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Can synchrotron micro-X-ray fluorescence spectroscopy be used to map the distribution of cadmium in soil particles?

Introduction

Cadmium (Cd) is accumulating in intensively managed agricultural soils in many countries (Sillpanaa and Jansson 1992), due to accession rates--mainly from phosphate fertilisers and organic amendments--that exceed the rate of removal and to strong Cd retention (Sumner and McLaughlin 1996; Jinadasa et al. 1997). Plants take up Cd from the soil, and plant products are the major route of Cd exposure for large segments of the human population. The resulting health risks are becoming better known (Satarug and Moore 2004).

The total concentration of Cd in soils is a poor predictor of plant uptake (McLaughlin and Singh 1999), and improved understanding of the behaviour of Cd in soils is needed to minimise its uptake by food and fodder crops. To this end, considerable effort has been focused on Cd (de)sorption (Elzinga et al. 1999; Sauve et al. 2000; Milham et al. 2004) and on selective chemical extraction (Ho and Evans 2000). Less attention has been paid to the distribution of Cd in soil and its effects on bioavailability and mobility (Boekhold and van der Zee 1991 ; Boekhold et al. 1991; Bottcher 1997; Seuntjens et al. 2002; Wu et al. 2002). Moreover, it is reasonable to suggest that heterogeneity also occurs at the micro-scale; however, the low concentrations of Cd in typical agricultural soils (Sillpanaa and Jansson 1992) have precluded relevant observations on this scale.

The brilliance of synchrotron sources has increased the sensitivity of X-ray fluorescence spectroscopy (XRFS) sufficiently to expose the local atomic environment of metal atoms in various materials (Xia et al. 1997; Rouff et al. 2004; Bohic et al. 2005), including Cd in natural organic matter (Karlsson et al. 2005). In addition, recent applications of micro-XRFS have revealed the distribution of Cd in plant roots and in biosolids (Naftel et al. 2001; Hettiarachchi et al. 2006). Cadmium was present at much high concentrations in the latter 2 studies than occurs in most agricultural soils; nevertheless, these successes encouraged us to test whether synchrotron micro-XRFS may be sufficiently sensitive to map the Cd distribution in small particles of agricultural soils.

Materials and methods

The samples

The soil samples were obtained from the 0 to 0.15 m layer at 4 sites on the peri-urban fringe of Greater Sydney, New South Wales, which is the oldest and most intensive horticultural region in Australia (Gillespie and Mason 2003). The samples were from 3 farmed sites (6, 7, and 17) and 1 unfarrned site (17A), part of a larger set prepared and analysed as described by Milham et al. (2004). The acid-extractable Cd concentrations for samples 6, 7, 17A, and 17 were 0.3, 0.9, 1.9, and 6.4 and mg/kg, respectively. This range is typical of intensively managed agricultural soils (Sillpanaa and Jansson 1992). A subsample of 17A was spiked to ~100 mg Cd/kg. A sample of phosphorite recently mined on the island of Nauru was also ground. This sample contained ~100 mg Cd/kg. Nauru phosphorite was included because it was a major source of Cd accessions to the soils on vegetable farms in Greater Sydney during the 20th Century (Jinadasa et al. 1997).

Grain mounts

Powdered soil or Nauru phosphorite (5-10mg) was placed in a cylindrical mould (~20mm diameter) and vacuum infiltrated/dispersed in ~2 mL of a low viscosity, low volatility epoxy resin (Epo-thin[R], Buehler Pty Ltd, USA). The resin was allowed to cure, then the block was thinned to 150-200 [micro]m and the surface was polished. The resulting grain mount contained at most ~1 mg of sample.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Synchrotron micro-XRFS

Synchrotron micro-XRFS measurements were performed on the grain mounts at beam-line 2-ID-D of the Advanced Photon Source, Argonne (Yun et al. 1999; Cai et al. 2000). The incident radiation (27.0 [+ or -] 0.003 keV) was focused to a spot ~1 [micro]m in diameter using a gold zone plate of 150 p.m diameter with a focal length of 337.5 mm (Fig. 1). A pin hole of 25 by 25 [micro]m, made from polished tungsten blades, was placed ~5 mm upstream of the sample to reduce zero- and high-order diffraction from the zone plate. Grain mounts were supported at the focal point of the incident beam, with an angle of ~15[degrees] between the incident beam and the normal to the sample plane. The germanium fluorescence detector (100[mm.sup.2] LEGe, Canberra Industries, Canada) was placed as close as possible to the grain mount at 90[degrees] to the incident beam, upstream of the mount (Fig. 1). The flux of transmitted X-rays was monitored downstream of the grain mount using an ionisation chamber, and during the 72 h of the study, the count rate was ~1 x 105 counts/s, with a range of [+ or -] 5%. The beam emerging from the ionisation chamber was intercepted and visualised using a CdW[O.sub.4] scintillator crystal (~500 [micro]m thick). The downstream locations were satisfactory because the samples did not absorb/scatter an appreciable proportion of the incident beam. The incident beam, sample, and detectors were enclosed in a hood that was continuously purged with helium gas.

[FIGURE 3 OMITTED]

A scan consisted of a set of stepwise fluorescence intensity observations at points on a rectangular grid. The grain mount was driven in the X-Y plane using computer-controlled motors with linear encoder feedback. The range of a scan could extend to several mm and the step size within the selected range could be adjusted in 0.05-[micro]m increments. The counting time could be varied at will between scans but was fixed within a scan. Counting times and step sizes are presented with the images.

During a scan, the signal from the fluorescence detector was processed through a shaping amplifier and pulse height analyser. Regions of the spectrum corresponding to the characteristic energies for Cd and several other elements were monitored at each observation point (pixel) and the data were stored in a computer file. An image was constructed for each element in a scan from the stored intensity data by representing the relative intensities of the pixels on an artificial colour scale: red (greatest intensity) > orange > yellow > green > blue > grey (least intensity). The numeric scales on the axes of the images are distances ([micro]m), in the X-Y plane, from an arbitrary reference position. In interpreting these images, 2 facts are important. First, for small spherical particles of uniform composition, the fluorescence for an element increases in intensity from background to a maximum as the particle is scanned radially from the edge to the centre. Scans that deviate from this pattern indicate deviations from spherical geometry and/or uniform distribution. Second, clay-sized particles are <2 [micro]m in diameter.

Results and discussion

Soils

Figure 2 shows a strong Cd [K.sub.[alpha]] signal for one particle from #6. No other particles in the grain mounts of the unamended soils had Cd signals above background. The particle that triggered the Cd signal was ~10 [micro]m in diameter, and the peak intensity was ~0.5 x [10.sup.3] counts/s. This peak occurred near a peak for Zn, i.e. with approximate X, Y coordinates of (823, 278). Peak signal intensities for Y, Ti, and Ca also occurred nearby (not shown). In contrast, the strongest signals for Fe, Pb, and Cu occurred closer to the putative rim (Fig. 2), as they did for Si and Mn (not shown). The data for Pb contain a contribution from As; however, the effect was small because the As concentrations were low. The distribution of the signal for K was bimodal, with the major peak located centrally and a smaller peak towards the upper centre of the image. Therefore, assuming that the particle was approximately spherical, of these 11 elements only Cd, Zn, Y, Ti, and Ca may be distributed relatively uniformly throughout the particle (see Materials and methods). Finally, Fig. 2 includes areas outside the presumed particle boundaries, where the signals for Si, K, Ca, Ti, Mn, Fe, Cu, Zn, Sr, Y, Cd, and Pb each produced no more than ~5 counts/s.

We were surprised that the only particle to emit a considerable Cd signal was observed in the grain mount of #6 (Fig. 2), because this was the sample with the lowest bulk Cd concentration (0.3 mg/kg). Only ~1% of the area of the grain mounts was scanned and each mount contained at most 1 mg of soil; nevertheless, the low bulk Cd concentration in #6 indicates that such particles must be rare. If this particle is a contaminant, it is more likely to have been introduced in situ than during/after sampling, either from soil amendments (Jinadasa et al. 1997) or road dust (Adachi and Tainosho 2004). Site 6 was within 100 m of a minor road.

The maximum Cd signal intensity of many particles in the grain mount prepared from #17A that had been enriched to ~100mg Cd/kg was also ~0.5 x [10.sup.3] count/s. One such a particle is imaged in Fig. 3. Despite the concern that the binding of Cd may vary with load (Xia et al. 1997; Ho and Evans 2000), some comments about the data for this Cd enriched subsample are in order. For example, there was an appreciable Pb signal with a distribution similar to that of Cd (Fig. 3). An association between Cd and Pb is not unreasonable, since both bind to soil organic matter and hydrous metal oxides (Adriano 2001). It is noteworthy that the Cd-Pb hot spot occurs at a location separate from several strong localisations of Fe. That is, there was no consistent co-location of strong signals for Cd and Fe as reported for a sewage sludge that contained percentage concentrations of Fe (Hettiarachchi et al. 2006). The concentration of Fe was almost 10 times that of the (hydrous) Fe oxides in our soils (Milham et al. 2004). This difference may have sufficiently affected the Cd-Fe distribution to make the association observable in one study and not the other. Alternatively, a Cd-Fe association may not be general in soils, even though it has been inferred from desorption studies (Milham et al. 2004).

Nauru phosphorite

This mineral consists of a Ca, F, O, and P matrix. Of these elements only Ca was observable in this study and differences in the Ca signal intensity (Fig. 4) relate to particle thickness (see Materials and methods). Cadmium was present at a bulk concentration of ~100 mg/kg and was readily imaged (Fig. 4). The relative intensities of the Ca and Cd signals are distributed similarly. That is, Cd too was relatively uniformly distributed through the matrix, which is consistent with the data for an apatite from Africa (Sery et al. 1996). Consequently, Cd may generally be relatively uniformly distributed in apatitic minerals.

[FIGURE 4 OMITTED]

Conclusion and future prospects

Grain mounts allowed the observation of Cd and several other elements in small particles of soil: the backgrounds were low and leakage into the resin appeared minimal. The resolution of the X-ray microprobe at beam line 2-ID-D of the Advanced Photon Source was adequate; however, Cd mapping in unamended agricultural soils may require at least a 10-fold increase in sensitivity. This is achievable and we are planning studies that will complement new micro-XRFS data by mineralogical information at a comparable scale.

Acknowledgments

This work was undertaken with financial support from the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. Use of the Advanced Photon Source was supported by the USA Department of Energy, Basic Energy Sciences, Office of Science, under contract No. W-31-109-Eng-38. We thank Arthur Day and Graeme Smith for preparing the grain mounts, Peter Holmes and Gary Kuhn (Incitec-Pivot Pty Ltd) for the sample of phosphorite from Nauru, and David Harland for input to the text.

Manuscript received 21 December 2006, accepted 30 October 2007

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Paul J. Milham (A,B,E), Timothy E. Payne (C), Barry Lai (D), Rachael L. Trautman (C), Zhonghou Cai (D), Paul Holford (B), Anthony M. Haigh (B), and Jann P. Conroy (B)

(A) NSW Department of Primary Industries, LB 4, Richmond, NSW 2753, Australia.

(B) Centre for Plant and Food Science, University of Western Sydney, LB 1 797, Penrith South DC, NSW 1797, Australia.

(C) Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia.

(D) Advanced Photon Source, Argonne National Laboratory, 9700 S Cass Avenue, Argonne, IL 60439, USA.

(E) Corresponding author. Email: paul.milham@dpi.nsw.gov.au
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Title Annotation:Short Communication
Author:Milham, Paul J.; Payne, Timothy E.; Lai, Barry; Trautman, Rachael L.; Cai, Zhonghou; Holford, Paul;
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Dec 1, 2007
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