The exquisite sensitivity of scanning x-ray microfluorescence (µSXRF) to trace elements makes µSXRF an ideal tool to map trace element distribution within a heterogeneous matrix. After µSXRF has identified the location of trace and major elements within a matrix, the new technique of scanning x-ray microdiffraction (µSXRD) identifies and images the distribution of mineral species in the nanoscale particles that are the most reactive towards the trace metals. Then, the nature of minerals hosting a particular trace element within the heterogeneous matrix is deduced from the comparison of elemental and mineral species maps (from µSXRF and µSXRD, respectively). Finally, with micro-extended x-ray absorption fine structure (µEXAFS) spectroscopy, the structural relationship between metal and mineral host is examined. Specifically, the coordination chemistry of the metal is determined, and hence its incorporation mechanism within the mineral host is identified.

The µSXRF elemental maps of the nodule show that manganese, iron, zinc, and nickel are unevenly distributed at the micrometer scale. Iron and manganese have no detectable correlation, and zinc and nickel are both strongly correlated with manganese and not with iron. The highest zinc and nickel amounts are observed in the manganese-rich core; the outer regions also contain significant amounts of manganese and zinc but are depleted in nickel. The partial nickel­manganese association suggests that manganese is present in at least two forms, with only one containing nickel. The comparison of µSXRF and µSXRD maps clearly shows that nickel and the mineral lithiophorite have the same distribution, indicating that nickel is exclusively bound to this particular manganese phase. The goethite map does not match the zinc and iron elemental maps, which means that this constituent is devoid of zinc.

Where the Metals Go in Soils

Heavy metals are components of hazardous waste at many industrial and government sites. They can exist in combination with other species in soluble (dissolved in water) and insoluble forms and, unfortunately, cannot be completely broken down. In other words, once a metal, always a metal. As a rule, the less soluble a chemical species, the less mobile and less toxic; the more soluble it is, the more mobile and more toxic. Manceau et al. focused on nickel because certain nickel sulfates, sulfides, and oxides are suspected carcinogens. Industrial nickel sources include metal processing operations, coal and oil combustion, and sewage sludge. In soils, after first undergoing many chemical transformations, nickel waste often finds its resting place bound (sequestered) in compressed aggregates or nodules that are rich in iron and manganese and are stable over long periods of time. By using three x-ray techniques in combination, the researchers were able to determine at the molecular scale where and how nickel naturally hides itself in soils, leading to the eventual ability to (1) predict with greater accuracy the evolution of metal chemistry in a contaminated environment (or contaminated sites); and (2) modify and control a metal's form to maintain soil quality and frame effective, site-specific waste-cleanup (remediation) strategies.

Combined µSXRF­µSXRD measurements recorded on a soil iron­manganese nodule. The four images on the top are elemental maps obtained by µSXRF, and the four images on the bottom are mineral species maps obtained by rastering the sample in an XY pattern and analyzing the diffraction patterns.

The sequestration mechanism of nickel inferred from µSXRF and µSXRD was confirmed by nickel K-edge µEXAFS measurements from selected regions of interest in this nodule and others from the same soil and from various soils from different countries across several continents.

A µEXAFS examination of the sequestration mechanism of nickel in a soil ferromanganese nodule, showing a nickel K-edge µEXAFS spectrum from a nickel "hot spot" of the core of the nodule seen in the µSXRF image (solid line), compared to a spectrum from the literature for a nickel-containing lithiophorite reference (dotted line).

Unlike nickel, the zinc map does not resemble any of the mineral species maps obtained from µSXRD, nor can it be reconstructed by a combination of several. Therefore, the nature and proportion of the zinc host phases were determined by analyses of five µEXAFS spectra recorded in different spots chosen to vary the proportions of component species (principal component analysis and least-squares fitting analysis). These analyses indicated the presence of three predominant species, lithiophorite, birnessite, and poorly-crystallized iron oxide (e.g., ferrihydrite). Finally, bulk EXAFS spectra were recorded in order to verify that these assessments truly represent all of what is found in the entire sample.

A µEXAFS examination of zinc sequestration in a soil ferromanganese nodule, showing zinc K-edge µEXAFS spectra collected from five points of interest having variable manganese/iron ratios, as indicated in the µSXRF image.

Research conducted by A. Manceau (Université Joseph Fourier and CNRS, France, and ALS), N. Tamura, M.A. Marcus, A.A. MacDowell, R.S. Celestre, R.E. Sublett, and H.A. Padmore (ALS); and G. Sposito (Berkeley Lab).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES), and Berkeley Lab Laboratory Directed Research and Development. Operation of the ALS is supported by BES.

Publication about this research: A. Manceau, N. Tamura, M.A. Marcus, A.A. MacDowell, R.S. Celestre, R.E. Sublett, G. Sposito, and H.A. Padmore, "Deciphering nickel sequestration in soil ferromanganese nodules by combining x-ray fluorescence, absorption and diffraction at micrometer scales of resolution," American Mineralogist 87, 1494 (2002).

ALSNews Vol. 219, April 2, 2003