EXAFS and XANES in Practice

To illustrate the utility of EXAFS and XANES analysis, we will examine a study of soil from Cornwall (a county in southwest England) that was polluted with very high concentrations of arsenic (1,000 jmg g-1 or more). This occurred due to mining activities that took place in this area from Roman times up to the beginning of the twentieth century, and today it poses a health risk across a large area of Cornwall (van Elteren et al. 2006). Especially dangerous are the so-called "hot spots," where the arsenic concentrations are higher than 10% and the potential mobility in the soil could result in serious water pollution and thus the poisoning of plants, animals and people. The arsenic in the soil from former industrial sites is potentially mobile because of its association with amorphous iron oxides, which can be solubilized under reducing conditions. The fixation of arsenic in these soils has been hypothesized to occur through either precipitation, which corresponds to the formation of secondary minerals/phases (such as iron arsenates), or the sorption of arsenate (As(V)) on iron (hydr)oxides, with the arsenic oxyanion immobilized by iron (hydr)oxides. The risk of mobilization is more pronounced for arsenite (As(III)) than for arsenate, since arsenate binds more strongly to minerals. In some instances the leaching of organic arsenic compounds, such as monomethylarsenic acid (MMAA) and dimethylarsenic acid (DMAA), can occur.

The potential leaching behavior of arsenic is usually studied using sequential extraction protocols, which provide insight into its association with the soil phases. However, such an approach only yields very crude information on the binding of arsenic and gives no structural information on the surroundings of the arsenic atoms themselves, which is crucial to assessing its potential toxicity and the risk of mobility. To obtain a better understanding of the physicochemical form of arsenic in the soil and its potential mobility and uptake by plants, As K-edge XANES and EXAFS analyses were used to retrieve molecular information on the arsenic; i.e., its oxidation states and the local structure around the As atoms in the soil. In this way, the most abundant modes of As bonding were identified (X-ray absorption spectroscopy experiments described in this text were performed in various synchrotron laboratories, including ESRF in Grenoble, ELETTRA in Trieste, and HASYLAB, DESY in Hamburg, with the financial support of European Community Contract RII3-CT-2004-506008 (IA-SFS)).

Normalized As XANES spectra of the soil and of reference arsenic compounds and minerals are shown in Fig. 6.10 (right), and include metallic As, trivalent As compounds (realgar [AsS], orpiment [As2S3], NaAsO2 and arsenolite [As2O3]), pentavalent organoarsenic compounds (monomethylarsenic acid [CH3AsO(OH)2] or "MMAA," and dimethylarsenic acid [(CH3)2AsO(OH)2] or "DMAA"), and pentavalent As minerals (scorodite [FeAsO4-2H2O] and pharmacosiderite [KFe4(AsO4)3(OH)4-6-7H2O]). From Fig. 6.10, it is evident that the edge position in the soil sample coincides with the position of the As(V) minerals scorodite and pharmacosiderite, which clearly indicates that the As in the soil is predominantly in a pentavalent form. In addition to the K-edge position, the shape of the As K

edge of the soil sample is very similar to the shapes of the two reference As(V) minerals scorodite and pharmacosiderite, where the As atoms are tetrahedrally coordinated to four oxygen atoms, which suggests a similar local symmetry for the As atoms in the soil.

A more detailed insight into the local structure around the As atoms in this soil can be obtained from the EXAFS analysis for the As K-edge. The k3-weighted EXAFS spectra for the soil sample and the reference As(V) mineral scorodite are shown in Fig. 6.11. Some information about the neighborhood of the As atoms can already be obtained from the FTs. In the spectrum of scorodite, which is the most likely candidate for the As carrier in the soil, there are two distinct peaks that represent the contributions of the first two shells of the neighbors around the As atom. In the spectrum of the soil sample, only the first peak of the nearest coordination shell is similar to that in the scorodite, while the characteristic strong peak of the second shell is absent. This clearly shows that the As in the soil is predominantly not in the form of crystalline scorodite.

Quantitative EXAFS analysis is used to determine the structural parameters of the local As neighborhood: the radii and Debye-Waller factors of the nearest neighbor shells, together with the chemical species and the average number of atoms in the shell. Upon modeling the EXAFS spectrum of the soil sample, four oxygen atoms were identified in the first coordination shell at a short distance, 1.69 A, and with a small Debye-Waller factor, which is characteristic of a tight As(V)-O bond, such as that present in scorodite. In the second coordination shell, iron atoms were clearly identified at a distance of 3.34 A, similar to the findings for scorodite. This is not surprising considering the elemental analysis data, which showed that As and Fe concentrations in the soil are strongly correlated. However, the number of Fe neighbors for each As atom was found to be significantly lower than in the crystal structure of scorodite. This suggested that secondary arsenic compounds were formed as a result of weathering, by the (co)precipitation of arsenate, which led to the amorphous or poorly crystalline FeAsO4.

In addition, one Si atom at a distance of 2.54 A and about four oxygen atoms at 3.48 A were observed in the local neighborhood of the As. Among the possible structures, only As5+O4 bonded to a SiO4 tetrahedron in a bidentate mononuclear complex, with an As-Si distance of 2.6 A, matched well with the short As-Si distance of 2.54 A observed. These results strongly suggest that arsenate is partially adsorbed onto quartz or aluminosilicates, such as the abundant clay that is present in the soil.

The presence of Ca or Mg atoms as candidates for As coordination were excluded by the EXAFS analysis. Also, there was no evidence of organic As ligands, so the presence of organoarsenic compounds in the soil was also excluded.

Thus, using these As K-edge XANES and EXAFS analyses of the contaminated soil, it was possible to determine the valence state of the As and its most common modes of bonding. The results show that the arsenic in the soil takes the form of the pentavalent oxide, which is less mobile, and hence potentially less bioavailable and less dangerous to the health than the trivalent form. However, the As5+ occurred predominantly in amorphous phases and sorbed species, and was not bound in the inert crystalline form, as expected, which could lead to arsenic leaching and thus a higher environmental risk.

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