X-ray Microscopy and Imaging: Environmental Science

Introduction

X-ray microscopy presently makes major contribution to a wide range of environmental research and scientific studies. Some key examples include:

• Knowledge of fundamental environmental processes particularly in the food chain
• Effects of heavy metal contaminants and their remediation in the environment
• Efficient and clean energy production and the development of renewable energy sources

The techniques used are predominantly x-ray fluorescence mapping and micro-XANES for chemical state mapping.

Within the x-ray microscopy group we can use both hard and intermediate x-ray and have some of the highest resolution microprobes using Fresnel zones plates available. Spatial resolution of 100 nm on hard x-ray beamlines and 50 nm at the intermediate beamline can be achieved allowing unprecedented environmental studies to be achieved.

Useful links

Envirosync website

Research Examples

We have a diverse range of environmental research programs being conducted by users and group members

Fundamental processes

• Environmental systems contain complexity and heterogeneity on the micro- and increasingly nano-scales
• aim to understand complex processes and pathways
• Microscopic marine organisms are at the base of the world’s largest food chain
• Trace metals and the global carbon balance
• Role and importance of phosphorus speciation
• The microchemistry and bonding mechanisms of constituents such as sulfur and phosphorus to soil particles are critically important

Pollution and bioremediation

• Pollution and heavy metal contamination
• Fly ash from power plants
• Cadmium in soils
• Bioremediation efforts will expand
• Remove from our environment and safely lock up
• For example the uptake of arsenic from soils by fern species
• Contaminant transport mechanisms
• geo-biochemistry of microbes and their interplay with mineral surfaces
• For example the formation of Sphalerite deposits in natural biofilms of sulfate reducing Bacteria [1]

In research published in Science [2] , APS scientists used high-energy X-ray fluorescence measurements for mapping and chemical analyses of single free-floating, or planktonic, and surface-adhered, or biofilm, cells of Pseudomonas fluorescens. The results showed differences between the planktonic and adhered cells in morphology, elemental composition and sensitivity to hexavalent chromium, a heavy-metal contaminant and a known carcinogen. The biofilm cells were more tolerant of the contaminant, while it damaged or killed the planktonic cells.

Figure 1. False-color micro-XRF maps of qualitative spatial distributions and concentration gradients of elements in and around planktonic P. fluorescens microbes harvested before (A) and after (B) exposure to potassium dichromate [Cr(VI)] solution (1000 ppm) for 6 hours [2].

Spectromicroscopy in the 1–4 keV region: new capabilities and opportunities for environmental and biological science studies

Unique opportunities are opening up in an increasingly important energy region for environmental, biological, and advanced materials studies. Using brilliant focused x-ray beams at the Advanced Photon Source, spectromicroscopy can be achieved in the 1–4 keV region at beamline 2-ID-B. This energy region has traditionally been difficult to access, but recent inclusion of multilayer coatings to the grating monochromator has increased available flux by 20 fold at key absorption edges.

Phosphorus and sulfur are key elements in environmental and biological studies. Chemical state analysis using near-edge spectroscopy combined with spatial resolution of 70 nm allows probing and resolution of new science questions.

Recent examples of research at 2-ID-B include the study of phosphorus within dissolved and particulate organic materials. Phosphorus in these forms is a significant, but poorly understood, source of bioavailable phosphorus in many aquatic environments [2]. Figure 2 shows high resolution fluorescence maps of dissolved marine organic material locating phosphorus rich regions . The chemical state of the phosphorus in sub-micron areas can then be determined using spectromicroscopy as shown in Figure 3.

Figure 2. High resolution fluorescence maps of dissolved marine organic material. Absorption mapping of marine organic phosphorus particulates has shown that high density regions are not necessarily the phosphorus rich regions. Once the higher concentration regions of phosphorus are identified the chemical state can be determined using spectromicroscopy.

A second example concerns sulfur which is an indispensable nutrient for plants and microorganisms. Sulfur’s speciation in soils is intimately linked with the chemical state of the soils, such as redox potential and acidity [3]. Changes in the speciation of sulfur in soils caused by pedogenetic processes and changes of the physicochemical temporary environment (e.g., water logging) can therefore result in considerable changes of soil fertility. In both these examples knowledge of the chemical speciation at the sub-micron level and in-situ is critical and can be provided by x-ray spectromicroscopy.

Figure 3. Fluorescence spectromicroscopy from 100 nm areas of dissolved marine organic material. Fluorescence signal is used when the concentration of phosphorus is too low to be detected by absorption contrast.

Questions ?

References

[1] M. Labrenz et al., Science , 290, 1744 (2000) .

[2] Kenneth M. Kemner, Shelly D. Kelly, Barry Lai, Joerg Maser, Edward J. O'Loughlin, Deirdre Sholto-Douglas, Zhonghou Cai, Mark A. Schneegurt, Charles F. Kulpa, Jr., Kenneth H. Nealson , Elemental and Redox Analysis of Single Bacterial Cells by X-ray Microbeam Analysis, Science , 306, 686 (2004) .

[3] J. A. Brandes, C. Lee, S. Wakeham, M. Peterson, C. Jacobsen, S. Wirick, and G. Cody, Marine Chemistry 92 , 107 (2004) .

[4] J. Prietzel, J. Thieme, U. Neuhäusler, J. Susini, and I. Kögel-Knabner, Euro. J. Soil Sci. 54, 1-11 (2003).