Environmental sciences at the ESRF

28 Vol. 23, No. 5, 2010, SYNCHROTRON RADIATION NEWS

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Environmental Sciences at the ESRF

M. COTTE1,2, M. AUFFAN3, W. DEGRUYTER4, I. FAIRCHILD5, M. A. NEWTON1, G. MORIN6, G. SARRET7, AND A. C. SCHEINOST8 1European Synchrotron Radiation Facility, Grenoble, France 2Centre of Research and Restoration of French Museums, Paris, France 3CEREGE, CNRS/Aix-Marseille Universite, Aix-en-Provence, France 4Section of Earth and Environmental Sciences, University of Geneva, Switzerland 5Geography, Earth and Environmental Sciences, University of Birmingham, UK 6Institut de Minéralogie et de Physique des Milieux Condensés, Paris, France 7LGIT, Université Joseph Fourier, Grenoble, France 8Forschungszentrum Dresden, Rossendorf, Germany

In the past two decades, environmental sciences have increased theirshare in the research portfolios of European synchrotrons, not least fortheir topicality from a societal point of view. As this field overlaps withmany other disciplines, the ESRF decided to establish, in 2005, a dedi-cated Review Panel for Environmental Science and Cultural Heritage.

This facility report summarizes recent trends and results from theESRF to highlight the different disciplines, techniques and topics. Itshould also be noted that environmental studies encompass both a betterunderstanding of natural phenomena and monitoring the impact ofhuman activity on nature.

Better understanding the natural environment Volcanic eruptions can be benign effusive outpourings of lava, but

also violent explosions with dramatic effects. Which outcome an erup-tion takes is determined by the composition and dynamics of the magmaas it rises through the volcanic conduit from the magma chamber to theEarth’s surface. Decompression during ascent of water-rich magmaleads to volatile saturation, followed by nucleation and growth of gasbubbles. This bubbly flow accelerates upward; the bubbles becomedensely packed and coalesce. Permeable bubble networks form and gascan escape from the magma. When this outgassing process is efficient,eruptions lose their energy and may effuse as lava flows or as domes.However, when exsolved gas cannot escape easily, the magma foammay abruptly fragment to form a turbulent gas flow with pumices(highly vesicular fragments of solidified magma) in suspension. Themicroscopic structure of these pumices provides key insight into theeffusive-explosive transition of volcanic eruptions, as it captures asnapshot of the state of the magma foam just prior to fragmentation.

A recent experiment [1] investigated pumices from past explosiveeruptions using X-ray microtomography imaging at beamline ID19 toquantify magma outgassing as a function of the pumices’ internal topol-ogy and geometry. To this end, a single-phase fluid flow through the

pumices was calculated numerically, using the 3D images as boundaryconditions (see Figure 1). Compared to laboratory measurements, thisapproach has the advantage that the velocity distribution can be linkedto the microscopic structure of the pumices. The results show howmagma outgassing depends on connected porosity, tortuosity, and spe-cific surface area, and they validate a simple permeability model foroutgassing. The main drawback is the need to capture a representative

Figure 1: Visualization of simulated gas flow through pumice sampleKPT32c produced by the Kos Plateau Tuff eruption (Aegean Arc). Thepumice walls (black) are partially shown and superimposed on the flowstreamlines. The color gradient of the streamlines ranges from blue (mini-mum velocity) to red (maximum velocity). Courtesy W. Degruyter et al.

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volume at high spatial resolution, which is now possible thanks torecent progress in X-ray tomography and computing power.

Knowledge of the oxidation state of sulfur in inclusions or naturalglasses in volcanic rock is of great interest as these solid bodies areconsidered as frozen pictures of their molten counterparts [2, 3]. Parallelin-situ measurements are essential to fully understand the geochemicalbehavior of chemical elements in magma. For example, the partitioningof Ba, La, Y, Sm and Eu between aqueous solutions and alumosilicatemelts at 750°C was recently studied for various pressures and salinities.Because these magmatic fluids are not quenchable, the trace elementconcentrations were determined in-situ at high pressures and tempera-tures using a diamond-anvil cell and micro X-Ray Flourescence (µXRF)analyses [4]. This has now become possible thanks to the high energybeam (50keV) with μm spot size at beamline ID22.

Paleo-environmental studies at high resolution The atomic structure and composition of an object may contain

information about the Earth’s past environment. Ice cores are well-known examples, but also speleothems, for example stalagmites fromlimestone caves, are of great interest. They form by dripping of mineral-ized water and deposition of calcium carbonate and can reveal informa-tion on past seasonal rainfall and draught.

Recent experiments [5] at beamlines ID21 and ID22 used µXRF on anannually laminated calcareous deposit from Ernesto cave (Italy) to estab-lish that the visible fluorescent lamina is characterized by the delivery ofa suite of trace elements (including Y, Pb, Cu, Zn, Br, and P). Enhancedlevels of these ions appear to characterize both seasonal flushing fromthe soil zone and longer periods when vegetation cover is disturbed.

The structure of autumn flushing events has been revealed in thestudy of a similar type of speleothem calcite, but enriched in Zn and Pb,from Obir cave (Austria). Within a general period of metal enrichment,micron-scale concentrated zones appear to represent events of only afew days duration (see Figure 2). The element supply (horizontal struc-ture in Figure 2) is complemented by an oblique structure, moststrongly visible in the P and Mg maps, representing crystallographicallycontrolled differential element incorporation [6].

Several years ago, work at beamline ID21 demonstrated that spele-othems can also preserve information on past atmospheric sulfur fluxes.This has now been complemented by a detailed study of sulfur incorpo-ration in conifers close to the Ernesto cave [7]. µXRF mapping andXANES studies at the S edge reveal that S is concentrated in the innercell wall in the fir tree Abies alba and that a faithful record of the 20thcentury sulfur pollution can be obtained once localized hotspots arediscarded.

Human activity and the environment The chemical speciation of contaminants and nutrients plays a

pivotal role in determining their transfer in soils and sediments and theiruptake by living organisms. Likewise, understanding how microorgan-isms and plants accumulate contaminants and cope with toxicity isimportant for food safety and decontamination alike.

Many plant and soil scientists are now using synchrotron tech-niques. For a recent review, see [8] where details of the experimentsbelow and further references can also be found.

Figure 2: X-ray fluorescence mapping of Obir cave stalagmite. Analysesperformed at ID22 (left), and ID21 (right) with an excitation of 23keVand 2.9keV, respectively. The left figure shows several annual bands of Pband Zn enrichment, which are imaged at various excitation depths belowthe surface, and so is a pseudo-3D image. The right figure is an enlarge-ment of a single annual band with a small depth of X-ray excitation [6].Courtesy I. Fairchild et al.

Figure 3: Quantitative µXRF maps showing the distribution of Au, Ca,Cu, Fe, S, and Zn in an individual cell of Cupriavidus metalliduransCH34. The concentrations (errors) given in the images relate to thedelineated areas. Courtesy F. Reith et al.



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The ESRF beamlines BM30B (FAME), ID26, BM08 (GILDA),BM20 (ROBL), and BM26 (DUBBLE) have used X-ray absorption tech-niques (bulk EXAFS and XANES) to provide insight on the fate of metalliccontaminants in soils, plants, and microorganisms. Characterizing dilutedelements in heterogeneous matrices and in complex systems in general isoften based on structural knowledge obtained on simplified systems,including single minerals and organic matter.

The combination of bulk and laterally resolving techniques(µXAS, µXRF, and µXRD) provides a more complete and reliabledescription of complex systems. Beamline ID21 (spectromicroscopywith soft X-rays, µXRF, and µXAS) has been largely used to studysoils, plants, and wastes; for example, the relationship between Fe andS in soil aggregates and the localization and speciation of Cd in plantsamples. New insight on sulfide alteration was obtained on ID21 andID18F.

The nano-imaging endstation ID22NI was used to show that abacterium, Cupriavidus metallidurans CH34, can transform toxic goldcompounds to their metallic form using an active cellular mechanism[9] (see Figure 3). CH34 rapidly accumulates toxic gold complexesfrom a solution prepared in the lab, a process that pushes the bacteriumto induce oxidative stress and metal resistance clusters as well as an as-yet uncharacterized Au-specific gene cluster in order to defend its cellu-lar integrity. This leads to active biochemically mediated reduction ofgold complexes to nano-particulate, metallic gold. This was the firstdirect evidence that bacteria can be actively involved in the cycling of

rare and precious metals, a result which opens the door to, for example,the production of new types of biosensors.

Arsenic – ever more dangerousArsenic and many of its compounds are potent poisons. Their

presence as water pollutants in many parts of the world is a major envi-ronmental concern, mostly linked to volcanic sources, weathering ofAs-bearing rocks, mining and industrial operations, and the use ofarsenical pesticides. Extensive research has revealed that As sorptionand redox reactions play a crucial role in As cycling and toxicity.

However, accurate predictions of As transfers in soils and aquifers suf-fer from the scarcity of data addressing molecular-level speciation ofarsenic in field samples, especially in soils, as well as in sulfidic and reduc-ing environments. Better understanding of the surface reactivity of nano-sorbents for As is also needed to improve remediation strategies. RecentXAS experiments at beamlines BM30B (FAME), BM08 (GILDA), BM20(ROBL), ID26, and BM29 have closed some of these gaps: for example, in-situ XAS studies of As speciation in samples from polluted sites demon-strated that As binding to ferric-(oxyhydr)oxides and -hydroxysulfates is theultimate sink for this element after weathering of arsenate minerals in soilsor of sulfide ore minerals in acid mine drainage [10, 11] (see Figure 4).

XAS also revealed important effects of the microbial reduction ofAs(V)-sorbed iron oxyhydroxides on As mobility in anoxic conditions[12, 13], which is a key issue for understanding contamination ofgroundwaters.

0.0 0.5 1.00

10

20

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th (

cm)

As (wt%)

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ArseniosideriteCa2Fe3(AsO4)3O2 2O

k (Å-1)

Soil 20 - 30 cm

Soil 0 - 5 cmoxalate treated

Soil 15 - 25 cm

k3(k

)

Figure 4: Vertical evolution of arsenic speciation [10] in a heavily polluted sandy soil near a former arsenical pesticides processing plant in Auzon (France).After weathering of the calcium ferric arsenate mineral arseniosiderite in the topsoil horizon, coprecipitation with — and/or sorption to — poorly orderednano-sized iron oxyhydroxides as ferrihydrite are the ultimate natural attenuation processes that delay As migration downward through the soil profile.Courtesy B. Cancès et al.



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Synchrotron XRD and XAS at beamline ID11 recently permitted usto understand how arsenate substitutes to sulfate in gypsum [14].Another XAS laboratory study on BM20 distinguished for the first timeamong various dissolved thio-arsenic species still poorly investigated infield studies, which may influence biogeochemical cycling of S and Asin volcanic springs and mine drainage [15].

Finally, EXAFS investigations of the sorption modes of the mosttoxic As(III) species at the surface of nanomaghemite have helped toexplain the exceptional sorption efficiency of these compounds [16, 17].The high capacity of nano-materials to sorb metals like As has thepotential for new nanofiltration and adsorption techniques for waterpurification.

Nanoparticles: dangerous or useful? The technological, scientific, and commercial potentials of the

“nano-sciences” point to a new industrial revolution with a substantialsocietal dimension. The promises, notably to combat pollution and dis-eases, must be confronted with the fact that the reactivity and small sizeof nano-materials may also have negative environmental and biologicaleffects.

Studying the environmental behaviors and implications of nanoparti-cles requires multi-scale and interdisciplinary approaches. The improved

instrumentations of BM30 (FAME) were essential to probe unique andhighly reactive adsorption sites at the surface of manufactured nanosphere[16] and natural aluminosilicate nanotubes [20]. These sites have a strongreactivity towards pollutants in solution (e.g. arsenic) and in natural volca-nic soils. They are able to adsorb about 82% of the total nickel present inthe soil of La Reunion island (France) [20].

For example, the arsenic-adsorbing compounds described aboveare not alone: interesting vacant sites were even observed at the surfaceof natural aluminosilicate nanoparticles present in natural soils at LaReunion (France), which adsorb about 82% of the total Ni present in thesoil [20].

As their properties differ from that of bulk materials, the issuewhether “new” surface properties of nanoparticles may lead to “new”toxicity is central. This is the case of the CeO2 nanoparticles whichare used commercially for fast Ce4+/Ce3+ exchanges linked to oxygenstorage. However, such valence exchanges also occur in mammaliancells culture media [18] and bacterial pure culture (see Figure 5).This reduction leads to oxidative stress, lipid peroxidation, DNAlesions, and chromosome damage [18]. Carbon nanotubes canalso induce chemical modification of macrophages after internaliza-tion as demonstrated using the ID21 micro X-ray fluorescencemicroscope [19].



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Nuclear waste: where do radionuclides end up? The safe geological disposal of nuclear waste is one of the great

scientific and technical challenges for 21st century society. For anylong-term isolation of radionuclides from the biosphere, a wide range ofphysical, chemical, and biological processes must be understood(see Figure 6), which are a main topic of beamline BM20 (ROBL).Here, XAS is used for the study of radionuclides at the molecular level.

An important aspect of BM20 operation is safety; handling radio-nuclides, even in minute quantities, poses specific challenges to whichother beamlines cannot easily respond, as the few examples that followillustrate.

Waste matrices for radionuclides have to resist high radiation levelswithout releasing the enclosed radionuclides. In the case of americium-doped zirconia-ceramics, it could be shown that this radiation tolerance isdue to specific structural re-arrangements reducing strain and volume [21].

While waste containers inevitably corrode over long geologicaltimescales, the secondary minerals that form in this process catalyzeredox reactions, thereby enhancing radionuclide retention at the surfaceof the containers or within the clay backfill materials. This effect wasdemonstrated for selenium, thorium, and plutonium [22, 23, 24, 25].

Outside of engineered barriers, radionuclides are retained in thegeosphere predominately by absorption to mineral surfaces. Good coin-cidence between surface complexation modeling and in situ spectro-scopic experiments has been obtained, a critical requirement for reliableprediction of radionuclide migration [26].

Because they possess several oxidation states, actinides formnumerous aqueous species with corresponding diversity in geochemicalbehavior. The discrimination and identification of these species is a dif-ficult task, which has only recently been tackled by combining XAS

(twice) with electrochemistry [27 28], quantum chemistry [29], chemo-metrics [30], and new XAS analysis approaches [31].

Besides their radiotoxicity, the scarcity of some heavy elements isalso a challenge. For example, the structure of californium (III) hydratewas unknown until very recently [32].

When radionuclides migrate to the biosphere, biotic processes becomeimportant factors for their mobility. Bioremediation could therefore be alow-cost approach to their immobilization. Whereas single biotic processes

Figure 5 (left): TEM picture of human fibroblasts incubated with 7 nm-CeO2 nanoparticles. Vesicles containing nanoparticles are observed. Right: In-situXANES measurements determining the oxidation state of the CeO2 nanoparticles at the surface or within the cells. 21% of the Ce4+ surface atoms arereduced into Ce3+ after 24h of incubation with the cells. Courtesy M. Auffan et al.

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in this direction are now increasingly well understood, experiments withmore complex real-life systems still show unexpected results [33, 34].

Finally, it has now become possible to study the uptake, retention,and release mechanisms of radionuclides by higher organisms [35, 36].

Catalysis Catalyzed gaseous reactions are at the core of many environmental

technologies, and much research is directed towards developing bettercatalysts for specific reactions – whether to convert toxic gases intomore benign emissions, or to store and release specific gases, forinstance in hydrogen fuel systems.

The “static” notion of “storage capacity” is often only half thestory. For practical operation both the gas uptake and release functions

need to be kinetically suited to the application. Therefore, experimentsmust provide, in operando, the structural, functional, and kinetic assess-ment of the storage and release functions [37].

To this end, recent work at beamlines ID24 and ID15B has coupledtime-resolved X-ray techniques with infrared spectroscopy (IR). IRprobes the surface molecular speciation and complements extremely wellthe intrinsic structural sensitivity of the X-ray methods. Starting fromcombining IR with time-resolved energy-dispersive EXAFS [38, 39, 40],this approach is now extended to couple IR with very hard X-ray diffrac-tion (HXRD) for the dynamic and in-situ study of catalysts [41].

Time-resolved HXRD enabled the observation that Pd nanoparti-cles in a car exhaust model catalyst, alongside the surface chemistryconverting CO to CO2 and N2O to N2, can also dissociate CO and

Figure 6: Geochemical processes that may lead, over time and space, to the migration of radionuclides (RN) from a nuclear waste repository to thebiosphere. Courtesy A. C. Scheinost.



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transiently store the resulting atomic carbon. Direct cross-referencing tothe simultaneously acquired IR spectroscopy then revealed that this pre-viously hidden carbon storage has a significant effect on how molecularCO adsorbed on the Pd nanoparticles partitions itself between the dif-ferent possible adsorption sites (see Figure 7). This effect has a poten-tially high significance for the conversion efficiency of toxic gases suchas CO or N2O.

Conclusions These examples illustrate that today the ESRF confers prime impor-

tance to cutting-edge environmental sciences. Even if the systems studiedare diverse, they have in common the need to probe interactions: interac-tions between metals and plants, between metals and microbial biofilms,between minerals and aqueous solutions, and more general solid-liquidinterfaces, interactions between gases and solids, etc.

Beyond simple elemental identification, access to molecularcomposition is also crucial as the solubility, stability, and toxicity of an ele-ment depends on its chemical environment. Spectroscopic techniques, inparticular XANES and EXAFS, are therefore at the forefront of research.

Several trends are clearly emerging for the future: in-situ analyses,where experiments reproduce a natural environment as closely aspossible, including control of temperature, pressure, and atmosphere;nano-probing, whether to characterize nano-particles and their interac-tion or for a better observation of microscopic systems; and finally,time-resolved techniques. Knowing the kinetics of a chemical reaction iscrucial not only for catalytic processes but in all cases where “stability” ofa system is questioned.

The ESRF will respond to these trends with two Upgrade Beamlinescurrently under construction. TEXAS and NINA will soon enable newfrontline science at the service of the environment.

Acknowledgments We thank all co-authors of quoted articles for their contribution, andJean-Louis Hazemann and Gema Criado-Martinez for fruitfuldiscussion. Guillaume Morin is especially indebted to L. Alvarez, B.Cancès, G.E. Brown Jr., G. Calas, F. Juillot, V. Laperche, G. Ona-Nguema,D. Polya, O. Proux, D. Vaughan, and Y. Wang. The text was copyeditedby Claus Habfast.

Figure 7: Left (a): Time-dependent behavior of supported Pd nanoparticles during CO/NO cycling at 673K (10 cycles), exhibiting a periodic “sawtooth”variation in the lattice constant of the nanoparticles according to their reactive environment. Right (b): Data from four probes - DRIFTS, EXAFS, MSand HXRD ([311] reflection from Pd, straddled by 2 Al2O3 reflections) - that may be applied in situ, and pertain to a single NO-CO-NO cycle. TheX-ray techniques may be used, in time-resolved mode, simultaneously with the DRIFTS and MS. Courtesy M. A. Newton et al.



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