. Experimental data and their kinetic modeling evidenced that transfer towards less labile bearing phases are controlled by slow processes limiting the overall sorption of Se in soils. These results were used to estimate time needed for (77)Se to reach the distribution of naturally present selenium which may extend up to several decades. Ambient Se speciation accounted for 60% to 100% of unidentified species as function of soil type whereas (77)Se(IV) remained the more abundant species after 2-years field experiment. Modeling Se in the long-term without taking account these slow sorption kinetics would thus result in underestimation of Se retention. When using models based on Kd distribution coefficient, they should be at least reliant on ambient Se which is supposed to be at equilibrium.

Stable isotope tracing: a powerful tool for selenium speciation and metabolic studies in non-hyperaccumulator plants (ryegrass Lolium perenne L.).

Selenium is both essential and toxic for mammals; the range between the two roles is narrow and not only dose-dependent but also related to the chemical species present in foodstuff. Unraveling the metabolism of Se in plants as a function of Se source may thus lead to ways to increase efficiency of fertilization procedures in selenium deficient regions. In this study, stable-isotope tracing was applied for the first time in plants to simultaneously monitor the bio-incorporation of two inorganic Se species commonly used as foodstuff enrichment sources. Occurrence and speciation of Se coming from different Se sources were investigated in root and leaf extracts of ryegrass (Lolium perenne L.), which had been co-exposed to two labeled Se species (77SeIV and 82SeVI). Although the plant absorbed similar amounts of Se when supplied in the form of selenite or selenate, the results evidenced marked differences in speciation and tissues allocation. Selenite was converted into organic forms incorporated mostly into high molecular weight compounds with limited translocation to leaves, whereas selenate was highly mobile being little assimilated into organic forms. Double-spike isotopic tracer methodology makes it possible to compare the metabolism of two species-specific Se sources simultaneously in a single experiment and to analyze Se behavior in not-hyperaccumulator plants, the ICP-MS sensitivity being improved by the use of enriched isotopes.

Iodine distribution and cycling in a beech (Fagus sylvatica) temperate forest

Radioiodine is of health concerns in case of nuclear events. Possible pathways and rates of flow are essential information for risk assessment. Forest ecosystems could influence the global cycle of long-lived radioiodine isotope (129I) with transfer processes similar to stable isotope (127I). Understanding iodine cycling in forest involves study of the ecosystem as a whole. In this context, we determined the 127I contents and distribution in soil, tree compartments and atmospheric inputs during a three years in situ monitoring of a temperate beech forest stand. The iodine cycle was first characterized in terms of stocks by measuring its concentrations in: tree, litterfall, humus, soil, rainfall, throughfall, stemflow and soil solutions. Main annual fluxes (requirement, uptake and internal transfers) and forest input-output budget were also estimated using conceptual model calculations. Our findings show that: (i) soil is the main I reservoir accounting for about 99.9% of ecosystem total stock; (ii) iodine uptake by tree represents a minor fraction of the available pool in soil (<0.2%); (iii) iodine allocation between tree compartments involves low immobilization in wood and restricted location in the roots; (iv) translocation of excess iodine towards senescing foliage appears as an elimination process for trees, and (v) litterfall is a major pathway in the I biological cycling. In our soil conditions, the input - output budget shows that the ecosystem behaves as a potential source of I for groundwater.